Flathead's Combee TomCat

[Flathead]

OK, I'm not too lean. Changes jets to make it richer and it got worse. So I ordered some smaller jets and we'll see what happens. Basically what the problem is if I set the prop pitch too high, them the motor can't get past 4500rpm's. If I back off on the pitch, then the motor gets past 4500 rpm's but rev's past my 6500 engine rpm max.(way past) There is no happy medium. The color of my plugs are telling me to lean it out, but I'm already using considerably smaller jets that what it recommended. What the heck, it's only a motor. They'll make more.......

I'll get er'

Bruce

[buster]

Hey Bruce,

Sounds like you got yourself what we used to refer as a "Problem Child". I never been too good at diaganosis, but I know a guy, maybe you heard of him, goes by the name of Duane. He generally can come up with reams of info about most any problem a guy might have.

If he does, be prepared to to a lot of reading. He will most likely have the answer, but it may take you a while to sort it out. He's a good guy to know when you need to find tech info about something!

Dave

[Duane Scarborough]

Hey Bruce,

Like Buster said, you may have yourself a problem child.

The problem may or may not be mixture related. It could even be timing related. But what you've described sounds mixture related.

You may have a problem with the mixture being rich at one RPM, about right at another, and then lean at another. The mixture may change several times as the carb transitions from one circuit to another. (if you're using a multi-circuit carb)

If that's the case, you could play with jets a very long time, and never stumble upon the right setup.

I'd suggest that you buy (or borrow) an air/fuel ratio monitor. With one of these installed, you can readily see what your mixture is at any given RPM.

You can buy meters that you permanently install, or temporarily install, depending on what you want.

Here's a couple of links:

http://www.knfilters.com/airfuelmonitors.htm

http://www.jetlink.net/~okayfine/su/airfuel.html

Cyberdyne makes a few inexspensive models, but you'll need to buy the O2 sensor separately.

http://www.summitracing.com/search/D...erdyne-Gauges/

If you feel adventurous, you can even build your own fairly cheap, though I wouldn't suggest it.

http://www.scirocco.org/tech/misc/afgauge/af.html

Like I said, it may or may not be mixture related. If nothing else, an air/fuel ratio monitor would answer that question.

Have fun with it !

Duane

[Flathead]

Thanks Duane,

Unfortunately, the K&N product will not work with gas mixed with oil (2 stroke). K&N cautions the oil contaminates the O2 sensor and gives false readings. What we gernerally do with our sled here in MN is use the color of the plug to gauge air/fuel mixture. Though not as accurate as an O2 sensor it does work very well from a practical standpoint. I experimented with richening the engine, even though my plug condition would indicate I should lean it out, because fatter jets was all I had on hand. The only take minutes to change, so I decided to experiment a little. I have a selection of leaner jets coming that I hope will solve the problem. 2 stroke are notoriously fussy about being too rich.(or too lean)

Wish me luck!

BTW is there anyone out there using a rotax 582 with Mikuni carbs? What size main jets are you using?

Bruce

[Duane Scarborough]

Bruce,

Sorry that my feeble attempt at help was no good.

You'll get it figured out. Good luck !

Duane

[Flathead]

Duane,

I am only familiar with the K&N product. It may well be one of the others would work in my situation. If my new jetting does not do the trick, then I'm sure I'll be looking deeper into a sensor. Thanks for taking the time to help!

Bruce

[n4wsp]

I don't use any oil on my k&n filter. It is not like you are going to be running a airboat down a dusty road. I have it on there just to keep the bugs out and the tree branches out.

Steve

[Flathead]

Tried the leaner jet and it did run better, but the flat spot is still there. After more research, I'm back to suspecting the exhaust is causing the flat spot. Interestingly enough, exactly as it is supposed to, IF it was in a snowmobile. Sleds of that era typically are clutched and geared to allow the motor to rev up to around 4500rpm's before the clutch engages. Then the engine comes on the pipe and produces maximum power in it's upper rpm range where most of the riding is done.

Solution? Find a OEM pipe designed to run on an 582 Rotax ultralight aircraft application. Supposedly produces more mid range power, (eliminating the flat spot caused by the snomobile tuned pipe) but sacrifices a bit of top end power output. Anybody got one I could try? Rotax exhausts are big bucks. A bit to spendy to buy one HOPING it will solve this problem.

Details, always details......

Bruce

[buster]

One place I would watch is craiglist. I noticed there used to be a number of ads for snowmobile repair. If they are still in there, give 'em a call, you never know what used parts they might have, or know where one is.

Good luck with that tune-up problem,

Dave

[Papee]

Depending on how the pipe is made you may be able to tune it yourself. I put some Harley mufflers on my 084, pulled out the guts and tuned the output hole size until mine ran the best. When I was young and racing motorcycles we had tunable exhausts, there was a snap ring in the end that held in washers, you just took out the snap ring and put in the washer with the hole size needed for that day.

[Duane Scarborough]

Bruce, (sorry)

I was talking today with a co-worker who has some Rotax experience. After I described your problem, he asked if your engine had R.A.V.E. valves in the exhaust ports. I know nothing about Rotax engines, so I asked him to explain.

His explanation (or maybe my understanding) wasn't great, so all I really got out of it was that they somehow shift the power band, according to vacuum. That, and if they're acting up, they could possibly cause what your describing.

When I got home, I did a quick search, and found this:

http://www.rotaryforum.com/forum/showthread.php?t=21100

Read the first post, and tell me if that's not the same thing you're experiencing.

Duane

[Flathead]

Duane,

My engine does not have the R.A.V.E. valves although all my snowmobiles do. They work very well by the way, squeezing 25 more hp out of the same engine I am using in the fatcat.

My current train of thought is still with my tuned pipe causing the problem. Although it is helping the engine to breathe at certain higher rpms it is actually inhibiting the motor from breathing at certain lower rpms (causing my flat spot). That's the trade off using a tuned pipe. Since I have two pipes, I am going to gradually start cutting the end off of one of the pipes and see if the flat spot disappears. If my understanding of how a 2 stroke tuned pipe works is correct, then the flat spot should go away.(of course i also loose the extra power gained by the tuned pipe.) Should be a fun experiment....

You know, if I was really smart(and had money to burn) I would just buy a set of bing carbs and a Rotax exhaust and set it up exactly as rotax says to do it and be done with it. But where's the fun in that? Besides, fall is here in MN and I figure there's only a couple of weeks of potential boating weather left. I will have all winter to get it dialed in.....

I will be running it in the snow, by the way. I'll let you all know how that works out.....

Bruce

[buster]

Hey Bruce,

Sorry about your 2 cycle problems, wish I had some good advice for you. Problem is, I never was great at tune up diagnostics in 4 cycles, and I have very little experience with 2 cycle.

Keep us informed about you snowboating experiences, I hope they go better than mine did. You might want to talk to me about them before you start, 'cause it can bite you in the a$$ real quick! Have you got a good place picked out close by?

Dave

[Duane Scarborough]

Hey Bruce,

I figured that you knew about R.A.V.E. valves, since you're snowmobile savvy, but I thought it might be worth a shot.

2 cycle engines, and tuned exhausts have always seemed like black magic to me. Way back, when I was riding dirt bikes, I shied away from 2 cycles because of that. I kinda' understood 4 cycle engines, so I stuck with them.

I remember WHY now.

Good luck with your experiments. Let us know how it turns out.

BTW, you really should talk with Buster (on the phone) about his snowboating experience. I almost peed my pants hearing that story.

Duane

[Flathead]

So far, so good.

I cut sections from my extra tuned pipe shortening it until the tapered end of the pipe was essentially gone. All I had left was a 'megaphone' shaped pipe. and yep, the flat spot finally went away. I was able to add 2 degrees to the pitch of my prop as well. ran much better through the midrange and revved up faster too! Unfortunately, It's loud enough to wake the dead (and the baby across the street).

So, I think once I find a semi-tuned Rotax pipe designed for an ultralight aircraft, I can buy it and have some confidence the money will be well spent. (I'm going to hold out for a used one since it's almost winter here.....;peek.

I'm not real good at pasting links, but for those of you who may be curious, here is the address of a good article, with neat graphics, explaining the basics of how a tuned pipe works.......check it out.

http://en.wikipedia.org/wiki/Expansion_chamber

Bruce

[buster]

Hey Bruce,

I was just wondering what you did about mounting your gas pedal?

I can't very well use a new style pedal on mine since it will be mounted right on the bottom of the boat, or very close. Even if I used the original pedal mechanisim, which mounts on the rib, It seems to be in the wrong place, no matter where I try to put it. (That is because the driver will sit in the center when alone, and on one side if a passenger is along.)

Let me know what you came up with a.s.a.p.

BTW, I finally have all the major parts on my Tomcat, and I'll pull it outside and take a couple pictures to post in the next few days.

Thanks, Dave

[Duane Scarborough]

Hey Bruce,

I was just wondering what you did about mounting your gas pedal?

I can't very well use a new style pedal on mine since it will be mounted right on the bottom of the boat, or very close. Even if I used the original pedal mechanisim, which mounts on the rib, It seems to be in the wrong place, no matter where I try to put it. (That is because the driver will sit in the center when alone, and on one side if a passenger is along.)

Let me know what you came up with a.s.a.p.

BTW, I finally have all the major parts on my Tomcat, and I'll pull it outside and take a couple pictures to post in the next few days.

Thanks, Dave

Buster,

Here's just a thought... Have you thought about using a motorcycle style twist throttle, mounted on the rudder lever?

Duane

[buster]

Duane,

Actually I did think about a twist throttle, but I haven't seen any lying around. I will have to do a bit of looking if I decide to do it that way.

I'm not sure that it would have the right "feel" for an airboat, mounted on a verticle stick that moves back and forth, or side to side. It will never be at the same angle very long. Any thoughts about that? Maybe it would have a better feel if I mounted it horizontal on top of the stick, ....or maybe not?

Dave

[Flathead]

Dave,

What I am using is brake lever from an old snowmobile mounted to the steering stick, upside down. It's not very clean looking, but works great! I've attached a pic where you can see it. Can't wait to see your boat! I found my exhaust on Ebay, looking forward to it getting here and trying it out...

Bruce

[Duane Scarborough]

Duane,

Actually I did think about a twist throttle, but I haven't seen any lying around. I will have to do a bit of looking if I decide to do it that way.

I'm not sure that it would have the right "feel" for an airboat, mounted on a verticle stick that moves back and forth, or side to side. It will never be at the same angle very long. Any thoughts about that? Maybe it would have a better feel if I mounted it horizontal on top of the stick, ....or maybe not?

Dave

Buster,

Hmmm.... I guess that you're right, especially on a side to side moving rudder stick.

I was thinking about how the throttle on a helicopter is mounted on the collective stick, which is on the pilot's left side. It's not vertical, it's more like horizontal. Plus he doesn't make either throttle or collective pitch adjustments nearly as often as you'd need to move the rudder, or adjust the throttle on an airboat.

Maybe my idea wasn't so good, huh? I think that it "could" work, but might take a bit of getting used to.

Something like Flathead has may be a much better idea.

Now ya'll got me rethinking my own setup.

Duane