New muffler mixture question.

[GS750GUY]

I just recently installed a set of brand new 4 into 2 Sito mufflers on my 78 GS750EC. The instructions that came with the pipes said these are very high quality Italian made mufflers designed and tuned at the factory specifically for my bike and the instructions went on to say all I would need to do is replace the mufflers and no re-jetting or mixture adustments would be necessary.

The pipes look absoulutely fantastic and the fit and finish is superb. The pipes produce a slightly quieter sound with a nice low growl.

There is only one issue that I am somewhat concerned about: When I had the old pipes in place, I notice when stone cold I would set the choke and the bike would immediately start right up with just a bump on the starter button and the engine would immediately begin racing and I could easily control the engines warm up idle speed by simply moving the choke lever to various positions to keep the fast warm up idle speed below 2500 RPM until the bike warmed up.

Now with the new pipes in place I will set the choke and the bike still immediately starts right up with just a bump of the starter button, but the engine does not immediately take right off with a very fast idle speed like it did with the old pipes. The bike starts right up but no matter where I place the choke lever the bike only idles very slowly at around 1200 RPM for a couple of minutes until the bike begins to warm up. Then after the engine begins to warm up after two or three minutes the engine will then begin to increase its idle speed and begin racing and only then after a 2 or 3 minute warm up can I begin to lower or raise the idle speed by positioning the choke lever.

Does anyone have any thoughts on what is causing the warm up idle speed condition I am describing above with my new pipes?

I hope I have not created a too lean condition that could cause an excessively hot running condition.

I apprecieate your input.

GS750GUY

[Don-lo]

The GS750 I had was very sensitive to exhaust back pressure. I tried some turn-out mufflers, but the bike ran terrible and fouled plugs. Returning to stock mufflers fixed that. Running a Kerker header required extensive rejetting. It sounds like you may need some rejetting. How are your plugs after riding? Has the quality of power changed? You want to be careful here, because running lean can quickly burn new pipes. If the startup is the only symptom, you may be ok.

[salty_monk]

The choke overrides all the other carb settings anyway so your jetting won't matter in this instance.

Mine can do this similar thing if I haven't started them for a while. You'll find if you get it going, rev it on the throttle & then let it go back onto the choke it will prob behave like it did with previous pipes.

Did you leave it for a while without starting it (a couple of weeks) whilst fitting the pipes? Is it colder there than normal (this would cause it too).

Dan

[1978GS1000custom]

it sounds like your back pressure changed. your old pipes were made for economy and low noise and had a specific back pressure. the new pipes have more back pressure causing the bike to not be able to breath as clearly. check your air filter to make sure its not restricting anything or perhaps a good aftermarket filter to let a bit more air in. the new muffler company made those pipes and tuned them to a test bike most likely a European model so you may have to open (CCW) your air mixture screws a 1/8 to 1/4 turn and try it cold again. i am sure one of these carb pros on the site will kick in any time with better advise on the carb end. when i install drag pipes on a bike i always have to do the opposite and close the screws a bit. hope this helps you. cliff

[GS750GUY]

The choke overrides all the other carb settings anyway so your jetting won't matter in this instance.

Mine can do this similar thing if I haven't started them for a while. You'll find if you get it going, rev it on the throttle & then let it go back onto the choke it will prob behave like it did with previous pipes.

Did you leave it for a while without starting it (a couple of weeks) whilst fitting the pipes? Is it colder there than normal (this would cause it too).

Dan

This happens after the bike has sat for about a week. I do set the petcock to its prime position to be sure plenty of fuel is reaching the carbs and then set the petcock back to the run position. The temps here have been in the low 50's when starting up. The air filter is a new aftermarket foam filter just like the original and only has 1800 miles on it.
I have not done a plug reading since I installed the new mufflers. I have approximately 200 miles on the new mufflers.
GS750GUY

[GS750GUY]

Are there any other carb / new muffler gurus out there with more advice on this issue?
Thanks,
GS750GUY

[GS750GUY]

I finally got around to taking a plug reading and found that plugs 1,2,&3 appeared to be running slightly lean and plug 4 sligtly rich. I richened 1,2,& 3 up by turning the air screw in by 1/8 of a turn and leaned up plug 4 by backing the air screw out by 1/4 turn.
This did seem to improve my cold warm up performance. The bike fires right up and now I immediately have a fast idle and I can now immediately adjust my idle speed with the choke lever. And the bike seems to warm up quicker.
GS750GUY

[twiggy2cents]

umm out is rich in is lean

[Steve]

umm out is rich in is lean

On the CV carbs like on your '85, yes, that is correct.
You have only a mixture adjust screw that adds more of a pre-set mixture.
Turning it out adds more, richening the mixture.

However, GS750GUY says he has a '78, which has the VM carbs, and they do have an air screw on the side.
Turning it out allows more air, which leans the mixture, just as he has done.

.

[twiggy2cents]

lol i guess thats what i get for trying to know it! lol well i learned something today