Topic: Cylinder 4 not firing

[sindrenv]

Hi, so I am trying to finally get my Cb400f '75 model on the road again. It has been in storage for 2 ish years and when it put it away it only fired on 3 cylinders. This problem has persisted, even though I have pulled out the carbs and cleaned them. Before I put it away I checked the compression and all four cylinders were around 100.

What I have done so far:
- Cleaned carbs and replaced slow jet and main jets along with the floats
- Changed to new spark plugs
- Adjusted valve clearance to 0.05mm in and out on all four cylinders
- rough synchronized by adjusting the sliding body that holds the needle (can't remember the name) as it's says in the manual, at full throttle 0-1mm of the sliding body shows when looking from the inlet.
- set the float height to spec 21(or 22?) mm
 
when running:
- swapped plug cap between cyl 1 and 4 to see if the problem moved(it did not) => unlikely to be timing issue or coil issue as cyl 1 & 4 have the same timing and coil. the plug cap should not eb an issue as well
- check fuel level in float bowl(it was roughly equal between cyl 1 and 4), so there is fuel in the float bowl
- Check synchronization with mercury gauges: cyl 1, 2, 4 where approximately equal while cyl 3 was slightly higher (cyl. 3 is firing so thats not the problem)
- checked spark on cyl. 4, there was spark
- I pulled the other plugs and it was obvious that they were running rich, but they were running


Other info: When giving more gas it sounds like cyl 4 starts to fire, but it's difficult to tell. It also started to fire for a while when i switched from the alternative gas source and removed the mercury gauges. which where used to test the synchronization, but it quickly stopped firing.

So at this point of time the next step would be to check cyl 4. compression, take of the carbs to check if the slow jet is blocked, and maybe try to lift the sliding body in the carbs to check if it is to low. But I am sort of at a loss to what to do.

Do you have any suggestions? Something obvious I forgot to check?

[M 750K6]

Not sure synchronising the carbs when one cylinder isn't running, will have helped. Check them on the bench and get them as close as you can, that should be fine for diagnosing the firing problem and have the screws at the factory setting.

Is this problem once the motor warms up, off the choke? One of mine wants to drop out when stone cold, but runs consistently after 50yds down the lane.

Possibly
- swap plugs, to rule them out, even though new
- check the carb rubber manifolds for a leak?
- I find setting float heights on the 750 a bit of an art form, requiring practice. Possibly double check you are doing it correctly and the float and float needle are clean, the two floats sit level with each other and aren't sticking.

If cyl 4 chimes in on >1/3rd throttle, it suggests the low speed / pilot circuit is the problem, as you suspect. IDK what compression should be on the 400, 100psi seems low, but shouldn't stop it firing.

[sindrenv]

No synchronising them might not do a lot by itself, but the thought was that if it starts firing when giving throttle then maybe the sliding body is set so low that it fully blocks the airflow when the bike is idling. So the thought was that you could check it with the mercury gauges.

Checking for air leaks is definitively a good idea, I forgot to mention it but I did try at the end with some soap water. And there did not seem to be any air bubbles forming on the rubber manifolds. But if I get some air leak detection fluid maybe it will be easier to see.

On the note that setting the float height being an art form is definitively correct. but I felt I got a fairly good fit at the end. I would be more skeptical towards the valve clearance adjustment I did haha.

But yeah I am going to pull of the carbs again and have a look if there are any obvious problems and flush the jets with air and carb cleaner.

Thanks for the answer.

[M 750K6]

Not just the jets, the carb body has passages that need to be clean. I blow the carb cleaner out with compressed air.

[newday777]

No synchronising them might not do a lot by itself, but the thought was that if it starts firing when giving throttle then maybe the sliding body is set so low that it fully blocks the airflow when the bike is idling. So the thought was that you could check it with the mercury gauges.

Checking for air leaks is definitively a good idea, I forgot to mention it but I did try at the end with some soap water. And there did not seem to be any air bubbles forming on the rubber manifolds. But if I get some air leak detection fluid maybe it will be easier to see.

On the note that setting the float height being an art form is definitively correct. but I felt I got a fairly good fit at the end. I would be more skeptical towards the valve clearance adjustment I did haha.

But yeah I am going to pull of the carbs again and have a look if there are any obvious problems and flush the jets with air and carb cleaner.

Thanks for the answer.

You won't see bubbles. The air leaks will be vacuum sucking inward, not pushing air outward to blow soap bubbles.