Intermittent Ignition cyl 2 - No sign of vacuum leak 1978 CB750 K8

[tdubsy]

Ahh shoot my bad went back to size 35. Anyways went back to the 40.

I cut the ends of the wire and recrimped, still had the same issue. so I swapped the ignition coils. And then the same issue appeared on 1 or 4 with the 1/4 spark plug wires attached. depending on which wire was attached. so essentially three different wires had the same issue and it seems like it is all out of the same port on the ignition coil. I assumed the two ports were wired together? The coils were replaced recently so maybe I have a dud?


Have you seen this happen before?


The floats all look identical but I will try throwing one of the old ones back in or ordering a new one

 

[dirtdigger]

sounds like bad coil

 

[tdubsy]

Ah my bad #35.

Finally received new coils and now all 4 cylinders are working. So far this is the happiest i've seen the bike

I had to adjust my mixture screws with the additional cylinder burning gas every stroke instead of every few.


I was able to get reasonable throttle response out of a #35 pilot but the #40 with the screws all the way seemed a bit better. (although at idle this is very rich looking at the colortune and the rich smell is very noticeable) It does lean out once the RPMs go up though

Is there any overlap in the ranges of the pilot screws or should I order a #38, Would it also be smart to get a digital tach to help with adjustment? I'm currently doing it by ear and testing the throttle in increments


Currently when I blip the throttle fast the bike hesitates than catches up after I back off. If I do it a touch slower it seems fine. Looking at the colortune it goes from rich to completely lean to the point where I can see the combustion stop when the throttle is opened up quickly. The Accell pump is working and the backflow valve on it is not jammed open.

Any suggestions on getting the engine more fuel? or do these bikes just not like to be blipped?

 

[dirtdigger]

If it hesitates with the stock jets then more then likely the accelerator pump is working correct on all 4 carbs. Check make sure you have a squirt out of all 4.

 

[tdubsy]

Accel pump 100% works on all 4. I am able to squirt fuel right out the front of the carbs when I have it out of the bike. I have heard people have some luck with the older carbs without the Accel pump. If they raise the needle.

Would that actually help or just cause other issues if I shimmed the needles?

 

[Medyo Bastos]

Have you changed plugs?

 

[amc49]

If there is even the slightest bit of slack in pump linkage you can get a balk even if the pump is working right. It must work instantly.

Engine condition can in unusual conditions make a balk too, slow timing, engine in less than good shape, plugs black from fouling due to not sparking can do it too. If not a pure carb problem only, then it can balk due to engine refusing for a second to take the opening throttle due to something wrong with it.

 

[72 Blue]

Hey Everyone,

Noticed a bit of an issue today.

I was using a colortune system to adjust my mixture and I noticed that cylinder 2 was firing intermittently below 2000 RPM. You are able to see the spark but there is only ignition every few cycles.
There wasn't any obvious signs of a vacuum leak as I sprayed both engine and intake boots with WD-40 and the RPM's didn't even flinch.


I did notice a popping whenever it would ignite and I could see the carb boot expand when this happens (Had to set the pilot screw a touch rich to prevent this)

I have removed the carbs and cleaned them a few times also done some re jetting.

Is there anything I should triple check?


Videos below

Cyl #4 Acting Normal at idle

Cyl #2 Igniting intermittently

List of things I have done so far
- Rebuilt Carbs with new floats, gaskets,seals
- New Carb boots
- Installed dyna electronic ignition
- New ignition coils
- New spark plugs and wiring
- static and dynamic timed ignition
- adjusted valve spacing to OEM specs
- Synced carbs
- 115 Main jet (up from stock 105)
- 40 Slow jet (up from stock 30)
- Checked cylinder pressure (within 5 psi of stock value)

Thanks


Thomas

I,m wondering if your a bit lean on idle screw adjustment , it taking a few cycles to "pop" intermittently ?

 

[dirtdigger]

This post is 5 years old and the original poster hasnt been back in as long.