Stihl 4 mix engine conversion with carb issues - RC Groups
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Feb 17, 2013, 10:23 PM
Registered User

Stihl 4 mix engine conversion with carb issues

Hi all, Its been a while since I have actually logged in here but I have an interesting project and an interesting problem too. I have a stihl 4 mix engine I have converted for aircraft use on a cub. I have it running but it will not run at full throttle unless the choke it mostly on. I have researched this issue and some are using 2 stroke carbs with smaller venturis and connecting the impulse port as needed or making a gasket with smaller porting. Others have drilled the main jet larger and some have used the stock carb without problems. I would like to use the stock carb if I could without modifying it but since I bought the engine and carb separate off of ebay there is no "original" carb. The model numbers match up though so I know its the right carb for the engine. My question is why am I, and others, having a problem using the carbs that match the engine? Why do people need to go to the troubles of swapping, modifying, fanangling carbs with their engines to get them to work? Dont the weedeaters run fine with the stock carb? Anyway there are very precise instructions on the maintenance of these engines like leakdown rates, pressure and vacuum values..etc that I am working on figuring out. What I have noticed is that adjusting the high needle on the carb does nothing to the engine. I have cleaned the carb and it does not seem clogged in any way. What on earth do I do now?
Thanks much
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Feb 17, 2013, 10:47 PM
Registered User
earlwb's Avatar
Well a four cycle engine needs to use the pulse carbs in a different manner than a two cycle engine. A two cycle engine has strong crankcase pressure pulses as well as the opposite low pressure pulses to drive a pump diaphragm with.

But a four cycle engine basically has no crankcases pulses as you want the bottom end to have more of a vacuum in order to reduce internal drag against the piston. So they took to using the intake manifold to get the pulses from the intake cycle on the engine. They have used a spring in some pumps to help simulate the vacuum part of the cycle when the intake valve is closed. So you have the spring push the diaphragm back after the intake pulse has opened it up more.

But the problem with a pulse pump is when the engine is under a heavier load or at full throttle. As the engine advances towards full throttle the intake valve opens and closes faster and faster, the pulses become weaker and blend into each other, so the pump effectively stops working. A similar thing happens under a heavy load too, as the throttle is open all the way and the pressure differential moves farther forward towards the front intake of the carb.

Now in the trimmer engines they do not run full throttle and they don't have a heavy load on them either. So they can get away with the deficiencies that the pulse pump has. As they do not run wide open on the throttle and under a heavy load too. Plus they usually have a speed governor system on the engines to ensure they don't too.

I have a four cycle engine conversion project I am working on. But I need to finish it, and I am expecting a similar problem with the carb on it too. But my engine has a throttle barrel version Walbro carb on it, and the bore is pretty small. So the engine may be able to draw its fuel more strongly without depending on the fuel pump so much. Thus at full throttle it may simply be drawing its fuel more than the pump can pump it.

Anyway, I am not sure there is a simple easy fix to it or not. It may require using something like a Perry four cycle pump on the engine. The Perry pump uses vibration to operate the pump diaphragm instead of engine pulses.

I ran into the same problem with scooter engines using pulse pumps to provide fuel to the engine. At full throttle or going up a long hill the engines would stall and die on you as they ran out of fuel. I switched them over to using an electric fuel pump and that worked great, but that isn't exactly a good fix for model airplane engines though.

One thought is using muffler pressure on the engine. That may help assist the carb in drawing fuel and thus alleviating the problem. You can use muffler pressure and the odds of it starting a fire or exploding aren't any more worse than they are with glow engines or model diesel engines (they use lots of ether too) using muffler pressure. The Evolution 10cc gas engine even required muffler pressure too. Anyway the fuel pressure regulator will even it out for the carb as well.
Feb 18, 2013, 01:56 AM
supreme being of leisure
ZAGNUT's Avatar
no experience with the 4-mix but since it (and the shindaiwa) pumps through the crankcase just like a YS i would assume that it would have enough pressure pulse to work just fine with a regular walbro carb.

can you check somehow if the carb is actually pumping fuel? like maybe add a T to your pulse line and connect up a separate carb and see if it pumps while the engine runs?
Feb 18, 2013, 08:15 AM
Registered User
earlwb's Avatar
Zagnut does bring up a good point. I have a couple of Walbro carbs that are designed to not have a fuel pump in them. They even went as far as to machine the pump side of the carb smooth too. I assume they used a gravity feed and or maybe some pressure build up inside of the fuel tank.
Feb 18, 2013, 09:07 AM
Hope to get out of life alive
kenh3497's Avatar
Did the engine run before the conversion started? If it did, then there has been a change someplace in the fuel system. Like Zagnut says, part of the fuel goes through the crankcase to lube the bottom end.

The carb does use a crankcase pulse to operate the pump in the carb. If the seals on the crank leak or the valve cover leaks the pulse will be diminished and the carb pump will not work.

Which engine are you using? (which Stihl product got cannibalized for the motor)

Feb 18, 2013, 11:13 AM
Stick Head
munzy's Avatar
Since its running but not at full throttle. Had this problem with the same issues. Have you checked the timing. Your setup have the coil and module. Here is a site with some info that may help;, Munzy
Feb 18, 2013, 11:27 AM
Registered User
DarZeelon's Avatar
It is my impression that cannibalized agricultural engines will always be heavier, awkward and noticeably less powerful than purpose built engines; even if they are based on identical internal parts.

Yes, they are often cheaper... but what gives?
Feb 18, 2013, 11:37 AM
supreme being of leisure
ZAGNUT's Avatar
Originally Posted by DarZeelon
It is my impression that cannibalized agricultural engines will always be heavier, awkward and noticeably less powerful than purpose built engines; even if they are based on identical internal parts.

Yes, they are often cheaper... but what gives?
that's only true with "homeowner" grade tools but even then not always. high end saw engines do very good, look at the BME 50 as a good example....other than slapping a prop hub on there's nothing to modify or improve for RC use.

but remember also that the OP is talking about a piper cub, no need for high performance and extra weight might even be welcomed.
Feb 18, 2013, 12:01 PM
Registered User
Why carve on the guy for doing something you don't like ?
Constructive you are not
So leave it alone or do you just like to push your opinion on everyone ?
Feb 18, 2013, 09:55 PM
Two left thumbs
I'm surprised that a Stihl distributor's information is on the internet. Stihl doesn't publish this stuff except for their dealers.

Because the crankcase pulse is weak on the 4-mix engines, the carbs use a booster pump diaphragm, which is the extra bulge on the bottom of the carb. You MUST use a carb with the boost diaphragm to get adequate fuel pump operation on a 4-mix. The boost diaphragm can get brittle, since the hot air/fuel mix from the crankcase blows directly on it via the vent tube. A stiff diaphragm = poor performance.

As for 4-mix engines being too heavy, the smallest ones are pretty anemic power-to-weight wise, but the larger ones are good. Having a full crankshaft instead of the single-sided variety of the cheap lawn & garden engines creates a weight disadvantage, but imparts a strength, reliability, and smoothness advantage.

BTW, Stihl Inc. is very litigation conscious, and does not permit any dealer or distributor to promote its products for use in any way other than that intended.
Feb 19, 2013, 03:12 AM
Registered User
DarZeelon's Avatar
Originally Posted by Machzx
Why carve on the guy for doing something you don't like?
Constructive you are not.
So leave it alone or do you just like to push your opinion on everyone?

This is a discussion forum, where people express their opinions, as well as stating facts... That's what it is so live with it, even if you don't agree with some things written here.

Since what I wrote upsets you, it must have at least some truth in it.

I agree some would convert a chainsaw to power their large model, because it is their passion.

When you buy an engine designed to power your R/C model, you only get what you need for it to run.
You obviously don't need all the ancillaries that would make it a good chainsaw/leaf-blower/weed-whacker/Etc.. And you obviously don't need to pay for all that useless stuff.

A couple of decades ago, before giant scale became so popular, there were no purpose-built gas engines available; so cannibalization was the only way to go, if you needed one.

This is no longer the case.
Feb 19, 2013, 09:00 AM
Registered User
Earl: That makes sense about there being lower impulse pressures on a 4 stroke. I will be getting a new carb diaphragm kit but I am intrigued about what you said about the load issue. A weedeater is not under full load like a airplane engine is so those pulses from the crankcase blending together makes sense too. I will have to try putting exhaust pressure in the fuel tank to see if that does anything.

Ken: I did not run the engine since I got it basically as a short block off of ebay. I can post some pics of the serial number that was printed on the flywheel housing and the carb (which I had to buy separate). Below are some of the numbers you will see in the photos soon.

Munzy: That is some good info on the link. I found that my carb is not referenced under what the weedeater should be. Here are the numbers straight off the carb: S131A and 931A. the shroud around the flywheel had 265174766 laser stamped in it but the local stihl dealer wasnt able to look anything up with that.

DarZeelon: I take no offense since people I go to school with (yes I am an aviation maint. student=poor) and I have to explain to them why I take these things apart and convert them. 1. Its fun
2. It is cheaper for someone with machining experience and more time than money on hand.
3. Its a cool feeling thinking I am into this engine less than 100 bucks and to get an equivalent I would have to spend 5-800 bones.
4. With prop, ignition, and everything needed to run it, she weighs in at 3.8lbs. Not bad I think.
Feb 19, 2013, 09:07 AM
Registered User
Sorry you were helping the man solve his issue how ?
Feb 19, 2013, 10:50 AM
Registered User
Here is what I plan on doing.
1. Leakdown check
2. Vacuum check
3. Replace carb gaskets and diaphragms
4. Recheck timing and valve clearance
5. Put pressure port on header and port it to gas tank
6. If no joy with those attempts then try to modify a small carb off of a 2 stroke engine.
7. Maybe explore fuel injection or mechanical fuel injection. Does anyone know where to start??
Thanks all!!
Feb 19, 2013, 11:02 AM
Registered User
DarZeelon's Avatar

I agree my remarks did 'take the wind out of the guy's sail', so to speak.

They also sidetracked the discussion somewhat, but are closely related to it.

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