Cylinder #1 is 'cold'-- Help!

[MarkD]

Hi all, I have a '79F, and #1 cylinder is cold. I will bullet mark all pertinent info. I have been wrenching on bikes for over 50 years, but I am stumped. Thanks for your help.
--all cylinder show nearly 145lbs compression.
--I cleaned the carb 3X (correctly)
--vacuum lines are good, and correctly connected
--spark plug is good. I have connected a plug (outside the cylinder) when running, and it's firing a white spark appropriately.
--ran fine when I stored it last season, and I got it ready to go for some Fall rides a couple weeks ago, so it sat since last winter
--put hand on carb throat inlet, and didn't affect the running too much, and some fuel leaked from it. Carb is getting fuel, as I drained the bowl each time I removed carb bank, and bowl is full each time.
--Put hand on other carbs, and RPMs slowed way down

Any more info, and I'll pop back on and add it. Could it be an intake valve issue? I am stumped!

[bikerphil]

When cleaning the carbs, did you remove the pilot jet (one down in the tunnel) to clean it and verify, holding up to the light, making sure the nozzle is clear? How about over 4K RPM, does it heat up then?

[MarkD]

Yup-- emulsion tube was clean, I ran a thin wire through just in case-- all was clear. Another point, less suction on #1 carb inlet than others, showing less vacuum, which could be telling it's not getting fuel to whole carb jets, just the float bowl.

[bikerphil]

Not the emulsion tube, the low speed pilot jet, as shown...

[MarkD]

Oops, yes that is the part, I removed it, and it was clean. I blasted cleaner through all openings. I considered it could be a faulty diaphragm on top of the slide that's causing the weak vacuum- but held it up to the light showed no holes. I have considered a stuck or cartooned up intake valve, too, but with the good compression, I am doubting that now.

[bikerphil]

Ok, again, does the cylinder come back on line when high revving to 4K?

[DiverRay]

1. Easy, Reverse the #1 and #4 plug wires and try again. If #4 is now cold, you probably have a bad spark plug cap. To verify, unscrew both caps and test with a VOM. Should be about 5K Ohms between the wire side and the plug side.
2. Harder, check the intake boot for leaks. You can try the propane method, with an unlit propane torch try and see if it comes online when you have the propane between the boot and head.

[MarkD]

I have revved it up to 3-4K, and it hasn't fired. The 'dummy' spark lug I connected does fire white when connected to the plug wire, so it appears the cap and wire is okay.

[MarkD]

I can try spraying WD 40 on the carb manifold boot, but they are fairly new boots, and look good. I know looks can be deceiving, so I can try that tomorrow to rule that out. I can also try the unlit propane torch aimed at carb boot.

[cajun31]

I believe Phil is on the right track. That pilot jet can look clean and still not be. I had this same problem on a set of carbs I was working on and until I actually replaced that carb's pilot jet it would not fire. Something you might want to try is to simply swap the pilot jet from another carb to it and vice versa and see if the problem persists on the same carb. By the way no one has asked. Are all your jets oem mikuni brand or aftermarket?

[MarkD]

All jets are genuine Mikuni proper jets. I learned the hard way a long time ago to use only genuine Miks.

[DEEBS11]

Check compression in that cylinder just to make sure.

[MarkD]

In all, I've checked compression on all cylinders approximately 8-9 X with a new and my old tester. All readings consistently reading about 145lbs. Losing patience with this issue!

[DiverRay]

Compression, fuel, and spark is what you need to fire.
You have compression. You have spark, you need to look at fuel delivery. I hate to say this, as I've been there myself, but you should probably pull carbs and start with a "bench test".
Set the carbs level in a vise to hold them. Plumb a tank of ATF to run through the carbs. Use a hair dryer set to low/no heat to blow through each carb with a sheet of white paper about three or four inches in front of the engine side of the carb. Start at #4 and work your way to #1, at idle setting, and see what the splatter pattern look like for each carb. Now pull the enrichment lever and go again. Finish off with about 1/2 throttle and look to see how much ATF shows up with each carb. The patters should be close, if one is off(yes #1 carb), you now know what needs to be done. If they all look the same, then you must have a spark or compression problem.

[MarkD]

Thanks for detailed steps in bench testing the carbs, Ray. I'm glad these pages are archived, as I will use the bench test one point if needed. Now-- an update. I ran the bike for a while at 4K RPM, again, but for a longer period of time, as per Bikerphil's suggestion. The #1 cylinder seemed to be firing as per usual after a time. It was up to temp, in comparison to the other cylinders, so I road tested it. It seems to be running normally now. Is it possible that there was a piece of crud that worked its way through the carb? That is my only explanation. If anyone has a thought, that would be great. Many thanks to all who suggested different approaches to address my issue. I will road test again tomorrow, as the weather is supposed to be nice again. Thanks again, guys!