1984 750FD only firing on left hand 2 cylinders

[Maurits]

Hello Chaps,

My 1984 CB750FD is only firing on the 2 left hand side cylinders (as sitting on bike). There is terrible backfiring every so often. The 2 right exhausts are cold, indicating no fire. Thing is, it was fine 2 days ago riding into work, ran like a top! I have had this problem intermittently before, but not as bad, probably only 1 cylinder not firing. Plugs have all been replaced. Bike has been standing for a while, with only short rides now and then.

Compression is fine, carbs are fine as when it runs it is 100%. What could be the problem, coils? Condenser?

Many thanks and cheers,
Maurits

 

[Maurits]

Further to my post above, I know the sparking system is divided into cylinders 1/4 and 2/3. What would the cylinder numbering be from left to right, when sitting on the bike? 1/2/3/4 or 1/4/2/3 ?

Cheers,

 

[dan1951]

Maurits, When setting on bike LT. to RT. 1,2,3,4,

 

[Maurits]

Thanks for that Dan, that kind of eliminates my idea that it was one half of the ignition system components, as then either 1/4 would be cold or 2/3 cold. But with 3 and 4 both cold, further investigation is required.

 

[Maurits]

I am leaning towards carbs. Will strip them out, clean and see what happens. Will post an update when I resolve the issue!

 

[dirtdigger]

I would say no fuel getting to the two carbs. Check the fuel T.

 

[Maurits]

Allright, problem solved. It appears as if it was a combination of HT leads and spark plugs! Got the bike home and tested the spark on cylinder 4 with a spare plug, and very weak spark. Cylinder 3 had no spark at all. Bought new HT cable and rewired all but then only 1 and 4 fired up! Re-checked the HT wiring job I did and made sure that all was screwed in properly etc. Only 1 then fired up, was at wits end. Then tested all 4 HT cables individually with a spare plug and spark was present on all 4 cylinders. Then tested each of the 4 plugs in situ and found that only 1 had spark. Replaced all 4 plugs and the bike runs better than it ever has. Took a great 50 mile ride straight after and really cranked it! So answer was new HT wires and plugs! Something so simple.

Question now is , how can plugs just go so rapidly? They were replaced a few months back and bike hardly run since. Can damaged HT wires blow plugs?

 

[dan1951]

Glad You have it running proper, Wires will not blow plugs they were probably fouled by fuel when it was not firing constant.