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Old 07-24-2009, 08:54 PM   #1
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Default Propane Carburetor

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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Old 07-24-2009, 10:08 PM   #2
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Default Re: Propane Carburetor

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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Old 07-24-2009, 10:14 PM   #3
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Default Re: Propane Carburetor

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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Old 07-24-2009, 10:35 PM   #4
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Default Re: Propane Carburetor

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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Old 07-25-2009, 02:57 AM   #5
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Default Re: Propane Carburetor

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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Old 07-25-2009, 12:49 PM   #6
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Default Re: Propane Carburetor

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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Old 07-25-2009, 06:05 PM   #7
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Default Re: Propane Carburetor

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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Old 07-25-2009, 07:51 PM   #8
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Default Re: Propane Carburetor

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,
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Old 07-25-2009, 08:15 PM   #9
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Default Re: Propane Carburetor

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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Old 07-27-2009, 02:48 AM   #10
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Default Re: Propane Carburetor

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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