Issues after work-over on Makita 6401

[NPKenny]

As a background, I bought two Makita 6401's for cheap with the idea of building a 7900 and a 6401. Both were in rough shape, but ran fair. I figured I would get a feel for one before I build the 7900.

I outfitted one with a bar the other day with the intention of just getting it set-up to take as a back-up saw. I ran if for one hard cut in some firewood and it stalled out and I couldn't get is started again. So...I decided to tear into it to tinker with porting, compression ratio, clean-up, etc.

I just got it wrapped up after cleaning and replacing diagrams in the carb, cleaning off the flywheel surfaces, performing a mild porting on the intake and exhaust (mostly widening), dropping the base gasket out and creating a somewhat pop-up piston by sanding/grinding on the squish profile. My compression was 135psi to begin with and I ended up with 160psi. It fired quickly upon start-up and idles and accelerates okay, but not as well as I had hoped for. I've tried slight carb adjustments using the tach and cuts to try and find the tuning it performs well at. I also knocked in the screen and drilled 8 1/4" holes in the muffler.

1. The throttle response is poor. It seems to fall on it's face coming off of idle prior to picking up and getting to running rpm's.

2. Top end rpms are 12,300 at max. I tried coming all the way up from 10,000 rpms and cannot get it to read past 12,300. When I pull the plug, coloring looks good to slightly lean. Could this be an impulse line issue? I would think a lean condition would get this saw up to the rev. limiter at 13,500.

3. The saw does not pull as well as I had hoped. I didn't get a chance to really make cuts before hand to see how they compare. I was able to bog it down on a 12" milling type stump cut just as I was testing it.

4. Do I just have too high of expectations for the 6401 comparing it to my 361?

Thanks to all.

 

[blsnelling]

I would have recommended getting it running right before modding. At this point you don't know if it's your mods or something else. There's no way to compare this saw with a 361 until it's running right. Any saw should accelerate quickly from idle. I'm not sure what might be wrong. Sorry I can't help.

 

[RiverRat2]

Did you change the fuel inlet screen under the fuel pump??? and check for fuel flow through the screen and into the metering compartment via the needle and seat or fuel inlet valve,,,,, I remove both covers, then hold the carb with the inlet screen side up fill the inlet screen port with fuel mix,,,, then reach under the other side and depress the inlet valve to open the needle valve,,, the fuel should empty fairly quickly,,,, it is imperative that you do that to insure you are getting enough fuel then rebaseline all of the carb settings,, I did not notice your mentioning the final squish #'s did you check your squish????

 

[NPKenny]

I pulled both of the carb plates, but did not specifically put fuel mix in at the needle and seat. I pretty much just blasted carb cleaner through it. Likely not a complete enough cleaning. That is easy enough to do again.

I did not check final squish, but the preliminary measurements should have put me right in the .018" - .020" range. Out of curiosity, what are the effects of too tight of squish???

I am such a newbie with 2-strokes. Yes, clearly in hind-sight I should have had the saw running properly prior to modding, and I was telling myself such as I began, than I was modding. This is a sign of an irrational, uncontrollable addiction.

 

[huskydave]

First check squish. If it is less than .018 the piston could hit the head and you will have serious engine damage. I would clean the carb properly and rule out the small things first starting with the fuel filter. Squish is different on each saw that is why it is important to check it after mods.

 

[NPKenny]

Final squish results in.....

I am and IDIOT.

It appears no damage is done to the piston or head, but the squish is .008" at the back of the piston and .010" near the exhaust port. Looks like I will be making a gasket of somesort as described in the thin gasket material thread.

Is the the likely culprit???

Glad to be able to run to this site.

 

[RiverRat2]

invert the carb after you have once again removed both covers,, gaskets fuel pump diaphram and metering diaphram and set them aside,,,,put the fuel mix in on the fuel pump side opposite the needle and seat or metering side,,,,, there should be a cavity that the fuel inlet wire mesh screen is in,,,,, if the needle and seat or fuel inlet valve is holding you should be able to fill the cavity with mix,,,,,, then reach under and depress the inlet lever that resides in the metering chamber,,,,,, the fuel should flow through pretty freely if it doesnt then change the inlet screen,,, they get fine trash in the pores and slow down fuel delivery best toi change them any way,,, does the L & H screws have limiter caps if so remove the limiters,,, thenn gently bottom out each screw and then remove them.... set them on a clean paper towel or shop rag and mark the rag with an L and H where each resides now take the carb cleaner with nozzle attached and insert in both L&H carb adjustment screw holes & blast/flush out the passage you should have a good solid stream into the venturi bore there may be two fine little holes for the idle circuit (some carbs only one) that must flow freely for the idle funtion to work If they dont open up I use a piece of piano wire to rod from the venturi bore side,,,,,after you established flow then put it all back together,,, bottom out the L& H and back them out to 1 turn each,,,, maybe 1 +1/8 on the H since you modded it the sqish sould be no tighter than .018
you must have clearance to allow for expansion once the engine heats up and the piston grows..... it will damage the motor if it is too tight,,,, if it is too loose your compression will suffer and go down...

 

[Erick]

NPKenny,

I would not make a paper gasket .010 you would be better off finding some shim stock as was mentioned in that thread. I am still working on the aluminum foil gasket and should have a few pic's up later tonight but it is way to thin for what you need. The reason why my paper gasket in that thread works is that it is thin enough that it cant be beaten much flatter than it already is if you were to try to make a thicker paper gasket it would likely suffer the same fate as the factory paper gaskets and get beaten down to the point that the head bolts may become loose. Haven't measured an aluminum can yet but that might be a possibility for you also.

Just my thoughts.

 

[RiverRat2]

I am and IDIOT.

It appears no damage is done to the piston or head, but the squish is .008" at the back of the piston and .010" near the exhaust port. Looks like I will be making a gasket of somesort as described in the thin gasket material thread.

Is the the likely culprit???

Glad to be able to run to this site.

Good thing you checked it!!!!!!!!

Are you checking both front and back at the same time,,,??? as you will get some piston rock that is normal and just split the difference sounds like you need about .010 worth of gasket and to make sure the carb is fixed.... it will only take a few more minutes,,, are you sure you have the carbs metering diaphram and gasket in the right order???? did you change the needle and seat valve???? if so did you check the height of the fuel inlet lever as well!!!!

 

[NPKenny]

You must have clearance to allow for expansion once the engine heats up and the piston grows..... it will damage the motor if it is too tight,,,, if it is too loose your compression will suffer and go down...

OK, I think I got the carb cleaned to results you described, all but the fuel mix one. I will do that tomorrow.

The squish is a definate issue as i had plenty of unburned fuel. I can see where no expansion = no atomization = no explosion. Makes sense after the fact. I will shoot for .018-.020".

 

[Freakingstang]

As a background, I bought two Makita 6401's for cheap with the idea of building a 7900 and a 6401. Both were in rough shape, but ran fair. I figured I would get a feel for one before I build the 7900.

I outfitted one with a bar the other day with the intention of just getting it set-up to take as a back-up saw. I ran if for one hard cut in some firewood and it stalled out and I couldn't get is started again. So...I decided to tear into it to tinker with porting, compression ratio, clean-up, etc.

I just got it wrapped up after cleaning and replacing diagrams in the carb, cleaning off the flywheel surfaces, performing a mild porting on the intake and exhaust (mostly widening), dropping the base gasket out and creating a somewhat pop-up piston by sanding/grinding on the squish profile. My compression was 135psi to begin with and I ended up with 160psi. It fired quickly upon start-up and idles and accelerates okay, but not as well as I had hoped for. I've tried slight carb adjustments using the tach and cuts to try and find the tuning it performs well at. I also knocked in the screen and drilled 8 1/4" holes in the muffler.

1. The throttle response is poor. It seems to fall on it's face coming off of idle prior to picking up and getting to running rpm's.

2. Top end rpms are 12,300 at max. I tried coming all the way up from 10,000 rpms and cannot get it to read past 12,300. When I pull the plug, coloring looks good to slightly lean. Could this be an impulse line issue? I would think a lean condition would get this saw up to the rev. limiter at 13,500.

3. The saw does not pull as well as I had hoped. I didn't get a chance to really make cuts before hand to see how they compare. I was able to bog it down on a 12" milling type stump cut just as I was testing it.

4. Do I just have too high of expectations for the 6401 comparing it to my 361?

Thanks to all.

The off idle performance sounds too lean, give it more fuel. (low jet)

Did it do this before you did any mods, or just after?


How much material did you remove and where from?

where are your base line carb jet settings? 1 turn out, 2 turns, etc?


If you dropped the jug (with no gasket) and didn't raise the ports at least back to stock location, you have essentially retarded the stock timing.

 

[NPKenny]

The off idle performance sounds too lean, give it more fuel. (low jet)

Did it do this before you did any mods, or just after?
I didn't run it very long prior to mods, but I remember there not being any lack in acceleration off throttle prior.


How much material did you remove and where from?
I widened the ports mostly on the inside of the cylinder, raised the exhaust slightly, lowered the intakes slightly and shaved the circumfrecne of the piston slightly in an attempt to get squish clearence. I then polished the ports and piston.

where are your base line carb jet settings? 1 turn out, 2 turns, etc?
1 turn out for L and 1 1/8 for the H as baseline. I worked in and out from there. I removed the limiter from the H jet.


If you dropped the jug (with no gasket) and didn't raise the ports at least back to stock location, you have essentially retarded the stock timing.
I think this is the biggest issue on more fronts than one. How would I compensate for the retarded timing? Move the ignition pick-up or degree the flywheel?

Thank you for the input. I have a few things to work out to get a better hold on what is going on.

 

[romeo]

I think that you are also working against a rev limiter on your saw, you might need a different cdi/coil. Also sounds like you were going to lean to me, that is the biggest culprit of a flat spot. .008 is too shallow, the reason for leaving it at .020 is for the heat expansion of the piston, the minimum I would ever go is .014.

 

[RiverRat2]

Thank you for the input. I have a few things to work out to get a better hold on what is going on.

Take another .005 - .010 off of the top of the piston and recheck your squish,,, how far in toward the center of the piston did you begin your radius,,, you could do that and stay with the thin gasket,,,, remove your rings and keep polishing sanding and checking till you get it where you want it..... then when you get it there,,,,, reassemble!!!
and how are you keeping it uniform?????

Just curious????

 

[RiverRat2]

I think that you are also working against a rev limiter on your saw, you might need a different cdi/coil. Also sounds like you were going to lean to me, that is the biggest culprit of a flat spot. .008 is too shallow, the reason for leaving it at .020 is for the heat expansion of the piston, the minimum I would ever go is .014.

+1!!!!!

 

[PES+]

You have a dilemma

Port timing is NOT the same as ignition timing Kenny.

Is this the saw that the squish was asymmetrical when you measured it?

That is quite odd and can be for a number of reasons.

You have retarded the valve (port) timing by lowering the ports when you removed the gasket.

I suspect you have a distorted piston from long running hours.......the skirts stretch out.

You need to go back in and measure the piston to bore clearance and find out why the squish was so whacked out when you measured it.

You may have a mis-machined cylinder or the cylinder base flange may not be centered over the crank center but I tend to think it is just a worn out piston.

Did you install new rings?

With the work that you have done and dropping the exhaust port height your PSI should be higher than what you are reading with a good piston and ring seal.

 

[Freakingstang]

Thank you for the input. I have a few things to work out to get a better hold on what is going on.

You say that you lowered the INTAKES slightly


But the word intake with and S on it, that to me means more than one. Did you lower the transfers? The two ports on the sides that are divided in three little "chambers"?


And what is a little bit? 1/8" 1/4", etc

get your squish on target and lets go from there. The rev limiter shouldn't kick in until 13K unless it is worn out or starting to go south.

 

[RiverRat2]

You say that you lowered the INTAKES slightly


But the word intake with and S on it, that to me means more than one. Did you lower the transfers? The two ports on the sides that are divided in three little "chambers"?


And what is a little bit? 1/8" 1/4", etc

get your squish on target and lets go from there. The rev limiter shouldn't kick in until 13K unless it is worn out or starting to go south.

Good to see you posting again Steve!!!!!! and a good point to ponder,,, hes got a mess ehh????

Merry Christmas!!!!!

 

[NPKenny]

You say that you lowered the INTAKES slightly


But the word intake with and S on it, that to me means more than one. Did you lower the transfers? The two ports on the sides that are divided in three little "chambers"?

TYPO. Just one intake on this saw. And I only smoothed it out toward the bottom. I don't know how much I effectively lowered it.

On the transfers, I only polished back the factory casting flaws.

On the piston top, i began the radius about 1/4" in from the edge of the piston. I can't really say that I kept it terribly uniform. I used calipers to check as I went and never hogged out any material. More or less sanded along the edges.

I used a .010" piece of flashing for the gasket with copper sealant in an attempt to get the squish back in line. Again, non-uniform squish: .020" at the back of the cylinder on the intake side and .026" near the exhaust port. I likely added to the difference by working the piston slightly, but after PES+ post, I am thinking the piston is worn out and the ring at the very least. The saw's initial compression was only 135psi. With the .010" gasket in the saw, the compression is back at 135psi.

I can get it run, but it wants to die once it is warm and it still has the dead spot. With the air filter off, I can see fuel puddling in the intake shroud, So I am suspecting the carb is not acting as such.

On the carberator, I took it all back down and noticed a few things.
1. The o-ring on the L needle screw is shot
2. The oval shaped insert on the bottom of the carb leaks when I spray carb cleaner through the H-side seat. (Both of my camera's batteries went dead while trying to take a picture of the area I am talking about.) On the carb diagram, I can't see if this oblong part listed. I don't know if it can be removed/replaced.
3. The filter is clean and functioning.
It is a ZAMA carb.

This saw was in much worse initial shape than I realized. RiverRat2, it appears I "got a mess, ehh."

Thanks for all of the posters thus far. You do realize you are all partially to blame for the condition of the saw and operator. And I mean that with the sincerest appreciation.

 

[RiverRat2]

TYPO. Just one intake on this saw. And I only smoothed it out toward the bottom. I don't know how much I effectively lowered it.

On the transfers, I only polished back the factory casting flaws.

On the piston top, i began the radius about 1/4" in from the edge of the piston. I can't really say that I kept it terribly uniform. I used calipers to check as I went and never hogged out any material. More or less sanded along the edges.

I used a .010" piece of flashing for the gasket with copper sealant in an attempt to get the squish back in line. Again, non-uniform squish: .020" at the back of the cylinder on the intake side and .026" near the exhaust port. I likely added to the difference by working the piston slightly, but after PES+ post, I am thinking the piston is worn out and the ring at the very least. The saw's initial compression was only 135psi. With the .010" gasket in the saw, the compression is back at 135psi.

I can get it run, but it wants to die once it is warm and it still has the dead spot. With the air filter off, I can see fuel puddling in the intake shroud, So I am suspecting the carb is not acting as such.

On the carberator, I took it all back down and noticed a few things.
1. The o-ring on the L needle screw is shot
2. The oval shaped insert on the bottom of the carb leaks when I spray carb cleaner through the H-side seat. (Both of my camera's batteries went dead while trying to take a picture of the area I am talking about.) On the carb diagram, I can't see if this oblong part listed. I don't know if it can be removed/replaced.
3. The filter is clean and functioning.
It is a ZAMA carb.

This saw was in much worse initial shape than I realized. RiverRat2, it appears I "got a mess, ehh."

Thanks for all of the posters thus far. You do realize you are all partially to blame for the condition of the saw and operator. And I mean that with the sincerest appreciation.

When you are checking the squish are you using two peices of solder at the same time one on th eintake side and one on the exhaust side,,, 180* out???

Also Im thinking 1/4 inch piston radius may be not quite enough to clear the edge of the squish band in the combustion chamber????