My GS850L falls asleep over 5K, help!

[bison head]

I have an 81 GS850L with 42000km, I bought it for 300 bucks fixed a butchered engine and became a mikuni carb expert getting it running. I have had the head off, so timing chain has been touched. My problem is that around 4500rpm it starts to sputter a little and then it is smooth to redline but no power. It idles and starts awesome, low end torque is plentiful and acceleration is great then I hit 5000 rpm and zzzzz... My compression is around 135 psi for all cylinders. I am wondering if any of you have heard about maybe having a cam a tooth out of time or something else. I have a clymer manual but it was terrible for telling where the timing marks are and which to use. I am an automotive journeyman and this thing is starting to drive me nuts. I checked fuel flow today : all good. Ignition system seems fine beause low end and idle are great. All stock intake and exhaust gear except no intermediate muffler; it was rotten, I took it out. Any help is appreciated greatly, I can ride the bike but I really want to go rip up some asphalt.[-o<

[chef1366]

Get a Stock Exhaust and it would probably run ok. You will need to rejet the carbs for the exhaust mod. It's probably leaning out causing the lousy power band. Sometimes even when it appears to have good fuel flow the petcock can be an issue at higher rpms.

[bison head]

I have modified the petcock so the vacuum isn't needed for fuel, so I know it doesn't have a fuel flow problem. I have everything apart for fuel today and re checked it. The mufflers are stock and it ran the same before I took out the intermediate muffler but I thought I just needed to clean out the fuel system. As for jets, the mains are 115's and torque and power are great at W.O.T. until around 4500 - 5000rpm.

[chef1366]

Replace your intake o-rings. Do you have straight flow from your pipes or do you have some kind of baffle. If the old one was rusted out then you still had improper air flow. Do a full throttle chop and check your plug colors.

[bison head]

I'm not sure what o-rings you are referring to. If you could elaborate that would be great. I'll do a plug reading tomorrow. Would the proper way be to run the engine at where the problem is and then pull the plugs and check? I can read a plug for a car whether rich or lean but I have never had to mess with jetting before on anything. I appreciate the back and forth, thanks!

[chef1366]

The o-rings are between the motor and the intake boots. Mark you throttle at 1/4, 1/2 and 3/4. To do a throttle chop you need a good stretch of road . Use new or clean plugs. Take off on your bike, twist to full throttle as long as you can and hit the kill switch without releasing the throttle. Pull the plugs, they shouil be a light tan. Do this at the throttle positions noted and state the results. Before you do this check the intake boots with the WD40. You will get inaccurate readings with bad intake o-rings.

[semerm]

are you running the stock air box or pods?

[bison head]

I am running stock everything except a removed intermediate muffler. Bike ran the same before I removed the muffler. I had a few air leaks in the exhaust so I welded the whole system and ditched the I-muffler cuz it was almost rotten through. I'll check for vacuum leaks around the carb boots but I doubt it's the problem because the idle is great (too rich if anything) and the low end is great where the engine generates the most vacuum. In theory there is almost no vacuum at w.o.t. where I am having the problem. Bike is great around town riding but I noticed it the first time I took it on the highway and went to pass it had no power. I have heard of a scenario on a '04 malibu car dohc where the exhaust cam was out one tooth and the engine would idle great but have no top end. I used my clymer manual to line up all timing marks but the diagrams were crappy at best and I figured that someone else out there maybe goofed the cam timing before and ran into this problem. Nevertheless I will check for vac leaks and try a plug reading later today when I get a chance. I just can't see it being jetting because w.o.t. up to 5k rpm is great; unless it starves after that? This a great site and I have learned alot already from all of my reading, Thanks alot everybody.:-D

[JHawkins]

I am not sure if that engine run points or electronic ignition, if it has points, the manual spark advance in the ignition may be stuck. That could give you that kind of result.

Edit: took a look at bike bandit, looks like electronic ignition with a mechanical advancer. I would make sure the advancer is free.

[semerm]

It sounds to me like your mixture is off for some reason (muffler mod?, air leaks?). I have a 1981 gs850g that had pods on it when I got it. It ran very similar as you describe. It would not do over 5000 rpm in neutral either. I replaced the pods with the stock air box and 8000rpm no problem now. Mine was stubborn cold starting though (When it was cold out). Now it runs like a new machine.I hope this info helps you. have a great day!

[tconroy]

[QUOTE=bison head]I'm not sure what o-rings you are referring to. If you could elaborate that would be great. I'll do a plug reading tomorrow. Would the proper way be to run the engine at where the problem is and then pull the plugs and check? I can read a plug for a car whether rich or lean but I have never had to mess with jetting before on anything. I appreciate the back and forth, thanks![/QUO Have you tried removing the 2 air lines on your carbs to see if that will help?If you have a mod exhaust that might be way to resovle the problem.OR look up posts by Kieth Krause on this subject.

[bwringer]

Here are the intake o-rings that have been referred to:

http://bwringer.com/gs/intakeorings.html

(Please note that the part numbers are for 80-83 GS850 only.)

also:

http://cycleorings.com/intake.html

Probably not your problem at the moment, but definitely worth checking and knowing about.

You have some unknown modifications to the exhaust, and I can't help but think that could be causing some problems. If the exhaust flows more freely it would be leaning out significantly and falling on its face.

I'm just guessing at this point, but it actually sounds more like the exhaust is plugged up somehow. The bike will start, idle, and putt around just fine, but won't rev because it can't breathe.

I've seen the same thing in cars with impact damage to the exhaust pipe or clogged catalytic converters. It will idle but it won't go... just a thought.

[robinjo]

Did you check your diafragms for tears?
Did you fit them perfectly in the grooves?

I solved the same symptoms on a 454LTD and a GS1150, first one had a rip and the second were incorrectly mounted, both result inpower upto a certain point and thenit stalls because not all the slides lift.

[blackwolf]

I have a gs850l too and mine sputter around the higher rpm's and I replace my pilot jet and that was that, I also have a vance\hines 4\megaphone,accel coils and Dyna s ignition so I had changed the pilot jet to a no.40

[bison head]

I think I will do a back pressure test in both sides of the exhaust and see if I have a plugged muffler or something. It's pouring rain so I haven't had a chance to do a plug reading yet, oh wait thundering and hailing too. I'll check for any intake leaks too just to be sure,I can do it under cover. I noticed the carb section in my clymer manual is terrible and though I am good at cleaning these mikuni's I can't find any info on what the different jets do. I know the main jet is the one for 1/4 to full throttle in a carb. Isn't the pilot jet for the idle and transfer circuit. And I have no idea what the jet in the air filter side does. I know a plug reading will help you guys help me and I'll get one done as soon as I can.
Thank you very much

Did plug readings in the problem rpm ranges and found white plugs! read plugs when running arond town and idling and found dark tan plugs.
did a backpressure test and found 3psi peak on both banks when revving up and trying to open the throttle as much as possible. Tried to find any intake leaks and found none. I love the weather here, Don't like it; wait ten minutes! Bottom line, I am running #115 jets for main jets and I am wondering what to try for a new main jet number, any ideas? anyone?