Cylinder 3 not firing

[loxley007]

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?

 

[brettp]

Sounds like you have everything covered. Check the float valves to make sure they don't stick

 

[Loose Chain]

What does that one spark plug look like? It will look different than the rest.

 

[Old_Crow]

Mine did this a while back, except on cylinder #2. Seems the 45-year-old boot between the carb and the head had cracked.

 

[loxley007]

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?

 

[brettp]

Your compression test would be impacted by bad valve shims. It would read poorly.
Have you seen the spark plug fire?

 

[Loose Chain]

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.

 

[loxley007]

Your compression test would be impacted by bad valve shims. It would read poorly.
Have you seen the spark plug fire?

Yes, I had the plug out and saw it spark.

 

[loxley007]

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.

 

[Loose Chain]

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.

 

[loxley007]

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.

 

[Loose Chain]

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.

 

[loxley007]

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.

 

[Loose Chain]

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.

 

[loxley007]

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.

 

[Nylundarn]

Good problem solving this far, I think we can rule out ignition since it would most likely affect both Cyl 1 and Cyl 3, at least if the problem were spark units or coils. A nice looking spark isnt always a good spark but Im sure you have switched plugs around to see if the problem moves? It should not be spark plug wires either.

Are you running pods or airbox? Were the carbs rebuilt as well? All slides moving nicely with carbs on the table?

Also, is this problem occuring at idle or are you riding the bike?

 

[plantguy]

If you have an ohm meter check wires 2-3 and 1-4 should be around 23k ohms.

Check my post in this thread.

https://www.cb750.com/threads/cdi-ignition-module.9296/

 

[loxley007]

Cheers everyone. I put a spare set of carbs on it and cylinder 3 fired. I then pulled the carbs and had a look at carb 3. for my 79, Murray’s rebuild guide suggests slow speed jets are pressed in. Mine are slotted (maybe a later set of carbs) and the slow jet on 3 was fully clogged. I’ve cleaned it up and I’ll test it this weekend.

 

[loxley007]

 

[Loose Chain]

I hope you are going to do all the carbs since you have them off. I'm sure the rest will be just about as bad. I thought you said you had done a complete carb rebuild?