1&2 head pipes cold?

Working on a 81 750 LTD for a friend, has compression readings of 100,95,100,105 psi on a cold engine. Has spark on all 4 holes, but after getting her started, and roughly idling for 2-3 min, I notice that head pipes 1&2 are cool to touch, whereas 3&4 are scorching hot. I assumed at first that it must be a fuel delivery issue, with a blockage at the fuel inlet tee between carbs 2&3, but loosening the fuel bowl plug screws, clean fresh fuel flows freely and plentifully... So I got the carb rack off the bike, on the bench, fully disassembled, and all jets and needles are clean as a whistle... Any specific ports in the carb bodies I should look into deeper? Thought I'd check here before splitting the rack and having to start at square one. Thank you gents! And remember to keep it rubber side down!

Are the spark plugs wet with gasoline in the non-combusting cylinders?

Could perform clear tube test to assure fuel level is within specs, as a too high fuel level may produce the reported symptoms.

Good Fortune!

The plugs were not wet, but I did wonder about float height level, as #1 carb appeared to leak ever so slightly when on side stand, what is the recommended fuel float height for this bike, with kei hin carbs, with plastic floats?

daveyg wrote: The plugs were not wet, but I did wonder about float height level, as #1 carb appeared to leak ever so slightly when on side stand....

A carb may leak for different reasons and from different locations, but I don't suspect float height level in #1 carb as being related to the mentioned leak.

Absence of gasoline on spark plugs in a non-combusting cylinder with satisfactory compression indicates failure of fuel mixture being drawn inside the combustion chamber from the carb.

Here's a link from KZr's FILEBASE to Keihin carb section:
www.kzrider.com/filebase/doc_download/47...-shop-manual-via-omr

If the carbs at hand are the style that are supposed to have a plug at the bottom of the pilot jet, the plug might be missing. And if missing, would prevent the pilot circuit from properly functioning.

And/or the small passage into the pilot jet may be clogged, as noted in this illustration:










Good Fortune!

Thank you for the pictorial that is quite helpful! All 4 carbs DO have the plastic plugs in the pilot hole... But very possible that the circuit is plugged inside the carb body.

The plugs I have are somewhat different than the ones in your pics though, they r plastic, with an o-ring. Should I replace all 4 with fresh o-rings for assurance that they plug properly?

daveyg wrote: The plugs I have are somewhat different than the ones in your pics though, they r plastic, with an o-ring. Should I replace all 4 with fresh o-rings for assurance that they plug properly?

If the o'rings look okay and seem to fit snugly, they're likely sealing all right.

Would concentrate on assuring carb spray cleaner flowing into the carb from the pilot circuit (as shown in the illustration), which would indicate that fuel mixture would also be drawn through the pilot circuit into the carb bore and sucked into the combustion chamber.

Good Fortune!

Well I feel a little stupid right now... Got the carbs back together and on the bike, hooked up my sync gauges, started her up, sounded to be running on only 3&4 still, then began backfiring heavily on cyl 2, thru the vacuum nipple, and into my clear hose! Turns out the plug wires for 1&2 were backward... And were ever since the bike was dropped off at my house... So now that she is pumping on all 4 pots, now I just gotta tune for the leaned out aftermarket slip on mufflers that are on it and sync and we'll be back in business!

Just kidding! Not out of the woods yet, cyl #2 head pipe is still ice cold upon warmup. And now if I give her a little rev, blue flames shoot from the left muffler (1&2) making me wonder if #2 is flooding out so bad, it's running fuel straight thru the cyl and into the pipe, where it is then being ignited... Would leaking idle plugs (o-ring) be enough to cause that much enrichment? I guess I will pull the carb rack once more, and replace all the o-rings on the idle circuit plugs just to rule that out...?

never mind misread post