This morning, my SuperHawk developed what we in the auto repair industry refer to as a "driveability issue."

I hammered it to pass a car and it fell on its face around 5,500 RPM. With a bit of experimentation, I found that I can sneak up on higher revs, but if I hit it hard it flattens out from as low as 4,500 RPM. If I back off just a bit, it begins pulling again. As the revs get higher, it pulls less and less. This isn't accompanied by any popping or backfiring. By the way, the bike ran great before this morning.

Anyway, I've looked around here and I'm concerned I have a petcock issue. I lifted the rear of the tank today and popped the fuel line off of the petcock. I didn't anticipate any fuel flow, since the engine wasn't running. However, fuel ran freely from the tank. The bike was recently in the shop for a carb sync (about 100 miles before the problem began), so I'm also concerned the technician screwed up the vacuum line or fuel line routing.

Am I right in assuming the petcock shouldn't flow any fuel with no vacuum supply? Is it possible for the petcock to jam partially open so it won't close, but also won't flow enough fuel? What should I look for with this issue?

Thanks in advance for your help.

-Aaron

 

There is a diaphragm in the petcock, so if that got damaged it could be the issue. Also make sure the vacuum hose is connected to the rear of the petcock, not the bottom as the bottom is the vent. either way I don't think it should flow with the engine not providing vacuum to the petcock.

 

No, it's one of the carb slides sticking.

 

It happens almost every time I take my tank off. Gas flows freely. It just means it's sticking open. That shouldn't have any impact at all on driveability. Verify the small vac line is not going to the small nipple on the bottom of the petcock. It goes in the rear of the petcock.

Beyond that it's possibly a vacuum leak or a sticky slide as 'bling suggests, maybe even a torn/pinched diaphragm.

 

I would think it would be unusual for the slides to suddenly begin hanging up. If they are, is there a common cause/fix for this?

 

That's why it can be so hard to diagnois.

One day I was doing a hard run thru the gears when all hell broke loose. I thought I had lost a CCT or something. I slowed down and bike ran fine. Give it gas, it would fall flat on it's face, popping, sputtering, and basically sounding like it was running on one cylinder.

I replaced coils because I had a new spare set from ebay. No change. Cleaned petcock, no change. Got another used petcock, no change. Tried a different tank (thinking vent was blocked), no change. Changed out ICM with used, no change. On the test runs, I could even get it to run well (pull hard through the gears) once or twice but then the condition would return.

Finally pulled airbox and saw one slide sitting higher than the other. Pushed it up and it returned much more slowly than the other. Took it apart and the grease had dried out. Cleaned it all, lubed diaphragm with silcone spray and lubed slide with dry graphite powder (manual gives no recommendation). I also lightly dressed the edges of the slide for any nicks with some steel wool. Bike was back to normal.

So I recommend you pull the airbox lid/filter and check the slides for either hanging or unsmooth operation first. Also push them up together and release at the same time for comparison.

Causes? Dirty or dry carb slides, diaphragm is damaged or not seated properly, weak spring.

 

Not saying this is the cause but I've had two petcock diaphragms go South on me so trust me it does happen....

 

Oh for sure they can go bad. That's why I went to all the effort to check/replace them first but you can first rule out the slides in a matter of minutes with a quick look/check.

 

yes, always start with the easy and obvious stuff, no doubt...

 

Checked my petcock diaphragm at lunch today with my Mity-Vac. It holds vacuum okay. Also checked TPS resistance. Idle is 987 ohms, WOT is 5,660 ohms. Across the outer terminals is 5,770 ohms. I could occasionally find a dead spot (no continuity) around 4,000 ohms. Probably not enough to cause an issue, though. TPS does need adjusted.

After work, I'll check the carb slides.

-Aaron

 

As crazy as this may sound, another possibility is that one of your main jets is either loose or has fallen out completely, this is more likely if you've recently had the carbs off for rejetting/cleaning etc.
The affected cylinder will have a nasty looking sparkplug and it'll blow black smoke out of the exhaust when you blip the throttle.
I know that this is way out there but I've seen it a couple of times over 30 years of wrenching, once on a used truck I bought and twice on CB750's, both in the same week no less.

Kai Ju

 

I checked the slides and both seemed to move smoothly. However, to be sure I removed both slides, checked the diaphragms for tears and cleaned and lubed the slides. I also disassembled and checked the petcock and set the TPS (498 ohms with the metal inserts removed).

After reassembling the carbs, the slides seemed to move more freely, but there wasn't a huge difference. I left work and cautiously accelerated through the RPM range. Everything seemed fine, so when I turned the corner and traffic cleared, I hit it. It pulled hard right through 9k RPM through three gears. Bingo!

By the way, I'm not sure if the TPS adjustment made much of a difference, probably because it was running so poorly before the carb work.

Anyway, thanks for all your help and suggestions. It definitely made diagnosing the bike much quicker and easier.

-Aaron

 

This was something I didn't mention previously as it's only a thought but.....

Mine weren't EXACTLY the same on return before cleaning/lubing and I think that while under load it's enough to cause a serious intake imbalance which leads to a further "hanging" on the slide as the throttle is open further.

Or I could just be talking out of my you know what!