XJ900 dead in the water in Switzy-land

Hi all

I'm new to this forum and actually new to Yamahas, having had and worked with Hondas for the last 25 years (just hobby-wise). Beside my current main ride (1985 Honda Shadow VT1100 "Balrog"), I just picked up my next project, a Yamaha XJ900 1983 which has been standing around for at least four years. Condition, as far as I can tell kind of okay, but needs a lot of work to get it to run.

I'm thinking to work in two stages. First stage is learning and trying to get it to run at all. Second stage - provided it's gonna run at all - is to optimise and get it running well and out on the road again. So for the first stage I just want to see if I'm wasting my time or whether it's saveable at all.

The information on this site is great and I've learned so much that I actually feel confident stripping the carbs down and rebuilding them. Which I'll do next.

I have a maybe dumb question, though. I understand the need to ensure proper valve clearance before synching the carbs (once they're clean and rebuilt). But for the first stage, wouldn't checking the compression in each cylinder tell me roughly whether the valve clearance is approximately okay? I'm assuming that if the valve clearance is out of spec, the cylinder in question will not be able to hoget good compression. Or am I thinking totally along the wrong track? In the second stage I'll obviously do it properly. But as I said, I'm only trying to get it to run at all in the first stage and although I don't want to take short cuts, I want to find out soon whether I need to scrap it or whether it's worth doing it properly. If so, I'll take all the time needed.

Cheers, Dani

 

Were talking shims on the XJ, so they would get tight, in 8 times out of 10. That wouldnt affect pressure in a bad way, just other important components.

But for sure, make a compression test, wont hurt, but for accurate readings, on a hot engine.

To check valves, thats a 30 min job, no worrys. And good to do when engine is cold *hint*

 

Ah... Getting the engine hot could be a bit of a problem, as it doesn't run. So short of using a blowtorch... :?

I guess I'll go for the valve spacing and do it right first time. Thanks for the feedback!

Cheers, Dani

 

I don't know who or when it was suggested that Checking Valve Clearances should be done before doing a Compression Test.

That puts the Cart before the Horse.

You want to KNOW what the Compression Numbers are before you need to check anything else.

Just like Human Machine.
When you go to the Doctors Office for a Check-up; you have a seat and the Health Professional checks your Blood Pressure and Pulse.

The 000.01, A#1, 1st Test you need to do to any Bike you are thinking about buying or is your responsibility to get running after having been laid-up is to KNOW what the Compression Numbers are and write them down, ... as a reputable shop does, ... to show you further work isn't a waste of time.

 

It doesn't hurt to check that all the valves have at least SOME clearance, that would give the comp' test some validity.

 

The Compression Test lets you know if the problem is Rings or Valves.

By watching HOW the Cylinder "Pumps-up" and looking at the results of the initial "DRY" Test; a profile of what you are dealing with begins to take shape.

As a rule of thumb, most engines should have 140 to 160 psi.. of cranking compression with no more than 10% difference between any of the cylinders.

Low compression in just one cylinder usually indicates a bad exhaust valve.

Low compression in two adjacent cylinders typically means you have a bad head gasket.
Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

To check compression, all the spark plugs are removed. The Ignitor then disabled or the Plug Wires grounded. The throttle is also held wide open.

The engine is then cranked for a few seconds (using a remote starter switch or a helper) while a compression gauge is held in or screwed into spark plug hole.

The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders. The individual cylinder readings are then compared to see if the results are within specs (always refer to a manual for the exact compression specs for your engine because they do vary from the ballpark figures mentioned earlier).

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little oil into the cylinder through the spark plug hole and repeating the compression test.
The oil temporarily seals the rings.

If the readings are higher the second time around, it means the rings and/or cylinder is worn.

No change in the compression readings tells you the cylinder has a bad valve or to investigate for the possibility of a Valve needing adjustment.