Carburetors

scottessex

Sling-Wing Pilot
Joined
Nov 12, 2003
Messages
11,240
Location
central, ga
Aircraft
Dominator 582
Total Flight Time
200+
After doing extensive research on the internet, (it rained today) I have found out the my carburetors are too big.... The engine 4 strokes up to about 4000 rpm and then smooths out. So after finding some arctic cat microfiche online, and checking different years configurations and part numbers.....I have come to the conclusion that my Mikuni VM38's are from the twin pipe sled.

I need the VM 34 carbs from the single pipe sled since that is the exhaust that I have. I have a spare set of BING 54's that rotax uses, they are 36mm.
I can get new mikunis for $116 each, and then get new carb spigots for $25 each, and I should have the correct set up.
The bings fit the carb spigots right now,
So I guess I am asking opinions on throwing the bings on jetted for a 532-582, or just getting the correct mikuni's.

Bing's are $300 new and the jets and needles are twice the price of mikuni parts, but I have 2 now.

Suggestions? Thanks.

Details here: http://www.rotaryforum.com/forum/showthread.php?t=21578
 
Ha…yeah…Scott, 36's are closer than 38's.

I am fed up with those dumb carb's. I don't even understand why an aircraft engine like Rotax doesn't have decent ones that you can lean in flight. I'm rebelling and am not going to use those suckers, or that dumb spaghetti muffler! All I need to do is……
 
Hello Scott,

I would expect the porting for the single exhaust, smaller carburetor sled would be different, is that the engine you have?

I have not had good results trying to mix and match two stroke exhausts.

In my opinion a larger bore carburetor has a weaker signal over the emulsion tube. I feel this makes it harder to jet.

From the symptoms you describe I feel the problem is not in the carburetor being too big because with the slide down, the throat has reasonable velocity over the emulsion tube unless it is a butterfly carburetor.

In my opinion a constant velocity carburetor can generally be larger than a slide carburetor because it opens according to pressure differential and that gives it an effectively smaller throat most of the time.


My advice based on my general two stroke experience would be to first make sure you have the correct exhaust for the engine and after trouble shooting everything, use the Bings.

Thank you, Vance
 
Vance,
Wouldn't the big throat carb, loose its ability to properly meter fuel/air as the slide is increased. It seems as though the smaller exhaust would not flow enough air fast enough through the carb to draw the fuel. Would not this lead to a lean mixture?
 
Hello Mark,

There are many good books about two stroke tuning. I am not in a position to write one here. Gordon Jennings wrote a particularly good one.

In my opinion yes, as you open the slide at low rpm the signal becomes weaker and less consistent. This does not necessarily make it richer or leaner, only less consistent.

I feel that a carburetor is about pounds of air per pound of fuel and the signal over the emulation tube is what tells it how much fuel to add.

The jetting tells it what to do with the signal.

In my opinion, single or dual exhausts on a two stroke are not about flow, but the ability to tune the reflected waves independently. That is why the porting would probably not be the same for both exhausts.

In my experience, generally speaking, an exhaust restriction that causes back pressure will make the mixture go rich. This is about more than just the carburetor.

I feel that one of the most important things I learned about high performance engines is that they are a system. The more highly tuned the engine the more critical each part of the system is.

In my opinion, because a two stroke doesn’t have valves the exhaust timing and resonance are particularly important.

Thank you, Vance
 
Scott, jetting is likely to be the problem, your air cleaners are different to the sled that this engine is from, so if you fit the 34s, you will probably have to tweak the jetting on those too

What color is the plugs?

125cc 2 stroke dirt bikes usually have 38s.

Ken
 
I am aware of the large carbs on small engines, BUT our aircraft engines are operated differently, we are not spooling up to 9000 RPM, and requiring blistering acceleration.
A lycoming aircraft carb is really small for the displacement, you won't see a holley 4 barrel sized carb on a large aircraft engine.


Anyhow I put the BING's on, (bing 54, 36mm) It runs much smoother and idles a whole lot better, I may have to scan Ebay for some of the 34mm Mikuni's or just see how this works out. The carbs are jetted and set up at factory settings for the rotax 532-582.
the work continues, hopefully it won't be long now!
 
Top