Another Carb thread (Harley Davidson Keihen CV40)

[John B]

I didn't want to hijack the other carb thread, but I also have been struggling with carb choice for the rebuild of my Sonerai IIL The airplane has been flying since 1992, but due to life, hasn't flown for about 10 years or so.

So I started off in 1992 with a POSa carb. After many hours fighting with it on the ground, including both modifying and machining custom needles, and then 10 or so flight hours in, I had finally had enough. I felt so highly of my experience with the POSa that it came off and went into the garbage can.

I then tried a Solex 34 PICT carb that came off of an actual VW Beetle and I believe it would have worked great on about 1835 or smaller displacement, but just didn't flow enough for my 2276cc engine. It would also have needed carb heat.

I wound up with a Dellorto 2 barrel side-draft carburetor that was infinitely tuneable and it worked great. It should have had carb heat but I instead ran it where it is drawing air that had already been through the cylnders. At this point the airplane has sat for several years and the last fuel it had in it was auto fuel. Needless to say the carb is pretty much crap now since I can't get parts for it anymore. This carb worked well, but the two carb throats weren't ideal with one throat feeding the left bank and the other feeding the right bank. I probably could ahve rebuilt this carb by buying NOS parts for a couple hundred dollars but I decided to try something different.

I have seen the AeroInjector in person and it is a nice looking piece of equipment. Well made, and I do believe that even though the basic theory of operation is the same as the POSa that the problems the POSa had would likely be solved. It does have one issue that the POSa had that I never did like and that is the fact that if the fuel valve is on fuel is going somewhere. I just would really prefer a carb that doesn't leak. I still am considering this a fallback option, but...

The Great Plains Zenith carbs are fine, but they are a basic venturi style carb that can make ice and they really have no benefit over venturi type carbs. I have a side draft Harley Bendix carb and if I were to use an ice-maker carb I'd probably use this one.

To combat the icing issue while still having a flat equipped carb, I decided to give a Keihen CV40 constant velocity carburetor as used on early 2000's Harley Davidson motorcycles a try. The benefit of the CV carbs is that they are a variable venturi constant velocity carburetor and this supposedly makes them less prone to carburetor icing, which was always in the back of my mind. Being a vacuum operated CV carb they are also somewhat altitude compensating. I have the new intake fabricated and have the engine running well from idle to about 2400 rpm. Past 2400 rpm it is leaning out too much and I have parts on order to combat this. Hopefully this will get me going soon. If anyone has Keihen CV40 tuning experience let me know. I think I have it pretty well figured out but maybe talking to someone who has specific knowledge about the idiosyncrasies of tuning this carb would probably help.

Thanks

John Brannen

 

[schines]

Using a CV carb seems like an interesting idea. I'd like to see your intake set up. Keep us posted on your progress. It sounds like you already know this, but there are plenty of different sizes of jets that should help with fuel flow. Another tip is to buy a manual mixture screw for easy mixture adjustments. CV Performance has all that stuff.
-Scott

 

[John B]

Well I'm about at my wits end. The engine runs well enough at idle and I can advance to about 1/2 throttle travel with it running well. I can actually get 3100 rpm out of it running fairly well at 1/2 travel. If I open the throttle any more, the burping and farting starts and the mixture goes really lean. What I don't know is where the vacuum operated slide is at this point. I would think at 1/2 throttle opening that I am pretty well off the low speed jet and into the main/needle/needle jet range. I shimmed the needle A LOT and it helped a bit, but still not good enough. I also had a spare needle jet and a spare main jet, so I drilled both of them out. The needle jet went from 0.1135" to 0.116" (#32 drill) and the main jet I drilled out to about a 205 (#46 drill). That didn't change a thing.

At this point I think I'm going to shelve this idea for a bit but I think I will come back to it at some point. Monday I'm going to order an Aerocarb even though I really don't like one aspect of them. That is that they leak when the engine isn't running. I'll deal with that though as the most important thing right ow is to get this airplane back in the air.

 

[soneraifred]

An AeroCarb/AeroInjector shouldn’t leak when the engine isn’t running. When you pull the mixture control out to the idle cutoff position, the fuel flow is completely shut off. At least it is on my AeroCarb.

 

[John B]

An AeroCarb/AeroInjector shouldn’t leak when the engine isn’t running. When you pull the mixture control out to the idle cutoff position, the fuel flow is completely shut off. At least it is on my AeroCarb.

Well that's fantastic news. I was under the impression it leaked all the time like the POSA. Even read the manual and it says:

"WARNING: Fire hazard. Failure to turn off the main fuel valve may result in fuel flowing from the AeroInjector after shut-down."

I believe what you are saying. I will still use the fuel valve but it is nice to know that there isn't a constant leak like the POSA.

I still think the CV40 or some other constant velocity carb has merit because it would be icing resistant, but also if one could get it running right it should be altitude compensating as well. I may revisit it later. I'll still have the intake Y I made for it. Going to make a new one for the AeroInjector.

 

[John B]

I ordered an AerInjector this morning. Hopefully it won't take too long to get here. In the meantime I am going to get the airframe/engine as ready to accept it as possible. so I have a few questions.

1. My previous carb required a fuel pump and the airplane currently has dual electric fuel pumps running through a spring type mechanical fuel pressure regulator. I read in the AeroInjector manual that my setup can be used, but from other AeroInjector users, would I be better off switching to gravity feed?

2. How sensitive is the mixture control system? My last carb had no mixture control so I'm starting from scratch here. I have a high quality locking push-pull cable but will I need a vernier type cable?

3. Does the mixture control have a spring to hold it open (rich), or does it rely on a stiff wire bowden cable to go to the rich position (ie: Don't use a flexible stranded cable.)?

4. Since fuel can run from the carb if the mixture control is not idle cutoff, I am trying to decide my preferred mounting angle. I could mount it angled down (up-draft) so that any fuel that runs out of the carb runs out of the throat, but then any fuel that runs out it is inside the cowl (possible fire hazard). If I put an elbow on and mount it horizontal (side-draft) then any fuel that runs through the carb will sit in the elbow behind the carb possibly creating a flooding potential. I'd like to hear how others have theirs mounted and their experiences.

 

[eschrom]

1. Gravity works fine for me. I've never had a fuel pump for comparison.
2. A simple push-pull cable with detents.
3. Bowden cable, no spring.
4. Throat down.

I'm not necessarily saying this is the best way to install an AC but it has worked well for me. One small drawback in my experience is that even the slightest power adjustments require a mixture adjustment also to keep the air-fuel gauge happy. But if I didn't have the AF gauge I would never know the mixture isn't perfect because the engine always sounds good.

Ed