Carb talk for Beginners

[GreenTwin]

In the early 80's the world was evolving into fuel injection. My 1983 VW GTI had a mechanical air flow meter before the throttle body that worked fine with the stock cam but when a cam with more overlap was installed the more erratic pulses from the cam change caused the flow meters plate to flutter uncontrollably and lost control of the air flow metering.
The popular fix was to strip off the fuel injection system and install some type of carb instead. I went with 2, side draft Dellortos, similar to weber DCOEs. The venturies and every circuit was tunable. The fussiest component was the idle circuit. it was against the rules to drill out jets, but I would have had to order them and waited a week to get them so drilling was the order of the day. I drilled too big occasionally so I began making the idle jets out brass. That jet size was so fussy. I found that tapered jet reamers were available and worked great for finding the best size jet orifice.
As Programmable Fuel injection came into vogue for modified engines I built a Mega Squirt kit to step into the new age, but I never installed it. It was a whole new world of data logging and re-mapping fuel and timing tables. I wish I could have gotten excited about it as I know there is drivability and increased power from Programmable EFI, but there is a personal satisfaction from fighting fueling issues mechanically. I could make or modify many of the carbs and linkage systems and it bothered me that I couldn't keep that simplicity with Computerized engine management.

I purchased a 1967 VW Beetle, just to have a car that I could actually work on and repair, with NO electronics.

I got in a pinch and sold that car, but I am beginning to regret doing that.

I am somewhat old-school, and like to be able to repair mechanical things.

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[Klrskies]

I purchased a 1967 VW Beetle, just to have a car that I could actually work on and repair, with NO electronics.

I got in a pinch and sold that car, but I am beginning to regret doing that.

I am somewhat old-school, and like to be able to repair mechanical things.

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I used to rebuild and modify the air-cooled engines. I have the case align boring tool still. Nearly all machining can be done in a bridgeport mill since the engines are so modular. I learned about some carb issues on the air-cooled engines.

The single, center mounted PICT carbs required several sources of heat to keep the fuel from falling out of suspension as it left the carb and traveled down those long runner tubes. The carbs could easily frost at their bases without the preheated air tube to the carbs air filter intake and the exhaust gasses manifold heat tube to the base of the intake manifold functioning correctly and not getting plugged up with carbon.

Switching to dual carbs, with the carbs sitting directly over the 400-degree heads got rid of many tuning issues as the head provided plenty of heat to keep the fuel in suspension traveling down the short runners.

I still have a 69 bug that I took the body off of to replace the passenger side floor pan. I hope I get to get it running again. Simple is good as far as maintaining a car goes but drivability and clean emissions suffer. But it can be pretty good if properly sorted out.

I also had 69 fastback that I stripped the fuel injection off of and fitted with an aftermarket intake with a center mounted, synchronized, 2 barrel carb from an early Porsche that also suffered tuning issues that would not resolve until I finally kept the base of the carb warm with a homemade aluminum carb spacer that had had hot engine oil circulating thru it and the factory-style exhaust heat tube warming the base of the intake manifold tubes sufficiently to keep fuel from falling out of suspension.
All those modified bugs running around out in warm, Southern California that were so popular in the magazines of the day didn't have to deal with Midwest winters!

 

[GreenTwin]

I had an early 60's Fiat 400 with the suicide doors that I got from the junk yard.
Never got it running, but always wanted one that ran.
Its not as fast or versatile as a VW Beetle, but I like the Fiat a lot better, and the engine seems more accessible.

If I ever win the lotto, I will get another Fiat 400.

(Not my photo)

 

[GreenTwin]

When I built my 302 V-8 engine I played around with several different designs and sizes of carburetors. What I settled on was, at the time, the smallest O.S. carb available. It has a bore of about .200 (5.10mm). I then measured the critical sizes, bore, air bleed port, barrel diameter etc. and started drawing a reasonable scaled version of a factory carb for this engine. This would be the Autolite/Motorcraft 2100 2bb. carb. My carb has all the looks of the full sized carb but uses the functionality of the O.S. carb. The fuel control needle is hidden under the choke spring body.
The carb works throughout the rpm range of my engine and quite cleanly I might add. The rpm ranges from idle which is about 1500 rpm to 7800 rpm. The idle is high because the engine is limited by the scale size of the flywheel. The engine bore is 1.00 (25.0mm) and the stroke is .90 (23mm) My fuel supply is a remote tank that has the top of the tank level with the fuel control needle valve. Once started the engine will run all the fuel from the tank which is 1.00 in depth. The picture show the most important aspect of the carb, the venturi bore. I didn't use calculations to arrive at this size it was by trial and error to get the best performing carb.

That is a really neat and clever carb design.
Thanks for posting that.

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[Roger B]

A lot of the design and complexity of the carburettor design depends on it’s intended purpose. As has been said on here if the engine just has to idle at an exhibition for a few hours/days it can be quite simple.

If the engine is coupled to a load it becomes more difficult. There have been many model engines used to drive aircraft or boats but a propellor is a very benign load as the speed drops the torque requirements drops quadratically so stalling is usually avoided (it also acts a good speed limiter in the case of model aircraft engines).

Very few model engines have been used to drive land vehicles, ETW designed the 1831 locomotive and the Model Engineering road roller. A few railway locomotives have had homebuilt engines, Roy Amsbury built a Hymek powered by a 120 cc V8. The problem here is that as the load on the engine increases, reaching a gradient for example, the engine speed drops as does the airflow through the carb. This reduces the depression by the jet so the fuel delivery also drops, resulting in the engine stalling. If the throttle is opened this may further reduce the air velocity which make the situation worse.

Full size engines either use multiple jets to compensate for this or use a system to keep the air velocity fairly constant, constant velocity or constant depression as George mentioned. ETW designed a carburettor with air flow compensation for the 1831 and Roy Amsbury built a version of the SU CD carburettor for his V8.

I decided to try a small version of the SU CD type for my 12cc four stroke twin (the SU type does not like the pulsed flow from single cylinders. For full size one choke per cylinder balance pipes were used).

Keeping the air valve and jet parts concentric was tricky, mostly using small end mills as boring bars but in the end after some experiments with piston springs it works quite well and will go on and off load without stalling.

I decided to use a fuel reservoir with an overflow and a diaphragm pump rather than trying to make a flow chamber at this size.

There are some more pictures on Flickr after this one:



Some trials here:

 

[Steamchick]

Hi Kirskies.
I had a 1972 Volvo with the Bosch mechanical air-flow metering for single point fuel injection - and 10.5:1 compression pistons! What a brilliant set-up. NEVER any problems. 2.0l 125BHP compared to a colleagues 1976 similar ((But next generation model) that had a 2.2l engine with twin carbs, 9.5:1 compression and only 98BHP.
He asked if he could buy my fuel injection system as I was getting 28mpg compared with his 26mpg best. And my earlier car went MUCH quicker (his words when he borrowed it to try!). Mine needed 5 star fuel though, which was becoming a bit scarce in the late 1970s.
And I could get as low as 18mpg if I really used it!
K2

 

[CaneSelvatico]

@Roger B Sometimes it can be surprising how simple carburetors can perform very well even for more wide range of situations. Dell'orto company produced a series of small carbs for small 50cc mopeds and Vespas (named SHA and SHB) with throat diameter from 10mm to 13 for the SHA and 14 to 19 for the SHB. They only had a flat slide valve and a single jet without metering needle (the bigger series had also an idle jet), the fuel metering was done through a vertical pipe with two small holes right in front of the valve. I used them a lot even with tuned 75cc engines they performed quite well.

 

[Steamchick]

The most curious, but not most flexible, carb I experienced was a guy in the 1960s who ran a drag bike that took a bleed of exhaust gas to a jet that blew over the carb jet.... Apparently it helped him win medals drag racing in South Wales... for 500cc single cylinder engines. He reckoned the hot exhaust jet timed before the inlet valve opened drew a cloud of fuel vapour into the intake manifold that was followed by the air when the inlet valve opened. He was using a larger carb than anyone else to avoid the venturi restricting the amount of air the engine could ingest. But the large carb could not suck the fuel effectively - hence the exhaust "blow" to suck the fuel. He said it took a few years of development to get it right, but the trophies proved it worked.
But only at full throttle on a highly tuned engine?
K2

 

[Vietti]

Enjoying this thread! I have made a handful of model carbs and none were outstanding.

It hasn't been mentioned yet(?) but I like the OS carbs and the Traxis type carbs that have a drum type air intake that rotates on a spiral ramp and lifts the needle to supply more fuel as the airway opens up. I wonder if anyone has designed one like this and made plans for making one??

 

[GreenTwin]

The most curious, but not most flexible, carb I experienced was a guy in the 1960s who ran a drag bike that took a bleed of exhaust gas to a jet that blew over the carb jet.... Apparently it helped him win medals drag racing in South Wales... for 500cc single cylinder engines. He reckoned the hot exhaust jet timed before the inlet valve opened drew a cloud of fuel vapour into the intake manifold that was followed by the air when the inlet valve opened. He was using a larger carb than anyone else to avoid the venturi restricting the amount of air the engine could ingest. But the large carb could not suck the fuel effectively - hence the exhaust "blow" to suck the fuel. He said it took a few years of development to get it right, but the trophies proved it worked.
But only at full throttle on a highly tuned engine?
K2

Primitive fuel injection.
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[GreenTwin]

Enjoying this thread! I have made a handful of model carbs and none were outstanding.

It hasn't been mentioned yet(?) but I like the OS carbs and the Traxis type carbs that have a drum type air intake that rotates on a spiral ramp and lifts the needle to supply more fuel as the airway opens up. I wonder if anyone has designed one like this and made plans for making one??

I remember the rotating drums on my OS carbs, and I also noticed the lifting needle.

Someone posted a model airplane style carb design concealed inside of an auto engine looking carb, on this thread.
I will have to look back and see what they did.

(see George's post #37).

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[Klrskies]

Hi Kirskies.
I had a 1972 Volvo with the Bosch mechanical air-flow metering for single point fuel injection - and 10.5:1 compression pistons! What a brilliant set-up. NEVER any problems. 2.0l 125BHP compared to a colleagues 1976 similar ((But next generation model) that had a 2.2l engine with twin carbs, 9.5:1 compression and only 98BHP.
He asked if he could buy my fuel injection system as I was getting 28mpg compared with his 26mpg best. And my earlier car went MUCH quicker (his words when he borrowed it to try!). Mine needed 5 star fuel though, which was becoming a bit scarce in the late 1970s.
And I could get as low as 18mpg if I really used it!
K2

Impressive indeed! Was that the Bosch D Jetronic system in use?
VW first released the Bosch D Jetronic on the 1968 VW type 3 models with a center mounted throttle body and individual runners going to each cylinder. It was the world's first computerized, electronic fuel injection system with some integrated diagnostic capability. It was quite phenomenal in its day and functioned quite well as long as VW techs maintained it. Some owners and independent mechanics would strip off the fuel injection system and replace it with the 1967 or earlier dual, single barrel carb set up or aftermarket dual two barrel webber or solex carbs so they could work on or modify a system they could understand.

When VW decided to leave the air-cooled world and begin producing the water cooled Golf or Rabbit and also switching over to fuel injection, they implemented the Bosch K Jetronic, or as it was also known, the CIS system. Totally mechanical with its air vane metering the air intake volume. The fuel pump was the only thing electronic.

This worked well but was intolerant of cams that introduced enough overlap to cause disturbing pulses in the intake tract and caused the vane to flutter excessively. On the MK1 GTI's once one switched to dual sidedraft carbs, the 1.8 liter engines could be modified and tuned to the 125-175 HP range... lots of fun in an 1800 lb car.

Hi Kirskies.
I had a 1972 Volvo with the Bosch mechanical air-flow metering for single point fuel injection - and 10.5:1 compression pistons! What a brilliant set-up. NEVER any problems. 2.0l 125BHP compared to a colleagues 1976 similar ((But next generation model) that had a 2.2l engine with twin carbs, 9.5:1 compression and only 98BHP.
He asked if he could buy my fuel injection system as I was getting 28mpg compared with his 26mpg best. And my earlier car went MUCH quicker (his words when he borrowed it to try!). Mine needed 5 star fuel though, which was becoming a bit scarce in the late 1970s.
And I could get as low as 18mpg if I really used it!
K2

I liked that level of tech back then! Nothing like the complexity of today.
VW kinda did it backwards... In 1968 they produced the type 3 vehicles with an electronic, D Jetronic system from Bosch, a computerized system with limited diagnostic capability. That system would be refined and finally implemented into the beetle in 1975. It was ahead of it's time. Some owners and independent mechanics didn't fully appreciate the well-functioning fueling system after the warranty ran out and often stripped the f/i system off and replaced it with original or aftermarket dual carb systems, similar to the 1967 and prior dual carb systems for type 3's.

Then, as fuel injection was being utilized on the later, water-cooled VW Golfs, they used Bosch's K Jetronic, or as it was better known, CIS mechanical fuel injection system. This was the one with a flapper plate in the intake system to meter air intake which couldn't tolerate cams with much valve overlap due to pulsations. Once the GTI's already perky 1.8 liter engine was equipped with proper, dual, side draft carbs from weber or dellorto, the engine could then be readily modified from it's stock 100 hp up to150~200 hp with aftermarket cams, high compression, stroker cranks, big bore pistons, head work and exhaust systems. Much fun in an 1800 lb. car in a simpler, analog time.

 

[Steamchick]

The Volvo had the CIS system. I had motorcycles, so although the Volvo was fast, it's main reason for being was carrying 2 Timpani and full drum kit around with the brass band. For speed I used motorcycles... Cars simply don't compare in my view.. But we all differ in our choices.
I did think the Volvo shifted better then most cars, and the fulling was so good, when the Bosch regulator for the alternator began to give me problems and leave me with a flat battery, I could bump start the Volvo - from a parking space in a multi-story car-park - just by pushing the car (door open) a few strides, hopping in and engaging gear and clutch! Then never a mis-fire, more economical than the carburetted similar large estate cars, then faster than almost everything on 4 wheels when you wanted to... Away from traffic lights, or up the motorway in comfort with a car load of mates! - Even towing a large sailing boat or car on a trailer didn't bother it. It was the emissions laws that nailed the lid on the coffin of cars like that. Fastest Tank/Wardrobe on the road!
https://www.autoevolution.com/cars/volvo-145-1967.html#aeng_volvo-144-2000-20-126-hp
K2

 

[Klrskies]

The Volvo had the CIS system. I had motorcycles, so although the Volvo was fast, it's main reason for being was carrying 2 Timpani and full drum kit around with the brass band. For speed I used motorcycles... Cars simply don't compare in my view.. But we all differ in our choices.
I did think the Volvo shifted better then most cars, and the fulling was so good, when the Bosch regulator for the alternator began to give me problems and leave me with a flat battery, I could bump start the Volvo - from a parking space in a multi-story car-park - just by pushing the car (door open) a few strides, hopping in and engaging gear and clutch! Then never a mis-fire, more economical than the carburetted similar large estate cars, then faster than almost everything on 4 wheels when you wanted to... Away from traffic lights, or up the motorway in comfort with a car load of mates! - Even towing a large sailing boat or car on a trailer didn't bother it. It was the emissions laws that nailed the lid on the coffin of cars like that. Fastest Tank/Wardrobe on the road!
https://www.autoevolution.com/cars/volvo-145-1967.html#aeng_volvo-144-2000-20-126-hp
K2

I had motorcycles too, but never used anything on them other than stock or a very similar replacements. I wanted to try putting 4 motorcycle cv carbs on a cars 4 cylinder but never got that far. I am thoroughly impressed with the amount of power current automotive engines can produce... and with catalytic converters the absence of smells. It would still be fun to play with aftermarket programmable aftermarket engine management... data logging and map editing.

In the late 90's I got involved with downloading maps from bmw m20 6 cylinder engine's e-prom chips with a cheap, e-bay chip reader/writer and using an home-cooked editor that a Porsche enthusiast had created for a similar Bosch system they used, then flashing the e-prom with the altered code. Sometimes the checksum wouldn't be correct and it wouldn't work, and the programmer that created the editor didn't want to spend the time sorting out the differences between the Porsche and BMW DMEs, but that was really fun and cheap. My daughter was into hondas at the time and there were a few good free editors for their e-proms available. She had a 2 liter prelude at the time and for some reason it's trigger wheel in the distributor was different than one that was in the distributor that was on the integra engine that was so popular and who's engine management computer was supported with the free, available editors.

I had a wire EDM at the time and made a new timing wheel for the prelude's distributor so we could use the integra's readily available, editor-supported computer to run the prelude. I thought it might be a popular conversion, but it really wasn't as it lost some functionality and diagnostic capability of the preludes ECU and Prelude owners didn't seem to want to modify their cars too much. About that time Mega Squirt and Hondata were becoming popular for aftermarket engine management system re-mapping for the popular Honda engines... although not inexpensive.
What a fun time for re-mapping up until then and it wasn't expensive, difficult to modify or troublesome to access the e-prom chip's code... I think the e-bay chip reader and cable was like 40$ from eastern Europe! The cars were out of warranty and dealers didn't monitor for edited chips like they do today. I still liked carbs better though without all the electronics.

 

[Steamchick]

Working with new models through the emissions lab and with on-road real driving studies, I saw first hand how much effort from design went into creating a map that functioned well in all aspects of driving, as many conditions as we could verify (towing caravans over the alps in sub-zero temperatures, cold starting at or near -30C in Finland, Hot evaluations at near 50C in the desert near Dubai, coping with English Belgian, Spanish and a host of other countries' traffic circumstances, etc. so believe the Manufacturers' data to be the best compromise for all markets.
I am sure the tinkerers can do better for any specialised condition, but also pretty sure that emissions are compromised in the process, and probably economy too. Just try blowing a bit of propane into the air intake? - Or NOx? Either of those will affect the performance and tailpipe emissions as well.
Better still, on ANY car over 3 years old with an airflow meter, try a thorough clean with carb cleaner aerosol, or whatever, to clean the "hot-wire"? It makes a difference with all my cars... because these hot-wire sensors do get dirty and that affects the calibration.... but it takes more than 3 years use before the calibration goes out enough to make a significant difference. (because the law says they have to be that good).
It is not illegal to change what the manufacturer has set, but it can be illegal if the manufacturers have different settings for the emissions tests and the road version.... (I believe certain German manufacturers have been caught having done that in the past...?). Likewise illegal in some countries to sell a car that knowingly has been modified such that it changes the tailpipe emissions. If it is sold for use on the public roads to the original emissions certification. So please be aware of the implications of tuning manufacturers' stuff.
But you knew all that anyway, I guess?
K2

 

[Klrskies]

Yes, of course, and today all ECU's are much more difficult to crack into and edit but the aftermarket tuning companies have become able sell switchable map tuning systems that can be uploaded thru the diagnostic port and returned to the stock maps for emissions testing or a visit to the dealer. A sharp tech can still detect if a modofied tune is available to the system but most don't.

Up until Labor Day 2020, many Volkswagen dealers were registered dealers and installers for VW approved, APR stage1 and 2 software for their cars. Of course, as warranty claims came up on the cars, Volkswagen initially could void the warranty for any reason but many used common sense and didn't want to damage customer relations.

Yet eventually after several court cases, it was determined that VW could only void the warranty if the damage could directly be proven to be specifically caused by the performance software. It came down to dealer deciding if they wanted to repair the car under warranty or not. It was troublesome enough they finally ended their relationship with APR and began having mechanics begin checking damaged engines and transmissions for modification to the factory software and if modified, the info went into the corporate database record for that vehicle.

Tuning companies responded by making their software include a switchable tune that could be returned to stock tune before the vehicle was connected to the dealers or emission testing facilities computer.

The bump in performance was significant even on stage 1 with 70 ft. lb. bump in torque on the otherwise stock Mk6 GTI. If one had made additional performance mods that were not taken into account on the software the owner needed to have the care tuned on a chassis dyno to optimize the maps as no VW dealer had dyno facilities. VW also had a hefty $175 dollar upcharge every time a tune was upgraded or changed and many of the non-VW APR dealers were much cheaper or free for a few times as well as having dyno facilities and stage 3 level tuning as well.

The cars have held up surprisingly well if not undermined by using inappropriate fuel grades or pushing the engine too hard before fully warming to temp and good maintenance observance. All the knock sensing remain active.

Who could blame VW for not warrantying obvious engine abuse? Yet many did if the mechanic didn't report the installation of modified software... many of them ran modified tunes on their own personal cars and helped protect the owners... hence VW eventually choosing to get out of installing modified tunes on their customers vehicles.

 

[EyeTrip]

Just look at Bernoulli's principle, you will get it

 

[GreenTwin]

I vaguely recall from the ancient days (pre-1985) that I studied non-electrical topics in Physics I & II, and as I recall we were forced to take Thermodynamics, which I hated.
We called it the dreaded "Thermo" class, and it seemed more like Chinese water torture than anything useful for EE's.

I am sure I never had any fluid dynamic classes.

I remember PV=mRT

As far as Bernoulli's principle, I have never been able to wrap my head around that.

I can understand electrical, basically because it cannot be compressed, and it acts like water flowing in a river, with the branches all summing to equal the flow in the main river channel.

Voltage is analogous to pressure, amperage is analogous to current flow in a river or a conductor.
Resistance is like dikes and obstructions in the river.

Electrical transformers typically have a higher voltage (pressure) on one side, and a lower voltage (pressure) on the other side.
Reducing the voltage (pressure) increases the current (flow).

I visualize the Bernoulli image as a large pipe (wire) on one side, which intuitively means high current, low pressure (voltage) in the electrical world, and the small pipe (wire) would be high voltage, low current.
But this is the opposite of the actual Bernoulli lower pressure on the smaller pipe size.

So this is the opposite of what visually is seen in the electrical world, and thus difficult for me to wrap my head around.

I don't think you actually change the speed of electrons in the electrical world, and so there is no velocity change.

I can visualize the effect of power factor in the mechanical world, which is a surge tank.
A surge tank acts exactly like a capacitor or inductor, ie: they all store energy.

If I crimp the end of a garden hose, I can spray water a long distance.
The same water flows out of the hose (I guess) whether I crimp the end or not.

If I crimp/bend an electrical wire, the same current flows through the wire, so no analogy there.

It is a bit of a culture shock to go from the electrical to the fluid dynamic world.
Sort of like driving your car, but turning the steering wheel left moves the car right, and vice versa, or stepping on the brake pedal makes the car go faster, and stepping on the accelerator pedal slows the car.
Its all mixed up in the fluid world.

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[EyeTrip]

Sir, I was referring to carburetors, I discovered that it helped to understand carbs.

I would like this to be in a separate thread. This my be long, on an unrelated topic

 

[GreenTwin]

Yes, I understand that it was referring to carbs, but just like relating electricity to water in a river, if I can't find something physical to relate to Bernoulli's principle, then I can't apply it to a carburetor design.

You can post anything in this thread.
This is a "no holds barred" thread, and I am not the type who tries to limit how wide my topics can expand in scope.

Its all related, so post away.

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