Makita 6401 conversion low compression

[Griffbm3]

Hate to be a pain...

This is crappy advice, but switch it back to a 6401, and try it then. Run it in a log or two and make sure it works well. Then re-assemble. If it's bad, call Baileys. That prevents you from having to figure out how to do a pressure test on the crankcase and what not. Since a switch job is a pretty fast job, and you just did it...

See what I mean though, it's not good advice.

Wish I had more.

Jason

 

[RiverRat2]

This is crappy advice, but switch it back to a 6401, and try it then. Run it in a log or two and make sure it works well. Then re-assemble. If it's bad, call Baileys. That prevents you from having to figure out how to do a pressure test on the crankcase and what not. Since a switch job is a pretty fast job, and you just did it...

See what I mean though, it's not good advice.

Wish I had more.

Jason

Wow Jason!!!!!!

Are you serious???

 

[Griffbm3]

All right, I said it was bad advice...

All right, it was bad advice... We know the following: Carb good/ran good prior to switching. There is an impule line from the right rear portion of the dolmar head to the right side of the carb on that saw.

The saw worked just prior to the teardown. Therefore, I would surmise that the bottom end is fine as well. I realize that this is a gamble not being there, but I will take the internet chance that it's probably okay.

When you put it all back together, and lined up the four head bolts, did they go in easy/straight? Does your carb appear to be nicely in its' place, or is it "mashed" into its' position?

The carb has limiters on it, should be fine for a start up. Everything is coated with 2-stroke mix. Are you getting spark to the spark plug?
I know you said the compression is low, and it won't start, but I figured we could cover everything.

ONE OTHER THING!!! I did this ONCE, and yes I will admit to it.

When you put the new piston onto the connecting rod, DID YOU REMEMBER THE BEARING? I didn't once, and the wrist pin fit REALLY easily, and it fired, ONCE!!! Never again, and the compression was really LOW. Just a thought.

Jason

 

[slnranger]

yep,everything lined up, went together well. Yes on the wrist pin bearing.

It did run ok before but I don't remember the compression being as strong as the saws I looked at today. So it may have been weak before. Assuming I did the top end ok, are there things on the bottom end that would cause low compression?

 

[Griffbm3]

Well, no...

It's all relative. You say it ran good as a 6401. But the compression wasn't feeling as "strong" as the ones you checked out today at the depot. But you replaced the decomp button, good move by the way.

I usually don't hold much stock in the "drop test" with compression. nor does a compression test tell the whole story. But, when we are talking about the bottom end, and it ran before the top end rebuild, the bottom end shouldn't really fit into the equation here.

Compression tests are for the top end, pressure and vaac tests are for the bottom end in the most basic of principles. The fact that it is not running is what has me.

Carb boot tight after rebuild? New or untorn impule line from right lower portion of head to carb? Fuel line to carb in place? Spark to the plug? 4 head bolts lined up properly and seated (preferably torqued, but who has a torque wrench when you need one?), fresh mix in the tank? Should fire right up.

Since it doesn't and it feels "weak", Hmmmm. By any small chance, did you drop the cylinder or piston just before assembly?

As my first post gave, it might be crappy advice, but I might switch it back to a 6401 just to try it out as a rule out. But I get freaky when things don't work right...

 

[wanab]

if you say the saw wont start and your decomp was pressed that may account for the low reading.

check your plug, you may have got too much oil on the top end.

 

[Jack_Shaft]

As my first post gave, it might be crappy advice, but I might switch it back to a 6401 just to try it out as a rule out. But I get freaky when things don't work right...

That's where I'd be heading.

 

[ericjeeper]

O would not mess with the 64cc jug

Pull the carb off remove the jug. remove the ring from the piston. place the ring into the bore. measure the End gap. report back. Also get some soft solder, Lay four pieces one at noon 3,6 and 9 on top of the piston bolt the jug back on do a squish test.Do this without the ring.
Or this could all be possibly caused by a severely flooded saw.
You could have a 73cc piston in a 7900 jug.

 

[Griffbm3]

Great post

Pull the carb off remove the jug. remove the ring from the piston. place the ring into the bore. measure the End gap. report back. Also get some soft solder, Lay four pieces one at noon 3,6 and 9 on top of the piston bolt the jug back on do a squish test.Do this without the ring.
Or this could all be possibly caused by a severely flooded saw.
You could have a 73cc piston in a 7900 jug.

Nice thought Eric:

I didn't even think of the 7300/7900 possibility. The squish test may work, but many people won't bother because a lack of knowledge with the information. And many people lack a micrometer to measure the end with. That also holds true for the end gap on the ring. I was just thinking of what he can do with what he has on hand. If someone doesn't have a micrometer, and they do have the tools to switch out the head, I say do the head, then buy the tool... But that's because I am cheap. Great post, forgot to mention the flooded saw part from assembly.

Jason

 

[blsnelling]

I liberally coat the P&C when assembling a motor and have never had any issue starting them. I seriously doubt that's your problem. Check for spark first, then check your squish. I wouldn't make this any more difficult than necessary.

 

[slnranger]

I will report back on the ring gap tonight.

I don't have a micrometer so I'll have to skip the squish test.

A couple have mentioned a flooded saw or too much oil could be my problem. Do you mean the "not starting" or the "low compression" problem?

I'm fairly sure it's not a 7300 piston. It came in a single box from Baileys & is a snug fit in the bore. I didn't install the sparkplug upside down either!

 

[retired redneck]

Is it posible threads around comp realese leaking ?squirt some water around it pull starter rope look for bubbles......

 

[cjnspecial]

Pull the muffler, slowly pull on the pull rope and listen for air escaping past the rings. Honestly, I would just take the jug off and check for cracked rings or other damage.

 

[3fordasho]

any resolution?

Sure would like to know before I attempt a cylinder/piston swap on my 6401...


MS210 & 361
5100s & 6401
Just had the 210 then I found this site

 

[1rockinacman]

Sure would like to know before I attempt a cylinder/piston swap on my 6401...


MS210 & 361
5100s & 6401
Just had the 210 then I found this site

+1

 

[Wortown Mick]

Sure would like to know before I attempt a cylinder/piston swap on my 6401...


MS210 & 361
5100s & 6401
Just had the 210 then I found this site

Just do it.

Its really astonishingly simple.. really

 

[1rockinacman]

Just do it.

Its really astonishingly simple.. really

It does look pretty straight forward. My gasket is on backorder from Bailey's. I'll tell you though, it's going to be very tough to put the cylinder on there with all those casting imperfection in the transfers. LMAO..... I got it bad.....

 

[Wortown Mick]

It does look pretty straight forward. My gasket is on backorder from Bailey's. I'll tell you though, it's going to be very tough to put the cylinder on there with all those casting imperfection in the transfers. LMAO..... I got it bad.....

Its just normal sand casting marks.

Blueprinting a two strokes never a bad idea so everything flows smooth. I cant see the rough surface helping flow any.

 

[slnranger]

Update

After screwing around with this for 2 weeks. I did a little cleanup on the ring , one side of the ring gap was not square. The net results was compression of 120#.
Better but not much.

I shipped it off to Scott at Cutting Edge. It should get there today or tomorrow. Hopefully he will have a simple (cheap) solution that won't make me feel like a retard.

Peter

 

[Hugenpoet]

After screwing around with this for 2 weeks. I did a little cleanup on the ring , one side of the ring gap was not square. The net results was compression of 120#.
Better but not much.

I shipped it off to Scott at Cutting Edge. It should get there today or tomorrow. Hopefully he will have a simple (cheap) solution that won't make me feel like a retard.

Peter

Sending the saw to Scott, regardless of what he may find and do, should hardly make you feel like a retard. Quite to the contrary, it is a darn smart course of action.