If you have to use an air compressor to start your engine, the venturi might be too large!!

Seriously, I'm sure the engine guys will comment on this but if it gets too large, starting will be difficult because it will not draw fuel well.  The smaller the venturi, the better the fuel suction--to a first approximation,

It will get inconsistent in running, and needling will be difficult.  Fuel consumption will become usually so high that your tank wont be large enough.

FWIW

Curt

Hi Stan:

If all else is normal, I would say not likely--but I would look at other stuff to be sure.  Plug or loose backplate--could do that.

Need more info tho.  Engine, plug, fuel,  did it ever work right?

Curt

Once asked Lew Woolard what happens if you lower the compression too far and his answer was something like "It just quits" might check to see how many head shims are in the engine...

I'm trying to get some sort of handle on the compression - venturi relationship. I understand you can use a larger venturi if you lower the compression but that statement by itself doesn't tell me much.  My experience so far has been pretty limited to the following.

Reducing the venturi on a ceramic sleeve Fox 35 (actually a .30) cured a run-away problem.

Reducing the compression on a ceramic sleeve ST 46 to try and solve a run-away problem made it "just quit". Upping the nitro fixed the quitting problem but the run-away came back. Maybe the venturi is too big??? Still not comfortable with this motor and would like to be..

Mean time I am breaking in a stock ringed ST 46 with stock sprinkler venturi to replace the ceramic engine so I will have a base reference.

So deal or no deal.. oops wrong subject.. Too big or not too big? Too low compression or too high compression? Thems the questions?

Hi Curt,

Hi Stan:

If all else is normal, I would say not likely--but I would look at other stuff to be sure.  Plug or loose backplate--could do that.

Need more info tho.  Engine, plug, fuel,  did it ever work right?

Curt

It's a sq. head Magnum .36, I believe either an OS A3 plug or a Thunderbolt (hobbypeople) std long. Stock number of head shims, none added.

Fuel is my standard mix, Powermaster 10% (18% lube) with 8 oz Klotz Castor added.
Top Flight 11/4 prop, launch RPM was 9800.

I did run it on another profile with good success and no problems. Original venturi was .261 with a PA needle and spraybar.  Venturi was opened up to (I think, but I'll check) .278. MACS 1 piece muffler but no muffler pressure.

I remember that get an annoying burp on a hard outside corner like the inverted turn in the wingover or 2nd corner on the outside square. Didn't quit then.

Thanks!

Curt,

Actually, I use my own spreadsheet for fuel mix calculations. The numbers work out to 22.82% lube  (about 49.49% syn. 51.51% castor) the nitro percentage drops from 10% to 9.41% and methonal is 67.76%. The lube content may be a little high, but it's been a good all around mix for my ABC, ABN, or AAC motors.

I don't believe backplate screws or head screws are loose or leaking, but , I haven't checked them.  Yes, the spraybar goes through the venturi, it's not a sprinkler type.

I can try changing the plug and reducing lube content, but it has to wait until I have something to test it on. The profile it was mounted in is a little bent because of structural failure caused by exceeding the decelaration design limitations. An unfortunate result of a flame out!

I cured a K&B 4011 of wind up by inserting an aluminum tubing sleeve in the venturi.