Engine basics > Engine set up tips
O.S. Max-s 35 wants to shut off?
Jerry Reider:
I have a Brodak 38 Special with an O.S. Max-s 35 on it. The plane flies great and I feel I could do anything with it but....the engine stumbles and misses at 12 o'clock on the 2nd loop of a lazy 8. It has shut off for me one time while doing an outside loop at the top of the loop. It also will stumble coming out of a tight inside loop at the 6 o'clock position. It is fine on large inside loops and if I make the 2nd loop of the 8 large it runs fine with no sign of a miss. It runs the same flying inverted as it does in level flight. It has plenty of power in any manuever, I have a light smoke trail and plenty of castor residue on the plane. I was running 10% nitro 25% oil 50% syn. 50% castor but have recently changed to 5%nitro 28% oil 26% castor 2% syn. It stumbles the same with either fuel. Today I changed from the 11x5 Zinger to a 10x5 to increase the RPM's; no change, still stumbles, same places. I'm using Zingers because they have less pull than my preferred Master Airscrews. The plane is too fast with a Master Airscrew. The O.S. has a .281" dia. venturi and the hole in the spray bar is set to the forward position at 5 o'clock to the vertical center line of the venturi. The C/L of the tank is 1/8" higher than the C/L of the engine. I had the tank mounted lower and raised it by 1/4". The fuel line that goes over top of the fuselage goes to a 90 deg. piece of brass tubing that faces the front of the plane. I only make one change at a time so I'll know what worked. So far nothing has made a difference. This looks like a fuel delivery problem to me but it's got me stumped on how to solve it. I'm considering going to a metal tank. Any ideas?
Dennis Adamisin:
K&B Idle Bar Plug, long reach, with two washers.
2nd choice: Fox Idlebar (not the 2V type)
Use the 10% FUEL WITH 50-50 oil, better still try some 10% Omega (less Castor)
Robert Zambelli:
What Dennis said should work. Try it first.
I have quite a bit of time on the Max 35, both profile and inverted mounting.
I have NEVER had acceptable results with a clunk tank. The last Max 35 I used was on a Banshee. It ran flawlessly with an old Perfect 4 ounce wedge tank, over/under venting (no uniflow).
Also, your venturi sounds a bit small. The best results I had were with the stock venturi, around .250 as I recall.
Bob Z.
Jerry Reider:
--- Quote from: Dennis Adamisin on March 18, 2009, 08:28:47 PM ---K&B Idle Bar Plug, long reach, with two washers.
2nd choice: Fox Idlebar (not the 2V type)
Use the 10% FUEL WITH 50-50 oil, better still try some 10% Omega (less Castor)
--- End quote ---
Thank you for your reply Dennis. I have a Fox R/C long w/ idle bar glow plug in it. What do you think of trying a Fireball Hot long? But it doesn't have an idle bar. Why 2 washers, to decrease the compression? I changed to the castor after reading Randy Smith's post titled "Fuel , GLO-plug and running tips" that is posted towards the beginning of this section. He stated that the older O.S.'s have smaller bearing surfaces and he felt they needed more castor oil. I had bought some of that because I have a few Fox 35's that I use. I can certainly go back to the 10% nitro 25% fuel if you think that will help solve my problem.
Jerry Reider:
--- Quote from: Robert Zambelli on March 19, 2009, 05:24:41 AM ---What Dennis said should work. Try it first.
I have quite a bit of time on the Max 35, both profile and inverted mounting.
I have NEVER had acceptable results with a clunk tank. The last Max 35 I used was on a Banshee. It ran flawlessly with an old Perfect 4 ounce wedge tank, over/under venting (no uniflow).
Also, your venturi sounds a bit small. The best results I had were with the stock venturi, around .250 as I recall.
Bob Z.
--- End quote ---
The venturi is the one it had on it when I got it. I was wondering if it was the right size. I'll check with my local hobby shop to see what he can get. I know the clunk is not hitting the back of the tank and I believe it's mounted correctly. I was considering going to a Brodak 4oz. wedge because I had some doubts about the clunk tank. As you stated I was looking at trying the tweaks first. Hopefully Dennis' advice will solve it.
Thanks,
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