Please help, spent too much time trying to diagnose

Thoruss

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Hi there, I'm somewhat new here, have lurked for a while but rarely post as most things have been covered, I cannot however find this particular problem in the search.

I've owned this 76 cb750f for 2 years now. it ran ok...when i bought but wasn't charging. fixed that with new/proper rectifier/regulator and a minor harness issue....problem is while i did that i also replaced the coils(upgraded), plug caps/resistors(2 failed and were open), plugs, main harness, resistance and load tested the remaining sub harnesses. Replaced both handle bar control switches, rebuilt the carbs and redid the points with new ones. I performed the valve lash adjustment and probably a few other things I've forgotten. the idea was to make the bike good as new however i did a few too many things (not all of these were at once but i cant remember exact order and don't know when the issue really occurred or i would redo whatever system i did before) I have also sprayed brake clean around the carb boots to look for splits and inspected them as well, no leak i can see.

So that brings me to my problem. When i go to start it cold, it starts up after a couple tries then idles ok. after warm it starts up immediately. however when i give it revs even just small ones it backfires and pops....and if i really try to rev it up... it starts to hard sputter and miss at 3k rpm... if a really slowly roll into it i can get it to go a little higher but not much, maybe 4k. (that's not something I do regularly just trying to get an idea of what could be causing it.

I've re gone through the carbs, checked all ports and ensured the jets are clean as a whistle. i even went in a third time and put back the original k stamped jets to see if the new ones weren't correct, I've heard these carbs can be picky.

I've checked for voltage at the new coils, they had lower voltage than i would expect (10.5V) so i ran a relay with a dedicated circuit straight from battery positive (fused) and use the original circuit as the power signal. No change in revs. I have double checked points gap and timing, looks ok. resistance tested from the points to the coils, 0-0.1 ohms. performed load tests and voltage drops on all spark related circuits, resistance is always good, however the voltage drop is always worse than i expect ( I'm a journeyman mechanic but don't usually work on bikes, this was supposed to be a fun side project that is easier to store than a car)

For reference, the bike is a 1976 CB750f, unknown mileage, air filter pods (didn't come with original), 4 into 1 pipes with no resonator, its leaking at the head a tiny bit between the headers and the head (will deal with after it runs ok unless this could be the problem but i don't think it is) i believe the jets are set to oem with #40 and #105 or #110 jets. have not yet completed the carb synchronization.

Can someone with experience with these bikes please please give me some input or point me in a direction. I can complete any and all repairs that are necessary but I'm really at a loss. Honestly don't know if the problem is electrical or fuel. I feel like I've been trying to figure this out for far far to long, could be something simple I'm missing or something i totally have to redo.

Thanks for the help...i hope
Just wanna ride this year....
 

dirtdigger

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Have you actually tried to ride the bike or only rev it? These carbs are the keyhole not round tops correct?? Keyholes are jetted leaner than the round tops because the keyholes have an accelerator pump. Make sure the accelerator pump is working and also spraying into all 4 carbs. If its good take it for a ride. When you get to a spot that starts running bad start closing the choke. If it gets better with the choke it is lean. Stays the same its jetted close. Gets worse then its rich. For the mods you have it probably needs larger mains but you never know till you try them. Timing and point gap good but how about the dwell....important. Make sure the ignition advance is working correct.
 

Thoruss

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this is round top keihins, no accel pump. I've taken a peak while reving and the advance appears to be operating correctly and spin out, i have a new advance system to put in but I'm honestly just tired of working on it right now... i know the dwell is off just slightly as they wouldn't set with proper gap, had to move the timing plate and split the difference on the gap....I assume this would lead to slightly off dwell. That said i wouldn't expect it to be enough to dead stop it at 3k. plan was to redo the points again with the new advance, new oem points, new condensers...Ill do that right now i guess... I'm just a bit burnt out from going in circles over and over again with it... as with the jets I went back to stock setting as it should at least run ok, I can tune it properly once I have found out why its running the way it is. I have ridden it and same story, sputters and pops and gets mad if I don't keep it between 1800-2800 with real slow accel.
 

Thoruss

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was the points... or condensers somehow.... parts or install error.... i like triple checked them so may have been bad...dont know but revs up properly.... now i have to start plug chops and see where my fueling is at....also balance the carbs....
 

dirtdigger

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Good that you figured it out. I never do plug chops anymore, I can get spot on by feel with the choke. But what ever gets you the correct jetting is all that matters.
 

kirky

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plug chops are running the bike on different throttle openings and stopping the ignition at different throttle openings and inspecting the plugs to see how it's running on the carb jets it has in it
 
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