Compression difference

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Gerryjackman

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Ok so I finally got my sled all back together newly plated cylinders new rings, didn't do the pistons as they were ok. I took it for a short ride and did a compression test after this is what I came up with all with throttle open and cold
Cyl1 has 120
Cyl2 140
Cyl3 120

Why would this be I'm at a loss I just spent quite a few dollars did everything correctly as far as I know and I'm still not bang on where I should be, can anyone help me with this I'm at a loss here why would one have that much more And the others not?

Is this something that can still be rode the way it is?
 

Couple questions. What was compression before engine work? The thing that jumps out at me is possibley the 120lb cylinders ring are seating just yet. Or you have a 2000 and up style head on that 140lb. Did you check squish when assemblying?
 
20lbs is a lot of differance between cylinders...I have never had an engine show that much differance after a fresh top end.

Some things to check:
-ensure throttle is held open[KILL SWITCH + KEY OFF]
-check power valves are either all open or all closed
-ensure compression guage is sealing properly on each cylinder
-pull it over at least 5 times each cylinder at the same brisk rate

If you still have a 20lbs differance:
-ensure the cylinder head volume is the same on each cylinder...did you change or exchange a head with a new or used one? cc the cylinders to comfirm they are the same
-is one cylinder ported differently from the others? A higher exhaust port will give a lower cranking compression

If all of the above checks out then I would say you have an issue with the rings on the low cylinder and you may have to tear it down...but that does not seem likely as everything is new.

Let us know..
 
The compression prior to tear down was if I remember right right at 100 maybe 105 across the board I'm not sure what squish is?

I have done all the proper steps to checking it the o ring on the compression gauge seems iffy but it does hold no problem and have no issues in past with it.

I did not change head it is the same one.

The middle cylinder is stamped Y2 and they are new cylinders well refurbished I should say but in mint shape. My old cylinders also had a Y2 stamped on the middle cylinder. I'm not sure how to check cc

The cylinders as far as I was told were not ported or altered in anyway.

Could it be possible that they would have not fully seated yet?
 
Are you using the same compression guage after the rebuild that you used before?

If it were me...I would get my hands on another gauge and repeat test before I did
anything else.

Did you have any trouble during assembly that you could have bent or hooked a ring?

I really think you have to sort out the cylinder head volumes...if you search checking squish it will get you started.
 
Are these the original cylinders replated? Or exchanged? If they where exchanged I would measure port heights on all 3 cylinders. Verify deck height as well to make sure the cylinder hasn't been cut.
 
I did not change head it is the same one.

Are you running an aftermarket (peak..etc) one peice head? Or do you have 3 individual heads? Search "squish"
 
I searched squish think I can do it. I just don't understand why "squish" would be any different. As far as measuring port and what not is this possible with it all back together?
What should I be trying to achieve getting middle cylinder down to others or bringing outside cylinders up to where middle is. I have the stock heads 3 individual joined by the coolant bar( bad terminology I know sorry)
 
Put some miles on the engine like about 20-50 miles. Then I would do a leakdown test to actually check the piston ring sealing. Do a squish test now as it is easy enough.
 
I just reviewed your post and realized only the one cylinder is 140 and two are 120...I initially thought it was the other way around.

120 - 130 is pretty normal for a post overhaul compression, depending on the guage, 140 is high for a stock engine.

You don't have a cylinder thats too low you have one that is higher then normal. The opposite of what I initially thought.

Like most other people, I still think you have a different cylinder head volume, deck height, or porting spec on the high cylinder.

If the head has been cut or is off of the 2000 - 2002 SRXs it will have a tighter squish measurement then the other two.

Are you running all stock pistons or at least same brand in all cylinders?

If you decide to run it the way it is you may need to jet that centre cylinder richer on the main and needles so it won't detonate.
 
Yeah all the pistons as far as I know are stock yamaha I didn't change them out they were ok I can check the squish tonight. I have to go buy a verneer caliper is that what I should be or is there a better measuring tool?

The casting numbers are 8dn00 two low cylinders are stamped y1 an the middle is still 8dn00 but stamped y2
 
A digital verneer is the best way to measure the solder after a squish test.

Since you picked up your SRX used and it was jetted for Mountain riding and it is 15 years old with probably numerous owners...it probably has been appart +/or modified in the past and you now have mixed + matched parts. I would bet you have one cylinder that has a head on it that has been cut for more compression or you have three heads that have been cut on two ported cylinders and one stock cylinder...either way you should sort it out and try to match them up.

A lot of mountain sleds run higher compression heads to keep the power levels up at the higher altitudes so it would not be unlikely that one or more of your heads have been modified.
 
Ok I understand what your saying but my pre tear down compression numbers were dead even across the board and since I used the same heads and swapped cylinders I'm gonna guess its in the cylinders where do I start to look and what do I do.

Can you give me in your opinion the best way and things needed to check squish? I think I have it just want to be sure on sizes more input the better!

Do you think I should tear it down or run it as is for a short while and see if anything changes?
 
Remember also these are refurb cyls that I ordered from chrome USA they were suppose to be all stock no porting no shaving etc. maybe ill give them a call back and make sure!
 
K I went down to the auto store they have 3/32" which is 0.094 sder but its hollow with rosin core I guess hard to find lead solder here will this work?
 
Gerryjackman said:
K I went down to the auto store they have 3/32" which is 0.094 sder but its hollow with rosin core I guess hard to find lead solder here will this work?
Click to expand...

If thats all you can get it will do.
-Keep the solder peace you are using long enough that it won't fall into cylinder.
-Have the piston near the top when you insert the solder[about an inch or so down, just low enough to get solder to the edge without interfering with the piston]
-Shape the solder like an "L" and have the bottom peace just long enough to reach from the centre of the spark plug hole to the inside edge of the cylinder parallel to crankshaft.
-Turn the piston through top dead centre by hand on the clutch...you will feel the piston resisting as it squishes the solder.
-Only turn over enough to squish then free the solder then remove it and measure the squished part...don't reuse the same solder but use a new peace for each cylinder.

It will make pretty good sense when you start to do it...you are only really interested in the measurement at the outside edge as this will be the minimum clearance between the piston and head.
 
Last edited:
Bob I agree with your procedure but suggest the squish band be checked at the outside edge as well as about 8 mm's in from the outer edge.
I have seen many squish band angles changed when modding heads.
Ideally I would prefer to know the overall volume of the head rather than the squish.
 
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