1980 750 LTD 4 cylinder odd choke issue

1980 750 LTD 4 cylinder w/ CV34's.

Bike has been running great! I have 1000 miles on already in about 2.5 weeks. I've been experiencing an odd issue though when I choke the bike. It's done this since I've had it. When I start it with the choke it fires up and idles fine. The INSTANT I take the choke off, #1 cylinder sputters and fires. EVERYTIME this happens (when cold). The bike runs perfectly fine and #1 fires just fine when off choke. My plugs all look pretty decent. Things I have done:
1. Cleaned carbs. Verfified all passages and holes are good.
2. float heights are perfect
3. idle screw is adjusted properly and oring in good shape(i've tried tinkering with it too, no change)
4. polyester plug is very snug fitting.
5. Good spark, good compression
6. timing is correct, valves adjusted to spec.

Seriously guys, the bike runs like a champ, but it bothers me it doesn't fire on #1 when choked. I'm not sure if it's too much gas or too little... any suggestions? As for now I'll keep riding it!

apbling wrote:

1980 750 LTD 4 cylinder w/ CV34's... bothers me it doesn't fire on #1 when choked. I'm not sure if it's too much gas or too little... any suggestions?...

If #1 resumes combustion upon being un-choked, it's likely being over-choked (receiving too much gas -- meaning overly rich fuel mixture) while choke is applied.

As choke flaps are presumably functioning okay, would suspect a pilot circuit issue.

Good Fortune!

I would too... the choke is an actual REAL choke, so I wouldn't see how too much gas would be getting through, unless the jet is too large (it's stock) or the plug is leaking (it fits really snug) or that idle screw is goobered up (it looked ok). I'm not used to pilot circuits allowing too much fuel....

Could experiment by leaning #1 carb pilot air screw, say 1/2 turn, to see if it makes any difference.

Good Fortune!

tried that too. I've gone from 1.5 turns to 2.5 turns. Makes a difference on how the bike runs with choke off, but not with my choke issue.

Looking at your year parts diagram, you have Mikuni CV34's?
Well if so I would pull the float bowl on the offending Carb, this can be done with the rack still installed. Then on the bowl there should be a brass tube with a small hole in it, this sticks up when the bowl is off but right side up, so you look into the bowl. This where the choke circuit gets its feed of fuel, make sure its clear of being plugged. Other wise from the top of the carb you can take off the lid for the diaphragm and pull out on the choke knob, you should see the plunger open up a passage in the top inside of the chamber. If you got a 90 degree air fitting you can blow out the passage here while the knob is pulled out.

Edit: The more I look at the diagram the more I think you have Kehins?

A simple flap style choke (as opposed to a separate enrichener circuit) mechanically reduces air intake from the airbox without otherwise affecting carb function.

Nothing else is coming to mind besides an already over-rich carb pilot circuit.

Perhaps the pilot jet is too large, or might be reamed out from overly aggressive cleaning, whereby the pilot circuit is too rich regardless of the air adjustment. And application of choke is overwhelming the engine's ability to combust the already over-rich pilot circuit mixture. Or perhaps the pilot air jet (if applicable) is partially clogged.

Just guessing here.

Good Fortune!

apbling wrote:

...Good spark, good compression...valves adjusted to spec...runs like a champ...doesn't fire on #1 when choked....

If not already done, would verify correct gap on #1 spark plug.

And also test with a brand new spark plug in #1 cylinder.

Thinking NGKB8ES. But should follow plug recommendation per FSM.

Good Fortune!

They are Keihins CV34's.
I agree Patton, it does sound like a pilot issue. The jet is actually a new to me/used one. The original was plugged tight, so I thought I struck gold when I found this. Only thing it didn't change my choke issue, but made my overall idle and off of idle performance much better. Verified pilot passages are clear.... This is really a stumper. I'll check with the spark plug. They are new wires, but old caps...could this old cap be an issue? I was assuming it was a carb issue since it seem to be 100% related to the choke.

Could experiment ---

Swap #1 and #4 spark plug wires to see whether the problem moves to #4.

Swap the #1 carb pilot jet with one of the other carb's pilot jet to see whether the problem follows the pilot jet.

Results may be helpful to further diagnosis.

If not already done, would remove #1 spark plug, hold its base against the engine head (to ground the plug), and visually observe spark quality while spinning over the engine. Look for fat blue sparks -- not wimpy thin orange sparks.

Am presupposing that compression and valve clearances on #1 cylinder are within specs.

Good Fortune!

apbling wrote:

...I'll check with the spark plug. They are new wires, but old caps...could this old cap be an issue? ....

Weak spark could be caused by an imperfect cap, or poor connection between cap and plug wire.

Sometimes helps to trim back the plug wire, which allows a fresh bite by the cap.

I prefer solid core plug wires and non-resistor caps and non-resistor spark plugs. Just a straight shot high intensity loop without any resistance other than jumping across the plug tip.

But this does cause radio interference, if that matters.

In any event, would assure against having more than one resistor in the high intensity loop. Meaning, only the wire, or only the cap or only the plug.

Good Fortune!

well, I have solid core wire. Maybe I'll see if I can't find a non-resitor cap. It shouldn't matter about radio interference if I don't have one, right?

That wouldn't cause the plug to run hotter, causing long term issues with my combustion components, would it?