Dolmar PS 7900 timing

[rhhold0812]

Hi All,

I am writing in hopes of some expertise on a Dolmar PS 7900 advanced timing set up.

I have a PS 7900 which was build by a gentleman who has passed away so I cannot go back to him for help. Here is what I have been told has been done to it.

Finger ports, 272 coil, opened up muffler, domed piston, advanced timing, possibly more but I do not know.

My issue is I broke a fin off of the fly wheel. (Tried to block the flywheel to remove the clutch, dumb of me I know). I have a new flywheel which has the factory crankshaft key in it. The old flywheel I removed has the key removed.

My question is how do I find the correct orientation when installing the new flywheel? I found a mark on the coil saying, “26 degrees”. I am assuming being written on the coil that this is the angle which is the setting below DTC (this is just a guess but was planning to use it as my starting point).



My plan is to scribe a line where the factory key is on the new fly wheel, file off the key, align as close as possible, put on a timing wheel, advance 26*, reassemble, and test. Are there flaws in my thinking?

There appeared to be red lock-tite on the crankshaft, is it recommended to put lock-tite on the shaft before reinstalling?



Any and all advice is appreciated, I have almost no experience in saw building but trying to learn.


Thanks!
Robb

 

[wcorey]

What I've found to be a good starting point at least, is with piston at tdc, line up the trailing edge of the flywheel magnet block to the trailing edge of the leading coil leg as in the pic. Different coil but works the same on a 272 coil conversion. It's nothing new, tried and true for some years now, there's a good drawing of it floating around the forums somewhere.
In my experience 26 degrees is pretty conservative, 30-33 degrees can offer additional performance, ymmv though. I usually keep advancing until I start hearing the that distinctive popping in exhaust note while in the cut, then back off a couple degrees.


All that said, if I have a broken fin on a flywheel, I just break off one on the opposite side to maintain balance, particularly on something with as many fins as these dolmars have.

 

[rhhold0812]

What I've found to be a good starting point at least, is with piston at tdc, line up the trailing edge of the flywheel magnet block to the trailing edge of the leading coil leg as in the pic. Different coil but works the same on a 272 coil conversion. It's nothing new, tried and true for some years now, there's a good drawing of it floating around the forums somewhere.
In my experience 26 degrees is pretty conservative, 30-33 degrees can offer additional performance, ymmv though. I usually keep advancing until I start hearing the that distinctive popping in exhaust note while in the cut, then back off a couple degrees.


All that said, if I have a broken fin on a flywheel, I just break off one on the opposite side to maintain balance, particularly on something with as many fins as these dolmars have.

Thank you wcorey.

 

[rhhold0812]

What I've found to be a good starting point at least, is with piston at tdc, line up the trailing edge of the flywheel magnet block to the trailing edge of the leading coil leg as in the pic. Different coil but works the same on a 272 coil conversion. It's nothing new, tried and true for some years now, there's a good drawing of it floating around the forums somewhere.
In my experience 26 degrees is pretty conservative, 30-33 degrees can offer additional performance, ymmv though. I usually keep advancing until I start hearing the that distinctive popping in exhaust note while in the cut, then back off a couple degrees.


All that said, if I have a broken fin on a flywheel, I just break off one on the opposite side to maintain balance, particularly on something with as many fins as these dolmars have.

Is it possible to damage the saw if I reassemble the saw with incorrect flywheel location? My fear is if i am off 10 - 20 degrees for example it will damage the saw or the flywheel.

Thanks again.

 

[wcorey]

Is it possible to damage the saw if I reassemble the saw with incorrect flywheel location? My fear is if i am off 10 - 20 degrees for example it will damage the saw or the flywheel.

Thanks again.

Very unlikely, though if it's too far advanced it can have a tendency to rip the starter handle out of your hand and in the instance of a saw with a weak recoil system design the kickback could damage the starter pawls.

 

[rhhold0812]

Very unlikely, though if it's too far advanced it can have a tendency to rip the starter handle out of your hand and in the instance of a saw with a weak recoil system design the kickback could damage the starter pawls.

Hi wcorey. I wanted to thank you for your help. I got everything put back together and working properly.
I went with your recommendation as a starting point which seemed to be too far advanced. My sign was it was almost impossible to pull, very hard to pull, could not get to start. Disassembled, turn back a few degrees by eye balling and reassembled. Started right up after that with a normal pull on the cord. Checked with a timing light with advance programmed and kept increasing until my marks aligned.
My light is an Innova 3568a, I made the assumption that I needed to divide by two to get correct advance number. My clues were that my advance number was around 46* to 44* (which I thought the saw would never start with 44* advance) dividing by 2 would put my advance number around 23* to 22* (I was shooting for 26*, i was happy with 23* - 22*). Other test was that my rpms at idle were close to 6k, assuming idle was around 3k dividing by two would be correct)

Do my assumptions and numbers make sense?