suggestions for dolmar 6400 / makita 6401 big bore kit?

[bikesandcars]

I have searched AS and google and have learned a lot about the available Dolmar BB kits. I finally found a candidate makita 6401 and would like to do the upgrade.

I've heard some folks say the mahle / dolmar 7900 cylinder and piston is the way to go, a quick search shows these around $180+

There seem to be 3 BB kits available in the $110 range, the Bailey's NWP, the Hymar kit sold by watsonr (does he still sell these?) and one I found on ebay by a place called HLSUPPLY.. there are probably others, those are just the ones that came up.

To me one of these kits sounds like the best value, but the question is what kit to buy and how to set it up.

Reading all of you saw-junkie posts, you smart folks seem to have varying opinions, but none of the threads actually conclude with a recommendation, or at least not one that lets me go one way or another.

I'm really curious about what people recommend for a big bore kit and how to set it up. Do you run the gasket (squish numbers seem low from what people say), is there any consesus about these kits and what would be the best one to buy and put-in?

I don't mind cleaning up some non-cylinder port areas but don't want to mess with the cylinder wall at all. Just looking for a quality drop-in upgrade.

Sorry for beating this to death, it's just all the threads have pieces of info but I couldn't put my brain around an actual recommendation

 

[w8ye]

All three of those Big Bore kits you mentioned are made by the same company in Taiwan although there may be some slight differences in them.

 

[bikesandcars]

All three of those Big Bore kits you mentioned are made by the same company in Taiwan although there may be some slight differences in them.

now THAT is useful information! Thanks.

Is that a good or a bad thing?

Your statement makes sense because it seems that several kits have evolved almost simultaneously and all in the same price range. There is very little known about where they come from, and honestly how big could the dolmar BB market be to require several different companies casting and machining these. I wonder about the machining, do you folks think they be machined to different specs by the same or another company following casting?

The biggest single spec that confuses me is the squish people are seeing, it seems to vary wildly.

 

[watsonr]

Dolmar PS 7900

They are produced by the same company, machined the same. Differences are in the rings. Prices are nearly the same, shipping could be a factor:msp_rolleyes:

Great kits, easy to install, nothing extra needed as long as the fuel system is good-to-go and so are your seals. These are a direct bolt on and have produced exceptional results!

 

[nstueve]

I've had several of the kits mentioned and if all your wanting is more than stock power to run a longer bar; well these will do just fine.

The difference between stock and AM cylinders basically comes down to port timing, casting quality, and when your talking BBK's you might think about transfer volume too.

Big Bore Kits (BBK's) are a over-bored version of the stock 79cc cylinder. When you bore out the cylinder it takes some volume away from the transfers. You can grind out the transfers and do some extra cylinder mods to take care of this volume problem but it's not necessary.

The casting quality is hit and miss on all Asian made cylinders but the 84cc Makita/Dolmar kits seem to be pretty uniform in having good casting quality. This is probably because they are all made in the same place. Beware of painted cylinders as I have found some of the ones that have a black coating on them have been doing that as a cover-up on the poor casting. I can personally speak to the quality of the cylinders that watsonr has because he has been activly looking for porters and chainsaw professionals to rate his aftermarket cylinders and give him the low down on them.

Watsonr is weedeaterman.com posted above

others are
Chainsaw Parts (no longer a sopnsor) :msp_razz:
and baileysonline.com

Lastly is the port timing problem. None of the AM cyclinders have optimal porting timing. IF your a noob here port timing is like changing the cam in a V8 engine; it's the "brain" and where a lot of power can be lost or gained. Most AM cylinders having non-optimal port timing means they need to be ported for maximum power for their displacement. In the world of the 6400/7900 and their AM kits. The port timing is not optimal but its not seriously hideous either... The result is the extra displacement and non-optimal port timing leaves the 84cc AM kits about dead even with the 79cc OEM top ends. If you were planning on porting the saw in the future I'd encourage you to pay the extra $80 for the OEM but if not, the AM kit will do just fine. Spend the extra money on a HD air filter!

Heavy Duty Filter Kit for Dolmar PS-6400, PS-7300, PS-7900, PS-7310, PS-7910 - [957173400] - $69.95

Oh the last note is that watsonr is correct the difference is the rings. Cabre's are the best on the market so if you put an AM top end on make sure you don't use some really cheap rings on it during install (ie: the Taiwan cheapo rings). I believe Watsonr provides Cabers with his AM top ends... :wink:

 

[bikesandcars]

....all that good stuff..

Thanks a lot! I really appreciate your time putting all that down in writing!

Is squish / compression really important with this kit? should I forgoe the base gakets with these or should I measure first?

 

[Fifelaker]

Thanks a lot! I really appreciate your time putting all that down in writing!

Is squish / compression really important with this kit? should I forgoe the base gakets with these or should I measure first?

Measure first.

 

[nstueve]

Thanks a lot! I really appreciate your time putting all that down in writing!

Is squish / compression really important with this kit? should I forgoe the base gakets with these or should I measure first?

my BBK squish was at 0.019 without the base gasket but these kits are far from exact and each should be measured individually. I've see some as tight as 0.015-0.016 which is pretty tight for a 80cc saw. Small displacement saws are ok to run a little lower like that but every day work saws are more comfortable around 0.020. You'll probably have a bolt on compression between 155-165psi. You'll see a small bump in that by dropping the squish. The stock 79cc kits with new cabers bring 177-183psi on my saws with set squish at 0.019-0.022. I've done about two dozen BBK 84cc & stock 79cc. Depending on your bar length you probably won't notice the little bit of extra power until you're running 28-32in bars in hard dry wood.

Speaking of which if you leave a longer bar on this saw it will stretch the spring AV mount (between the cylinder and 1/2 wrap). Leave a 2x4 block under the bar to support the extra weight and this won't happen as fast. You can also flip the AV spring 180* so that it pulls the spring both directions.

 

[bikesandcars]

Dolmar PS 7900

They are produced by the same company, machined the same. Differences are in the rings. Prices are nearly the same, shipping could be a factor:msp_rolleyes:

Great kits, easy to install, nothing extra needed as long as the fuel system is good-to-go and so are your seals. These are a direct bolt on and have produced exceptional results!

Thanks, I got anxious and placed an order based on good up-front customer service

 

[MCW]

They are produced by the same company, machined the same. Differences are in the rings. Prices are nearly the same, shipping could be a factor:msp_rolleyes:

Great kits, easy to install, nothing extra needed as long as the fuel system is good-to-go and so are your seals. These are a direct bolt on and have produced exceptional results!

Hi Randy.
There are two manufacturers of the BB kits (or were?) and one was exceptional quality (both were Taiwanese). A number of members here have seen and/or own and run these kits.
Unfortunately the better quality kits seem to have gone AWOL and my supplier hasn't been able to get these for a while now. Squish was tight, they came with awesome rings (or ring!), and I didn't have one single problem with the dozen or so I sold.
Will Parris and Andyshine have had these kits although Will's Makita has since been sold to another member. Wayne (gmax) also has a saw fitted with one of these kits.
Anyway, sorry to butt in

 

[watsonr]

Hi Randy.
There are two manufacturers of the BB kits (or were?) and one was exceptional quality (both were Taiwanese). A number of members here have seen and/or own and run these kits.
Unfortunately the better quality kits seem to have gone AWOL and my supplier hasn't been able to get these for a while now. Squish was tight, they came with awesome rings (or ring!), and I didn't have one single problem with the dozen or so I sold.
Will Parris and Andyshine have had these kits although Will's Makita has since been sold to another member. Wayne (gmax) also has a saw fitted with one of these kits.
Anyway, sorry to butt in

I'm not, nice to hear from you Matt... and good stories of good saws! This one kit has been around a little and has no issues to speak of, or any that I've heard at least.

bikesandcars,

Thanks for the purchase, your going to like that kit!

 

[redoakneck]

I have a BB kit from watsoner that I just bolted on a 7300. I have a well broke in 7900 also. These 2 saws are very, very close in power. The 84cc BB is just now getting broke in at 10 tanks. They pull a 24" bar with an 8 pin and can hold 9500rpm buried in hard dry oak. I use rm chain.


That 7300 was very strong and got better gas milage that the 84ccBB, but it is staying on the shelf for now (the p&c)


Just my two scents (flatulence and BO)

 

[bikesandcars]

I'm not, nice to hear from you Matt... and good stories of good saws! This one kit has been around a little and has no issues to speak of, or any that I've heard at least.

bikesandcars,

Thanks for the purchase, your going to like that kit!

I'm really looking forward to it, this is my first "hot rod" saw. Probably more than I need but it seems like a great value for the size saw I'll end up with. I'm looking forward to the build and how it goes afterward and will be sure to post how it turns out.

 

[bikesandcars]

I got the BB kit and air filter yesterday so I couldn't resist doing the swap last night.

Overall it went pretty smoothly.

Note the vastly different port and runner design between the 6401 cylinder and the 84cc big bore.

View attachment 308608

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I finished tuning it this morning but I had a tuning question:

I had reset the initial mixture at 1 turn out L and 1.25 turns out H. At this setting I was only getting about 12,000 rpm and it was noticeably 4-stroking. at 13000 rpm I was about .9 turns out, and at 13,500 I was about .75 turns out. This is a little further in on the screw than I'm used to.

I settled on about 12,500 rpm and ran it through some test wood. It cleaned up while cutting, it seemed about perfect. The power wasn't great yet but it's not even barely broken in. This saw doesn't 4-stroke nearly as hard as the smaller cc saws I'm used to, it's a lot more subtle.

I was afraid to screw the mixture in further and go too lean. Do I have something going wrong here?

side note: the 6401 cylinder still has the cross hatching on it! talk about a low-use saw I'm going to save it in case this bb kit doesn't work out like I want.

 

[redoakneck]

Mine is the same way, tuned at 12500 and running slight fat for first 5-10 tanks. I will lean mine to 13 once it is broke in.


Nice job. That saw will get stronger as the rings get seated.

 

[bikesandcars]

Mine is the same way, tuned at 12500 and running slight fat for first 5-10 tanks. I will lean mine to 13 once it is broke in.


Nice job. That saw will get stronger as the rings get seated.

I just fiddled with it and though it is running well, I'm all over the place with the L adjustment.

I traditionally lean the L so it idles a little high and stumbles when I gas it, then tune richer until it doesn't stumble.

On this saw It seems to die (lean) before it stumbles hard accelerating. I started with 1 turn out and 1/4 turn either way doesn't seem to make a big difference until the saw just dies.

Am I missing a subtle hint on the L adjustment? I'm always open to improving my tuning game

Other saws I've rebuilt seem to be difficult to adjust right after a rebuild, but after a couple tanks they seem easier. Is it me or are saws easier to tune after they are broken in?

 

[redoakneck]

Put a couple tanks thru it should help. A lot of times I will try to tune a saw that is still cold after a rebuild and have some problems. The rings are super leaky at first so put some gas thru her to seat them in, just make sure you are on the richer side of the tune. Makes sure it idles without surging, if you have surging check for an air leak.

 

[bikesandcars]

Man, the more I play with it the more confused and frustrated I get. '

Is there a larger L and H cross-tie on these saws?

Setting the mixture on this saw is driving me nuts, adjusting the H seems to mess with the idle, but adjusting the L doesn't seem to do anything with the H. (Yes I'm turning the correct screws )


It seems to idle and run ok at 1 turn out on each screw. If I play with the L it doesn't seem to matter until the saw just dies. If I adjust L around .75 it runs OK but won't start.

Is the mixture too rich or too lean if you need to give it full throttle to start it?

If I try to lean for 13,500 rpm I never get there ,my tach is now showing about 11,800 WOT with 1 turn out on H. If I adjust H in it doesn't go above 12K, but then the saw won't idle.

Compounding this is the silly adjustment screws that I have to hunt for each time.

Maybe I need to step away or something.. I can make it run but I can't find the sweet spot where it starts on the first pull after sitting 15 minutes. It seems to be 4-stroking well on the top end though.

It's not surging at all except when I leaned the mixture on the H and it surged on the way down (too lean), definitely not lean sounding in general.

shoot, should I just post a video?

 

[redoakneck]

Did you make sure the limiter caps are trimmed and not hanging up??? I don't recall for sure but I think I took the whole cap off mine and turn the metal screw that was capped, so many carbs, so little memory:msp_sad:

 

[bikesandcars]

Did you make sure the limiter caps are trimmed and not hanging up??? I don't recall for sure but I think I took the whole cap off mine and turn the metal screw that was capped, so many carbs, so little memory:msp_sad:

Yea, I actually grind the tabs off the H cap because at least the H cap has a screwdriver keeper on it that makes adjustment easier. I don't have a dolmar carb adjustment nut driver.

I think I just need to put some gas through it. My 5105 was tough at first too but after a few tanks it was very easy to adjust.