Board index Flathead Power-Technical Questions, Answers, and Suggestions Evo's Thru 99 Gas fumes from exhaust when going up hill

Gas fumes from exhaust when going up hill


Posts: 21
I have a stock 1991 FLT. When under load (like pulling a hill riding two up) I get bad exhaust fumes from exhaust. I rebuilt carb a year ago and at that time no fumes. Recently took carb back apart and float level was slightly high and I adjusted it to specs. This bike has 72,000 miles and this just started in last 1,000 miles. I cannot not see any holes in the rubber diapham for the slide. Changed plugs several tiimes and new air filter. Bike runs great. What in the carb could be worn to allow two much gas or too little air at 2,000-3,000 rpm just when pulling hard (on hill mainly). Could a weak coil or weak coil wire do this. I have not tested the coil. Any help appreciated, Thanks!!

Pa Site Admin

Posts: 5843
Location: Ohio USA

Possible exhaust pipe leak up front ?


Posts: 21
The fumes are coming out the rear of the exhaust pipes, the person riding behind me can smell the fumes when I am going up a hill.

Pa Site Admin

Posts: 5843
Location: Ohio USA

Sounds more like the pipes themselves or something hindering the pipes from exhausting directly to the air behind the bike. This does not sound related to your bike issue but I have owned many a 4 wheeled vehicle such as Chevy Blazers, Ford Broncos, station wagons, etc., which when the rear window was opened even slightly, the exhaust fumes would enter in through the opening. How the air is areodynamically passing over your bike and riders may be the cause.


Posts: 21
I did a dry and wet compression test with cold motor. Dry is 120 psi on both cylinders. Wet is 145 psi on front and 130 psi on rear. A person on another board told me he thinks I have ring blowby. I guess I could be wrong were the fumes are coming from. Could be coming from crankcase vent, as opposed to exhaust pipes. Just know the fumes are strong when pulling a hill. I am getting ready to tear the top end down. Does this sound reasonable. Thanks

Pa Site Admin

Posts: 5843
Location: Ohio USA

I never even thought of crankcase venting. Before you do the tear down though, why not extend the pipes temporarily with make shift adapter/adapters and take a ride with a passenger on board. Just make sure the pipes exhaust much further behind the bike with the made up extension/extensions. If you still have exhaust fumes then, you know what you must do then.


Posts: 21
I tore top end down and found galling in both cylinders. Machinist says it needs bored for .010 pistons. I guess I found the problem.


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