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Discussion Starter #1
I have a weird question that applies to carb tuning vs engine load.

If you tune a carburetor for a correct mixture at cruise, but then increase the load, is the engine now going to be running rich?

For discussion’s sake:

472 BBF
2500 rpm
65 mph
22 in vacuum at cruise
Very little throttle
13 mpg

472 BBF + 4k lb. trailer/tractor
2500 rpm
65 mph
18.5 in vacuum at cruise
About 3/8" more thorttle (at the pedal)
10 mpg

I keep going round and round: With the increased load, the throttle is opened more which should be supplying more air/fuel. However, with the same rpm, the engine is (theoretically) pumping the same amount of air, but with the decreased vacuum with the load it is moving more air (less vacuum = less restriction = more air), but the fuel metering in the carburetor is a function of the velocity of the air passing across the booster venturies, so it should be “self metering”….

Round and round we go! :beek:

I think it is running rich (and based on the soot coming out the exhaust, I’m fairly confident that it is…) but is it? And if so, why? Is it like taking a properly adjusted carburetor off a small, light car and then putting it on a heavier car? It isn’t going to be tuned correctly for the application.

I friggin’ HATE carburetors!!!! :redmad:

So is it running rich when loaded with the trailer or is the mixture still correct. Or do I just have it adjusted too rich all the time?. :2huh:
 

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Can't say much on the carb, other than.. what type of carb?

As for your round and round.. I think you're letting the RPM confuse you. The only time the engine is really operating at it's max volumetric efficiency for a given rpm is at WOT. All other times, the air filling the engine at a given rpm is completely dependent on the throttle. It's probably in the fine tuning of the carb.. I just don't have enough experience with any to help you much. I know most Holley's have a power valve, but I don't think your vaccuum is anywhere near enough to zero to cause it to open.

Sounds like you're wanting to use a wideband O2 :p
 

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Discussion Starter #3
Yeah, it's nothing fancy, just a good ole fashioned Holley 750 vacuum secondary. The power valve comes in around 10 in and I can definitely feel it comes in with a power increase, which sort of makes me think I'm a little lean on the primaries...

As for the rpm, yes the air flow is based on the throttle. But the manifold vacuum is also an indication of the amount of airflow (I think). If you have less vacuum, the engine is actually pumping more air through it. Or at least that is the way I understand it to work.

I have an A/F gauge and a standard O2 sensor, but I just haven’t hooked it up yet to see if I’m even in the ballpark.

I may just give it up and get it dyno’d and pay someone that knows how to tune carburetors and say “Here’s the money…” :bawling:
 

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I made up a page on how to set up a carb for E85 that covers most of what you need to know. A carb is designed to cover the fuel needs from idle to WOT. You need to know which part of the carb handles what load.

1. Idle circuit
This is easy, it controls the idle, but on most carbs, is still in play even at WOT,

2. Transition Slot. (Tslot)
Controls the air/fuel from just off idle to around 22-2800 rpms.

3. Main Jets
Controls the a/f when the tslot stops feeding. This is when the carb starts feeding from the boosters in the top of the throttle bores.


4. Power Valve
This adds fuel under load when the engine vacuum drops to the preset point of the power valve.

5. Secondarys
These feed added a/f when the primary throttle blades reach a predetermined point around 1/2 throttle on most.

This covers mainly the E85 conversion, but the last half covers a lot of the basic stuff.

http://members.tccoa.com/392bird/carbtech.htm
 

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Discussion Starter #5
Good write up for convering to E85. :thumbsup:

I've tuned many Holley carburetors over the years with no problems, but never in an application where the load can change so much, i.e. in a towing application. It is like I need to tune the carburetor (and ignition) for two completely separate vehicles, one weighing 4000 lbs, and another weighing 8000 lbs. :mad:
 
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