Lean Spike with High Load

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gradedcatfood

1972 Plymouth Duster 440
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Hey everyone, I have a Holley 750 Double pumper with mechanical secondaries and i'm having a carb tuning problem.

I'f i'm cruising in 3rd gear, punch it, the car will downshift but will have an initial lean spike (up to 16-17 AFR) but will then settle back down to 12-13 AFR within a second or two. Also, it will backfire through the carb.

Is that something to worry about? I was thinking that maybe it needs a different power valve but I'm not sure if I would need to replace the one on the primary or secondary side (maybe both?)

This seems to be the only issue while driving as all other driving conditions yield a consistent and steady AFR of about 11-12 AFR. For example, from a dead stop, if I punch it all the way through the gears its totally fine.

I'm a bit of a newbie with it comes to carbs so any advice is appreciated. Thanks
 
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I had an issue with my 750 DP. I have to use a 6.5 power valve, because my idle vacuum in gear is only about 8.5. (rule of thumb 2 numbers below idle vacuum) If I give the car say half throttle I will get a short lean spike, because the vacuum does not drop enough to open the power valve. Not a problem if I give more throttle and the vacuum drops below 6.5. I went up one primary jet size and the lean spike is not too bad. I run an AFR around 12.5 to 13 at cruise which seems to be as lean as I can go without worsening the part throttle issue. By the way, my secondary does not have a power valve.
 
qkcuda said:
I had an issue with my 750 DP. I have to use a 6.5 power valve, because my idle vacuum in gear is only about 8.5. (rule of thumb 2 numbers below idle vacuum) If I give the car say half throttle I will get a short lean spike, because the vacuum does not drop enough to open the power valve. Not a problem if I give more throttle and the vacuum drops below 6.5. I went up one primary jet size and the lean spike is not too bad. I run an AFR around 12.5 to 13 at cruise which seems to be as lean as I can go without worsening the part throttle issue. By the way, my secondary does not have a power valve.
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What's your vacuum at cruise?
 
qkcuda said:
I pull about 17-18 at cruise.
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Then get a 10.5 or even a 12.5 power valve in it if you can find one. If that doesn't clean it up start increasing the squirter size.

The de facto 6.5 power valve isn't correct for much of anything.
 
So does anyone have a recommendation of what power valve to fiddle with? Primary or secondary? Thread sort of got hijacked
 
Rule for power valve is 1/2 of your vacuum. Not 2 points.

I would go for more squirt from the acceletator pump to fix it.

Power valve for WOT throttle
 
fklskv said:
Rule for power valve is 1/2 of your vacuum. Not 2 points.

I would go for more squirt from the acceletator pump to fix it.

Power valve for WOT throttle
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That rule is wrong and always has been wrong. A couple of numbers under CRUISE vacuum is what you need. Using idle vacuum is wrong and always has been.

Mark Whitner made a short video of how wrong that old rule is and many people still use it. It's wrong.
 
yellow rose said:
That rule is wrong and always has been wrong. A couple of numbers under CRUISE vacuum is what you need. Using idle vacuum is wrong and always has been.

Mark Whitner made a short video of how wrong that old rule is and many people still use it. It's wrong.
Click to expand...
 
gradedcatfood said:
So does anyone have a recommendation of what power valve to fiddle with? Primary or secondary? Thread sort of got hijacked
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The thread isn't high jacked. You want to correct the stumble don't you? Work with the correct power valve first. Most don't run a power valve on the secondary side because you are not cruising on the secondary side. If you have a power valve on the secondary side, plug it and add 8-10 jet sizes and then start your tune up.
 
yellow rose said:
The thread isn't high jacked. You want to correct the stumble don't you? Work with the correct power valve first. Most don't run a power valve on the secondary side because you are not cruising on the secondary side. If you have a power valve on the secondary side, plug it and add 8-10 jet sizes and then start your tune up.
Click to expand...

Of course I want to fix the issue, I wouldn't of posted otherwise. If people start asking questions of their own within a thread and others discuss around that reply within a topic it can become difficult to read and keep track of the original post and questions that follow it, that's all.

I appreciate the reply and will report back when I open up the carb.
 
11-12 AFR is really fat in other driving conditions. Cruise should be much higher numbers than 12, more in the high 13 or 14 range at a minimum.
 
I was not attempting to hijack your thread, I thought your problem was similar to mine, and it might help you troubleshoot your issues.
 
I run a 10.5 on a 10.0 idle; worksnice, and and 8 number split front to rear.
I got no AFR. When I punch it in second gear, it will rip the tires to shreds.
Here's how I know 10.5 is right for my combo;
I disabled the secondaries, and installed a plug in the front PV hole. Then went out and drove it, with a vacuum gauge stuck in the wiper where I could see it. Every time I depressed the throttle to see 10.5 on the gauge, the engine started to nose over, run out of gas.
So I pulled out my trusty Morroso PV tester, and Tee'd that same gauge into it, and started pulling vacuums on my collection. When I pulled on the 10.5, it seemed about right, so in it went, and the roadtest verified it.
So yeah as to all the other methods,I'd have to agree with
YR, they're just plain wrong.
I run the primaries slightly fat on the MJ, hence only the 8 number split. Fat because the Eddie headed 367 likes it,lol, and I don't care anymore about fuel economy.
 
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