1978 GS 500 E - no power, many checks

[Guest]

Hello,
After spending about two weeks just reading through the posts and testing different things I decided to start posting my findings and perhaps get some advice, as there is not much more that I can think off that could be worth chcecking.

So my bike is 1978 GS 500 E (it's a sleeved down version of 550 for Swedish market) four cylinders, VM22ss mechanical carbs, points, cast wheels with double rotors.

My problem is the power. The bike starts right away, idles without problem, and accelerates rather OK up to about 4k rpm, then it flattens out, it's smooth and revs to redline on 2nd but develops no more power. In any gear its maximum speed is 120 kph (ca. 75 mph).

These are the checks/maintenance/parts I did

1. All valve clearances within 0,05-0,01mm
2. Carbs stripped and cleaned 3 times, I followed VM carb rebuild. Boiled in acid solution, and also in ultrasonic cleaner, they are clean. New orings.
3. Manifold rubbers replaced with new ones incl. orings.
4. Stock airbox and very lightly oiled new foam filter. I do not have the lid, but made one out of cardboard to about right opening size.
5. The exhaust is 4into1 Marving. Tried running without it with no noticeable difference (so it's not clogged)
6. The points were cleaned with 500 grit sand paper and gaped at 0,3mm.
7. The condensers were checked by disconnecting them briefly - the bike run bad with either one disconnected.
8. Coils checked for resistance, primary ca 4ohm, secondary ca 35kohm, tried running with gsxr 1100 '89 coils/caps - no difference.
9. Caps and plugs are new, also cut off some 10mm HT cables where caps attach.
10. Tried running with a wire between a main braker to white/orange double connector for the coils - no difference
11. I had some problem with valve timing adjustment as I couldn't get the #1 mark to point at the gasket face (either above or below). In either case the engine is running the same (which is really confusing). Now I have the #1 mark slightly below gasket face.
12. The petcock is giving a lot of fuel, tried draining tank to a bucket, so no problem there, checked for "On" position.
13. I had the fuel filter for a while, but recently got rid of it just to eliminate a possible fault, no change after that.
14. Carbs are synced with manometers at idle speed.
15. Needles at middle position with thick nylon washer below the shim
16. Pilot jet #15 as per spec, main jet #102,5. Tried running with #92,5 jet, and it made the bike worse I think. tried also #134 and that properly bogged the motor.
17. Choke is as it should be, meaning when cold - it make rpm higher, when warm it bogs the motor.
18. There is no difference when I pull the choke while at max speed (75mph) no matter what gear/rpm I'm in.
19. Timing and advancer working fine (checked with strobe light). Although the strobe is not flashing above ca 4k rpm. But I blame it on weak strobe pickup, besides the bike run much higher rpm, and the headers all have same temperature.
20. The compression is low at 90psi (warm, WOT) on all cylinders. (they have been lightly honed as I had suspected glazing, did ca 30 miles after that). The rings are not stuck, I had the cylinder off, and rings are free, and gap within spec.
21. While I had the head apart, I checked valve spring free length (it was well within spec), and did a leakage test with ethanol on both in and ex side (no leaks).
22. The internals looked normal, no signs of burning, or excessive carbon deposits, or friction etc.
23. The float bowl levels were adjusted on the bench to ca 3 to 4mm below the carb body split line. No fuel leakage from the carbs.
24. The electrical system is OK. The stator was re-winded (did it myself) and is charging OK. Went through stator papers. Also cleaned all connections with spray cleaner.
25. There is no drag/binding in the drive chain or wheels (brakes).

What else could/should I check? I don't really want to do the piston/rings/bore before I am absolutely sure all other things are working as they should be. I suppose a low compression problem is more pronounced in the low rpm region so then it would not apply to my case.

I could make a run with a camera, to illustrate the problem perhaps.

Regards,
Dezert.

[GSCafe]

Hmm, I've a 1979 GS550 and I'm running #100 main, #17.5 pilot, raised needles with pods and pipes. I could probably use a #102.5. I've tried #124 main with the stock #15 pilot and it was boggy on the high-end, #87.5 main was way too lean. Assuming that there is no other problem, the devil is in the details. Every adjustment affects the whole range to a certain degree on the old slide carbs. I think you might make note of the positions and try to get your mix right with the air and fuel screws. You also might have to move the needles again if the transition between idle and WOT bends toward too rich or too lean as you open the throttle. I may not be a representative wrencher, but I struggled with getting the mix right on the old slide carbs.

[dezert]

I have yet again disassembles carbs. Bench synched them not more than ca 0,2mm difference between carb slides.
When I then tried measuring vacuum (as with normal sync) I discovered that cyl #1 has ca -400kPa, while #2,3,4 have ca -210kPa at idle.
What could that be caused by?

I even sprayed some start fluid around the new manifold boots, but noted no change in rpm or sound.

[JTGS850GL]

That's why you need to do a vacuum sync.

[dezert]

When I did the vacuum synch, #1 carb slide was ca. 5mm higher than all others - that doesn't even look right.
Will try bigger jets on Saturday (i have got 110 and 117,5) and post back the results.

[78550GS]

When I did the vacuum synch, #1 carb slide was ca. 5mm higher than all others - that doesn't even look right.
Will try bigger jets on Saturday (i have got 110 and 117,5) and post back the results.

Try the 110 first.

[dezert]

An update:
I have checked following main jest:
92,5 - lean bike went max 105km/h
102,5 (were on the bike when I bought it) - bike goes max 120km/h, perhaps a tad lean
110 and 117,5 - seem too rich, (plug chop tests show some carbon deposits), max ca 110 km/h

So to summarize, it seems that changing jets doesn't have much impact on the power problem.
The ignition advancer was cleaned and lubed, works ok.
The points are flat and gapped. although to set the timing right I must retard them all the way in slots, and still get maybe 1deg advance - shoudnt make such a difference.
The bike goes smoothly to redline in 1,2; in 3 and above the speed (ultimately power) is the limit.
I have run with coils directly to bat.
I do have low compression - but that would give rough idle (which is not the case) and it shouldn't affect high rpm ??
I guess due to many WOT tests and general bad combustion I got ca 20mpg! (went through 2 tanks in 250km)

[dezert]

Changed the condenser and points and got the timing perfect now - no change !
I guess I have to blame it on low compression then, even though it seems that low compression should have more impact on low revs - which is the opposite in my case.
Sad as the total rebuild is rather cost prohibitive here.

[tom203]

Did you try running without foam air filter ? Seems like this thing is running out of mixture as load goes up and starving.