# What PSI is "good compression"



## rwoods (Mar 14, 2010)

I purchased a ProMac 8200 off eBay. It does not have a compression release and it cranks very easily. In fact you can turn the flywheel with your hand and you can barely feel the compression. On the other hand, I have a ProMac 700 and it is very difficult to pull the starter cord unless you use the decompression valve. You can turn the flywheel on the PM 700 with your hand but it will move the saw and it builds up a lot of resistance way before top dead center. It feels like it has more compression with the valve open than the PM 8200 has. However, using the same new chain on a new 28" sprocket bar in 36" red oak the PM 8200 outsaws the PM 700 maybe 20% faster (just made 2 cuts first one with the PM 8200 then one with the PM 700 - so the PM 8200 had the advantage of the freshest chain but that shouldn't be too much of an advantage; and yes the PM 700 seemed to be running right). I thought the 12cc difference would be effectively cancelled out by the huge compression difference. Long introduction to how much psi equals "good compression" as I see these terms tossed about on this site? If I can find a gauge to borrow, I'll measure the two. Replies appreciated.:newbie:


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## rmh3481 (Mar 14, 2010)

150 psi or better is considered 'good'. Of course the greater the psi, the bigger the pop up to a point. Maybe you need a set of piston rings? Have you removed the muffler and inspected the piston and cylinder?

Bob


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## rwoods (Mar 14, 2010)

Haven't removed muffler yet as it appears I have to dismantle the bottom frame to remove it. Here are the results of my compression tests (bearing in mind the guage is borrowed, is of unknown quality and probably not designed for low readings*):

PM 8200 - a hair over 10 psi (yes, ten)
PM 700 DSP open - 70 psi
PM 700 DSP closed - 130 psi (it is very difficult to turn over with the recoil start; actually pulled it sideways and whacked my knee pretty good. I've own this saw since new - January 1982 - and it has always been this way.)

How does the PM 8200 cut with such low compression? Would rings alone account for this? I thought about putting some motor oil in the cylinder and see if that made any difference - would such a test be worth the trouble in trying to decide whether or not to spend any more money on this saw? 

Thanks,

Ron  :help:

* I checked the guage against two other air pressure dial guages; tied all three together to an air compressor. The test guage read about two pounds low until it stuck at 90 psi. I took the pressure up to 100 psi and it shook loose but was about 5 psi low.


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## sloch24 (Mar 14, 2010)

A saw won't run with 10 PSI compression... Check to make sure that your compression gauge has a schrader valve in the tip. A standard automotive compression tester will not give an accurate reading on a small engine.

You're not getting accurate readings with that gauge.

I'm sure others will chime in shortly.


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## barton174 (Mar 14, 2010)

rwoods said:


> Haven't removed muffler yet as it appears I have to dismantle the bottom frame to remove it. Here are the results of my compression tests (bearing in mind the guage is borrowed, is of unknown quality and probably not designed for low readings*):
> 
> PM 8200 - a hair over 10 psi (yes, ten)
> PM 700 DSP open - 70 psi
> ...



How many cycles did you run it through? Probably take at least 10 to get the pressure in the line and gauge up enough to even out...

ETA: It does have a check valve of some kind, right? So it holds pressure after it's unplugged or you stop cranking?
Mike


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## rwoods (Mar 14, 2010)

The guage does have a check valve so it will hold the reading. I'll take it to a saw shop this week and let them check the compression. Is there a rule of thumb as when you should scrap a chrome cylinder saw? Thanks, Ron


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## thomas72 (Mar 14, 2010)

It really depends on the saw. The McCulloch sp125 is just about at its best at 175 psi / depending on which type combustion chamber you have. The Poulan 71A is just about at its best at 135psi and will still run on 90psi. Compression is relative to a lot of factors to that particular engine. I have a promac 800 and it has 180psi. I really do not care about compression too much if I feel the saw is making the power needed to get the job done.


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## rwoods (Mar 14, 2010)

thomas72 said:


> It really depends on the saw. The McCulloch sp125 is just about at its best at 175 psi / depending on which type combustion chamber you have. The Poulan 71A is just about at its best at 135psi and will still run on 90psi. Compression is relative to a lot of factors to that particular engine. I have a promac 800 and it has 180psi. I really do not care about compression too much if I feel the saw is making the power needed to get the job done.



I have been led to believe that the PM 800 and the PM 8200 are essentially the same except for a three piece crankshaft in the PM 8200. So I expected 175 to 180 psi. It seems in the Mac literature some 800's had DSP and some didn't; this is also consistence with what I have encounted while looking for a "new" saw. Since my PM700 is almost impossible to hold down and crank without opening the DSP, I kept wondering during my search for a more powerful MAC how in the world was I going to crank an 800 even given the foot pad. I have even checked out the parts manual for a hidden automatic compression release but I don't see one (nor have I ever heard of one on a MAC).

I bought this rough looking PM 8200 after just missing some really good looking saws in the last two months: a 800, a 805 and a 850 - I guess I am just too cheap.

Any advice or sources for a better MAC would be appreciated. I'm not interested in a MAC without a chain brake. I still have by grandfather's old gear driven MAC bow saw which send me to the hospital 30 years ago as a result of me showing off with it and I haven't run it since. It could have decapitated me but I hung on to it and it went over my head. The efficiency of a bow still tempts me, but I just look at the old saw and I get over it.

Ron


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## thomas72 (Mar 14, 2010)

All I can say is look at your local saw shops. I found my 800 at a saw shop 4 miles from my house and paid $10.00 for it. I am like you, I do not spend a lot of money on something unless I can get a worth while profit off of it.


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## rwoods (Mar 14, 2010)

barton174 said:


> How many cycles did you run it through? Probably take at least 10 to get the pressure in the line and gauge up enough to even out...
> 
> ETA: It does have a check valve of some kind, right? So it holds pressure after it's unplugged or you stop cranking?
> Mike



Okay, I put the saw's recoil starter back on the PM 8200 and gave it about 15 pulls in a row - gauge reads 131 psi. I repeated this several times with same results. So I hooked the PM 700 to the gauge and tried several times to replicate but couldn't get a good rhythm going as it is too hard to hold down and pull; nevertheless I got my previous readings of 130 psi every time. Since they both read the same, that would explain the performance differential. But what does it mean, if any thing, that I have to build compression with the PM 8200 while the PM 700 appears to go to full compression immediately? Should I be looking at a rebuild, a new saw or just finish replacing the missing pieces and use it as is?  Thanks, Ron


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## Diesel Pro (Mar 14, 2010)

Gauges will vary based on the style of the end, etc. I consider my Snap On Motorcycle compression gauge kit to be of very high quality yet it reads lower than some I have seen here.

All cold with throttle wide open:

372XP (new just 2 tanks through) 145 psi
359 2002 model low hours excellent condition 145 psi
Echo 351 moderate hours excellent condition 145 psi
Homelite Super 2 moderate hours runs quite nice (low end quality saw) 125 psi


I've seen a 359 with much higher psi and feel that it is due to an "easy" gauge or difference in the adapter. I have a Milton gauge as well and plan to try it soon for comparison sake.


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## rwoods (Mar 14, 2010)

Diesel Pro said:


> Gauges will vary based on the style of the end, etc. I consider my Snap On Motorcycle compression gauge kit to be of very high quality yet it reads lower than some I have seen here.
> 
> All cold with throttle wide open:
> 
> ...



Does this mean I need to test it again with the throttle open? 

I have run saws for years but other than bolt-on stuff I have always relied on saw shops. No one wants to work on my Macs any more (they are older than most of the mechanics) so I am trying to learn to fix them myself and thought what the heck if I have to do it why not buy an old saw and fix it - thus the PM 8200 (not really an old saw as it will only be 18 this year). But as you can see I have gotten over by head real quick. Help and advice is welcome as I am still wondering whether I should just look for another saw instead of spending a lot of time and money rounding up parts for an obsolete saw that may be worn out. I love the sound of Mac's, the fond memories of my dad's first big Mac that he used to cut pulp wood, the American innovation, the looks you get from other cutters when you fire one up and let her rip, etc etc. but warm and fuzzy emotions* don't get the job done. Ron

* Mac's that won't start also stir some other pretty strong emotions for me, as well as they did for my Dad (died at age 81), who switched to Husky's in his later years. I have switched for light work to an 036 Pro and can't bring myself to spring for a big new Stihl for felling and bucking the bigger stuff. Given the abundance of replacement plastic parts that can make a saw look new, I am leery of buying a used modern saw as I don't know enough to know whether or not the mechanicals are any good as evidenced by my questions here.


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## Diesel Pro (Mar 14, 2010)

I don't think it is necessary to open the, just an old rule of thumb. Should get your peak number more quickly.

A better test for ring seal would be a cylinder leakdown, but you'd have to lock the crank from turning.


FYI My Harley is 190-195 psi if memory serves reduced from 205-210 by running larger cams to tame it down.

Most of my experience is 4 stroke stuff...


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## rwoods (Mar 14, 2010)

Diesel Pro said:


> I don't think it is necessary to open the, just an old rule of thumb. Should get your peak number more quickly.
> 
> A better test for ring seal would be a cylinder leakdown, but you'd have to lock the crank from turning.
> 
> ...



Thanks. I may rig some thing up next week and try a leak down test.

To me Macs are to chainsaws as Harleys are to motorcycles as radial engines are to airplanes - nothing else sounds right. Too bad the employees didn't buy McCulloch.

Ron


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## barton174 (Mar 15, 2010)

The pressure should be higher with an open throttle... or at the very least, go up faster... Should be higher, though...

Mike


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## nmurph (Mar 15, 2010)

is the valve at the tip of the hose?

i have a near-new PM700 and it only registers 130.

opening the throttle will not have any bearing on compression. it might make it rise quicker, as some has suggested.


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## J.W Younger (Mar 15, 2010)

nmurph said:


> is the valve at the tip of the hose?
> 
> i have a near-new PM700 and it only registers 130.
> 
> opening the throttle will not have any bearing on compression. it might make it rise quicker, as some has suggested.


In my experience the throttle has no affect and if you look at it the exh port is wide open to atmosperic pressure .
I quess it dont hurt to set the fast idle to make sure theres no vacuum in the crankcase pulling the air out before its compressed.


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## barton174 (Mar 15, 2010)

J.W Younger said:


> In my experience the throttle has no affect and if you look at it the exh port is wide open to atmosperic pressure .
> I quess it dont hurt to set the fast idle to make sure theres no vacuum in the crankcase pulling the air out before its compressed.



Fair 'nuff... I know it does matter on motorcycles (L4, large displacement), but then, the intake tract is smaller on a bike, compared to the piston size, anyway...

Mike


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## Saw Dr. (Mar 15, 2010)

If your 8200 runs, cuts, and idles like it is supposed to, what is the problem? I would not tear into that thing if it is working properly. There are several parts on those that are not easy to find. The piston is the hardest to come by, and it is not quite as easy to change as others, due to having to press the needle bearings into the piston. It would be very easy to create an air leak on that one since it is a clamshell engine. Air leak = junk unobtanium piston. I would just run it and have fun if it were me.


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## rwoods (Mar 21, 2010)

nmurph said:


> is the valve at the tip of the hose?
> 
> i have a near-new PM700 and it only registers 130.
> 
> opening the throttle will not have any bearing on compression. it might make it rise quicker, as some has suggested.



Valve is at the base of the guage.


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## rwoods (Mar 21, 2010)

B200Driver said:


> If your 8200 runs, cuts, and idles like it is supposed to, what is the problem? I would not tear into that thing if it is working properly. There are several parts on those that are not easy to find. The piston is the hardest to come by, and it is not quite as easy to change as others, due to having to press the needle bearings into the piston. It would be very easy to create an air leak on that one since it is a clamshell engine. Air leak = junk unobtanium piston. I would just run it and have fun if it were me.



I appreciate your comments. I didn't know what a clamshell engine was until today. I'll be careful if I tear it down to replace the rings. The reason I bought this size of a saw is to fell large oaks - I get my firewood from dying oaks that are usually 24" or greater at the base. I usually use a ProMac 700 with a 28" bar. Occasionally I wish I had a saw with more power. Once the tree is down, I usually buck it with a Stihl 036 Pro with a 25" bar. I also always have a back up saw when felling large trees. I don't like walking behind a tree as I fell it; sometimes the Stihl is too short (and not strong enough to pull the next size bar). I have always been led to believe that the 5.0 cubic inch Mac's were hotrods compared to the 4.3 cubic inch Mac's. I just want to get the potential out of the 8200 and since the post office lost a few critical pieces I didn't want to spend money replacing everything just to have a mediocre old saw.


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## spudulike (Mar 21, 2010)

IMO
130psi - poor
150psi - pretty good
175psi = new saw or good old saw
175psi+ - has someone removed the base gasket

I ALWAYS look for 150psi + on all saws and will re-ring/new piston at 130psi after inspection shows wear or thin rings from wear.
Spud


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## rwoods (Mar 22, 2010)

spudulike said:


> IMO
> 130psi - poor
> 150psi - pretty good
> 175psi = new saw or good old saw
> ...



Thanks


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## pyro2 (Mar 22, 2010)

My Husqvarna 268 is 160psi - Has plenty of power

My Poulan 3400 is 90psi (3rd pull) 105psi (5th pull) - No power at all. After 5 min of running it can barely cut through a 8 inch log

My Poulan 3700 is 130psi (3rd pull) 150psi (5th pull) - Umm, not running yet so I don't know! But sounds like its in good shape.


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## thomas72 (Mar 22, 2010)

B200's advice is good. If your saw will pull a 3' bar like it is why take it apart. Parts are just getting harder and harder to find for those old saws. The saw will either cut what you want or not. If it gets the job done what else can you ask of it. If it were my saw I would run it like it is and on the side start buying replacement parts. Then if it wears out to the point it will need to be rebuilt all the parts will be on hand and ready to use. More than likely unless the saw suffers a catastrophic failure it will probably never wear out under your use.


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## SawTroll (Mar 22, 2010)

Impossible to answer imo, some are very good at about 145, others need a lot more!


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## Stihlverado (Mar 22, 2010)

Ive been playing with a 575 shinny and was wondering if those compression rates would compare to my saw as well?


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## 056 kid (Mar 22, 2010)

Compression is a relative thing. I have run old 044/046s that would fall like a brick when you do the starter rope test on them but still cut good enough for me to produce. I have a sp81 that had killer compression until not long ago, now I can hear the fuel bubbling past the rings and compression feels lower. That really pisses me off, but you would never notice while running the saw...


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## gmax (Mar 23, 2010)

My sp 81 has 165
S10 with new rings 130 :censored:
Oly 999 175 
XL 12 120


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## rwoods (Apr 2, 2010)

Thank you to you who encouraged me to leave well enough alone. But before I took this advice, I took the PM 8200 to a saw shop. The sales lady said the mechanic said the compression was good. I ask how much did it have. She inquired and relayed back that it had 120 psi; and they don’t work on Mac’s (afraid they might break something that can’t be replaced). The other mechanic happened to be walking by, overheard the psi statement and said to me, “120 psi is not good, a saw needs to be rebuilt at 130 psi.” Took the saw home and checked it and my PM 700 again – read 131 psi and 130 psi respectively just as before. Out of curiosity, I checked my Stihl 036 Pro – 120 psi. Checked it several more times 120 psi every time. My Stihl is just like the PM 700 – very difficult to turn over with the decompression valve closed. So I decide to forget this chapter of my life and get on with repairing the PM 8200 and leave the internals alone. To the credit of the USPS, they quickly and without any hassle paid my claim for the lost chain brake/side cover, the bar and chain (I forgot to include the dogs and I didn’t ask for anything for the muffler since it was cracked already). I recovered enough to buy a new bar and chain, new dogs and a used chain brake/side cover off a PM 850 (couldn’t find one for a PM 8200 nor a PM 805 which appears to be the same). I have more to do to it: bar bolts are stripped, muffler needs to be welded back together and a little paint. Below are some pictures of what I bought, what I got after the package was destroyed in transit, and a picture of it today when I was using it to fell a crooked white oak. I don’t know how it is supposed to run but I put it in a section that was a tad over 20” and it cut straight through it as if I were cutting limbs (no engine bogging and no rocking of the saw) and that’s good enough for me.
Ron Woods


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## rwoods (Apr 2, 2010)

Rest of pictures. Sorry I can't upload any of them as each one is too big. Ron Woods


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## rwoods (Apr 2, 2010)

I cropped the pictures so they would fit. Can't see much now.


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## Simonizer (Apr 2, 2010)

In 20 years I have never used a compression gauge on a two-stroke single. It can tell you if it is bad but not if it's good. I have seen good readings from a cold saw that has sat for awhile. The fuel mix evaporated from the cylinder wall and the film of oil is left behind. Acts like new rings. On multi-cylinder two-strokes it is excellent as a comparator between cylinders. On four-strokes it is a useful tool, but not as effective as a leak-down tester. On a chainsaw, a visual inspection of the piston/cylinder is far more informative. If you poke a tiny led light through the spark-plug hole you can see the exhaust side. If you are ever suspicious, remove the muffler and look inside. Cheers, Simon.


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## Cantdog (Apr 3, 2010)

Yep as 056 said "compression is a relative thing" especially on saws. It will vary greatly from saw to saw depending on the design and build. Single ring, two ring, domed piston, flat top or dished etc. All of these will affect what you get for readings. Remember you are testing at a piston speed of what?, maybe 100 RPM by pulling the cord. And it idles at 2700 RPM and revs out at 10,000. Also what may feel like wimpy or really strong comp on the cord will have a lot to do with the starter pully size or even the size of the dia. of the cord itself. A friend of mine has a Dolmar 133 that is the GDest arm breaker/finger tearer I've ever started but the most we can get on a gage is 175 PSI. A lot of it was in the starter. It had a small dia cord so you were pulling from a leverage point smaller and closer to the crank. I replaced the small dia cord with one that would that would just fit in the pulley and completely filled the the pulley when recoiled. You still have to treat that saw with respect when starting but it pulls quite a bit easier now. I agree with the general theory "if it ain't broke---don't fix it" especially on old saws that parts are difficult or impossible to find.


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## jesus lord (Oct 29, 2010)

*does compression matter in a 2 stroke?*

one thing that occurs in a two stroke piston ported motor/engine is that piston speed alters the compression ,second is there are pipes on some that leak down some air .third factor that comes to mind is the case if leaking somewhere like end seals , alters the cranking speed test & affects the breathing +/or squish at running speeds. Fourth . Lastly,if the rings cause an effective seal power can be great in a saw that slowly sinks to the floor when suspended by the start chord. :greenchainsaw:SO, as Simoniser says check the cylinder through the plug .yes indeed! Its the exhuast side that shows wear first & worst.Check the piston through the exhuast port . One is not always the same as the other(pist.cyl.).Although they may make the right song it can be a different story in the wood where yu dont want it to go wrong.New second hand ? Shove it toward its task & then you get an idea?AND STILL after half an hour of fair performance some second hand saws I had would show a breakdown & start to give trouble only to run well when next started cold. And ported saws I have heard two ported supposedly the exact same way..one that goes like the clappers +the other ruined by the effort? IF IT AINT BROKE DON"T FIX IT!


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## chugbug (Oct 30, 2010)

This compression issue is hard to figure , I thought it would be cut and dry . The 359 that I purchased recently used and hard to start is running fine now , fixed the boot , no air leaks and its at 110 lbs.starts easy every time its a 2003 , for comparison my like new 346 NE 2009 is at 120 lbs. , My 25- 30 year old homelites are at 130 and 150 lbs . Maybe like many of you have said its not a perfect situation where we can come up with a rock solid figure . But how can a 30 year old homelite have 30 lbs more compression than a brand new husky ?

I guess what I'm after is a figure that I could by when buying used saws that would tell me if the saw is going to need rings and maybe a piston or if the cyl. is scored and that will need replaced also .


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## blsnelling (Oct 30, 2010)

If you're getting 120-130 PSI readings on 346s and 359s, then there's something wrong with your guage or your saws. That's not typical. You should be getting 150+.


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## chugbug (Oct 30, 2010)

Gauge is only a year old , wouldn;t think its giving me a false reading , the old homelites read 130 and 150 , I'll try to get another gauge and compare them .


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## chugbug (Oct 31, 2010)

The only thing I can see after checking it out again is I'm not getting a prefect seal at the threads , the fitting on the gauge looks like its tapered , and it does snug up a bit but if I spray some soapy water on the threads I can see it suck the water right into the cyl. , I never really worried about an air tight seal when checking compression , could this make a big difference , I can remember some gauges having a o ring to help seal but this gauge doesn't and its a good brand name gauge.


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## 056 kid (Oct 31, 2010)

one thing I have wonderd about is weather or not a motor will build compression back after having lost some due to being run hot or something.


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## chugbug (Oct 31, 2010)

I would think being run hot or more like overheated could hurt the rings enough to hurt compression .


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## chugbug (Oct 31, 2010)

Went out to the shop and compared my compression gauge to my air compressor like Brad suggested, the gauge came up 20 psi under the compressor . That would put my new 346 at 140 psi and if I can seal the thread leak maybe I will have found the problem . But first I'll compare it to a couple other gauges and see what I get .


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