Help: Engine cutting out after sustained high speed

[Watcher]

Do you have an inline fuel filter?

[mr72]

Apologies I haven't read the whole thread, but...

this sounds like a fuel delivery problem. Watcher is right about the inline fuel filter, also whoever said the vacuum petcock may be the culprit is also right to look there. Could also be low float height, stuck (closed) slide, clogged main jet (orifices in carb, not likely the actual jet), severe vacuum leak like a tear in the carb diaphragm(s) or missing o-rings or vacuum port caps, super tight valves, very bad intake boots or intake boot o-rings, carbs loose from intake boots (not seated properly before tightening hose clamp)...

I have had all of these problems happen

Basically the symptom is when it needs a certain amount of POWER (which is what it takes to go over 75mph) then it can't deliver, which means it just can't get enough fuel to make that much power. Everything that can cause it to not get enough fuel can be the cause.

You can rule out the petcock by running it on prime. If it works fine on prime then it's the petcock or vacuum supply to the petcock. If it does the same thing on prime, the problem is in the carbs.

My money is on:
1. low floats
2. a kinked/bent fuel hose
3. fuel hose with a crack or air leak somewhere so it sucks in air
4. vacuum leak in the petcock vacuum line so you get double whammy: not enough vacuum to fully open the valve and a vacuum leak that prevents the slide from operating correctly.

Either way the fix is to set the float height and while the carbs are apart replace every single o-ring and all of the hoses (vacuum, fuel), including the intake boot o-rings. That's cheap and easy, way easier than trying to troubleshoot everything. This is a case of: even though it might not be broke, fix it.

[Rallyfan]

Thanks for that. I suspect float level - boots - kinked fuel line too. I've replaced the two lines between the two petcocks; my next step therefore is to replace the main line from the frame petcock to the carbs, the carb boots, and any rings/hoses I can find in there too.

I'll post back once I get a chance to open it up; it may be a while with work but it's on my mind and I want this sorted out.

Thanks!

[Rallyfan]

Update: Main line replaced, all vacuum lines checked (used eyballs and blowtorch off while the bike was running etc.) Carbs removed from bike, cleaned, floats set again.

Fixed!

Likely the floats all along.

BIG THANKS TO EVERYONE!!!!!

[user11235813]

@SBW: "Ohh that is something I've struggled with too... why oh why couldn't there be just a little bit more space there?? Very frustrating!!!"

That's another benefit of the OEM fuel lines which remember are different sizes on each end.

[Rallyfan]

True! OEM is the way to go for fuel lines! Bike Bandit to the rescue. May as well pay a little extra and get the clamps; saved me the worry.

[ShowBizWolf]

I did go ahead and buy them (lines and clamps) a couple weeks ago. Got them in the mail already