Leak Down Test on GS1000ST

[terry]

OK, after replacing a burned out exhaust valve in my 1981 GS1000ST I put the engine back together, and since then my two compression testers are both reading only 80-90 psi right across all four cylinders. Before I tore it down, apart from the cylinder with the burned out valve, the other cylinders were returning a compression reading of 135-140 psi.

Martin (Brit 7.11) suggested a "leak down test", so I built a leak down tester using some (new old stock) air fork kits I had in a box for the air valve and guage. Anyway, the thing works, I used an old S&W syringe type pump to pump in 30 psi, and after 5 minutes there was no loss of pressure at all, but after an hour it was down to 15 psi.

I think Martin suggested to listen to where the air is escaping, but I couldn't hear anything, and I guess the leak is that slow that I wouldn't anyway. I tried brushing a little liquid detergent around the head gasket joint to watch for bubbles, but couldn't see any. Does anyone know where I should go from here? I tried re-torqueing the head down, but it made no difference at all, so I guess the head has to come back off, so I can pay more attention to the joint faces, boo hoo.

If you'd like to see the leak down tester I built, email me at sherriffbuck@yahoo.com and I'll send you some pics.

[Hap Call]

Reconfirm your cam timing. Did you by chance pull the cylinders?

Hap

[Anonymous]

Hi Terry,

The 30psi to 15psi in an hour thing, I wouldn't worry about, as the cylinders aren't designed to hold pressure for longer than a few milliseconds.

However, have you run the engine since the build? Most gaskets won't seal well until they have been heated. Also rings and valve seats will need to settle themselves again. Also re-torque the head when cold.

When I did my leakdown test recently I used 100 psi and it fell to 0 within a few seconds due to a bent valve.

Good Luck!

[Anonymous]

It's a bit strange that the low readings are even across four cylinders. A leaking gasket will normally effect one cylinder but not all of them. The same thing goes for valve seating.

Because of the even levels I'd go for valve timing as any difference will be common over all cylinders. Check your cam chain settings again.

It is easy to set the arrows up correctly (or what seems to be correct) only to find that when the cam caps are tightened and the tensioner is installed, the cams are retarded.

Another thing that can happen is that you set them up to the firing mark instead of the TDC mark on the timing disc. Check that you are looking at the correct mark through that little hole.

These are assuming that you have set the cam arrows correctly. My GS100 manual lists two cam sets - but I forget which bike you have. On the GS1000 the arrows are 19 pins apart on cams numbered 451 (intake) and 49S (exhaust) and 20 pins apart on the cams numbered 490 (both cams have the same number).

Check these before you take the head off - it might not be necessary.

Kim

[cbxchris]

I agree you have the cam timing wrong....hope you haven`t bent a valve or two. APE has a very inexpensive leak down tester with several adapters for different bikes. On it you use 80psi....a good drag engine will test out around 1-2% and a street bike may even test up tp 10-12%. A hot engine will usually have less leadown. On doing the compression test you did open the carbs up when doing it? A weak battery that turns slow will affect things also.

[terry]

Okay guys, but buggar! I've already ordered a new head gasket and cam cover gasket! Anyway, I'll take your advice and try the valve timing again, (that'd explain why it's apparently not leaking oil or air around the gasket area) the timing did look okay when I put it all back together, and once I'd set the timing, torqued the cam caps down, refitted the tensioner etc, I wound the crank around by hand a couple of times and it still looked ok.

BUT I didn't install the same cams, I fitted the "ST" cams that I took out of my other engine when I fitted the yoshi cams as there was a bit of wear on the journal surface on the original ET exhaust cam in this engine, plus I wanted that little bit extra from the (slightly) higher intake cam lift (okay Martin? ha ha) so maybe I should have set the cams 19 pins apart and not 20? Trouble is, I'm working off either a factory manual or a clymer, but both are dated 1979, and this particular engine is a 1980 (big port) with 1981 ST cams, so maybe I've missed something, like those different cam sprocket numbers? Thanks again fellers, I appreciate your help.

Today at work I was talking to a "Bush Mechanic" about the leak down tester, and he said another thing you can use one for is holding pressure on your valves so you can change your valve springs without pulling the head off? Sounds like a neat trick to me if your cylinders will hold the pressure for a few minutes, but I'd be interested to learn how to compress the springs to refit the collets without disturbing the valves? Any thoughts? :twisted: