GS850GL Carb Question?

[joejeweler]

Hi all,

As i had outined in a previous post my great looking 1982 GS850GL needs a liitle tweeking in the carbs. (11,500 miles on her, coming out of long time storage as the brown brake fluid would attest to before i changed it)

Although the PO had someone do a carb cleaning and rebuild, it was done in 93 degree weather, i was told,......and in the cooler weather i'm running in now i have a distinct bogging down hesitation between 2,500 and 3,200 RPM. After that she pulls great, and idle at 1,000 RPM is smooth on up to the aformentioned problem area.

As the PO had not registered the bike nor ridden it except for a brief test run (a suedo-dealer i presume as the title was simply signed over to him with the buyer section left blank for me to fill in),.........this problem had not shown up at that time. I was planning on taking the carbs apart this weekend and changing the main jets up one size to, i believe, size .020. They are at home so i'm going on memory. I think the PO said he had .015 mains put in on the rebuild, in deferrence to the mechanic's advice on going with the larger size.

Anyway,.....last night after a few weeks of running it over about 400 miles, i pulled the new plugs the PO had just put in to examine the electrodes to get some idea where the problem might lie.

Cylinders 1 and 2 looked just about perfect in a nice light brown color, (sitting on bike the 2 left cylinders)......but plugs in cylinders 3 and 4 had a light black sootiness to them, and also along the base and insulators. They are not fully fowled and still fire OK,......but they certainly don't have the nice clean burn appearence of the 1st two cylinders. And it doesn't appear oily in any way,.......perhaps a bit rich running which surprised me as i assumed i was a bit lean as when the hesitation occured i did a test and slid the choke over and the bike perked right up!

As i recall one coil fires cylinders 1 and 3,......the other coil cylinders 2 and 4,............so a weak coil should show up on alternating cylinders and not two in a row like cylinders 3 & 4.

Any thoughts would be helpful to point me in the right direction.

thanks,
Joe T

[GS850_Coug]

Actually, the coils fire 1-4, and 2-3, but that still doesn't explain your problem.

You will need to do a throttle chop to determine the true nature of your problem. Because the bike apparently runs fine through most of the rpm range, I would check to see if it acts worse under certain throttle conditions. As far as the rich looking plugs are concerned, the PO may have tuned those cylinders more rich to cover up the issue, but after riding, the problem has gotten worse. I would suspect that there is probably a plugged port in each of those carbs (especially since the other two plugs looks fine).

This link gives some excellent guidelines for tuning the carbs (they also work great for troubleshooting once you understand the why's).

[joejeweler]

Thanks for the response GS850 Coug.......

By a "throttle chop" are you referring to such things as running for a few miles at highway speeds and killing the engine to read the plugs immediately, and doing it again at various sustained RPM ranges?

[flyingace]

First thing (if not done already) is a carb synch. If this was not done after the carb work, it could explain the symptoms.

[joejeweler]

I should have already mentioned it but the guy that sold it to me said the carbs "had" been synced. But i may just bite the bullet and pick up a set of guages.

[flyingace]

I should have already mentioned it but the guy that sold it to me said the carbs "had" been synced. But i may just bite the bullet and pick up a set of guages.

If out-of-synch is the cause of this problem, you don't even need to buy gauges. You just need to do a bench synch to make sure all the throttle butterflies open at the same time. Gauges are more for precise idle and off idle tuning.

[joejeweler]

At idle it seems to run VERY smooth at 1,000 RPM,...........no complaints there. And off idle up to the aformentioned 2,500 RPM start of a bogging down seems normal also.

So when i get them off this weekend i plan to run them thru my professional jewelry repair ultrasonic cleaner one at a time. One thing i don't have any longer is the steam cleaning machine i'd used over 15 years. I stoped doing jewelry repair work 3 years ago and left the old unit at the store.

It was pretty rusty at that point,........but when i'd used it before on carb cleaning duties it worked great blowing 80 pounds of steam thru all the orifaces and ports! I did 4 automobile carbs over many years and 3 bike carbs,............and they always worked great afterward. All my bike carbs up to this point had been on Honda XL500S single cylinder engines and i believe i also did a Honda twin carbs at one point 20 years ago,.......

...............but this will be my first go-round with a four banger. I'm hoping i can just go thru them and check the adjustments such as float levels and screw settings,......along with checking that all ports are clean,......because i'm depending that the supposed rebuild by the previous owner did indeed replace all the needles and jets correctly. I haven't picked up a carb rebuild kit following the assumtion i don't need one,..................just hate to get her apart and find the previous guy cut corners somewhere and tie up the bike while i order a kit.

My previous GS650GL had always run perfect and when i needed to sell it when i was moving and couldn't store it,.........it was still running great so i never needed to tear the carbs apart on that one. But as i said this is my first four banger, and i want to do it right!

I've always been pretty mechanically inclined.............so i'm hoping i haven't lost the knack now that i just turned fifty! \\/

On thing i plan to look at VERY carefully is any munute differences between carbs # 1 and #2 (the ones that seem to have a good looking plug burn),..........and carbs # 3 and #4. (the ones where the plugs look sooty)

[GS850_Coug]

You understand the chops correctly. That will help to "prove" the carb circuit is/is not the problem.

As for the vacuum sync, most of the experienced techies here on the forum will do a vacuum sync at the engine speed they cruise at to guarantee optimum performance at the most used RPM. It is possible that this could solve the issue, since if your throttles are opening at different levels, your mix ratios will also vary.

Because your problem is at one specific RPM range though, I'm guessing it is probably at one particular throttle range also. This would point to a plugged circuit/incorrect size jets in two of the carbs.

Follow that link I gave above for further explanations of the different circuits in the carbs.

[joejeweler]

Thanks for the mention of the "link above" again.

Dummy me missed it the first time while i was at work! It looks like some very good info and should help in narrowing down the problem. I saved the link to my favorites and advise all GS'ers to do the same.