Choke Problems - One cylinder not running.

For the past month or two (lost track) I've been having issues cold starting the bike. It seems one cylinder is not running when on the choke so the bike will not idle correctly when warming up. I have to give it gas to keep it going. As soon as I push the choke down, the dead cylinder fires right up.

When this first started it was the #2 cylinder. So, I thought to myself this could be either a spark, air or fuel issue. So, I checked the spark and it seemed fine on the dead cylinder. I also swapped 2-3 plug wires to double check and the problem still remained on the #2.

So, I thought the enrichener circuit on #2 must be blocked. I removed the carbs and cleaned out using carb cleaner, Seafoam, wires and compressed air. I see everything coming out the correct holes as it should. At this point, just on a whim, I swapped some parts between the #1 and #2 carbs (i.e. pilot jet, floats, float needle and pilot air screw.

Reinstalled everything and now the problem was on cylinder #1!!! Weird! Must be one of the parts that I swapped over. Thinking it must have been the float and it wasn't at the correct level and this was causing issues. So I went and adjusted the float levels on all carbs using the clear tube method. Problem still on #1, so I decided two swap parts again. This time only the pilot jets and pilot air screws. Problem moves again to #2!!!!!! Must be one of these parts. I swap the air screws since they are easy to do, and this doesn't do anything. I then think it must be the pilot jet, I remove them both, clean them and swap them over. Problem is still on #2.

At this point I gave up. I can't figure this thing out. Is there anything I'm missing or anything else I can try?

This is a '78 KZ650 SR with VM24SS carbs and recent work on the bike includes:
- multiple carb cleanings
- carb bench sync
- vacuum carb sync
- points cleaned and gapped
- ignition timing set
- new spark plugs
- floats set using clear tube method
- sealed gas tank with POR-15

Any suggestions would be appreciated. Are there any tips or techniques I can try to further track down the source of the issue? I'd just like to resolve the problem before I put the bike away for winter. Prior to this problem starting the bike was flawless on the choke, requiring no user input.

Thanks

Nobody??

I still can't figure out what's wrong. I noticed today that choke plunger on the #2 carb wasn't fully up, like the rest, when the choke was up and figured that was the issue. Fixed it but no luck, not sure what else to check.

Any suggestions would be appreciated. I'll be putting it away soon for winter and would love to figure this out prior to that.

Sometimes visual appearance of the spark is misleading. Can't help being suspicious of ignition with coils/caps/plugwires being 30 years old, and that plugwires and/or caps might be leaking voltage. Thinking new green Dyna coils and Dyna solid core plug wires would be helpful. Also, if not already done, would assure good battery with full charge, tight clean connections to terminals and good connection where negative cable attaches to engine or frame.

But for now, let's say the ignition and spark are perfect, and move to the #2 carb, which exhibits the following reported symptoms:

cylinder is not running when on the choke so the bike will not idle correctly when warming up. I have to give it gas to keep it going. As soon as I push the choke down, the dead cylinder fires right up.

Am thinking the #2 cylinder is being "over-enriched" during choke application, which might not be fault of the enrichener circuit. Could be an excessively rich pilot circuit, which of course continues its own full separate independent functioning during operation of the enrichener circuit.

Would first experiment by leaning the pilot circuit (turn out the pilot air screw 1/2 turn or more) and see what happens. If same symptoms persist, would suspect either obstructed pilot air passage or damaged pilot jet. The pilot jet might be too large, or maybe damaged (enlarged) by over-zealous cleaning. Sometimes there's an air jet at entrance of the pilot air passageway (located at rear of carbs alongside the main carb throat). If fitted with an air jet, the air jet might be obstructed.

If using pod air filters, perhaps the #2 is clogged, dirty, or over-oiled. Could remove the pod and test run for effect.

Let us know if details about how to attain a "perfect as-new pilot circuit" might be helpful.

Good Luck!

Has compression test been done resulting in good psi figures, and also have valve clearances been checked to assure being to specs?

HerrDeacon wrote:

... Prior to this problem starting the bike was flawless on the choke, requiring no user input....

Forgot to ask --- did this problem appear suddenly, out of the blue?

Thanks Patton for the great responses, really appreciate it. To answer your last questions first, yes compression has been checked and numbers are good across all cylinders. Valve clearances have been checked and intake valves are out a bit. I'll be taking care of these over the winter (as well as getting the dyna goodies you mention above).

Yes, the problem did arise all of a sudden. I believe it happened after I had the carbs off when I sealed the tank. I was getting crud from the tank so I sealed it and got a new petcock. I then cleaned the carbs to ensure the crud wasn't in them. I believe this is when the choke issue started. Was perfect before this. I must have messed something up in the process.

I think you may be on to something regarding overfueling of this cylinder. The reason being is that after I start it on the choke I give it some gas and then take it off the choke. At this point the #2 cylinder comes on line and you can hear all the unburnt gas igniting (exhaust pops) for a few seconds, and the revs shoot up. I'll try leaning it out a bit to see if this helps.

I had the pilot jets out and they visually look fine. They are new this year, not to say that this one is not damaged or enlarged though. I'll probably get some new ones anyway when I order the ignition parts.

I do have pods but I already tried running without them to no affect.

I'd really appreciate the details around "perfect as-new pilot circuit", that would be great.

Thank you very much.

Patton, just did a search for cleaning the pilot circuit and found a great post you made to help someone else. I'll go through that and see how it helps.

Also some thing to keep in mind when cleaning jets, never use wire or metal, use a wooded toth pick instead the metal can enlarge the holes and you visualy wouldn't notice it.

Took the carbs off again last night to go through them again. Everything looked fine but I cleaned the pilot and enrichening circuit again. I just went out and tried a cold start to see if there was any improvement...none.

I had to still give it gas to keep it going. I had the pod filters off so while I was doing this I tried a few experiments. I put my finger over the enrichment opening and surprise the idle picked up beautifully. Let go and it almost dies again.

I then do the same with the pilot hole. While finger was on it never had much affect, but when I let go the idle would speed up for a second. Like it got a burst of fuel from the sudden rush of air.

So what is this telling me??? Is something wrong with the enrichment circuit so not enough fuel is being "pulled" up under normal circumstances? Seems when I blocked it the increased vacuum must have drawn in more fuel.

Or is it that the pilot circuit was too "open" which was minimizing the effect of the enrichment circuit?

I'm now waiting for the bike to cool down so I can further experiment. I may try turning in the pilot screw to richen it up to see what effect this has on it.

Thought I was post my findings while I'm waiting for the bike to cool down.

Am still thinking "over-enrichment" during choke application.

If not already done, would check integrity of the starter jet located at bottom entrance to starter pipe.

As known, the starter jet allows a metered quantity of raw fuel from the float bowl into the starter pipe. The raw fuel in the starter pipe is joined by some air from the float bowl to provide a mixture which then passes through the starter fuel passage up to the plunger chamber, where more air from the starter air passage is sucked along with it through the starter mixture passage into the main carb bore.

Perhaps the starter jet has been inadvertently enlarged by over-zealous cleaning.

Took the carbs off again last night to go through them again. Everything looked fine but I cleaned the pilot and enrichening circuit again. I just went out and tried a cold start to see if there was any improvement...none --- Dadgumit!

I had to still give it gas to keep it going. I had the pod filters off so while I was doing this I tried a few experiments. I put my finger over the enrichment opening and surprise the idle picked up beautifully. Let go and it almost dies again. --- likely just reduced effect of choke

I then do the same with the pilot hole. While finger was on it never had much affect, but when I let go the idle would speed up for a second. Like it got a burst of fuel from the sudden rush of air --- with or without choke being applied? But likely just worsening excessive richness by preventing air in the pilot air passage.

So what is this telling me??? Is something wrong with the enrichment circuit so not enough fuel is being "pulled" up under normal circumstances? Seems when I blocked it the increased vacuum must have drawn in more fuel.-- Am thinking too much fuel as opposed to not enough.

Or is it that the pilot circuit was too "open" which was minimizing the effect of the enrichment circuit? --- any effect from leaning out the pilot circuit using the adjustment screw?

I'm now waiting for the bike to cool down so I can further experiment. I may try turning in the pilot screw to richen it up to see what effect this has on it. ---Am guessing condition will worsen.

Imo, when removing choke allows the cylinder to resume combustion, it is a sign of being over-enriched during choke application.

Good Luck!

Regarding the starter jet, in my VM carb its just a long thin tube which doesn't look like it can be removed. Is this correct? Meaning it can't be replaced like the main and pilot jets? If it was inadvertantly enlarged is there anyway or fixing this?

There was no noticable affect from leaning out the pilot, but I turned it in (richen) a half turn and this seemed to help a bit. It would at least idle roughly without me using the throttle. I may have to continue to play around with it a bit, only problem is it takes so long to test since you have to wait for the bike to cool down.

Thanks again Patton for your insight and listening to my troubles. I'll post back with any other findings I have.

A quick check of the pilot circuit - - -

With the engine warmed and idling at normal operating temperature, turning the pilot adjustment screw either way is supposed to result in a noticeable change in rpm.

If turning the pilot adjustment screw has no effect on the rpm, there is probably still something wrong with the pilot circuit inside the carb. Which is of course independent of the separate choke circuit.

I don't understand why removing the choke during initial warm-up would allow combustion to resume in the affected cylinder, but keeping the choke applied while simultaneously enriching the pilot mixture during initial warm-up would also allow combustion to resume in the affected cylinder.

May well be wrong, but given the existing symptoms and test results, remain way more suspicious of the pilot circuit than the choke circuit.

Thanks for the progress reports, and we'll keep pecking away at it.