GS750 BREAKS UP at 5000 RPM's

[Anonymous]

My 1981 GS750L breaks up at 5000 rpm's. It will not go past it. It runs great below 5000 rpms, with good throttle response, good power, but once it hits 5000 rpms it is like it hits a wall. It does not seem to depend on how far open the throttle is (was thinking it may need re-jetting, but strange that it is fine from idle to full throttle until it hits 5000. Shouldn't this 750 wind up to 8000-9000 rpms???

[RacingJake]

Sounds like points and condenser's

[Anonymous]

I have had the GS750 for about 2 weeks now.....don't have a manual and am just getting familiar with the bike. Installed a new starter solenoid today and took off the tank to check out the wiring running around the backbone under the tank....was thinking the break up might be due to a worn wire or loose connection.
How do I service the points and condensors?????? Never done it before....but willing to learn. You'll have to explain it to me like I am an amateur....I am one you know.

[moto_dan]

I have had the GS750 for about 2 weeks now.....don't have a manual and am just getting familiar with the bike. Installed a new starter solenoid today and took off the tank to check out the wiring running around the backbone under the tank....was thinking the break up might be due to a worn wire or loose connection.
How do I service the points and condensors?????? Never done it before....but willing to learn. You'll have to explain it to me like I am an amateur....I am one you know.

If you're going to work on your bike, then you really need a manual. Either a Haynes or a Clymer will suffice.

Your '81 doesn't have points and condensars so eliminate that as a cause.

Possiblities: Low battery voltage. If it is under 12v then your bike can display the symptoms you describe.
Other possiblities:
1: Do you have the original air box/filter installed?
2: No load stator output voltage a 5k rpms <70vac.
3: Air intake leak.

[Anonymous]

I would also check the fuel line. A clogged fuel filter or a too-small or improperly routed hose could cause fuel starvation. If your fuel hose is clear you can see what happens to the gas flow when you rev over 5000.

[Anonymous]

The fuel filter is clean. The gas line is in good condition and is not pinched, correct diameter/size, pliable. There is no blockage in the line as I pulled both ends off.
I had a friend ride it today and he wondered if it has a 5000 rev limiter. I showed him that it will easily and smoothly rev to 9500 - good throttle response, revs right up there - as long as it is in neutral or clutch pulled in, so no electronic rev limiter.
Put it in gear and try it as you are going down the road and it breaks up right at 5000.
I asked him if he thought it might be the points and he said they are usually good or bad, no in between.
Don't know if it is too rich or too lean...seems perfectly jetted until it hits the wall at 5000.
The bike is really starting to wake up and then all a sudden....no more revs. Really want to be able to launch it hard and rap it up to 9000 and try some accel runs before winter hits.
What do you fellas recommend checking first?
Should I pull the carbs? Check the points? Condensors (don't know anything about any of this....)

[RacingJake]

Do the easy stuff first,

Remove the timing cover and check the timing with a gun.

Still sounds like a electric problem since you can rev it without a load but once on a load it starts to act up.

Once you get the cover off you'll be able to see if it's points or has pickups and you can check the mech advance too.

[earlfor]

You bike does NOT have points or condensors. It has a non adjustable electronic ignition. You cannot change the timing. That it revs freely in neutral to redline. indicates that it is not a timing problem. I had the same problem with my 750 when I got it. It turned out the fuel filter although clear and clean was too restrictive for when the engine reached its powerband and needed more fuel. I think there is a good chance that if you will just fhrow away that fuel filter and install a solid length of 5/16" I.D. fuel line, your problem will be solved. There is a very fine filter screen inside the gas tank that is sufficient to remove debris from the gas. Those damn little filters have caused more problems than they could possibly ever eliminate. :-)

Earl

[Anonymous]

Someone pointed out in another thread that the engine draws more gas on the highway at a given rpm than on the centerstand. This makes sense to me. So if it's a fuel flow issue you should be able to rev it over 5000 rpms in neutral before it reaches maximum fuel flow and won't go higher. Test this out and if you get it over 5000 I would think it's a fuel flow issue.

[Anonymous]

Exactly what is happening to my bike, she'll still wheelie out of the hole but as soon as I get much over 6k in 1st gear or push 5k in the upper gears, it falls flat on its face. It wasn't doing this before, so I think that it's probably a fueling issue. I pulled the carbs and they looked showroom, but as I've said before, I don't know crap about carbs.

One thing to try that I did is to pull the air filter and run it (making sure the airbox cover is still on and that you're not bombing down a dirt or gravel road :P ) When I do this, the bike rips, redline in every gear every time. While there is still an obvious problem, I LIKE to think that I have eliminated the electrics as being a culprit (can I get an AMEN here...please...anyone?) I was worried that it was the ignitor or the coils, but when she sung like new without the airfilter, I felt it wasn't an electrical problem. Might work for you as well.

If you're running fine with no air filter (ie more air into the motor), than you either run too rich with fuel, or there's a restriction in the airbox where the filter is in place. I drilled some speedholes in my airbox to facilitate more flow, it helped a little bit but didn't fix the problem. I'm gonna try to work on it today, I'll let you know what I find out! Good luck.

[Anonymous]

I broke down and bought a Haynes manual online today. The bike is currently down as I sent the shocks away to have them lowered 2". I am waiting on this stuff before I try to run the bike up my the street without an airbox - hope that it makes it past 5000 rpm's! I have to get new tires, make a new seat pan and seat, new rear fender, paint, turn signals, tailight. She is going to be slammed and hopefully have some revs....just know that the bike will scream once it gets past 5000......
If anyone can come up with an idea in the meantime, it would be helpful because I have spent a lot of time thinking about the fix on this.

[ToddB3]

Where did you send your shocks off to? I'm looking to lower my bike as well but wasnt aware the stockers could be modified.

[Anonymous]

Shocks were sent to Pete Rambo, www.sotmc.com
Sons of Thunder Metric Cycle
For 67$ he will lower the bike however much you want, that includes cleaning the shocks/polishing and sending them back to you. I found out about him on www.chopperunderground.com
I am on that forum also as I am bobbing my GS750 (cutting off a lot of weight and making it a low, lean machine, new fenders, seat, etc.).....
They should receive the shocks Tuesday and said they would work on them the day they received them and send them out the next morning. Quick turnaround time! We'll see how the work turns out as this is my first dealings with SOTMC.

[ToddB3]

I put a mid 70's Sportster rear fender on mine and am looking at re-doing my seat similar to the modern sportster seats, maybe a homebuilt supercharger or turbo later. Let me know how the shocks turn out.

Thanks!

[Anonymous]

Well, I think I got her figured out. Took the airbox assembly apart and inspected the filter - viola! THat pig was covered with a whitish cotton hair (weird stuff...looks like it was really restricting the air flow). Filter looked really old. Sat there peeling out the cotton-like material (is that what the stock filter is made out of....cotton? THought it looks like a paper-composed filter when you look at it in the inside, but the outside looks like cotton. Anyway...the filter looked plugged with years of dust, dirt, and as said before, strands of cotton-like material..
Cleaned it up and reattached. Warmed it up and rode around the neighborhood for a while. Tried to rev it past 5000 grand and made it up to 6500. I am going to replace the air filter once the local shop opens up (closed on Sundays)...