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

sevensandeights

sevensandeights

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Hard to say but for $15 (hopefully that's all a rebuild kit costs) I figure it's worth it to do or at least have it readily available in case the saw starts acting up again. All I know is that the H screw had no adjustability until I backed completely out of the carb.

When I get back in town I plan to run it as is with the carb cover on and see what happens. I'm just worried that whatever caused the problem initially isn't completely gone so I want to be prepared.

I think someone has definitely tampered with the carb because there are no limiter caps and the boot that guides the carb adjust tool into the H screw is not the same as it is on the L screw.
 
sevensandeights

sevensandeights

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Back in town one day early so I was able to cut today before I leave again tomorrow. Burned through one full tank cutting various limbs and bucking up some 20-24" dead ash - the saw performed great. Easy to start, idled well, good throttle response and plenty of power. Weather was cool today compared to last time but it made it through the entire tank without any issues with the carb cover in place. I have the H set at 13k max RPM at WOT. I still need to tune in the cut. Also plan to get a carb kit and clean/replace everything just for peace of mind.

Hoping the scored piston holds up . . .
 
Paul Silvestri

Paul Silvestri

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Back in town one day early so I was able to cut today before I leave again tomorrow. Burned through one full tank cutting various limbs and bucking up some 20-24" dead ash - the saw performed great. East to start, idled well, good throttle response and plenty of power. Weather was cool today compared to last time but it made it through the entire tank without any issues with the carb cover in place. I have the H set at 13k max RPM at WOT. I still need to tune in the cut. Also plan to get a carb kit and clean/replace everything just for peace of mind.

Hoping the scored piston holds up . . .
So what did you do differently this time?
 
sevensandeights

sevensandeights

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So what did you do differently this time?
Nothing - ever since I backed the H screw all the way out of the carb the saw has worked fine. Before that the saw appeared to be running fairly lean because no matter how many turns out the H screw was my RPM still maxed out at 13,500 WOT. It was almost by accident that I completely removed the H screw as I was counting how many turns out I was and the screw fell out after about seven turns I put it back in the carb and turned it all the way in, backed it out 1-1/8 turns and started the saw. I could then get the saw to 4-stroke and the RPMs to drop. I could also lean it out as I turned it in and get back to that 13,500 number. I set it at 13,000 at WOT. Went out to cut wood and it did just fine. Initially I cut with the carb cover off because I still had my tach connected to the spark plug wire. Yesterday I took the tach off and put the carb cover back on and was able to cut through a full tank with no issues. I have no idea what happened for sure but my guess is that something was blocking the H circuit in the carb causing the saw to run lean. When I took the screw all the way out the blockage cleared.
 

U&A

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Well either way it sounds like you got it going. Good deal. Id now take this opportunity to check the compression before you consider it to be goog to go.

JMO


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SteveSr

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Well either way it sounds like you got it going. Good deal. Id now take this opportunity to check the compression before you consider it to be goog to go.

JMO


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Forget the compression tester and just take a look at the piston through the exhaust port. If t here is any damage you will see it there. I have seen several two strokes with good compression but scored pistons. The compression test is not foolproof.
 

U&A

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Forget the compression tester and just take a look at the piston through the exhaust port. If t here is any damage you will see it there. I have seen several two strokes with good compression but scored pistons. The compression test is not foolproof.

Thanks for the advice

But in the end isn’t good compression......well.......good compression


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Oldsawnut

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I would keep an eye on the piston... If you change it out now you can prob get a 54mm AM one for 30-40. But the current one may last quite a while. Just don't let it ruin your cylinder so I suggest replacing it or looking at the piston on a semi regular basis to make sure its not getting worse over time. Might last a really long time.... or not. Unless you want to change it to an OEM P/C its hard to know exactly what you are going to get. Although the aftermarket ones I have seen lately especially the hyway have looked much better than they used to.
 
sevensandeights

sevensandeights

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Deja vu all over again!

Went to start the saw this morning and it wouldn't idle and had a poor throttle response. Had to go cut wood so I grabbed another saw.

When I came back I pulled the carb. Took everything apart and sprayed it down with carb cleaner. The L screw is nearly twice as long as the H screw and the end has a sharper point (see pics).

I Ialso released the pressure on the fuel line and removed the filter in the tank. Blew compressed air forward and backward through the line.

Put it all back together. Started with L screw out 1.5 turns. Set idle at 2700 RPMs and WOT RPMs at 12,900. Filled the tank and made 15ish cuts in 16" cherry and ash logs. Saw began to bog again. Hard to keep it idling. Certainly acted like a fuel issue. Checked the tank and was at least 1/2 full (interestingly, no pressure when I removed the cap). Checked the plug and it was wet! Let it dry and and turned the cylinder over a few times with plug out. Put plug back and it fired up immediately but bogged quickly while idling. Repeated this process 3 times with same result - plug was wet every time. Removed the fuel line at the carb and there was no pressure?

Brought it back to the garage and pulled the muffler. No change to the cylinder.

Stumped and discouraged!
 

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SteveSr

SteveSr

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When I came back I pulled the carb. Took everything apart and sprayed it down with carb cleaner. The L screw is nearly twice as long as the H screw and the end has a sharper point (see pics).
This is normal. Normally they are somehow "different" so that they can't be mixed up.

Put it all back together. Started with L screw out 1.5 turns. Set idle at 2700 RPMs and WOT RPMs at 12,900. Filled the tank and made 15ish cuts in 16" cherry and ash logs. Saw began to bog again. Hard to keep it idling. Certainly acted like a fuel issue. Checked the tank and was at least 1/2 full (interestingly, no pressure when I removed the cap). Checked the plug and it was wet! Let it dry and and turned the cylinder over a few times with plug out. Put plug back and it fired up immediately but bogged quickly while idling. Repeated this process 3 times with same result - plug was wet every time. Removed the fuel line at the carb and there was no pressure?
Wet plug says that you likely have a leak through the needle and seat. This can be confirmed with a pressure test on the fuel inlet which should hold 7-10 PSI indefinitely. Sounds like you may have more crud floating around inside the carb. Is the sealant around the welch plug(s) flaking off and getting into trouble?
 
sevensandeights

sevensandeights

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New carb on the way. Found an original Zama for $50 and seemed like the best route instead of $15 for a rebuild kit. Thought about the Walbro hd12 as well but just want to get this thing running consistently before changing anything else. Unfortunately I'm going out of town for a week and the carb won't be here before I leave.

What I really need to invest in is a compression tester and vacuum/pressure pump!
 

rmihalek

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I had a 6401 that I put an 84cc big bore kit on. I could not get it to run right. It was always too lean even with the H needle backed nearly out. I got my wire drills (I forget which size now, maybe it was 0.8mm or something like that) but I made the opening in the main jet a few tenths of a millimeter larger. That fixed the problem. H needle adjustability was restored and the saw runs fine. I'm not saying this would solve the problem with your saw since it seems like you have some response when adjusting the H needle. With my saw, it just seemed like the 84cc big bore was demanding too much fuel from the carb that was originally designed for a 64cc saw.
 
Paul Silvestri

Paul Silvestri

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I had a 6401 that I put an 84cc big bore kit on. I could not get it to run right. It was always too lean even with the H needle backed nearly out. I got my wire drills (I forget which size now, maybe it was 0.8mm or something like that) but I made the opening in the main jet a few tenths of a millimeter larger. That fixed the problem. H needle adjustability was restored and the saw runs fine. I'm not saying this would solve the problem with your saw since it seems like you have some response when adjusting the H needle. With my saw, it just seemed like the 84cc big bore was demanding too much fuel from the carb that was originally designed for a 64cc saw.
But don't they use the same carb on the 7900s? At least that's what my findings were... I could be wrong.
 
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