Howdy all-- I'm an old time rodder now building an experimental airplane and find myself approaching things a bit differently than most of the airplane guys (due to the rodding background). For example, I have a 470 cubic inch Continental aircraft engine (which I rebuilt) and want to use a Rochester 2Gv (off a big Pontiac V8, has the large base) instead of paying $900 for the stock carb. HERE IS THE QUESTION: This engine makes all of it's 230 HP at about 2600 RPM and it will necessary to be able to manually lean the mixture at higher al***udes. I plan to do this by making basically 2 air bleeders in the manifold that can be controlled from the ****pit. Since the 2GV powervalve opens when vacuum is lowered, any air that enters the manifold through my bleeder will lower the vacuum and therefore enrichen the mixture, negating what I'm trying to do. Would it be better to remove the PV and jet it upward or modify the pv by cutting some of the spring off? Thoughts are appreciated!
The carb on an airplane is probably the last place I would try and save a buck! On a car, if something doesn't work, you coast to the side of the road, park it and walk home. Not so painless in a plane.
You probably don't want to hear this, but don't do it. An aircraft carb is completely different from an automotive carb for several good reasons. First, aircraft carbs need to have the mixture *manually controlable* from *inside the ****pit*. You need to be able to go to 'full rich' for take-offs and landings and then be able to lean the engine out for cruising and additionly to compensate for varying al***udes. You can't do this with a normal automotive carb and the system you have propsed won't work for the very reasons you've already pointed out. Also, the power valve in an automotive application is only designed to add a very small amount of additional - just enough to momentarily smooth out the transtion when the throttle suddenly gets wacked open from an idle - but not so much as to greatly affect the overall mixture Even if if you were able to manipulate and control the power valve to work the way you want, it won't p*** enough additional fuel to affect the overal mixture - anywhere enough for what you need to be able to do..That's what the main jets are for and you can't vary or change them from inside the ****pit on a Rochester 2-bbl. .Secondly, aircraft carbs are 'up-draft- type' and mounted *underneath* the engine - again for a reason - to prevent engine fires and engine failure from flooding! With a standardc top mounted automotive-type down-draft carb. if the needle & seat sticks, or the float malfunctions, raw fuel will literally pour down the throat of the carb, into the intake tract, flooding the engine and causing it to quit, or if it somehow does manage to stay running, at the very least, cause a severe and sudden loss of power. Secondly, with a top mounted carb, any fuel leaks caused by either a fitting or line coming loose - or again, a needle & seat or float malfunction - will cause fuel to be dumped or sprayed down onto a hot engine causing a fire! Needle & seat and float problems can occur with a bottom-mounted 'up-draft- type' carb, too, but at least in the case of a needle & seat sticking or a float malfunctioning, the fuel will run out the mouth of the carb, away from the intake tract, rather than pouring into it. You may have a loss of power, but at least the engine probably won't quit completely on you. The same with the fire issue too - because the carb is mounted below the engine, any spilled or leaked fuel will run out, downwards, and away from the engine, rather than onto it. Mart3406 ======================================
Boys, you might be interested to know: --There are hundreds, maybe thousands of experimental aircraft in the US that use automotive engines and automotive carburetors; some guys are trying to modify fuel injection on these auto engines and there have been many accidents due to injection/cpu issues, and typically no carb problems. The FAA has no issue with using an autmotive carb on an experimental aircraft. --Airplanes do climb and bank, just like cars going up/down hills and into curves as far as carbs are concerned. The fuel slamming around in the fuel bowl of a circle track car or in a 10 sec quarter miler would probably greatly excced anything that would happen in a climbout. --Cost is only part of the issue. The aircraft carbs that have mixture controls are hard to find for the 0470 and often are screwed up, requiring rebuilds upwards of $1500 and wait times of months. --Robert McNeilly makes a commercially available "leaning block" for this very type of usage, but because of the effect of the power valve on the mixture, I'm trying to learn about THAT ASPECT of the carb so that I can avoid problems. Several other guys with automotive engines have struggled with this, although if they leave them alone they tend to run okay below 5000ft with the most notable effect being high fuel consumption. Thanks for all your help/opinions.
hmmm, how about a cable, like a speedometer cable, going to the top of the carb to adjust the fuel enrichment circuit (power valve) stop,.. open would be full rich, then closed would be lean, (hold power valve down) i looked at a 2g carb. this looks like and easy solution.. i have a simular adjustm,ent cable on my atv, (racer) so i can adjust the mixture screw while riding : ) just have to be creative, that's all. let me know what ya think.. would only require a threaded insert above the power valve piston,and experiment with the power valve orifice.. (restrictions on bottom of power valve0 then power valve would work normally, on take off, manifold vacuum drops, valve opens,,( richer ,more torque..)then at higher al***udes (no oxygen) you could simply adjust the power valve stop to leaner conditions.. man this would be alot easier on a q-jet, with the metering rods.. : ) we do alot of 2 barrel (2g) carburetors for limited stock car racers, and we have learned some whacky stuff about them, (including what doesn't work) but i really think that this one would be fairly simple.. also, we could test it on the dyno, (hope we have an engine on there when we need to test this carb..)
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