Does this sound Like fuel starvation?

[Rusty]

My 1980 gs450 will cruise along at 35mph in the top 3 gears just fine( lower rpm), when you rev it high in any gear for any time( highway or high rpm in a Low gear), after a short time, it will start to stumble and surge. Same thing when you accelerate hard from a stand still, Really powerfull accelleration for the first 2 gears, then it seems to start to stumble and surge. Starts up and idles fine.

I have a aftermarket inline petcock on it( it was for a lawn mower, the vaccume petcock is garbage) that has large barbs on it, but I now wonder if it has a small valve in it and can't flow enough.

Does that fit the profile of fuel starvation? I'll have the carbs off replaceing boots sometime, so I may pull the carbs apart to clean them again. Hopeing to get his cleared up soon, the bikes going to be getting alot of use.

[chuck hahn]

The typical OEM petcock has approx. a 1/4 inch hole in the petcock and nipple...so whats yours measure? Have you also capped off the vacuum port that would normally be attatched to the petcock? How clean is the air filter?

[tom203]

Make sure your gas tank is venting, so a vacuum doesn't occur.

[Rhinedog]

Tank Vent

Sorry for maybe a silly question, but where does the fuel tank vent on a 79 GS850? All I could imagine is via the fuel tank. If so how can I check for blockage?

[Rusty]

The typical OEM petcock has approx. a 1/4 inch hole in the petcock and nipple...so whats yours measure? Have you also capped off the vacuum port that would normally be attatched to the petcock? How clean is the air filter?

The air filter Is brand new OEM. Think it may be restricting airflow? Never thought of that.

1/4 or 3/16" is the ID of the Inline Petcock, not sure if it's the same all the way though though.
I have a vaccume line running from the vac petcock to the nipple on the carb.

I've pulled the cap out after my ride to school( as soon as I stoped in the parking lot) and I dident hear any vaccume like noises.

Edit: After thinking about it, I'll pull the air filter for a test tommorow. As soon as I Installed it, the bike now starts with no choke, when It needed it with the old one. That makes sence in a round about way. Could be restricing airflow and over fueling when the main jets come on. Dident even think about that.

[tom203]

Sorry for maybe a silly question, but where does the fuel tank vent on a 79 GS850? All I could imagine is via the fuel tank. If so how can I check for blockage?

Not silly at all! some tanks have little hole in filler opening. My locking cap seems to have two liitle holes just below the cap gasket that likely vent to outside the gasket, so air can sneak by as fuel heads to carbs. If you rode a while, stopped in a quiet place, and removed cap, you might hear a whooshing sound if your vent was blocked.

[Blastr2783]

Mine was doing the same thing, I put in new petcock, new fuel line, and new vacuum lines, rode with fuel cap off, rebuilt the carbs. Finally I just took it to the shop, they still got it. Gonna rejet all 4 carbs.

[whiterabbitt]

and check your float height, if its set too high there wont be enough fuel in the carbs for sustained high speed operation

[Rusty]

Ok, i'll check the float levels when I have the carbs off.