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'77 gs750. Coils, valves and leaks... oh my!

  • Thread starter Thread starter erch
  • Start date Start date
E

erch

Guest
Hi everyone,

Been working all summer on restoring my '77 Gs750 and through all the work I've done it still seems the engine is responding sluggishly.

My #4 cylinder is intermittent, sometimes cutting out entirely and the plug is looking oil fouled. On the highway it'll feel like the engine's tugging at me occasionally and any more than about 1/2 throttle isn't accomplishing anything. To make matters worse it seems I can't go any higher than about 5500 rpms without aggrevating the problem.

And here's the weird thing:

About a month ago, I decided to take a 400km trip and before I left I swapped out the plugs for new ones (and gapped them). For the first 100km or so the bike was running like new: took off like a bandit and higher amounts of throttle were accompanied by the power you'd expect. The only issue was as it ran better, it started to leak oil... pretty badly.

By the end of the 800km round trip, I had gone through about a liter of oil, lost the power, and cylinder #4 was back to cutting out. Thankfully the oil hasn't leaked any more since then, but checking the plugs regularly shows that the new ones are looking exactly like the old ones did (#1 carbon fouled, 2, 3 fine, 4 oil-fouled).

This has gotten me thinking that my issue could be a compound of a couple of things:

1. I'm fairly certain the oil leak was simply old gaskets. Checking after I cleaned the engine, it appears the head gasket and one of the crank-case gaskets are the culprits. I don't think it's the cause of the power loss but when the engine was running well I think it might have gotten hotter than usual and caused extra leaking.

2. Valve clearances are off. I'm thinking that the reason #4 is mis-firing could be from low compression.

3. Worn points, bad ignition coils (both are original), and generally weak spark in general. I think the brand new, clean plugs were compensating for my weak ignition system and once they had worn in it went right back to how it was.


Anyways, can anybody confirm or deny my suspicions? Just don't want to start ordering parts or taking apart the engine until I'm warranted in doing so.
 
Instead of wondering, do a compression test and a leak down test. These tests should give you some insight into the condition of the engine.

Have you done the basic maintenance like valve adjustment, replace points, plug wires..etc?

If you plan on keeping the bike, it may be a good idea to replace your leaky head gasket, replace valve seals..etc. How many miles are on the engine?
 
Last edited:
almarconi said:
Have you done the basic maintenance like valve adjustment, replace points, plug wires..etc?
Click to expand...

The bike is more than 30 years old so it's a good idea to do all the basic maintenance: carb rebuild with new O-rings, valve adjustment, clean air filter, check the plug wire caps, new points, etc. Doing these things will allow you to build a baseline toward a reliable bike.
 
Thanks for the quick replies guys.

I already rebuilt the carbs and know they're in good shape, plug wires look like they're at least not original but the coils definately are. I've also got a new air filter on it.

I'm planning on checking the valve clearances and points but I've never done either procedure so I was waiting till it's off the road for winter to give myself the time to not rush it.

If I'm going to check the clearances myself, do you think it's still worth taking over to the shop for a compression test?
 
erch said:
Thanks for the quick replies guys.

I already rebuilt the carbs and know they're in good shape, plug wires look like they're at least not original but the coils definately are. I've also got a new air filter on it.

I'm planning on checking the valve clearances and points but I've never done either procedure so I was waiting till it's off the road for winter to give myself the time to not rush it.

If I'm going to check the clearances myself, do you think it's still worth taking over to the shop for a compression test?
Click to expand...

How you are describing the way it runs sounds like a carburetion issue, what exactly did you do when you rebuilt the carbs?
Never take it to a shop for anything, they will ruin it and take your money.
Check points, five minutes, if they need replacing maybe a half hour or so.
Valves can be checked in an hour or so, first time maybe two hours.
Compression test, do it your self, five minutes of your time and a ten dollar tool required.
 
tkent02 said:
How you are describing the way it runs sounds like a carburetion issue, what exactly did you do when you rebuilt the carbs?
Never take it to a shop for anything, they will ruin it and take your money.
Check points, five minutes, if they need replacing maybe a half hour or so.
Valves can be checked in an hour or so, first time maybe two hours.
Compression test, do it your self, five minutes of your time and a ten dollar tool required.
Click to expand...


Carbs: new mains, new pilots, blown out with compressed air, soaked in carb cleaner, new rings and now run for about 2500kms on high octane.

Funny story about shops: I went in for a safety on my '83 honda and a fork seal went on the way to the shop. I figure I'd see if they'd pass it on my word that I'd replace the seal. They tried to convince me that my forks were pitted and both needed replacing and wrote me up a quote for $1700. I told them I'd consider it and never went back there. Another shop passed it first try with a new seal.

Back on track though: thanks, I didn't realize you can do a compression test at home. I'm actually about to head to a shop to grab new points (even if they're not worn I might as well replace them and be sure). I'll go pick up a compression meter while I'm out.
 
I wouldn't even bother with a compression test until you check the valves. Tons of GS bikes bite the dust because the owners don't adjust the valves and then they burn. The photos below show what a good valve should look like and what happens after the valve adjustment is neglected.


IMG_1020.jpg


IMG_1017.jpg
 
Thanks for the reference. I've just got the points exposed and it looks like they're in good condition (very minor pitting, had to really look for it) and gapped correctly. I decided to check the timing while I was at it and it looks like 1 and 4 are both oil fouled (making me think it's the ignition). Curiously enough though, while 3 looks fine, 2 looks carbon fouled.

I'm going to give them a little scrub to clean them up but I haven't been able to spot the spark while I rotate the crankshaft. I've checked that ignition is on, kill switch is off, they're touching the heads and that the garage is dark but I still can't spot it.

I'm going to take a look at the valves asap too, I have a suspicion that even if they're not what's causing my issues, it's still worth a look.
 
All the other advice given already is very relevant, but also check the voltage on your coils and the wires, HT leads and plug caps, especially the coil for #1 and #4.
Measure between the orange/white wire on the coil side of the coil connector and the negative terminal of your battery. It should not differ very much as to what you get across the battery terminals.
 
Well I've lost light for the day so I'm calling it quits but I've almost gotten the heads off. I can peek in and the pistons look pretty caked with carbon. Once I get a close look in the light I can tell if there's any damage.

On that note: the only thing left to loosen is the tach cable. I've got it off at the gauge end but it's not flexible enough to get around the frame. Any advice for a better way to get those stupid threaded connectors off?
 
Well, I've got bad news and worse news:

Here's #3 and 4. 3 was definately the best running cylinder but it looks like that's toasted it pretty nicely. 4, on the other hand, looks nice and fouled.
DSC00082.jpg




Here you can see 1 and 2, 1's definately oil-fouled and 2's caked with carbon.
DSC00083.jpg



Finally, here's an close-up of piston #4. Most of them are in similar condition though:
DSC00081.jpg
 
Well, I'm thinking the valves should probably go. In the meantime I should polish up the heads and remove any deposits. What kind of abrasive or solvent should I be using for the job if that's the case?
 
Tkent02, sorry it's out of focus. My camera phone won't do extreme close-up but all those speckles are thick deposits.
 
tkent02 said:
Clean them off. I see nothing unusual.
Click to expand...

Excellent! So what should I be using? Brake cleaner, fine grit sandpaper and rags?

Also, do you think the deposits are from cold plugs? Rich carbs? Or valve adjustment issues?

edit: also, how can I clean the top-side of the valves without a valve compressor?
 
erch said:
Excellent! So what should I be using? Brake cleaner, fine grit sandpaper and rags?

Also, do you think the deposits are from cold plugs? Rich carbs? Or valve adjustment issues?
Click to expand...

The deposits are from running, they do that. Does not hurt anything unless they get really big and thick.
The shininess on the head may be from oil getting past the rings.
Or from not firing for whatever reason.
How are the sides of the pistons and rings, and how are the cylinder bores?

Is this engine way over 100,000 miles? 200,000?

How was the compression before you pulled the head?
 
Last edited:
tkent02 said:
The deposits are from running, they do that. Does not hurt anything unless they get really big and thick.
The shininess on the head may be from oil getting past the rings.
Or from not firing for whatever reason.
How are the sides of the pistons and rings, and how are the cylinder bores?

Is this engine way over 100,000 miles? 200,000?

How was the compression before you pulled the head?
Click to expand...

I'm thinking the oil is from misfiring, each of the oil fouled cylinders has definately been misfiring.

I'll take a look at the pistons and bores in a second.

Forgot to do a compression test before I started stripping.

Engine's got about 39,000 miles on it.
 
Are you sure about that #3 exhaust valve though? It looks pretty fried to me.
 
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