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Nighthawk 750 Fuel Starved at top end

SteelerDan

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I've rebuilt my carbs, replaced the needle valves, ensured that everything is clean. At the higher end of revving, when the rpm's get over 6k, it acts fuel starved.
the bike starts right up, and gets gas on both the fuel and reserve settings of the petcock, is it possible that the vacuum is not staying constant and the petcock is closing at the higher end?

Anyone ?
 
Nope, the vacuum fitting plumbs directly into the intake port, it increases vacuum as flow increases there, similar to the slides. Besides that, easy enough to bypass it to test the idea out.

Compression test first, they can break up at higher rpm or not want to rev fully out if valves are leaking. Nobody ever sets valves on them because it requires shims to do it and a common issue with used ones. Then almost everybody goes after the carbs.............sound familiar? Why you find them 9 times out of 10 with the carbs off and in pieces.

If fuel starved it will run slightly better at max load by slightly backing off the throttle. If the breakup is missing can be rich from having pods on it. Common too, guys like them pods. Look for black smoke then.
 
Thank you! That sounds like good advice. My 750 has 95000 miles on it. The man I bought it from kept up on it's maintenance, I just started to notice the issue recently, tuned the carbs, (very dirty) and changed the plugs.
I'm
I discovered today that there was too much oil in the bike, about two quarts more than needed. I drained it off.
The Bike running better but does run slightly better when backing off the throttle, and No, I don't have pods on it. All Stock.
I'll run a compression test later today.
thanks again.
 
If that mileage is correct you are up there in the stratosphere, I've only seen one other make it that long. The norm is around 40k and dead and even more so in a hot climate as the tensioners crumble into pieces around there.

That oil issue might be why plug has fouled on one cylinder.
 
Sexy bitch odomoeter.jpg
I take good care of my girl.
 
Nope, the vacuum fitting plumbs directly into the intake port, it increases vacuum as flow increases there, similar to the slides. Besides that, easy enough to bypass it to test the idea out.
Re: petcock diaphragm

I have a 1983 CB550 SC and it is exhibiting this same problem: seems to be starving for fuel at higher RPM or when under a load. How does one go about bypassing the petcock diaphragm mechanism?

ETA: I guess my actual question is: Is the vacuum-operated petcock open or closed with no in-between? Meaning, can the petcock diaphragm have enough vacuum to open only partially, which may supply enough fuel to the carbs for idle or low-rpm low load riding, but not open enough for high-rpm high load riding?
 
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Yes to the last question. The diaphragm opens up all the way normally but tends to open less and less as the rubber gets harder depending on time and how close to engine heat it is.

You simply run a fuel line from tank directly to the carbs and plug the vacuum port to get rid of it. But learn to cut the petcock off EVERY time you get off bike, the valve stopped fuel leaking to run into the engine to cause engine locking up and damage. It's easy with bike on the sidestand. Cutting petcock off is a learned biker skill and for the last 50 years.

There is often a standpipe vertical filter between the petcock and the tank that can cause the issue too.
 
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