Removing carb vent lines solves starvation mystery

[tconroy]

I know what you mean dave,my piehole opens more than it should sometimes too!Its the concern that counts though buddy!!

[Jackyl]

I knew I wasnt the only one

I have a 1980 GSX750E, mikuni carbs and ](*,)

I have been having the EXACT SAME PROBLEM as described in the first post in this thread, I recently cleaned the carbs, and, because I hadnt found this thread yet, capped those vents. It wouldnt even idle, now I have 2 tubes, one on each side about 8" long that go behind the transmission, I still have the dying out/starving out problem, now though it is worse, not just at high speeds but anytime its under a load, let it drift down to about 30mph, or just get off the throttle for a minute or so and it runs great again. So now Im wondering if maybe I have a clog between the nipple and where-ever-the-hell those tubes go in the carbs, I looked at the schematic at Old Bike Barn and it shows the other side of the carbs, not the side that these 'vents' are on. any ideas would be great, like this guy, I was ready to give up and just find another bike, I dont want to, I like this one more everyday, and if it would just run right, I would love it .

[KEITH KRAUSE]

The brass float bowl vent nipples simply lead to the float chamber. Each hose supplies venting to two carbs. You should be able to blow into them. But blowing into them wouldn't prove they are 100% clear. Flush and use high pressure air as you would in a rebuild/clean up.
The jets need to draw fuel easily. If the float chamber isn't vented well, the jets can't easily draw fuel from the bowl.
I don't know if this is your problem but it could be. You may also have a clogged gas cap vent or a petcock issue.

[Jackyl]

I feel like such an idiot now

Ok, after 2 yrs and a new charging system, re-jet, pods, a year w/o riding, 3 times in the shop, kicking, screaming, cussing, and hair-pulling, I checked and cleaned everything, twice ! ! So there I am trying the last couple things suggested, I have the fuel cap off, still does it, vent lines off, still does it, turn the petcock to pri.

fixed.


If you see me out riding, please be so kind as to not point when you giggle, snicker or laugh. Thanks.

[KEITH KRAUSE]

Keith, why does it make a difference only when using pods?

Well, some days my brain works better than others and today my brain has the day off. I wonder why I didn't see your post for all this time?
I've tried to give my opinion on this a few times in the past and I'm sure someone could use the right words to explain better than me or actually knows the entire sequence of events that explains what's happening.
I just know that when you greatly increase air flow into the stock carbs, you're also changing the vacuum and the pressure. These changes will effect the venting and the fuel flow. These changes effect some models more than others but it effects them all. Some models only seem to have problems in crosswinds or other windy conditions, while others will have serious fuel starvation just cruising down the street on a calm day.
I called up Dynojet many years ago to ask why they suggest removing the float bowl vent lines when running pods (part of their stage 3 and 7 jet kit info). They said "a vortex will compromise the air flow into the vent if the lines are left on." That's about it.
I don't know how much of this problem is related to air travelling across or around the pods or how much is related to increased air flow INTO and THROUGH the carbs but on a few of the bikes I've tinkered with, the problem was there regardless of outside wind speeds.
I can see how a vortex could be created at the line end, such as blowing across an open tube. A vortex would create a resistance in the tube and any resistance could compromise the venting and this would make it harder for the jets to draw fuel from the bowl. I don't know why this same vortex wouldn't be as bad when acting upon the vent nipple.
I've read where this vortex can be strong enough under some conditions to actually create a vacuum at the vent line and this will draw on the bowl and defeat the venting to at least some degree, if not completely.
I've also considered that the stock venting system, just a float chamber and tube, could be inadequate to allow for the increased flow the carb is experiencing with pods. If more fuel is exiting the bowls then the stock venting may not be adequate to keep up with it under some conditions. In this case, the best thing we could do is remove the vent hose and that would at least MINIMIZE resistance to best venting. Line/tube length obviously factors in but I've had no luck by "shortening" the lines either. It's either remove them completely or don't run pods on many models.
It could be a combination of things or something else than the above. You'd be amazed at how much air your bike sucks in and it relies on proper venting.
I do know that even with very good jetting, most pod equipped bikes will show some mild hesitation and fuel flow problem while in stronger crosswinds. It's not a true mixture problem. It's because the jets can't draw fuel easily under this condition. Fuel starvation.
I remove the lines because it usually must be done if you want the bike to run well. I or my friends have never had a problem with the open nipples getting bugs or dirt in them or water either. Obviously, it could happen though. I've never had a serious problem from crosswinds either, but I've felt hesitation and even got a few "spits" out of the filters at the time. Just a minor annoyance in my case. I know what it is and that it will pass quickly. Even if expecting it, it can be difficult to tell if any hesitation is from the pods or just the wind hitting you and buffeting you around.
Hope this helps.

[KEITH KRAUSE]

Ok, after 2 yrs and a new charging system, re-jet, pods, a year w/o riding, 3 times in the shop, kicking, screaming, cussing, and hair-pulling, I checked and cleaned everything, twice ! ! So there I am trying the last couple things suggested, I have the fuel cap off, still does it, vent lines off, still does it, turn the petcock to pri.

fixed.


If you see me out riding, please be so kind as to not point when you giggle, snicker or laugh. Thanks.

Many have done the same thing. The '80 petcock is infamous for failure. Actually, the factory manual says the '80 petcock still needs engine cranking to initiate fuel flow EVEN IN THE PRIME POSITION. Almost like it has two "ons". Never really understood its operation even after reading it. Those petcocks belong in hell.
They should make a diaphragm/rebuild kit for your valve but I like the Pingel fuel valves much more. When they're on they're on and when they're off they're off.

[ericox]

Answer me this, does this apply to the vent tubes coming off the "T" fitting between the 1-2 and 3-4 carbs on my 82 750T? In not having any problems to speak of, but im just curious. Bone stock bike.

[Jackyl]

Actually, the factory manual says the '80 petcock still needs engine cranking to initiate fuel flow EVEN IN THE PRIME POSITION. Almost like it has two "ons".

When in the 'Pri' position, mine is definatly "on", after just a few moments I have fuel on the ground under the bike. I am going to replace it but Im not sure wether to go stock (OEM) or the pringel (sp) petcock. Im not so much concerned with cost at this point (too late for that lol ) but I know I have to get one ordered today as I dont have the self discipline to not ride now that its running .

Thanks for all the help, as me and my antique start racking up the miles and long trips Im sure we will be here a lot

[KEITH KRAUSE]

Answer me this, does this apply to the vent tubes coming off the "T" fitting between the 1-2 and 3-4 carbs on my 82 750T? In not having any problems to speak of, but im just curious. Bone stock bike.

I can't see your carbs of course, but yes, on a stock bike, any vent tubes should be attached and routed so there are no kinks.
Any bike with pods/major air box mods should remove the tubes and leave the ports open to breath.

[KEITH KRAUSE]

When in the 'Pri' position, mine is definatly "on", after just a few moments I have fuel on the ground under the bike. I am going to replace it but Im not sure wether to go stock (OEM) or the pringel (sp) petcock. Im not so much concerned with cost at this point (too late for that lol ) but I know I have to get one ordered today as I dont have the self discipline to not ride now that its running .

Thanks for all the help, as me and my antique start racking up the miles and long trips Im sure we will be here a lot

If you have the stock petcock, I believe it's the one where you turn the slot. There's only an on and a prime on this type. Most people hate 'em.
I suggest a Pingel valve regardless of your vacuum petcock type. Mine was about $90 shipped, with adapter plate. The dealer wants about the same or more for the stocker. Money isn't the issue here as you said. Peace of mind is.
Only "con" about the Pingel valve is it should be turned on and off in my opinion. I know the float needle valves SHOULD seal against excess fuel flow, but I just don't take the chance of fuel entering the crank. I always turn it off after each ride. It's not an annoyance to me.

[Jackyl]

Pingel vs. rebuild vs. stock

Kieth, I had already ordered a rebuild kit ($45 US) when I saw your reply, I was looking at the pingels and mine is about $90 also. The question I have, what do you do with the vacuum line if you eliminate the need for it, does it have to be capped or left open (ya know, venting lol)? Do you think simply rebuilding the stock petcock is going to work, I dont mind paying out the $90 for the pingel, but if rebuilding the stock one will work then Im going to wait until I paint the bike this fall to change it to the pingel. And I also have no problems with turning it off after riding, I have a feeling Im going to get quite used to that in the next week or two

[KEITH KRAUSE]

When switching to a non vacuum operated valve, you just cap the vacuum port so it can't leak.
The rebuild kit should work if your petcock is failing to operate right.

[Jackyl]

When switching to a non vacuum operated valve, you just cap the vacuum port so it can't leak.
The rebuild kit should work if your petcock is failing to operate right.

I found the stock petcock at bikebandit.com, but in the diagram and order area I couldnt tell what I was looking at, it looked like 2 different part numbers ( 1 & 2 ) for different sides of the same part, so oldbikebarn.com came to the rescue again

on a side note, not to promote any one site or anything, when I first got the bike it had a broken tail light lens, oldbikebarn had one in stock for under $20, I have been kind of a fan ever since

[duaneage]

I think the carbs on these bikes will leak when the bike is on the side stand and the fuel is on PRI. Some carbs, when tilted, allow the fuel to get by through the bowl vent or another passage, this drops the level and causes the flood. Not an issue when the engine is running since fuel is travelling through it anyway.

My CM400 Honda does not have a vacuum valve on it and it never seeps. If i put my GS650 on the center stand and PRI it, no seepage. On the sidestand and it will start. The carbs have a very high fuel level typically and that could be a factor.

The Pringles are nice but some bikes need to have mods done to take the valve on the tank.
Just my .02

[Jeffrey650]

After having the aforementioned problem of my bike loading power at over 70 then I would shutdown for about 10 mins and could usually be restarted. I clean the petcock filter and that didn fix it. I did some research and heard about removing the hoses.. I didn't have to do that although I may shorten them maybe. The PO had the carb vent hoses routed wrong. Seems to run smoother with better throttle response.