Jett .35 surging (Full Version)

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Strat2003 -> Jett .35 surging (6/4/2006 9:13:16 PM)

I'm not sure this is specifically a Jett problem, but I need some advice on high performance engines in general.

I have a Jett .35, rear exhaust, free flight venturi in a SAM LER Playboy. I'm running it on crankcase pressure to a metal tank. Lately when I start it up it surges. I can see fuel spraying out of the carb as it's running slow, then it picks up and sucks it all in.....and then goes through it all again. Sometimes it cleans up whe I go richer, sometimes when I go leaner! Once it cleans up it runs pretty well. So......is it starved til it sucks some of those droplets in, or is sucking that fuel spray in making it go rich?

Or something else?



bob27s -> RE: Jett .35 surging (6/5/2006 2:21:33 PM)

Hi,

Thanks for writing.

Are you running an exhaust system, header, or extractor on the engine?

Crankcase pressure is almost always tricky. Its a balancing act of sorts.

You have an engine trying to warm up, and tank pressure dependant on the engine rpm. Rpm goes up, more pressure, more fuel, engine goes rich, slows down, gets really rich until the tank pressure equalizes, then it leans out again..... cycles.

That is what tends to cause the surging. Takes practice to get a needle just right.

The one way I know to deal with it - get the engine warm, running steady state - clean - like you want it. Leave the needle alone. When starting, don't mess with the needle (or at least remember exactly where it was).

With F-1 I remember it being a nightmare until I went to using the pressure-bladder (balloon) type tanks, at least that was consistant pressure - and eventually added a regulator too.

Let me see what I can find out from Dub. He might have a suggestion or two.

Bob




bob27s -> RE: Jett .35 surging (6/5/2006 6:23:50 PM)

I just was speaking with Dub about it. He said basically the same thing I described.

It was once called pipe stutter. You see it with tuned pipe running pipe pressure. I suspect you also see it with engines running crankcase pressure.

You start the engine. It is rich from the prime. The pressure in the tank is low, so there is not much fuel going in. The engine leans out and builds up pressure in the tank. More fuel rushes in and makes the engine richer. RPM falls and lowers the pressure in the tank and the process starts all over.

The solution is to have the engine set leaner when you start it up.

You do get spitting when it is rich because the timing is fairly advanced on the crank; thus at low rpms you are blowing the fuel out the front before the crank opening closes. This is normal.



Strat2003 -> RE: Jett .35 surging (6/5/2006 6:38:22 PM)

So, I'm going to start it a little leaner, then richen it to my running setting?

Let me run something by you. I didn't have this problem last season, then I went to a smaller tank so I wouldn't worry so much about losing the airplane if the shutoff should fail. (a one minute run would put the plane far out of sight) Do you think the smaller tank is more sensitive to the pressure fluctuations?

Thanks for your prompt reply.



bob27s -> RE: Jett .35 surging (6/5/2006 9:47:08 PM)

That might be part of the reason. Id imagine that with a smaller pressurized volume, it would not take a big change in engine rpm to bounce the pressure in the tank around a bit.

Just a suggestion - perhaps try a restrictor in the pressure line. You will still get the pressure, not the volume. Maybe the changes will not be as rapid.



Strat2003 -> RE: Jett .35 surging (6/5/2006 10:09:45 PM)

Thanks, Bob, I'll give that a try.



Strat2003 -> RE: Jett .35 surging (6/13/2006 12:15:22 AM)

Just an update to anyone following this thread...I made a restrictor for the pressure line per Bob's suggestion, and also went back to the larger tank. My first run was with the tank partially filled to see if that would cushion the pressure changes and it seemed to work. I've had several good runs since and haven't needed to try the restrictor.



bob27s -> RE: Jett .35 surging (6/13/2006 2:06:42 PM)

Im glad you have it running again.

I guess the larger tank works a bit like an accumulator.



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