1988 L6 115hp idles and runs well at half throttle and WOT, but falls flat when put in gear at idle if I don't goose the throttle immediately

[tomconnolly]

Just rebuilt the motor. Didn't have this issue before rebuild. Carbs were rebuilt 2 year ago by PO. But the bottom carb idle screw has little effect from lightly seated to 2 turns out. I've set the timing to 5 atdc, then 0-2 btdc. Adjusted idle timing a dozen times trying to solve this problem. No positive improvement. Tested spark on all 6 cylinders with timing light. Compression was under 100 in most of all cylinders when tested after rebuild, about 1 hour of run time, but I was hoping rings weren't seated yet. Haven't checked it again.

I rebuilt fuel pump when I rebuilt the motor, but the online fuel filter between carbs and fuel pump gets air in it, over half air, when idling. Pumping bulb fills filter then it somehow gets half full of air again as it idles. Filter fills at full speed. Is this air in filter issue abnormal or ok? No leaks between pump and filter. Replaced bulb 6 months ago. Fuel lines appear nearly new black glossy jacket.
Some ideas I have:
* Pull gas line out of tank in case there is a filter on the end that is clogged. Is there typically a stone or other filter on the end of fuel line in the tank?
* Install fuel pressure gauge after fuel pump and see if I have pressure problems at idle.
* Replace fuel filter. No leaks apparent.
* Rebuild the bottom carb.
* Redo link and sync. Arrow is 1/4" right of alignment line when timed at 0-2 atdc...
Any advice is welcome. I'm still struggling with this issue after hours of using muffs and 5 times in the water since the rebuild.

 

[Chris1956]

Your issue is likely hat the carb idle mixture is too lean. Set the idle pickup timing to 0-9*ATDC. Set max spark advance to 21*BTDC. Don't change them.

Set the carbs to 1-1/2 turns open. Using fresh 50::1 fuel, adjust the carbs one at a time. So, with carbs open 1-1/2 turns, try to accelerate. If she stumbles or stalls, open top carb idle mixture screw 1/8 turn. Try to accelerate. If she stumbles or stalls, open mid carb 1/8 turn. After that adjustment, try to accelerate again. if she stumbles or stalls open bottom carb 1/8 turn. Repeat whole sequence until she runs in gear at idle long enough to suit you, and will accelerate smoothly.

The carbs will all have different idle mixture settings. That is normal. You are trying to achieve a reasonable idle, plus fast acceleration. it is a compromise as the carbs do not have acceleration pumps.

 

[tomconnolly]

Your issue is likely hat the carb idle mixture is too lean. Set the idle pickup timing to 0-9*ATDC. Set max spark advance to 21*BTDC. Don't change them.

Set the carbs to 1-1/2 turns open. Using fresh 50::1 fuel, adjust the carbs one at a time. So, with carbs open 1-1/2 turns, try to accelerate. If she stumbles or stalls, open top carb idle mixture screw 1/8 turn. Try to accelerate. If she stumbles or stalls, open mid carb 1/8 turn. After that adjustment, try to accelerate again. if she stumbles or stalls open bottom carb 1/8 turn. Repeat whole sequence until she runs in gear at idle long enough to suit you, and will accelerate smoothly.

The carbs will all have different idle mixture settings. That is normal. You are trying to achieve a reasonable idle, plus fast acceleration. it is a compromise as the carbs do not have acceleration pumps.

0-9? Atdc? That's a broad range. Did you mean 0-2 atdc? I've adjusted the carbs secret times similar to your directions, but the lower carb concerns me. I'll try again with timing and 1.5 turns and more. Thanks?

 

[Chris1956]

0-9*ATDC is the range, broad or not. Make sure the motor is warm and the fuel mixture is spot on. This assumes the carbs are clean, carb floats, inlet needles and gaskets are good and adjusted properly.

 

[racerone]

The engine must respond to the low speed mixture adjustment !!-----So take bottom carburetor apart.-----Do not run motor again.-----A plugged carburetor means a blockage can starve 2 cylinders of oil !

 

[444]

You rebuilt the engine but skipped the carbs because some previous yahoo did them!? I would open them up, make sure everything's clean, floats adjusted right, needle and seat not leaking and new gaskets. Then do a proper link and sync as per the service manual.

 

[tomconnolly]

Right on. Good advice! Thank you.