Odd thing going on with my Mikuni

[HMBAtrail]

So on the 350 race bike I got it running and was doing shake down rides to get the pilot jetting correct. It was hunting a little but from 1/8 throttle on to WOT it pulled great and clean. So in the midst of swapping to a different size pilot jet the bike started acting up. Now the bike will settle into an idle better than before but up around 5k and about half throttle the bike just falls on its face. Almost like I'm hitting the kill switch. Rolling from half throttle to wide open throttle yields no improvement. I have to close the throttle and then the motor will catch back up.

Brand new Mikuni carb. Brand new jets. Earlier in the day it was fantastic from 1/8th throttle to WOT. Now it falls flat on its face and stalls out unless I pull in the clutch, close the throttle and slowly bring the revs back up.

I checked float level and it looks right on spec. Just to make sure I slightly increased and slightly decreased the float level. No change. I took the bike back to the last pilot jet I had in it that the bike ran well on. No improvement. It's so weird, it was running great and then all of a sudden it wasn't.

Here are the particulars for what it's worth:
350 springer
Mikuni VM38 carb.
The ONLY change I was making was a pilot jet swap. Nothing else changed.
Electrex race ignition. I changed ignition maps, no improvement.
Pulled the carb and blew compressed air through all the ports. No improvement
I mean, surely it isn't something that suddenly went screwy with the Electrex and now the ignition isn't advancing, right? The chances of that being the culprit seem so remote. This has to be a carb issue seeing as how that's what I was working on, right?

[Jordan]

Getting good flow of fuel to the float bowl?
- Fuel tank cap breather hole obstructed?
- Fuel tap not clogged?
- Using an inline filter? (Don't.)

[HMBAtrail]

Fuel cap fine
Fuel tap (Pingel) brand new and clean
No in-line filter

Plenty of fuel delivery.

[Jordan]

Can you temporarily fit another carburettor?

[graeme]

Manifold air leak?

[HMBAtrail]

Grame, I checked that as well. Sprayed around all joints to check for change in idle. If there is an air leak I haven’t found it yet.

Jordan, I don’t have another VM38. All the other carbs I have are VM32. But I’m considering this.

[Jordan]

If you get good running with another carb, you can eliminate an ignition problem.
Then concentrate on your 38.

In the '70s my cousin raced a 350 with a 42mm Dell'Orto.

[double diamond]

I’ve had symptoms like this due to insufficient tank venting. How does your tank vent? Through the gas cap or a vent tube? If a vent tube, do you have a one way valve installed?
Also encountered similar symptoms with a needle that was far too lean but since it was running ok initially, not likely.
Matt

[HMBAtrail]

The gas cap is vented. The screw on type that came with the tank. I got the tank from Airtech.

Yeah, I moved the needle up a notch and back down a notch just seeing if anything would improve. The part that is the most telling is the bike ran well from right off maintenance throttle up to WOT and then all of a sudden it didn't. And not just a bog or a flat spot either. As if the kill switch was hit. No sputtering, no backfiring, no sluggishness, etc. Just "click" and it's gone. I'm able to save it if I quickly pull in the clutch, chop throttle, and slowly re-introduce revs.

I ended up ordering another VM38 body. I have plenty of new jets so I will build up another carb and swap it over. If the problem is still there I know it isn't a carb issue and can stop swapping out jets looking for the answer. If the problem is still there with another carb I have to think it is a timing issue, yes? What other problem would present symptoms like I'm seeing?

[double diamond]

One possibility is that it’s draining the float bowl and not refilling quickly enough. Your symptoms sound a lot like what I encountered due to a tank vent one way valve that had too stiff a spring. Replaced it with a valve with a weaker spring and symptoms cleared up. I’m not suggesting this is specifically your problem since it’s not clear what your tank vent arrangement is, but the larger issue I.e. fuel delivery is something I would recommend you look very closely at. Tank venting must allow enough air into the tank to let fuel flow freely out. Petcocks must flow adequate fuel and not be blocked. New fiberglass tanks can have debris inside that can get into the petcocks and block fuel flow. Are your float bowl vents clear and the plastic tubes (if installed) not pinched by zip ties or such. Is the float needle valve of adequate size? Are the floats and float arm installed correctly (not to be ridiculous but i’ve Installed floats upside down myself). Consider every component of fuel delivery and confirm 1)adequate fuel flow and 2) tank and float bowl venting.
If you’re sure fuel flow is ok, my next guess is a needle with insufficient taper or a main jet that is far too small. A needle jet that is too small is also possible. Your engine is in a pretty wild state of tune and is going to be very sensitive to settings. Have you considered putting it on a dynamometer to get the settings right? Even if you get it running acceptably well, you would still benefit from results of a dynometer session.
Matt