1981 cb750 custom backfiring

[Andy De]

Hi, my son bought the above bike and it backfires/pops out the right side only. The bike only has 10000 kilometers on it and a few years ago it was taken out of a 15 year storage. He just bought it this year. We checked the compression and from left (while sitting) to right was 148psi, 142psi, 132.5psi and 138psi. We put new spark plugs in and new gas (with carb cleaner) and new inline filter. After about 500 kilometers and on the 3rd tank of gas, the spark plugs from left to right look good except the far right. It is fouled and mostly dry, carbon fouled. The fouled plug has better compression than the plug beside it with only 132.5 psi. I suspect that the 4th carb is the culprit. A couple of questions: 1) should we do a complete carb kit on all four carbs? 2) could it be the ignition? 3) should we just do the carb in question?

Another thing is the bike is totally original except the pipes have been changed. They are the same on both sides. We want to take a long trip this summer and need some quick advice if possible. Thanks very much with any help you can provide.

Andy

[vern401]

There are a couple things that I can think of that would make it backfire. Download the manual and follow it to check the Valve clearance. I would give a very thourough cleaning of the carbs. you probably dont need a rebuild kit but any bad gaskets should be replaced. Use this as a guide for cleaning the carbs. http://honda-cb750-s.456789.n3.nabble.com/DOHC-carb-cleaning-procedure-td867809.html#a867809 It probably is something caused by a plugged port in the carb. If its mainly on deceleration the it may be the air cut off valve.

[Andy De]

Thanks! I will clean them up just like the manual. I'm just curious, would you expect that cylinder to be lean or rich? thanks,
andy

[vern401]

dry black plug you would say it's rich. But I have seen a Carb that did that same thing clear up after cleaning it. Take your time on them and get them as clean as possible. All to often it takes more than one cleaning to get it all cleaned out. Make sure your plug gaps are correct too.

[Re-run]

After cleaning the carbs, run a tank of seafoam too which helps remove junk hard to see and get too.