Jett .35 surging  
View related threads: (in this forum | in all forums)

Tower Hobbies
Enter up to 4 keywords or Tower stock numbers
Logged in as Guest



Users viewing this topic: none
  Printable Version
       



All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Engine Manufacturers Direct Support Forum >> Jett Engineering Support >> Jett .35 surging Page: [1]

Login
Message << Older Topic   Newer Topic >>
Jett .35 surging - 6/4/2006 9:13:16 PM   
Strat2003


 

Posts: 604
Joined: 3/9/2003
From: Mt. Pleasant, OH, AFGHANISTAN
Status: offline 		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?
       Post #: 1

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



Posts: 4588
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		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




_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to Strat2003)
       Post #: 2

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



Posts: 4588
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		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.


_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to Strat2003)
       Post #: 3

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


 

Posts: 604
Joined: 3/9/2003
From: Mt. Pleasant, OH, AFGHANISTAN
Status: offline 		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.

(in reply to bob27s)
       Post #: 4

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



Posts: 4588
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		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.


_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to Strat2003)
       Post #: 5

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


 

Posts: 604
Joined: 3/9/2003
From: Mt. Pleasant, OH, AFGHANISTAN
Status: offline 		Thanks, Bob, I'll give that a try.

(in reply to bob27s)
       Post #: 6

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


 

Posts: 604
Joined: 3/9/2003
From: Mt. Pleasant, OH, AFGHANISTAN
Status: offline 		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.

(in reply to Strat2003)
       Post #: 7

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



Posts: 4588
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		Im glad you have it running again.

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


_____________________________

Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host

(in reply to Strat2003)
       Post #: 8

Page:   [1]
All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Engine Manufacturers Direct Support Forum >> Jett Engineering Support >> Jett .35 surging
Page: [1]





Jump to:


Google 



Search | Marketplace | Event Calendar | Local Clubs | Magazine | Product Ratings | New Products | Discussion Forums

Photo Gallery | Member Video Posts | RCU Video Gallery | Instructor Search | Field|Track|Marina Search

Advertisers | Hobby Vendor Resources | Rate Manufacturers | Sign In/Sign Up

SITE MAP!   : :   FORUM RULES

© 2001 - 2007 24-7 RC, LLC, all rights reserved.

Charities we support that also need your help
Yorkie Rescue | Humane Society | ASPCA | Crohn's-Colitis America

Kaango.com Classifieds

1.063RCU1