mrbigg

My Feedback: (21)

Centering the Carb Butterfly? This is what you're supposed to do right? I had it set so that when you looked into the carb, with the choke fully open, the butterfly was perfectly matched with the choke. The engine is a used G62 and I went out to run see what kind of numbers it would do. Wasn't even close to what it should be so I took the end point adjustment up with the engine running. At 100 points the butterfly was fully open. I took it to 150, which is over rotated past center, and it gained about 350 RPM!

I haven't had a chance to take the cowl off and check it out yet, but I'm heading out to the garage in a little bit. I'm going to see if I can lengthen the pushrod and see if I can get any more out of it.

pe reivers

Adjusting the mixture probably would have had the same effect.

mrbigg

My Feedback: (21)

Nope, no dice with that one. Remember, I just bought this engine. The only way to start a "new to me" engine is to start it rich and lean it out from there. I leaned it to peak and back it off a little. Then started playing with the EPA.

Just got back from the garage and pulled the cowl. It's definitley past center now. I adjusted the pushrod so that I now have maximum travel at full throttle-the bellcrank is hitting the mechanical advance. This is way past center. I'll fire it up and see if I gain anymore RPM. I tried to get some pics but the butterfly is in too far.

Geistware

My Feedback: (16)

I have had this happen to me as well.
Not on all engines but my Brison 2.4 was like this.
If I over rotated the throttle butterfly, I picked up more RPM.
It must have something to do with the airflow going into the carb past the choke.
I now enable all my engines for max throttle travel and not for butterfly position

mrbigg

My Feedback: (21)

I hear you. I think I'll do the same as you now.

davewallace

My Feedback: (29)

MrBigg,
That is a very interesting finding. If you think about it, the butterfly should be aligned with the center of the flow into the port. The port opening is progressive, with the bottom opening first and staying open for the intake duration. As you go up the port is open for a progressively shorter time. Since the flow would not be evenly distributed accross the port, why should the butterfly be centered for peak RPM?

Dave

Zeeb

My Feedback: (41)

It's a not a bad idea to give this butterfly position a quick check when setting things up.

I too have experienced it with one engine and in my case, it actually ran faster with the butterfly slightly closed from a centered position...

And no, it wasn't a mixture problem as I checked that possibility first! I just couldn't believe what I was seeing and thought it had to be a situation of the HS needle being too lean, but that was not the case.

tkg

My Feedback: (1)

Most carbs have a positve stop at WOT just set you EPA untill it wont move any more.

mrbigg

My Feedback: (21)

Just went out and checked it. Gained about 600 total. I'm very pleased. Should check my ZDZ also. Maybe a coulple of hundred to gain there. Don't really need it though, it's a hoss of an engine.

dbcaster

mrbigg,
I had the same issue, I just set my throttle up for max throw and back off the end point until I reached max rpm. I am not really getting the RPMs that people are talking about though. I am only getting 6300 from a 22x10 Xoar.

mrbigg

My Feedback: (21)

That's exactly what I did. 6650 with 22x10 Beila, I think. It's painted white with red tips. Slimline pitts muffler.

Geistware

My Feedback: (16)

So am I hearing that if the butterfly is more in line with the porting angle, the RPM should be higher than if it is not in line with teh porting angle? Is that correct?

pe reivers

Mr. Bigg,
In the pictures you show the choke valve being rotated past full open. Other posters speak of the throttle valve. this confuses the issue. Which valve is it?

Diablo-RCU

My Feedback: (1)

Does this engine have throttle coupled mechanical spark advance? If so, that explains why you have more power when you over rotate the butterfly. More ignition advance.

mrbigg

My Feedback: (21)

yes it has the coupled advance. the choke is fully opened and not past center. I'm talking about the throttle butterfly.

pe reivers

I am inclined to go with the spark advance idea. If the butterfly position was important, it would probably work both ways from dead straight, which it does not. The Walbro carb is well known for the lack of rpm reactions near full throttle, so the reason must be somewhere else.
I usually set the trottle stop arm 5mm (7/32" ) from the stop at full throttle. On EI, This gives the same best power, and better throttle response to transmitter inputs.

Geistware

My Feedback: (16)

OK, this would definatel explain my Brison 2.4
Never though about that.
Thanks

mrbigg

My Feedback: (21)

I'm only getting 6300 with a Biela 22x10 on the G62. I'm wondering if I need more spark advance. It seems to have liked more when I over rotated the butterfly. What do you all think? Should I advance the timing a little more and see what it does? All I have to do is shorten the linkage a little. I can start it really easy now, so I don't think a little more timing at idle is going to hurt it.

pe reivers

It's worth the try, because you can always undo it.

mrbigg

My Feedback: (21)

That's what I'm thinking. I'll see if I can get to it early this week. I'll let you guys know what I find out.

rmh

watch yer fingers------------

pe reivers

I got wise trying the MVVS "pinky start" once too often. My thumb nail is all black, and about to part with my thumb [:'(]
After all these years one would expect to have grown wiser?

RTK

My Feedback: (1)

mrbigg--Is that an RCIGN1 modified 62?
From talking with Ralph, the way he has his mechanical advances set up there should be no increase in rpm like that when over rotating the butterfly. That is if my memory serves me right.
I would double check the total advance in your set up. As Dick mentioned, if you try to start that and have the throttle fairly open after adjusting the timing, you might have a little kick back

Antique

My Feedback: (4)

Advance past 28 or 30 past BTDC is no help at all with rpm....The advance is set for 28 with the throttle plate exactly parallel to the throttle bore...If over rotating the carb makes more rpm the advance was not set right in the first place...
An engine with syncro spark will show that over rotating the carb will NOT gain rpm, the advance is fixed at 28 or 30 no matter what the throttle position is ...Over rotate it far enough and the engine will slow down....

mrbigg

My Feedback: (21)

Yes, it's Ralph's motor that I bought used. I can start it at half throttle and don't get a big kick back at all. I think that it's been messed with.