Makita 6401 conversion low compression

[slnranger]

I just put a 7900 jug & cylinder from Baileys on a home depot makita. It ran fine as a 6401.

Everything seemed to go smoothly but it won't start. compression is only 100#. I assume my tester is ok because my 044 is at 150#.

I'm stumped as to where to go next?

 

[Tzed250]

Did you break a ring putting it together?

 

[slnranger]

I don't think so. I pulled the jug off after I got my first low reading to check the ring. It appeared to be fine.

 

[RiverRat2]

I don't think so. I pulled the jug off after I got my first low reading to check the ring. It appeared to be fine.

You did coat the bore with fresh premix????? prior to assembly Right,????

 

[slnranger]

I coated everything with 2cycle oil

 

[cjnspecial]

Check the compression release

 

[retired redneck]

Did you get new gasket with 7900 p/c old 6400 is to small......

 

[slnranger]

i did use the new gasket. I did get some oil on it though. Is there a right side up?

 

[retired redneck]

Some saw pistons have an arrow on piston points to exust port.....

 

[John Deere]

Is the new gatget thicker than the old one

 

[Wortown Mick]

Hm, I thought my saw was reading low compression as well but blamed it on the tester..
May just have to do some investergatin.

I dont even wanna talk about where I messed up in 3 key areas on assembly.

Not to mention my "break in" was more akin to trying to blow it up.

 

[Hugenpoet]

This site is really invaluable for me at times. Had convinced myself that I could actually do the conversion on my new 6401 and make myself a nice perfectly functioning 7900-and have a grand sense of self satisfaction in the balance.

Now thinking that it is more probable that I will turn $620 worth of new saw and parts into a pile of spare parts so for a saw that I no longer have.

Starting to think that there is nothing wrong with the 6401 as a $430 firewood saw, particularly because weigh doesn't matter as I usually only cut close in on the tree lines on my property and let the tractor carry the saw to and from the trees.

 

[Saw Bones]

I just put a 7900 jug & cylinder from Baileys on a home depot makita. It ran fine as a 6401.

Everything seemed to go smoothly but it won't start. compression is only 100#. I assume my tester is ok because my 044 is at 150#.

I'm stumped as to where to go next?

Is the stroke the same, and the piston? or the rod length? It may be that the compression ratio is different with the new jug. Just a thought.

:monkey:

 

[ericjeeper]

If it has low compression

has to be a bad tester, a broken ring. compression release is fubar. Once you get it figured out you will love your new powerhouse.

 

[cuttinscott]

Is It A OEM Piston & Cylinder????? the crank is the same between the PS6400, PS7300, PS7900. As well as the Base Gasket being the same.


Scott

 

[ericjeeper]

Good point Scott

Is It A OEM Piston & Cylinder????? the crank is the same between the PS6400, PS7300, PS7900. As well as the Base Gasket being the same.


Scott

I never thought about it being a non Dolmar part. Could be the tolerances are just out of wack

 

[slnranger]

it is OEM, I bought it from Baileys, it says Mahle on the side.

I did point the arrow to the exhaust,

I may have twisted the cylinder slightly when sliding it on. could this have hung up the ring on the little nub on the intake side (where the gap goes)

 

[cuttinscott]

sln, PM Sent

Scott

 

[Wortown Mick]

it is OEM, I bought it from Baileys, it says Mahle on the side.

I did point the arrow to the exhaust,

I may have twisted the cylinder slightly when sliding it on. could this have hung up the ring on the little nub on the intake side (where the gap goes)

Usually you tend to notice when a ring hangs up in a port.
Rebuilt a indy 650 triple and that was a headache. :bang:

 

[slnranger]

Is there anything else that could cause low compression? My ring looks good. I put on a new decomp. tester works fine (150psi on another saw).

I've been thinking the problem needs to be above the ports. That leaves ring, plug, decomp.

Could it be something in the crankcase or the intake boot? I stopped by Home Depot to look at their rental saws today, The compression on them seemed to be much more than mine before this project.