1978 gs400x no power at 40mph+

[icemanX]

i recently purchased a 1978 suzuki gs400x, it has new coils, wires, plugs, tires and oil. my problem is i can't accelerate past 1/2 throttle or beyond 45mph....the bike makes a stumbling type sound with occaisional backfiring/popping, also the idle seems random, it goes from around 1000-1600rpm.... i use premium gas and have cleaned the air filter....any suggestions would be greatly appreciated, thanks

[UncleMike]

Have you checked for intake leaks?

[icemanX]

not yet, i have only had the bike for about 2 weeks, it runs great at low rpm up to around 40mph... would spraying starter fluid around the intake be a good/bad idea to check for leaks? ( i remember my dad doing that on his old cars)

[UncleMike]

Go with WD-40 or fine misting of water instead.

If rpms rise with the WD-40 or fall with the water, you've got leaks.

But honestly, the only way to be sure is to pull your carbs and check the boots visually for cracks and stiffness. If they're original, you might as well replace them anyway, just to be sure. And don't forget the intake boot o-rings as well.

[Nessism]

Those intake boots are expensive, $25+ each for my 550, so I'd check them carefully before replacing them unless you are loaded. The O-ring is another matter though, it's a pretty good bet that they are leaking, or will be soon. Plus, they are cheap to replace so I say go for it.

[t3rmin]

Check your plugs again -- they can foul very fast under the right (wrong) conditions.

Make sure you're getting a good spark at various RPMs.

What it sounds most like is clogged jets. You probably want to get those carbs off and clean/rebuild 'em.

[icemanX]

well, i just finished completely cleaning the carbs, and i installed new gaskets between boots and spacers, also set the timing with a light.....runs fantastic!...wont idle below 1400 without dying though.....

THANKS so much for all the help and advice!

[UncleMike]

Glad you got it going.

[West]

Your idling problem is probably carb synchronization.

[icemanX]

well.....i fixed the idling problem...sort of... apparently the vacuum caps for the carb sync were missing......the bike sort of has a "sticky" throttle though...it takes a few seconds to return to idle from any rpm....someone suggested a new diaphragm....how much do these cost? and what online stores do u recommend? ...finally, what are the nipples for on the bottom of the carbs? are there any lines supposed to be connected to them?

once again, THANKS so much for the help, you guys and my local mechanic have been super helpful! Thanks again.

[Nessism]

well.....i fixed the idling problem...sort of... apparently the vacuum caps for the carb sync were missing......the bike sort of has a "sticky" throttle though...it takes a few seconds to return to idle from any rpm....someone suggested a new diaphragm....how much do these cost? and what online stores do u recommend? ...finally, what are the nipples for on the bottom of the carbs? are there any lines supposed to be connected to them?

once again, THANKS so much for the help, you guys and my local mechanic have been super helpful! Thanks again.

Sounds like you still have a vacuum leak of some sort if the idle hangs up before settling down. Try spraying around the carbs and boots with the starting fluid to see if the rpm increases.