Author Topic: Brodak .40 oscillates between 4- and 2-stroke in level flight (Read 1532 times)

Kim Mortimore · « **on:** July 07, 2007, 03:46:36 PM »

Following up on Steve Scott's Single vs Dual NVA spraybar holes topic, I have a Brodak .40 (early, lower power version) that likes to shift back and forth between 4-stroke and 2-stroke in level flight, when the needle is set anywhere richer than wide open. It likes to "ride the 4-2 break point". This happens whether mounted upright or inverted, and with any prop I've tried from 4" to 6" pitch, 10" to 11" diameter. Tried both one and two shims. Stock muffler and venturi, Uniflow muffler pressure, all-tubes-forward tank, both with and without air filter...nothing unconventional in the setup.

This doesn't seem to bother a lot of people. In fact I get comments on my good engine run! But it bothers me. I like the older Brodak. It's docile, but powerful enough for smaller planes. It seems like a good power match for my 38 oz, 45" span flapless Old Timer. It wasn't enough power for a 46 oz Oriental ARF.

Any ideas what causes this attraction to the break point?

Thanks for reading,

Kim Mortimore

Steve Helmick · « **Reply #1 on:** July 07, 2007, 09:08:10 PM »

I recall the venturi being .302", with the .156" spraybar. That's really large for a .40. I'd suggest trying a Bruline Fine air filter, or about 3 layers of pantyhose to reduce the air flow. I'd also wonder if this is one of those upwind/downwind things? Randy Aero has a solution for that...put a 90 degree elbow on the uniflow vent. Or, you might try muffler pressure, tho I don't think it's a good solution...potential for meltdown keeps me from it.

Steve

Bradley Walker · « **Reply #2 on:** July 07, 2007, 09:13:49 PM »

Quote from: Steve Helmick on July 07, 2007, 09:08:10 PM

I recall the venturi being .302", with the .156" spraybar. That's really large for a .40. I'd suggest trying a Bruline Fine air filter, or about 3 layers of pantyhose to reduce the air flow. I'd also wonder if this is one of those upwind/downwind things? Randy Aero has a solution for that...put a 90 degree elbow on the uniflow vent. Or, you might try muffler pressure, tho I don't think it's a good solution...potential for meltdown keeps me from it. Steve

I actually agree with Steve.

The word is that the new Brodak 40 s set up pretty powerfully. The venturi is big and the muffler exit is pretty big too.

The best way to tune the Brodak 40 is to plug holes in the tongue muffler.

I vent to the inside of the fuselage. There is no wind there.

Ralph Wenzel (d) · « **Reply #3 on:** July 08, 2007, 08:39:37 PM »

Kim,
If that "lousy engine run" disturbs you sufficiently, why not just sell it to some poor sucker (Ahem!) and let him sort it out? I, uh, kinda like that type of run . . .

Email me if you want to sell or swap . . .

Glen Wearden · « **Reply #4 on:** July 10, 2007, 05:23:25 PM »

I'm afraid I'll come across as a total "who is this guy" neophyte, but your run makes me wonder if your fuel is foaming. Glen

minnesotamodeler · « **Reply #5 on:** July 10, 2007, 06:18:08 PM »

Who is this guy, Neophyte?

(Sorry, I just can't help biting on these things...)

Glen Wearden · « **Reply #6 on:** July 10, 2007, 07:18:06 PM »

He's a guy who butts into a thread periodically to satisfy his urge to appear knowledgeable; which, of course, he isn't.

Glen

Kim Mortimore · « **Reply #7 on:** July 10, 2007, 07:40:34 PM »

Yeah, who is that neophyte guy anyway? He's got some nerve posting on MY thread! Why I oughta....hey, wait a minute....fuel foaming, huh?....hmm....not bad....not bad at all. It would explain why prop size and needle setting have so little effect. Also, we have been hearing some sort of vibration in flight, even tho the front end is tight as a drum, and about all I can figure is maybe a resonant pushrod vibration thang or a loose nugget of 24K gold bouncing around inside or something. Maybe I'll give Armorall (sp?) a shot. My friend Larry Fernandez was showing me the jug of Armorall he uses recently. I forget how much he said to use--is it 16 oz per gallon of fuel, or 32 oz?

Oops, I got so carried away with the emoticons that I pushed the Post button instead of the Preview button. So the rest here is edit. I run Thunderbolt R/C long and 4S plugs, which are about as hot as they get this side of fine-wire Merlins, 10% nitro. I cleaned everything out, the spraybar, filter, replaced all fuel tubing in case of pinholes. By the way, I bought a 30-foot roll of Prather pink tubing because I got tired of constantly running out of tubing (I like to change it frequently) with those 3' packages, and last weekend heard that the Prather stuff is no longer available. Don't know for sure. I won't need any for another week or two.

Where was I....oh yeah, I will check the cylinder alignment, thanks Ty. I'm running muffler pressure and have had nothing but good luck with it for a long time, so... The venturi does seem large. A machinst friend made me a delrin venturi, about .250, a bit too far in the other direction, so I will ream it out to .270ish range and see if that makes a difference. I have quite a bit of power to spare on this plane, even with the older version B40, which is the only way to go IMHO--allows you to experiment more freely. Will report back if anything useful results from experiments. Thanks for all the suggestions--especially Glen!

Kim Mortimore

Glen Wearden · « **Reply #8 on:** July 11, 2007, 08:33:27 AM »

Kim, we just put a couple of drops of Armor All in a gallon of fuel. Glen