The Thunder Tiger GP-42 and air bleed carburetors
 

I've run a number of air bleed carburetors over the years, and I rather like them. They're not overly finicky and they're easy to get set up for good, reliable performance. I own a number of Enya, O.S. Max, and Thunger Tiger engines with air bleed carburetors, and they are all fine running engines that are powerful and reliable.

Normally, when I set up a new engine with an air bleed carburetor, I simply open up the air bleed screw so that the air bleed hole in the carburetor is about half open and half closed. This is usually very close to an ideal setting for most of the engines I've run, and it is close enough to let the engine start and run reliably while I begin the break-in procedure appropriate for whichever particular model I'm running.

I've encountered a couple of Thunder Tiger GP-42 engines recently where this setting was not appropriate. With the air-bleed hole half open, the engine ran very hot at idle and ran erratically. The air bleed hole had to be mostly closed (three-quarters or more) for the low speed operation of the carburetor to be sufficiently rich for cool, reliable operation.

I'm posting this for two reasons. I want to pass this tip along specifically for the Thunder Tiger GP-42; if you're having trouble getting one to run correctly, try closing off the air bleed hole significantly to try to get the low-end mixture sorted out. I'm also posting this to hear from other glow engine users if they've also run across this issue with the GP-42, and whether the "half-and-half" rule for initial engine set up normally works well for them, or if I've just been lucky having this work so well for me with my engines.

Just curious what your experiences with air bleed carburetors have been like.

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

I have had similar experiences but never had my GP42 s give me any special grief. I am also a big fan of the simple reliable air bleed carbs and wish more manufacturers would build them instead of (cheap) poorly made twin needle carbs that are almost impossible to set up. I think the Aviastar engines would have been good engines if the carbs had been so.

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

Hi Big Ed,

I like the air bleed carbs too.

I start with the hole half open as per the OS instructions, but always end up with the hole fully open for a slightly rich idle.

I have considered opening the bleed hole up a bit but so far haven't bothered.

The reason mine are different to yours is probably due the fuel I use (18% synthetic 0 nitro), although I am running an OS 20 on diesel and that has the bleed wide open as well.

Dave H

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

TT42 GP engines are great but like other TT engines, the weakest point is carb/HSN./fuel nipple  installation.Threads are so fine causing strip easily.Also looses easily.I dont like at all TT fuel nipple/HSN system.But GP 42 is a great engine,  fly my  Champion 30 L for years.Run homebrew ,no nitro fuel.

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

The Enya and other better engines airbleed carbs are metered carbs with overlapping slots that regulate fuel flow above idle. Others like the TT .42 have carbs that simply don't control the fuel flow other than by the varying air flow through the carb. This works pretty good on engines like the Saito .30 and the OS LA .10. The Fox EZJust carbs, (like the Enya) are very good metered carbs with an airbleed trim.

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

I do not have a TT airbleed carb, but I do have a TT pro .46 with a 2-needle carb. Yesterday I had the first weird problem that has me a tad stumped. Using 15%nitro 20% oil fuel, 9x6 or 9x7 prop, Jettstream red muffler, and MC59 plug I had this problem: Set the main needle to a tick rich of peak (started out quite rich since it hadn't ran for awhile). It went from 12k very rich to on the pipe and screaming at 17k in a matter of a couple clicks of the needle. Throttle back to idle to set the idle mix if it needed adjustment.. Peachy as it almost always is - just a tick rich on the idle mix too... Idle was around 2800rpm and transition was instant. Put it out to run it up and halfway through its throttle up ( I punched it WOT) it started to burble and run so rich it was as if I magically richened the needle 5 turns. I'm baffled as to what is causing this. The tank is at the "tank rule", I'm using a felt clunk, the plumbing is good inside the tank, the carb is clear, tried a different glow plug, and it still acts funny. The pressure tap on the tank is a normal setup, tube set to the top of the tank, no clunk on it. No check valve on pressure line. Only had this sort of problem using the VP30 fuel pump which is no longer used. Backplate tap is plugged.

Any thoughts? Never had this problem with this engine when not using the pump... I have to prop it lightly to get the engine in the pipes tuning window of 15-17k FYI.

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

Like Dave H I always find that I need the air bleed holes open all the way because they usually tend to run rich at idle regardless. That's for my Enya and OS two strokes. But I have not yet tried my TT GP 42. When I do, I'll keep your experience in mind.

Is it possible your GP 42 is not fully broken in yet? The usual symptom of a lean idle is that the engine instantly dies with no smoke when you advance the throttle. I'm surprised it would be noticeably hot...unless it is not quite broken in.

Jim

RE: The Thunder Tiger GP-42 and air bleed carburetors
 

I don't know what happened, but I cannot edit my previous post... I was going to change/move it.

If anyone has any idea why my 2-needle TT carb is acting funny, do share. With the main needle peaked and idle mixture perfect, it loads up horribly bad in the midrange. If you punch it after peaking everything, it winds up fine. If you let it idle for a few seconds, it burbles and basically 4-strokes. It will not clean out and wind back up. Re-peak the needle and the same result over and over. Tried it 5 times. The only thing I can think of being the problem is the single detent needle holder. I am going to swap a double detent needle holder and see if that helps. If not, I'll try another carb.

I have an OS 40 FP that i have heard comes with an air bleed carb but i bought the engine used and can't tell if there is an air bleed carb on there or not. How do you tell if you have an air bleed?

A photo would help. all my OS FP engines have had an air bleed carb stock.

http://www.rcuniverse.com/forum/atta...mentid=1932787http://www.rcuniverse.com/forum/atta...mentid=1932786


Also, what is that screw on top, is that an idle screw?

That carb is from a .25 FSR. It has a high and a Low speed needle. The other screw is your idle stop. it is for adjusting for the smallest opening in the throttle barrel you can set for a reliable idle. On these carbs it is usually one milimeter or close to 1/32 of an inch.

Quiksport. Your rich midrange maybe can be adjusted, but whenever you are using a pipe, it needs to be so rich on the high end, that it will always be lacking something in the midrange and low range. Normally the low range can be adjusted for, but not the mid. A different taper on the low range needle may help if you don't mind destroying a few to experiment. An airbleed carb is not adjustable that way. I am pretty sure the problem would disappear with a regular muffler. I know you don't want to hear that. (read that)

Thank you. I thought it was a low speed needle but wasn't positive. I have the motor on a plane so i guess that idle screw is useless to me. I have it set so the transmitter throttle trim sets the idle and then i can kill the engine by setting the trim all the way down. The way those carbs bolt on is a little crappy. The screws on either side leaked so much air that i couldn't even kill the engine with the throttle all the way shut but i sealed it all off with RTV silicone and it runs like a top now. Doesn't help that who ever put that carb onto the motor stripped out one of the screws

My .46Pro ran fine last summer. I've been using a 9x7 APC prop and Jett (red) muffler. This configuration with 5% nitro fuel ran really well. I could set the top end 400rpm rich and get the idle mix set for a perfect transition even after 30 seconds. When I encountered the weird midrange load-up, I had a 9x6 APC on it. I suspect the single needle detent clip to be partly to blame. Wiggling the needle changed the mixture somewhat. Though when I ran the engine last, it had been a year since it had run. It probably just needed a few tanks to loosen back up from sitting. It's sitting on the shelf for now, I'll pull the carb apart and clean it out.

You may consider using a cold plug with a piped engine, it allows you to lean the top end to a more optimum setting hopefully clearing up the midrange. However you may only need a low speed needle adjustment.

Its just a tuned muffler, it's not a full pipe. It doesn't behave the same on a muffler as it does on a pipe. With low nitro fuel like I use, a hotter plug is necessary. Besides, no other plug changed the phenomenon. I need to tear the carb down. I'm sure it's got a chunk of crap stuck or something.

Hi!
15% nitro is a lot!
My standard fuel is 5% and when using a tuned pipe (long or short mini pipe ) it's mandatory to use a colder plug.
I would use a Nova Rossi 4 or 5 or Rossi 4 or 5 when running an R/C Engine on 0-5% nitro and Rossi 6 when running 15% nitro.


OS 8 or Enya 3 is way to hot plugs when using a mini pipe and 15% nitro!

It is NOT mandatory to use a colder plug with higher nitro. Tuned pipe or not. That's a generalization that's been spread for many years IMHO.

Some engines need the ignition advanced as far as possible like Jett. Dub tests all of his engines with 15% nitro Powermaster fuel and hot Merlin (red) plugs. Though they recommend Merlin plugs, "any hot plug will do). I use a McCoy MC59 hot plug in mine on 15% nitro and it works perfectly.

What's even more counterintuitive is using 30% nitro in my RC car engines on hot MC59 plugs. Spool-up is crazy fast and plug life is good. I run the car engines on the ragged edge of the "perfect tune" and rarely lose a plug.