# Throttle position on compression test?



## brages (Oct 25, 2010)

I tested the compression in my 026 the other day. Pulled and pulled but I only got it up to 105-psi. Strange, because the saw starts easy and runs great. Now I see that you're supposed to open the throttle when you do the test.


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## FATGUY (Oct 25, 2010)

guess I'm an idiot but I've yet to see how having the throttle open closed or any other position will affect compression.....


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## Paul001 (Oct 25, 2010)

FATGUY said:


> guess I'm an idiot but I've yet to see how having the throttle open closed or any other position will affect compression.....



I'm still pondering that as well...


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## brages (Oct 25, 2010)

Uh-oh, maybe I'm not an idiot and my saw is just worn out... if I am an idiot, at least I'm in good company. 


If the throttle is closed, the cylinder fills with less than a full charge of air -- that's what the throttle is there for. So there's a vacuum (i.e. lower absolute pressure) in the cylinder. Then the piston compresses the vacuum, so the peak pressure in the cylinder is less than it would be if the cylinder had been allowed to draw in its full volume.

Thermodynamics: P1*V1/T1 = P2*V2/T2 (???) With a closed throttle, P1 is slightly less, so P2 will be less as well ???


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## Diesel Pro (Oct 25, 2010)

The "proper" way has always been with the throttle wide open to be sure that the intake gets it's proper "gulp" of air. That said, I have seen very little if any difference in the overall outcome, but I still clip it wide open.


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## timberwolf (Oct 25, 2010)

{Edit... LOL you beat me to it Diesel}

If the throttle plate is closed then the base does not fill as well and less is trasfered up the transfers to fill the cylinder and get compressed.

I never found it made much if any measurable difference though.


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## pioneerguy600 (Oct 25, 2010)

Never found it necessary at all but have heard the theory that small engines displace so little air through each stroke that it takes too long to fill the comp tester hose up to the gauge. All the saws I test seems to reach max compression around 5 pulls over with the throttle closed.Might the max compression be reached with 4 pulls with the throttle open? I don`t know as I have never done it.

Pioneerguy600


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## JustinM (Oct 25, 2010)

Diesel Pro said:


> The "proper" way has always been with the throttle wide open to be sure that the intake gets it's proper "gulp" of air. That said, I have seen very little if any difference in the overall outcome, but I still clip it wide open.



Exactly.

I know the "rule" is supposed to be "open" but Ive rarely see it make any difference in a test.


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## drkptt (Oct 25, 2010)

brages said:


> Uh-oh, maybe I'm not an idiot and my saw is just worn out... if I am an idiot, at least I'm in good company.
> 
> 
> If the throttle is closed, the cylinder fills with less than a full charge of air -- that's what the throttle is there for. So there's a vacuum (i.e. lower absolute pressure) in the cylinder. Then the piston compresses the vacuum, so the peak pressure in the cylinder is less than it would be if the cylinder had been allowed to draw in its full volume.
> ...



Since you used the wording in your poll, then yes you are an idiot. A 2-stroke will backflow through the exhaust port--there can't be vacuum in the cylinder. An accurate compression test can be done with the throttle closed.


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## brages (Oct 25, 2010)

I'll have to try it with the throttle open and see if it "helps"...


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## Paul001 (Oct 25, 2010)

The theory makes sense if we were talking a sealed area, we are not. The muffler and intake areas might be restricted, but they are not sealed.

As pointed out, it* might *be the difference between 4 pulls and 5...still doubt that.


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## timberwolf (Oct 25, 2010)

Does you gauge have a check valve (shreder Valve)? if not it will read low esp with smaller engines.

If it does then check that the valve is tight, o-ring good and gauge well screwed into plug hole then try it on another saw just to make sure it is reading right.

The if you are still getting 105psi your saw does have a problem and will run better if it is fixed, even if you think it is running ok now it will start and run better if compression is closer to 160psi. I like to see closer to 200psi on my small saws but thats just me.


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## blsnelling (Oct 25, 2010)

brages said:


> I'll have to try it with the throttle open and see if it "helps"...



It's not going to make "that much" difference. You have a problem some where else.


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## Diesel Pro (Oct 25, 2010)

brages said:


> I tested the compression in my 026 the other day. Pulled and pulled but I only got it up to 105-psi. Strange, because the saw starts easy and runs great. Now I see that you're supposed to open the throttle when you do the test.





blsnelling said:


> It's not going to make "that much" difference. You have a problem some where else.




Like Brad says it will not make a huge difference. My best suggestion is to:

A) Find a different gauge to test with

B) find another known good saw and do a comparison test.

The numbers can vary quite widely from gauge to gauge. The adaptor itself, the rubber sealing washer, the check valve condition, etc are all factors.

My gauge has a pretty thick rubber washer so if I screw it in way tight I can raise the effective CR by decreasing chamber volume. The key here is consistency and reference to a known baseline more than actual numbers.


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## redunshee (Oct 25, 2010)

blsnelling said:


> It's not going to make "that much" difference. You have a problem some where else.



Agree. I've never seen a big diff. between closed or open throttle. My 260 has 150 psi with less than 2 tanks through it. Might be time for rings.


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## blsnelling (Oct 25, 2010)

Stephen C. said:


> I get a 20 to 30 pound difference.



Are you say opening the throttle gives you that much difference?


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## nmurph (Oct 25, 2010)

i have tried it both ways more than once, and i found that it makes zero difference. there is more variation in how hard i pull the starter and how many times it takes to reach the peak reading, than there is with whether or not the throttle plate is open.


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## drkptt (Oct 25, 2010)

Stephen C. said:


> I think you will find that your results will be much higher with the throttle in the open position. Perhaps not so much if the crankcase seals are marginal.
> 
> As the piston moves up to compress the charge it puts the crankcase under negative pressure. Air has to find a way in there if is going to move up the transfer ports on the downstroke. The only place it can come from is the intake tract or the crank seals. If you starve the engine for air you can't get accurate results.



No. The engine is rotating very slowly when rope pulled relative to running. While the transfers are open air flows through the exhaust port. There is enough time for everything--crankcase, transfers, and cylinder--to reach atmospheric pressure before the exhaust port closes and compression starts.


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## Slamm (Oct 25, 2010)

I have tested several of my saws and other than it might have taken one more stroke to get to max compression, I couldn't see where it made any difference on the overall Max Compression, and I tried it several different ways on several different saws in a 20 minute time frame with the same compression tester.

Thats what I found,

Sam


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## Boskaerm (Oct 25, 2010)

In my heard the logic says: the saw rotates so slowly when you pull the rope, so it can get more then enough air! 
If it couldn`t get enough air an the closed position, then how should it idle?? 

Just my logic, could be wrong!
On a 4 stroke you also test with closed throttle.


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## lesorubcheek (Oct 25, 2010)

drkptt said:


> No. The engine is rotating very slowly when rope pulled relative to running. While the transfers are open air flows through the exhaust port. There is enough time for everything--crankcase, transfers, and cylinder--to reach atmospheric pressure before the exhaust port closes and compression starts.



Perfect sense here. Very well said. If anyone doubts the air pressure equalizing throught the exhaust, consider what happens when the engine is running. A much higher pressure exhaust gas is capable of exiting the exhaust port at very high RPMs and allowing the inlet mixture to enter the cylinder when the intake ports opens. If the pressure couldn't equalize to near atmospheric as the exhaust gasses escaped, no inlet charge could enter the cylinder. Now slow that down to cranking RPM and take out the combustion. The cylinder is gonna be immeasurably close to atmospheric pressure with just the exhaust open. That is unless the muffler is plugged up.

Dan


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## splitpost (Oct 25, 2010)

I always have the throttle set to fully open but thats just the way i was tought how when doing my trade , 
engine at operating temp,throttle wide open and crank each cylinder about 6 times on the starter to get a reading ,
Reason for WOT is to lessen the drain on the battery and starter motor and to get a cylinder full of air to compress,chainsaws might be a different kettle of fish but thats how i was tought


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## Andyshine77 (Oct 25, 2010)

If a saw can run at idle, you can check compression at idle.:monkey:


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## Diesel Pro (Oct 25, 2010)

splitpost said:


> I always have the throttle set to fully open but thats just the way i was tought how when doing my trade ,
> engine at operating temp,throttle wide open and crank each cylinder about 6 times on the starter to get a reading ,
> Reason for WOT is to lessen the drain on the battery and starter motor and to get a cylinder full of air to compress,chainsaws might be a different kettle of fish but thats how i was tought



Same here. Comes from a more automotive or mechanical background I guess.

Saws confuse me as to why the psi lowers. Ordinarily one would think that as temps come up and piston expands it rocks less and ring seal improves. I guess the cylinder grows and causes psi to drop.

Open throttle pulls easier and carries on longer in my experience for faster cranking speed on the test.


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## Fish (Oct 25, 2010)

My suggestion:

Remove all Idiots from the forum...........


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## brages (Oct 28, 2010)

*Update:* I tested my "known good" MS210 -- also 105-psi, throttle open or closed. :monkey:

I'm thinking this gauge is outta whack. Yes, it has a Schrader at the tip.


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## nmurph (Oct 28, 2010)

i checked a 7900 last night. i pulled until it maxed out at 170 with throttle closed. thinking of this thread, i then opened the throttle and pulled some more. guess what???? it didn't change at all. i have done this test bf with the same result.


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## Trigger-Time (Oct 28, 2010)

brages said:


> *Update:* I tested my "known good" MS210 -- also 105-psi, throttle open or closed. :monkey:
> 
> I'm thinking this gauge is outta whack. Yes, it has a Schrader at the tip.




Make sure the schrader it tight, I have had them to come loose and give
low readings. I have also had to replace one schrader as it went bad.



TT


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## blsnelling (Oct 28, 2010)

nmurph said:


> i checked a 7900 last night. i pulled until it maxed out at 170 with throttle closed. thinking of this thread, i then opened the throttle and pulled some more. guess what???? it didn't change at all. i have done this test bf with the same result.



Same here. Even without pulling the throttle at all, you're still getting air, just not as much. I usually set the saw to the fast idle position.


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## Winn R (Oct 28, 2010)

Well I don't know 'bout ya'lls motors, but the last thing to close on mine is the exhaust.

Ain't no way to shut it off so it just lets that air in there and don't care 'bout no throttle. But I'm just an idiot so what do I know?


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## Trigger-Time (Oct 28, 2010)

I always check with switch and throttle off.

Only way I can see that having throttle on would help
is maybe a one ring piston that is worn. As were the
extra mix in chamber would help seal ring better.



TT


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## mikerecike (Feb 27, 2014)

Looking at this from another angle....I have a husky 55 with low compression around 92psi which will only start if you hold throttle open as you pull recoil. Is this due to letting extra air in ? Creating better compression for a start? What does this tell you diagnostically about a saw or any garden kit that needs throttle open to start? - ps i do know how to tune a carb so that's not this issue.


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## nmurph (Feb 27, 2014)

mikerecike said:


> Looking at this from another angle....I have a husky 55 with low compression around 92psi which will only start if you hold throttle open as you pull recoil. Is this due to letting extra air in ? Creating better compression for a start? What does this tell you diagnostically about a saw or any garden kit that needs throttle open to start? - ps i do know how to tune a carb so that's not this issue.



It tells me it's time for a rebuild.


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## mikerecike (Feb 27, 2014)

ok agreed - to help me understand what's going on is it that more air will increase the compression and that increases the chances of ignition in a worn engine? cheers for your help


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## nmurph (Feb 27, 2014)

It could be that your saw's compression is right at the margin of what is required to run. An open throttle makes VERY little difference in a compression reading. It might take one more pull with the throttle closed vs open to reach the max number. That tiny fraction might be what allows your saw to start with one throttle position but not the other.


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## mikerecike (Feb 27, 2014)

thanks murph


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## Ozhoo (Feb 27, 2014)

Someone who cares more than me needs to block off the exhaust port and retest just to prove a point.


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## Matt81 (Feb 27, 2014)

Well i just went out and tried it....

Pulled around 5-6 pulls with throttle closed until the gauge won't go any higher.

Then opened the throttle and pulled some more.... same reading no difference at all on the gauge. 

Tried the 7900, 365XT, 5105, 346xp and the old macs.... same result.


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