394XP low on max rpm after port work

RD35 said:

I'd try removing the muffler and air filter complete and give it a try. Also, check your tach....if it is out of calibration you may be hitting your rev limiter. (just tossing ideas in the air...this is a very interesting problem)

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Mastermind said:

Yeah.......If it was raining titties I'd get hit with a big cock.

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srcarr52 said:

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.

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MattG said:

Hello again pal,

I got around to setting up the POP on my 044/460 hybrid. I serviced/clean the carb first off with new OEM bits. I had a go at setting the POP. As everyone who does this, must realize that it's quite tricky to get an accurate reading and seeing as it was the first time, I've done this, it took a while. When done I think the POP is somewhere between 22-26 psi.

I've had the hybrid running now a couple weeks, and I have to say, it's really awesome. Excellent throttle response, and now tuned to 4-stroke at ~14.9K. What I would say is bad though, is the idle. It's just about impossible to find a range of smoothness between rich and lean bog on the L screw. Eventually I get it fluttering around 3k, it does idle, but is very jumpy, and I think will eventually stall if left long enough.

Everything is basically mint on the saw, no air leaks, a mild port, high compression, so I'm starting to wonder if the motor is struggling to pull the lever open what with the high POP? The theory certainly makes sense to me.

Thoughts?

Wondered what you guys reckon, before I pull the carb out.

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srcarr52 said:

Nip the spring and try it. It could solve your problem.

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MattG said:

I'm good with just bending the lever a little to bring the pressure down.

So are you kinda agreeing with that perhaps the POP is just a shade too high for this particular saw?

Perhaps 15-18psi or something might work out better?

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srcarr52 said:

If they run out of fuel at idle or have the high side needle adjusted way out 4 stroke its usually too much pop off pressure.

Usually bending the lever can only change the pop off a few pounds.

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