Highest Compression Engine

[spudulike]

How about 230psi - I didn't believe it either, engine looks pretty standard on this Jonsered 621! It pulls over ok and the gauge is one I use all the time so I know it reads ok.

Spud

 

[JustinM]

Wow, thats damned high indeed!

I managed to get to an even 200 on my dolmar once (i think its usually around 185) but never over.

 

[chopperfreak2k1]

wow that thing should cut steel!!

 

[spudulike]

Full picture now included - has a massive 13" bar - it doesn't slow down as it cuts
Spud

 

[Kenskip1]

Compression

Sir, not to rain on your charcoal but I suspect that their is oil in the cylinder. Oil as we all know will not compress.I would start it and I think that it may smoke to burn all the excess oil out of the cylinder.Get it good and warm and re-test it. It will go down, Ken

 

[2000ssm6]

Sir, not to rain on your charcoal but I suspect that their is oil in the cylinder. Oil as we all know will not compress.I would start it and I think that it may smoke to burn all the excess oil out of the cylinder.Get it good and warm and re-test it. It will go down, Ken

Yup, some earl in the cylinder will skew results.

 

[procarbine2k1]

Bar oil in the cylinder, my thoughts... I had a saw pushing high numbers once that was most likely lean seized...

 

[Cantdog]

Is there not always oil in the cyl when starting??? Not arguing about if oil will increase compression values, but what is the condition to get accurate compression figures. Absolutely dry just assembled or the way the saw is when used?? I also have a 621 that repeatedly will hit those figures after being pulled until the guage does not go any higher. I also have an 80 that I've pulled to 238. These old Jonsereds are high compression motors. They a have a substantial piston crown (not just a few 1000ths but like 1/8th"-3/16") and two rings. Both of which will increase comp values at rope pulling speeds. (especially compared to modern flat top pistons with just 1 ring). If the rings are new and have close end gaps (0.005-0.006") these numbers are not uncommon with these old motors. Remember these saws were designed to run on real gasoline with extra lead added!! Not pitiful the unleaded, oxygenated, ethanol fuels of today. Comp figures will go down when a saw is warmed up from cold this is known. So what is the standard for getting these figures?? I know how I do it. How do you all?? Hot, cold, just assembled dry or checked with mix in the cyl, throttle wide open, high idle, closed??? Just askin???

 

[Arrowhead]

Is there not always oil in the cyl when starting??? Not arguing about if oil will increase compression values, but what is the condition to get accurate compression figures. Absolutely dry just assembled or the way the saw is when used?? I also have a 621 that repeatedly will hit those figures after being pulled until the guage does not go any higher. I also have an 80 that I've pulled to 238. These old Jonsereds are high compression motors. They a have a substantial piston crown (not just a few 1000ths but like 1/8th"-3/16") and two rings. Both of which will increase comp values at rope pulling speeds. (especially compared to modern flat top pistons with just 1 ring). If the rings are new and have close end gaps (0.005-0.006") these numbers are not uncommon with these old motors. Remember these saws were designed to run on real gasoline with extra lead added!! Not pitiful the unleaded, oxygenated, ethanol fuels of today. Comp figures will go down when a saw is warmed up from cold this is known. So what is the standard for getting these figures?? I know how I do it. How do you all?? Hot, cold, just assembled dry or checked with mix in the cyl, throttle wide open, high idle, closed??? Just askin???

:agree2:

 

[pele55]

high compression

i have worked on quite a few older jonsereds over the years.. i have tested them then ran them and tested them again.. i had a 601 jonsered read 205psi.. it is not uncommon at all in the older saws.

 

[leeha]

Is there not always oil in the cyl when starting??? Not arguing about if oil will increase compression values, but what is the condition to get accurate compression figures. Absolutely dry just assembled or the way the saw is when used?? I also have a 621 that repeatedly will hit those figures after being pulled until the guage does not go any higher. I also have an 80 that I've pulled to 238. These old Jonsereds are high compression motors. They a have a substantial piston crown (not just a few 1000ths but like 1/8th"-3/16") and two rings. Both of which will increase comp values at rope pulling speeds. (especially compared to modern flat top pistons with just 1 ring). If the rings are new and have close end gaps (0.005-0.006") these numbers are not uncommon with these old motors. Remember these saws were designed to run on real gasoline with extra lead added!! Not pitiful the unleaded, oxygenated, ethanol fuels of today. Comp figures will go down when a saw is warmed up from cold this is known. So what is the standard for getting these figures?? I know how I do it. How do you all?? Hot, cold, just assembled dry or checked with mix in the cyl, throttle wide open, high idle, closed??? Just askin???

Yep, Just what he said.
I have a Jonsered 80 that will hit 200 in a few pulls.



Lee

 

[MacLaren]

I have a jonsered 2094 and I have to drop start it.

 

[heimannm]

I don't know about compression as I've never bothered to check, but I will tell you that a 621 Jonsered saw in good condition is one fine running saw.

Mark

 

[bama]

The 621 was the most advanced saw of its time! It still rocks and takes a real man to work with it.

 

[DougMN]

I tested the 621 I bought the other day and it read 150. Maybe it needs a ring?

 

[bama]

I tested the 621 I bought the other day and it read 150. Maybe it needs a ring?

It is a little low for a 621, but if it starts and runs well, then don't worry about it. THat being said, it isn't hard to re-ring it, either.

 

[SkippyKtm]

Could be excess carbon build up in the combustion chamber. I had a Husky 266 with almost 200 psi comp. I pulled the cylinder and scraped all the carbon out of the chamber (and the exhaust port as well) and the compression dropped to the 170's.

 

[Cantdog]

I tested the 621 I bought the other day and it read 150. Maybe it needs a ring?

DougMN if you only have 150 on your 621 perhaps you should replace the other ring too you might get 300 with two new rings........Sorry........
bama you're a hoot too!!!
Skippy, been there done that. Carbon is not the issue. Again you are comparing a modern flat top, single ring piston (266) to a 1970's domed two ring piston, different motor, different ways to an end. 266 will run much less compression than , say a 621, prerhaps 50-60lbs but turns faster and cuts faster, get's the job done faster, must be better...right?? Perhaps....however the 621 may still carry much higher comp figures than the 266 but stiil come in second in the timed cut...a different way to the same end..not trying to argue here but the OP was probably correct in his comp measurements (+ or -couple lbs). As I said I have 5 or 6 Jonsereds that will pull over 200 but my fastest saw is my 61/268XP which pulls around 170 lbs. when tested (by me) under the same conditions

 

[Mr. Bow Saw]

Yeah the Jonsereds do have high compression.
My 80 has about 215 psi, I have a 90 too but have not
checked that one yet.

My Jonsereds 81 has about 256 psi,

 

[bama]

DougMN if you only have 150 on your 621 perhaps you should replace the other ring too you might get 300 with two new rings........Sorry........
bama you're a hoot too!!!
Skippy, been there done that. Carbon is not the issue. Again you are comparing a modern flat top, single ring piston (266) to a 1970's domed two ring piston, different motor, different ways to an end. 266 will run much less compression than , say a 621, prerhaps 50-60lbs but turns faster and cuts faster, get's the job done faster, must be better...right?? Perhaps....however the 621 may still carry much higher comp figures than the 266 but stiil come in second in the timed cut...a different way to the same end..not trying to argue here but the OP was probably correct in his comp measurements (+ or -couple lbs). As I said I have 5 or 6 Jonsereds that will pull over 200 but my fastest saw is my 61/268XP which pulls around 170 lbs. when tested (by me) under the same conditions

You know....if you only replace one ring at a time, think of the cost savings! 50% of the full job!!