Compression Confirmation

[Jason2tpa]

Can someone please double check what the compression is for my engine (model below). Does 105 sound accurate?

[cpostis]

Can someone please double check what the compression is for my engine (model below). Does 105 sound accurate?

The book I have for a 115 says 136 psi. A 150 HP carbed says 108 psi. No specs for my FI 150. From what I've always heard, it's more important to have close to equal compression within all cylinders within a percentage. however, lower cylinders may have lower readings.

I hope to get some good response on a post I made within your other thread " Compression Test 101".

Let's learn!!!

[Jason2tpa]

Couldn't find the psi in my manual. 106 sounds low, so it's probably 135. I'll search round lil more. Thx bud.

[rodbolt17]

when was the last time your gauge was tested for accuracy?

that is why a lot of specs do not show published data. yet they will state compression should about 10 PSI highest to lowest with the same gauge.
first OMC 225 looper I tested I came up with 80PSI on all 6.
borrowed a gauge and got the same results.
called a friend and he said sounds about right.

[boscoe99]

Does not sound abnormally low.

Compression ration is 6.5/1. If the ambient air pressure is 14.7 then 6.5 times that amount is going to be 95.5 psi. Now what will be seen on a gauge will be a tad higher due to the effects of temperature rise with the heat of compression and the effect of temperature on pressure.

Yamaha does not widely publish compression test values. Which rodnut says are a teat on a boar hog anyway. But here is what is posted for a VZ250 that has a compression ratio of about 6.2/1.

[rodbolt17]

but does ANYONE ever look at the footnotes?

kinda like the OR in 100 hours OR six months whichever occurs FIRST.

that footnote actually says for reference ONLY.
your results may vary.

[boscoe99]

That is why I posted the footnote. Reference only. Does not mean that all will be well at or above 81 psi or that all will be unwell if and when it is below 81 psi.

Just like 0% leak down will not always mean that all is well.

People want (and get hung up on) hard and fast values. One motor runs at 151.3 degrees and the other runs at 151.5 degrees. There is obviously a warrantable defect with the motor that is running too damn hot. Yamaha needs to pay to fix it. Or better still, give me a new motor.

[rodbolt17]

while 0% is really not possible I have seen it a few times.
I attribute it to the calibration of the gauge.
it is simply not calibrated to read small %.

kinda like the port engine trims faster or the stbd engine hits 5500 and the port only 5400.
I get that crap on a weekly basis.

[Jason2tpa]

when was the last time your gauge was tested for accuracy?

that is why a lot of specs do not show published data. yet they will state compression should about 10 PSI highest to lowest with the same gauge.
first OMC 225 looper I tested I came up with 80PSI on all 6.
borrowed a gauge and got the same results.
called a friend and he said sounds about right.

Went to use mine and discovered the needle valve is busted. Went to O'reily's to loan theirs...it too didn't work. Gave up on the comp test for now.

My spec compression ratio is also 6.5:1, as boscoe mentioned. So I guess that's what I'll use to compare, as a baseline so to speak. I understand the 10% rule, but a published baseline psi would be helpful. If all my 4 cylinders are within 10% of each other, yet the highest is 70psi....well I'd say that is poor compression compared to off the production line compression, right?

[boscoe99]

Went to use mine and discovered the needle valve is busted. Went to O'reily's to loan theirs...it too didn't work. Gave up on the comp test for now.

My spec compression ratio is also 6.5:1, as boscoe mentioned. So I guess that's what I'll use to compare, as a baseline so to speak. I understand the 10% rule, but a published baseline psi would be helpful. If all my 4 cylinders are within 10% of each other, yet the highest is 70psi....well I'd say that is poor compression compared to off the production line compression, right?

Who's rule is this? It is not a Yamaha rule.

10 backyardigans doing a compression test might very well have 15 different rules.

[rodbolt17]

typically if I have 1 cyl 10 psi different than the rest I will use my bore scope.

you MUST remember unlike a few other manufactures you compare a Yamaha bank to bank.
then they toss in the old inline 3cyl where the 10 PSI difference was just how they made them.
each cyl was about 5 PSI from the next.

myself, I would not worry about testing compression unless there is a running issue.

[Jason2tpa]

Who's rule is this? It is not a Yamaha rule.

10 backyardigans doing a compression test might very well have 15 different rules.

Good question....I don't know. Seems to be the standard though, albeit who's standard?

But rodbolt makes a good point i think I'll follow....if it ain't broke, why test it.

[TownsendsFJR1300]

Good question....I don't know. Seems to be the standard though, albeit who's standard?

But rodbolt makes a good point i think I'll follow....if it ain't broke, why test it.

Agreed 100%.

If it isn't broke, why fix it?

Often, trying to "fix" (or at least get to) the part that needs checking results in "opening up a can of worms" for no reason. Ie, lemme check a part and then you break off a bolt deep in the block, NOW you have something to fix!!

My neighbors old (1997 I believe) 140 Suzuki 2 stroke has a little bit of water trickling out in between the mid section and lower edge of the engine. It's not filling up the cavity with water, nothing major. It also has an internal noise, probably the lower crank bearing going south. Looking under the hood, lots of corrosion.

He's here a couple times a year, but to fix that engine, I guarantee, will have many broken bolts, the crankshaft a small fortune to send out, etc..

It runs great, does get flushed. $ wise, its not worth fixing. With no issues, it might be worth $500... If he runs it easy(which he does anyway), he'll get a couple more years out of it...

[cpostis]

Agreed 100%.

If it isn't broke, why fix it?

Often, trying to "fix" (or at least get to) the part that needs checking results in "opening up a can of worms" for no reason. Ie, lemme check a part and then you break off a bolt deep in the block, NOW you have something to fix!!

My neighbors old (1997 I believe) 140 Suzuki 2 stroke has a little bit of water trickling out in between the mid section and lower edge of the engine. It's not filling up the cavity with water, nothing major. It also has an internal noise, probably the lower crank bearing going south. Looking under the hood, lots of corrosion.

He's here a couple times a year, but to fix that engine, I guarantee, will have many broken bolts, the crankshaft a small fortune to send out, etc..

It runs great, does get flushed. $ wise, its not worth fixing. With no issues, it might be worth $500... If he runs it easy(which he does anyway), he'll get a couple more years out of it...

Well, another technique to change. I have compression reading records for the last 6 years, I may just keep on doing it to present to a future buyer for a "sales point".

[TownsendsFJR1300]

Well, another technique to change. I have compression reading records for the last 6 years, I may just keep on doing it to present to a future buyer for a "sales point".

My F150 OB, motorcycle have never had a compression tester on them, NEVER..

Now I have used the CT and leak down tester on small engines that had issues and helped me find the problem...

Doesn't take but one bad thing (bad overheat, oil leak, etc), to ruin those 6 years of #'s from the past (certainly doesn't hurt)..

If buying a used engine, I'd want today's #'s

Just my 2cts..