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Hi,
1994 fxstc, 32K, crane cam-(unknown type), Cycle shack M series, CV carb has thunder jet and dynajet kits, large aircleaner-rivera/primo.

Here is my issue, the rear cylinder does not fire evenly, especially when cold. At idle it will go “fire,fire,poof/pop”. When held at a constant low throttle setting it breaks up and you can hear it sort of missing when the rear cylinder goes poof. It’s been doing this since 20K when I got it but I thought it was due to the straight drag pipes it had. When you get on it, the thing runs great, plenty of power, no coughing through the carb, 40+mpg, easy starts, no oil consumption, in really good shape and both I and & the PO maintained it well.

What I’ve done and checked to try to fix it:
Autolite coppers look good, rear slightly richer at slower speeds but not much or bad.
Compresion is 149/151, leakdown was <5% rear and about zero front.
VOES tested working, hose good.
Intake gaskets changed, once by me, once by someone else.
New SE coil and wires, switching wires front to back does not change anything.
Tried different jetting and settings, finally got a brand new CV, absolutely no difference.
New b-series lifters and cam bearing @30K, timing good, rockers looked good and readjusted the revteck pushrods.
Two different exhausts and the original drags, new gaskets.
Swapped out the stock ignition for an SE modual.

Correct me if I’m wrong but I think I’m down to either possible trouble in the heads/valves or the cam has too much overlap for a street bike-(this thing really runs strong and has a seemingly endless top end-I never have to go WOT). If it were a head issue I think it would show in the compression, leakdown or make mechanical noises but I’m no engine builder. If I’m right about the cam then maybe getting a single fire ignition where I could adjust the rear cylinder offset would be the way to go.

Your thoughts please?? Anyone deal with this befor?

thanks,
60
 

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Me too

Hello,
I have 1988FXRP showing similiar problem,missing at idle on rear cylinder, runs fine at top . Had my topend apart to repair leaking base gasket and found crack between rear spark plug and inlet and exhaust valve seat. Awaiting parts to repair so don't know if this will fix it . Just my input i know you might not like this answer but anyway......
 

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Hi Man

I believe you are In-correct.. Personally I wouldn't touch it or fret..
I am basing that belief on the fact that all your comp and leak tests show sooo well..

It is the firing order of the our 45* motors that give them that Off-Beat sound at idle,,, some waaaay worse than yours, others not so much, partly due to what you guessed at,,, motor set-up.. Running drags/straights doesn't help and trying to dial it out will simply put a cluster-butt on it..

If you get a S-F ign the firing order will simply stay the same unless you plan on changing at what "time" the rear fires, as you stated..
I've only known a couple guys that do that,, they do it for extreme racing reasons and they know exactly why and what they are doing,,, up to you.........

I have run S-F and D-F and points in the same motors right after each other and Never felt any diff in any kinda way..
Again, up to you to experiment with..

That screwy idle sound gets to everyone till they get used to it..

have a ball
 
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