Question for the Dolmar folks

[heyduke]

Yep I did the upgrade, theres nothing tricky about it, just PM Nate fordf150 and tell him what your doing.
He'll sell you the bits you need. The actual modification requres the muffler to come off 3, T25 torx screws from memory. The shrowd around the air filter, bolt holding the top bar and AV mount to the cylinder head. Then remove the carb boot from the cylinder head. Then make sure everything is clean before removing the 4 cylinder head bolts. Don't let crud fall into the engine at this stage. Then the piston comes off by removing the spring pins on the gudgon pin.

Putting the new bits is the reverse except you will need a ring compressor which Nate can sell you, its just a cheap plastic thing.
This was the first time Id pulled a 2 stroke apart, they are simple and designed to pull apart quick and easy.
No fancy tools requried.
I have a couple of links on my work computer which helped me. Ill post them up when I get into work.
If you have'nt run the saw yet in its 64cc form I would before doing the upgrade you will find with a 20 inch bar its pretty dam good. In the 79cc form its a blast.
I also upgraded to the heavy duty air filter. You will also need to tune it up after doing any mods.

If you do the upgrade my understanding is you will have the highest power to weight production built saw In the world. I'm happy for anyone to correct me if this is not true.
Also correct me if i missed anything..... Its been a year or so since i did it.

Cheers
Jeff

no need to remove the carb, after removing all the plastic and handle bar, remove the impulse line from its barb on the cylinder (or you'll tear it with the torx wrench). lift the rear of the air filter holder to disengage it. unhook choke and throttle. i prefer to remove the muffler. unscrew the four bolts that hold the cylinder and lift everything, cylinder, intake boot and carb as a unit. you can have the entire unit sitting on your bench in fifteen minutes. and its easier and faster to swap the carb/intake on the bench instead of in the saw. reassemble in the reverse order after installing the piston. a plastic ring compressor/piston stop makes that job easy and quick. i like to leave the old cylinder gasket in place since it is thinner than the ones that come with new top ends. the stock dolmar gasket is .5mm. the old paper gasket is maybe .030mm.

you have your choice of 84cc aftermarket cylinder (i like the nwp from baileys) or oem dolmar 79cc. the dolmar is way better but costs an extra benjamin. with the dolmar cylinder, a mahle, and paper gasket, i get 150psi at 6300ft altitude. with the nwp and no base gasket i get maybe 130psi. i have heard that it is possible to adapt the older non-catalytic muffler to the 6421 but you might want to try it with the stock muffler first.

these are great, very robust saws even without the upgrade.

 

[HandLogger]

NWP big bore $129 with shipping we will call it $145, add caber rings for another $15 so that puts you at $160. 7910 OEM kit is $200 shipped. no brainer for me

Can you supply the blue cosmetic bits for the saw, as well, Nate? The photos of the ex-rental saw in question show a lot of etching done by the Home Depot box store folks and, quite frankly, it looks like something our dog leaves out on the lawn.

 

[fordf150]

Can you supply the blue cosmetic bits for the saw, as well, Nate? The photos of the ex-rental saw in question show a lot of etching done by the Home Depot box store folks and, quite frankly, it looks like something our dog leaves out on the lawn.

yes but i dont stock any of the blue parts....yet

 

[Michigan Escapee]

Except for one ex-Rental I bought, sold, and later learned had a bad bottom end (that I sold to a lying thief of an AS member who was down on his luck, so I shipped it to him on the promise he'd pay me and, surprise, he never did), the worst that any of my ex-rental buys needed was a new A/V spring or two from being wrenched on by idiots who stuck the saw in the cut. These saws are built tough!

lol! If spite was currency, most Michigan people would never be poor.

 

[Jeffkrib]

http://www.arboristsite.com/community/threads/makita-dolmar-6400-big-bore-photos.244247/
This link is somewhat helpful but listen to heyduke is more up to speed than me.

 

[heyduke]

http://www.arboristsite.com/community/threads/makita-dolmar-6400-big-bore-photos.244247/
This link is somewhat helpful but listen to heyduke is more up to speed than me.

i just finished my third bigbore conversion a few days ago. i should have recorded it with my camera but got lazy. i used the stock mahle 79cc top end. bought it locally from the dolmar dealer. everything went smooth, took an afternoon. broke it in the next day dicing big wood from a tree job into woodstove lengths. the only problems i've had with it were tweaked av mounts. the previous owner had used it for removing some large cottonwoods and must have gotten it stuck a few times. i could tell he thought he was ripping me off when i bought it. but it was easy to straiten everything out and i got it for $200. guess i should have showed him how to use wedges.

 

[heyduke]

NWP big bore $129 with shipping we will call it $145, add caber rings for another $15 so that puts you at $160. 7910 OEM kit is $200 shipped. no brainer for me

nate-

those are great prices. i paid $260 for the oem kit at my local dealer last week. it was under $200 a few years ago. my kit did come with the plastic to accommodate the decomp valve that has moved to the right side of the cylinder. however the fit of the plastic wasn't very good. it also came with an aluminum gasket that was a full .5mm thick. i reused the paper one from the makita. compression was so good that i installed a new starter rope before sending it out into the world.

 

[HandLogger]

Thanks for the posts, fellas.

Is it necessary to re-jet the carburetor after the 7910 OEM top end is installed onto the Makita DCS 6421 (blue) crank case?

Is the muffler modification simply a matter of splitting the muffler and removing one piece? What are the contributor's thoughts on the muffler mod, in general?

Once again, thanks for your time

 

[heyduke]

Thanks for the posts, fellas.

Is it necessary to re-jet the carburetor after the 7910 OEM top end is installed onto the Makita DCS 6421 (blue) crank case?

these saws come in three flavors, 64cc. 73cc and 79cc. as far as i know they all use the same caburetor. i don't know what you mean by "re-jet". i've never seen a chainsaw carb that has removable jets like, say, a mikuni. almost all have adjustable low and high speed jets. an increase in displacement will result in increased air volume and increased velocity thru the carb's venturi and will require adjustments. i like to start mine out with a rich air/fuel mixture then move to a mixture that provides optimum power as the motor gets broken in. There are useful discussions about carburetor adjustment on this site and others including youtube. google is your friend.

 

[HandLogger]

these saws come in three flavors, 64cc. 73cc and 79cc. as far as i know they all use the same caburetor. i don't know what you mean by "re-jet". i've never seen a chainsaw carb that has removable jets like, say, a mikuni. almost all have adjustable low and high speed jets. an increase in displacement will result in increased air volume and increased velocity thru the carb's venturi and will require adjustments. i like to start mine out with a rich air/fuel mixture then move to a mixture that provides optimum power as the motor gets broken in. There are useful discussions about carburetor adjustment on this site and others including youtube. google is your friend.

Thanks for the reply. I'm certainly no chainsaw mechanic, but I do have experience wrenching on motorcycle engines. Perhaps that's where the idea about higher displacement and new jets came from.

Do you have any thoughts on the muffler modification?

 

[heyduke]

Thanks for the reply. I'm certainly no chainsaw mechanic, but I do have experience wrenching on motorcycle engines. Perhaps that's where the idea about higher displacement and new jets came from.

Do you have any thoughts on the muffler modification?

i haven't done this retrofit on the 6421, which is the saw that i think you have. i have three 6401's two with 84cc nwp's and one, my most recent work, a 79cc dolmar oem. the 6401's don't use a catalytic converter and in my opinion do not require a muffler mod. 6421's like yours do have catalytics and that's another question. i'm a radical apostate on this site. i think most field modifications are of questionable value. sometimes muffler mods are a significant improvement, but often not. one solution for the 6421 is to replace its muffler with one from a 6401 (or dolmar 6400) the problem is that they have slightly different mounts. the two screws adjacent the exhaust port are the same but the third screw at the top of the muffler uses a different spacer between the muffler and the cylinder. it is either shorter or longer, don't remember which.

to do a proper muffler mod (i think the 6421 may require this) you need to cut open or uncrimp the two halves of the muffler then cut or grind out all or part of the baffles and cat, then weld it back together. with some mufflers you can drill or grind thru the apertures provided by the intake and outlet.

if you plan to work on federal or state land you need to be sure that your spark arrester remains fully functional. if you don't want to start a fire on private property, same. if you plan to use the saw in a business environment, especially if you have employees, you should be aware that field mods not authorized by the manufacturer can create liabilities.

those are my thoughts, hope they help.

 

[Rob Stafari]

Definitely some carb gains to be had.

http://www.arboristsite.com/community/threads/zama-carb-mod-cliff-note-version.275905/

That's the short version of the story. The original thread where it was being figured out is quite long but worth the read if you want to understand the whys and not just the hows. Many thanks to Poleman and Terry Syd for all their time and effort there. I haven't done it to mine yet, but can tell from the videos it really wakes up the saw. Much better throttle response and low end grunt.

 

[SteveSS]

Thanks for the posts, fellas.

Is it necessary to re-jet the carburetor after the 7910 OEM top end is installed onto the Makita DCS 6421 (blue) crank case?

Is the muffler modification simply a matter of splitting the muffler and removing one piece? What are the contributor's thoughts on the muffler mod, in general?

Once again, thanks for your time

The existing carb will be just fine with the 7910 top end. I just did one of these saws (6421 to 7910) last month the same way. A stated there are three flavors, and they all use the same carb. The muffler was a different story for me. Uncrimping and pulling out the catalytic guts proved to be easier said than done at my house. I tried everything I could think of to get it uncrimped with my meager arsenal of tools here. I heated it until it glowed with a torch and still it wouldn't budge. Those mufflers are tough. I finally took one to the grinder and ground off the crimp part way around to get the pieces apart, and I still ended up ruining a perfectly good muffler. It would have been quicker, easier, and cheaper in the long run for me to pay someone else to do it, or just buy a non-cat muffler. Luckily I had a spare, and it didn't get in the way of finishing my project.

Here's a picture of my completed project...

 

[fordf150]

nate-

those are great prices. i paid $260 for the oem kit at my local dealer last week. it was under $200 a few years ago. my kit did come with the plastic to accommodate the decomp valve that has moved to the right side of the cylinder. however the fit of the plastic wasn't very good. it also came with an aluminum gasket that was a full .5mm thick. i reused the paper one from the makita. compression was so good that i installed a new starter rope before sending it out into the world.

I have sold probably 100 cylinder kits in the last year or 2 and none of them have come with a base gasket so not sure which gasket you ended up with. 957531160 is the base gasket for the 7910 and it is a steel gasket with rubber coating??? and it is .013"/.33mm thick.

 

[fordf150]

the cat muffler needs gutted if installing it on a 7900/7910. Personally, all i do is uncrimp it, remove the cat plate and the spacer plate, remove the bird cage baffle and put it back together. stock size outlet is plenty big for my taste and i leave the screen in every muffler i do. This minimal approach IMHO gives them plenty of room to breathe and they are quiet enough to use even in city settings plus it retains the factory spark arrester screen.

muffler modding a 7900 or 7910 muffler is a different story. i enlarge the outlet on those muffler because for some reason they are smaller than the 6400 cat muffler.

its personal preference but i like to use the 6400 cat muffler over a 7900 muffler. It seems to be a better made/thicker/heavier muffler and it has the reinforcement bracket on the front of it which eliminates some of the cracked muffler problems that the 7900's had.

 

[heyduke]

I have sold probably 100 cylinder kits in the last year or 2 and none of them have come with a base gasket so not sure which gasket you ended up with. 957531160 is the base gasket for the 7910 and it is a steel gasket with rubber coating??? and it is .013"/.33mm thick.

nate-

thanks for the info. could be that the seller screwed up. mine came with two gaskets, both coated aluminum. one didn't fit, looked like it was for a 50cc. it was .3mm thick. the one that did fit was .5mm. my previous projects used nwp kits. they came with a steel spacer for solos and a gasket, if my memory serves well. the nwp's had excessive squish so i did what most hombres do, went without a base gasket.

the oem top end has better compression. i get 150psi at 6300 feet, equivalent to 180psi at sea level. oem also has much better rings the. nwp rings are cheap junk. anyway, i'm really happy with the dolmar parts. next time i'll order from you, though i have no regrets for patronizing my local dealer. he's not just a dolmar, echo and shindaiwa dealer (a rarity these days,) he's the best outdoor powered equipment shop in the area and carries a good selection of parts for the mentioned brands and also stihl.

maybe you could shed some light on how to mount a 6400 muffler on a 7910. my memory is that the three threaded holes in the cylinder are the same but the spacer at the top is different.

 

[heyduke]

muffler modding a 7900 or 7910 muffler is a different story. i enlarge the outlet on those muffler because for some reason they are smaller than the 6400 cat muffler..

i've noticed this on some other saws using enhanced scavenging or strato technology. i suspect that a slight muffler restriction might be needed in part of the cycle to make them work. the mufflers i've seen on poulan stratos are ridiculously restricted. th 5 series husqvarnas on the other hand have longer, more elaborate (and expensive) fresh air ducts and don't seem to need as much help from the muffler.

 

[fordf150]

nate-

thanks for the info. could be that the seller screwed up. mine came with two gaskets, both coated aluminum. one didn't fit, looked like it was for a 50cc. it was .3mm thick. the one that did fit was .5mm. my previous projects used nwp kits. they came with a steel spacer for solos and a gasket, if my memory serves well. the nwp's had excessive squish so i did what most hombres do, went without a base gasket.

the oem top end has better compression. i get 150psi at 6300 feet, equivalent to 180psi at sea level. oem also has much better rings the. nwp rings are cheap junk. anyway, i'm really happy with the dolmar parts. next time i'll order from you, though i have no regrets for patronizing my local dealer. he's not just a dolmar, echo and shindaiwa dealer (a rarity these days,) he's the best outdoor powered equipment shop in the area and carries a good selection of parts for the mentioned brands and also stihl.

maybe you could shed some light on how to mount a 6400 muffler on a 7910. my memory is that the three threaded holes in the cylinder are the same but the spacer at the top is different.

If you have a local dealer thats decent buy from him. I love the business but lots of us small guys are hurting with the box stores taking so much of our new equipment sales.

7900 muffler will fit the newer saws with just a different top bolt and spacer but after doing many, many, many swaps i prefer the newer mufflers over the old ones. also a note about the old 7900 muffler installed on a 7910 cylinder.....the ports dont match worth a hoot because of the SLR

you cant go gasketless with the OEM kits. with the base gasket you still only end up around .025" squish

 

[SteveSS]

the cat muffler needs gutted if installing it on a 7900/7910. Personally, all i do is uncrimp it, remove the cat plate and the spacer plate, remove the bird cage baffle and put it back together. stock size outlet is plenty big for my taste and i leave the screen in every muffler i do. This minimal approach IMHO gives them plenty of room to breathe and they are quiet enough to use even in city settings plus it retains the factory spark arrester screen.

How do you get them uncrimped? Maybe a pic of the tools that you use and the general process?

 

[Termite]

How do you get them uncrimped? Maybe a pic of the tools that you use and the general process?

I use a propane torch and a screwdriver. Two people make it super easy.