371xp runs with H jet closed - is that bad?

[Zippy]

I've fixed a bad air leak (replaced oil seal and o-ring) on the clutch side of a 371xp and put it all back together.

I've tuned it up - it starts easily, it idles great at 2700rpm (L set at 1.25 turns out) and runs WOT at 13500rpm (H set at 0.75 turns out).

I tried closing the H all the way to make it scream so I could hear it turn to blubber as I opened it but it never really screamed.

I closed the H all the way and it continued to run around 14000plus rpm.

Previous saws I've tuned would run up to 17000rpm if I closed the H too much and would die if I closed the H jet.

Is there something wrong with this saw or is this ok?

 

[FATGUY]

did you do a vacuum test on the saw before you fired it up? Sounds like you still may have an air leak.

 

[blsnelling]

Some saws will do this. In those cases, not all WOT fuel is metered by the H needle. An air leak would cause it to go too lean, not prevent it from leaning out.

 

[Zippy]

Pressure & Vac test ok

Yep - held pressure and vacuum perfectly.

 

[FATGUY]

Some saws will do this. In those cases, not all WOT fuel is metered by the H needle. An air leak would cause it to go too lean, not prevent it from leaning out.

so where would the air be coming from then? The low?

 

[blsnelling]

so where would the air be coming from then? The low?

It's simply that he can't shut of all fuel with the H needle. Could this possibly be a Tilly carb with a governor in the carb?

 

[Zippy]

That's what I was hoping....

Some saws will do this. In those cases, not all WOT fuel is metered by the H needle. An air leak would cause it to go too lean, not prevent it from leaning out.

At WOT I think all circuits (1 x H and 2 x L) supply fuel - but was hoping that with this saw it was just that the L circuits deliver a bigger % of the total fuel necessary?

Sounds like you've experienced this before so I guess it'll be ok.

 

[nmurph]

It's not getting extra air, it's getting enough fuel through the L side or the H needle is not long enough and not closing off that passage.

I would turn the L screw in some and see if that changes things.

 

[Roanoker494]

Some saws will do this. In those cases, not all WOT fuel is metered by the H needle. An air leak would cause it to go too lean, not prevent it from leaning out.

This is my thinking also, air leak would cause a lean condition and too many RPMs.

 

[Zippy]

It's simply that he can't shut of all fuel with the H needle. Could this possibly be a Tilly carb with a governor in the carb?

It's a Walbro HD6. I haven't taken it apart but it did pressure test ok.

It's not getting extra air, it's getting enough fuel through the L side or the H needle is not long enough and not closing off that passage.

I would turn the L screw in some and see if that changes things.

It's night time here so can't do anything now but could try tomorrow. When tuning the idle, closing the L screw starved the fuel. I can try the L at WOT and see what happens tomorrow.


I guess that providing it's not screaming and rpms are good, it's getting enough fuel so it's ok - just wondered if it was normal or if it was a sign of something else wrong.

 

[blsnelling]

It's not a matter of air. The carb venturi is wide open. If there were additional air entering the engine, you would have a situation where you couldn't get enough fuel. This is the opposite of that. Just make sure that it still 4-strokes slightly in the cut when you let off the load.

 

[Jacob J.]

Most likely what's happened to this carburetor is that someone damaged the adjustment needle seat on the high side at
some point and the high needle can no longer regulate the fuel as readily as a carburetor with undamaged needle seats.
I've seen this happen on many carburetors.

People get ham-fisted and reef on the adjustment screws and then the carburetor is never the same.

 

[Zippy]

That makes sense

Most likely what's happened to this carburetor is that someone damaged the adjustment needle seat on the high side at
some point and the high needle can no longer regulate the fuel as readily as a carburetor with undamaged needle seats.
I've seen this happen on many carburetors.

People get ham-fisted and reef on the adjustment screws and then the carburetor is never the same.

I guess so long as I can get it set right - and I have - then it should be ok.

Otherwise I could try a replacement carb - everything on EBay is aftermarket at the moment and I don't fancy splashing £100 for a new one from the main dealer.

Hutzl (Hong Kong) will deliver their carbs for a 371/372xp for £16. Anyone got any experience of their carbs? Any better/worse than anyone elses aftermarket carbs which go for around £30?

... or maybe I'll just stick to the walbro in the saw as it's tuned up ok?

 

[FATGUY]

did you do a vacuum test on the saw before you fired it up? Sounds like you still may have an air leak.

so where would the air be coming from then? The low?

:redface: Been a bad day, I don't know what the hell I was thinking...

 

[jockeydeuce]

A leaking welch plug can cause this too.....The gas leaks past the welch plug to the main nozzle.

 

[dk27]

I think Jacob J nailed it.
I have a 372 that 4 strokes just fine out of the cut with the high speed jet opened up a little over a 1/2 turn.
The exposed portion of the jet had to be bent back with needle nose plier just so I could make a full turn.