Need some advice for 77 gs550 owner.

[johnhend]

Hello,

I bought a 77 gs 550 that I probably shouldn't have for $300. Wouldn't start. Put about $500 in parts, labor into it (carb clean, new filter, plugs, brake lines tank clean, tires, etc, etc), got the right two cylinders to start, but the left two wouldn't. So, I took it to an ASE auto mechanic who works on older bikes on the side. He says the right compression is low (110 psi) and the left cylinders are around 135 psi. He says that it wouldn't be worth it to put in new cylinders and rings and possibly rebore.

At this point I figure that I am commited to finishing this thing. I can buy an 80 gs550 engine on ebay that runs perfectly for $375 including shipping and swap mine out. I've already gone this far, I've got to finish right? I can turn a wrench and read a manual, but I'm no mechanic. My wife is going to kill me when she see's how much money I am sinking into this thing.

[johnhend]

Oh by the way, I didn't even notice when I bought it but the speedo says the bike has 84,000 miles on it, yikes! And the mechanic says that the spark is low.

[johnhend]

Can anyone tell me what would cause a weak spark to plugs. I have new plugs and wire caps? 77 gs550.

[BassCliff]

Hi Mr. johnend,

A week spark can be symptomatic of a wiring harness full of corroded connectors causing extra resistance and less voltage to the coils. The "coil relay mod" is one of the favorites around here. But nothing beats a wiring harness with clean connections and grounds. There is lots of coil relay mod info on my site.


Thank you for your indulgence,

BassCliff

[tkent02]

Don't worry about the miles or the compression. They are not the problem.
Worry about learning to fix it yourself.

If that's not going to happen, run away.

[mike_of_bbg]

The bike will run with 110psi. Ironically, those are the two cylinders that are firing! A lot of times the compression will get better after the motor runs. Look at the spark and gauge for yourself if it seems weaker than the other two. If both weak sparks are on one side, then it's probably not as simple as bad coils. But coils can be replaced.

You can put in a 1980 motor, but there are some things to consider:

1. The carbs are different (not sure if the old VMs will bolt right up to the newer head or not). You can swap the heads if necessary.
2. 1980 was the start of electronic ignition. You'll have to retrofit your old ignition, get a wiring harness and ignitor from an 80-82, or go Dyna (probably with new coils).

Plugs on cylinders 1&4 and 2&3 fire at the same time. If you think it's ignition-related, I'd swap the wires on those pairs and see if your running cylinders move around.

[waterman]

Your bike still has points for ignition. May want to give them a check and see what they look like. Your coils may be shot, the wires could be leaky, etc.

Your bigger question is should you get another motor. Personally, it is hard to say. What shape is your motor in? Oil leaks, broken bolts, bunged up threads to spark plugs? If you can get it to run on two cylinders and the compression is at 110, I would bet that it will probably run on four and run pretty well barring that no previous owner has been into the motor. Low compression on a engine that has sat is pretty common. Usually, after running engine, it will gain some compression.

I do find it odd that you are running on the "low" compression side and not the higher side. This in itself tell you that your problem isn't compression related. Do you have the spark plug wires correctly routed, ie, 1, 4 on one coil and 2, 3 on the other? You also state new plug caps, did you cut off about 1/2 inch of spark plug leads before screwing on caps?

For your wife, a little white lie is okay. I think you are closer to getting it running correctly than you think.

[Big T]

As to the compression numbers, I've gotta ask

1. Did this bike sit for a long time?
2. Did it run before you got it?
3. Did you adjust the valves?

Your rings might be stuck and your valves may not be closing, both of which can cause low compression. Your cold compression numbers indicate the motor is not worn out yet.

Pull the plugs, put some Seafoam or Marvel Mystery Oil in the cylinders and rotate the engine a number of times. Then, adjust your valves and see what your compression is.

130 cold is pretty good

And, did you clean your carbs per the instructions found here? New Carb O rings

[mike_of_bbg]

A silly thought I just had (not that I put much stock in cold compression numbers on an engine that hasn't been run for a while)...

Could there be blockage on the left side exhaust? That could cause both a non-running condition and perhaps an artificially high compression reading due to back-pressure? BTW, service limit on the 77-79 GS550 motors is 90psi.

[johnhend]

Thanks for the replies.

1 Not sure how long it sat. I don't think the guy I bought it from ever had it running decent.
2 Didn't start until I installed the air box which was missing when I bought it.
3. I've never adjusted valves before.

I thought that it was funny that the side with the lower compression ran too. That made me think it was the left two carbs that were still clogged. The bike had a ton of rust in the tank before i had the carbs cleaned. I didn't clean them myself, but I know that the guy who cleaned them didn't put any new parts into the carbs like o rings.

I'll check that the spark wires are routed correctly when I get the bike back. Which cylinder is #1 if you are sitting on the bike?

The engine looks pretty good. turns smoothly. When i start it and the right cylinders are running it idles smoothly and there is no smoke.

[johnhend]

Could there be blockage on the left side exhaust? That could cause both a non-running condition and perhaps an artificially high compression reading due to back-pressure? BTW, service limit on the 77-79 GS550 motors is 90psi.

I've got a 4 into 1 exhaust, so I don't know how there could be any blockage.

[Steve]

Which cylinder is #1 if you are sitting on the bike?

#1 is always on the LEFT side.

Just for reference, any time a direction is mentioned on a bike, it is made as if you are sitting on the bike, ready to ride.

That means that #1 is under your clutch hand, #4 is under your throttle hand.

.

[mike_of_bbg]

Cylinder #1 is on the left hand side - i.e., under your clutch hand as you're sitting on the bike. #4 obviously is on the right. Check the routing on the spark plug wires - one coil fires the outers and one the inners.

Four-into-one is not stock - the first couple years of GS550 stock had two, 2-into-1s which is why I thought of exhaust blockage. I think starting in 80 they added a stock cross-pipe between the two to change the tone.

I think it's time for you to clean the carbs personally yourself. Check jet sizes, etc. Dip those parts for 24 hours and change all the o-rings.

[johnhend]

Plugs on cylinders 1&4 and 2&3 fire at the same time. If you think it's ignition-related, I'd swap the wires on those pairs and see if your running cylinders move around.

You mean swap 1 with 4 and swap 2 with 3 right?

[mike_of_bbg]

You mean swap 1 with 4 and swap 2 with 3 right?

Yes. First make sure the outboards (1 and 4) are going to the same coil, as are the inboards (2 & 3). If spark is your problem the firing cylinder should move with the wire.

If your poor sparks are on the same coil, then it's likely a coil or wiring harness problem. If you have two weak sparks and they're on different coils, the wires and boots or their interconnect are suspect. The GS series runs a "wasted spark" system, so the plugs fire near TDC on both compression and exhaust strokes. When the coils fire, they fire two cylinders at the same time.