Dead cylinders, sporadic firing....tell me what you think.

[Ludeykrus]

I already know what's coming, but thought I'd run it by you guys.

Just bought a '78 GS750. Ran great for the most part after I took it for a good little drive, aside from a sporadic miss on cylinder #4 (? the far right one when sitting on the bike). Sometimes, more rarely, #1 would go dead, and I have a feeling that it may just be a loose wire at the spark plug....haven't fully concluded this, though. So two sporadic cylinders made me check the compression first thing, just to eliminate things since I suspected the carbs.

Well the results and plug colors are below:
#1: ~90psi, a good healthy tan rich color
#2: ~90psi, no color, almost looked new.....not lean, though!?
#3: ~30-40psi, no color like above
#4: ~30-40psi, a bit rich, a good bit of black carbon and some oily soot

All plugs looked like they were fairly brand new, indicating the previous owner was trying to work through the problems himself. I let it cool down, then cranked it and touched the pipes. #1 got hot like normal, #2 just a HAIR slower, #3 got hot but very slowly, and #4 didn't hardly get hot at all. Compression test with a cap full of oil brought the compression up from 30ish psi to 65ish psi.

#1's occasional misfire is not explained, but apparently I've got two near-dead cylinders, even though #3 still fires strong. Pressure came up with oil added, but still not enough so I'd say it indicates bad valves? No visible leaks for a head gasket or anything. Anyone got any advice? I don't have a manual so I don't know what are acceptable compression numbers on this motor, I was just comparing between cylinders. I gotta try another compression tester tomorrow morning as well, to make sure my gauge isn't reading a bit low.

[p_s]

Those numbers are extremely low. My '85 GS550ES has 140 psi cold on all cylinders. I'm surprised yours runs at all. You might check your compression tester against a good engine--they aren't always accurate. You are opening the throttle all the way when you test it?

[Ludeykrus]

Those numbers are extremely low. My '85 GS550ES has 140 psi cold on all cylinders. I'm surprised yours runs at all. You might check your compression tester against a good engine--they aren't always accurate. You are opening the throttle all the way when you test it?

Yep. I have a feeling the gauge is bad/blown out partially as well, since I also didn't think it would run hardly with those numbers. So don't take them for face value, I worried more about the left cylinders compared to the right cylinders.

I gotta get a new gauge in the morning.

[tkent02]

At least two problems, 1-4 ignitions are common, something intermittent there, points or coil?

Low compression could be valves clearances too tight, check them before you run it any more. If tight valves are the problem and if you continue running it, burnt valves can result quickly.
Also while the valve cover is off check the cam timing. I only say this because the 750 I just bought had both cams installed one tooth too far advanced, a sure buzzkill in the power department.

The actual compression numbers mean nothing, the relationship between them is everything. There are too many variables in elevation, cranking RPM, camshaft, temperature, gauge errors, etc. All within ten percent or so is fine, within 20-30 percent will run OK but is not the best.
Two good and two bad is a problem no matter what.
Engines with low compression will usually run sort of OK at high RPM, but idle will be rough, low power running and cruising will be uneven, and tuning correctly will be nearly impossible.

Also you didn't mention the overall condition, is it a beat to **** high mileage low maintenance rat bomber, or a nice tight low mileage clean machine?

Two adjacent cylinders reading identical low compression is likely to be a compression leak between those cylinders, probably a failed headgasket. This will have no external signs, no oil leaks. The oil you added could have clogged up the leak a little, raised the compression for the second check. A leakdown check would tell you for sure, but if the valves are in spec and still the numbers stay the same pop the head off and look.

Poor man's leakdown, blow air through the compression gauge hose into one cylinder, with both valves closed. Stick a small hose into the adjacent cylinder and stick the other end into your ear. If you can hear the leak it's the gasket. You may need the assistance of your beautiful assistant for this one.
If you hear it through the carbs or exhaust, its the valves.
If you hear it through the oil filler cap, it's rings.

[Ludeykrus]

At least two problems, 1-4 ignitions are common, something intermittent there, points or coil?

That's what I forgot to ask. My 450 has the electronic ignition. I know 1/4 and 2/3 cylinder problems are usually linked via ignition, what are the likely suspects on this bike? What's the ignition system like on it?

Also you didn't mention the overall condition, is it a beat to **** high mileage low maintenance rat bomber, or a nice tight low mileage clean machine?

Mileage is unknown and most assuredly higher, but it appears to be in very good pampered shape. Everything was done RIGHT, all proper maintenance appears to be done.....aside from a few small cosmetic blemishes, it appears to be pampered. Definitely not a beat to hell rat, like my "other" bike.... 8-[ \\/

[tkent02]

That's what I forgot to ask. My 450 has the electronic ignition. I know 1/4 and 2/3 cylinder problems are usually linked via ignition, what are the likely suspects on this bike? What's the ignition system like on it?

Old fashioned points, condensers and coils. Dual system, 1-4 and 2-3. Standard old stuff.



Mileage is unknown and most assuredly higher, but it appears to be in very good pampered shape. Everything was done RIGHT, all proper maintenance appears to be done.....aside from a few small cosmetic blemishes, it appears to be pampered. Definitely not a beat to hell rat, like my "other" bike.... 8-[ \\/

Check the valves and give us a status report.

[Ludeykrus]

Compression tester gauge is officially blown. I got 110psi on number one after monkeying with it, then the check valve stopped working and it won't build on the gauge. New test results to come in the morning. I'm now hoping/betting on an out-of-spec valve on number 4 and a wiring/ignition problem across 1/4.

[Ludeykrus]

Took her for a spin, got a new compression gauge, and got her good and hot. New test results on a new gauge:

#1: 125psi - pretty rich plug, still healthy brown
#2: 105psi - plug clean as a whistle!
#3: 65psi - plug clean as a whistle!
#4: 115psi - plug is getting carbon coked, fouling out....possible the misfire?

Now seeing how #3's got a clean plug but bad compression, I think my bet is a valve out of adjustment. #4 has good compression but is blowing serious oil, so valve stem seal or rings....though I'd probably bet on the stem seal since compression's good. It is also not completely ignition related since when it dies out, I can pull the plug wire with it running and it makes no difference in the engine, but I can touch the lead back to the spark plug and hear the spark clicking, so it's getting spark and still not firing. Still a fouled plug? #1's misfire is not obvious to me from the test, so I'd say it's ignition related.

Anyone agree/disagree?

[Dave8338]

I'de set the valves and recheck the compression on a warm engine. At the least, you can rule out the valves this way. If the numbers are still all over the map, my guess would be a head gasket. The #3 plug being white and clean suggests a carb problem on #3. It should be fouled or at least black with that low of a number (assuming the carb is flowing correctly).

[Ludeykrus]

Figured I'd update this thread a bit.

I cleaned out the spare rack of carbs and slapped them in, it now runs like a raped ape, but doesn't like to crank. Once it's cranked, though....let's just say that the abundance of power is appreciated, and may cause me to be funding the local police station very soon

BTW: these are slide carbs.....what model/name do these go by?

[TheCafeKid]

VM26SS most likely if they are of the same design as the originals.

[Ludeykrus]

VM26SS most likely if they are of the same design as the originals.

Do they have a nickname? The guy made them out like they were high-demand carbs, and I don't know much about any others (roundslides, slingshots, etc...) than the BS's,

[TheCafeKid]

Post up a pic of them. He could have fed you the line about them being smoothbores, or he may not have known what he was talking about. They look very similar, i'll give him that. However, no zook came stock with VM29's (smoothbores) that im aware of. Least of that era. But i could tell you by seeing them. Stock, pre80 GS's i THINK all came with VM26SS's...good mechanical carbs. Smoothbores are a big deal tho.

[Ludeykrus]

Post up a pic of them. He could have fed you the line about them being smoothbores, or he may not have known what he was talking about. They look very similar, i'll give him that. However, no zook came stock with VM29's (smoothbores) that im aware of. Least of that era. But i could tell you by seeing them. Stock, pre80 GS's i THINK all came with VM26SS's...good mechanical carbs. Smoothbores are a big deal tho.

Cool, thanks. I'll get a pic if I can when I get home. the spare set was the same as the set that came off the bike, I know that.