Dolmar PS-420 help ... saw idles, but won't rev

[Farmer_Nate]

I have an older PS-420 Dolmar. I loved that saw until a year or so ago when it started acting up and ran funny. I have never been able to get her back to her old glory.


Symptoms and Information:

Saw will go from idle up in RPM smoothly until a certain speed, then shuts completely down, even if the throttle lever is released. It will not recover if a certain RPM is exceeded. Seems coil related? I'd hate to put a 3rd coil on this thing.

Saw will start and idle with a new Zama carb. I ruined the original Walbro trying to track down a check valve. Some on this forum mentioned trouble with a check valve in the carb on theirs.

https://www.arboristsite.com/threads/dolmar-ps-420-issues.369459/
Saw seems to have as good or better compression than my other saws, especially for only 42 cc. Piston seems in decent condition. I had the cylinder off at one point.
I replaced the original coil with a 421 coil, used, from Chainsawr.

Plug has spark, but not sure if it is working totally right.
I replaced sparkplug with another identical one, no better performance.

Thought maybe fuel tank vent not allowing enough fuel to be pulled into the carb?

I turned the running saw on it's side and cracked open the fuel fill plug. It still won't go up in RPM past a certain point, as if the kill switch is hit once it passes a certain RPM.

I took it totally apart. The impulse line and fuel lines are replaced and clear flowing. I even checked the impulse line fitting on the cylinder - it is clean and clear.

I like the saw and want to make it run. I converted it to a rim sprocket. It is a nice handling saw.

It holds vacuum and pressure, so I don't think it is a seal issue, at least not while cold.


Piston seems in decent condition.

A member on here mentioned he had trouble with a check valve in the Makita equivalent of this saw and fixed his with a new carb. This one has a new Zama.

 

[Farmer_Nate]

Is there anything to check on the flywheel that could cause this? The magnets?

BTW fuel filter is brand new and not clogged - I can blow through it.

 

[Farmer_Nate]

Thought maybe fuel tank vent not allowing enough fuel to be pulled into the carb?

I turned the running saw on it's side and cracked open the fuel fill plug. It still won't go up in RPM past a certain point, as if the kill switch is hit once it passes a certain RPM.

 

[cookies]

try putting a new spark plug in it, if you think its the vent run it on its side with the gas tank cap loose but be aware if it is over half full it will spill out from vibration. you can also empty the tank and filter of fuel then run a vac test through the fuel hose on the tank. if you have a timing light you can test the ignition to see if it is firing or even mark the flywheel and case at top dead center then remove the starter cover with it running to check for ignition timing adv.

 

[cookies]

90 day warranty, gus is a good guy to deal with over there
https://sawzillaparts.com/proline-r...ps420c-ps421-dcs3500-dcs3501-dcs4300-dcs4301/

 

[mountainlake]

clogged muffler, worn piston skirt on the intake side. Steve

 

[Farmer_Nate]

Thanks.

It runs the same with an open fuel fill - problem is not the vent.
New plug does not fix it, at least a different plug with the same part number does not.

Ignition timing advance...maybe that is what is not occurring.

 

[Farmer_Nate]

Muffler is completely gutted and clear.

Piston skirt on intake side could be an issue but I don't really think so. It ran great.

 

[Farmer_Nate]

I ordered that coil.

More updates later when it arrives.

 

[MMG]

I had a similar issue with Makita 6401. After a lot of messing around, coil, plug,carb…I changed the fly wheel and bingo, that was it. Still don’t know why or how it happened but the flywheel lost its timing. Mine has a fixed key way so that wasn’t it either.

 

[Farmer_Nate]

It really seems more to me now like the coil circuitry is somehow activating the rev limiter, but activating the rev limiter permanently once it is reached, and at much too low an RPM. It is very weird. It idles smooth and very nicely. But as the revs are increased to about half throttle it is as if the coil is getting too many pulses, as if the saw "thinks" it is running faster than it actually is an shuts down completely, as if the spark shuts off totally and does not recover.

 

[Farmer_Nate]

ProLine coil from Sawzilla fixed the problem.

I learned a bit about how coils work and what they do through this experience.
I never knew there were electronics potted in, sad to say.

Saw will be for sale soon.

I am thinking $325