Trouble Tuning Inexperienced

[huntincowboy]

Hi everyone, got one of Cliff's SR kits for my 17084226 carb that I'm putting on a mild 460 ford in an f250 with Comp 252H cam, mildly ported & polished heads, long headers, and an HEI distributor.

Finally got everything in and plumbed and did the break in, carb idles ok and as long as I'm just sitting, it will rev smoothly. Drove it out of the shop to clean up and as soon as it get's loaded up it has a bad backfire in the exhaust that only gets worse when you give it pedal. I knew it needed tuned so I went to start tuning the carb up and I'm having no effect from the mixture screws, I can screw them all the way down on one side and it doesn't stumble at all. It's drawing right around 14-15 in Hg at idle around 800 rpm. I had initial timing set at 12* but backed down 10* after thinking that may help (it didn't).

I think it's running pretty rich, but that's just a guess, not very experienced with carbs in general and this is my first quadrajet. I have a holley regulator giving it a steady 5psi from an electric fuel pump that is the stock in tank style for an 86 f250 w/460.

My jets are a 75 with 50C rods and the secondary rods are DA's.

Do I possibly have too much jet/rod or does it sound like another issue? I have the float set at 1/4" and other than that and Cliff's SR kit, everything else is "stock". Also, there are no vacuum leaks that I can find, if that helps.

[Cliff Ruggles]

During the rebuild did you remove the idle tubes or at least run a small wire thru them to clean them?

Did you verify the power piston hanger arms are level and EXACTLY even.

How many turns up from seated is the APT screw?

[huntincowboy]

I cleaned the carb in an ultrasonic cleaner, but did not remove or run a small wire through the tubes. The piston arms seemed level but I didn't check them with any tool to verify exactness. What would be the recommended way to check? I left the apt screw alone

[huntincowboy]

Ok I got the carb pulled and checked the apt adjustment, it was about 2-3/4 turns out. I have some wire I'll run through the idle tubes to check them. Still not sure how to exactly check eveness of the power piston arms, but they're not visibly uneven.

[Cliff Ruggles]

Pull the metering rod tips together under the piston to verify they are exactly the same.

I'd raise the APT some and leave the plug off over it until you are done fine tuning or tap it for a set-screw so you can make changes without removing the airhorn.

Are you running vacuum advance at idle?

[huntincowboy]

Ok I've been playing with the apt. I've managed to  get what I think is close but it's still giving a pretty bad exhaust pop (one) when you punch it in neutral as it passes through 1200-1500 rpm going up.

I was running vacuum advance but I took it loose and set timing at 12*. The mechanical advance has me at about 30 total at somewhere between 2-3000 rpm (I'm by myself can't check both, do rpm just by ear). With vacuum hooked up it's at like 53* in the same area. I probably need to back that way down.

The apt rods are even and I'm still getting no stumble on the right (driver side) idle screw when I turn it all the way in. The left one will get to a point that it affects idle.

Would something be clogged in that circuit?

[huntincowboy]

I did check the idle tubes with a piece of wire and they are clear btw.

I just did a short low gear pull (mechanical advance only) and it's still popping in about the same spot under load. (12-1500 rpm going up).

[Cliff Ruggles]

53 degrees would be a lot of timing on a relatively large engine with a tiny cam in it.  What is the compression ratio of the big 460?.....

[huntincowboy]

It's about 9.5:1. I found my backfire issue l, and it's my fault. I had 7 &8 backwards in the firing order. I missed it because the wires cross over each other and I was seeing it wrong. Backfires have all stopped now but the driver side idle adjustment still has no effect.

I removed the blades during cleaning and I'm wondering if maybe when I assembled them they were not reinstalled close enough to get the right signal on that side?

[huntincowboy]

Something else that I thought of that's probably useful info is that in order to clear the distributor with my air cleaner I have to run two 1" spacers. The top one is an open center spacer and the bottom one is a qjet to square bore that tapers. I'm not sure how much that matters either, but I drove the truck about 30 miles today and it ran great.

[Cliff Ruggles]

Assuming you are using a dual plane intake the spacers/adapters should have a divider in them to separate the two halves of the intake or basically keep the plenum areas separate.

It's OK to let one side see the other a little bit, but I've done a lot of testing in that area and the best results were always obtained keeping one side so it doesn't see the other very much.

The best results in all testing I've done were to grind about half a silver dollars worth of material out of the divider right between the secondaries as shown below.  On top of that I use a 1/4" open gasket.  Any time I've allowed much more opening than that one side to the other on any dual plane intake I started to see some running and performance issues......Cliff

[Cliff Ruggles]

On single plane intakes spacers work very well and best if they are open and well blended into the plenum area.

As far as no control with one idle mixture screw remove the baseplate and verify they plates are fully seated and indexed.  A slight twist in the shaft having one open more than the other can cause issues like you are seeing. 

You can also remove the mixture screws and hit the holes with high pressure compressed air to back-flush them and clear any debris that may be in the idle tubes or passages......