Figured why gs750 not starting

[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

[BassCliff]

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

Hi,

Go to one of the online parts fiches, like partshark.com, and order up the parts you need. I've never seen a guide for this procedure for your specific bike. Perhaps you'd like to write one?



|REF#| PART #| REQ| RETAIL| YOUR PRICE|

1 CHAIN, CAM SHAFT DRIVE (120L)
12760-45040 1 $93.31 $73.91

2 GUIDE, CAM CHAIN
12771-31301 1 $38.27 $30.88

3 GUIDE, CAM CHAIN
12782-47040 1 $106.29 $83.55

4 SCREW
02112-05127 (replaces 02112-05128) 2 $1.42 $1.18

5 TENSIONER, CAM CHAIN
12810-31301 1 $65.20 $52.42

6 CUSHION
12814-44101 2 $2.68 $2.24

7 ADJUSTER ASSY, TENSIONER
12830-33302 1 $127.74 $99.65

10 .SCREW
12838-45001 (replaces 12838-45010) 1 $5.08 $4.24

11 WASHER, SEAT DAMPER UPPER
08322-0108A (replaces 08322-11085) 1 $1.32 $1.10

12 NUT
08310-0008A (replaces 08310-11085) 1 $1.32 $1.10

13 .O RING
09280-21013 1 $1.05 $1.05

14 ..O RING
09280-06005 1 $1.05 $1.05

15 .OIL SEAL
09284-10006 1 $2.49 $2.09

16 NUT
08316-1006B (replaces 08316-16065) 1 $1.84 $1.54

17 GKT,TENSIONER A
12837-47010 (replaces 12837-47000) 1 $2.54 $2.13

18 BOLT (6X25)
09103-06212 (replaces 09128-06022) 2 $1.70 $1.42


Thank you for your indulgence,

BassCliff

[Big T]

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

You've missed the most important step here

Set the ignition timing marks
The arrow on the exhaust cam should point parallel to the gasket surface (this is what you missed, it times the cams to the crank)
Now, count back 20 pins to align the intake cam to the exhaust cam

Try that and see if your compression comes back. Then check the cam chain tensioner and rebuild

[baz666]

So align the timing marks to the 1-4 signal generator side and then look for the number 1 and the arrow on the exhaust cam? And that should be aligned to the cyl. head gasket surface? Now I count back 20 pins from the number 1 and its arrow? Am I correct so far?

Also, took out the carbs after some struggle and removed the cam chain tensioner. The pushrod spring appears very weak. I can push in the spring-loaded rod all the way in with very little effort, I mean like almost none and it pops back without much force. It's difficult to imagine the spring would have enough resistance to maintain any kind of pressure on the cam chain guide.
Other than that, the entire tensioner seems clean and the rod is not pitted or bent or anything. The knurled knob and its spring seem to work as indicated on the instructions.

One last thing, a local bike mechanic keeps insisting it's got to be bent valves. Wouldn't I feel some resistance while turning the engine by hand if it was a bent valve - and - what would bend valves in all four cylinders? I'm wondering if this guy knows what he's talking about. Sort of glad I never took a bike to him.

Any comments would be much appreciated.
thanks,
baz

[koolaid_kid]

If you have a bent valve the clearance would be way off due to its inability to move easily.

[mike_of_bbg]

The cam chain tensioner push rod spring doesn't need a whole lot of force. It will advance in the rare event that there is little enough push-back to prevent it from doing so. You should be able to push it in without too much effort - but only if you're turning the tensioner knob counter-clockwise (as assembled).

[baz666]

No, the rod pushes in with almost no force whether I turn the tensioner knob counterclockwise or not.

Also, since the valve clearances are all spot on, I think The Koolaid Kid makes sense when he said the valves are not bent but likely not fully closing, indicating a timing issue. I have to get the giant 30mm bolt off the back of the cam chain tensioner today to look at the internal spring - which means going to Crappy Tire (Canadian version of Sears Auto) and buy a 30mm socket. I've ordered the parts required for the rebuild. A week to 10 days I'm told. Looking at the almost warm weather is giving me added impetus.
In the meantime, since I have the carbs off and they look filthy, I'll check the float bowl height and clean out the jets, etc, give it a bath.
I have a container of Seafoam. Is that good for a carb bath?
thanks for all the help, I'd have given up without it.
baz

[tom203]

No, the rod pushes in with almost no force whether I turn the tensioner knob counterclockwise or not.

Also, since the valve clearances are all spot on, I think The Koolaid Kid makes sense when he said the valves are not bent but likely not fully closing, indicating a timing issue. I have to get the giant 30mm bolt off the back of the cam chain tensioner today to look at the internal spring - which means going to Crappy Tire (Canadian version of Sears Auto) and buy a 30mm socket. I've ordered the parts required for the rebuild. A week to 10 days I'm told. Looking at the almost warm weather is giving me added impetus.
In the meantime, since I have the carbs off and they look filthy, I'll check the float bowl height and clean out the jets, etc, give it a bath.
I have a container of Seafoam. Is that good for a carb bath?
thanks for all the help, I'd have given up without it.
baz

The units that I have, you could hold the 30 mm in a vice and tap adjuster body to loosen it- the knurl clears the vice jaws. As was said, you can push the rod in only when you turn knurled knob counterclockwise- release the knob and you can't push it in, but the rod can move out to take up chain slack.
You got the carbs off, you might as well give them the deluxe cleaning while you wait for your adjuster parts.

[Mekanix]

Once your done with the tensioner.

here are a few things you can try.

Since you already have the carbs off, pull all the spark plugs out and disconnect the exhaust or just move it aside.

Put a plug in # 4 and turn the engine over by hand.

When you feel the compression stroke listen to where the hissing is coming from.

At the same time look at your camshaft's . they shouldn't be pushing the valves down at all on the compression stroke.


This will tell you if its the valves,

IF your sure its its not hissing out of the intake or exhaust then open your oil cap and listen there.

If you hear it coming from there, than it narrow's it down the the rings

[baz666]

After disassembling the cam chain tensioner, I test-reassembled it as per the instructions from bwringer.com.
After returning the spring to the knurled knob adjuster assembly, I turned it all the way counterclockwise while holding down the pushrod and its spring, then I used the setscrew to lock the pushrod in place and setscrew bolt to secure the screw.

But, when I put the bolt on the end of the adjust assembly nut and try to tighten it with Locktite, as instructed in Step 12, I find I can turn the adjuster assembly shaft even further counterclockwise quite a ways
.
I haven't been able to get the 30mm bolt loose yet but does the above indicate there is something not functioning properly inside or is it something I'm doing wrong?
thanks for your patience,
baz

[Nessism]

You need to play around with adjustment on the external spring until you find the proper position. Push in the pushrod plunger, set the lock screw, and then start adjusting the external spring. You need to engage the spring with the hole in the large nut and the knurled knob, and then wind up the spring by turning the knurled knob before pushing it onto the shaft with the flats cut in it. Make sure you get nice strong spring force on the pushrod with the adjuster all the way out, and at the same time you don't want coil bind on the external spring when the pushrod is all the way in. Takes some fiddling but you will get it.

Oh, and you have it correct now about checking the cam timing: 1-4 T align, exhaust 1 mark level with the head, and then count the pins between 2-3.