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Low power, low rpms, backfiring

jcoop86

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Needing some help here, I have an 81 750f I picked up in a trade. I have been trying to get the motor squared away before I really tear into it over the winter. Here's what I've done so far. New rectifier has it charging great 14.7 at 5k. New carb boot and no vacuum leaks to be found. It does have pod filters. 115 main jets. At one point it seemed to be running great but then something changed. It doesn't want to hit high rpms and spits and backfires trying to get there. Idle is almost uncontrollable. And a weird sucking/huffing noise Is coming from the #4 carb when accelerating I keep tinkering trying to figure it out but nothing changes. The carbs also have a fairly fresh rebuild too. I have bench synced them while I wait for my manometer to arrive. I'm looking for suggestions for things to check that I might not have already. I'm at my wits end here trying to figure this out. Please help!
 
You're wasting your time until the valves are known set good at .005", why most people go for the carbs yet still can't fix it. The valves CLOSE UP on these to then be held open, you can't run right like that. VERY common. Hope you haven't burned any valves yet, when they get there they do it quick. Could be your #4 huff, backblowing from burned valve on compression stroke.

You always run a compression check on these before buying them.

Pull plugs, probably black or wet.
 
Haven't done valves or compression yet, #2 plug is dark but not wet or terrible build up. #1 3 &4 are very light so keep thinking lean. Will do the valve clearances and compression check asap
 
I'm in the same boat. Went all the up to 140's and 80's on the pilots, still a big flat spot after 1/2 throttle. 80 750F, New boots, Plugs are dry and tan. Compression is about 130 all 4. Got passed by a Harley today. Really embarrassing.
 
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