compression.

[muckyal]

anyone know what sort of figures I should expect compression testing a 350 ypvs f2?
ta.

[seahorse83]

above a hundred both sides is good.
Plugs out, throttle wide oped 10 or more kicks each side will do the trick.

[muckyal]

cheers!!!

[muckyal]

my carbs are off so im guessing thats same as throttles open.
exhausts are off too.
obviously cold engine im getting 100-right
105-left.
read on other forums 110-115 is good reading and 100 is when I should be looking at a rebuild.
can someone in the know please confirm this?

[muckyal]

my carbs are off so im guessing thats same as throttles open.
exhausts are off too.
obviously cold engine im getting 100-right
105-left.
read on other forums 110-115 is good reading and 100 is when I should be looking at a rebuild.
can someone in the know please confirm this?

[turbopresh]

glad i stumbled upon this.. been trying to find figures for ages.. after top end rebuild mines at 110psi  both sides not run in or anything.. should i expect this figure to go up once the rings have settled?

[muckyal]

beats me! ive seen figures banded about of 110/115 good.
100 start thinkin of rebuild...
90 starting problems from cold etc
ive got 98/105. rebuild time methinks.
must be someone who knows optimum compression psi?

[flames]

I think you need to put carbs and pipes back on.i don't think you will get a proper reading with them off because of lack of back pressure.
however,with the readings your getting with them removed,i wouldn't think you have much of a problem.someone will be along shortly to confirm of deny this.

[seahorse83]

I can expand a bit on the 100+. did a bit of a search 100 + is still good. 110 seems about average. Seen results on standard bikes on 4th oversize or 65mil pistons at 130psi.
On a past RZYPVS i owned I had 95 and 98, the bike ran crisp, I reduced the squish from 1.7 to .9 and got about another 7 PSI per side. The bike ran crisper.

I have read that if you have a difference of more than 3+ PSI you should investigate ( im not sure about this but I will test my current YPVS next week to check )

[turbopresh]

Sweeet.. good news.. I tested mine with carbs and exhausts etc off.. what your measuring is the compression of the air from above the intake up to the squish band/dome so off or on doesnt really matter.. mine hasnt had squish or anything set yet.. was planning to run her in first and then check and set it...

[steven]

hi
CP= 17-20 x CR.

CP = compression pressure.
CR = compression ratio.

Using that calculation tells us that the compression pressure should be somewhere in the region of 105 - 124 psi (7.1 - 8.4 bar)

Thats for a 4L1-4L0, a YPVS has a very slightly lower compression ratio.

steven.

[muckyal]

whats the 17-20 mean? between 17-20??

[steven]

hi.
17-20 = 17 to 20 times compression ratio.

so.. compression pressure = 17 to 20 times compression ratio.

so ...6.2 x 17 = 105.4 psi
6.2 x 20 = 124.0 psi

so... your theoretical compression pressure should be between 105.4 psi - 124.0 psi.

[muckyal]

ok got that...but obviously 105-124 is a big difference. so what is the 17-20 figure refering to? if I multiply by 20 im all good. at 17 its a lot lower, how do I know which figure to use for my engine or is the formula figure of 17 enough for any engine equation?

[steven]

Hi,
Ehhrrmm...... I interpretit it as 17-20 is the minimum and maximum.
Or the upper and lower limit kind of thing,its only something I read online while trying to find out what the compression pressure should be for my standard 4L0 engine.
So if your compression is inside this "Band" and not more than about 10 percent or so difference
between the two cylinders, all is well with your LC engine and indeed the world,lol.

This is only a theoretical thing, and Im sure none of us are checking these pressures with calibrated pressure gauges.
A "proper" compression test should be carried out with an engine at operating temperature.
As someone posted earlier, If my engine was only showing approx 100 psi, I would be thinking about looking into new rings and or a rebore, but there is probably a good chance it will run for ages just fine like that, but just not be as powerful.
From what I read, most of the LC,s, infact most production 2 strokes leave the factory with wide squish bands, due to the dificulties encountered acheiving production tollerances of engine components.
I checked my squish band, and ended up getting a new/other standard head as I was not happy with the first one.

From what I read also, a slightly wide squish band may not even be too noticeable, and will probably be fine for just normal driving, I also read that getting the squish band spot on makes it go like stink.
I just left mine as it left the factory, its completely standard, and is just over 2mm or so.
Some engines you can buy different thickness base gaskets and head gaskets for setting your squish band, seems to be lots for motox type engines, but I could not find any for LC,s.

I would like to have a go a building an LC engine and setting the sqush band to where it should be and seeing what it was like, the only problem I can see is that the reputable people that machine the head and combustion chambers seem to be very busy and have quite long turnaround times.

This time of year is good to find that ur LC has low compression...gives you all winter to sort it out ready for next year.

steven.

[bare]

Geez.. So much guessing and wayyy too much reading of internet drivel :-).
You will find that almost everyone who does a comp test will generate slightly different number even on the same machine.
Technique or lack of it has influence.. so what :-)
At 100 psi or slightly below start planning a rebore, ~125/135 is a fresh 'standard' engine reading.

[muckyal]

ok. that said, and my figures at 100 and 105 with bores that look great,
no scoring etc....same for pistons and no piston slap. even minimal blow past on top ring at peg only would you suggest a genuine re ring or still recorecommend a rebore.?
dont want to do unnecessary engine work.

[jessy03]

If it was me. if runs well with no noises and starts ok I'd leave it for another year, just my opinion though.
Jess

[muckyal]

starts ok. runs ok midrange? but only does 100mph???

[seahorse83]

Aug 17, 2013 5:26:36 GMT 1 bare said:

Geez.. So much guessing and wayyy too much reading of internet drivel :-).
You will find that almost everyone who does a comp test will generate slightly different number even on the same machine.
Technique or lack of it has influence.. so what :-)
At 100 psi or slightly below start planning a rebore, ~125/135 is a fresh 'standard' engine reading.

+1 with Bare 100 + plus your fine. Below 100 worth a check. 135 hmmmm could be a bit of internet drivel sneaking in there, ive never got above 125 on a fresh bore, cept on the Mito I got 140, but that is standard for them. Either my guage is rooted, my machinist is rooted, my eyes are rooted, or my technique is rooted. Guessiing all 4, but hey dont beleive all you read, it could be a trap.  

[tsa]

For the best and piece of mind below 100 Id re ring it at the least after you have checked for bore glazing and ovality. The accepted norm for a YPVS with the PV open is 110-115 on a fresh engine.

[simon5264]

Mine just rebuilt and i have 120 psi on each after running in hope that helps.

[turbopresh]

Yeah great help mate! Thanks..

[seahorse83]

Aug 17, 2013 11:23:12 GMT 1 simon5264 said:

Mine just rebuilt and i have 120 psi on each after running in hope that helps.

Tested my ypvs today about 1500 kms on a 65 bore, not easy clicks either. Come in at 120 exactly both sides.   

[tygertung]

I just get different thickness gasket paper and make my own gaskets, it's quite easy and then you can easily set up your squish.