Low Compression Update 9/21/26

[750turboguy]

Hey hey hey!
Whoever recommended opening the throttle for the compression test was right on! Thanx!

First, the reason number 4 was so good was because the nipple port on the fuel rail was uncapped and air was able to get in. This is the port I use for the fuel regulator.

After uncapping 1, 2 and 3...AND holding the throttle open...
1 is now 90 psi
2 is now 90 psi
3 is now 90 psi
4 is still up near 120 psi... 118 to be exact and that still concerns me.

Any input appreciated....


Okay guys...doing the rebuild project but I'm posting this here so more eyes can see this post.

Did a compression test today and...big disappointment...

Cyl-1 90 psi
Cyl-2 60 psi
Cyl-3 60 psi
Cyl-4 120 psi

Spent a little time thinking about my assembly process and checking my cam timing. Everything looks okay but
obviously something is up/leaking...

Anybody??????

[Würstchens-Turbo 810]

Do you know what a cylinder leak-down test is?
If you have the chance, I would do that first.
Then you'll know why the pressure is missing and where it's escaping.

[riturbo]

Did you check the valves. Are they adjusted correctly. Did you do any head work/checking while you had it apart.

How long have you run it ?

[Kawadoodle]

Try taking all the plugs out and redo compression test again on each cylinder in turn with throttle fully open. If reading still low, repeat with a teaspoon of engine oil in each. If the readings show significant improvement, the rings are leaking. If no improvement, it's the valves leaking. Leaking valves are possibly bad seats or insufficient gap.

[Kawadoodle]

That's a teaspoon of oil down each spark plug hole, but only one at a time and cover those you've done with a cloth before moving onto the next, to avoid residual oil spraying out. Temporarily the oil seals the rings.

[750turboguy]

I will try the oil trick. Apparently some of you haven't been following my rebuild videos in PROJECTS. follow the links to my Youtube videos.

What I really want to do is remove the cams and crank the engine with the valves all closed. Only problem is, supporting the cam chain as the engine turns. If I can do that, then I can at least rule out the cam chain timing. I'm pretty sure it's right (cam timing) but you never know.

[rangeroy]

cam timing fine (just looked again on youtube vid you posted) use kawadoodle oil trick or leak down tester as Horsh said.
good luck

[Kawadoodle]

Don't try turning it over without cams, it won't achieve anything and you'll run the risk of snagging the chain around the crank and busting stuff up. Anyway with the valves closed throughout cycle you won't get any air into the cylinder, so you're probably more likely to end up pulling a vacuum on the down stroke and have a low pressure reading on the up stroke which won't tell you much.
The oil compression test is quickest to rule out piston ring seal. And don't forget to ensure the inlets are not restricted, the butterflies in the intakes must be fully open. If compression is still low recheck the valve clearances to be sure there is the correct gap off cam. If all okay then recheck the timing. Note that the cams have two sets of timing marks, one is for non turbo engine and the other for turbo if I recall. You must use the markings as shown in the turbo supplement manual. If you took the sprockets off the cams, check they're reassembled correctly. Check and check again. If still unresolved try leak down test. But if it was me I'd probably lift the head, put plugs in and leak test the valves by filling the domed chambers with paraffin and looking to see if it drains freely out the ports. If it does then you have a valve sealing problem.

[750turboguy]

WOW...Great advice. Thanks Rangeroy for checking the video. You saw where Kawi mounted the sprocket wrong on the exhaust cam...assholes. They must have used the ZX7 mark at the factory.

Yeah snagging the cam chain is/was my biggest worry. I don't have a leak down test kit but I can get one. As for opening the throttles....I never thought of that. I watched several videos on Youtube of guys doing motorcycle compression tests and not ONE of them said anything about having the throttles open. That will be the next thing I do, and the oil trick.

Stay tuned.....the weather is getting crappy here. not may more days to work on the bike. Hate to put it off till next spring but now comes the time for fall projects and they DON'T include motorcycles.

[rangeroy]

As kawadoodle says throttle bodies fully open and I bet your problem will be solved

[Würstchens-Turbo 810]

As kawadoodle says throttle bodies fully open and I bet your problem will be solved

Well, it’s definitely getting better, but it probably won’t resolve the stark differences between them.

do you have a link to your videos ?

[JezZedexE1]

do you have a link to your videos ?

viewtopic.php?p=193569#p193569

A variety of links in this post.

[Würstchens-Turbo 810]

do you have a link to your videos ?

viewtopic.php?p=193569#p193569

A variety of links in this post.

thx

[Kawadoodle]

Hey,
It looks as though you had some success but gremlins are back. Where are you at now?

[750turboguy]

I'm not going to worry about the compression since it's all in spec now. Cyl 1 2 and 3 are at the bottom with 90psi and 4 is at 118psi. NOt perfect but doable.

My new focus is getting the valve clearances and making any adjustments there...if I can still get shims.

After the valves I'll redo the compression and look for improvements.

But...we just entered our rainy season so it's cold and wet right now. Might have to wait until spring.