Another vacuum problem (maybe)

[bvsilveira]

Hi guys, this is my first post here. I'm in Brazil and here we don't have very information about the GS, cause it didn't sold to many units arround.

I have a 08 gs500e, if i'm not wrong, it is the same as the 05/06 models in the US. The problem is a major flat spot at WOT, especialy at high gears (4th, 5th and 6th). I have k&n drop in air filter and a full exhaust kit. I have done some jet adjustment but still trying to find the best configuration. But regardless what jet I use, i have a MAJOR flat spot at WOT. A friend of mine have the same bike, whit basicaly the same configuration of air filter and exhaut and his bike don't have this problem, even whit the stock jets (20/62.5/135, we have 25% ethanol mixed whit our gas here, yes we are stupid and acept this and pay one of the most expensives gas in the world ). The only thing that I think can be causing this is the plastic piece that goes at the top cover of the carburetor, one side was broken, and the previous owner used epoxy to put it togheter. But inspecting the part, I've noticed that it's not firm, and probably there's a vacum leak there.

Question is, could this leak cause the diaphragm don't go all way up in WOT? Most of vacuum leak issues that I've read here talk about rough idlle and thats why I'm kinda confused. I'm planing to take all that PAIR shaZam! off the bike, but i can use any advice in this problem, cause it's pissing me off.

Sorry about any english erros, I'm rusty, don't use it very often.

[bmf]

A broken diaphragm, or missing oring underneath will kill the vacuum that raises the carb slide. You could well be running on 1 and a half pistons.

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[bvsilveira]

I'll re check the diaphragm, but i think they are ok. The o-ring that you mentioned, is that tiny one, that goes at the front side of the carb body? If so, I may be wrong, but it is no longer present in the newer models, the ones with the PAIR system.

[W201028]

Correct, the newer carbs do not have the little oring. I think the problem is the leak in the carb top, the slide will not open the whole way, this will not supply enough fuel at WOT. You could block off the plastic PAIR fittings on the carb tops, and block the vacuum lines that go to them. This will not affect performance, as I believe they are for emissions only.

[rcbedin]

Hey bvsilveira.

I would try increasing those main jets too. My GS also had a flat spot on high REVS when I installed a sports pipe and a K&N dropin. I've playing around with 137,5 and 140 mains, with and without the restrictor. The best results I got was using 140 without the restrictor.

Anyway, you should check out vacuum leaks first.