Topic: number 4 no worky cb750F

[78whiteorbs]

Got a 77 cb750f that isnt firing on the num 4
Cylinder. Has strong spark(iridium plugs with a dyna s iggy and dyna coils). Plug is wet to so its getting gas. Carbs are freshly cleaned....what could this be? What to do next? Have amother set of known good carbs to slap on there to see if it is
Carb related. What else can I test for?

[cheftuskey121]

I would swap coils and wires/plugs around to see where the problem follows. if it stays on the number four then it would point to cylinder or carb problems. swapping the coils and plugs will at least tell you if its with the electrical or not. that's what I would do at least

[Jerry Rxman Griffin aka MuthaF'er]

Is the plug cap good? How does it's resistance compare to the others? Trim the end of the plug wire back half inch or so and re-install.  As Chef suggests swap #1 and #4 wires to the opposing plugs. Swap the plugs after correctly gapping them. .

[78whiteorbs]

Tried all that. Brand new pkug wires and caps. Hot blue spark
From plug. Getting spark!thats for certain.

[78whiteorbs]

If not carb related what engine probs could exibit this symptom? Have good compression.

[tlbranth]

It's not the coil or #1 wouldn't fire. If you swap #1 & #4 plugs and wires and the problem remains with #4 AND you have good compression, the problem must be in #4 carb.

[Davez134]

I had a similar problem before and it turned out the rubber boot from carbs to intakes of head was the problem. The boots felt soft to me, passed all the tests I tried that I read about here. I went through everything. All tune up steps, repetedly cleaning carbs, triple checking ignition, and problem remained with cyl #3. Finally I said screw it and bought some new boots and clamps (clamps were aftermarket and wide too) problem solved 100%. Done.

[78whiteorbs]

Thanks guys, brand spankin new boots and brand new clamps.
Lookin like the carb but the plug is gettin wet with gas so ,weird.

[david 750f]

Sounds like the idle jet is plugged. Were the carbs properly cleaned? Any rust in the gas tank? Drop the float bowl on the carb and see whats in the gas.

[goldarrow]

Drop the bowl and see if the slow pilot jet is not fall of to bottom of bowl...?

[cheftuskey121]

that was my next suggestion. that idle jet is what is going to get the bike started for sure! they are so small and easy to clog up. especially if you have spark and good compression....and I am assuming air somewhere, ha. its odd that the plug is wet too

[tlbranth]

Did you synchronize the carbs? Try this: shut off the fuel, open the throttle a bit and see if #4 will go. By the way, I would add some 2-stroke oil to your fuel while you're doing all this testing. #4 is over-fueled and it's washing lubricant from the piston and rings.

[78whiteorbs]

Checked num 4 idle jet and she was clean as a whistle.
I did try syncing them just for giggles with my morgan carbtune.  As expected the number four was pulling way way more.  It is not an intermitant fire either. At all revs nothing test with temp gun on header and it like 100 degrees...

[seanbarney41]

fuel level in bowl too high?

[tlbranth]

Checked num 4 idle jet and she was clean as a whistle.
I did try syncing them just for giggles with my morgan carbtune.  As expected the number four was pulling way way more.  It is not an intermitant fire either. At all revs nothing test with temp gun on header and it like 100 degrees...

If #4 is pulling a much higher vacuum than the other 3, the throttle piston is not opening or not opening as much as the others. This would give you the symptoms you are seeing. Pull the air box, stick your left thumb and finger in 3 & 4 carb throats and slowly twist the throttle. Are the pistons  both moving at the same time? Is #4 moving at all? Oh, do this with the engine not running.

[78whiteorbs]

UPDATE
Pulled the air box and by "hand" choking the number four carb
it revs up so it seems its not getting fuel . Popped another set of know good carbs from my other 750 and it runs great on all four.... these were tge cleanest carbs that I have seen on the out side when I got them. Took them apart except for the top keyhole area. Jets were all pristine. This one has has me scrathching my head. It is confirmed definately carb related though. What to do with this carb rack.....

 This isnt my first rodeo either , I have clean many many sohc4 Carbs