Bike hesitating- I'm about to throw it in the lake. NEED HELP

[HMBAtrail]

I'm so frustrated I'm about to throw the bike in the river and go buy a bowling ball. I have a widecase 350 race bike. Here are the particulars:
Misano head. Piper 277 cam. 38mm Mikuni (45 idle, 2.5 slide, 6DH3 needle. 166P8 needle jet. 250 main. 0.5 air jet.) Electrex ignition for dual spark plug.

I built this bike up over the Winter and have raced it four times this season. For all four race weekends I've had the same issue. The bike pulls well down low and in lower gears (first and second) it will pull to about redline. But in 4th and 5th gears anything over 75% throttle and the bike will pop and hesitate. Rolling beyond 75% throttle results in the bike bogging and having zero power. I thought it was the carb jetting and went leaner on the main. No change. Richer on the main and no change. I've moved the needle up and down with no change. Tried a leaner needle and a richer needle. Tried a richer needle jet. None of it mattered. Then I thought maybe the ignition box was bad and replaced with a Saschse ignition. No change in problems. Then I suspected perhaps my exhaust was too long. Welded up a new exhaust (1.5" diameter pipe, 620mm long, paired with a 13" reverse meg). Again, no change. Then I pulled the head to see if I had possibly a bent valve. Lapped in the valves and used blue engineers dye and got a solid transfer from valve to valve seat. Pulled the valves and inspected on a granite surface plate. All checks out.

I have straight cut upper and lower bevel gears and the timing dots on the lower bevel gears appear one tooth off. According to the specs on the cam I should be at 54 degrees BTDC with the valves at running clearance (about 5 thou) and 20 thou lift. If I use the timing dots I'm at about 74 degrees BTDC. Rotating the lower bevel gear one tooth gets me almost spot on 54 degrees (again with running clearance and 20 thou lift). I can' t imagine the bike would even run with a 20 degree difference in valve timing but who knows. But even if it did would it behave like the bike is??

Like I said before, I'm so frustrated that I am losing sleep. I have Barber in early October so I have a little bit of time to figure this out but I'm at my wits end on what it could be. Two new Mikuni carbs. Two new ignition systems. Two different exhausts. Countless jet swaps.

The one constant variable in all this is me so I'm all ears if you think I'm doing something wrong.

[Jordan]

An air/fuel ratio gauge kit might help ?

[Eldert]

does the carburator get enough fuel ? could be pulling the float bowl empty .

is the float needle in the carb big enough ? dont know about Mikuni's but Dell Orto
have diff size float needles .

Eldert

[blethermaskite]

Hi,
It surely has to be a fueling issue, if it was ignition related it is unlikely to rev out clean in the lower gears, sound like a fuel flow problem.
Cheers,
George

[HMBAtrail]

I have the float set to Mikuni specs (18mm from base of carb to the brass float arms) but that spec may not hold for a race bike at WOT. I’ve been thinking more that maybe the float bowl is running dry at times. Easy enough to check I suppose.

[blethermaskite]

Maybe a silly and obvious point, but do you have a really adequate petrol tank breather system, and is it working? also a big bore fuel tap/ feed line to the carb.
Cheers,
George

[HMBAtrail]

Not silly at all! A good question. I’m using a single Pingel 3/8” tap. The tank breathes via the gas cap. A simple way to check would be to run the bike without the gas cap.

[double diamond]

To check tank venting, put a gallon or two of gas in the tank. Tighten the gas cap as you would when you’re running the bike. Disconnect the fuel line from the carb and direct it into a gas can. Open the petcock and let the gas run for about a minute. Take note of the fuel flow when you first open the petcock. Then watch to see if fuel flow diminishes. You should get robust flow when you first open the petcock. If flow diminishes over time, you might have a venting problem. If it’s really bad, fuel flow will diminish to trickle and might even vent the depression in the tank through the petcock. If you get venting restriction symptoms, take the gas cap off and see if it resolves the fuel flow issue. If flow is undiminished with the cap off, you have a venting problem. If flow still diminishes with the cap off, then your petcock is being blocked somehow. I would think a single 3/8 petcock would be adequate.

Matt

[etsjw]

Tank venting might be the culprit.
A dumb question: When you are tucked in at WOT, could your knee be blocking the carb intake?
I have never had any success with a reverse cone meg and the total length of your exhaust seems very short.
Check photos of the factory racers from late 60s/early 70s. They all have long slow taper megs with, maybe, just a hint of a reverse cone at
the end. Many years ago when I was racing my 250 MK3 I used a meg with these dimensions: Header pipe 1 1/2" X 21" long connected to a 32"
long meg tapering to 3 1/4". At the very end there was about a 1/8* long reverse for an exit opening of 2 7/8". This gave good hesitation free performance to 9K and beyond.
At continuous WOT in 4th/5th gear, the exhaust gas temperature doesn't get much of a chance to cool. This might be effecting the timing of the reflected pulse from your short exhaust and upsetting the intake charge flow.
John

[jbcollier]

I used a portable A/F meter to set up my carb. Made what could be a challenging task in the past super easy. I run a Mikuni TM33 on a 250NC desmo with a V2 race cam. It is very simple to weld in an O2 bung and then you know what's up, no more guessing.

As an aside, "popping and hesitating" usually means you're lean.