Author Topic: Dolmar 7900 Porting Info  (Read 3795 times)

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Offline EHP

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Re: Dolmar 7900 Porting Info
« Reply #20 on: January 07, 2012, 11:02:42 am »
compression goes back to the same problem, how much port timing did you put in the motor , less port timing the more compression you can run unless you run race gas of some kind , same thing with your flywheel timing the more advance you run the flywheel the less compression you can run cause its going to get real hot fast . I donot like running any saw on pump gas above 175 psi on my gauge cause first in most cases it will blow up second it will not be as fast or last as long as a saw running 160 to 170 psi that has had the other stuff do to it , Most high test pump gas is 91 or so octane now and at least 10% ethanol so you have to consider what your have for fuel cause its no good building a saw that has to run on C12 or something like that if the saw is to be a worksaw not a weekend cookie cutter , another thing  the higher the compression in most cases the shorter the main bearing last , I got 7900's that are out there with tree companys that are 7 years old and work pretty much every day , I did see 2 this summer come back for main bearings from 7 years of work so that is pretty good in my mind

Offline Old Iron Logging

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Re: Dolmar 7900 Porting Info
« Reply #21 on: January 07, 2012, 11:19:00 am »
Ed, what if the saw stock is running in the 190 lbs plus range to start with? We see more than a few saws at the shop in the 190-210 range stock.

Offline EHP

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Re: Dolmar 7900 Porting Info
« Reply #22 on: January 07, 2012, 12:56:31 pm »
I guess like I said it depends on the gauge , I have not see much of the newest strato stuff as most guys here are staying away from that , if you got 190 psi on my gauge and is running on pump gas then it has low porting and in most cases a large blow down time . I had 2-288's come in here last summer that were stock but they pulled the base gasket  and in real nice shape but were in the 180 psi, in 16 inch maple they were fine but in the bigger 40 inch plus stuff they would lay down in the cut bad, good thing the guys running them knew to stop and see what was wrong , I just lowered the compression to 170 psi and they said they worked great , Now mostly I blame this on our great gas we are getting now and if you have better gas then you most likely will not see these problems but believe me the more ethanol you get the worse this problem is going to get and another big problem coming is alot of 2 stroke oils will not mix with ethanol and the ones that do cost lots of money and hard to get in some areas . I know when guys from NY come up and play with their saws they have to richen them up to run our high test pump gas which tells me ours has more ethanol in it

Offline EHP

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Re: Dolmar 7900 Porting Info
« Reply #23 on: January 07, 2012, 01:21:15 pm »
another thing I have noticed and this is with saws like the 361 , if they were sold in this area they all are low compression with the thicker base gasket , if they came from up north where I'm from they were the higher compression with the thinner base gasket .

The higher the compression the more carefull the operator has to be on making sure he keeps the engine heat down .

Now back to dolmar , if you put a unlimited coil in it make sure to use lots of oil like 32-1 cause it going to turn alot of revs , 16,000 rpms  happens all the time with say a 20 inch bar , I have seen over 17,000 rpms on a woodported saw on a pipe and that was where it cut best at , One thing I have not seen yet on the 7900 is a crank go bad , I was playing with a 7900 on alcohol on muffler and was doing my cuts the the return spring on the 3120 carb broke so now the throttle is wide open , I got the top covers on the saw so no way to get at the carb, the off switch is not going to work in that state with the carb wide open cause the blade never came down after the run and now the plug  is glowing so in a split second I figured I wanted nothing to do with this problem so I threw the saw cause I figured the rod would be coming out to meet me  ;D, it ran about 5 seconds on the ground before the flywheel came loose and she finally quit , I checked the crank and I see nothing wrong , it did not spin on the crank pin so not as bad as I thought , I'm sure 20,000 rpms were passed cause it sure was screaming

Offline StumpysCustoms

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Re: Dolmar 7900 Porting Info
« Reply #24 on: January 07, 2012, 01:43:30 pm »
Sorry Ed,  I feel dumb,  what else is new. 
Thanks anyways for all the info.  I'm with Randy on this,  the more I learn the dumber I feel.

Offline EHP

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Re: Dolmar 7900 Porting Info
« Reply #25 on: January 07, 2012, 02:52:19 pm »
Believe this , if any one tells you that they know everything , tell them they are full of BS as new ways to skin the cat get figured out every day and if no one ever thought outside the box nothing would change  ;D, and donot worry Stump , I'm not the smartest tool in the shed  :o but I always think outside the box cause its more fun out there

Offline EHP

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Re: Dolmar 7900 Porting Info
« Reply #26 on: January 07, 2012, 08:48:03 pm »
Another thing I should put in here put I'm sure somewhere else I have said this , the small the cc's of the saw the higher you can run your BEMP, what psi you run in a 50-60cc saw and it can live you will not run in a 120cc saw with the same porting numbers and crankcase ratio , Just something to think about , if you take the crankcase ratio of a 3120 , which is in the 1.4's and make it 1.20  the amount of horsepower you will gain is unbelievable as 1.20 is the idea ratio but the same thing , your amount of fuel needed is almost double so just like everything else , how much power do you need and what is that power going to cost you  in fuel, shorten the life of the saw if you go to wild , make the saw not very user friendly as its a prick to keep running

Offline Andyshine77

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Re: Dolmar 7900 Porting Info
« Reply #27 on: January 08, 2012, 12:09:43 am »
Sorry Ed,  I feel dumb,  what else is new. 
Thanks anyways for all the info.  I'm with Randy on this,  the more I learn the dumber I feel.

Stump you speak the truth mang!!!!!

Offline Mastermind

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Re: Dolmar 7900 Porting Info
« Reply #28 on: January 08, 2012, 10:24:30 am »
The quote thingy ain't working for me...... :-[

Ed the ratio your speaking of, is this crankcase to displacement?
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Offline Cut4fun .

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Re: Dolmar 7900 Porting Info
« Reply #29 on: January 08, 2012, 10:31:25 am »
The quote thingy ain't working for me...... :-[

Ed the ratio your speaking of, is this crankcase to displacement?

Read the 2 ways to do it since update by server.   http://chainsawrepair.createaforum.com/general-discussion/%27quote%27-not-working/
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