Pop off question

Brian Eyley · Joined: 20 Nov 2007 Posts: 12 Location: Australia,

This is all great stuff I love it...................

If we can refer to the Tillotson 334AB............

To put things simple for us mortals, "cause and affect".

- High popoff does what? rich or lean
- Low popoff does what? rich or lean
- Fulcrum hight does what? ?????

Now testing all these adjustments everyone talks about, tune to the EGT.

What is the procedure to tuning to EGT.

I hope with some simple answers can help make it all a bit clearer.

Thanks......Brian
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al nunley · Joined: 13 Nov 2006 Posts: 3030

Pete Muller · Posted: Mon Aug 03, 2009 6:40 pm Post subject:

Pete Muller · Posted: Mon Aug 03, 2009 7:08 pm Post subject:

Something I'd like to add in to Steve's excellent post --

One "quirk" that we constantly have to deal with on a pumper-fed fuel system that works off crankcase pulse is that by its design, it probably tends to put out less fuel per stroke right at peak torque.

If you think about it... the pumper is a tightly-held piece of material (either some type of rubber-fabric or a phenolic). It has a very high "spring rate" because of that. It's logical to assume that the pump is probably the most efficient (greatest displacement per pulse) when the positive and negative pulses are closest to being equal. I'd be fairly certain that at peak torque, the crankcase shifts toward having the highest mean pressure (least negative pulse). When that happens, I envision the pulse as being more of a "more positive / less positive" pulse, rather than a "positive / negative" pulse, and because of the high spring rate of the material, total displacement is quite probably less. In simple terms: the pump side of the carb is the least efficient when the engine is the most efficient, and the pump is the most efficient when the engine is well above or below the revs of peak efficiency. Some of the things I have felt when running reed valve engines lead me to believe this is true.

How important is this? Probably not that important as long as there's "enough" fuel pressure and volume.

I have, however, run a KT100 with no pumper in the carb, and a just slightly pressurized fuel tank... and it definitely metered a bit differently through the rev range. I never pursued it much because it's illegal under everyone's rules, but having a very steady supply of fuel (or more accurately: a steady pressure in the fuel supply) makes the carb work a bit differently.

My gut feeling is that when Steve runs his sprint setup with high popoff pressure and the jets opened up a good bit more, this may tend to negate this issue with the pump, to some degree.

PM

Oscar Aguilera · Joined: 18 Jul 2001 Posts: 1614

Ok i'l give it one more shot while agreeing with al because as i stated earlier i do agree with al a little.

raise pop off does nothing, correct but you might find yourself leaning the mixture you have. ie you were at 1.5 turns on top and now your at 1.25 turns to reach the same egt 1200-1350 on the very top end.

you lower your pop off and it does nothing, but you might find your new high speed needle likes more fuel you were at 1.5 and now you need to go to 1.75.

you will find that the low speed needle will need the opposite.

so for the "mere mortals" you can set the pop off at 10 psi and run about 1.5 turns on each needle and it will run for the most part with some fine tuning.

on the leopard there are some guys that have figured out the perfect fulcrum setting to match what the leopard likes.

back in the day the motor builder built the motor and then tested different settings combined with the other settings he was allowed to mess with.

not needed with the leopard.

and yes al is correct that when the adj alt goes up then you will need to lean the motor and vice versa.

i will agree with al in respect to it doesnt matter becasue IMO the chassis is far more important. ssssoooooo

that's why i set the carb the way i was told. i completely ignore all the perfect explanation steve gave because i am a driver. but i did check the pop off on a carb that was freshly given to me by the motor builder so that i could maintain it. and yes i was very anal like pete and still am when it comes to my mixing of fuel, storage, and anything carb related. i always do myself. and i clean often.

i go back to none of this is something you can test at the track as good as on the dyno.

just my opinion and i managed to agree with everyone! Very Happy

mike collins · Posted: Mon Aug 03, 2009 7:19 pm Post subject:

al nunley · Joined: 13 Nov 2006 Posts: 3030

JIM SILVERHEELS · Posted: Tue Aug 04, 2009 1:37 am Post subject: WELL

Has anyone ever tried say setting the pop off at 6psi, run a f/a ratio test by setting the needles, then immediately change the pop off to 16 psi while not touching the needles to see how much the f/a has changed? Of course all at the same rpm? I don't have a f/a meter as yet, so anyone...
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Pete Muller · Posted: Tue Aug 04, 2009 7:32 am Post subject:

From 6 to 16?

No need to check the f/a ratio -- the engine would probably seize before you could get a good reading.

mike collins · Posted: Tue Aug 04, 2009 9:22 am Post subject: Re: WELL

Mike Goebel · Posted: Tue Aug 04, 2009 10:10 am Post subject: Re: WELL

JIM SILVERHEELS · Posted: Tue Aug 04, 2009 12:30 pm Post subject: THANKS MIKE

Thanks Mike for the response. Gonna buy those autolites today.
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mike collins · Posted: Tue Aug 04, 2009 2:53 pm Post subject: Re: THANKS MIKE

mike collins · Posted: Tue Aug 04, 2009 2:57 pm Post subject: Re: WELL

Frank Cire · Posted: Mon Aug 10, 2009 7:55 pm Post subject:

It might also be useful to note that the Walbro carbs were originally designed for chainsaw style applications and have only two metered fuel circuits. As such, the carbs really only operate in two modes by design, low RPM/closed butterfly and high RPM/full open, there is no midrange circuit. (Well, there is a third primer circuit that is unmetered)

Setting the pop-off pressure affects both circuits. A higher pop-off pressure means less fuel is available for metering, which is beneficial to setting low RPM circuit while detrimental to the high RPM circuit. A lower pop off has the opposite affect.

If you are learning to drive, or your driver is a newbie, you might want to run high pop-offs so the engine will idle cleanly and will not load up while the driver is trying to figure out what to do at each corner. As the driver develops, and spends more time at full open throttle, you might want to go to a lower pop-off to ensure the engine is getting enough fuel.
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