How to make your 7900 or 681 NON rev-limited

[tdi-rick]

Yeah, me too:greenchainsaw:

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[BloodOnTheIce]

I would not be at all surprised to see a 084 beat a MS880. Andre's 084 is not rev-limited. The MS880 is limited with both the carb, and the coil. Remember how Gink's 880 was constantly on the limiter that one time you ran it? Once you get rid of those, they'd probably be quite close.

Same thing happened to my 3120, with the adjustable carb and unlimted coil it acted like a completely different animal.

 

[blsnelling]

Same thing happened to my 3120, with the adjustable carb and unlimted coil it acted like a completely different animal.

No doubt! Makes all the difference in the world on those two models.

 

[spacemule]

Here's a vid of this saw shortly before I sold it. It's wearing a 28" B&C. The 7900 at the end is stock but only wearing a 20" B&C. Despite the longer B&C, the 681 is still about 30% faster.

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Here's another of it.

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2 points:

1. you're comparing 2 different makes/models of saws
2. the cuts are not the same type of cut. overbucking is consistently slower than a smooth straight cut like you were doing with the solo

For these reasons, the comparison is meaningless.

 

[CHEVYTOWN13]

Show off

Me no show off here no mo

 

[timberwolf]

I know this is an old thread but....

Either I am missing something here or there is some info amuck.

How to make your 7900 or 681 NON rev-limited
I finally got an OEM Husqvarna coil for my 681. It is installed on my 681 and running like a top. I thought several of you might be interested in knowing exactly what was done to make this happen.

1.Purchase a coil for any one of the following Husqvarna models. 334, 335, 336, 338, 339, 340, 345, 346, 350, 351, 353, 357, 359, 362, 365, 371, 372, 385, 390, 570, 575. There are a couple different OEM PNs that are used on these saws. I believe that the only difference is the length of the spark plug wire. I used PN 5033014-01.
2.Remove the air filter cover.
3.Remove the sparkplug.
4.Remove the recoil assembly.
5.Remove the flywheel.
6.Remove the hold downs for the spark plug wire.
7.Remove the original coil and spark plug wire.
8.Remove the metal core from both coils. It will simply press out. Take care so as to not crack the case of the coil.
9.Remove one layer of the laminate from the metal core of the original coil. It is too thick to go in the replacement. Take a pair of needle nose pliers and simply peel it over the rivets. Re-peen or crush the rivet heads to keep the laminates tight.
10.Press the lamite steel core into the replacement coil. Make sure to seat it all the way to the bottom. It will slightly stick out the far side of the coil.11.Fit a new section of spark plug wire to the new coil. It simply threads onto a stud in the coil. You will not be able to use the little dust boot where the plug wire goes into the coil.
12.Thread the new spark plug wire through the protective sleeves and grommets used on the original. Leave it long on the spark plug end for now.
13.Bolt the replacement coil into the saw, with the plug wire routed as shown below.
14.Connect the ground wire to the coil.
15.Lay the plug wire into the groove in the case and reinstall the keepers.
16.Position the full length of the new plug wire as it needs to be, and terminate it on the plug end.
17.That concludes the installation of the coil.

Here is a picture of a 7900 coil with a coil that would be found on 357 359 365 372 385 ect. The husky is a three poil coil with close spacing the 7900 2 pole with wider spacing. To me it looks like the amount of "pressing" required to fit a 365 husky coil onto 7900 Dolmar coil poles would kill more than just the rev limiter, also a bunch of fitment problems with the case.

272 or husky 50 coil yes that is a closer fit and stihl coil too looks close but not sure how it would be running a 3 pole coil on a 2 pole fly wheel? Also outlet for the plug wire is not in good location.

 

[rmh3481]

Did you try peeling back one of the layers off the lamenations? Maybe the third pole has no bearing on it?

 

[timberwolf]

The laminations are not the main problem, though it does look like the pole cores on the 7900 would be too thick as well.

The big problem is there is no way the actual coil part from the 365 would fit on the poles from a 681 or 7900. The holes don't line up with the poles. Even if they did it would take a bunch of grinding up the case to make room.

I guess its all just a matter of straining the information out of the BS.
Last two 272 coils I had went on an efco and a solo and don't seem to have found anything else to fit so this 7900 might just stay with the limiter. It is not cutting near the limiter so would have just been for the convienience of setting the carb.

<iframe title="YouTube video player" width="480" height="390" src="http://www.youtube.com/embed/a8EoL_ULZCA" frameborder="0" allowfullscreen></iframe>

 

[ECsaws]

Hi Brian, Hope all is well..
Nope I cant get a 7900/681 to pull above the limited either, I actually think they make more power on the stock coil.. (maybe its a advancing coil ? not sure)
If you want to change it, The 372 series coil wont work, the 272 series WILL.
There is also a real easy way to install them. If your interested let me know and I'll send ya a diagram.
Eric

 

[timberwolf]

Hi Brian, Hope all is well..
Nope I cant get a 7900/681 to pull above the limited either, I actually think they make more power on the stock coil..

Thanks Eric, good to know someone else was finding the same thing. I found the same thing too with husky 365/372 with the limited blue 365 coil, bolting on the black unlimited 372 coil did give more open throttle RPM but it lost a tenth of a second in 10 inch round. In that case though the blue coil fired a couple degrees earlier than the black.

Replaced my timing light so will give it a try tonight and see if there is some advance going on with the coil.

On the Efco and Solo I just pressed the coils off the poles and pressed them back on the other sets of poles, took a look at where the magnets and poles were in relation to TDC on both saws and set the timing by that. If there is a better way I'd be grateful to know.

Over 100 posts though before anyone picked up that info in the original post was off. Maybe Brad could go back and update it to save the confusion for anyone ordering the wrong coil.

 

[timberwolf]

By part numbers I'm looking at you will want a coil from husky 50, 51, 55, 61, 254, 257, 261, 262, 266, 268K, 272, 272K or Jonsered 625, 630, 2054, 2055, 2095.

 

[roncoinc]

Thanks Eric, good to know someone else was finding the same thing. I found the same thing too with husky 365/372 with the limited blue 365 coil, bolting on the black unlimited 372 coil did give more open throttle RPM but it lost a tenth of a second in 10 inch round. In that case though the blue coil fired a couple degrees earlier than the black.

Replaced my timing light so will give it a try tonight and see if there is some advance going on with the coil.

On the Efco and Solo I just pressed the coils off the poles and pressed them back on the other sets of poles, took a look at where the magnets and poles were in relation to TDC on both saws and set the timing by that. If there is a better way I'd be grateful to know.

Over 100 posts though before anyone picked up that info in the original post was off. Maybe Brad could go back and update it to save the confusion for anyone ordering the wrong coil.

He did update it in post #44.
the original post #1 had me confused also.

 

[weimedog]

By part numbers I'm looking at you will want a coil from husky 50, 51, 55, 61, 254, 257, 261, 262, 266, 268K, 272, 272K or Jonsered 625, 630, 2054, 2055, 2095.

I guess I need some help understanding why I would spend those dollars on a different color coil for a work saw. I've scratched my head on the no load RPM issue on work saws since I've been here...If my 72cc-89cc work saw will be cutting at say an arbitrary range (depending on the saw) ie. some where in the 9,000 to 11,500 range..why do I care if it can rev past 14,000 no load vs. focusing on gains in power at that actual working RPM range? It would take extensive mods to get that work saw to reliably cut at past 13,500RPM's in the wood, more than most targeted by these posts will ever do..if its even possible! Mod's looking to pick up an actual 500 to 1000 RPM's are much more probable in the cut or just hold RPM's under larger loads placed by longer bars , more aggressive chains, more teeth on the drive sprockets and more pressure by the operator etc....either way those work conditions are still under the rev limited 13K plus RPM's allowed by the coil in most cases. Also I think I would rather have that Rev Limiter in place after a woods porting anyway...protect the investment from "brain stool" moments showing off the saw making noise for a video or GTG! I guess if there are limited coils on the small bore saws and you might see higher than limited RPM's in the work there is a possible gain..or real racing saws....but not the typical saw I have or the typical target for this posting who would just swap coil's on there stock saw looking for gains. WHICH is why no one caught the errors..no one tried.

There is ONE area I would like to see some tech innovation types spend time and that's on the rev limited Carburetors! They are a Pain, especially on some of the old Homelite's where I would like to get closer to Modern saw RPM's out of them..and that little ball & spring deal needs to be disabled. So far epoxy is all I know how to do..but I ruined one anyway with bad eyes and fat fingers.

Another is understanding the actual differences between model numbers when modding things like the 61 thru 272 series. Can you retro different model carbs to help with mods and actually gain power? That would be interesting to read about. ie swap a HS-163a for a HS260a and see any difference? Or are the differences more about hookup and other things.

 

[blsnelling]

I was just looking back over this thread. It appears that I put bad info on what coil to use. I said to use the unlimited coil that fits saws like the 346, 357, 359, 372, 385, 390 etc. Obviously, that's not what's pictured on the Solo. I believe the coil you need is for 50,51,55,61,254,261,262,268,272,394.

Either I am missing something here or there is some info amuck.

Here is a picture of a 7900 coil with a coil that would be found on 357 359 365 372 385 ect. The husky is a three poil coil with close spacing the 7900 2 pole with wider spacing. To me it looks like the amount of "pressing" required to fit a 365 husky coil onto 7900 Dolmar coil poles would kill more than just the rev limiter, also a bunch of fitment problems with the case.

272 or husky 50 coil yes that is a closer fit and stihl coil too looks close but not sure how it would be running a 3 pole coil on a 2 pole fly wheel? Also outlet for the plug wire is not in good location. By part numbers I'm looking at you will want a coil from husky 50, 51, 55, 61, 254, 257, 261, 262, 266, 268K, 272, 272K or Jonsered 625, 630, 2054, 2055, 2095.

He did update it in post #44.
the original post #1 had me confused also.

I'm going to see if I can get a mod to change my original post to save some people some hassles.

 

[timberwolf]

I guess I missed post #44. Thanks.

The point of the coil swap is not to gain in the wood RPM. It will factor in some on a race where the energy stored in spinning the motor and chain up that extra couple thousand RPM before it hits the wood can be harnessed to cut wood.

The point is more just for setting the jetting on the carb, given the saw is ported or muffler modded the factory spec for a certain fraction of a turn past where the limiter is set to come on is of no help. The 7900 I have on the bench will sit on the rev limiter no matter where the HS is set 1/2 turn out or 3. It is quite possible to tune them decently by timing cuts and reading the plug but far from convienient.

Seems most of the rev limited saws I pull down show signs of being run rich.

 

[blsnelling]

I understand that there are easier ways of doing this, but there may still be some useful info this thread.

 

[blsnelling]

Six years later, and I'm just now doing this mod for the second time. This time it was on a 7910. I set the timing at 30° advanced past the factory flywheel timing.

I've marked 0°, 25°, 30°, and 35°.