Brodak 40 head gasket?

I was breaking in a Brodak 40 this weekend, and it seems to have an awful lot of compression. It isn't the fit, because it turns over fine with the plug cold. It seems to be pre-igniting when I try to start it. I can get it going with the reverse bounce technique, but have to get lucky with the mix to flip it normally.

Once running, it is just fine, and very responsive to the needle. I can set it and it will hold the 2/4 break point through the whole run, even though it only has 7 tanks through it so far. I am running 15% Nitro 11% Castor 11% Synthetic fuel.

So, the question: Has anyone found the need for an extra head gasket on this engine? Lower Nitro content? Should I look for a colder plug? Or just live with the reverse kick start if that is the only real problem?

Mr. Renger. My Brodak 40 is an excellent motor as stock with the stock muffler. I have some extra head gaskets for this motor (which also fit the OS 40). I see not reason to modify the Brodak 40. I'm running this motor set on the ground at a fast 4-cycle. It goes to a 2-cycle upright, as expected. If that regimen is what is expected, the Brodak 40 does it perfectly.

LAST EDITED ON Oct-11-04 AT 05:42 PM (CDT)
 

Larry,

I run the brodak 40 in my Heinz 57 and I always start it by hitting it back wards. It starts on the first hit almost every time. I do how ever use only 10% nitro and 22% oil half and half.Hope this helps Larry.

Gordy

LAST EDITED ON Oct-11-04 AT 05:58 PM (CDT)
 
Hi Larry,
I'm using PowerMaster GMA 10-22 in my B40s. They are indeed tight in the beginning, but the first one shocked me with a one-choke, two-flip start - - Almost lost my grip on the test stand I was so startled!

It may not be completely broken-in, but I think it's ready to fly after about 15 two-minute 4-2-4 bench runs. The first 10 bench runs were sans muffler. The last run required a full turn leaner to maintain revs, which should make it (cautiously) flyable.

Just getting ready to begin with the second B40.

>I was breaking in a Brodak 40 this weekend, and it seems to
>have an awful lot of compression. It isn't the fit, because
>it turns over fine with the plug cold. It seems to be
>pre-igniting when I try to start it. I can get it going
>with the reverse bounce technique, but have to get lucky
>with the mix to flip it normally.
>
>Once running, it is just fine, and very responsive to the
>needle. I can set it and it will hold the 2/4 break point
>through the whole run, even though it only has 7 tanks
>through it so far. I am running 15% Nitro 11% Castor 11%
>Synthetic fuel.
>
>So, the question: Has anyone found the need for an extra
>head gasket on this engine? Lower Nitro content? Should I
>look for a colder plug? Or just live with the reverse kick
>start if that is the only real problem?

Ron B.
F4Fguy

Larry:

Sounds like you're on the edge of detonation. If it runs to your satisfaction once started, live with it and start it "on the bounce",but listen carefully for signs of a "crackle" of detonation in the two stroke mode. As you get more time on it it should settle down,but if you're still nervous about it,either add a gasket, or try 10% nitro. Early ignition will kill an engine faster than anything I know.

From an engine guy's standpoint,lower nitro and higher compression is the proper way to go. It's more thermally efficient, fuel efficient, and cooler running than adding gaskets and nitro. I realize this is heresy to the high priests of stunt,but the higher the nitro content,the harder it is to control timing. In our engines the compression ratio is king. Nitro may give more "power" (whatever that is) but so will more compression.They both have the same end effect,increasing either of them advances the firing point,decreasing them retards it. It isn't magic and it ain't rocket science,but it's engines 101.

I have to agree with Ron B.

Fifteen percent is a lot for this engine and if you are running the stock set up, I would say you are detonating. The Brodaks are made in Europe and those guys always seem to set them up for FAI or low nitro fuel. Mine ran great during the hot summer months with 7.5 percent. I am now using 5 percent in the cooler weather with the same great run.

Brodak offers a great shim package for this engine. The shims are only .008 and I added one to mine. All it did was allow the engine to settle back to a four cycle faster after a maneuver. It doesn't seem to hurt the power at all.

All in all, this has turned out to be a great CLPA engine and I think you will like it a lot.

Ron King

I am surprised that nobody told you this. Go back and reread the instructions. Seven tanks of fuel is not even close to being broken in. I broke mine in according to instructions and ran two tanks in the ARF Nobler. Even taking needle in a little it didn't go into a two stroke until last few laps. Too much power for Nobler tho. Later, DOC

I had similar problems with two out of three NIB Tower 40's. I could not get them to do anything but backfire or ocellate. My weak starter wouldn't turn them over. I took my dremel to one of the heads and ground out some radial grooves to lower compression. This was an immediate fix on starting one, but not the second. I finally got hold of a powerful starter and started the second one with the decompressed head. I don't remember the sequence on the first engine, but the second one was yesterday. After the first run, it would hand start easily. Two more runs and I put the original head back on and it hand starts easily. I don't understand why those two engines would not start with the stock head, but once run, quickly became docile. Or why no problem with the third one (actually the first one I bought.)

>I had similar problems with two out of three NIB Tower 40's.
>(snip) My weak starter wouldn't turn them over. I took
>my dremel to one of the heads and ground out some radial
>grooves to lower compression. This was an immediate fix on
>starting one, but not the second. I finally got hold of a
>powerful starter and started the second one with the
>decompressed head. (snip) After the
>first run, it would hand start easily. Two more runs and I
>put the original head back on and it hand starts easily. I
>don't understand why those two engines would not start with
>the stock head, but once run, quickly became docile. Or why
>no problem with the third one (actually the first one I
>bought.)
>
>Jim

First off, Jim, there can be a tremendous difference in deck height from engine to engine that you get. This is true of almost any of the mass produced engines that are out there, but something I notice regularly on the OS and Tower engines. Simply stated, the compression ratio of one engine from the factory can be quite different from the next. But in every case but one that I have ever seen, the tolerance error gives lower compression, not higher. Thus your easier starting engine may have had lower compression to start with. Or the fit may not have been as tight. Several reasons here.

The fact that you have a squeaker tight fit, however, is good. But it does make some ABC engines a bit "cranky" to begin with. When broken in properly, however, they will become quite easy to hand start. And I might add, I have never had an engine that I could not hand start from day one.

I would strongly caution against using the Dremel tool on the heads, however. With the experimenting I have done, I have never found a head shape that works better than the one that comes with the engine. You might come up with something equally as good. But chances are you are going to hurt performance.

And how do you know with any consistency what you are doing with a Dremel tool? We tested head shapes that were turned on a lathe. We used a button head and numerous buttons to try different lay outs on these engiens. We tried with more and fewer head gaskets. And the stock head, at proper height, always equalled or bettered (usually bettered) anything we tried. So even today I still use the stock head on these engines (after setting the deck height to where I want it.)

But, again, I emphasize, what we tried was done with a lathe so we could repeat results to a thousandths of an inch (or better). When you start grinding on an engine with a Dremel tool, and have no idea what you are doing or where you are going, you are just using up parts. Use the Dremel tool for building airplanes, not modifying engines.

My dremeling was just to remove material to lower compression so I could start the engine. It worked. What puzzles me is that here is an engine that is overcompressed, but a couple of runs makes the problem go away. It must be a change in fit somewhere, but it is not obvious that there is much, if any, change in piston cylinder fit. The pinch seems the same before and after so far as one can tell by wiggling things around. Actually, isn't it somewhat remarkable that very small changes caused by breaking an engine in have such large consiquences in how the engine runs?

LAST EDITED ON Oct-12-04 AT 11:53 AM (CDT)
 
Thanks all for the input. I think I will drop down to 10% nitro for starters. I don't hear any pre-ignition in a 2 cycle mode, but I have never had an engine kick so hard and consistently on starting - even diesels!

I am, however very happy with the power and run quality. Setting it just where it was intermittantly switching between 4 and 2 stroke, it tached 11,600 on a 9x6 Rev-up prop on about the 5th run.

And I did read the instructions and don't plan to put it in a plane until I have the full 20 runs.