Figured why gs750 not starting

[baz666]

No compression. I mean like 30psi per cylinder.
It's a mystery since when I did a compression test last fall before putting the bike away for the winter it read between 149 to 151 across the board.
I didn't bother trying a compression test in the past week or so while trying to get the bike to start since I thought that was okay.
I checked the valves and they're all within spec.
The only thing I can think of is that when I first cranked the bike a couple weeks ago after taking it out hybernation, I heard one quick clack from the engine. I thought it was maybe the cam chain.
When I had the valve cover off tonight, I turned the engine by hand and nothing looks out of order but I'm no expert. Somebody said it could be a broken crankpin.
Maybe somebody on this forum has a few ideas.
Really a major drag. I love this bike but I'm wonder if this is the end of the road. I'm not sure I'm ready to take on an engine rebuild.
If there is something else that might be causing this I'm overlooking, please let me know.
thanks in advance,
baz

 

[Guest]

two things i can think of

1: cam chain jumped,

2: drop some oil in each cylinder and see if the compression increases.

 

[BassCliff]

Hi,

Oh my, that's bad news. Let's hope you can find a good used engine to drop in your bike. That would be fairly quick and painless.


Thank you for your indulgence,

BassCliff

 

[Flyboy]

Agreed, check the cam timing, line up all the marks and count the links between cam sprockets, check that the timing has not jumped a tooth.
Also pull the spark plugs and insert a long, thin screwdriver into the plug hole and SLOWLY turn the motor by hand to see if the pistons are traveling all the way up and down the stroke, although a broken crank pin would not affect all the cylinders, just those on the other side of the break.
Check that your valves are not sticky, turn motor over by hand and check that each one comes all the way back onto its seat once off the cam lobe.
+1 on a teaspoon of oil in each cylinder and re-check.

It is never the end of the road, if worst comes to worst, tear into it and repair, it is not as daunting as it seems, and there are many fine folk here, who will walk you through a rebuild, you don't have to go it alone.

 

[salty_monk]

Remember that you will be getting lower compression as it's cold engine... Not that low though.

Could the rings be rusted to the piston? I think that might cause what you're seeing, in which case I believe that introducing some diesel to the bore may help. Think I read that somewhere... Let it soak.

 

[Smokinapankake]

Like my drag racin' buddy Tim says: "If it's not hanging a rod, it's worth saving!"

 

[koolaid_kid]

I agree with Flyboy, it sounds like it jumped time. This is usually due to the cam chain tensioner needing a rebuild. Fairly easy job that can be done on the bike.

 

[baz666]

First, thanks for everybody offering their advice.
So, I take off the right hand engine cover, turn the engine so the timing marks are lining up and then count the links between the cam sprockets?
How many links am I looking for?

If it did in fact jump time, does that absolutely mean it bent a valve?

thanks for all,
baz

 

[koolaid_kid]

First, thanks for everybody offering their advice.
1) So, I take off the right hand engine cover, turn the engine so the timing marks are lining up and then count the links between the cam sprockets?
2) How many links am I looking for?

3) If it did in fact jump time, does that absolutely mean it bent a valve?

thanks for all,
baz

1) Yes.
2) Should be in your Service Manual. If you don't have one, look on BassCliff's web site. You will need to refer to it in order to line up the marks; the link count should be in the same section.
3) Absolutely not. Get everything squared away first, then re-check the compression.

 

[baz666]

Okay, thanks a lot. I'll do that today and report back.
thanks again for the help.
I'm not feeling so bad now that there's a small chance there might be some light at the end of the tunnel.

 

[baz666]

I checked the manual for the GS750E and ES from BikeCliff's website.
It shows how to do camshaft reassembly but does not show the number of links between sprockets while the camshafts are in place. The chain pulled tight after being removed from the camshafts should have 20 link rivets within 157.80mm or 6.231 in.

The manual also shows how the camshaft sprockets should be positioned on the camshafts.

Am I looking for the number of links between the sprockets or for the position of the sprockets relative to the camshafts?

Also, I noticed that one of the oil-feed line bolt is stripped, the one above the exhaust valves at cyl. #1. I can lift the head of the bolt out a tiny bit and see the broken off shaft of the bolt sitting in its hole. There does not appear to be any piece of it missing. I'm guessing that wouldn't have a huge effect on starting the engine.
Someone suggested the rings could be seized to the pistons. Is it really possible for all four sets of rings to do that after sitting for just 5 months?

Thanks in advance,
baz

 

[Guest]

Is it really possible for all four sets of rings to do that after sitting for just 5 months?

Its possible but very unlikely. Oil test will tell you that though

The facts we are working with to advise you are:

1 low compression on all cylinders But consistently about 30psi on all

2 that clack sound you herd points to the most obvious thing, The cam chain tensioner.

You can cause the timing to jump if you loosen off the tensioner and it sticks in the loose position. How this could happen is another story.

When you look at the manual there should be a diagram that shows the cams and the links in between and the timing positions. maybe not a writeup on it but just a diagram. ( thats what i've seen in a few manuals yours might be special)


20 links means that if you look at the cam sprockets there is an arrow on each. Count the pins between the arrow on one and the arrow on the other and it should be 20 links exactly.

All that means is that the cam's are in line with eachother, but it doesn't mean its aligned with the crank.

For that you have to remove the right timing cover look inside and find the timing marks and align them with the notch somewhere on the case, The manual shows what they are. There is a timing mark for TDC.


If it did jump, be cautious when your aligning them, IF you feel any resistance while you are turning the engine over by hand STOP, turn backwards and go back to where you started from. ( make sure the plugs are out so that isn't the resistance your feeling)

you will know when you are hitting the valves, It just doesn't feel right.

Once you have it timed correctly, do several rotations by hand easily before trying the starter.

Good luck

 

[baz666]

Okay, found the correct diagram in the manual. I was looking in the Camshaft section and it's in the Engine Reassembly section and used diagrams and text to explain in detail how to line up the camshafts with the crankshaft in the correct positions.
I'll examine the current positions and get back with what I find.
Thanks to everyone for all your help. Keep your fingers crossed for me!
baz

 

[baz666]

Well, I checked the timing marks after setting the T to the 1 and 4 signal generator mark, as shown in the manual. Arrows 2 and 3 were pointing straight up on the camshaft sprockets and the pins between the two arrows were exactly 20, also as shown in the manual.

I poured a teaspoon of oil into each cylinder and tried the compression test again. Once again it was 30psi in each cylinder, with a very slight increase in cyl. 4, to maybe 33psi but that just might be the gauge.

Although I did forget to turn the throttle wide open this time so that probably had an effect but I don't think it would make that big a difference, would it?

I can turn the crankshaft by hand and its silky smooth, nothing hanging up or sticking. What on earth in this engine could break that would affect all four cylinders equally and so dramatically? And break not at high revs or under duress but break while hitting the start button! I'm obviously no expert but it does seem very strange.
I have another bike, an old XJ650 that's apart and that I'm going to reassemble so I have something to ride this summer and then maybe I'll take apart the GS750 engine just to see what the heck happened.
I plan to take photos and post them so that whoever is interested can keep up and hopefully comment on what they see and perhaps with your help, I can discover what kind of catastrophic failure occurred.
It really is very sad, heartbreaking. Even though I only had this bike for one season, it ran like a top, had all the power I need and great handling for my limited riding skills and perfect for the city, where I do 90% of my riding. Yeah, really too bad. What a lousy piece of luck.

 

[Nessism]

Well, I checked the timing marks after setting the T to the 1 and 4 signal generator mark, as shown in the manual. Arrows 2 and 3 were pointing straight up on the camshaft sprockets and the pins between the two arrows were exactly 20, also as shown in the manual.

What about the exhaust cam position? Align 1-4 T, then check the exhaust cam position (where the 1 -- mark is pointing on the exhaust cam sprocket). Last step is to check the pins between 2 and 3.

Oh, and that broken bolt is for the oil feed to the head. They are very fragile bolts since the center is drilled out to allow oil to flow down the center. I wouldn't run the bike without the bolt being replaced, and don't just use an ordinary bolt since oil won't flow at all then.

 

[baz666]

Okay, I'll do as you suggest and check the exhaust cam.
Also, about the broken oil feed bolt. Does that allow the lubrication of the underside of the exhaust camshaft, is that the idea?
Since the bolt's shaft is broken off inside the cam cap, I'm guessing I will have to remove that cam cap then somehow remove the broken bolt shaft.

But that is secondary. I'd love to be able to figure out what exactly happened that was so catastrophic. The bike showed 149 to 151 psi last October then sat for 5 months and now has almost no compression. Very weird. The only thing was a loud clack when I first tried to start it after it sat in a heated garage all winter. After that, it cranks like an sob but obviously won't turn over with such low compression, 30psi across the board.
WHAT would possibly caust that to happen?
I've asked a few mechanics at local bike shops and have gotten various answers. The rings are stuck to the pistons, bent valves, timing off, big end bearing gone. I was asked if the bike has a fly wheel and counter balancer but I don't know and kind find a refererence in the manual.
Anyway, I guess I have to work on putting my XJ650 back together to have a ride but man, this is just driving me completely nuts!
thanks to all for their help,
baz

 

[koolaid_kid]

First, since this affects all 4 cylinders, it indicates a cam/valve issue. There are 2 basic places where the compression could be leaking - rings or valves. The head gasket is also another area, but, as mentioned, it is unlikely that all 4 pistons have stuck rings or that the head gasket is leaking around all 4 cylinders. The most likely culprit is that the cam(s) are slightly out of time, allowing the valves to be ever so slightly open when they should be completely closed.
I strongly suggest that you consider rebuilding your cam chain tensioner, using the instructions here: http://www.bwringer.com/gs/camchaintens.html
You may find that it is sticking just a bit, causing the cam chain to not be under consistent tension all the time. Just my 2 cents worth.

 

[baz666]

Sounds like a reasonable thesis and looking at the link, rebuilding the cam chain tensioner doesn't appear to be beyond my limited skills.
Thanks again for all this incredible support. It must get tedious to have me coming back all the time with the same issue.
baz

 

[baz666]

On the cam chain rebuild link it mentions that the parts required and their numbers are for the GS850 and that although almost all GS models share cam chain tensioner parts, to make sure they apply to my bike, an 83 GS750ES. Is there parts list somewhere on the site I can verify what I need?
thanks for any suggestions.
baz

 

[7981GS]

I have successfully interchanged parts from several years/models of cam chain
tensioners. I.E. : steeper angle on the plunger/rod, stiffer spring, longer spring, longer rod.
If it fits, basically, it will work. As long as it doesn't push against the chain
in the compressed position, you are good to go.

Daniel