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Discussion Starter · #1 ·
I have a 74 ironhead, bought as a basket case. Got it almost done, but i'm having misfire issues with the front cylinder. It will only fire under wide open throttle, other than that it it doesn't fire. The motor is fresh, .040 wiesco pistons, heads were done, new valves, ground seats, new guides. New points, gapped correctly. Valves are adjusted correctly, checked and rechecked cam timing. I'm running a 38 mm mikuni, 310 main jet, 30 pilot. i have a custom intake built for it, it comes off the motor, turns 90 degrees forward and runs about 4 inches towards the front of the motor and the carb is clamped to that, is this my issue? I would think it would affect both cylinders not just one. My coil tested fine, even tried a known good coil off a buddies bike, nothing. Someone please point me in the right direction!
 

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Did you adjust the valves with the bike in fourth gear and on the ground? if not then they are not adjusted correctly........Roach.
 

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Discussion Starter · #4 ·
I checked for vaccum leaks already, nothing there, everything is sealed up just fine. As far as the valves go, this is the third ironhead i've built, never adjusted the valves on the ground in 4th gear and i've never had a problem, care to explain why this is necessary?
 

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compression test both cylinders to rule that out. if you are in doubt of your custom intake, spend the 30 minutes to install a normal intake and see what happens. this is really basic, but are you sure you don't have a fouled spark plug? also, what are you running for ignition?
 

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Discussion Starter · #6 ·
Compression tests fine in both cylinders, motor is fresh. i'm running a stock ingnition, new points, gapped correct, checked a few times. I've tried two new sets of plugs to rule that out. First set was autolite, then NGK, then tried Accel plugs. i might pull the S&S carb and intake off my buddies 75 ironhead to try that out.
 

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When you are adjusting the valves, the first head you do will try and turn the motor over just a little while your trying to adjust the other head, same on big twins but not as bad, so With doing this for the last 20 years, I found that having the bike in fourth and on the ground, it keeps the motor at TDC so you can adjust the valves right on the bottum of the cam lobe........Thats why.

.....Roach.
 

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Discussion Starter · #9 ·
Roach
Thanks for the tip, i wasn't sure if you had an actual reason, or you were trying to razz the new guy ;) but it makes sense. I do how ever go back through after all 4 are adjusted and re check each, so i'm sure the valves are correct.
 

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May be nothing, but I would look at the lobe on the points advance unit, and measure the points gap on the sharp lobe, and then the flat lobe, and see if both are within spec. If one lobe is higher than the other, one rotation will fire, and other may be to wide or not wide enough to get good fire to the other cylinder. Also, the charging system may not be giving the coil a full 12 volts, until it is reved up, causing it to not fire at lower speeds, not likely, but possible.
 

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Discussion Starter · #11 ·
Thanks for all the responses, i finally got it runnin right today. After checking, and double checking everything again, i decided to build a stock type intake instead of the one pictured, fired up one kick purrin on both cylinders. I know ironhead are tempermental, but man. Once i get the motor broke in i might toy with the jetting and try to get the intake to work. Might be a waste of time, could be 90 thats screwing things up? or maybe the length? what do you guys think? I'll post some pics of it all completed tomorrow. Thanks again

Terry
 

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I tried running a 90 degree on a CH with a SU carb drove me crazy!!! I know of another one that wouldnt run no matter what was done to it all i can think of is the way ironheads work that the intake pulses throw off the air flow???? The other guy went to a VM38 round slide an i went to a re worked bendix (i still have the SU) So with you going back to a normal intake took care of the problem as with the other 2 sportys i would say try it an see how it works, who knows
 

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ran an su type 90 with a super b for a while on mine..no issues and pulled real good, i would bet that the large plenum is affecting the signal,or lack thereof at your carb, until the shear velocity at wot makes things work. ran a few dellortos on shovel this way too no issues..in short i think you should get playing with the jetting.
 
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