Makita 6401 to 7901 Plus porting the 7900 and the Solo 681

[Locust Cutter]

I would still love to have a cherry 681...

 

[wde_1978]

I would still love to have a cherry 681...

I wouldn't mind having a well used but fully operational and healthy Solo 681 to accompany my Dolmar PS-7900.

After all they are related to each other in a matter of speaking and it could be interesting running them side by side just for kicks.

 

[Chris J.]

They seem not to be beveled much, if any!?

...

My thought was "They left the factory looking like that?".

Randy, other than the more restrictive muffler, is the Makita DCS6421 any different from the 6401?

 

[RedneckChainsawRepair]

Brad I just remembered where a lightly used 681 still might be. You will know by this pic so it gives you inside track. @blsnelling

 

[Mastermind]

do these saws use similar #s to a 372 ?

Not at all.

Sp Randy are these a customers or are you selling them?

I'm selling the Makita.

How much room do you have before the ring would get into the intake?

After cutting the squish on the 7900 jugs there is about 4mm.

I would still love to have a cherry 681...

How about a brand new 665? We can use a 7910 jug......

My thought was "They left the factory looking like that?".

Randy, other than the more restrictive muffler, is the Makita DCS6421 any different from the 6401?

I've never been inside a 6421.

 

[mdavlee]

The 7900 jug shows what happens when a big bore is put in a smaller cylinder, the transfer tunnels run close to the bore and there is a tighter turn (and perhaps less cross section) for the transfer flow. The Solo appears to have addressed the porting problem and moved the transfers out from the bore. - A 5% increase isn't going to come from a 2 millimetre increase in stroke.

I was thinking my next big saw would be a 7900/7901, but that well designed Solo jug now has me thinking about the Solo.

Can the 681 jug be used on 7900 cases and still be able to use the increased area in the lower transfers?

You could use it with a 1mm spacer. The cost and just getting one would be the hardest part. The cranks were twisting on a few of them that were ported pretty hot. Stroking the dolmar may be a better option in the end for the stroke.

 

[Terry Syd]

The 681 jug looks to be the better jug to work with, but that extra two millimetre of stroke complicates things. Shifting the jug one millimetre changes the timing figures roughly 4 degrees.

Too bad some after market company doesn't produce a 681 transfers type of jug for the Dolmar length stroke. They'd probably sell a heap of them. Heck, the R&D for the transfers has already been done by Solo, they could just take castings of them and incorporate that into a slightly different jug. Maybe even turn it into an 84cc jug that could actually breathe.

 

[Locust Cutter]

Not at all.



I'm selling the Makita.



After cutting the squish on the 7900 jugs there is about 4mm.



How about a brand new 665? We can use a 7910 jug......



I've never been inside a 6421.

PM sent.

 

[wde_1978]

My thought was "They left the factory looking like that?".

Randy, other than the more restrictive muffler, is the Makita DCS6421 any different from the 6401?

Yup, that is how the jug looks like stock.

The way I see it the Makita DCS-6421 is identical to the Dolmar PS-640x - completely identical rated specs.
The parts list seems to show some parts replacements/changes where one specific part number is meant for saws built till xx-date and a successor part number from xx-date onward.

I think they (Makita/Dolmar) would be stupid to make mayor changes to this saw series.
The lower rpm limited coil and SLR muffler is taking its toll on the PS-7910 line in form of power loss - luckily the plugged muffler can be "fixed".

 

[Poleman]

On a stock 7910, after you take the time to get SLR chamber out to open it up, it will run a lot better. Next if you unlimited the coil it's like porting the saw in comparison......but if you port it on top of that it's unreal.

The unlimited coil is one of the best bangs for the buck for theses saw that remain stock and really brings them to life.

 

[Termite]

On a stock 7910, after you take the time to get SLR chamber out to open it up, it will run a lot better. Next if you unlimited the coil it's like porting the saw in comparison......but if you port it on top of that it's unreal.

The unlimited coil is one of the best bangs for the buck for theses saw that remain stock and really brings them to life.

Do you mean with just an unlimited coil or unlimited coil and timing advance?

 

[Poleman]

With the 272 coil to unlimite it you need to advance the timing from stock timing around 30°'s from where the key is on the flywheel. Stock timing goes out the window with the different coil.

 

[Poleman]

The other thing tat is nice about changing the coil, it work on either the 6400-7900 and 6401-7910 just the same. You keep what flywheel you have and it will work the same.

That's for those with 7900's with a bad coil and it's been sitting because of the need to purchase a 7910 coil and flywheel, which will be limited now at 12,5, instead of 13,5 originally. This is way cheaper with better outcome and you will personally love it!!!!

 

[benp]

The other thing tat is nice about changing the coil, it work on either the 6400-7900 and 6401-7910 just the same. You keep what flywheel you have and it will work the same.

That's for those with 7900's with a bad coil and it's been sitting because of the need to purchase a 7910 coil and flywheel, which will be limited now at 12,5, instead of 13,5 originally. This is way cheaper with better outcome and you will personally love it!!!!

Why yes, yes you will.

This was today in Red Oak with the saw tuned at 14.4k.

 

[Poleman]

Saaaaweeet!!!!!

Sent from my SAMSUNG-SM-G900A using Tapatalk

 

[166]

The other thing tat is nice about changing the coil, it work on either the 6400-7900 and 6401-7910 just the same. You keep what flywheel you have and it will work the same.

That's for those with 7900's with a bad coil and it's been sitting because of the need to purchase a 7910 coil and flywheel, which will be limited now at 12,5, instead of 13,5 originally. This is way cheaper with better outcome and you will personally love it!!!!

The Red 13,500 coils for the 6400-7300-7900 are not discontinued?? The PS-7310/7910 coil is limited to 12,800 and not 12,500.

I don't think we've had a complaint from someone that has 7900's and 7910's about the 7910 power. Most actually like the 7910 better.

 

[Poleman]

Not dogging the factory coils for the everyday Joe, but everyone on AS is interested in what to do to make saws perform better. We ALL like Hot Rods!!!!

I've also now heard conflicting information on the availability of the 7900 coils and the rpm limit on the 7910 coils. Last I knew they were unavailable.

Sent from my HP Slate 7 using Tapatalk

 

[benp]

Not dogging the factory coils for the everyday Joe, but everyone on AS is interested in what to do to make saws perform better. We ALL like Hot Rods!!!!

I've also now heard conflicting information on the availability of the 7900 coils and the rpm limit on the 7910 coils. Last I knew they were unavailable.

Sent from my HP Slate 7 using Tapatalk

This part. Especially when dealing with a modded one.

My plugs are nice and tan in the 14.5 +/- range.

 

[166]

I'm just trying to keep the information correct on here. Some will read this and think the saw runs terrible with the factory setup.

 

[fordf150]

7900 is limited at 13500 and 7910 is 12800. Last I checked the part number for the 7900 coil updates to a coil/flywheel combo.
The 13500 coil works fine for stock saws...mine likes to be tuned at 13200 but that is a bone stock saw. Ported I would imagine needing a few more rpm and if running the 12800 coil in smaller wood the saw would be against the limiter.