Stutter at full throttle.

[andmoon]

83 850.

New to me, came w/ a box of spares which includes a dyno jet kit which looks like it has been installed.

Intake is stock and the exhaust looks stock but doesn't say suzuki or have other markings lead me think aftermarket.

Up to ~1/2 throttle she pulls fine all the way to redline.
Past 3/4 throttle she stutters.

I did the push the kill switch at throttle and look see the plug thingie and they were a nice tan.

I am thinking carb and not electrical since she will pull full rpms at part throttle but I have no idea if there is an advance issue w/ the timing.

I am currently running on my first full tank of gas. I have seafoam and lotsa prayers that I don't have to remove the carbs.


OH mighty gs gurus....where my problem lay?

Don

[Dave8338]

Your problem lies in one circuit of the carbs...the Primary.

Either the jets are too small (you never know wht PO's have done to these bike) or the primay circuits themselves are plugged. A good carb clean (soak for 24hrs) and some new orings from cycleorings.com will have you flying happy.

Welcome to the GSR...

[tkent02]

Past 3/4 throttle she stutters.

I did the push the kill switch at throttle and look see the plug thingie and they were a nice tan.

How long did you run it at full throttle before looking at the plugs?

[chef1366]

If it surges at WOT then you are lean. You will need to up the main jet.

I would also check the valve clearance.

[andmoon]

Your problem lies in one circuit of the carbs...the Primary.

Either the jets are too small (you never know wht PO's have done to these bike) or the primay circuits themselves are plugged. A good carb clean (soak for 24hrs) and some new orings from cycleorings.com will have you flying happy.

Welcome to the GSR...

Is the Primary circuit the route the fuel takes going through the main jet?

[andmoon]

How long did you run it at full throttle before looking at the plugs?

~5 seconds then hit the kill button.

[andmoon]

If it surges at WOT then you are lean. You will need to up the main jet.

I would also check the valve clearance.

It is more than surging. It stutters similar to what happens w/ bad plug wires.

Valves have been adjusted. (These things were designed w/ small clearances!)

[tkent02]

It is more than surging. It stutters similar to what happens w/ bad plug wires.

Could be way too rich at full throttle. What you might expect if it was a stage three kit and stock intake/exhaust.
Too lean tends to surge, too rich sometimes pops and sputters.

~5 seconds then hit the kill button.

Not long enough to color the plug.

Is the Primary circuit the route the fuel takes going through the main jet?

Not with the throttle open, the primary is in use at low throttle openings, it has no effect at wide open throttle.

OH mighty gs gurus....where my problem lay?

Your problem lay with you not wanting to take the time and effort to get into the carbs and get them working correctly.

It is VERY easy to pull the carbs on that 850, the airbox comes out, carbs slide out, piece of cake.
Go through it all, dip them, clean every passage, replace all the O rings, adjust the floats. Do the intake boot O rings too, they need it for sure.
Why not put in the jets it came from the factory with, they should be close to what you need with this intake/exhaust.

[Dogma]

If it surges at WOT then you are lean. You will need to up the main jet.

I would also check the valve clearance.

A very leaky airbox will do this too. For example, if the airbox boots are so old and shriveled that they fall out.

Just sayin'.

[andmoon]

So this lazy arse got off my arse and removed the carbs.

Carbs had the largest dynojet main jets (I think dynojet #165). The inside was pretty clean so I sprayed carb cleaner thru all the passages and in my eye as well (for good measure). I installed the next size down jet and unwrinkled the one wrinkled diaphram, put the stock needles back in (dynojet's needle look identical) and....the stutter diminished but still there in 1st, 2nd & 3rd gear ~ 6500rpms.

Loaded w/ confidence, I removed the carbs a second time, replaced the dynojet #160 mains w/ 155s... all in 45mins.

One kid needed his bottom wiped and another was watching forbidden TV so I have to wait until tomorrow to check the results of the 155s.

Is the stock 112.5 (or similar low number) in the same scale as dynojets?
If so this 155 is still waaay too big. Of course the box w/ jets the bike came w/ only has 3 stock main jets.

Thanks for all the advise...especially the one telling me there was no lazy way out.


Now....what's causing all the popping on decel? Just suttle popping, no back fires. I have no leaks via the visual and spray wd40 test.


Don

[chef1366]

Look on the filter side of the carbs and make sure there is a brass jet inserted on the right side of the outer carb throat.

[tkent02]

Is the stock 112.5 (or similar low number) in the same scale as dynojets?

Different scale, but WAY too big anyway.

If so this 155 is still waaay too big. Of course the box w/ jets the bike came w/ only has 3 stock main jets.

Buy the 4th

Thanks for all the advise...especially the one telling me there was no lazy way out.

My pleasure

Now....what's causing all the popping on decel? Just suttle popping, no back fires. I have no leaks via the visual and spray wd40 test.

Fry the big fish first, then the little ones.

[chuck hahn]

TAKE DEEP BREATHS... If going full throttle scares you so much you stutter then back off it a little bit !!!!!

[1979GS750E]

Likely the intake is too free flowing

Before getting involved with motorcycles, we all heard that, with respect to air filters, the more free flowing and less restrictive, the better. That might be true on lawnmowers and cars, but with motorcycles the situation is different. Most motorcycles run poorly if the stock restriction levels are reduced. What happens is that they lean-out at higher RPM's because the vacuum is too low to suck fuel up the main jet. I know it sounds counter-intuitive, but sometimes you have to increase the intake resistance slightly (more oil on the foam, thicker foam), to keep the vacuum needed at RPM's above 4000. My bike will not run worth anything with the air filter off, but it runs perfect once re-installed. I tried using free-flowing pod filters, but the performance was worse than with the stock arrangement.

[andmoon]

TAKE DEEP BREATHS... If going full throttle scares you so much you stutter then back off it a little bit !!!!!

I am trying to move towards bikes that are closer to their limits at lower speeds. Stutter is good, the last 'UJM' I had was a ZRX1200...Never mind the stutter, it was change shorts time w/ that one.