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Carb Jetting 2

S

sigtauenus

Active Member
I can't believe its July and I'm still screwing with this. Winter was rough, weather too shitty to drive, and spring was a blur. I think I've flown 25,000 miles in the past 2 months.

Anyhow. To the issue:

Pulled the jets on the carb tonight.

What I'm working with is a 5.0 short block, ported 289 heads with 351W valves, long tube headers, shelby high-rise intake, holley 725 list 4118 (4150 with vac sec).

Car starts and idles just fine. rev's fine. drive's fine except for one thing.

I pull out in 1st, all is good, I shift into 2nd, push on the gas, and the engine stumbles.

With the car idling, the air stinks of gas. Plugs are a dark grey and wet with gas. I know you need to read them after cruising, but the car still shouldn't be running this rich at idle.

Floats are set where you can just see the fuel at the threads and it dribbles out if you rock the car.

Timing is set at 10* with 35* total at 3500 rpm.

Primaries are a 62, secondaries are a 71 jet size.

Idle is set at around 625-650 and it idles smooth.

Idle mixture was set by leaning it out until the vacuum dropped then turning the screw back towards rich 1/4 turn or so. Vacuum is reading around 19 in.

I know the carb is big for the engine, but its a 68 shelby replacement carb, and my engine flows better than the stock 302 did, so I should be able to make this work.

I have no idea what the original jets were, and nothing to reference.

Does 62 primary sound like too much?

I plan on just biting the bullet and buying a holley tuning kit, but would still like to have some assessment on where I'm at. I plan on initially dropping down to 60 and see what happens.

I'm more concerned about the stumble than I am the richness at idle, but I'm pretty sure the jets are responsible for both.

Suggestions?
 
Does it stumble off idle when just revving the engine? Does it have changeable squirters? I'm running a 650DP on my 289 and it has 67 fronts, 76 rears and 25 in all the squirters. I had to play with the squirter size to get rid of my stumble.
 
It doesn't stumble at all when revving with the car stationary, its only when the car is moving that the stumble occurs.

Primary squirter is a 31.

I have a spare 28 I can try.

Does the Holley tuning kit come with squirters too or just gaskets and jets?
 
If it doesn't stumble when you give it a wide open throttle shot, then it's probably not the squirters.

No idea on the kit contents.
 
Ok, let me ask this...

When I shift into second and step on the gas, is that initiating the secondaries? With the 62 primary and 71 secondary, could it be the secondaries are too big and causing the stumble?

As for the richness at idle, can I adjust the mixture screws down off the max vacuum or does that affect something else?
 
Another question.

Since my primary jets are on the low end as I'm understanding this, is there a way to find out if the stumble is from not enough gas or too much gas at throttle tip in?
 
Secondary opening should/would be at higher rpm. Stumble from to much fuel would show black smoke out of pipes.
 
So I need to drive the car around the block a few times, figure out exactly where it happens, and then have somebody stand there to watch, eh?
 
Reading up on this some more...

If I adjust the accelerator pump cam one way or the other, I may be able to get rid of the stumble.

I still need to read the plugs after driving the car at highway speed though to determine the correct jetting.

And I need to lean it out at idle a little bit.

Which of these should I do first?
 
I'm not sure how it works on a holley, but your secondaries are opening too soon.

As for the rich idle....Not sure, probably jets....

Pete is the man for this, follow his instructions.

Mel
 
Secondaries may not even be opening. I apply a bit of white grease to the vacuum rod and drive it. Then see if the grease has been disturbed. Drive it in different manners to check the opening time frame.
 
For my own reference as much as anybody else's, power valve (I assume that's screwed in on the back side of the primary metering valve) had 55 on it. I assume 5.5?

Put the carb together, started the car, checked for leaks. For some reason the idle dropped about 100 and is around 500 on the tach. I'll check it with the timing light (has more accurate tach on it). I'll adjust that back up a bit, was still smooth but the alternator light was flickering on. Although I did turn the a/c on, so I likely need to redo the idle adjustment with the a/c on.
 
Reset idle to 600 with a/c and headlights on. When compressor kicks off idle jumps to around 680-690 or so.

Put more tension on the accelerator pump cam (had some slop to it) and that took out a LOT of the stumble. There's still a bit of hesitation though as if its not getting enough gas.

I did not lean out the idle mixture yet, can't remember which way to turn those. Will look that up and adjust them 1/4 turn and take it for another drive.

I also have an annoying ticking sound that I didn't have before, assume its a lifter because the car hasn't been driven in about 6 months.
 
You said in an earlier post your vacuum at idle is around 19" and your power valve is a #55. Based on your vacuum at idle, you should be running a higher (numerically) PV. A power valve that's too low (low number) will cause a lean condition when the engine is under heavy load or at full throttle.
 
Found the specs on my carb.

http://tgsperformance.com/holley.htm

According to this, the original jets were 68/78 and PV was 8.5.

For Mark, can you clarify the stumble when revving? I just played with it, and although it does rev smooth, if I jam the throttle wide open it does hesitate momentarily. Very short period of time though. I tried the size 28 squirter and didn't really see a difference.

I leaned out the idle mixture 1/4 turn each side, and this time when I went for a drive, I had some popping out of the exhaust. Plugs appeared to be dryer than before but that's not a scientific analysis. Didn't drive it enough to clean them off, still grey/black near the tip. Before they were the same color but sopping wet. I turned the mixture back in 1/4 turn but haven't driven it again to see if that was definitely the popping. Is the popping a bad thing? After doing a search for it, it appears to be common with larger exhaust (magnaflow stainless 2.5")
 
Should I go to a 8.5 PV or a 10.5?

Also, the biggest change so far on driving it has been tightening up the lever for the accelerator cam. I noticed when I was playing with it (was curious) that the plastic cam can be reversed and has a different, more steeper profile on the other side if I flip it over. It is also labeled 1 and 2, and there is a hole on the metal linkage labeled 1 and 2. Right now the smoother profile is up, with the screw holding the cam in the bottom hole labeled 2 on both the cam and the linkage.

What are the implications of flipping that cam over to get the steeper profile?

I feel like the carb is very close at this point to being where it needs to be.
 
I would say put the idle mixture about 1/2 of what you tried it at.

The popping could be timing.....I would try a couple of degrees different. Pop out the exhaust is timing to late or not a hot enough spark and things not burning fast enough.

The accelerator cam will simply give you a different action and timing for that action. Some positions will increase the coming on time but decrease the flow near the end of the sweep. Others change things a bit different. Try all and remember the action of each to best pick the one that works best.
 
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