Author Topic: Dolmar7900 with 28" bar  (Read 677 times)

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Offline Cut4fun .

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Re: Dolmar7900 with 28" bar
« Reply #10 on: July 27, 2013, 06:34:09 pm »
I'm figuring the newer 7900 or the 8000 will be the quicker cut times if I had to guess. 

Only thing those 8000 weigh as much as a 066 which is a big neg in my mind.  Oh this 8000 has been muffler modded, squish tightened up with stock porting numbers. 

 The older 7900 I have ran with one of my rebuilt 372 71cc saws and they seemed the same in the cuts without timing.  Jim I think ran them both that day too.




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Offline 660magnum

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Re: Dolmar7900 with 28" bar
« Reply #11 on: July 27, 2013, 08:07:39 pm »
I think my new 7900 runs faster than the old one if you screw the needles down a little.
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Offline 660magnum

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Re: Dolmar7900 with 28" bar
« Reply #12 on: August 22, 2013, 09:47:23 pm »
The last time I used my older 7900 (2006 model) was in June. The Oiler quit. Upon examination, the wings of the worm gear were gone. The 7900 had been bad about draining all the oil out of the tank while setting so I decided to rebuild the pump and replace all the hoses while I was in there. As is the usual case, this area was packed solid with oily sawdust/dirt. Whatever . . . it was a black mess.

Taking it apart, it had the new style green hose from the pump up to the bar but the tank tube was an old black one. Someone had packed the pocket where the pickup tube goes through the tank with red RTV. That was a mess digging all that out. Getting the new green pickup tube in place was a chore but it finally slipped in place. I pushed with two dull screw drivers on the flanges of the tube to seat it in place.

On the oil pump I drove out the retainer pin just enough to get the adjuster screw out. The pin fit in there rather tight. The end of the old adjusting screw was worn off and there was little stroking of the pump as it went around. I put the tiny O-ring on the new adjusting screw and the big spring around the outside. I aligned the pointer of the adjusting screw in the correct direction and held the screw down while I squeezed the pin back into position with pliers. I was lucky and got it into the right position the first time. Then I inserted the pump shaft and I installed the spring, ball, and retainer screw in the other end. I had to remove the chain plate to remove/ replace the hose from the pump to the bar. I replaced the chain plate and was then ready to bolt the pump back into its resting position.

I put the little thin washer against the lip on the crankshaft that keeps the worm gear away from the crank seal and then inserted the new worm gear. On top of the worm gear was a large diameter washer that keeps the clutch shoes away from the wings of the worm gear. Screwed the clutch spider/shoe assembly onto the crankshaft left hand threads until it was had tight. There's a little thick washer that goes on next as a backup for the needle bearing in the clutch bell or brake drum. After that was the 3/8" X 7 tooth large hub rim sprocket, the retaining washer and snap E-clip. I would like to add here that there was a upgrade to the 6400/7300/7900 clutch along about 2008 where they used a larger needle bearing in the clutch. I upgraded this saw to the newer style needle bearing and clutch/brake drum.

What originally alerted me to my troubles with this 7900 was that it wouldn't shut off. On the old style 6400/7900, the shorting wire goes down to the lower right AV mount and the flexing breaks the wire. It doesn't help that the original wire gets hard as steel. I couldn't get to it for a repair and had to remove the tank . . .

I decided while going to all this trouble, I would replace the fuel line, fuel filter, and impulse hose. The front AV mount is a PIA as the retaining screw does not come completely out of the spring assembly. To remove the spring requires a giant metric Allen wrench or a metric bolt head to fit in the hex socket of the black plast AV spring pocket. With some careful maneuvering, removing and re-inserting the front of the tank into the right position can be done but it requires a few magic words. You cannot tilt the tank down enough to replace the fuel line. Thank goodness the other three AV screws were easy to completely remove.

Anyway, the new fuel line has to be inserted from the inside. I used a bicycle spoke and bent it a little to find the hole. I ran the fuel line up the spoke and out the hole. I grabbed the end of the fuel line, pulled the spoke back out and snapped the new fuel line in place. That was the easy part.

I installed the new style magneto shorting system in this chainsaw which includes a metal assembly that attaches in the original screw hole by the lower right AV mount  and the goes under the mount for the carburetor/air box floor where it has a tab for a spade connector. This comes as a kit from Dolmar with new wires and the metal assembly.

To get the tank re-attached, requires you to get the tank pretty close into position, connect the switch wires and they have to go under the throttle cable. The throttle cable has to be threaded into the guide coming off the trigger and into the hole in the carburetor floor. The fuel line has to be stuck up through the hole by the carburetor right side. Then it is time to pry the screw of the front AV mount into the hole in the front of the tank. Then the tank assembly can be swung up into position.
Oh, I forgot, there is a metal cup that goes over the right rear AV spring. Well the other screws go in easily enough and you can just tighten them down.

At this point you have to have the starter cover off so you can swing the needle adjusting guide out of the way to finish connecting up the throttle cable. The cable has to be treaded into a holder inside the air box and then brought up and over the nylon bell crank on the throttle arm. The fuel line is just a matter of pulling it up and slipping it over the carburetor nipple.

After that was done, I removed the top cover and installed a new impulse line from the cylinder to the carburetor. Just a matter of getting the job done. Dolmar sell you a one meter long package of impulse line that will last you the rest of your life.

Re-installed the HD air filter assembly, top cover, and starter cover. Bar & chain and she's ready to try out.

The hard parts were all the dirt and mess and getting the tank assembly back into position.

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That knowledge can live after us... and that "Pays It Forward".
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Offline 660magnum

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Re: Dolmar7900 with 28" bar
« Reply #13 on: September 02, 2013, 07:09:52 pm »
Thought I would run a compression check on this puppy today for it has had some use and looks like it hasn't been apart since some dirt.

My compression tester's extension hose has 14 & 18 on the end of it. It also has a 14 and 10mm long reach adapters.

I removed the spark plug and attached the compression tester.

I put the choke on and off to set fast idle.

Pull the saw through a few times and the gauge read 180 lbs.

So as far as I'm concerned, it's OK.
We should share what we know... someone may learn...
That knowledge can live after us... and that "Pays It Forward".
Be all that you can be . . .

 

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