Silly mixture setup question.....

[jasoncsc]

Hi,

Here is my boat that is in question:

Microburst 21

CMB Valvola with OS 20B carb, BVM remote main needle, geared at 1.5:1 reduction

CMB header and pipe set at 20.5cm

Octura X646/3 prop

30% fuel

I have the whole setup at less than 15 tanks of fuel. The problem is that I could setup the top end with no problem, but I just could not get the engine to run below top, not even at half throttle. Whenever I leave off full throttle in the water the engine would quit immediately if I set the main to the optimum speed or would bog and never come back up if I richen the main a bit. The idle mixture is now at 2 turns out. Should I lean or rich the idle needle? I need to get the boat running at idle or half throttle, and could pick up speed whenever I punch the throttle, because a race is coming next week and I need to set the boat up for running start. At full speed for 10 minutes of operation the engine head is hot but still could be touched.

Any help appreciated.

 

[Ian Inverarity]

Jason,

What plugs do you use? It could be that your plugs are too cold, try a hotter plug like a Mc8 or Mc59.

Ian.

 

[TimD]

Regarding setting the idle needle on the OS carb - when you have the main needle correct - start it on the bench and from idle blip the throttle to full momentarily - it should hesitate slightly for say 1/2 a second then clean out. if it is "snappy" like a car throttle the idle mixture is too lean....

 

[Ian Inverarity]

Tim,

did you see that it is geared? Thus the prop....

Nitrocrazed racing: listen to that gear-whine!

 

[TimD]

Doh! I should've known better. I'll edit it.

 

[TimD]

I might build another 21 geared mono one of these days 8) I have most of the parts to do it somewhere. Lighter hull this time tho'.

 

[Kez]

Jason,

I am not sure if it is entirely an idle mixture issue. You mentioned that the engine would not run anywhere but full tilt. The idle needle should not affect the mixture until it is below 1/2 throttle. My guesses:

1. Pipe too short for the prop. You are at about 8". I would start at 9".

2. Too much prop. You did not mention the strut depth. The 646 could be too much for your CMB if it is run too deep.

3. The idle needle position is way out of whack (too far in). If it is the case I would richen the mixture (back out the needle).

Regards,

Kez

 

[EatMyShortsRacing]

Hmmmm for some strange reason I have similar problems with a gear driven 21 in a hydro.... only snag is it is using an exhaust throttle.... maybe time to see Nitrocrazed for an engine...

 

[Ian Inverarity]

Fair go Mate! I'm flat out getting your other engines done! ;D And working two jobs....

Nitrocrazed racing: Twins are twice the trouble....

 

[jasoncsc]

Thank you all for your tips. Really appreciated. I would try your tips and come back to let you guys know.