Makita 6421 with NWP 84cc p&c bogs down after running fine for 20 minutes

[sevensandeights]

Picked up a Makita 6421 with a fresh 84cc NWP p&c from a seller here. Saw starts easily and idles well. Decent throttle response. I have run 3 tanks through it and after 15-20 minutes of cutting it bogs down and dies. This has happened each time I have ran the saw for 15-20 minutes. Limbing and bucking ash/soft maple. No milling or long cuts. Won't start again until it cools. After it cools, it starts again easily and idles fine.

Seller has been great and is helping me troubleshoot but we have run out of ideas.

More info on the saw and what I've done so far:
Brand new Tsumura 20" bar and 3/8" husky chain.
Non-cat 6401 muffler.
Seller replaced the tank/handle assembly and clutch cover.
Nylon air filter (have the fleece also).
Stock Zama carb with limiter caps removed. When saw arrived it started easily and idled fine (in other words, carb appeared to be tuned correctly).
Using 90 octane ethanol free gas mixed at 45:1 with VP racing full synthetic oil. Gas is no more than a month old and has been run in other saws with no issues.
Pulled the plug and it is light brown.
Checked the impulse line and it looks OK and holds vacuum (tested very crudely by sucking on the hose and occluded the other end!).
Pulled the top cover of the carb and cleaned out the square mesh/screen (didn't appear very dirty). I did not pull out the small circular screen.
Pulled the muffler. There is one small gouge that wasn't there before I ran the saw per the seller (see pic). Rest of the piston looks OK to me but I haven't looked at many in person before.
Started over with carb settings. 1 turn out for low and 1-1/8 out for high. L screw doesn't seem to do much both in terms of idle speed and throttle response. High screw does cause 4 stroking when rich and hits the limiter when lean.

It has been really frustrating. This saw starts easy, idles fine and has a decent throttle response. It cuts awesome for 15-20 minutes then just bogs and dies. Any thoughts?

 

[huskihl]

How many turns out is the H screw when the limiter kicks in?
These carbs settings are all over the place depending on which era the carb is. Some are 1 turn out, some are nearly 2.
It sounds like you’re either lean on the H jet, the tank vent is bad, or the coil is bad.

 

[sevensandeights]

I will check how many turns out but I'd bet I'm closer to 2 turns out than one. Will hook up my tach as well to help pinpoint when she's bouncing off the limiter.

Tank certainly builds a ton of pressure - way more than I'm used to with my 5100s. Lots of pressure when I pop the cap and I got splashed with fuel the first time disconnected the line from the carb. This is supposedly normal for these saws from what I've read.

 

[huskihl]

I will check how many turns out but I'd bet I'm closer to 2 turns out than one. Will hook up my tach as well to help pinpoint when she's bouncing off the limiter.

Tank certainly builds a ton of pressure - way more than I'm used to with my 5100s. Lots of pressure when I pop the cap and I got splashed with fuel the first time disconnected the line from the carb. This is supposedly normal for these saws from what I've read.

Pressure is ok. Make sure it’s not vacuum

 

[Rags]

I would try to run the saw until it dies, pull the plug & check if it still has spark.
Sounds like the coil could be the culprit.

 

[sevensandeights]

How do I check to see if it's vacuum? Sorry if that's a dumb question. I know when I pop the gas cap, it spits pressure out and does not suck air in.

Hopefully I'll get a chance to run it and check for spark tonight.

 

[Nex]

Sounds to me like it is a thermal related issue, so you have a couple of (more or less) easy checks:

When it stalls;
open the filler cap. (carefully) - is it pressurized? if so, check/clean fuel tank ventilation. - if there's (significant) pressure its shooting fuel under pressure in to the carb and thus flooding itself (hard to verify on sparkplug since it's hot and fuel evaporates really quickly)
Pull the airfilter cover and try restarting it. If that works, remove/flip the winter setting on the saw (usually a baffle/plastic tab) - its pulling too hot air and get incorrect mix
As Rags said, check spark. Thermal expansion can make the gap between the flyweel/coil just a little too large. use a business card or similar to set gap when cool.

Final measure, adjust the carb when it's hot and see if you can get it to run. Let us know the results, it'll give more info to go on.

 

[sevensandeights]

Thanks for all the suggestions so far. Winter/summer baffle is in the summer position and the carb/airfilter chamber is blocked off from the cylinder.

Tank is new (I have the old one also). How do you check the valve? It is not caked with sawdust or otherwise visibly dirty.

 

[huskihl]

How do I check to see if it's vacuum? Sorry if that's a dumb question. I know when I pop the gas cap, it spits pressure out and does not suck air in.

Hopefully I'll get a chance to run it and check for spark tonight.

Sounds like you’re certain that it’s pressure so you can probably rule out the tank vent

 

[huskihl]

The tank should have pressure, just not so much that it pops off the metering needle. To do that you’d need upwards of 10-15 psi in the tank which is unlikely. It would make the saw rich anyway, not lean

 

[sevensandeights]

Ok - I'm totally confused now. Put the tach on (cheap Hardline hour meter version). Fired right up. Idles at 2800.

I can't get this thing to come down from 13,500 at WOT. 5.5 turns out (1/4 and 1/2 turn out at a time) for the H screw and it's still at 13500! It will drop to 12800-13000 when turning the screw out (I think this is what I misinterpreted as 4 stroking before). However, let it idle a bit and rev again and it goes right back to 13,500. Doesn't really bounce around - stays pegged at 13,500. Had to do this all with the carb cover off since that's the only way to access the spark plug wire.

Thoughts?

Didn't bother going out to cut because I didn't want it to be turning that many RPM.

 

[sevensandeights]

Ok - it's getting weird now. After not having any adjustment on the H screw I decided to search the forms about the 6421 carb. I found several references to various carbs that were non-adjustable from the factory or required a special tool. Went back out to the garage to see what mine looks like. The H screw on my carb does accept a standard flathead carb adjusting screwdriver. The fit is a little sloppy but it works.(hard to tell if it was modified but I don't think so). I also noticed that the rubber boot that usually guides the carb adjustment tool to the screw is not there on the H screw (see pic). Idle screw is on top with rubber boot. L screw is on the bottom in the far field in the rubber boot (either boot or screw is longer but they are touching). H screw is in the near field inside the carb housing. The silver thing is my carb tool.

Regardless, I backed the the H screw all the way out to where it was out of the carb. I then put it all the way back in. Turned it out 1-1/8 turns and started of the saw. Now the H screw adjusted the saw as expected. Was able to get clear four stroking and also have it peg the limiter at 13,500.

I set the max wide open throttle RPMs around 13,000 and went out to cut some wood. Saw ran great and cut great. Was turning around 10,000 RPMs in 20-in diameter soft maple. I cut for the rest of the tank (spent at least half a tank messing around earlier) and had no issues. I did have the carb cover off though because I still had my tach wrapped around the spark plug wire.

Unfortunately I have to go out of town for a few days starting tomorrow and won't have time to mess with this any further. Would like to see if this was a fluke, the problem was the H screw or the problem is heat and was alleviated by me having the carb cover off.

I'm still open any suggestions because I don't feel good about this . . .

 

[Wood Doctor]

Did you change out the old fuel filter? In post #1 you said "nylon air filter" but did you change out the old fuel filter? You might have skipped a step.

 

[sevensandeights]

Yes - not OEM but came new with the saw.

 

[Wood Doctor]

That should be OK. Keep using the saw as is. It might correct itself or the problem you encountered might be solved and disappear. Stranger things have happened. My Makita 6401 runs like a champ, but now and then the owner (me) slides the on/off switch the wrong direction when trying to start it. These saws do not start well in the off position, which is the on position on lots of Husqvarna saws.

 

[huskihl]

I would disassemble the carb completely and clean it and install new diaphragms. When you opened your H jet all the way out it probably allowed a slug of something to pass and started flowing fuel again.

 

[U&A]

Isn’t your P&C already scored up?


Sent from my iPhone using Tapatalk

 

[sevensandeights]

Yes it appears to be. Still not sure what caused that but it hasn't affected the compression enough that it won't start and run.

 

[jetsam]

Ok - it's getting weird now. After not having any adjustment on the H screw I decided to search the forms about the 6421 carb. I found several references to various carbs that were non-adjustable from the factory or required a special tool. Went back out to the garage to see what mine looks like. The H screw on my carb does accept a standard flathead carb adjusting screwdriver. The fit is a little sloppy but it works.(hard to tell if it was modified but I don't think so). I also noticed that the rubber boot that usually guides the carb adjustment tool to the screw is not there on the H screw (see pic). Idle screw is on top with rubber boot. L screw is on the bottom in the far field in the rubber boot (either boot or screw is longer but they are touching). H screw is in the near field inside the carb housing. The silver thing is my carb tool.

Regardless, I backed the the H screw all the way out to where it was out of the carb. I then put it all the way back in. Turned it out 1-1/8 turns and started of the saw. Now the H screw adjusted the saw as expected. Was able to get clear four stroking and also have it peg the limiter at 13,500.

I set the max wide open throttle RPMs around 13,000 and went out to cut some wood. Saw ran great and cut great. Was turning around 10,000 RPMs in 20-in diameter soft maple. I cut for the rest of the tank (spent at least half a tank messing around earlier) and had no issues. I did have the carb cover off though because I still had my tach wrapped around the spark plug wire.

Unfortunately I have to go out of town for a few days starting tomorrow and won't have time to mess with this any further. Would like to see if this was a fluke, the problem was the H screw or the problem is heat and was alleviated by me having the carb cover off.

I'm still open any suggestions because I don't feel good about this . . .

Sounds like maybe a chunk of something passed through. I would pull the carb and clean it, and blow out the fuel lines, and have a good look in the gas tank.

13,500 is the factory rev limiter on that saw, so I keep a little south of 13,500 (though it won't matter when the saw is in wood).

My tach wire fits through those little slots on the carb cover, I leave it on when tuning because air comes through the pre-screen filter in that cover, so you do not have normal airflow when it is off. (Probably doesn't matter too much, though, the pre-filter isn't too restrictive if it's clean).

Vapor lock is also possible, but I've never had it on that saw.

 

[sevensandeights]

Any suggestions on where to get a good carb rebuild kit or possibly a new carb? No dealers anywhere near me.