394XP low on max rpm after port work

[MattG]

It should be above 20 psi.

I've used nail polish or super glue to reseal Welch plugs.

Can you explain the POP test a bit to me? It's not something I'm familiar with. I'm only familiar with the regular pressure leakage test for carburettors, e.g. like this :



Sorry to derail your thread @Timbco84 but I'd like it if someone gave a brief explanation of the POP test

 

[Adirondackstihl]

NOTE:
when performing the POP test, make sure the needle and seat are wet with fuel or WD-40.

 

[MattG]

NOTE:
when performing the POP test, make sure the needle and seat are wet with fuel or WD-40.

So do you pressurise the carb are per normal, but just at a higher pressure?

And what is that pressure? At least 20 psi like @srcarr52 states ?

 

[Adirondackstihl]

So do you pressurise the carb are per normal, but just at a higher pressure?

And what is that pressure? At least 20 psi like @srcarr52 states ?

Srcarr won't steer you wrong.
I believe @tree monkey said as long as there is a min of 15lbs, that's all that matters.

 

[srcarr52]

So do you pressurise the carb are per normal, but just at a higher pressure?

And what is that pressure? At least 20 psi like @srcarr52 states ?

You keep pressurizing the fuel inlet of the carb slowly till the pressure starts to bleed of through the needle valve. Like stated before make sure there is fuel or WD40 inside to get an accurate reading. You're measuring the pressure at which the needle "pops off" (I.e. opens).

 

[Jimmy in NC]

Pop off differs for application. An HL Tully feeding a 5hp Briggs we set at 5psi. A saw carb I look for a min of 15 but 20 and up is better. I fill a bit of fuel line with mix using a dropper. Pop it a couple of times. It will pump to a point, crack open and bleed down to another point, then you can pump it up again. Look for the same crack pressure a couple times repeated to make sure it is consistent results.

Sent from my SM-G955U using Tapatalk

 

[MattG]

Thanks for your help on that one guys.

 

[Timbco84]

Sorry to derail your thread @Timbco84 but I'd like it if someone gave a brief explanation of the POP test

No derail at all, It's great to read through a thread and find more than you were looking for. Great information being presented here. I appreciate it a ton.

 

[MattG]

So help me out (again) guys.....what exactly are testing here?

Firstly the manual page I piccied is testing to ~11.5 PSI, and then you guys are mentioning 15(to 20) PSI.

So, correct me, if I'm wrong, or confirm, but is it basically saying "testing to the lower pressure is a basic leak test", and "testing to the higher pressure ensures that the carb will not flood easily" (since it should take a lot to POP OFF) ??

So what's going on exactly?

 

[Mastermind]

So help me out (again) guys.....what exactly are testing here?

Firstly the manual page I piccied is testing to ~11.5 PSI, and then you guys are mentioning 15(to 20) PSI.

So, correct me, if I'm wrong, or confirm, but is it basically saying "testing to the lower pressure is a basic leak test", and "testing to the higher pressure ensures that the carb will not flood easily" (since it should take a lot to POP OFF) ??

So what's going on exactly?

If pop off is too low, pressure in the fuel tank from heating (like the saw sitting in the sun) can cause flooding. Aside from that, I've not seen POP cause many real issues in the type saws I build. I just test to be sure the carb will maintain 15 psi, and will reseal correctly.

 

[srcarr52]

So help me out (again) guys.....what exactly are testing here?

Firstly the manual page I piccied is testing to ~11.5 PSI, and then you guys are mentioning 15(to 20) PSI.

So, correct me, if I'm wrong, or confirm, but is it basically saying "testing to the lower pressure is a basic leak test", and "testing to the higher pressure ensures that the carb will not flood easily" (since it should take a lot to POP OFF) ??

So what's going on exactly?

Here is my typical procedure:

1) Test to 10 psi and make sure it holds pressure without bleeding down to test the state of the needle/seat, upper pump gaskets and any other way that side of the carb can leak.
2) Bump the lever arm to release some pressure and test that it seals back up quickly without a slow bleed after. This is also testing state of needle and making sure the lever arm isn't kicking the needle and wedging it into the seat.
3) Check pop off pressure by adding pressure slowly till the needle releases on it's own. There are some guidelines on what this pressure should be above but it's different for every carb/saw. Don't go over 50 psi, you can poke a hole in the pump diaphragm.

Pop off pressure has to be balanced with the engine (demand for air and pulse for pump), the fuel pressure inside the bowl is inversely related to the pop off pressure. So low pop off pressure is high pressure inside the bowl and thus will supply more fuel.

If the engine is starving for fuel with the high side needle all the way out the pop off pressure can be lowered to correct this. Another indicator of low pop off pressure is if the engine tunes fine but 5 seconds into a cut it goes lean because the bowl is being drained of fuel and the pulse of the pump can't open the needle enough to supply enough fuel.

In this case if the high side needle is almost all the way in but it still rich you may need to raise the pop off pressure because the internal pressure of the bowl is too high and provides too much fuel.

 

[Mastermind]

Here is my typical procedure:

1) Test to 10 psi and make sure it holds pressure without bleeding down to test the state of the needle/seat, upper pump gaskets and any other way that side of the carb can leak.
2) Bump the lever arm to release some pressure and test that it seals back up quickly without a slow bleed after. This is also testing state of needle and making sure the lever arm isn't kicking the needle and wedging it into the seat.
3) Check pop off pressure by adding pressure slowly till the needle releases on it's own. There are some guidelines on what this pressure should be above but it's different for every carb/saw. Don't go over 50 psi, you can poke a hole in the pump diaphragm.

Pop off pressure has to be balanced with the engine (demand for air and pulse for pump), the fuel pressure inside the bowl is inversely related to the pop off pressure. So low pop off pressure is high pressure inside the bowl and thus will supply more fuel.

If the engine is starving for fuel with the high side needle all the way out the pop off pressure can be lowered to correct this. Another indicator of low pop off pressure is if the engine tunes fine but 5 seconds into a cut it goes lean because the bowl is being drained of fuel and the pulse of the pump can't open the needle enough to supply enough fuel.

In this case if the high side needle is almost all the way in but it still rich you may need to raise the pop off pressure because the internal pressure of the bowl is too high and provides too much fuel.

Excellent post Shaun. Thank you.

 

[MattG]

Thanks mate! A really neat answer - told me a helluva lot. From this bit:

2) Bump the lever arm to release some pressure and test that it seals back up quickly without a slow bleed after. This is also testing state of needle and making sure the lever arm isn't kicking the needle and wedging it into the seat.

I've figured that you actually do this test with the carb opened.

Thanks buddy, have a cyber-space beer on me!

 

[Chainsaw Jim]

@Timbco84, how far are you from Springfield? I'll fix it for free if you want to bring it on by my shop.

 

[Mastermind]

Thanks mate! A really neat answer - told me a helluva lot. From this bit:



I've figured that you actually do this test with the carb opened.

Thanks buddy, have a cyber-space beer on me!

Watch you don't get sprayed in the eye. On some carbs you can tickle the metering diaphragm through a hole in the cover.

 

[MattG]

Watch you don't get sprayed in the eye. On some carbs you can tickle the metering diaphragm through a hole in the cover.

Thanks man, yeah gas in the eye would be bad. Fortunately I need to wear working glasses (to see anything at all!!), but 10-4 on the extra warning

 

[Timbco84]

@Timbco84, how far are you from Springfield? I'll fix it for free if you want to bring it on by my shop.

Thank you for the offer, I am hoping to figure this problem out so I can learn from the experience. Got a new carb on the way, if you have anything else I can check, I would really appreciate the info. Thanks

 

[Chainsaw Jim]

Thank you for the offer, I am hoping to figure this problem out so I can learn from the experience. Got a new carb on the way, if you have anything else I can check, I would really appreciate the info. Thanks

I'd have to see it in person.

 

[Timbco84]

Well I tried the brand spankin new Walbro carburetor out and it runs exactly the same.

Can someone tell me if it is a possibility that I gave the saw to much intake area/duration...I measured the saw stock @74* (148*duration), I lowered the jug to adjust squish and I took a little out of the floor of the intake and ended up @79*(158* duration). I originally measured 78* after port work, but think I got a bad measurement cuz of assembly oil film, rechecked and got 79*.

Is it possible that the intake is flat open to long and its drawing in too much air/fuel per rotation?

Is it possible for a coil to be weak and thus the week spark won't burn the mix well enough to get max RPM?

Is there any other reason why I can't get over the 11,000 rpm wall I am hitting?

 

[mdavlee]

79 is fine. Coil or flywheel timing is where I'd look next.