B&S carb issue

[cadman60]

I have a Gilson snow blower that has a B&S 8hp Sno/Gard engine on it; model: 190402; type: 0992-01; code: 78060611. It has a 2pc med Flo-jet carb on it that I rebuilt 5 years ago. It ran great until this year. It will run at full throttle, 3600 rpm's, but when I push the throttle to the idle position the rpm's drop down some, then the rpm's will fluctuate up and down a little. When the rpm's drop down it seems to be loosing rpm's at each cycle until the engine dies. I'm trying to get the idle set to 1750 rpm's which I hope is the correct idle rpm. At least that's what my B&S service manual has in it. Most of the time to get it started back up I have to choke it. Does this sound like it could be something in the idle circuit in the carb? I do use a fuel stabilizer in all my gas, but I guess that doesn't guarantee anything. What do you think the problem is and what should I look at first. TIA for all your help.

[KE4AVB]

When setting up the carburetor basically you follow the instructions in the service manual; just remember to hold the throttle valve against its idle stop when adjust the idle mixture. You need to adjust higher to get the initial idle mixture set and then lower back to spec @ 1750. Remember these engines usually have a governed idle speed so the idle rpm will vary from the 1750 rpm; hence, the reason for having hold against the stop when adjusting both idle mixture and speed.

[cadman60]

Thanks KE4AVB for the info. I tried to adjust the carb again at idle this time holding the throttle valve against the idle stop, but got the same results. This morning I started to trouble shoot. I took the carb off to make sure everything is OK with it. It looked clean as a whistle in there. Float and needle valve are OK. After doing that I decided to check valve clearance. Intake valve has 0.005 clearance. I couldn't get the 0.006 gauge in there though. I then tried the exhaust valve. I can't get any feeler gauge in there. Note: I had the piston at TDC when I took these measurements. Would the exhaust valve being held open, (with no gap between the valve stem and the tappet), cause this engine not to idle and die?

[RoyM]

It sounds you have the piston on the exhaust stroke. Rotate the flywheel a full revolution to bring it up on the compression stroke then a quarter turn more.

[NO0C]

It sounds you have the piston on the exhaust stroke. Rotate the flywheel a full revolution to bring it up on the compression stroke then a quarter turn more.

Quarter turn? I always thought they mention 1/4" of piston travel.

Why not just turn the engine until the intake valve is depressed? Then measure and set the exhaust valve clearance.

[KE4AVB]

To answer both questions.

1st yes it is a 1/4" pass tdc of compression stroke and not a 1/4 turn pass.

Why not just turn the engine until the intake valve is depressed? Then measure and set the exhaust valve clearance.

That works too.

2nd L-heads do tend to close valve clearances over time and this can cause problems when idling slow and sometimes the engine will not even start as the decompressor can be partially active. It usually the exhaust that closes the gap but it can be the intake sometimes too.

I just had L-head where the intake was @ near zero clearance and would not even start due to lack compression. That engine actually pass the leak down test as the valve was fully closed at TDC.

[cadman60]

Thanks everyone for the replies. I did have the piston on the compression stroke when I took the measurements the first time. After reading the replies here I rotated the engine until the piston was 1/4" down past TDC. I measured the clearances again and got the same results as I got the first time. 0.005 on the intake and zero on the exhaust. Looks like I have some valve work to do. Thanks everyone again.

Also, just for grins and giggles, I rotated the engine, as suggested above, until the exhaust valve was open. I measured the intake clearance: 0.005. I then rotated the engine until the intake valve was open and measured the exhaust valve clearance: Zero. Now I have to go dig out my small engine tools!!!

[Arkie]

Yes, you need to correct the valve clearance, but I suspect you will still have a CARB idle issue after getting valve clearance correc.
Flow jet carbs are easily cleaned up if they have not corroded inside. Low speed jet area is probably dirty restricted.
Even just a soak in 50/50 H20 and vinegar and then low pressure air will work miracles with the old flo jets.