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

[sevensandeights]

Currently trying to work out a compromise with the seller. I think the saw has an air leak causing it to run lean, especially once it warms up. Could be the base gasket or the decomp valve or something totally different. Need to get it to someone familiar with these saws that can perform the appropriate tests before any more money gets thrown at it.

 

[sevensandeights]

Anyone have a link to a video or pictures of how to do a leak test on this saw? Anything to look for in particular?

I have the saw torn down and equipment on the way. I watched some generic Youtube videos but most of those saws were completely torn down. I still have the flywheel on but that limits access to the base gasket for testing. Speaking of base gaskets, it doesn't look there is one on this rebuilt saw. Seems to be some sort of sealant - again I can't tell for sure though with the flywheel in the way. No gasket might not be an issue from what I've read since the aftermarket P&C kits tend to have high squish numbers.

 

[rmh3481]

Pressure test-


Have you cleaned out the tank vent? If your not sure where its located, look on the schematic.
In order for the fuel to flow up to the carb, the liquid volume needs to be replaced by air. Additives in fuels mess with the plastic that the vent is made from. After the vent melts internally from the alcohol in the fuel, it wont pass enough air to keep the engine running. This may be the reason for the big bore kit to begin with?

Ignitions really dont have any parts that you can replace. When they get older heat bothers them. The internal capacitor seems to not be able to hold its primary voltage which makes it hard for the secondary spark voltage to overcome the Resistor in the plug. The R on the side of the plug has nothing to do with radio interference. The Resistor is in the plug to make the hottest spark possible for air pollution reasons. Pull the boot and look at the numbering on the plug. If you see an R, switch to a plug without the R like a regular CJ6 or CJ6Y Champion. Parts stores are used to handing out the R plugs so instead ask by part number. ngk? - BPMR7A? You want the non R which is BPM7A. So again no R plugs.

Are the carb diaphragms new?

On the pressure test your watching the gauge so you only have to disassemble far enough to get your gauge to work. For instance, you dont take the flywheel off unless your gauge indicates a leak or you see bubbles from soapy water behind it.

 

[SteveSr]

Ignitions really dont have any parts that you can replace. When they get older heat bothers them. The internal capacitor seems to not be able to hold its primary voltage which makes it hard for the secondary spark voltage to overcome the Resistor in the plug. The R on the side of the plug has nothing to do with radio interference. The Resistor is in the plug to make the hottest spark possible for air pollution reasons. Pull the boot and look at the numbering on the plug. If you see an R, switch to a plug without the R like a regular CJ6 or CJ6Y Champion. Parts stores are used to handing out the R plugs so instead ask by part number. ngk? - BPMR7A? You want the non R which is BPM7A. So again no R plugs.

This is TOTAL GARBAGE. The resistor plugs were originally developed PRECISELY to deal with radio and other RF interference, now including to computerized ignition and engine control systems. For this reason you should ONLY use the spark plug type specified by the original equipment manufacturer.

 

[sevensandeights]

Edited my post above to say no base gasket might NOT be an issue due to high squish numbers with aftermarket P&C kits.

Carb is brand new.

The saw came to me with a new tank installed per the seller (certainly looks new and I have no reason to doubt him). I have looked at the vent from the outside and it did not appear to be caked up but I did not remove it (seems difficult to do - not much access to the inside of the tank through the gas fill hole).

I will be able to pressure test this evening.

 

[sevensandeights]

Well - I found the leak. Pumped the cylinder full of air and could hear an audible leak around 5 PSI. Spark plug and decomp valve were fine. Base gasket under the exhaust port and behind the flywheel were fine. Found the leak when checking the base gasket under the intake port BUT it wasn't the base gasket. The impulse line nipple was leaking like crazy. When I went to remove the impulse line the entire nipple came out!!!!!!!!!! I read no less than 100 threads today at work thinking about what the issue could be. I did see one reference to poorly seated impulse line nipples. Not sure if this is a common problem or not.

So, good to know what caused the lean condition but now I likely need a new P&C. Really want to go OEM. Best price I have found online is $195. I plan to call Cutting Edge tomorrow to see what he has. I think I need 038-130-070 (also found 038-130-031) for left side decomp valve or 038-130-071 for right side.

Can anyone confirm the part numbers for the base gasket (965-531-160), impulse (?) and fuel line (038114052)?

 

[sevensandeights]

Ordered the following from SLE Equipment:

038 130 070 P&C kit
001 131 150 decomp valve
965 531 121 base gasket
965 452 382 impulse line

Spoke with the guys at The Cutting Edge and they were a big help as always. They are out of "plain" P&C kits (but did have one kit with the decomp valve and new cover with hole on left side). I needed everything sooner rather than later so I ordered from SLE.

 

[sevensandeights]

Parts came in today and I put the saw back together. Runs like it should now. Only ran 7/8ths of a tank through it though bucking up some 16-20" ash.

On a side note - I got the old paper gasket. Not sure how I got the part numbers mixed up but SLE equipment only has the 965 531 121 part number on their website anyways and that who I ordered from. The metal gasket is 965 531 121. I called the guys at The Cutting Edge and they said don't worry about it and run it. I can't saw enough good things about that shop.

I have quite a bit of bucking to do this weekend. I tuned the carb with a tach today but will tune it in the cut this weekend.

 

[Oldsawnut]

Wow 150 for a OEM 7901 PC kit is a great deal.

 

[sevensandeights]

Parts came in today and I put the saw back together. Runs like it should now. Only ran 7/8ths of a tank through it though bucking up some 16-20" ash.

On a side note - I got the old paper gasket. Not sure how I got the part numbers mixed up but SLE equipment only has the 965 531 121 part number on their website anyways and that who I ordered from. The metal gasket is 965 531 121. I called the guys at The Cutting Edge and they said don't worry about it and run it. I can't saw enough good things about that shop.

I have quite a bit of bucking to do this weekend. I tuned the carb with a tach today but will tune it in the cut this weekend.

Can't figure out how to edit (even though I've done it before!). The correct part number for the metal gasket is 965 531 160.

 

[sevensandeights]

Saw runs great after new OEM P&C and intake boot. Set the high a little rich for the first few tanks. Tuned in the cut and blew through some 24" ash logs today! Truly a joy to run this saw now.

Caveat emptor - lesson learned . . .