Puzzled: Power Cap around 5k rpm; won't break 75mph

[Guest]

Greetings!

To make it brief,

81 gs1000 - Attempted to return to stock jets after PO "adjusted" them w/ a drill.
Actual Jet sizes presently forgotten, will recover numbers when I get back to the shop.

What's happening is - Powerful start, Choke functions, bike requires minimal warm-up, carbs sync'ed correctly, air-mix set properly ( as near as I can tell) - Strong takeoff, great acceleration, but I quickly lose power at the top end; too much throttle bogs down the engine.

No airbox mods, no exhaust mods that I know of. Rebuilt and very thoroughly cleaned carbs. Potential debris bypassing filter, pending closer examination.

What I am wondering is... Could this be the result of bad clutch plates? Bad fuel? I am certain the plates are in need of replacement, but I figured worn plates would just slip and cause the engine to rev higher, not bog down.

Could this simply be incorrect jetting - again? Does elevation make a difference when selecting jets? I do have the stock jet list from BassCliff, am going to compare this against my current jets when possible.

Presently I'm just looking for ideas, possible causes. Any suggestions greatly appreciated!

[JTGS850GL]

Explain what you mean by "Potenial debris bypassing filter"?

What main jet size is in there now?

Higher elevation requires smaller jets.
Here's a nice chart that helps understand it:

http://www.jetsrus.com/FAQs/FAQ_rejet_elevation_and_temperature.htm

Sounds like a main jet size or a fuel flow problem to me. Doubt that it's clutch related.

[OldVet66]

There is no point in re-jetting if your exhaust and air box are stock. They need to be replaced with new stock jets since the PO destroyed them. You need to replace the intake boots and O-rings and possibly the boots to the air box if they do not seal tightly. The lid to the air box also needs to seal. Weather stripping is good for that. The clutch is not your problem. You would figure it out pretty quick if it was slipping. Altitude makes no difference with CV carburetors. The older VM series of carburetors that ended in '79, would require re-jetting for higher altitude, not yours. Have you adjusted your Valves? That should have been the first step. The only fuel filter that should be on that bike is the one in the tank on the petcock.

[Call Me Ahab]

There is no point in re-jetting if your exhaust and air box are stock. They need to be replaced with new stock jets since the PO destroyed them.

That was the intention, but there are no less than 5 sets of jets for this model and year.

You need to replace the intake boots and O-rings and possibly the boots to the air box if they do not seal tightly. The lid to the air box also needs to seal. Weather stripping is good for that.

Done, one of the very first things I did.

The clutch is not your problem. You would figure it out pretty quick if it was slipping.

I was thinking as much but wasn't positive - thanks for confirming that for me!

Altitude makes no difference with CV carburetors. The older VM series of carburetors that ended in '79, would require re-jetting for higher altitude, not yours. Have you adjusted your Valves? That should have been the first step. The only fuel filter that should be on that bike is the one in the tank on the petcock.

Good to know; will disregard altitude as a potential factor here. Valves were all within spec when I got the bike, they were leaning toward tight but all within the tolerances listed in the manual. The fuel sock on the petcock is ancient history, sadly... The original petcock had to be trashed, and neither that one nor the replacement had a sock. Had to drop an in-line filter in, but when I installed it I think I tore the element...

Explain what you mean by "Potenial debris bypassing filter"?

As listed above, I -might- have torn the element when I installed the filter.. Gotta get out to the shop to check.

What main jet size is in there now?

Found my numbers written down in the manual's notes pages!

Higher elevation requires smaller jets.
Here's a nice chart that helps understand it:

http://www.jetsrus.com/FAQs/FAQ_rejet_elevation_and_temperature.htm

Sounds like a main jet size or a fuel flow problem to me. Doubt that it's clutch related.

Great! Now if I can determine which set of jet configurations is appropriate for me

This is good news - getting closer and closer to pinning this down.
Now, for the jets:
160 air jets,
pilot 40's,
115 mains

These are the numbers I pulled off an old catalog that was distributed for the UK; I was told at that time it didn't make any difference... However, this makes me think otherwise.

***I have a set of numbers from US: the only difference is 170 air jets... this make any difference?

[OldVet66]

The jets are available. You could order them from a dealer or online by model number. External filters are always a problem. The line can kink and collapse (not much room under there) if the filter is big enough for gravity flow, or restrict flow if they are small enough. The Suzuki petcock is available but expensive. Bikebandit wants $93.00 for one. A clean tank and good petcock is necessary.

[Call Me Ahab]

The jets are available. You could order them from a dealer or online by model number. External filters are always a problem. The line can kink and collapse (not much room under there) if the filter is big enough for gravity flow, or restrict flow if they are small enough. The Suzuki petcock is available but expensive. Bikebandit wants $93.00 for one. A clean tank and good petcock is necessary.

Right, I picked up some jets from an online dealer but they screwed up the order - I picked up some old ones from a local shop and just cleaned those up.

I solved the external filter kinking problem by chopping the lines and dropping in a few "strategically" placed brass *90 fittings... It looks kinda weird, especially the way I had to route the lines, but it works! I think... Unless I actually did tear that filter element... In which case it -Didn't- work, but whatever.

Yessir - after much hunting and considering the issue, I settled on replacing the damaged petcock with a different model and boring out one side, plugging the other. Not gonna go back with the stock petcock, yeah I can get it new, but the '82 model petcock I've got on now is half the price and much less complicated, lower profile, and just looks better. Requires minor modifications, but meh - I can live with it.

SO! At this point it looks like I just need to get back out there, tear off the carbs, swap out my 160's for a set of 170's (which, by the way, would these actually be causing my problem or not?) and start over at square one... Alas, it's raining today, and my street is underwater. I don't feel like stomping two miles down the road, up to my arse in water, and in the rain no less.

I'll update with what I learn in the shop, thanks to everyone who has posted here, much appreciated!

One thing I need further clarification on: I know you (OldVet66) said that altitude is not a factor here, but in my quest for the US jets, I did find mention of a gentleman who had to compensate for altitude on his '82 gs1100. I̶ ̶c̶h̶e̶c̶k̶e̶d̶,̶ ̶a̶n̶d̶ ̶t̶h̶e̶ ̶1̶1̶0̶0̶ ̶h̶e̶ ̶r̶e̶f̶e̶r̶s̶ ̶t̶o̶ ̶u̶s̶e̶s̶ ̶t̶h̶e̶ ̶C̶V̶ ̶c̶a̶r̶b̶s̶ ̶a̶s̶ ̶w̶e̶l̶l̶,̶ ̶a̶l̶t̶h̶o̶u̶g̶h̶ ̶i̶n̶ ̶s̶o̶m̶e̶ ̶p̶l̶a̶c̶e̶s̶ ̶t̶h̶e̶y̶'̶r̶e̶ ̶c̶a̶l̶l̶e̶d̶ ̶B̶S̶ ̶c̶a̶r̶b̶s̶ ̶-̶ ̶w̶h̶a̶t̶ ̶a̶m̶ ̶I̶ ̶s̶e̶e̶i̶n̶g̶ ̶h̶e̶r̶e̶?̶ ̶I̶ ̶d̶o̶n̶'̶t̶ ̶u̶n̶d̶e̶r̶s̶t̶a̶n̶d̶ ̶w̶h̶a̶t̶ ̶t̶h̶e̶ ̶d̶i̶f̶f̶e̶r̶e̶n̶c̶e̶ ̶i̶s̶ ̶b̶e̶t̶w̶e̶e̶n̶ ̶B̶S̶ ̶a̶n̶d̶ ̶C̶V̶:̶ ̶t̶h̶e̶ ̶V̶M̶'̶s̶ ̶a̶r̶e̶ ̶o̶b̶v̶i̶o̶u̶s̶,̶ ̶I̶'̶m̶ ̶s̶o̶m̶e̶w̶h̶a̶t̶ ̶f̶a̶m̶i̶l̶i̶a̶r̶ ̶w̶i̶t̶h̶ ̶t̶h̶e̶i̶r̶ ̶d̶e̶s̶i̶g̶n̶ ̶a̶n̶d̶ ̶o̶v̶e̶r̶a̶l̶l̶ ̶f̶u̶n̶c̶t̶i̶o̶n̶,̶ ̶b̶u̶t̶ ̶B̶S̶ ̶a̶n̶d̶ ̶C̶V̶.̶.̶.̶ ̶w̶h̶a̶t̶ ̶g̶i̶v̶e̶s̶?̶

Okay! BS = CV; one literally means the other, so there's that.

[JTGS850GL]

The carbs on your GS1000 are BS34SS carbs that are CV carbs. The BS simply identifies the model of Mikuni. Hope that clears it up.

I'd still get rid of the inline fuel filter altogether. The petcock has a filter built into it. No need for additional filtering. Some of those inline filters do cause problems and the problems are just like you described. Fuel starvation at higher throttle positions. You might also want to see if the problem persists with the fuel tank cover removed.

[gustovh]

If after checking everything already stated, make sure that you have the right amount of links between cams, an extra link on the intake side will limit your rpm's at the top end. There should be 20 pins between cams.

V

[Call Me Ahab]

The carbs on your GS1000 are BS34SS carbs that are CV carbs. The BS simply identifies the model of Mikuni. Hope that clears it up.

I'd still get rid of the inline fuel filter altogether. The petcock has a filter built into it. No need for additional filtering. Some of those inline filters do cause problems and the problems are just like you described. Fuel starvation at higher throttle positions. You might also want to see if the problem persists with the fuel tank cover removed.

The inline is necessary, there is no longer that sock attached to the petcock - the original, leaking petcock one had decayed or just fallen apart; the replacement didn't have one attached. I had rigged up one of the filter cartridges as a... odd-looking filter within the tank, but when I pulled it off again that filter fell off too, wasn't worth fighting it so I just dropped in the in-line so I'd have -something- in there. The filter itself, I am almost certain, is not restricting fuel flow. The gas tank lid is something I'll certainly try.

As far as the timing goes, when I did my valve work and oil seals etc etc - the timing is something I'm extremely anal about, I'm as certain as I can be that it is correctly set. I had one nightmare of an experience a while back where I was given the wrong direction by a "mechanic" who knew nothing about my bikes, and tried to get me to time it according to his instruction. I went out and bought an original... not Haynes, the other one... Whatever, that manual and followed the timing directions to the letter - no problems. Since then, I always double, triple, sometimes quadruple check to make sure I don't screw that up. I don't take chances w/ the timing. However, regardless of how many times I checked my links - I am only human. If I exhaust my other alternatives, this is certainly something to re-visit.

Something else occurs to me: I had to heli-coil spark plug no. 3 because the PO had stripped the threads and then, I kid you not, glued a plug in there... It's possible that my own re-thread is a little shallow; would the retarded spark / incorrect plug depth have any real impact here?

[tom203]

Best to ditch inline filter and get new petcock with its filter- you need plenty of fuel flow as load goes up. This guy has them- as good as original and made in japan

http://www.georgefixs.com/brand-new-suzuki-gs300-gs450-gs550-gs650-gs750-gs850-gs1000-gs1100-petcock/

[Flatline_Racing]

Alot of the inline fuel filters do not flow enough to support these beasts at higher load/rpms so the float bowls will empty and you will run lean.

[JTGS850GL]

At lease take the filter out to verify that it's not the problem.

[hillsy]

I ran an in-line paper filter on my 1150 for years with no problems.

But I agree - try it without one to rule it out.

[Call Me Ahab]

Sorry guys, been a rough couple of days, I have not been able to get back to my shop to check on my bike.

A few things though: I will certainly remove filter asap to check it for a tear...

I used paper filters for a while but it looked like they were deteriorating, I was often finding fibers in my carburetors - looked just like the filter material... This got so bad that I threw them all away and upgraded to the 3/8 nipple glass-enclosed nylon filters. So far they've done a good job, as far as I can tell. However I do intend to replace the in-tank filter ASAP to verify that this issue isn't stemming from my filter. A very good point, thank you gentlemen - one I had not even considered.

Also - Thank You for posting that link to George's store! I've been watching eBay for a good deal, but these look to be around the same price if not cheaper.

You all have a wonderful week, I'll update here whenever I can get to my bike again.