At my wits end. HELP!! - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
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post #1 of 63 (permalink) Old 08-09-2009, 12:48 AM Thread Starter
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At my wits end. HELP!!

Guys-

Been working on/fighting with my VN750 for weeks now. It is running, and pretty strong, but it still hesitates and misses between 4k and 5k on the tach. No matter what I do, the whole powerband gets better and more solid yet there is still that hiccup. I want to get it perfect!

92 scoot, earshaved and coastered, jetted up, straight pipes (stockers reamed and de-goated), about 12k on the odo. It was running strong a couple of months ago when I developed a fuel leak. Decided to do the rattlecan paint. Brother died. Decided to bolt her up and go. Had rust in the tank I put back on. Sucked the crud through the carbs. Hasn't been right since.

What I have done:

Petcock rebuild. Thoroughly cleaned although no internal parts were changed. It all seemed OK.

Muriatic acid/Naval jelly treatment to the tank to remove rust.

Installed inline fuel filters just above carb intakes.

Tore down and rebuilt carbs thoroughly. Again no gaskets or anything were replaced but they all seemed fine.

Set fuel/air mixture screws carefully to 2 1/4 turns each.

Balanced carbs with home made 2/bottle manometer

At this point I am pretty sure the fuel delivery is solid so I went on to spark and just put in my iridiums that have been waiting. Torqued them to 12 ft/lbs.

It starts well, idles great, has wonderful torque and bottom end. It can make highway speed no problem, but it starts hesitating around 4k and honestly it's never 100% right after that, although the worst of it seems to be between 4k and 5k.

Left to try:

CRobins says I can take the spring out of the petcock and make it gravity feed. I might try this. That way if the problem is somehow the vacuum calling for more gas we can eliminate that.

Bigger in-line fuel filters. These are new since the fuel leak so maybe I just need bigger ones to pass more fuel?

New spark plug wires. I have a set sitting waiting. Been running the stockers and they are cracked and worn.

Dyno2000 CDI. I am starting to wonder if maybe it's the CDI. Maybe it's not calling for the right spark at that time somehow? I don't have another OEA one, but I could replace what I have with the Dyna.

And that's about all I have for ideas. Seems to me the fuel is there and the only way that could be the deal would be if when it suddenly wants more fuel (cranking on it, getting it quickly up to 3k-4k) the fuel can not get there for some reason. Occlusion (too small an opening?) or a vacuum issue? Or if the fuel is there it's not getting burnt- ie ignition issue/CDI or spark cables. But if it were the cables I would expect the problem to be all over and not so consistent. So that doesn't seem likely.

Any other ideas, either for solutions or diagnosis/troubleshooting?

I'm really going mad. I want to get this girl SOLID before I start working on the 1500.


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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post #2 of 63 (permalink) Old 08-09-2009, 06:09 AM
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My bet is that it's the petcock or, less likely the fuel filters. Can't you remove the filters? I don't think my 2005 750 has any.

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post #3 of 63 (permalink) Old 08-09-2009, 07:05 AM Thread Starter
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Petcock, really? Meaning the vacuum isn't opening enough to send the fuel in in a hurry? Is that because something is wrong with it or because it just isn't capable of passing enough fuel by design? Somehow that seems unlikely as other ppl are running earshaves and degoats and making more than stock HP on these girls.

Do you think just taking out the spring and going gravity feed will solve it? I will certainly try that, and if it fixes it I can live w/ that as a long term solution, even.

But I have 3 petcocks and a rebuild kit sitting waiting so I will probably try to get at least one going strong from the parts of the three I have.

What do you think is wrong w/ the petcock? I would love it if that's the answer. 'Cause this scoot is going to RIP when I get this bug ironed out. I mean just be viscious.


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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post #4 of 63 (permalink) Old 08-09-2009, 07:12 AM Thread Starter
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Oh and the reason why I quit barking up the petcock tree is this- it passes no fuel when the engine is off. As it should not. This means the diaphram is closing it. Then of course it runs, so that means the diaphram is opening when the engine calls for fuel, so it is functioning.

So the question is- is the diaphram simply on or off (fuel or no fuel) or are there degrees- 10% of capacity, 40% of capacity, 75% of capacity, etc. If there are degrees then its possible the diaphram is not operating correctly and not getting the fuel where needs to go, if say it's not capable of getting above like 60% capacity or something. Whenever the engine enters a condition where it needs more than that, it never gets it, and that *could* describe what is happening.

Also I could mention that it does seem worse under load- ie on a hill or something. The hesitation starts sooner on the tach in those conditions.

I'm going to buy bigger fuel filters and swap them out next time I'm down in there just on the principle that you can never have too much fuel available.

An no, the stockers don't have them. The filter/screen in the petcock is a joke. It's rarely present and intact. Given the experience I've endured w/ crud getting into the carbs, I want some protection against that. It's awful when it happens and there is nothing in the stock system to safeguard against it.


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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post #5 of 63 (permalink) Old 08-09-2009, 10:34 AM
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There's no "partial" on the petcock - so she's either supplying fuel or not. One other thought that comes to mind - how do your ignition coils and wires look? I know you're changing the plugs and stuff, but check the wires really carefully and make sure they're ok. Sometimes you can't determine a problem there until the bike's under load.

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post #6 of 63 (permalink) Old 08-09-2009, 10:51 AM
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I've had the same problem going on, when I got my last inspection the dealer replaced one of the diaphrams in one of the carbs (the cheaper diaphram) and it seems a little better now, but still not quite right. I'll be interested to know if you find your fix. I've been wanting to change my plug wires also. Probably wouldn't hurt either way.

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post #7 of 63 (permalink) Old 08-09-2009, 11:15 AM
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It has to be a carb, or carbs. I am having the same problem with my maxim. I am going back into the carbs when I have time. Mine seems to be one carb with the problem.

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post #8 of 63 (permalink) Old 08-09-2009, 11:38 AM Thread Starter
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Good to know there is no partial. Was just going to go out and test that. (It's raining here, so no riding today, so I went back to bed!) So my petcock must be working, although I will certainly try your "take the spring out" trick if needed. No way to test it in this downpour so probably won't be today.

My wires are crap which is why I have new ones waiting. Just didn't put them on last night because I didn't have time to take the tank off again as I was meeting the gf at the movie so I did the iridiums instead since that didn't require a tank removal. So today I'll go in there, take off the tank, and put on the new wires. Won't know if that was it until it quits raining, though.

But if the problem w/ bad wires shows up under load that would explain it. They are the stockers and they are cracked and in horrible shape.

Quote:
Originally Posted by Crobins365 View Post
There's no "partial" on the petcock - so she's either supplying fuel or not. One other thought that comes to mind - how do your ignition coils and wires look? I know you're changing the plugs and stuff, but check the wires really carefully and make sure they're ok. Sometimes you can't determine a problem there until the bike's under load.


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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post #9 of 63 (permalink) Old 08-09-2009, 11:43 AM Thread Starter
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Yeah, the diaphrams aren't cheap. Any way you can find out which one he did? I would imagine the small one (air cut off I think they call it) would be cheaper. However those seemed well and fine on mine. The larger ones around the needle valve were the ones that seemed a bit questionable on mine. I had to work them and knead them to get them to fill the round groove where they go. They didn't just want to sit there- there had been some distortion or warpage of the rubber.

CRobin- can you tell us what each of those diaphrams does functionally and thus which is most likely to be the culprit, here? The needle jet is the idling jet, is that right? So I would think the diaphram around that would have something to do with idle and coming off the line smooth, both of which are strong on my scoot.

And while we're on the subject of carb parts- gaskets. Is it safe to assume that if it doesn't leak gas, they are OK? If they were deteriated to the point they let in air (which would be a problem) you'd know because they would also leak fuel, right? Otherwise if it could be the gaskets, gaskets are cheap. Cheaper than diaphrams!

Quote:
Originally Posted by shark88 View Post
I've had the same problem going on, when I got my last inspection the dealer replaced one of the diaphrams in one of the carbs (the cheaper diaphram) and it seems a little better now, but still not quite right. I'll be interested to know if you find your fix. I've been wanting to change my plug wires also. Probably wouldn't hurt either way.


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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post #10 of 63 (permalink) Old 08-09-2009, 11:55 AM Thread Starter
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FYI- just looked up the diaphrams quick on cheapcycleparts. W/out comparison shopping the large diaphrams are $100 each and the small ones $35. That makes $270 for a set. And they sell the entire carb assembly for $785.

Is anyone still thinking Crobin's rebuilds at $340 aren't a good deal?!


Current VN: 03 VN1600A

Past VNs: 92 VN750A, 98 VN1500A

Future VN: 94/97 VN1500A basketcase/bobber
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