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rider
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213 Posts
Discussion Starter #1
hey vulcanites.

not even sure why i'm posting... i've read pretty much every thread there is to read across vn750.com and the yahoo groups site about why our bikes won't idle. just frustrated. weather is nice... i want to ride... and i'm getting really sick of playing with my bike in the garage.

i took a nice ride this weekend... about 200 miles or so. bike runs great above 3000 rpm... under that i have to be really quick with the clutch and throttle to keep it running.

rebuilt the carbs a couple months ago... reinstalled... it just won't idle. problem is there are too many things which could be the problem... something i missed during the rebuild... vacuum leak... carbs out of sync... incorrect air/fuel screw adjustment... float valve stuck... weak battery... incorrect throttle cable adjustment... i've been through every one of them and just when i think i've made some progress i let the idle drop down below 2k and... dead... and it's back to the drawing board.

if you've made it this far... here are the latest symptoms: above 3000 rpm, the bike runs fine. the one issue i'd mention above 3000 rpm is slow deceleration. the engine winds down very slow when i pull the clutch lever in and drop the throttle and this makes shifting a bit awkward. if i drop below 3000 rpm... sometimes i can get the bike to idle around 2000 rpm... sometimes it just dies altogether. lately, it dies way more than it lives. if i CAN get it to idle for any period of time around 1500 - 2000 rpm, the moment i wind up the throttle (to say 5000 rpm or so), the idle will then STAY there... and i'll have to completely back off the idle screw to bring the idle down. i've double checked the throttle cables and have sprayed wd40 on the carb linkage... that part is smooth. there's something else going on.

my guess is that there's a vacuum leak, but i just tried spraying some wd40 around the boots and the engine didn't make a sound... so i guess that rules that out.

since the carb replacement, both cold and hot starting have been a bit of a problem... lots of cranking and eventually the engine catches. i've noticed popping from the rear cylinder at 2000 rpm and decel.

anyway... just some more ranting. honestly, i'm just sick of screwing around with this thing. there were some carbs from a 2004 bike on ebay a couple weeks ago... i should have bought them. i've learned a lot messing around with my bike, but at this point i'm thinking about just getting something newer. if i could find a local mechanic who'd be willing to work on my bike at a reasonable rate, i'd say go for it... but no one wants to screw around with a 91 bike with carb issues... there are just too many variables.

the thought of pulling these carbs and putting them back in again does not sit well with me... took me hours to manipulate these things both out and back in.

so... guess that's it. just needed to get this off my back. thanks if you made it this far.
 

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Premium Member
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3,505 Posts
Clearly at low idle you're starving for fuel, so checking for a vacuum leak was a good move. You still have your airbox? Go back in and check all the lines from that to your canister, too. It's easy for those to come loose under the seat, behind the tank. All it takes is one stupid line to slip off and the whole thing is a mess.

Hang in there - the more often you pull your carbs, the easier it gets. :)
I'd suggest doing that one more time and just see that things really are where they're supposed to be. Check your throttle linkages, the seating of the needles, the floatiness of the float valves, and the operation of the choke plungers. There are two springs that are a pain in the behonkus to reinstall correctly - the one on the choke plunger rod, and the one on the throttle linkage plate. Make sure those two are operating smoothly and easily. I'd also try to "pre-sync" the carb plates, getting them "close" to each other before reinstalling on the bike and using the manometer to do the final sync.

I don't know what else to offer except a lot of moral support and encouragement - Go, Paul, go!
 

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Super Moderator
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11,865 Posts
Just a shot in the dark here but sounds like a clogged jet(s) ..there is an idle circuit and the jet has a very tiny hole...

I went through this with a scooter. The jet was not clogged shut, but had a thin coatining of crap on the inside walls..thusly decreasing the diameter of the hole. I had sprayed the crap out of it with carb cleaner, but only pulling a steel wire off a fine cable and using it to ream out the hole worked.

You might also want to check the airscrews to make sure they are not clogged or damaged somehow.


KM
 

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Undercover Sportbiker
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1,097 Posts
Before you pull the carbs, order the carb cleaning tool from hell I forget where I saw it. But it's a tool with several stiff wires in different sizes to clear out all the little carb holes that, as KM said, can be clogged but not fully blocked. Just my .02.
 

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rider
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213 Posts
Discussion Starter #5
i've decided not to give up on this one. here's the latest:

based on a recommendation i saw on this site, i drained all the fuel out of the carbs and sprayed gumout carb cleaner up the drain hole... held the hole with my fingers... then let it drain out. repeated two times.

i also pulled out the two pilot needles. interesting find: the needle on the right side came out with spring and small rubber o-ring intact. the needle on the left side came out but the spring and (presumably) the o-ring stayed in the carb. not sure if this is noteworthy but i thought i'd mention it.

update: the bike WILL idle at 1200-1500 RPM (and even sounds pretty good)... but only when the choke is 75% - 100% engaged. if i apply throttle while dialing off the choke, the engine sounds like it's starving for fuel and i see white smoke... from exactly where i'm not sure... looks like it's coming out the sides of the engine where the pipes attach... loud popping... and the engine dies quickly.

so... i feel this is progress. thing is... i thought smoke meant a rich condition... but if so, why does it sound like the engine is starving for fuel? i have the pilot screws turned out about 3 turns. any advice?
 

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Kawika
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441 Posts
update: the bike WILL idle at 1200-1500 RPM (and even sounds pretty good)... but only when the choke is 75% - 100% engaged. if i apply throttle while dialing off the choke, the engine sounds like it's starving for fuel and i see white smoke... from exactly where i'm not sure... looks like it's coming out the sides of the engine where the pipes attach... loud popping... and the engine dies quickly.

so... i feel this is progress. thing is... i thought smoke meant a rich condition... but if so, why does it sound like the engine is starving for fuel? i have the pilot screws turned out about 3 turns. any advice?
My bike did something similar a while back, and it turned out to be my petcock being partially plugged....the gasket swelled or something. I would suggest a sanity check by checking the fuel flow from the petcock. Rebuild kits are around $25
 

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Premium Member
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3,505 Posts
White smoke usually means you've got coolant getting into your system... How does your coolant level look? What's your oil looking like?

For your air/fuel adjust screw whose spring wasn't sprung, you might shoot a little cleaner in there and see if that'll loosen up the spring and o-ring. It could be kinda gummy in there, which would also screw up your ratio.
 

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rider
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213 Posts
Discussion Starter #9
here's the latest:

as i've already stated, the bike idles fine if the choke is engaged.

i tried turning the petcock to OFF and i pulled the two vent tubes coming from the fuel tank (this is a california model) and i shut off the choke and the bike started idling at around 1500. not totally steadily... but i definitely considered this progress.

can someone tell me again what those vent tubes are on the california model? can i leave those tubes off of the tank? i know i've run the bike in the past with those tubes off.

per davewex's suggestion, i disassebled the petcock. everything looks all right... some metal shaving debris... but the diaphragms look all right. still, i'm going to replace all rubber/plastic parts in the petcock.

also... there is definitely a small vacuum leak somewhere, contrary to what i said earlier. i sprayed some carb cleaner on the carbs while the engine was running and the engine did rev up... but only very slightly... maybe an extra 500 - 1000 rpm... and then dropped back down.

as for that white smoke i mentioned earlier... it's coming out of the front cylinder... looks like i don't have the pipe cranked down tight enough. it's not really white... but i definitely can see it when the bike is running.

so... that's the latest. thank you to everyone who's responded so far.
 

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Premium Member
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1,647 Posts
It still sounds like a clogged jet. Last week Ursula started running on only the rear cylinder when the RPMs got above 3k. Anything below and she ran normal. After that and the front cut out. This last weekend I pulled the carbs and took them apart and discovered that the high speed jet for the front carb had vibrated out of it's holder!! When I first took the cover off, I didn't even see the jet drop, it appeared as if it had disolved or something. A little looking around and I found the errant jet and reinstalled it and voila, she runs great again.
The idle jet has a diameter about the size of the period at the end of this sentence so any kind of small obstruction could cause your problem.

Cindy is right though, if you take the choke actuators apart, getting that spring reinstalled will cause you to develop a new vocabulary. It must have taken me the better part of an hour to get the beggar back on.
 

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Premium Member
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1,647 Posts
As far as the vent tubes on the rear of the tank, one is fuel overflow and the other goes to the evaporative emissions canister.
The overflow is self explanitory.
The evaporative emissions is to filter the gas fumes into an activated charcoal can so the fumes don't make it into the atmosphere and pollute the air. It shouldn't affect your idle at all as it is a passive system that is not tied to the carb system. The only caveat to that is that if you overfill your tank and liquid fuel gets into the canister, it can cause hard starting.
I still am leaning toward the clogged idle jet theory though it does sound like you have a vacuum or air leak as well. With the vacuum leak, one cylinder should still run at idle while the other is off unless both intake boots are loose and sucking air. My .02.
 

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rider
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213 Posts
Discussion Starter #13
thank you again, everyone.

i've ordered some new carb boots and clamps (which i'm going to try my hardest to install without pulling the carbs entirely out of the side of the bike) and new petcock diaphragms. i've also replaced all of the rubber hosing (vent tubes and vacuum lines) that i could get to.

should have the parts in a week or so. i'll give this a shot and then if it comes down to it i'll pull the carbs again to check the jets.
 
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