I got a scale model from another club member that came with an ASP .91 red head (which is not the current model). I've considered replacing it because of the trouble I've had with it but would rather not change the installation. I finally got a good run on the test stand, but there is something odd: I have the high speed needle turned way out, and I cannot get it to run rich.

It's had fuel draw trouble right from the start--runs were erratic and I couldn't keep the engine running for long. I put pipe thread compound on every joint on the carb and replaced and checked all fuel lines inside and outside the tank. All that helped and I am now getting steady consistent runs, but the needle is out five turns and close to falling out.

I can get an acceptable idle, with a little hesitation with sudden full throttle even though the pinch test at idle does not result in any speed up.

I'm using 10% nitro, K&B long plug, 14 x 5 prop and two head shims.

Any ideas?

thanks,
Jim

The stock carb is way oversized on this engine. There are several ways to fix this.

1. Use the stock muffler, this increases the backpressure thus increasing muffler pressure.
2. Increase backpressure to the existing muffler. If using a pitts muffler closing off one of two stacks is sometimes enough.
3. Add a restrictor to the carb.
4. Use another smaller carb.
5. Use a pump. A Clines regulator may not work because of the lack of sufficient muffler pressure.

I think that sums it up.

Opps, forgot one, possibly the easiest. Simply adjust the carb so that it does not fully open. Test by opening the needle as much as you can, and close the carb till it runs rich then set that at the max opening and then adjust the needle normally.

Richen up the low speed 1/4 turn, and see if it is better.

FBD.

I forgot to mention it has a Pitts muffler with one vent closed off, and I do not have the stock muffler to test it against.

Jim

Hi!
Was it not the big ASP.90 that could not stand nitro... anything over 5% nitro was too much for it!?
Regards!
Jan K
Sweden

Thanks guys. I tried richening the low speed needle 1/4 as suggested and I think it helped. The hesitation from idle to full throttle is no worse that way, maybe better, and I was able to screw in the high speed needle quite a lot with no ill effects.

It's still odd, because the engine scarcely seems to respond to the high speed needle at all--it was open five turns and I screwed it in at least two turns but the engine ran the same. But it seems to run fine now no matter where I put the high speed needle.

I know about the reputation of the carb and need for low nitro, but the engine did run wide open, and with the two head shims I don't think the 10% nitro is a problem.

Between the new fuel line, the pipe thread compound on the carb parts, and a different glow plug (K&B 1L) I have no idea what actually made the difference. But it wasn't a science experiment; I just wanted the engine to run, and now it does, quite well.

Thanks again,
Jim

Sport Pilot summarized your problem and the solutions very well. I have two ASP 108s of the same vintage, and went through the same problems. Not a cheap fix, but the OS 7D carb will totally tame the engine down. A cheaper fix that will probably work is the Magnum 91 carb, or even a Perry Carb. They made the carb too large, trying for top rpm, then put a clone of the OS 61 FSR muffler on it to increase the back pressure to get it to run. With the 7D carb you can run a Pitts muffler with no problems.

Clair

I think you are almost there. If you slowly close the carb you may notice little differance in power from full open to about 3/4 or so. If that is the case then adjust the carb so that it will open just a little more than that. Then you may notice more response to the needle setting. This way it will not be as likely to go too lean when in the air. Most engines, will lean out when the prop unloads.

The carb on the ASP and Magnum engines 91's are not "too large".
In fact that same carb is used on the .61, .75, 91, and the 1.08, and they
all run fine....I have personally run all those engines.

The carb did look "largish" on the .61's, but ran good, in fact the stellar
performance of the .61 was due largely in part to the big carb.

Some of the old OS .61 FSR's used the huge 7D carb....the carb off the
1.08 FSR, and were the fastest of the fast FSR's. They idled OK too.

FBD.

This is the ASP redhead. It has a larger carb than the later ASP's and Magnums.

The reason high speed needle not respond can be the idle screw is overtighten to block the spray nozzle.Try to unscrew the idle needle first to have a good high speed needle responce then adjust the idle screw.I hope some improvement.

Sport_Pilot....what is the actual throat size then ???

I don't know off hand, but remember that the size was reduced on the .91, I think it remained the same on the 1.08, but not sure.

The Magnum site is listing this carb for the original XL 91 and 1.08, as well as the current 1.2. However the later .91 and 1.08 are differant. They don't show the throat sizes, and I don't know if the earlier ASP is the same.

http://www.hobbypeople.net/gallery/280005.asp#More

Buzzard Bait,

Years ago, I had the ASP 1.08 redhead and symptoms similar to yours. I also could not get a really rich setting on the needle with a pitts muffler, although there was no problem with the stock muffler. I also plugged one of the 2 exhaust stacks on the muffler, that helped. What I found was that in my ongoing effort to eternally tweek the low end for a lower idle, I had turned the low end needle in so far it was interfering with the ability of the high end to allow fuel into the carb. I backed out the low end needle a LOT and started to retune the settings. Surprise! I was able to get a "too rich" setting on the high end I couldn't get before. Maybe that is going on with yours, as well. [8D]

Good luck.

Ah ha! Turk! and Jim D are telling me the same thing and it makes sense. I remember being under the impression the idle was rich quite awhile ago and I kept screwing it in. When I took Flyboy Dave's advice and turned it back out 1/4 turn it helped--maybe I need to do it a little more. Thanks guys!

Jim

buzard....

....either there is a clog in the carb nozzle....there is not enough fuel pressure,
or the low speed is so far off you cannot adjust the carb properly.

Keep turning the screw 1/8 turn at a time untill it gets perfect. At that point
the main needle should be 1-2 turns out. Dont worry....the carb is not
"way oversize for this engine".

FBD.

Maybe just oversized? I would say that a carb that is sufficient for a 1.2 is oversized for a .91. I had fuel draw problems with this engine. Had to run it way too rich with a full tank to prevent going too lean with an empty tank, and that was with the stock muffler.

OK guys, after richening the idle a little the high speed adjustment was normal. The engine runs quite well now from idle to full power with a pretty good transition. As a result, I finally flew a 1/5 Thomas Morse Scout built by another club member more than 10 years ago, which had never had a successful flight because of both engine and radio problems.

So there really is something weird about this engine--the low speed needle is quite tolerant in its effect on the idle, but it is very sensitive in its effect on the high speed needle. So, like the old Fox carbs, you actually have to adjust the low speed first and then the high speed, but it is a bit of a guessing game because the low speed needle is pretty insensitive and it's hard to tell when it is right for the idle.

The engine problems are fully resolved; there is still a radio problem, but that's another story and another forum. The Scout was absolutely beautiful in the air. Thanks for all your help.

Jim

The normal tuning procedure is to have the low speed a little on the rich side.

Adjust the high speed need

The peak the low speed. The engine will not transition out of idle without quiting.

The enrichen the low speed needle just to where you get a good transition to high speed