The Makita Monster I Think I've Created

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rta01

rta01

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Finally got around to rebuilding a Makita 6400 (6421?) I mentioned in a post about a year ago:
Ended up buying a used Home Depot rental, Makita DCS6421 - 20", several years ago. Needed some initial love, but has worked well. Put 24" bar on for some of the bigger trees. Recently started leaking (really pouring out) fuel. Found it to be the flywheel side seal. Replaced the seal and the leak stopped for a very short while, but started again same place. New seal was destroyed. Found crankshaft to be wobbly. Bearing shot. Plan to completely rebuild with big bore. Do I really need this big of a saw? No, but, ..... well, you know. But still need a bigger saw than the Stihls to keep working on property now.
Had collected all the parts I thought I needed and got an aftermarket 7900 cylinder and piston. Been putting off jumping into rebuilding, but took the opportunity the last couple of days since we were having so much rain and got it put back together mostly. That was an adventure, remembering where and how everything went together. But got it there nonetheless late today. Decided to give it a whirl, see if it would start/run. That's where the monster part comes in. Set the choke, made sure the stop switch was not on stop, pushed the decomp button, and pulled the cord. About broke my wrist. I can only pull one compression cycle. Checked the decomp button and tried again. Same results.

First thought is I had something messed up internally. Removed the spark plug and pulled the cord. Spun as nice as you please.

So started looking back through all the various revisions of parts lists I have accumulated - here's what I found. One of the lists shows a Type 1 and Type 2 decompression valve. I reused the one I had and it is type 2 078-118-180. The type 1 is 001-131-150. Type 2 is for use with the black hood and Type1 is for use with the gray hood. (Decomp on right vs decomp on left.) I think all the 7900's had decomp on left - the aftermarket I got did. So I had gotten the gray hood as well. Never saw anything about a different decomp.

Am I on the right track?? I'll certainly never get it started with how I can pull it now.
 
wcorey

wcorey

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Not sure but if the decomp fits and pushes in/out without interference it should work, I don't usually even bother using them though.
Maybe check that you didn't shear the flywheel key, a bit too advanced on the ignition timing and it will do that to you.
 
rta01

rta01

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Not sure but if the decomp fits and pushes in/out without interference it should work, I don't usually even bother using them though.
Maybe check that you didn't shear the flywheel key, a bit too advanced on the ignition timing and it will do that to you.
The flywheel key is integral with the flywheel.
 
rta01

rta01

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Holding the saw upside down pulling it over without the plug would help clear it out. It is possible that the carb needle is leaking and flowing fuel back into the case.
I'll give that a try tomorrow. The carb should be fine. I pressure tested it before I put it in. Held 7 psi on the fuel input
 
rta01

rta01

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Yup, can still shear just the same, actually easier than a separate one as it's fairly weak in both design and composition compared to a steel key.
It can also get mushed over if the flywheel is tightened down with it partially out of the keyway...
Well that would certainly be a bother. In all my searching around for parts for this saw, I did run across info that there had been a recall for the flywheel on certain models of this saw. There had apparently been a couple of reported incidents where the flywheel flew apart. And, of course, my saw serial number fell in that range. Think Makita would still spring for a new flywheel?
 
rta01

rta01

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OK. Pulled the hood back off so that I could see the decomp valve clearly. It was not pushing in all the way - bottomed out on something inside the cylinder. Backed it out until I could fully engage it. Was then able to pull the cord. It was still pretty heavy on compression, but was able to get it through 3 or 4 cycles on one pull. Not sure how this setup will work long term - probably need to spring for the other valve. Anyway, did get it to "cough" once, so it is firing. Wore out pulling, so taking a break.

Checking the spark, it did not look as strong as I think it should. New spark plug. For the tear down and rebuild, I did not mess with the coil at all. My thinking being that part ran fine before, don't mess with it.
 
rta01

rta01

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My 6401 will flood easily if I am not careful.I usually pull about twice with the choke on then push it in to 1/2 throttle.
This one's the same way. I would end up pulling the plug and clear the excess fuel. I saw a thread here that mentioned a different seat spring for the carb. Gonna look into that.

BTW - it's running. Rough adjusted the carb. Haven't put the bar/chain on yet. Got it going by partially screwing it out the decompression valve, but then it loosens on up when it's running, so figured no point until I have the correct one. Tried just plugging the hole, but compression is still too much for me to start.
 
rta01

rta01

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It lives! New decompression valve came yesterday. The threads are a good 1/4" shorter - so difinitely the problem. Just finished completely putting it back together. Put the 24" bar and chain on it. Set the high end at 12500. Will run it there for a bit for break in. Then may go up to 13500.
 
dirtcurt

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Yup, can still shear just the same, actually easier than a separate one as it's fairly weak in both design and composition compared to a steel key.
It can also get mushed over if the flywheel is tightened down with it partially out of the keyway...
I had one shear and it drove me crazy! It was a Echo 590 Timberwolf. It wouldn't lean out, had low power and nothing seemed wrong with it.
 

MMG

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Oh yeah. Mine has a 85cc kit on it. I can't pull it over at all without the decomp. Got nothing to prove at this point and it runs great, pulls hard & fast. I think you will be impressed once you get that thing dialed in. Too bad Makita is getting out of the chainsaw business. Dolmar saws have been around since 1923 and have now succumb to the UN Globalists and have gone electric. What a shame.
 
dboyd351

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Oh yeah. Mine has a 85cc kit on it. I can't pull it over at all without the decomp. Got nothing to prove at this point and it runs great, pulls hard & fast. I think you will be impressed once you get that thing dialed in. Too bad Makita is getting out of the chainsaw business. Dolmar saws have been around since 1923 and have now succumb to the UN Globalists and have gone electric. What a shame.
Agreed on that
I always wanted a 7900 or a 6400 with a big bore jug
but not so sure its a good idea now
 
old CB

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I bought a Dolmar 7900 some years back, and been quite happy with it. I put a 24" bar on it, and since I rarely have anything to cut that requires that size bar/chain, that saw rarely gets used.

Picked up a toasted Dolmar 7300 about a year ago and put a 7900 piston & cyl. on it (Hiway?--can't remember but it was a good aftermarket outfit), and it's become one of my go-to saws, w/ 20" bar & chain.

I don't use the decomp on any of my saws. The Dolmars (and your Makita is a Dolmar with blue paint) are some of the most challenging to pull over, but mine start.
 
rta01

rta01

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I bought a Dolmar 7900 some years back, and been quite happy with it. I put a 24" bar on it, and since I rarely have anything to cut that requires that size bar/chain, that saw rarely gets used.

Picked up a toasted Dolmar 7300 about a year ago and put a 7900 piston & cyl. on it (Hiway?--can't remember but it was a good aftermarket outfit), and it's become one of my go-to saws, w/ 20" bar & chain.

I don't use the decomp on any of my saws. The Dolmars (and your Makita is a Dolmar with blue paint) are some of the most challenging to pull over, but mine start.
Since I got it running and have run it some, it is a little easier to pull w/o using the decomp. I still don't think I could pull it through enough cycles to get it to start, though. And my wrist and shoulder do not appreciate me trying.
 
dboyd351

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Since I got it running and have run it some, it is a little easier to pull w/o using the decomp. I still don't think I could pull it through enough cycles to get it to start, though. And my wrist and shoulder do not appreciate me trying.
Don't know your age, but particularly if you are getting up there, I'd say discretion is the better part of valor. Use the decomp and save your body.
 

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