CHEAP DOLMAR 7900 COIL SWAP

[Poleman]

I had been watching the threads on the newer Dolmar coils from the 7900 to the 7910 that needed a new flywheel along with a new down graded limited coil. (12,800 rpm's)

I had also followed the unlimited coil threads with pressing off the coil part and reinstalling on the laminated part.

My thoughts was could it be done without doing the extra work......well it can!!!!

I started out with a an aftermarket 272 unlimited coil, removed the coil wire that came with it....then started fitting. The only things that needed to be changed is the screw holes needed to be notched to get the coil to fit and be adjustable. Use the stock coil wire and screw it in before you install the coil after you have it fitted. It will be under the coil when installed. You will also new to bent the ground tab wire on the coil so it doesn't ground on the case.

Next you will need to find TDC and make an indicated mark on the case for refferance, then using a degree wheel from TDC make your first mark at 25 degrees BTDC and mark every degree to 35 BTDC.Next remove the flywheel and grind off the tab. Having the piston at TDC install the flywheel to the 30 degree mark (that's where mines set) tap the flywheel down and tighten without moving it. Reassemble saw and test.

My saw was initially set at 27 degrees Btdc and it was a little boggy so I bumped it to 30 degrees and it straightened right out. I then retuned and cut.

Sorry I couldn't get my pictures to load but go to Brad thread of doing his unlimited coil. I used his method of marking the degrees on the case. You may also need to play with your timing to get your particular saw dialed in.

A cheap way for those having coil issues on a 7900 and an easy way of unlimiting the 7900 also.

 

[Fly By Night]

Rich

That is all

 

[cgraham1]

Nice job Rich!

 

[Poleman]

Thanks guys.....did this cuz I got in a panic about ever changing to a slower rpm coil.....and just because!!!!

 

[Mastermind]

Pics......we need pics. lol

 

[Poleman]

I know Randy....I'm trying to figure out how to get them on...never had a problem before..... I need to reduce there size. My phone takes to good a pictures.....

 

[Mastermind]

I'm not sure what you'll have to do......never posted from a phone.

 

[Poleman]

Just like computer....just able to take pictures and post....usually.

Sent them to Clint, hope he can figure it out. Usually its easy least....

Only four pic's, stock 272 coil, installed and timing marks on case.

 

[Mastermind]

The unlimited coil swap is a great thing on this beast........just think of the cant saws that can be built.

 

[Poleman]

Ya no kidding. I was thinking of coil failure and the change out....this makes it way cheaper....oh and better!!!!

 

[cgraham1]

Pics......we need pics. lol

 

[Poleman]

Thanks Clint!!!!

I hope this helps...

 

[Mastermind]

I see.

Thanks Clint.

 

[mdavlee]

Yep. That's the method I was shown before.

 

[Poleman]

Finally got to really check out how this saw worked today....and it really works good!!! Had a 28 on and cutting mostly 24+inch hard woods. Saw ran very strong and I could dog it pretty hard. I had non load speed set at 13,800. I really liked the speed in the cut.

I also did a little cutting in 8-16 inch pine......the saw really ran through that and didn't even breath hard.

I think from this experience I will be doing more Dolmars as I really like the way it ran. It may be my imagination but it seemed to make an already Smith saw when smoother. I also had the 30-35-55 nodded carb on this saw and it performed flawlessly. Started cold in three pulls, idled and reved very good and restarted with about a half pull when warm/hot. The timing was set at 30 degrees BDC.

 

[AlfA01]

Old thread, but good info for those of us that are getting into the 7900 upgrade realm a little late. I'm beginning a 6400 to 7900 project as soon as the parts arrive. I've got a 2007 6400 bone stock and new in the box. I recently ordered the 7910 P&C kit. Any need to know information from you guys that have already done the upgrade?

 

[kevin j]

Very interesting.

I have wanted to change to unlimited coil just for ease of tuning, not for more rpm. But not wanted to do the laminations changes and all the cut and fit work of the (Solo?) coil option.

I am not good at tuning the limited coils, just have a hard time hearing rich vs limiting. I do see the tac go funky so assume that is limiting mode.
Right now I am running about (I think from memory) 13 to 13.3 k, limiting at 13.5 to 13.8.

I have a 64/79 Mahle conversion 7900 maybe 15 years old, running 20 inch, 3/8 full chain, 8T (but 7T seems the same) and it seems rich when under load.

The 272 coil lists a dozen Husky models that use the same coils on ebay/Amazon, and are $10-15 so I’d like to try this.

I don’t understand how you know when the triggering spark point is. Obviously no points to check with voltmeter (yes I am old). Marking TDC is easy, degree wheel to -30 is easy, but how do you know what coil to flywheel alignment means spark will trigger?
Can I assume that the coil laminations are exactly the same as the Dolmar and aligning the edge of laminations should be exactly the same?
Or, is it get it close, then use timing light to find spark point and timing, then readjust the flywheel?

Link to Brad’s old post might help, I am not having much luck finding. (Yes, I am old tech here too....)

Not a rush, but for under $15 seems like a neat improvement.

 

[Poleman]

The triggering point is when the fly wheel passes the poles on the coil. The advance/retard can be changed by moving the flywheel in reference to the crank shaft changing timing either advancing or retarding. To much advance and the saw for a better explanation makes a potting noise.... when to retarded it’s has slow throttle response and very doggy in acceleration.
This all can slightly vary due to compression, fuel used and port timing.

 

[kevin j]

In concept, yes, when the magnet passes the poles, but I guess my question is how to define that point exactly, because a mm or two would be several crankshaft degrees.

As I’ve thought more about it, maybe I can use the strobe timing light first on the stock coil and see where the marks indicate the relationship of flywheel to poles.