Pioneer 620 - Won't rev up clean to full throttle

[cdahl383]

An air leak usually makes them run lean and faster

But would it cause it to run for a bit then die out? Seems like it would just run lean and a little faster all the time right?

Seems like it could be some sort of fuel starvation issue?

 

[buzz sawyer]

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[WoodmanSr]

But would it cause it to run for a bit then die out? Seems like it would just run lean and a little faster all the time right?

Seems like it could be some sort of fuel starvation issue?

So the Pioneer 600/620 are early Piston Ported design, right. Not reeds valve of the era. What is piston condition?

Okay, so I'm wondering something. Any chance you have extreme worn or cracked piston intake skirt? Could symptom be related to Intake free-porting condition. Intake stroke not sealing off sufficiently after few minutes run from metal expansion effects. Maybe behaving as weird port timing.

Or, did previous owner botch a porting job; intake too low or transfers way to high. Take a look down Intake & Exh Ports for evidence of grinding work or trashed piston skirt.

If you don't mind a flooded crankcase & drying it out.
- when skirt drops below Intake Floor (should make semi 'closed' port). Should hold a pool of gas mix on the intake port, should not immediately drain thru to crankcase. Yes, leak slowly thru piston clearance & transfer channels path. But, if not "test pooling" a moment than may suggest very poor intake stroke closing. (Builders/Porters please comment... just what I'd evaluate, DIY)

Just saying because done it... broken reed test. A tight Reed Valve motor on it's down stroke will hold "test pool" over intake boot a decent time (15-30 sec). Only freely shallow after rolling crank.

I had a similar rich restricted speed issue on twin B&S. Alot different but about the same symptoms. Only turn half RPM as dead rich, but idle fine. Finally found previous owner had installed Camshaft mistimed to Crank (off by 7 teeth). Compression was 165. Motor just couldn't breathe to make RPM where a okay Carb mixture & correct crankshaft basis Ignition Timing.

 

[Old2stroke]

But would it cause it to run for a bit then die out? Seems like it would just run lean and a little faster all the time right?

Seems like it could be some sort of fuel starvation issue?

Normally fuel starvation will first show up when the engine requires maximum fuel flow, which is when it is cutting under full load at WOT. Do you have a test log that you can do serious cutting on? If you can make several consecutive cuts at full power without bogging, it's not fuel starvation. Just running the engine up to maximum rpm is NOT a load test.
To clarify:
Does the engine bog while cutting at full power?
Does it bog only when accelerating from idle?
Does it bog if just left to idle for a long time?
When it bogs and quits, do you have to use the choke to get it started?

 

[cdahl383]

So the Pioneer 600/620 are early Piston Ported design, right. Not reeds valve of the era. What is piston condition?

Okay, so I'm wondering something. Any chance you have extreme worn or cracked piston intake skirt? Could symptom be related to Intake free-porting condition. Intake stroke not sealing off sufficiently after few minutes run from metal expansion effects. Maybe behaving as weird port timing.

Or, did previous owner botch a porting job; intake too low or transfers way to high. Take a look down Intake & Exh Ports for evidence of grinding work or trashed piston skirt.

If you don't mind a flooded crankcase & drying it out.
- when skirt drops below Intake Floor (should make semi 'closed' port). Should hold a pool of gas mix on the intake port, should not immediately drain thru to crankcase. Yes, leak slowly thru piston clearance & transfer channels path. But, if not "test pooling" a moment than may suggest very poor intake stroke closing. (Builders/Porters please comment... just what I'd evaluate, DIY)

Just saying because done it... broken reed test. A tight Reed Valve motor on it's down stroke will hold "test pool" over intake boot a decent time (15-30 sec). Only freely shallow after rolling crank.

I had a similar rich restricted speed issue on twin B&S. Alot different but about the same symptoms. Only turn half RPM as dead rich, but idle fine. Finally found previous owner had installed Camshaft mistimed to Crank (off by 7 teeth). Compression was 165. Motor just couldn't breathe to make RPM where a okay Carb mixture & correct crankshaft basis Ignition Timing.

Piston looks like new. Cylinder looks good as well. Compression about 145-150psi cold and hot.

I’ll try the fuel pooling test and see what happens.

 

[cdahl383]

Normally fuel starvation will first show up when the engine requires maximum fuel flow, which is when it is cutting under full load at WOT. Do you have a test log that you can do serious cutting on? If you can make several consecutive cuts at full power without bogging, it's not fuel starvation. Just running the engine up to maximum rpm is NOT a load test.
To clarify:
Does the engine bog while cutting at full power?
Does it bog only when accelerating from idle?
Does it bog if just left to idle for a long time?
When it bogs and quits, do you have to use the choke to get it started?

I have not cut anything with it yet. I’ve just been test running it with no bar or chain.

It will rev up good at first. After running for a minute or two then it will start not revving as good. Eventually it starts to idle faster, then cuts out and dies. It sounds like my trimmer when it runs out of gas.

It will bog after idling for a while like a few minutes.

When it dies, I’m able to restart it almost immediately with no choke.

 

[Old2stroke]

I have not cut anything with it yet. I’ve just been test running it with no bar or chain.

It will rev up good at first. After running for a minute or two then it will start not revving as good. Eventually it starts to idle faster, then cuts out and dies. It sounds like my trimmer when it runs out of gas.

It will bog after idling for a while like a few minutes.

When it dies, I’m able to restart it almost immediately with no choke.

Since it starts right up without using the choke, it means that the fuel chamber still has fuel in it and it did NOT run out of fuel, however, as it seems like it did, maybe there is just not enough fuel being supplied by the jets. Back both the L and H screws out 1/2 turn and see if it makes any difference