• Hello there guest and Welcome to The #1 Classic Mustang forum!
    To gain full access you must Register. Registration is free and it takes only a few moments to complete.
    Already a member? Login here then!

Carb Jetting 2

For street driving the rule of thumb is half of your idle vacuum. The 8.5 should put you where you need to be. Be sure to only make one change at a time to ensure you know what worked and what did not.
 
Popping from the exhaust could be several things. To tie it into the carb problem you have been having, check that your plugs are not fouled. You mentioned earlier that they were very wet. This could definitely cause popping through the exhaust. Also check your plug wires.
 
"737IP" said:
For street driving the rule of thumb is half of your idle vacuum. The 8.5 should put you where you need to be. Be sure to only make one change at a time to ensure you know what worked and what did not.
Click to expand...

Should I jump right to the 8.5 as a pretty sure bet, or also pick up a 6.5 and a 7.5?
 
"sigtauenus" said:
Should I jump right to the 8.5 as a pretty sure bet, or also pick up a 6.5 and a 7.5?
Click to expand...

The lower numerically you go the longer it will take for the power valve to come into play.

Put the 8.5 in it and move on. A 5.5, 6.5, or a 7.5 just give you more delay. Personally I'd run a 9.5 in it if you have 19 inches of vacuum at idle.
It would not hurt to bump your curb idle (w/o ac on) to 750.

Do the power valve change and before you make any more adjustments.

As far as running rich you may have to jet it down a step or two. I could not get mine to lean out at idle until after I jetted down the primarys. Idle mixture should be controlled by the idle screws but if the carb is too big I suspect the jet size plays a role. A big hole is a big hole right?
 
That's what she said.

Now that that's out of the way (couldn't resist)

I'll have more some updates on Weds after the parts come in.
 
Amazon let me down. 2 day delivery was supposed to be here today, tracking shows the power valve in Kentucky still as of 3PM, probably not going to be here tonight. Guess I'll report back tomorrow... and hopefully the weather here clears up a little bit for me or I won't be test driving it anyhow. Been raining all week, forecast isn't any better at least for the next 2-3 days.
 
Installed the 9.5 PV, wow what a difference. No noticeable hesitation at all. I think I can still fine tune the timing a bit, but the car drives much, much better.

So although its running rich, it wasn't getting enough gas on acceleration. Interesting.

Ok, I'm going to try driving it like this a little bit and see how it affects the plugs. I know going down the local road a couple miles at 45-55 isn't the same as getting it out on the highway. I have one more new set I can throw in there, but don't want to waste them just yet.

I did still have some popping out the exhaust on hard acceleration, but I read that that was normal with these 2.5" magnaflows. Is that anything to be concerned about? Is that the rich mixture and will go away if I jet it down?
 
Popping in the exhaust sounds like timing to me. Ford's like advanced timing. Makes 'em harder to start when hot but they like it when running hard. You want 10, 12 initial with a mild cam and you want it all in pretty early. With a big cam or lots of duration it can be as much as 20 degrees initial. The timing advance mechanism should not begin advancing before 1,000 rpm and, usually, full advance will have occurred before 3,500 rpm. SBF engines with a compression ratio of ~9.5:1 will, in most cases, respond well to 36 degrees of ignition timing (initial timing plus the mechanical advance), plus a further 10 degrees from the vacuum advance mechanism, making a total timing of 46 degrees.
I don't know what distributor you have but the curve is important as well. A rich air/fuel mixture requires less advance timing than a leaner air/fuel mixture. If the advance is relatively slow you can change the springs to make it quicker. If it has too much or too little travel there are bushing that control or limit the amount the timing advance can move.
 
I haven't heard that about the 46 total. I've read a lot about 35-36 total. My understanding was total advance is always initial setting plus mechanical, and the vacuum advance just adjusts the timing within that window for efficiency when cruising. What I did before was rev the engine to 3500 and set the timing to 35* total. The distributor I have had a factory mechanical advance of 21*, so initial timing was at 14*. However, with the vacuum advance on the front carb port pulling vacuum full time, advance at idle is actually 24*. I thought that was too much advance on the low side, even though my engine breathes really well (ported 289 heads, 351W valves, long tube headers, 2.5" exhaust). I switched out a bushing changing the total mechanical to 28*. Total remains 35*, but initial is set to 7*, plus vacuum is 17* at idle, which is in the ballpark you mentioned. Actually, the other day I bumped it up to 10* initial so its sitting at 38* total, and 20* at idle. I was playing with the vacuum line on the carb to see which one it was driving better with, and need to confirm which line it is on.
 
The reason an engine can handle the additional spark advance provided by the vacuum advance mechanism is because a leaner air/fuel mixture at light throttle cruising speeds, takes longer to burn than a richer air/fuel power mixture. In most cases, when using a "quicker" mechanical advance curve, the desired amount of additional advance generated by the vacuum advance mechanism is around 10 degrees.
7* initial seams low to me. 10 even lower than I like. I am running 14* initial and even ran that much on my stock 5.0 HO in my T-Bird. The vacuum advance is of course based on engine vacuum.
 
I think now that I have the stumble figured out, I'm going to back to the factory bushing in the distributor (setting mechanical advance back to 21*), and will set initial timing to 14*. And then go from there.
 
I was going to mention that the popping could be too much timing advance at high rpm.

Get it set to 35 total and you should be set.

Why are you running a vacuum advance? I'd shit can the vacuum, set the base timing at 12 (might be a bit much for hot-soak restarts) let the dizzy pull it's 21 degrees and be done with it.

That's only 33 degrees but a good place to start.
 
I reset the curve to 21*, set timing to 14* initial and 35* total. I have a dist with a knob on it to control timing adjustments so if 14* is too much when hot I'll dial it back .5* at a time. Also removed the vac advance.

Drove it, it felt good. It's only got about 150 miles on it, think I need to pull the valve covers and readjust the rockers and then retorque the headers. Should be set.



Sent from my HTC EVO using tapatalk.
 
Got home and had to turn off the car to go in and open the garage door (haven't ordered an extra opener for the fastback yet) and when I went to start the car again it wouldn't start. Dialed the timing back a few clicks and it fired right up. I'm thinking I'll set it at 12* and see how that works. (Oddly enough, that was suggested by above).
 
Some ignition modules (like mine) retard the timing during starting/cranking. What are you running? MSD?
 
You must log in or register to reply here.
Back
Top