'75 std motor problem

[s clarke]

Hi group, have been away from list for some time, but have returned for "help" from anyone who can solve this problem. Have just put std back together from "resto/modo", and when I get it running, it is not running on one cylinder. Equal compression all across, good, new plugs, carbs all done, coils checked, no breaks down by vacuum advance, I can see a constant puff of black out the "can", when it is idleing, and when I took it for a ride, it was difficult to get it up to speed, but once up, it was great from 2 to 5th, but weak on hills and any "pull". It feels like one cylinder is not working, which confuses me, and since each coil runs 2 cylinders, it should run a lot worse if a coil were gone. Any input would be muchly appreciated. Chiao.
SQUID

[GNEPIG]

Sounds like the carb on that one cylinder is flooding: black smoke = rich. Your spark plug should be black on this cylinder.
To make sure that all plugs are firing, pull the caps off of one spark plug at a time, plug an extra spark plug into it and lay the plug on the head, then start the bike and look for spark. Or do all four at the same time.
good luck

[prometheus578]

"Or..."

If the plugs don't give enough indication(They still might be self-cleaning)

Get a spray bottle and spray a little water on the header pipes at idle.
Which ever cylinder doesn't sizzle, that's the fizzle.
If all plugs spark, then it's carbs

[TADracer]

Had the same prob a few years ago. Probably float level on one carb is just a hair off from the others. Float level affects low and mid range throttle openings alot. I raised the float height 1 mm (lowering the fuel level) and the problem ceased.

This is worth a try first since it takes no parts or $ to do.

[Dsxs11]

Just had the exact same problem and it was a float problem. Mine was cracked.

[Ken Talbot]

Hey, Sid, good to see you back tinkering! Just leave all those big 'fat' jets in the drawer this time round, eh?

For a quick diagnosis, get it running for a few minutes and spritz a bit of water on the pipes. This should point out a cool one if that's actually what's happening. It could be a simple as a bad sparkplug cap, or some corrosion where the cap screws into the end of the plug wire. Sometimes just snipping about 1/4" off the end of the wire and reattaching will clear up a bad spark.

[s clarke]

Me and the STD....again

Well, I checked all spark, good,.. took carbs off and using digital caliper, brought all floats to within specs in manual, reassembled, fired to get warm, it was ragged, and still that "puff" of black,..consistently, all exhaust pipes "hot", but it was "backfiring" out of carb, opposite choke lever side, and would spit, fart, etc., and would not readily come to power on throttle "blip"! This has been a brutal fruitless "tuning" day, and I might try once more tomorrow, or just push it back into shed, and ride "zonker"! Thanks for the help.
SQUID
PS Deep in my soul, I think the problem is either a coil going "dead" ...slowly, or the main fuel inlet jet, (controlled by float action), is still sticking, at high, (flood), or low,(starve), and letting fuel fluctuate as it does so, but still should not backfire through carbs.

[GtrMn]

but it was "backfiring" out of carb

BE CAREFULL! If it is a stuck float...I had that happen and gas got into the air box and it caught on fire...Almost blew up the bike and the house at the same time.

[Pat Kelly]

Maybe a tight valve (intake or exhaust).