Seat cover vs carb tuning

[email2r2]

Hi guys,

I've experience a real real weird problem and I have to share it with you. I restored an '83 GS750Es last winter and part of the restoration was to cover up the seat. The guy did a good job but the original grooves in the seat were not replicate as you must had already figure out.

When I started the bike this spring, I found out that it was skipping after 4500 RPM. I've done all sort of investigation and tuning around the carbs but the problem was persisting. You wont believe it but it was the seat restoration that caused the problem.

When I sitted on the bike the front part of the seat was prying, leaving a ½ in. space between the seat and the fuel tank. This space was wide enough to let air flowing directly to the air box opening. That was modifying the air/fuel mixture and caused the bike to hunt after 4500 RPM.

I've installed a baffle attached to the seat pan to break that air flow and everthing came back to normal.

Never thought that a simple mod like this could change the bike tuning so drastically. And now I undestand why designers of the bike put those grooves on the seat originally.

Now you're aware...

Thanks

[first timer]

do yo uhave bad/worn out rubber boots or air box boots that just aren't flexing? can you just trim you seat pan? is it fiberglass?

[first timer]

do you have bad/worn out rubber boots or air box boots that just aren't flexing? can you just trim you seat pan? is it fiberglass?

[Mark whiz]

I came across a real simple fix for this problem years ago. Turn the rubber inlet horn on the airbox so that it faces the rear, rather than the front. Also trim off the top section of the airhorn one "rib". This will solve any inlet problem that can be caused by the seat pulling away from the tank.

[email2r2]

Final result

Took a vacuum hose hanger. Made it fit the seat pan. The advantage it that it's made of black sturdy plastic similar as the seat pan.

Here's the result...

[mf70]

I'll bet this bike is naked. There's a lot of air pressure on your chest at speed.

Interesting observation though. I hadn't thought of it before.

[email2r2]

Hi mf70

The bike is totally original as it was in 83 (GS750ES) except for a Supertrapp exhaut and a restored seat. Damn, I must dismantled those carbs 20 times looking for a clue. I even tried another ignition box, coils, another range (heat) of plugs all kind of jets, needle combinations. Putted back original exhaust. Removed the air filter element. Tried to run without the cap on the gas tank. Finally it was what you already know...

That's why I decide to put it on the forum. Not much people have experience this kind or problem.

[Regent]

Lack of factory ribbing in the seat isn't the problem. It's only there for decoration. If the seat pan is in good condition, then there's a few things that could be wrong.

The vinyl was stretched too tightly front to back when it was applied, or it isn't pliable enough for a seat with that much of a curve to it.

The old foam is too soft from age, or the new foam used isn't firm enough - allowing you to sink in too far, causing the seat to pop up in front.

I'd recommend taking it back to the upholsterer and having it fixed. You paid for a good job, so get one. It'll help if he has the bike there to dry fit it as he goes along, to make sure things are headed in the right direction.

I'm not knocking your upholsterer - it can be difficult to cover a curved piece that's supposed to fit into a specific area - but he should know that he needs the bike to ensure a good fit on a custom job.