# Propane Carburetor



## Rustkolector (Jul 24, 2009)

I know this subject has been posted before, but the posts never quite addressed the problems I am having. Most previous discussions were on hit and miss engine applications. I have a 4 stroke throttled engine (not hit & miss) that will run (skipping badly) on my demand regulator and any number of OS or home made carbs. They all seem to perform about the same. On occassion it does smooth out, but not for long. It is a bear to start, and won't run even poorly until it warms up. I can vary almost every variable, but no combination seems to work. My question is, has anyone been successful in running a single, or multicylinder, slow speed throttled engine on propane? I have shelved this engine for some time, due to this problem. I would now like to resolve this fuel issue, or else switch it to camp fuel. It is too cute to sit on the shelf. 
Jeff


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## Blogwitch (Jul 24, 2009)

Maybe you should have a look at Jan Ridders vapour system.

It seems to cure most engine running problems.

Have a look around his site and you should soon find out how it works.

Here is one of his engines, using the vapour system

http://heetgasmodelbouw.ridders.nu/Webpaginas/pagina_linford_tweetakt/linford_frameset.htm

Blogs


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## steamer (Jul 24, 2009)

I am told that long intake runners get cold and make the fuel vapor coalesce into droplets. This makes starting running a bear.  How far away is you carb from the intake port....got some pictures?

I have heard nothing by good things about Jan's vapour systems....I concurr with Blogs....check out his site....but do post a pic if you could, there are some real experts here.....

Dave


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## RobWilson (Jul 24, 2009)

Hi Jeff
I work with due fuel engines (LPG,PETROL), could you please explain your demand regulator .
Is you demand regulator vacuum operated ?
Too much gas will cause erratic running,hard starting and it is the most common fault with LPG gas systems, leaks in diaphragms , vacuum system etc
ignition needs to be retarded to run on propain, as a rough rule of thumb by 4 Deg 
Regards Rob


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## Rustkolector (Jul 24, 2009)

Rob,
Yes, the demand regulator is vacuum operated. It appears to be quite sensitive. I have even added a balancing spring to the regulator to make the sensitivity even greater. Varying the pressure to the demand regulator seems to have little effect. I have checked for vacuum leaks in the fuel and intake manifold system. I have a variable timing lever that allows retarded to advanced timing adjustment. I have had this engine running great on this system, but for only a little while. Just don't know what happened. I suspect I have missed something, but what?

I have been very interested in Jan's Vapor Carb for some time, and will be building one for my next engine, but I want this particular engine to run on propane. The original designer's engine run's fine on the same propane system. But mine won't and I haven't found anyone else that has successfully used propane on a throttled engine. It can't be that difficult.....
Jeff


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## putputman (Jul 25, 2009)

Jeff, you may have already read my post on getting my engine running on LP, but if not:

http://www.homemodelenginemachinist.com/index.php?topic=2561.30

After working on the demand valve for awhile, Dave suggested I work on the intake as LP uses a lot less air intake. I ended up making an adjustable valve for the intake and it made all the difference in the world. Might be worth a try.


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## Rustkolector (Jul 25, 2009)

Putputman,
Yes, I have read your posts on your LP system. I believe yours was a hit & miss engine. My carburetor is a rotary barrel type which, in effect, gives me the means to vary the air intake which I have been doing. It is fairly closed to hold the speed range I want. I used my thumb to vary it further, but it doesn't improve operation at all. I can get close, but it still skips ryhmically, and will occassionally smooth out for a short period and then it resumes skipping. I thought adding a little load would help, but it quickly kills the engine instantly. It has taken load rather nicely before, but not now. I am beginning to think it is something other than the fuel/air mix. 

This is a one cylinder 1" x 1.125" engine with a tiny crankcase, and an atomspheric intake valve. I had a restrictive crankcase vent that was pushing oil past the rings and accumulating on top of the piston. This caused smoking and plug gap resistance issues which gave me the exact same skipping symtoms. I opened the vent up a bit, and the oil problem seems resolved. The smoking in gone and the piston is almost dry on top now after running. Compression is very good. Spark looks good. I had issues with the intake valve spring tension early on, but varying tension now on either valve spring provides no change. I am sure it is something simple, but what have I overlooked.
Jeff


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## Kermit (Jul 25, 2009)

I'm inclined to think it is a problem with the 'spark'. 

Either the spark is not hot enough or it is being insulated by hot gases around the plug tip. Are there other plugs or coils you could substitute in and check with? The coil might have short circuited some of the turns of wire and isn't giving as high a voltage as it did in the past. Everything else being the same, the spark would 'look' good but wouldn't be hot enough to insure combustion.

Always poking my nose in it,
Kermit


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## putputman (Jul 25, 2009)

Jeff, have you tried running without the demand valve. The demand valve is necessary on a hit & miss so the gas does not flow during the miss cycles. It is kind of a saftey feature also. 

I have heard of people running without it but you dare not walk away from it because if the engine stops, the gas contues to flow. It might be worth a try if nothing else has worked.


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## Rustkolector (Jul 26, 2009)

Well, thanks for the help and comments. Nothing mentioned, or that I have come up with has worked so far. I don't want to own any shelf pets, so I intend to keep working on it. I am going to give it a break for awhile, and then try again. 
Jeff


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## black85vette (Jul 26, 2009)

I am far from an expert on this and this may have already been mentioned but the thread was interesting. My dad used to be a propane dealer and ran all of his trucks on propane. I recall how hard they were to start on propane so he had them run both gas and propane. He would start them on gas and wait until the engine heated up and then he had a switch that would shut off the gas and open the propane. I seem to recall a unit that set near the front of the engine compartment that had the hoses from the radiator attached to it and then the propane line ran through that and into the plate below the carburetor. I think that unit was used to vaporize the propane and bring it to a consistent temperature. Kind of like what the generator does on a Coleman stove, I think.  Don't know that any of this is of any use but it certainly took me way back in my memory banks. 

Not really related, but back in the late 60's my dad paid about five cents a gallon for propane at the refinery near us. Since it was unusual to have street legal trucks running propane the state put the road tax on an honor system. You were supposed to keep track of the gallons you used and then send them a check for the tax on it!

Sorry for the diversion off into propane trivia.


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## Rustkolector (Jul 27, 2009)

As an update to this thread, I decided to remove the propane carb and install an RC carb I had laying around, and connected it to a small fuel tank using Coleman camp fuel. It didn't start right up, but after 5 minutes of tweaking, the engine ran pretty well without any skipping. So the engine appears to be ok, as I suspected it was, since it has run well before. It would appear there is something wrong in the propane fuel mixer. 
Jeff


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## Harglo (Apr 15, 2021)

Hello Jeff
Ive had very similar problems with the propane mixers. Seems the choke plate adjustment needs to almost be like a  micrometer as the tiniest change can kill the engine. A question for any body that has built carbs for propane. Should the air intake/horn be long or short.
Harvey


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