372xp Rebuild Stumbling

[quincyhoke]

I have been fighting a 372xp-xt rebuild getting it to run. The current symptoms are similar to an air leak where I'm having a hard time tuning the carb. I can get it to idle once warm but has poor performance in the cut. If I get the cut tuned OK it won't run at idle. The saw has no throttle response and stumbles if the idle low enough to stop chain spin. It often will die after coming of WOT.

The saw and carb are both holding pressure and vacuum. I did have a leak around the oil pump bushing but have resolved that with little improvement. Carb has a new Walbro kit in it.

Any leads are greatly appreciated, Thanks.

 

[cuinrearview]

Tough to troubleshoot over the net, but sounds like it might be short on fuel. Maybe the line is pinched or something isn't right in the tank. Other things to check would be running it without a filter and a clogged spark screen.

 

[Maintenance supervisor]

Impulse line could be blocked by just the tiniest trash,especially after a rebuild.
You should maybe pop the covers on the new carb just to make sure the needle is set right, I know it sounds ridiculous on a new one but I've found production trash in the screen.

 

[Robin Wood]

They're notorious to leak between carb and intake boot, that throttle holder makes it weak. When you depress the throttle it leaks even more by flexing, get an AM metal holder that'll rule out one trouble

Sent from my NOH-NX9 using Tapatalk

 

[quincyhoke]

Tough to troubleshoot over the net, but sounds like it might be short on fuel. Maybe the line is pinched or something isn't right in the tank. Other things to check would be running it without a filter and a clogged spark screen.

I may try pulling the spark arrestor just to try. I have run with and without a filter previously and haven't seen a noticeable response. I did throw in a new fuel line and filter after running it for a while hoping the issue was a leak in that line but didn't get anything from that either.

 

[quincyhoke]

Impulse line could be blocked by just the tiniest trash,especially after a rebuild.
You should maybe pop the covers on the new carb just to make sure the needle is set right, I know it sounds ridiculous on a new one but I've found production trash in the screen.

I'll check the impulse at the cylinder, I haven't done that yet. I did check the needle and even tried bending the metering lever up a little but I didn't see any improvements from that.

 

[rmh3481]

Keep using it and sooner or later the problem will show itself. Even if you just come home from work and start it out in the garage and let it run for 10 minutes. Simple checks up front. Did the saw run right before you started working on it? (This is no indication of your ability, only a reference as to if others had problems before you.) Could the muffler be plugged? Ditch the screen until you get it running right. Did the case pass a pressure and vac test? If you need to borrow a tool, ride up to Auto Zone and borrow one of their Brake Bleeder hand pumps. When you put the ignition module on, did you use a 'Business Card' to set the air gap? Replace the spark plug and this time try a non R plug. IE, CJ6Y or CJ6 Champion instead of the resistor plug RCJ6 or RCJ6Y. In the NGK line its BPMR7A, so the non R plug would be BPM7A. Cross reference to be certain.

Some of the older ignition modules just wont generate enough voltage to fire 'R' plugs. Resistor spark plugs have nothing to do with radio interference. They are about air pollution. The Resistor keeps the plug from firing unless/untill the voltage exceeds 20K volts. This guarantees a hot spark. As ignition modules age they seem to have trouble getting over that 20kv threshold. Ive seen several engines that wont fire an R plug? Put in a regular plug, and away they go...

 

[bowenp]

I have been fighting a 372xp-xt rebuild getting it to run. The current symptoms are similar to an air leak where I'm having a hard time tuning the carb. I can get it to idle once warm but has poor performance in the cut. If I get the cut tuned OK it won't run at idle. The saw has no throttle response and stumbles if the idle low enough to stop chain spin. It often will die after coming of WOT.

The saw and carb are both holding pressure and vacuum. I did have a leak around the oil pump bushing but have resolved that with little improvement. Carb has a new Walbro kit in it.

Any leads are greatly appreciated, Thanks.

 

[bowenp]

I have seen the fuel line pinch where is comes out of the tank and turns to go into to the bottom of the air box. I will not use bulk fuel line anymore, on my 371's. I only use the factory fuel line.

Your issue sounds like and could be a pop-off pressure.
The needle must open between 12.5 and 14 PSI for best performance.
Pop off pressure is like fuel pressure. The pressure and vacuum is generated the piston, ports and the pressure changes in crankcase .
The lever is equal to a float bowl setting. Set the lever correctly, using the Walbro W gauge.
Turning testing lubricate the needle with a light spray lube or 3 in 1 oil. If the pressure is high, trim the spring. I generally trim the tight coils to the last one on the first end and retest. If still high, trim the other end and retest. If still high, I will clip 1 coil at a time and retest.

 

[cscltd]

Is it possible that something went wrong with rebuilding? Oem cylinder Kit used or a $25 special?

 

[Vintage Engine Repairs]

A video of you starting it, letting it idle and running it in the cut will help diagnosis a lot.

 

[quincyhoke]

I may have resolved my issue but did so much at once I can't say for sure what the issue was.
Rebuild was with a Hiway P/C. I had an old OEM jug that had a high corner so I cleaned up that jug base and slapped it on. Re-gapped the coil and put in a new plug. I saw a bit of improvement here but still had some dying issues after WOT. I backed out the Low another 1/2 turn and adjusted the idle to match. It seems fixed now but don't have much time on it.

 

[Old2stroke]

I have seen the fuel line pinch where is comes out of the tank and turns to go into to the bottom of the air box. I will not use bulk fuel line anymore, on my 371's. I only use the factory fuel line.

Your issue sounds like and could be a pop-off pressure.
The needle must open between 12.5 and 14 PSI for best performance.
Pop off pressure is like fuel pressure. The pressure and vacuum is generated the piston, ports and the pressure changes in crankcase .
The lever is equal to a float bowl setting. Set the lever correctly, using the Walbro W gauge.
Turning testing lubricate the needle with a light spray lube or 3 in 1 oil. If the pressure is high, trim the spring. I generally trim the tight coils to the last one on the first end and retest. If still high, trim the other end and retest. If still high, I will clip 1 coil at a time and retest.

Nothing should open the inlet valve except the action of the diaphragm operating on the control lever. It should NOT be forced open by pressure from the fuel pump or pressure from expansion in the fuel tank. Pop-off pressure is a test to ensure that this doesn't happen or flooding could occur. If you are cutting on a hot day and sit a hot saw down, pressure in the fuel tank can build quire high, some operators claim the pop-off pressure should be at least 30psi but usually around 15psi will do.

 

[bowenp]

Nothing should open the inlet valve except the action of the diaphragm operating on the control lever. It should NOT be forced open by pressure from the fuel pump or pressure from expansion in the fuel tank. Pop-off pressure is a test to ensure that this doesn't happen or flooding could occur. If you are cutting on a hot day and sit a hot saw down, pressure in the fuel tank can build quire high, some operators claim the pop-off pressure should be at least 30psi but usually around 15psi will do.

 

[bowenp]

I may have resolved my issue but did so much at once I can't say for sure what the issue was.
Rebuild was with a Hiway P/C. I had an old OEM jug that had a high corner so I cleaned up that jug base and slapped it on. Re-gapped the coil and put in a new plug. I saw a bit of improvement here but still had some dying issues after WOT. I backed out the Low another 1/2 turn and adjusted the idle to match. It seems fixed now but don't have much time on it.

Ok, it sounds like a crankcase leak between cylinder and crankcase.
I test my saws in a dunk tank and look for bubbles if it does not hold pressure. (rotate crank a couple of times while testing)
I made this tool (see picture) to insure flat cylinder bases and lower cylinders to increase compression ratios.
The size allows for sandpaper to be held down with the screws and not interfere with the cylinder.
1/4" aluminium and 3/4" Vantec plywood.
I use thicker sandpaper made for sanders.
Range paper targets and paper bags can be found in different thickness for tweaking piston squish.
I always wet down paper gaskets with light machine oils before assembling.
Use a torque wrench.

 

[quincyhoke]

Ok, it sounds like a crankcase leak between cylinder and crankcase.
I test my saws in a dunk tank and look for bubbles if it does not hold pressure. (rotate crank a couple of times while testing)
I made this tool (see picture) to insure flat cylinder bases and lower cylinders to increase compression ratios.
The size allows for sandpaper to be held down with the screws and not interfere with the cylinder.
1/4" aluminium and 3/4" Vantec plywood.
I use thicker sandpaper made for sanders.
Range paper targets and paper bags can be found in different thickness for tweaking piston squish.
I always wet down paper gaskets with light machine oils before assembling.
Use a torque wrench.

I'm not sure this was it, prior to pulling the cylinder I didn't have any loss of pressure or vacuum. I even tried dunking it with pressure and never found anything that would indicate a leak.

 

[Isidro]

I have not had much luck rebuilding Walbro carbs. If the carb is old, not sure what the magic age is, I now I just buy a new one. Not cheap but less drama.

 

[quincyhoke]

Update:
Just got a few minutes back on the saw and I definitely didn't get the issue fixed. The last time it ran it was working well. This time on a cold start was hard (usually 3 pulls on choke then one off). It acted like I had too much fuel on the low jet. Once it warmed up it ran at a very low idle and eventually stalled. Brought the idle up a hair to keep it alive and the idle would climb and come back down just like an air leak.

I'm guessing this has to be in the carb somewhere??? Tank vent is clear, impulse clear, can't find an air leak from the case/cylinder/intake boot.

 

[huskihl]

The idle coming up and down could be the coil advancing timing above 3K RPM or so, and retarding when it falls below that number. You need to set the mixture probably a little bit leaner and idle the saw down

 

[quincyhoke]

The idle coming up and down could be the coil advancing timing above 3K RPM or so, and retarding when it falls below that number. You need to set the mixture probably a little bit leaner and idle the saw down

I'm not familiar enough with how these coils work. How does the coil itself adjust the timing?