Carb nozzle?

Steve Sampson · #1 11-20-2008, 02:48 AM

I have an 160hp O-320 with a 10-5217 carb in an RV4 and c/s prop.

At full power, on initial climb out EGT and CHT head up quite quickly. Because I pull power back early to control temps, and climbing is not a problem in a -4, I can control CHT to 400, and the combined cht+egt to 1825. Both rules of thumb for engine longevity I understand.

It has been suggested to me however that I might want to have the main carb nozzle increased in size from .096" to .104". (Because the VANS airbox is very efficient.)

My questions are these:
1. Is this commonly done?
2. Is this the 'normal' amount of increase? Those numbers are tiny, but its a 17% increase!
3. Am I correct in assuming it only controls the amount of fuel at full power, after that it is controlled by the mixture control, provided I lean appropriately.
4. I need to get hold of a diagram of th carb. Is it easy to change the nozzle?
5. Is there any downside?

Anything else I ought to know?

Cheers!

mahlon_r · #2 11-20-2008, 04:34 AM

I think I would go maybe .004 rather then .008. There are no production nozzles, you have to ream yours. If you go to far, it will make the engine run overly rich and rough at all power settings as making the nozzle larger effects all power settings not just full power.
Good luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

FrankK90989 · #3 11-20-2008, 05:36 AM

I never used more than 6 gph with the same carb. I drilled mine, was #42(.093) to #39 (.099). This did cool it down, and now I burn around 8gph@2400 rpm

jonbakerok · #4 11-20-2008, 06:07 AM

I had your same setup in my 6 -- 160hp O-320 and C/S prop. I had the crossover exhaust, though and if I remember your web site, you've got the 4 stack, right? That could make a difference, but you could always go a little further if necessary.

My stock nozzle was #43. I went to #40 and it did the trick. More than that would have been too much because afterwards I found that I had to lean for hot starts. I didn't see any change in fuel consumption, but that's probably because I'm a cheapskate and lean agressively.

Don't you guys use wire size bits in the old country? I have no idea how many millimeters #40 is, but I'm sure you can order one on-line.

By the way, I think the Vans exhaust has as much to do with the problem as the airbox. The certified O-320 is intended to be installed in a Piper, with its big muffler and contorted exhaust. Any Harley owner will tell you that if you bolt on a set of drag pipes you have to re-jet the carb richer.

Mel · #5 11-20-2008, 06:39 AM

Quote:

Originally Posted by jonbakerok

Don't you guys use wire size bits in the old country? I have no idea how many millimeters #40 is, but I'm sure you can order one on-line.

Jon, They're talking inches, not millimeters. A #40 drill is .098" in diameter.

Steve Sampson · #6 11-20-2008, 07:29 AM

Quote:

Originally Posted by mahlon_r

I think I would go maybe .004 rather then .008. .............. If you go to far, it will make the engine run overly rich and rough at all power settings as making the nozzle larger effects all power settings not just full power."

Mahlon, thanks for the advice. Could you expand a little. Why if the hole is a little oversize, does the mixture control not control the mixture at all the intermediate power settings?

Thanks, Steve.

dmeloche · #7 11-20-2008, 06:15 PM

Bart at Aerosport opened mine to .104" if after I couldn't control CHT's on takeoff after trying all the other mods (baffles, plenum, etc). I does help a lot but I can't do a run-up full rich or the engine has large RPM drops when switching mags. I have to pull the knob out about .75" to get a good mag drop. In the pattern or at low power settings the engine will let you know if the red knob is full forward by running a little rough and stumbling if you increase power. My standard procedure is run-up with the knob out about .75", takeoff full rich (my home field is 400msl) and then lean as I climb out or reduce power and essentially leave it there until I land or initiate a go-around. I now reserve full rich for takeoff and WOT operations at lower altitudes. I have noticed no effect on fuel mileage. I'm no carb expert but I believe the jet that is opened up is only in the fuel circuit when full rich. Thats why the negligible effect on fuel mileage IF you lean. There are lots of carb experts on this forum who will correct me if I'm wrong.

Regards,
Doug
RV6 270hrs

jonbakerok · #8 11-21-2008, 06:33 AM

Quote:

Originally Posted by Mel

Jon, They're talking inches, not millimeters. A #40 drill is .098" in diameter.

Duh! Sometimes I'm a real dumb ***. Steve's in England, and I thought... well, never mind.

Anyway -- I was just trying to point out that it might be easier to order a drill bit online than mess around with conversions.

The topic has be discussed a bunch and the rule of thumb seems to be to go two or three wire-size bit sizes larger than whatever you started with.

Bryan Wood · #9 11-21-2008, 08:55 AM

Steve,

I have an 0-320/Hartzell and like others had the same problem as you are having. Maylon offered the same advice to me back then as he is offering you now and it allowed me to get on the rich side of peak on takeoffs. If you decide to drill yours you might want to order the locking tabs before you go into the carbs so you will have what you need on hand. Others re-use these tabs, but if a corner breaks off and gets ingested it could ruin your day.