Still can't get past 2500 RPM

[sissonk]

I have a gs850 81 I have just finished taking the carbs off again and cleaning and very carefully checking everything to do with the pilot system and the needle jet . the bike idles ok . I put on the vac gauges and first thing was that the vac level seems about 60 percent of what it was before this problem.
the compression is 100psi per cylinder on a warm engine no oil put in . the vac after pleeping the troutle drops off to nothing and takes a long time to come back. What could cause low intake vac levels other than air leaks and bad compression ? The valve clearances have also just been set and within spec.
This is the most frustrating problem I have ever known..

[mark]

Is your airbox on when you check the vacuum? Its gotta be on. Also check to make sure you're vacuum line is hooked up to your petcock, or plugged when you check as well. Your compression is barely within spec., but should be O.K.

[sissonk]

yes everything is back on the bike nd the vac line is pluged . I tried the test of spraying WD40 all over the inlet rubber couplings with the engine at idle and nothing changed.

[mark]

Tell us more of the history. What was the initial problem with your bike? Did you do a total tear down of the carbs when you cleaned them? If you didn't, what did you do? Floats adjusted correctly and working good etc.? All cylinders getting good spark? Plenty of gas getting to the carbs?

Does it backfire when your trying to rev it? What color are you sparkplugs?

[sissonk]

this was the initial problem it just started one day. the float levels are set correctly and work fine I have good fuel flow , and the plugs are slightly black at idle . With a color tune plug in I get a nice blue flame and can see the ignition spark , as the revs go up the flame changes to white then disapears . this would be around 3000. I have a Dyna S ignition and used an oscilloscope to observe the spark pulses everyone is there through the revs. the strange point is that the fault came about out of the blue, the engine before ran perfect. also as I said earlier the vac gauges hit the bottom as the engine trys to pass 2500 rpm . this is why I ask what causes low vac , it must be low vac that can't pull the fuel up.
Any help very much appreciated.

[bradleymaynar]

Is there an inline fuel filter between the p/c and carbs? Could be clogged. Also, what condition is the p/c? Is the tank rusty? It sounds like a fuel issue, though the black plugs would indicate a rich condition.

I'd drain the tank and pull the p/c. It could be clogged with sediment.

Brad bt

[chef1366]

Check the mechanical advance under the ignition cover.

[Suzuki mad]

Diaphram on the carbs go a small pin hole in it?

Vacuum pipe or joints between carbs leaking?

Suzuki Mad

1981 GS1000ET
1983 GS(X)1100ESD
2002 GSF1200K1

[49er]

this was the initial problem it just started one day. the float levels are set correctly and work fine I have good fuel flow , and the plugs are slightly black at idle . With a color tune plug in I get a nice blue flame and can see the ignition spark , as the revs go up the flame changes to white then disapears . this would be around 3000. I have a Dyna S ignition and used an oscilloscope to observe the spark pulses everyone is there through the revs. the strange point is that the fault came about out of the blue, the engine before ran perfect. also as I said earlier the vac gauges hit the bottom as the engine trys to pass 2500 rpm . this is why I ask what causes low vac , it must be low vac that can't pull the fuel up.
Any help very much appreciated.

Your compressions are low. However, having been able to start it, you should still be able to rev your engine higher than your stated 2500 -3000rpm, even with stuck mechanical advance weights/faulty springs.
You appear to have a problem with the vacuum controlled slides on your CV carbs. Recheck that you have assembled them correctly.

[sissonk]

thanks for the help.
There is no rust in the tank and no fuel filter , the tank is spotless and no fuel flow problems . I have checked the mechanical advance with a timing light and it is ok. I did look for holes in the diaphrams and didn't find any , but did notice that 3 of the slides don't thunk against the bottom when U lift them up and then let them free fall . don't know what that indicates but I also have a set of 4 carbs off a Yamaha and they all give a nice metal thunk when they fall and those carbs work very well on the Yamy. My though is going with maybe to much wear in these old carbs although the may only have 28k miles on it .although that doesn't sound like a problem that would appear out of the blue . also there are know signs of wear, only some chips on the sides of one of them. I have checked the condition of all the couplings and vac pipe is new and plugged .

[denydog]

I don't really know what's up here, but after I cleaned my carbs, the bike would run to about 2,500 rpm, then just sputter and stumble. I went through the carb's for three straight days, about ready to pull my hair out. Finally I happend to notice that the holes in a couple of the main jets weren't quite as large as in the others. After I REALLY cleaned the main jets, it ran great.

[Steve]

Seems that everybody is shotgunning here, so this is my thought:

Have you checked the valve clearances? Clearance tends to decrease with normal wear, and eventually the valves do not spend as much time as they should on the seats. This could contribute to your low compression.

Without sufficient time on the seats to cool down, the valves eventually burn, causing a drastic decrease in compression for that particular cylinder. Drivability and performance are reduced at the same time.


.

[Mike McNaney]

The key to your problems is the low compression and vacuum readings. this and not being able to rev above 2500 means it can only be one thing.

Ignition or more specifically, Timing. Late timing. I promise.

Dont screw with your carbs or you'll have to fix those when your done.

Dont know how many miles but you may have skipped a tooth on one of the cams or crank. Tensioner issue. (maybe)
Or maybe your ignition plate lost a hold down screw. (probably) Check your dyna and advance plate.

This is a basic diagnostic scenario.

After looking back at the posts, Chef was on it.

Glad to help.

[Mike McNaney]

This post is a day or two old. i hope you come back and check it before you go to far in a different direction.

BUMP it up.

[sissonk]

Thanks everyone for your help . It seems that my title is not accurate. the engine will rev to 6k+ but the problem is getting through 2500-3500 rpm. the timing and advance system works and is as per the manual using a timeing light the marks line up exactly and are dead steady. I just replaced the head because I originally thought this problem was due to poor compression , so I got new valves and guides but the machinist destroyed the head. the used head now on the engine is within spec and I just lapped the valves, much quieter than the old one. but the exact problem existed before I changed the head. From the color tune I can see it going lean around 2500 rpm on all 4 cylinders Why?
I think the vac falls off because the engine can't speed up to sustain the vac, with the butterflys open. I checked the diaphrams of all carbs by using a vacuum cleaner sucking air through the carbs outlets, they go up and down very uniformly, so it seems that that system is sealing itself and working. I am missing something but what...