parts saw Dolmar PS-7900 re-build

[meglodon]

Here's the latest project I've picked up. I bid on ebay (probably to high) on what appeared to be a very dirty but mostly complete saw with a blown top end. Looking through the pictures it was missing a few things but mostly complete. It arrived and I have started cleaning to see how bad things are and try and determine how it blew up.

Although it's dirty, underneath the sawdust it seems to be in decent shape.


I initially took a quick look at the cylinder and exhaust/intake sides of it looked fine (it was pretty dirty) but there appears be some damage around the transfers, I think (based on what I can tell) that either the plating wasn’t chamfered enough around the transfers and it caught a ring or... the carb also appears to have a loose screw, so maybe it dropped a screw through the intake (I have't dug into the carb to deeply)

Before realized how bad that was, I had ordered a new piston for it… so of course, now I’m wondering if I should have just pony up for the 54mm big bore kit. On closer inspection I think it is possible to clean up the damage at the transfers. So, I'll do that and port it and if it completely bombs out, I’ll upgrade to the 54mm top end.


On a positive side although it ingested something(s?) the bottom end is tight, crank rotates smoothly and I haven’t found any bits and pieces floating.



from what I see only one side has damage above the transfer which might raise them while porting. I'm curious if others have had success fixing chipped plating and what the longevity of it might be? figured it'd be worth trying.

 

[meglodon]

as I'm pulling things apart and cleaning I found that the oil pump drive had a broken tab. So I added that to the list of parts on order.

Finding parts for the saw has been a bit of a treasure hunt. It's been a combination of ebay, sawagain.com, jackssmallengines.com referencing the IPL from herehttps://www.makita.de/explosionszeichnungen.html

Here's how I left it last night, waiting for more cleaning and parts to arrive.

 

[meglodon]

A bunch of parts showed up in the mail today.
- seals
- bearings
- piston

Now that I've torn things apart and have a better idea of what I think is ok I may not use the seals, bearings and will have them as spares.

Over the holidays I bought a 90 degree foredom tool off aliexpress and it finally showed up. Perfect timing for this project! It needed some slight modification to the base to make work on the existing foredom cable I have.

With that in hand I took a run at cleaning up the damage around the transfers with a diamond ball over my lunch break. Seems to be working pretty good so far, I'm happy with how it's turning out.

Since the piston was in the box of things that arrived, I hope to get all the burs knocked out of the transfers, clean the cylinder and get some intial timing numbers and see where land.

 

[EchoShindaiwaMakitaDolmar]

From my perspective this is a pretty cool project on a great model saw.

 

[ballisticdoughnut]

Don’t buy the 54mm top end they’re junk.

 

[meglodon]

Don’t buy the 54mm top end they’re junk.

Good to know!

As noted: Seems to be cleaning up ok. Here's some before / after side by side shots

 

[blades]

The oiler assembly and the rebuild kits are in very short to unobtainment supply for the 64xx/79xx units.

 

[meglodon]

The oiler assembly and the rebuild kits are in very short to unobtainment supply for the 64xx/79xx units.

Yeah, that's what i'm afraid of. I'm looking at my order from Jacks small engine parts, and although they claimed they had in stock. It appears that in fact, they do not. I'll let it sit for a bit but probably will have to follow up on that.

I was curious if it a husqvarna 372 oiler might be retrofitted to this (some of the husqvarna parts appear to interchange in design anyway)

 

[blades]

I went that route and one is close but the shaft dia. bore is smaller. I haven't had any time to mess with them. thought about 3d printing but no time to follow up. got that idea from from one of vids with J. Leno's car collection. i would need to to take somewhere to get it done- no idea cost wise at this point.

 

[meglodon]

I went that route and one is close but the shaft dia. bore is smaller. I haven't had any time to mess with them. thought about 3d printing but no time to follow up. got that idea from from one of vids with J. Leno's car collection. i would need to to take somewhere to get it done- no idea cost wise at this point.

Nice, I do have a 3d printer that's collecting dust so maybe that's an option if I'm not successful with the part from Jacks small engine. Do you remember if the diameter of the 372 shaft or the 7900 was the smaller of the two?

Although I should probably focus on getting it running first ...

 

[buzz sawyer]

FWIW, I put the big bore kit from Bailey's on my 6401. almost ten years ago. No problems, lots of power. Currently running a 28" bar.

 

[meglodon]

FWIW, I put the big bore kit from Bailey's on my 6401. almost ten years ago. No problems, lots of power. Currently running a 28" bar.

Nice, that's also good to know.
From what I see, there are a variety of different options out there.
28" bar is a good length for sure! thanks for sharing the vid!

 

[meglodon]

Making progress.
Piston fits pretty good and I was able to source some cylinder bolts.
Numbers are: (if I'm reading this digital timing wheel correct)
squish - 0.0205
exhaust - 104.4
transfers - 134.4
intake - 80




Those seem to track with the stock numbers Dyno Joe has in the video he did: (Note: I'm still learning how to use this digital degree wheel and it's not 100% the same saw)
https://www.youtube.com/watch?v=tU4U2zXV_0M
Intake 77.5
Exhaust 107
Primaries :134
Secondary: 131

 

[meglodon]

Last night I spent time doing overall clean up and started to put things back together. I realize is not 'eat off the plate' clean... but I decided
1) I'll be investigating a parts washer and ultrasonic cleaner to add to my tool pile.
2) This is what I consider a shake down build

With that in mind, I took the oil pump out and cleaned a bit in there. Things were dirty in there, it's hard to say if it was leaking or not so I used threebond 1184 to on the oil line around where it goes into the case. I figure that would help seal things up there. The pump itself seems to work well just spinning the pump mechanism.




Then I took a run tank and cleaned a bit on that. Found a crack near the foot plate but it doesn't look to be into the tank itself. I put some pressure on the fuel hose and it seemed to hold however, I didnt' fill the tank with fuel yet to tempt fate. (and leak fuel all over just before I decide to go to bed)



cleaned a bit on the top cylinder cover plastic, flywheel and carb deck. All of which look much better than they did.




got the wiring for the coil rerouted and coil mounted.





Then put things back together to prep for the cylinder porting.

 

[meglodon]

The saw arrived without the back air filter cover, which I've sourced. so I figured i'd 'mock' up how the new vrs old plastic looked. Not to bad.



The next puzzle is the carburator setup. It's missing a few parts i'm sourcing. Bolts and the throttle cable plate are the major missing parts. I wasn't having luck finding oem so I have a farmertec 372 metal throttle cable plate on order and I found a used plastic one that I had in my parts pile I'll be mocking things up with.

Then took a quick look at the Carb. This is going to be a problem, it's got a broken spring on the throttle plate. I'm looking through a few carbs I have to see what I can canabilize. However, I also have a chinese husky 395 carb which I bought for a rainy day, and I may be putting it into service. I also believe I found a 372 xtorque carb. Which might be a power producing beast if the Ironhorse video is anything to go by (search for The Ironhorse - Dolmar 7900 /066 mag hybrid! Carb and intake! on youtube.)

But I never saw an update on that project so it may not have worked as expected.

The 395 carb I have may just be what goes in for now if I can't get spring things figured out.

 

[meglodon]

I spent this evening so far doing a lot of reading around porting theory and 7900 and where blowdown and longer stroke saws with tight crankcase volumes play. So for me, I think the numbers I have are a good baseline to start out at. I plan to open up the muffler and allow for more flow, and once I have the saw running I'll have more of an idea of what things will need adjustment. So I focus on cleaning up and chamfering the port openings to start and get to reading up on porting theory (e.g. finding a physical copy of https://vdocuments.net/two-stroke-tuners-handbook.html?page=1)

That being said, I'm waiting on a cheap 372 oil drive gear I bought off ebay to see what the differences are and few more parts for it which are shipped but not here yet.

 

[meglodon]

Dug into the carb, it had 2 busted springs (not sure how that occured) but I had a few parts carbs that I pulled apart and got a couple springs (and I think** it now works as intended.) I pulled the caps and checked the diaphrams, both sides look good and the needle moves freely.


** while it pressure and pop tested I guess I won't know until I get it into the saw.

The 372 intake cable block appears to work as hoped, once the parts show up I'll be able to make some forward progress there.




and I have a 52mm caber ring from another project that I didn't use it in but I think will make a nice addtion to this project.



Then I took some time and cleaned up casting flash on the piston, hard edges.


i'll heat treat it tomorrow (450 degrees for four hours then into an ice bath.) This is what Ironhorse does in his videos and says it seems to make them be a bit more abrasion resistant. I figure I'll just add that in as one of the steps I do.

 

[meglodon]

Had the piston ready to go and was excited to start assembly, and then a stag as I was doing some more cleaning to put things back together.

I found that the crankshaft has a crack in it.



Which sucks... but I guess that it’s better to find it now and not after the damage would be done.

Since I'm this deep in may as well move forward. I found a short block on ebay with the following description:

OEM Crankcase assembly with piston. Bearings are good, comes with oiler pump and gear, Piston is good.

Once that arrives this project can move forward.

 

[buzz sawyer]

Well, that stinks! Like the vintage Clinton I restored. Got it apart and found one side of the crank had broken off but was welded back together. They actually didn't do a bad job - only about 0.010" runout, but I was able to find another parts saw with good crank.

 

[meglodon]

Well, that stinks! Like the vintage Clinton I restored. Got it apart and found one side of the crank had broken off but was welded back together. They actually didn't do a bad job - only about 0.010" runout, but I was able to find another parts saw with good crank.

Yeah. Bummer for sure. (both for you and for me...) But, sounds like they did do quite a good job on that repair, especially to get so accurate on the runout! Glad you were able to find a good crank. I'm glad parts for this are still semi-easy to find and for the cost of the short block, I doubt I would be able to cover the cost of a machine shop to repair this one.