Author Topic: 3700 low compression  (Read 208 times)

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Offline dano

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3700 low compression
« on: April 10, 2018, 02:04:18 am »
I replaced the rings on my poulan 3700.The cylinder was worn but clean. There isn't any marks on the piston or the cylinder.I ordered new nos oem rings deleted the head gasket and put her back together. The saw runs and idles ok, but my compression is only 90 lbs. I did not check the new rings with the old rings. they may have sent the wrong rings.Is 90 lbs normal before break-in?
thanks
Danny

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Offline pete

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Re: 3700 low compression
« Reply #1 on: April 10, 2018, 06:46:54 am »
If I had a saw that ran and idled well at 90 lbs compression I would try another gauge or try my gauge on another saw
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Offline dano

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Re: 3700 low compression
« Reply #2 on: April 10, 2018, 06:53:33 am »
I did on a 3400. Got 135 lbs. Is it normal for low compression until break in?

Offline Cut4fun .

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Re: 3700 low compression
« Reply #3 on: April 10, 2018, 09:34:16 am »
Did you fit a ring in the cylinder and check the gap?   After putting ring on piston did you check fit with ring compressed?

Just ideas. 

How was the cylinder wore like you say above?
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Offline dano

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Re: 3700 low compression
« Reply #4 on: April 10, 2018, 01:37:56 pm »
No I didn't check anything. I ordered a new set of rings last night. Would it hurt anything if I ran it with rings with too much end gap for a few days until I get the correct rings? I noticed in the pictures the rings I ordered have a shelf or notch at the end so when they butt against the locating pin the outside edge of the ring overlaps the pin. The rings I just installed were cut square strait across allowing a gap where the pins are.What is the correct end gap? The cylinder is worn a lot. In some places the chrome plating is worn off. But there was no score marks, everything was really clean. I wasn't expecting super high compression, but I think it should be more then 90 lbs. It was late and I was tired when I put it together last night.  Thinking about it now There is no way it could have good compression with the rings leaving the locating pins exposed. That's a really big hole for the gasses to escape. It would take a really badly scared piston to have gaps that big.

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Re: 3700 low compression
« Reply #5 on: April 10, 2018, 10:35:02 pm »
Those Poulan 3700's with the two thin rings usually run about 150 psi and then with a deleted base gasket you should get at least 150 to 155psi. 

That cylinder sounds pretty worn if some of the plating is gone.    After putting on your new rings if the compression is still low I would consider another cylinder.
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Offline dano

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Re: 3700 low compression
« Reply #6 on: April 11, 2018, 12:13:58 am »
This is my first 3700. I cut a little wood today, and I was impressed with this saws power and torque. The saw is nearly 40 years old and even with it's low compression it cut rings around my 50cc saws.Do you know where new cylinders and pistons can be purchased.

Offline Cut4fun .

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Re: 3700 low compression
« Reply #7 on: April 11, 2018, 07:06:49 am »
ebay or find parts saws.  Just watch ebay prices some folks like to gouge you on the prices.

If you used sets you could use  3400 smaller,  3800 about the same,  bare bores and 3700 same,  4000 bigger cc  chrome bores

Roger since I dont know this series that well. Can you tell by the numbers if this is a 3700 or 3800 cylinder?   

530012069

I think it is a chrome bore 3700 375 385 cylinder.  my WAG

https://www.ebay.com/itm/NEW-POULAN-3-7-CYLINDER-FITS-3700-3800-CHAINSAWS-530012069-OEM/172977455083?epid=7009387137&hash=item2846427feb:g:Nt4AAOSwj99aCQfZ
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Re: 3700 low compression
« Reply #8 on: April 11, 2018, 04:50:02 pm »
That is the correct part number for a 3700 but that is also a crazy price I would not pay.   

The cylinder for a 3800 will not fit a 3700 so for them to list it that way is also wrong.

If it were me I would be patient and look for a used cylinder for a 3700 on ebay or one of the guys that sell used parts as a business.
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Offline dano

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Re: 3700 low compression
« Reply #9 on: April 12, 2018, 04:33:45 am »
Well I think it's to much to,but parts are scarce. I waffled back and forth. Finally I said the heck with it and I hit the button. I bough a new piston also. I plan on replacing the seals and the sprocket, sanding and painting it. I hope to bring it back to it's original condition.
Thanks everyone for your help. I will let you know how it turns out.
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