2 stroke twin won't start

[grizelli]

The title sums up my problem, but this is not your average engine

Its a 2 cylinder 2 stroke, running on petrol, with horizontally-opposed pistons and the spark plugs in the centre of each cylinder. It uses a small vane-type supercharger to scavenge the cylinders, and the inlet piston lags the exhaust by around 15 degrees in order to give a small amount of supercharging to the charge after the exhaust ports are closed and while the inlet ports are still open. For those interested, the design is loosely based on the Tilling Stevens TS3 engine which was built in the 1950s and 60s in the UK. The carburettor is a stock model carb (made in the UK by Just Engines) with a bore of around 7mm, and the ignition is provided by a Rxcel CDI box. The engine is quite large, bore is 30mm and stroke 30mm.

I've been spinning the beast with a drill for several days now, and all I can get is encouraging pops and bangs and the occasional flames out of the exhausts, but it won't pick up and fire continuously and won't run on its own. Its set to fire at around 15 degrees BTDC, but I have tried various settings from 0 to 30 degrees BTDC and it seems to try hardest at around 15. A clue may be that it pops and bangs most enthusiastically with the throttle around 1/3 open, pretty dead either side of that point - I've played with the mixture a bit and it seems to like it lean rather than rich, but since it won't run its kinda hard to tell.

The pictures give an idea of the engine, please note that the oil pump works surprisingly well and accounts for the pile of clean-up rags . I know that there are a lot of knowlegeable people on here, and several of you have experimented with unusual designs so I'm hoping for some suggestions to get the beast started. Help!!!!

 

[canadianhorsepower]

Nice looking set up.
In my opinion not seeing the engine plans
#1 you are missing a carb
#2 7mm carb for 22cc engine is way to small.
I would take the plenum of and spray gas on both cylinder with a bottle of gas mix
and see the response
Good luck

 

[pickleford75]

Almost sounds like it has low compression

 

[Cogsy]

Are you certain about the ignition timing? Flames out the exhaust suggest a possible valve or ignition timing issue and I know from my experience with an Rxcel clone that they can automatically advance the timing massively as RPM increases on the starter. I set my ignition turning over by hand and was something silly like 50 degrees out from where the engine would run (I wasn't aware of the auto advance at the time). Sorry if you know this already and have accounted for it but just in case I thought I'd mention.

I disagree that the carb is to small to start and run though, in general you can run a decent size engine on a tiny carb, you just can't make much power from it, so I would expect the engine to at least start up even if the carb is too small.

I do agree that it looks like a great engine.

 

[petertha]

First off, very cool engine! I hope we get to hear it run & learn more about its design & construction.

Re model carb sizing, here is something I’ve been messing with FWIW. It might be all BS. Some folks building gasoline fueled engines outfit RC methanol carbs to them. But consensus seems to be that methanol throat sizes are not a good match to similar displacement gasoline engines. Some also feel that throat size is only one parameter, the jet/orifice equivalent also plays a role.

I wish I had a good, reliable tabulation of carb sizes used on commercial RC engines spanning the displacement range and for both methanol & gasoline 2S & 4S engines. If any of you find a link, let me know. In the absence of that I went to Perry website which is an aftermarket supplier of RC methanol carbs. Let’s assume they did their sizing homework because they market carbs to many specific RC engines.
http://www.perrypumps.com/prod01.htm

On the gasoline carb front, seems to be even spottier data trying to cross reference carb part numbers, unlisted throat sizes & variable engine class. I found this link which had a Walbro sizing correlation method for recommended minimum & maximum throat sizes. According to their equation, carb throat is a function of a (high or low range) constant, engine rpm & displacement.
http://www.billetboard.com/showthread.php/9036-Carbs-Venturi-Size-and-Formula-For-Proper-Carb

Carb sizing of gasoline burners has been discussed a bit on the forum. Here is a link I saved from master builder George. http://www.homemodelenginemachinist.com/showthread.php?t=24003
I’m going out on a limb interpolating his data point. His gasoline engine is 1.0” bore x 0.90” stroke = 0.71 CID (25mm bore x 23mm stroke = 11.6cc nominal). He mentions he based it on an OS 2A RC methanol carb, which I think was used on OS 2-stroke .20 & .25 CI engines = 3.3-4.1 cc. I can’t find reference to that older model#, but eyeballing Perry’s sizes, I guestimated 7mm throat. If so, his gas engine runs a carb normally associated with a methanol engine of 28-35% his displacement.

To assemble these 3 sources into picture form, I made this spreadsheet chart showing Perry methanol carbs (blue scatter points) and the min/max calculated gasoline carbs (solid curves) assumed at 5,000 rpm. I overlaid the George carb which you can see plots nicely to the high side gas carb curve. In the second plot I increased rpm to 7,000 & you can see the effect. But its also probably safe to say the upper displacement methanol engines are lower rpm, so some apples & oranges in that mix.

Punching the numbers for the subject engine 21.2cc yields a range from 6.7 – 9.3mm = 8.0mm average if 5,000 rpm was a reasonable. All bets off with the supercharger business, above my pay grade!

I don’t want to dilute your troubleshooting with carb talk. If there is interest I can re-post & maybe some other folks with proven gas runners can throw their numbers in the mix to compare.

 

[Parksy]

Thanks for posting that petertha. Is that graph only applicable to single cylinder engines? Or if multi cylinder engines, just the displacement of a single cylinder?

 

[grizelli]

First off, many thanks to all for the suggestions and ideas. I was a bit concerned that the carb was too small, but I reasoned that it should be ok to get it going even if restricted the revs. The information from Petertha is absolutely priceless in this regard, so thanks again - it also looks as if the carb may not be as undersized as I thought.

The supercharger's main purpose is to blow the exhaust gases out of one end of the cylinder using fresh mixture blown in from the other end, and the amount of actual charging is limited. However, it may be part of the starting problem, as I discovered today that the shaft for the blower had come loose from the rotor, so I had to dismantle it and nail it back together. First few tries after that, the engine sounded a bit happier, but still won't quite start, so I'm wondering if the vanes in the blower are too snug a fit and they are not sliding out properly to capture the air. The blower needs lube, so I'm using a very weak oil mix and I wonder if this is making the vanes goo up in their slots. Too late to tackle this today, but another strip down tomorrow is on the cards.

Has anyone else built a working vane supercharger? It would be interesting to compare notes.

Also interesting is the point about the Rxcel ignition automatically advancing the ignition with engine revs. I built the engine with the ability to adjust the timing by around 30 degrees, and have also tried moving the magnets to a different position to give another 20 degrees - so far, I have tried all of the above, and the engine sounds most lively at around 15 degrees BTDC. Not alive, just lively.

I'm happy to provide more design and construction details about the engine if people are interested, though until it gets started I'm afraid its more a collection of bits of metal than a real engine

Once again, many thanks to all. Further instalments to follow.

 

[petertha]

Thanks for posting that petertha. Is that graph only applicable to single cylinder engines? Or if multi cylinder engines, just the displacement of a single cylinder?

That's a great question & I purposefully left it open ended, mostly out of personal ignorance. Most of the referenced methanol engines are singles. I've never checked but seems to me RC methanol 4S carbs are similar or maybe a tad smaller than 2S. But the sizing formula inputs rpm as a factor & many 4S are lower rpm rated. The site referencing Walbro correlation pertained to gas scooters so presumably singles and I suspect many utility engines like weed whackers & chainsaws etc. But the carb referenced in the George link is for his V8.

I know some RC gasser twins & quads actually use the same or very similar carbs, and this may be because the carb is serving essentially once cylinder at a given time based on induction timing. When you start getting higher cylinder counts with gas flow delivering to more cylinders simultaneously.. I'm really not sure how to scale throat size. Sometimes you see one carb on a 9-cyl manifold. Other times (maybe different reasons) multi-carbs serving partitioned manifolds.

Its a good point because the OP's engine is 2-cyl 2-stroke & I'm not entirely clear how the induction timing works.

 

[petertha]

Sorry grizelli, I think we posted nearly simultaneously. Is this the gist of your induction/exhaust?

https://www.google.ca/search?q=Tilling+Stevens+TS3&biw=1649&bih=985&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjrh6_WwufPAhUmqVQKHTSdBMwQ_AUIBigB#imgrc=l3KRYlrvHy1C0M%3A

- when you say 30mm stroke, is that 15mm per side (ie. the displacement figure we were calculating is actually correct?)

- what is the compression ratio?

- the answer is probably no, but is there any convenient way to temporarily 'unhook' the supercharger hardware just to see if it will pop continuously with no backpressure or obstruction & then introduce that into the mix? Even on my 2S engines specifically ported for reflective wave tuned pipes, they manage to fart at low rpms open exhaust. They don't run well or sing, but my gut tells me something fundamental: breathing, ignition, CR...

 

[grizelli]

Yes, thats the engine that mine is based on - I used to work at the company that made them waaaay back in the sixties.

To run the engine without the 'charger would require a new manifold of some sort for the carb, as the charger sits between the carb and the inlet ports. Also, I'm not sure if it would work at all, as the charger provides the only means of shoving air in and exhaust out, unlike a normal 2 stroke where the crankcase pressure does all the pumping. It might be possible to hook up a compressed air line to the carb as a substitute.

I plan to strip the charger down tomorrow anyway, loosen up the vanes a bit and maybe put some more ooomph into the leaf springs. If that doesn't work, I'll try your idea with a spot of compressed air.

The stroke is 30mm per piston, so 60mm between the pistons, and a compression ratio of around 10 to 1. Although feeding the cylinders alternately, that points to the carb being pretty too small, but I'm hoping its big enough to make it run - if I can get the engine going, I will probably make my own carb.

Thanks again

 

[Parksy]

Thanks Petertha.

Grizelli, you raise an interesting point about the supercharger and it's role of intake and exhaust. Do you have the convenience of experimenting with different pulley sizes?

 

[canadianhorsepower]

The stroke is 30mm per piston, so 60mm between the pistons, and a compression ratio of around 10 to 1. Although feeding the cylinders alternately, that points to the carb being pretty too small, but I'm hoping its big enough to make it run -
Thanks again

these math Is why I said " you are missing a carb Or it's way to small":fan:
now on an experimental motor like yours it's easier to get the unit running with two carbs. #1 you don't have to worry about the plenum volume both cyl are sharing. #2 and the ability of one cylinder to burn fuel more easily then then other one.

As for all the info on carb size...... any good running set up with pump fuel will need to have 20% more fuel to run on methanol to get the power increase you are looking for. THAT SIMPLE all we have to worry about is fuel ratio gas is 14.7 and methanol 17.64 so 20% more fuel.

example you have a main jet of .060 thou the area would be .060*.060
=.0036 x.7854=.00283 that is your mane area.

now you want methanol so you need 20% more and that will be .00334
area.
so .00334/.7843=.004320 and square route of .004320=.00657

and that is all the fuel you need :hDe:
of course if you have a multi needle adjustment barb do the same math for all channel
cheers

 

[XD351]

Just out of curiosity , is the blower supplying enough air fuel flow for the engine ?
You may not be getting enough flow to give good scavenging

 

[grizelli]

Some progress to report. I stripped the blower down and discovered that the fuel seems to have swollen the tufnol vanes enough to make them bind a bit, and that the leaf springs needed some slight changes to the form to allow them to sit amost flat when the blade is fully in. Back together again, and the beast makes a lot more encouraging noises, but still wont quite run.
Ive played around again with the timing to find the point where the most firing seems to happen, so I'm going to discount that for now and concentrate on fuel. Some interesting points.

First, it fires best with the throttle about half open. This may well be because the carb can't draw in enough mixture at smaller openings. Second, it gets very excited if i fully choke it with a finger over the carb and the release it slightly, which would suggest that the mixture is too weak. However, opening up the mixture jet give the opposite effect, so not sure what is going on. All suggestions are welcome!

To respond to other points made. I could change the pulleys to speed up the blower, but i would have to make new ones. I think these blowers work best when run fast, current ratio is around 4x crank speed. Its pretty much impossible to tell if the blower is delivering enough puff, though when spun while off the engine theres a significant amount of puff coming out of the pipes and i think if it wasnt doing anything much there would be no firing at all.

I'm coming to the conclusion that i may need to build a new carb with a bigger throat, though I'm a bit concerned that this would introduce another potential failure point. I know the current carb works as i have used it on a couple of previous engines. Are there any diy carb designs around that i could take a look at? Alternatively i could simply copy the carb that i have i guess.

This is very frustrating, as it is 99% running, just won't quite get up and go on its own. I'm in danger of falling into the stupidity trap of keep repeating the same thing in the hope of a different outcome. Need some out of the box thinking

 

[mcostello]

Did You try a squirt of fuel while cranking? Extra fuel might tell something.

 

[t.l.a.r. eng]

My opinion is that vane type units make better air motors than air compressors. I built a small 4 vane unit from plans in Strictly I.C. plans and decided to fit it to the first 4-stroke I built from scratch. Ugly but runs good.
The air vane unit seems to work so much better as an air motor than it does trying to move air.
It is turning 5 to 1 belt driven and while the engine runs, the rpm is limited to about 5000 rpm. the vane unit is working as there are bubbles of fuel ecaping under the upper manifold bolt while it is running.
Runs much stronger on nitro with glow normally aspirated.
I suspect your unit is too small and/or turning too slow starving the engine for air. My own observations bigger is allways better with vane type units.

 

[gbritnell]

Hi Grizelli,
Lets talk about an engine first, any engine. If the compression is good and the spark is at the right time then by introducing fuel to the cylinder should produce results.
Here's an example. A commercially built two cycle engine won't start. You check the spark and it's good so you don't question the timing because it would be correct for that engine. The compression is good as you pull on the starter cord. That leaves the final part of the three elements for an engine to run and that is the fuel. The fuel tank is full, you put the choke on but still nothing. At this point this is what I do before dismantling the carburetor I pull the spark plug and put a small amount of raw fuel into the cylinder. Now this amount of fuel is way too much for the normal mixture of 15:1 but that's ok. The spark plug is installed and the engine is pulled over. Usually by the third pull the engine will start and run for ten or so revolutions. That part is questionable because the engine will rev up and then die. Now you know that the engine will run and it's not getting fuel so from there you remove the carb and do the normal maintenance.
Now why did I explain all this, it's because most engines will run with the wrong amount of fuel. Maybe not well or long but with a dry spark plug they will run.
You could try this with your engine to see if it will run. If it does then I would work backwards toward the carb. I'm just guessing but in all reality I'm thinking that the supercharger has something to do with it.
As for carburetors for an engine of your bore and stroke you should be able to get away with a carb with a 6-7 mm bore and have it run fine. A lot of my engines have a bore of 1.00 (25.00mm) and run just fine with a venturi of .210 ( 5.33 mm) The small diameter won't prevent the engine from running it will just limit the rpm range of the engine.
gbritnell

 

[grizelli]

Thanks again for all the suggestions and advice. I have tried shooting a squirt of juice into the carb, but it seems to make it worse rather than better. The plugs look ok as well, dry and clean.

The engine does run, just not on its own. The best results I have had are from cranking it at around 2500 revs using an elecric drill, while working the throttle with the other hand - it fires pretty much constantly, so much so that its kicking at the drill the whole time, and I get a decent clean transparent blue flame from the exhaust ports. However, it will only do this with throttle more or less wide open, and if I pull the drill it stops straight away.

Its a big engine, the two pistons per cylinder give it about 45cc swept volume per cylinder, firing at 180 degrees to each other, and although the carb is only feeding one at a time the information I have collected on here and from others lead me to think that the carb needs to be more like 15mm diameter than the present 7.

Of course, if the blower isn't blowing enough either, that will also starve the engine, but making a bigger blower is a fair bit of work so I plan to make a bigger carb first and see if that helps. If not, then a bigger blower is on the cards, probably geared up to run faster than the present 4:1 ratio. I've also noticed that one of the rocker arms has started to crack around the welds, so I will make a new set from solid.

I've had a lot of experience with different types of full-size carbs, and I think that the easiest for me to make with my equipment will probably be the slide type. Anyone have any views ( especially of the 'ooh, don't do it missus' variety) on these, or tried making their own?

It looks like it will be a while before I get back to the starting process again. Thanks again for your collective help, any further ideas will be more than welcome.

 

[canadianhorsepower]

[quote I get a decent clean transparent blue flame from the exhaust ports][/quote]

Ho HO
Timing is way off, cam timing or ignition timing.
this is not a torche so the nice BLUE flame should be inside

cheers

 

[petertha]

Well, I really should re-post my carb sizing chart separately to try & solicit some data points from other folks who have built running engines & can provide more measurable data at their disposal.

I came across another engine, Ron Colonna 1/4 Scale Offenhauser (on YouTube). It’s an inline 4-cyl running 9.5:1 CR on 90-100 octane gasoline. Each cylinder is 26.2mm bore x 27.8mm stroke, 15cc displacement. It has 4 dedicated RC methanol carbs, one-per-cylinder. He spec’s Tower Hobbies T46BB carbs. http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXJA92
I looked it up, another oldy. They say 12.8mm bore. That 15cc, 12.8mm coordinate would plot way-way high on the graph which makes me suspicious. I thumbed through some other .40-.46 size carbs and suspect what Tower calls bore in this case might be ID of the engine crankcase for fitting purposes vs. the actual throat ID. 13mm is a manhole cover even by similar displacement race engine standards. Does anybody know? For example here is a similar OS carb http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCA08&P=ML
They say OD=10mm ID = 6mm. Maybe that’s more like it, 10mm = body & 6mm = throat. If I plot 15cc + 6mm, I get the plot as attached.


I stumbled on some vids of Schillings V8 80cc engine (back to methanol fuel). It has 4 carbs for 8 cyl. Not quite sure but maybe 1 carb serving one cylinder at any given time the manifold looks? I can’t find carb specs but let’s guess same .40 CI (6mm) carb eyeballing the pic. So that would be a 10cc, 6mm combo (80cc/10).
http://how-to-build.xyz/tutorial/Schillings-V8-Modelengine-Engine-80ccm-Glow-Methanol-Nitro-DOHC-F1/rVgJODHXyvc.html

Rather than build a carb, there seem to be many reasonably priced Walbro’s (or what look to be clones under different engine trade names). If you could get your hands on one, ideally correctly sized to your application, it would save a lot of fabrication effort.
http://www3.towerhobbies.com/cgi-bin/wti0095p?FVPROFIL=&FVSEARCH=dle+carb
Here is a link that details some of the inner workings.
http://www.helifreak.com/showthread.php?t=326599
On one hand I think Walbro type might offer more mixture control. OTOH, more parts & ports to mess with without some underlying knowledge. My understanding: most of what ‘pumping’ translates into on these carbs is delivering fuel to the carb body and/or timed opening of fuel passage to induction demand. That’s important when your engine is moving around, especially a model flipping around in different orientations & working the throttle range throughout. But I’m not sure that applies to a stationary engine with a fixed gas tank (hydrostatic head). I think in both methanol/Walbro style, fuel draw & atomization is still dominated by the venturi throat size, air mass flow & fuel orifice/needle valve opening size. But maybe I’m wrong

Some folks mess around with reducers like this which is another way to tweak different setups on a common carb body within certain bracketed ranges.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXWLC4&P=7