CB72 L pot not firing

masunari · Post by **masunari** » Thu Feb 04, 2010 4:24 pm

The forum helped me last year when I was rebuilding my 1966 CB72. Now I am having trouble with the L cylinder pot not firing. I am hoping someone may be able to help. The basic facts are
1 Sometimes (not always) when starting the L pot reuses to fire. After a few minutes there are a few backfires and off she goes.
I believe that the timing is correct as she was working fine until this problem occurred.
I have checked the spark. Sometimes There is a healthy blue/white spark at the electrode sometimes not. Sometimes when I earth against the cylinder there is no spark. When I hold it about a centimetre away a huge spark will jump from the outer electrode to the cylinder head.
I have checked the carb; sometimes there is no fuel in the bowl. When I pull the line from the petcock there is fuel coming out. when I replace it fuel then passes into the bowl. Is there an airlock?
This morning the usual happened and the pot would not fire. I put the choke lever midway and it immediately fired. It is starting to get me down a little. Especially as the other day when it backfired it blew open the silencer weld!!.
Sometimes I think it is an electrical problem, sometimes a carb problem. At the moment I am tending towards a carb. problem. What does the forum think.

Goodbrew · Post by **Goodbrew** » Wed Feb 02, 2011 7:05 pm

Mate
I just found your post after putting up one myself for the same issue.
Im leaning towards the carb as I note that if i remove the hose at the petcock breifly the cylinder will run OK.
Hopefully we may both find an answer - maybe it is something particular to the LH CL/CB72 carby??
Have you cleaned your carb?
Dean

Goodbrew · Post by **Goodbrew** » Sun Feb 20, 2011 3:45 am

see my post "miss on left cylinder"

masunari · Post by **masunari** » Fri Feb 25, 2011 4:06 am

Dear Dean,
Thanks for the reply; yours is the only reply so maybe its not such a big issue. I am still experiencing the same problem but by fiddling with the spark leads I can get it to work. I was wondering whether the answer was in a poor spark. By resting the spark cap on the plug there would be a small gap. In order to cross this the spark is intensified. In fact there are commercial spark intensifiers available that do the same job. But this does not explain why it is just the L hand pot and not both.
However, sometimes I still wonder if it is a carb problem. The reason for this is that I have just finished rebuilding a CB160 which seems to have similar problems. It is almost as if one pot becomes dominant, especially at low speed, preventing the other from working. There is fuel and a good spark.
Basically I am still struggling to find a complete solution. Let mw know how you are getting on.
Best Wishes
Marcus

Goodbrew · Post by **Goodbrew** » Fri Feb 25, 2011 7:12 am

Hi Marcus
Check your points are very clean and if you can get your spark on that side to jump maybe 8-10mm in the open air and its a nice blue colour then I think it would be fine.
If spark is OK have a look at the fuel supply and carb on that side.
Have a look at the posts about carb float levels and check the bowl vent on the carb is clear- it is connected to the little brass rod sticking out the side of the carby. Use a piece of very fine wire and some carby cleaner and compressed air to carefully probe it .
Check to see the banjo bolt - fuel inlet bolt hasn't still got a gauze filter on it as that can cause issues and make sure fuel runs to the carby in a smooth flow from the petrol tap.

Have look at Loud mouse's tune up procedure and float level check as that is good to follow

Good luck - is your'e still stuck keep asking - plenty of people here can help

Regards Dean

Dgormley · Post by **Dgormley** » Fri Feb 25, 2011 12:46 pm

Marcus, try trimming the spark plug leads a 1/4 inch and then reinsert them into the caps............D

Phil-UK · Post by **Phil-UK** » Fri Mar 18, 2011 11:47 am

Hi

May also be worth checking the wiring to the points isn't cracked. Had something similar and found the wire had hardened over the years and when "waggled" it broke.
Also could try swapping the leads at the coils/condensers so that you actually use what was the R/H coil and condenser to fire the L/H pot and vica-versa.

One other thought - you time the R/H cylinder and then adjust the L/H points to get it to fire at the right time so theoretically you could easily end up with a big difference in the dwell.

Phil