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Re: Horrible Off Idle Stumble [Re: cjskotni] #2793580
07/06/20 05:34 PM
07/06/20 05:34 PM
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RJS Offline
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Some good advice and some I don't agree with but MOST IMPORTANT thing is do you know how to adjust a pump-shot? I am surprised by the amount of people that have no idea.

If you know this and have done this then my apologizes: With a homemade tool or another person helping engine off have the carb. at full open, now push the pump arm down fully and check with a feeler gauge between pump arm and the bottom of adjustment screw for (Holley says .015) I like .006 clearance. Just make sure you are not over stressing the diaphragm and ripping it. You could have .000 if you want just not past that. I can't tell you how many cars I've seen with stumbles because of this step not being followed. Also everytime you change or reposition a pump cam you have to check and or adjust this.
Ron

Re: Horrible Off Idle Stumble [Re: cjskotni] #2793834
07/07/20 09:07 AM
07/07/20 09:07 AM
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Originally Posted by cjskotni
Originally Posted by lewtot184
lazy pump cams and big squirters don't work for light to part throttle tip in. driving on the street vs running on the track require different accelerator pump tuning.


So are you saying the red cam + 45 nozzle isn't a good choice for a street car? If that's true what would you suggest? A more aggressive cam and smaller nozzle?


There are several charts out there showing pump cam lift curves. AFAIK, the most aggressive cam is the BLUE cam and I think it’s got the number 427 on it, but I’d have to go out and look to say for sure. But it’s definitely blue.

I’m not saying your combo doesn’t need a 45 squirter. It very well may. I tuned a front engine dragster a long time ago and even though it was light, it didn’t have the correct converter so it took a squirter in the high 50’s. It was a foot brake car, and you couldn’t get a big enough nozzle on it.

I your case, I’m still suggesting to get a vacuum gauge on it, and see where you are for manifold vacuum at a cruise. Your 8.5 power valve is better than what most use, but there are several things I’ve learned lately.

One is it’s a bummer you can’t buy a 12.5 power valve any more. Most guys could use it. I know I could. Another is when you test the power valve, the vast majority of them open LATER than what is stamped on them. I tested an 8.5 that didn’t start opening until 6 inches of vacuum. That can definitely affect your tune.

As for the correct method of setting power valve opening, Holley, most authors and the vast majority of end users, because of the aforementioned people, have been incorrectly taught and have over the decades been doing it incorrectly. In fact, the vast majority of end users (and evidently Holley because they still teach it) have no real idea how the power valve (which is really an economizer valve, but that doesn’t market well) circuit functions.

When you are at a cruise, you are running on the primary main jet. Holley (and most very other company producing Holley clone carbs) sends the carb out with very rich jetting. So at a cruise, they are just slobbery rich, they get horrible fuel mileage, kill plugs and are just generally miserable to drive. That’s why many guys won’t use a Holley. Horrible fuel mileage, hideous rich idle and all that. That’s where the power valve comes in.

At your CRUISE, the power valve is shut (or should be) and all the fuel is coming from the primary main jet. And it’s most likely rich. Then, you accelerate and the power valve OPENS and ADDS fuel, and you are now pig rich, wasting fuel and most certainly giving up driveability. The amount of fuel added by the power valve circuit depends on the Power Valve Channel Restriction holes, which are the small holes you see when you remover the power valve. I forgot to mention that as you accelerate, you also add pump shot too, which is more fuel added to an already rich condition.

So, the correct way to tune the carb is to keep reducing the primary main jet until, at a cruise you get a SLIGHT lean stumble or surge. At that point you add a couple of sizes to the primary main jet and move to tuning the power valve, keeping in mind you’ll never touch the primary main jets again, unless you are tuning cruise fuel issues.

The two tuning options you have for the power valve circuit are WHEN it opens, and how much fuel it flows when open. That’s why cruise vacuum is how you set the opening. The later you open it, the later the engine gets the added fuel it needs because of the added demand by the engine. So you need to get the opening time as close to cruise vacuum as you can. Certainly you don’t want the power valve open at a cruise, but it needs to open quicker than what Holley has guys doing. My engine has about 9 inches of idle vacuum so by the de facto setting, I’d need a 4.5 power valve. I have about 15 inches at a cruise. So I use a 10.5 and would use a 12.5 if I could get one.

That later opening requires a bigger pump shot to cover up the lack of fuel because the power valve is opening so late. Running more nozzle than you need just wastes fuel. If you get the power valve/main jet set correctly they run cleaner and make more power.

So now you have the power valve opening at the right time, you have the primary main jet set for best clean cruise but now at WOT you are lean. The first thing guys want to do is grab a screw driver and ADD main jet to the primary side and maybe add some main jet to the secondary side and that is DEAD WRONG. You have now made your cruise air/fuel ratio much richer than it needs to be.

When tuning for WOT after you have the primary main jet correct for cruise and you have the power valve opening as correct as you can get it, you adjust WOT air/fuel ratio by changing the size of the power valve channel restriction (from here on I’ll type out PVCR so I save some strokes). On the “better” carbs, the PVCR’s are threaded holes and either brass set screws or little 6/32 threaded jets are installed in the PVCR holes so you can easily adjust them.

Of your carb doesn’t have the holes threaded for brass, you can get some 6/32 x 3/16 brass set screws from McMaster/Carr. A box of 50 is about 5 bucks. You'll need the correct 6/32 tap and drill, a pin vise and a set of 80-61 number drills and 60-1 number drills. Any decent hardware store will have the 80-61 drill set and pin vise and Harbor Freight has the 60-1 drill set for about 20 bucks or less IIRC.

Then you just drill the PVCR holes to the correct number drill size (I forget what it is off the top of my head) and then thread the holes just enough so the set screw is flush or just below flush and you’re done with that. Now it’s a simple task to drill some brass and tune the PV circuit.

Now at WOT on the primary side you either make the PVCR larger to add fuel or smaller to reduce fuel, never touching the primary main jet. That way, you still have a clean, crisp cruise and the correct amount of fuel at WOT.

The next question is how do you know when to add WOT fuel to the primary side and when to add it to the secondary side. The best answer I have is to disconnect the link from the primary to secondary throttle shaft, wire the secondary throttle shaft SHUT and only tune in the primary side until you get it nailed. Then hook the secondaries back up and tune for WOT only using the secondary main jets. You don’t touch the primary main jet or the PVCR’s, because you already have it nailed.

All this takes time, but the payoff in the end is well worth it. Of course, like everything else Holley, once you get that dialed in, you can start tuning the idle circuit to clean that up. Tuning the idle circuit requires some of the same work...you drill out the idle feed restriction and tap it for a 6/32 set screw,but that’s a discussion for another thread.


Hope this helps clear up some of the bad information out there on how to tune the main jet/power valve circuit on a holley carb. It’s harder to type out than it is to do.


Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
Re: Horrible Off Idle Stumble [Re: cjskotni] #2793887
07/07/20 11:45 AM
07/07/20 11:45 AM
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Originally Posted by cjskotni
Originally Posted by lewtot184
lazy pump cams and big squirters don't work for light to part throttle tip in. driving on the street vs running on the track require different accelerator pump tuning.


So are you saying the red cam + 45 nozzle isn't a good choice for a street car? If that's true what would you suggest? A more aggressive cam and smaller nozzle?
yes. for OP's carb i'd try a .028" squirter with tubes and a green pump cam to start out with. those lazy cams are ok for the secondaries but work poorly for light tip-in which most street driving is. if all you do is race and your just stabbing the throttle and go then big and lazy can work. I want my cars to drive nice with no spits, hiccups, or flat spots and I don't do the big squirter slow action accelerator pump stuff. big squirters and lazy pump cams don't provide enough pressure in the accelerator pump circuit to push fuel into the air stream at light throttle. generally fuel will dribble down the side of the venturi and not mix in the air stream creating a flat spot. the old 950 3bbls (which have the same throttle bore and venturi on the primary side as an 850) used a .028" squirter with tubes with the blue 427 cam and drove great.

Re: Horrible Off Idle Stumble [Re: lewtot184] #2795372
07/11/20 07:39 AM
07/11/20 07:39 AM
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cjskotni Offline OP
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Originally Posted by lewtot184
Originally Posted by cjskotni
Originally Posted by lewtot184
lazy pump cams and big squirters don't work for light to part throttle tip in. driving on the street vs running on the track require different accelerator pump tuning.


So are you saying the red cam + 45 nozzle isn't a good choice for a street car? If that's true what would you suggest? A more aggressive cam and smaller nozzle?
yes. for OP's carb i'd try a .028" squirter with tubes and a green pump cam to start out with. those lazy cams are ok for the secondaries but work poorly for light tip-in which most street driving is. if all you do is race and your just stabbing the throttle and go then big and lazy can work. I want my cars to drive nice with no spits, hiccups, or flat spots and I don't do the big squirter slow action accelerator pump stuff. big squirters and lazy pump cams don't provide enough pressure in the accelerator pump circuit to push fuel into the air stream at light throttle. generally fuel will dribble down the side of the venturi and not mix in the air stream creating a flat spot. the old 950 3bbls (which have the same throttle bore and venturi on the primary side as an 850) used a .028" squirter with tubes with the blue 427 cam and drove great.


Wow so much information but I would expect nothing less! thumbs

I went ahead and swapped to the green cam (#1 position), adjusted the pump arm (.015), and dropped to the #28 nozzle (no tubes). I took her out and part throttle tip in is good as is nailing it while already moving. However, if you nail it from a standstill, it does have a slight bog. However, this is much less than it was before I started all this tinkering.

The bog is slight enough that I think going up a nozzle size should fix it. When the car cools down, I will swap to a #30 nozzle and take it back out. Once I find the right nozzle, is it really worth it to order one with the tubes?

Thanks as always for the input everyone. This fall, I will likely take the car to the dyno and get it tuned which will address and issues with PV or jetting (which I think are already close). drive

Re: Horrible Off Idle Stumble [Re: cjskotni] #2795411
07/11/20 10:09 AM
07/11/20 10:09 AM
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Originally Posted by cjskotni
Originally Posted by lewtot184
Originally Posted by cjskotni
Originally Posted by lewtot184
lazy pump cams and big squirters don't work for light to part throttle tip in. driving on the street vs running on the track require different accelerator pump tuning.


So are you saying the red cam + 45 nozzle isn't a good choice for a street car? If that's true what would you suggest? A more aggressive cam and smaller nozzle?
yes. for OP's carb i'd try a .028" squirter with tubes and a green pump cam to start out with. those lazy cams are ok for the secondaries but work poorly for light tip-in which most street driving is. if all you do is race and your just stabbing the throttle and go then big and lazy can work. I want my cars to drive nice with no spits, hiccups, or flat spots and I don't do the big squirter slow action accelerator pump stuff. big squirters and lazy pump cams don't provide enough pressure in the accelerator pump circuit to push fuel into the air stream at light throttle. generally fuel will dribble down the side of the venturi and not mix in the air stream creating a flat spot. the old 950 3bbls (which have the same throttle bore and venturi on the primary side as an 850) used a .028" squirter with tubes with the blue 427 cam and drove great.


Wow so much information but I would expect nothing less! thumbs

I went ahead and swapped to the green cam (#1 position), adjusted the pump arm (.015), and dropped to the #28 nozzle (no tubes). I took her out and part throttle tip in is good as is nailing it while already moving. However, if you nail it from a standstill, it does have a slight bog. However, this is much less than it was before I started all this tinkering.

The bog is slight enough that I think going up a nozzle size should fix it. When the car cools down, I will swap to a #30 nozzle and take it back out. Once I find the right nozzle, is it really worth it to order one with the tubes?

Thanks as always for the input everyone. This fall, I will likely take the car to the dyno and get it tuned which will address and issues with PV or jetting (which I think are already close). drive
too much fuel can cause a bog as much as too little. try the .030" if you want but you'll be stepping back towards the tip-in problem. try a white 218 cam first. green and blue cams are pretty aggressive and sometimes they can be too aggressive. the white 218 cam was always the first step in pump cams back when holley was holley. float levels being too high or too low can create off idle issues. trying to run the carb too lean, especially on the idle circuit, can create a low speed stumble.

Re: Horrible Off Idle Stumble [Re: lewtot184] #2795517
07/11/20 02:39 PM
07/11/20 02:39 PM
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cjskotni Offline OP
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I had the red cam on it before and it bogged horribly with a #35 so badly it would cut off if you nailed it. I had to go to a #45 to get that bog gone. The white cam seems to be very close to the red cam from the charts I see so I don't see how dialing it back down won't take me backwards.

I don't see how going to a mild-ish cam and a small nozzle won't make this motor bog like heck. I just installed the #31 nozzle (green cam) and I will let everyone know if it helps (or hurts). At least I know one winning combo that worked well (red cam + #45 nozzle) thumbs


Last edited by cjskotni; 07/11/20 02:39 PM.
Re: Horrible Off Idle Stumble [Re: cjskotni] #2795549
07/11/20 03:57 PM
07/11/20 03:57 PM
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If the GREEN/031 combo gets you closer but you need more, test the ORANGE/031 combo next 👍


1970 YO7 A66 [Canadian Export] F8 Challenger
340 (SL.P. 226/238@.050,270/288 dur,.538"/.528" w/Yella Terra 1.65 rockers, 9.7:1, 63cc Sidewinders (Bowl Ported),RPM intake, QF SS-750 Annular DP, MSD Street Fire Ign., & TTI's. 727/Dynamic 9.5" 3800, TF-2, & 3:91's
Re: Horrible Off Idle Stumble [Re: YO7_A66] #2795583
07/11/20 06:02 PM
07/11/20 06:02 PM
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North Carolina
cjskotni Offline OP
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The Holley cam charts seem to make the orange cam look slightly lazier than the green one. The only more aggressive cam I see is the blue one.

Re: Horrible Off Idle Stumble [Re: cjskotni] #2795598
07/11/20 07:02 PM
07/11/20 07:02 PM
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Originally Posted by cjskotni
The Holley cam charts seem to make the orange cam look slightly lazier than the green one. The only more aggressive cam I see is the blue one.


The blue cam, number 427 is the most aggressive cam. If you have .015 between the pump arm and the lever that’s too much. I set them so I can just wiggle the pump arm a skosh and run it. More than that delays the pump shot.


Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
Re: Horrible Off Idle Stumble [Re: cjskotni] #2795607
07/11/20 07:48 PM
07/11/20 07:48 PM
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""The Holley cam charts seem to make the orange cam look slightly lazier than the green one.""

If you look at the attached Holley chart, reference the throttle percentages between the GREEN and the ORANGE. The green has a shorter duration which is completely used up by 50% throttle. It does have more cc's but only at 40-50% throttle. The ORANGE actually provides more cc's from 10% up to 40% and the ORANGE has a longer duration up to 70% throttle.


Attached PDF document
Holley Pump Cams 2.pdf (6 downloads)

1970 YO7 A66 [Canadian Export] F8 Challenger
340 (SL.P. 226/238@.050,270/288 dur,.538"/.528" w/Yella Terra 1.65 rockers, 9.7:1, 63cc Sidewinders (Bowl Ported),RPM intake, QF SS-750 Annular DP, MSD Street Fire Ign., & TTI's. 727/Dynamic 9.5" 3800, TF-2, & 3:91's
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