Cylinder 3 not firing

loxley007

CB750 Addict
Messages
177
Reaction score
67
Points
28
Location
England
Hi all, I’ve done a complete rebuild and the bike turns over well but cylinder 3 isn’t firing (the exhaust isn’t getting warm at all).

I have checked for spark and all is good. I have checked compression and all is also good.

I did a carb rebuild (including new floats and jets) so my next step is to pull the carbs and check no 3 to ensure the jets are unclogged and there is fuel in the bowl.

Any other advice whilst I’m pulling them?
 
Update:

Spark plug 3 has no carbon compared to other ones.

Cylinder 3 has good compression.

Carb 3 had fuel in the bowl and all the jets were clear and the float valve was seating well.

The boot has been checked (it was new) and looks good.

My thinking is that the valve shims may be off for cylinder 3 as I had them all out.

Any other ideas?
 

Attachments

  • IMG_4766.jpeg
    IMG_4766.jpeg
    469.4 KB · Views: 10
Last edited:
If you have the carbs off, remove the top cap and slide and confirm that item #3 is in the carb. You should be able to see it by looking down the barrel that the slide runs in. The needle on the end of the slide fits down into item #3. Confirm that it is there. It's #3 in the top diagram.


1718664065505.png
 
If you have the carbs off, remove the top cap and slide and confirm that item #3 is in the carb. You should be able to see it by looking down the barrel that the slide runs in. The needle on the end of the slide fits down into item #3. Confirm that it is there. It's #3 in the top diagram.


View attachment 18338

Thanks, item 3 is definitely in there. The needle was visible and moves when the carbs are inverted.
 
I'm not talking about the needle. I'm talking about the small part that the needle slides into. Item #3 on the top diagram. Item #3 on the lower diagram is the needle. It is called a needle jet and it goes in before you put in the secondary main jet emulsion tube.


1718690306408.png
 
Last edited:
I'm not talking about the needle. I'm talking about the small part that the needle slides into. Item #3 on the top diagram. Item #3 on the lower diagram is the needle. It is called a needle jet and it goes in before you put in the secondary main jet emulsion tube.


View attachment 18340

Ahh, I’ll check that out this evening when back from work. Thanks for the clarification.
 
Yes look at the picture looking down the barrel where the slide goes (as I described earlier) and the red arrow pointing to it. It won't be the first time I've seen someone loose one of them.
 
Yes look at the picture looking down the barrel where the slide goes (as I described earlier) and the red arrow pointing to it. It won't be the first time I've seen someone loose one of them.

I’ve had a look and it looks like the needle jet is there. I think I’ve exhausted all avenues with the carbs unless anyone has other thoughts.

Next step is checking valve tappit clearances.
 

Attachments

  • IMG_4804.jpeg
    IMG_4804.jpeg
    256.8 KB · Views: 8
Okay, good you checked. It would be a good idea to check valve clearances even if that doesn't cure the problem. Tight valves are not good. Did you happen to look into the intake to see if there isn't a small rag in there from when you did the rebuild. I came across that more than once where someone had put rags in the intake during rebuilding to keep things clean only to forget one. I know it's a long shot but worth a look.
 
Okay, good you checked. It would be a good idea to check valve clearances even if that doesn't cure the problem. Tight valves are not good. Did you happen to look into the intake to see if there isn't a small rag in there from when you did the rebuild. I came across that more than once where someone had put rags in the intake during rebuilding to keep things clean only to forget one. I know it's a long shot but worth a look.

Cover off and micrometer purchased (I used digital verniers last time and they were not accurate enough)…going to check valves tomorrow evening!!

Checked intake and all clear from rags!!

Thanks for staying on the thread and helping! It is much appreciated.
 
Back
Top