cylinders 2-3 wont fire

[parksbuddy]

Hello All, Im fixing up a 1980 Cb750k into a brat cafe style bike, obviously, and Ive run into an issure with cylinders 2-3 not firing. First off Im running stock airbox and 4-2 exhaust, carbs have been cleaned and rejetted with dynojet stage 1 kit. Carbs were bench synced to the best of my abilities. The bike starts and idles pretty smooth on cylinders 1-4, 2-3 headers are cold. I checked the coils (ohmed) and even flipped them to make sure they are firing, and I appear to get spark when holding a plug to the frame, so i guess they are good. when i pull plugs 2-3 they are wet, not oily, so im getting gas and air. Compression seems decent (thumb check).  I tried to start the bike on only 2-3 cylinders and I get some exhaust rolling out of the pipes and the odd backfire pop, but wont start.

so to sum it up. I have spark, fuel, air, and compression. whats goin on!?

DONT WORRY THE PODS ARE GONE!!! but they do look cool.
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[Re-run]

Might be that it is too much fuel. The plugs really should not be wet So there is too much fuel. Is there a particular reason you put a jet kit in and did you increase the size of the jets? If the old jets are useable, I would put those back in and try again.

I would also check how far the idle mix screws are turned. Too far out makes things richer as well.

Tools will also tell you to make sure the carbs are very clean, especially the idle circuit.

[parksbuddy]

The reason for rejetting was that I had intended on using pods, and had purchased the dynojet stage 1&3 kit, but scrapped the pods and figured i might as well use some brand new jets. I do believe they give slightly better performance with a stock bike. And like I said 1 & 4 cylinders run and respond pretty good. I think I will remove the carbs and make sure again everything is clean and clear.  

would valve clearance have anything to do with my problem? I am new to this bike tinkering. My normal ride is a 2010 ktm exc that I have minimal maintenance, so from what ive been reading I hope I havnt bit off more than I can chew with the 750.  

[gkmaia]

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[parksbuddy]

When changing my jets I just followed dynojet's instructions.  Enlarged the slide lift holes, changed the needles washers on the 3rd groove and swapped the main jet to the dj096. And I turned my mixture screws out 3.5 as requested by dynojet.  


When I was troubleshooting, I took the plug caps from 1-4 (they fire no prob) and put them on 2-3 plugs and used the spark plugs from 1-4 so there is defiantly a spark,  and I can't get the bike to start. Just backfires and pops and some exhaust rolls out the pipes.

[Re-run]

Yeah that is an awful large amount of fuel for a virtually stock set up. The engine can only take so much fuel before it floods. I would still back things off, the idle screw settings at the very minimum.

[parksbuddy]

That does make sense to me but if all the pilot screws were set the same and the carbs were bench synced half assed I'm running the stock air box, why would carbs 2&3 be pushing too much fuel?   Just curious, I intend on pulling the carbs and checking for blockages and turn the pilots in to 1.5? I believe that's in the manual?

[Re-run]

Blockage is still possible for sure. Could also be a weak coil. You got spark but it can be hard to tell if it is enough. Certainly clean the carbs again though. It is likely the source, turn back the mix screws too. Even then, it may still be too rich.

[parksbuddy]

could it possibly be the pulse generator that is causing 2-3 to not fire. I took the cover off to paint, might have bumped a crusty wire? Im trying to think of any other things it could be before wrestling with that dang airbox and carb separation nightmare.

[TOOLS1]

Switch the wires on the pulse generators, and see if the dead cylinders move to 1-4.
TOOLS

[parksbuddy]

OK! so i swapped the spark units around and i got 2&3 cylinders to fire, so it looks like one of my spark units is too weak. I ohmed them both out and they read damn near exact resistances but it has to be the problem. with both spark units connected and all spark plugs pulled out i tested for spark and the 2 sparks plugs on the assumable failed spark unit are very weak.  Is it safe to assume that I found my problem? what is the best replacement for this component. ( 1980 cb750k stock)