Carburator Tuning Question?

[needabronco]

OK I finally got around to tuning my carb and dizzy.

First off, I've got a fresh 302, Edelbrock Performer cam, intake, and AVS 650 carb(I know it's a little big). MSD distributor with one blue and one silver spring to go full advance at or near 3000 RPMs. I live at approximately 6000' elevation, and I'm using the stock .095 primary jets, and a set of .068 x .047 rods.

When I had the timing set at 10* advanced, with the idle mixture screw (IMS)out 2 full turns from full lean, I got 14" if vacuum. Then I tried 6* advanced and with the IMS about a 1/2 turn richer I get between 12-13" of vacuum. The idle is set at approximately 750-800 RPMs. I tried .092 jets and it ran worse... I called the Edelbrock tech line and they told me that changing jets or rods wouldn't help, but suggested bigger rods anyway...

The 6* tuning feels better than the 10*, and it runs waaayyy better than it did, but does this sound right? I'm not really a carb guy so any help is absalutely appreciated.

Casey

 

[broncnaz]

You vacuum reading seems low for that setup. If it runs good at 6 then leave it at 6. Are you taking this for test runs to see how it drives? Tuning small engines with big carbs at elevation can be a PITA 2 things happen one you have to run richer jetting for the carb to work but then its always to rich because of the elevation you should be able to play around with it and find a good combo of jets and rods. 650 cfm is not all that big but a 500cfm would be a lot better and easier to tune.

 

[needabronco]

I did test drive it alittle and 6* did feel the best, it does run rich, which I can't stand. I hope I can drive it a while like this and then finally get around to completing my efi swap that's been collecting dust. I still need some parts for that to work though...

 

[DirtDonk]

I would have thought that more advanced timing would run better at higher altitudes. Especially if you're rich to begin with.
When you say it runs better at 6°, what exactly are the differences?

And how did it run worse with the smaller jets?

Paul

 

[needabronco]

"I would have thought that more advanced timing would run better at higher altitudes. Especially if you're rich to begin with.
When you say it runs better at 6°, what exactly are the differences?
And how did it run worse with the smaller jets?"
Paul,

I had the distributer advancing farther(smallest bushing for advance in an MSD), and with the timing set at 10* and it started pinging pretty bad at part throttle. The throttle actually feels crisp, and returns to idle faster at 6*. Now I can accelerate in any gear and feel power all the way up in the rpm range. At 10* there feels like some sort of hesitation in the mid rpm range. When I put the smaller jets in it seemed to sputter and fall on it's face throughout the rpm range, it didn't 'feel' right either. Does that make sense?

It started pretty well before I tuned it, but now all I have to do is tap the starter and it purr's away. My wife even commented that it's alot quieter now(?) Casey

 

[DirtDonk]

Well then it sounds like 6° is the magic number for now.

Maybe that change to the .092 jets was just a little too much? Really doesn't sound like it, but I wonder. Do you have other rods to experiment with? At least they're easy to mess with.

Just out of curiosity, what year model are the cylinder heads? Is your PCV system fully hooked up and functional? What gear ratio and tire size are you running? Are you running vacuum advance too?
You know, get us a little more familiar with the total setup.

Have you been re-adjusting the idle screws as well? They don't need to be a certain number of turns out and they don't need to match the number of turns to each other. it's just for reference. When you adjust the screws you can usually "feel" when it's running just right. And thankfully they thought to make the heads nice and big and at a decent angle so you can easily get to them with a screwdriver and even adjust them with your fingers when you need to! It's a great setup.

Paul

 

[needabronco]

Paul, no I don't have any extra rods, to try out. As far as specs, the heads are fresh 71, only thing not new are the valve springs because they checked out good. PVC system is all new everything, gears are stock 3.50's(for now) 31" tires, it's all pretty stock with the exception of a warmed up engine. I do run a MSD distributer with vacuum advance (it is not the one that uses the separate computer). As I said earlier its fresh with about 300 miles so far, Edlebrock Performer cam, intake, and their 650 off road carb. If memory serves me the compression is right at 9.5:1. I have been adjusting the idle screw all along and have been between 750-800 rpm's. Thanks again. Casey

 

[broncnaz]

you might try going with less mechanical advance and bumping up the inital a little more. Or the springs might be to light for a bronco. might want to try something a little heavier to slow the advance rate down just a little. What exhuast are you running? The stock system is a choke point I would at least run larger pipes from the manifolds back it will help. You've added air intake but it cant get out.
I'd also get a tunning kit for your carb most carbs are setup to run at sea level.