Problem exists only at highway speeds?!!?

[Anonymous]

My 82 gs 750 starts fine and runs well until about 45-50 mph. Over 50 mph the engine seems to cut in and out. The bike will start to slow down so I will give it more throttle and that usually doesn't help, then it will suddenly gain speed again. It acts as if you were to continue getting on the throttle and then letting off the throttle, repeatedly. The bike starts easy and runs with no apparent problem at the lower speeds reguardless of the RPMs, but once you get to highway speeds it starts up again.
I got caught with it in a down pour and rode approx. 20 miles in the rain and the problem did start to appear at the lower speeds. Any ideas where to start checking for the problem. My guess is an electrical problem, but why only at highway speeds? - Mike

[TheBigRed]

hmm.. sounds familiar

Hey,
This sounds about like the problem I was having with mine. (see my Oh, Crap post)
In my case it was the diaphram of the petcock being punctured and allowing gas through the vacume hose. I replaced the petcock and took the bike for a spin last night to fill the tank. She's pulling just like she used to.

Amos

[daveo]

definitely sounds like not-enough-fuel-flow related

[Don Lobacz]

It is common to find fine rust in the bottom of the early 80s GS float bowls - I think the inside of the gas tank is more prone to this fine rust than earlier years. You may also have water in there too since you had a run-in with the rain. I would drain and inspect the bowl contents. I cut the bottom 1 1/2" off of a white plastic oil bottle for this (wipe out the oil, sprayout with brake or carb cleaner). You need a 10mm wrench to pull the drain plug. Often the contents resemble chocolate milk! If it's really bad you may need a carb clean, but often you can clean the loose silt out just by shooting a bunch of carb clean up the drain hole and rinsing until clean. Keep in mind that this will continue to re-occur until you take care of the problem. A fuel filter is a temporary help (and there is very limited space for it) but the real solution is lining the tank. Kreeming the tank is a pain in the butt, but it works well when done right.

[Anonymous]

Okay, here is a little more history on the bike. It has a Vance and Hines 4 into 1 pipe on it, and it has been jetted. I bought the bike from a friend who owned it from 1992 to 2002 (he bought an '87 Honda Hurricane FOOL!!). He started to have this problem with the bike so he took the carbs apart and cleaned them. There was no difference. He then took it to a repair shop who took the carbs apart again, and raised the clips on the needles. Does that sound right? I think there intent was to richen the mixture. Still has the problem. I bought the bike in October 2002 for $500 knowing it has the problem. Well once I took it out on the highway and wasn't sure if I was going to make it home, I knew what problem he was talking about.

The bike has a full tank of gas so I can't inspect it for rust until I siphon it out. I will drain the carb bowls to check the condition of the gas that comes out. I don't think this is the problem because the problem existed before and after 2 carb cleanings. What am I looking for when I take off the petcock and disassemble it? Also, should I get fuel through the vaccuum hose off of the petcock? What is the purpose for the diaphram and how does it work? Thanks -Mike

[moto_dan]

What am I looking for when I take off the petcock and disassemble it? Also, should I get fuel through the vaccuum hose off of the petcock? What is the purpose for the diaphram and how does it work? Thanks -Mike

You should not get any fuel from the vacuum line connected to the petcock, if so you will be sucking raw fuel into the intake causing an extreme rich condition on the cylinder where the vacuum spigot is located and your petcock needs rebuilding or replacing. The purpose of the diaphram is to automatically open the petcock with vacuum supplied when the engine is running and when the engine is cut off it automatically shuts off the fuel flow. One of the more serious complications of a malfunctioning petcock along with a malfunctioning needle seat valve in one or more carburators is the possiblity of filling your crankcase with fuel and causing the engine to hydraulically lock which in worst cases can cause massive engine failure.

[frosty5011]

........and raised the clips on the needles. Does that sound right? I think there intent was to richen the mixture.

??????????????????

No, Mike.....that really don't sound right......unless you meant to say ...."raised the needle"

[Anonymous]

No, Mike.....that really don't sound right......unless you meant to say ...."raised the needle"

What I meant to say was lower the clip to raise the needle. Thanks for keeping me in line!! - Thanks

[Mr. Jiggles]

I agree with Daveo - fuel starvation might be the problem. At highway speeds you need more engine power to maintain your speed because of increased wind resistance. This means increased fuel consumption compared with that at lower speeds. If your petcock isn't delivering what it should, its possible that at highway speeds your engine is burning fuel faster than it's being delivered by the petcock. If I were you I'd check the following - whether turning the petcock to "prime" helps; whether the diaphram is working correctly; and whether the flow rate thru the petcock is OK. The last two could be tested by simply running the fuel line from the petcock into a cup and sucking on the petcock vacuum line (**provided that it's not leaking gas). Gas should come gushing out of the fuel line. If all of this checks out then I'd start looking at the carbs.

BTW is there a fuel filter between the petcock and carbs?

Good luck,

Jeff

[Don Lobacz]

It could also be the gas tank vent malfunctioning. You can test this by taking a ride with the gas cap loose.

[KEITH KRAUSE]

cheetah-bike. If you still have the problem, do you have pod filters?