Tuning help needed

I reset the plugs to .041 and it made a big difference. I can drive it like this.... I think I'll just keep breaking it in.. 2 hours later and another romp and it's back. Crusing sputter, associated with pressing down on the accelerator. I hear it stumbling, Stomping it, it stumbles a little, but when it gets to 4000 rpm it works great and busts the tires and gets squrliey. I hit the 6k limit for the first time. Stumbles back to the shop. Float level looks ok. I had a tad of gas seeping from my banjo fitting that sucks fuel from the tank so I resurfaced the copper. Could it be bubbles in the gas? I dunno? Maybe the plug type matters? These are Autolite 24 or 42 Bill 1362

I would suspect the carb on this first. MSD second. Make sure the power valve isn't blown. Check the idle air bleeds for debris. Make sure you are idling on the idle circuit and not the mains. Idles could be clogged.

I would of course suggest to do the full process on www.tuningmadeeasy.com.

But my guess is also carb more than ignition. Above 4000 is secondary circuit, that seems to work fine, but primary and idle circuit (idle circuit contributes all the way up to 50 mph) seems to be the problem.

First, check for problems. Take the carb off, and look inside. Any dirt clogging anything? Powervalve smooth? PVs are a terrible invention, they can burst, but they can also just harden, then the calibration is totally gone. Test it by pushing it against another PV of the same vacuum, they should move evenly, like a mirror. If nothing wrong is found, adjust the carb, as per my home page. Yes it takes time, but it gets you there instead of keeping on guessing.

Thanks No Quarter, Great site! I think I have the air screws adjusted. Now I have instant hesitation, As soon as the vacuum drops to 5 it bogs. I bought a carb kit. The 2 power valves supplied are the same, they have 3-5-and 7 on them, each. How do I know what they are? Perhaps the one valve can be either size? Bill 1362

Thanks lastpushbutton for the kind words.

PVs usually come in x.5, standard in most carbs is 6.5 and it tells at what vacuum the PV closes. An engine with a wild cam would have a lower vacuum, so maybe a 4.5 PV. They're usually stamped e.g. 6 and 5, meaning 6.5. I sounds like your kit has a 3.5 and 7.5 where the 5 is so weak you can't see it?

However, if the engine now works fine except an immediate hesitation, I would guess an accelerator pump issue. Most obvious, is it adjusted so that as you move the throttle there's instantly a spray of fuel? There should be. If yes, then maybe the amount of fuel is too small, need a bigger shooter? But first, adjust end play as described below, that's free.

Paste from www.tuningmadeeasy.com:
Tuning the accelerator pump
Again here we would say the set up when buying would in 80% be perfect. But if you bought too big a carburetor as many people do, then there’s a bigger “hole” to cover when you smash the pedal to the floor, so more pump shot is needed. There are many things you can change on the accelerator pump. The total volume, standard is 30cc, a bigger pump can be bought pumping 50cc. We’ve so far never seen an engine needing more than the 30cc though. Secondly the speed of the “arm” can be adjusted by changing or moving the colored plastic cam on the throttle shaft. It determines the volume in relation to throttle position. A Holley kit contains the following cams (least pump shot->white, blue, red, orange, black, green, pink, brown->most pump shot). Thirdly the size of the holes in the shooter determines how fast the fuel is squirted into the bores. So adjustments are infinite. Which makes things easier but also more complex. This is not just a screw you can turn.
One thing before we start. Regardless of which colored plastic cam and size of pump (30/50cc) it’s critical to adjust end play when throttle is fully depressed. If end play is too much you lose pump action, if it’s to little you end up destroying the pump. End play is determined by opening the throttle fully (engine off) and then seeing how much further you can press the accelerator pump at the are situated on the fuel bowl. You should be able to create a gap of 0.4mm, adjust spring just above to obtain that.
You need to tune the accelerator pump based on two sets of info. Is it too little or too much? Is it too soon or too late? This must be based on experience when flooring the pedal. First though, let’s get rid of a myth. People that experience a stumble when suddenly accelerating often describes it as “it wont take the gas”. It is in fact the opposite that happens, the engine doesn’t get enough gas to cover the “hole” when the pedal is pushed down, throttles open and vacuum drops. So what’s your experience? Does it stumble or is there smoke out the exhaust? And is that condition immediate or after few seconds?

Immediate stumble:
Here the accelerator comes in with too little to late. This can be solved by either changing the cam to one that provides more shot earlier, or a shooter with bigger holes. Notice the spring on the accelerator pump arm? Fluid can’t be depressed, so when the pedal is floored, the spring depresses and only extends as fast as the holes in the shooter allow. So if you need to get more shot faster, it’s the cam or the shooter size. The cam comes in kits, normally with data on each cam. If not, put them on top of each other, you can clearly see which has the most aggressive profile. Shooters are starting around size 0.025”. Go up approx 0.003” at a time until the problem is cured. What’s best, cam or shooter. Well a cam kit is a lot cheaper than various shooters, and in 90% on the cases it solves the problem.

I pulled the carb, bought a kit, and the new power valve has a lighter spring, not sure what value or size it is, BUT IT WORKS! All I did was put in the lighter power valve! 1362 is back on top! The way I understand it, your feeding the engine with the fuel from the jet size, When the vacuum drops, the power valve opens and lets more fuel in through a third orifice. Here are the 2 valves, I'm pointing to the light one that works like it should.Thanks for your help guys Bill

Great. I'm happy to hear your problem is solved. But I don't understand it. If you put in a PV with a softer spring it'll stay closed more (it's the spring that pushes it open). I would have guessed you needed a harder sprung PV.

Never mind. Go burn some fossil fuel for us

I got to thinking about this, I put the 2 different valves end to end, and mashed them together between my fingers to see what was the lightest. They seem almost the same, BUT the one I'm pointing to and using has more travel, thus letting in more fuel. Notice the spaces between the spring coils. The old motor seemed a bit rich with this carb, I must be pretty close to the right jet size with the extra cubes' . I'm watching for my pipes to lighten up.

There is a number on the side ... a 6.5 is usually what comes with the carb ...a 55 45 34 is lighter usually depending on the vacumn you would go lighter usually 1/2 the vacumn guage ... a 6.5 would mean you have 13 inches of vac because a HP cam creates less vacumn ... your TURBO BOOST was actually the power vlave kicking in and adding Additional Fuel to the System all at once at higher rpm ... you would rather it come on in a smoother fashion ? check the vac pressure then divide by 2 and install that power valve.

R

I got a lot worked out, I just did a hard pull burnout in front my friends son. The cluth is slipping just like I figured. He said it was popping under the heavy load. It that a sign of being lean? Bill 1362

Can't be slipping that much if you can light the tires up.
What kind of clutch?

It is a crappy Centerforce, and it is basicly new. The guy at the shop that built the motor said to put the front bumper against the wall and break in in. 'm afraid I would shorten the car! It does not slip on a hard start, maybe when I hit third, Maybe the top of my shoe was not letting the pedal off. I don't hear the popping either, Just going by what he said. I'm almost positive I was not on the limiter. Need more testing, barefoot, so If my toe gets pinched I'll know it. I put in an o2 gauge and it says rich. Bill 1362

Rich will make it pop too because you are blowing carbon off of the plugs from a rich mixture. That is probably what happened to your power valve. They are very sensitive to any kind of a back fire.

As far as the Centerforce goes, if it is the dual friction, it really isn't crappy, just a little strange. I've had three of them in three different cars. The Taurus SHO ate it up in 8,000 miles.

Have a Chevy 11inch in my 68 GT350 and it hooks up really nice.

The Pantera has one too and is the Pantera dedicated 10.5" unit.

Not that I am an expert on anything but I can tell you that, yes, they sure do need to break in and yup, fresh out of the box you can make them slip with not much problem at all.

There is an old adage about breaking in an ENGINE the way you want to use it. This just ain't so with this Dual friction unit. You want to drive it normally, if not even baby it for a couple of hundred miles.

I'll bet you a nickle that if you pull this thing apart right now, not that you're gonna', you will see the nice little (about the size of a quarter) hot spots you just put on the flywheel?

Patience there bunkie. The damn things do work if you give them half a chance but like I said, they are just a little unique as far as a break in procedure.

The popping can be two things, a backfire in the carb or unburned fuel igniting in the hot exhaust.

Backfire: Usually because of too late timing, but be sure to check your Total mechanical advance (4000 rpm, vacuum disconnected and plugged) is not above 36 degrees, if it is, don't just advance the timing anymore, or it could be expensive. Let me know if it is, and I'll try to set up your ignition advance curve from remote. And yes a backfire can destroy a powervalve, in fact the old Holleys usually destroyed the powervalve on the first backfire, I had a Dominator that did that. But modern Holleys (and my preferred carb: Summit) are protected by one-way valve systems with a check ball, so it shouldn't happen.

Pop in the exhaust: Yes, too much fuel indication a rich mixture, could be powervalve, could be idle circuit, could be primary jets, could even be float level. My previous Pantera did it a lot, I liked the sound, and never did anything to get rid of it

Clutch, I know nothing about...

I have been changing jets. I now have 76 in the primay, and 80 in the back. I can tell the engine really comes on hard when the secondaies open. More now than when I had smaller. That's why I put the 76 in the primary from 70. I figure it likes more fuel. No smoke, but it kinda lopes. I installed an A/F gauge but it stays on RICH, execpt for the instant I hit the pedal and open the carb ... The engine works great...but? Is it optimum?

A stock 750 double pumper comes stock with 75/76 mains. For the 351, even running 12:1 compression and a 294 duration cam, I had to jet it down to 72/75.
Considering that you are running so many cubes, the 75/75 should be about right. That is a BB jetting.
You didn't say what you are running size wise on the carb Bill but a stock 4179 should be right on for your engine right out of the box.
In the OLD days of the Ford Power Parts program, an 850 Holley double pumper was recommended for a high compression Cleveland and was said to be worth about 15hp over the 750.
Your monster could easily handle that. Think of it this way. If the town you live in ever wants to bring back the trolly, they could hire you and the Pantera to cut the grooves in the road for the steel tracks?

quote:

I can tell the engine really comes on hard when the secondaies open. More now than when I had smaller. That's why I put the 76 in the primary from 70. I figure it likes more fuel. No smoke, but it kinda lopes. I installed an A/F gauge but it stays on RICH, execpt for the instant I hit the pedal and open the carb ... The engine works great...but? Is it optimum?

Sounds a bit rich...
Two things. An engine that feels like it's suddenly coming on strong, is not optimum. The reason it suddenly feels strong is that it was relatively weak before. Since we're talking feelings here like a girls' forum, don't conclude too much, but an ideal engine feels equally strong all over. My Pantera felt ok below 4200, and very strong above 4200, would suddenly spin its wheels. I thought it was the cam etc., but eventually found out that the initial timing and curve set by previous owner made it that way. I've changed that now, and now it feels strong all over. Unfortunately I didn't measure, but I'm sure it's overall faster than before.
Second thing, I played with A/F gauges some years ago, it's impossible to fully remove a lean spot when you hit the pedal. If it doesn't bog down, you're OK, and no need to keep it rich to avoid that lean spot on your A/F gauge, it's unavoidable

Good luck, glad it runs better

That's funny. That's the EXACT rpm at which all hell would break loose for me. At 4,200 the tires would literally come off the ground on mine too. I attributed that to the Edelbrock Torker power curve with the big 4v iron heads. Up to 4,200 it was a little flat but this helped in launching the car and minimizing tire spin.

The A3 heads are different. They have more torque off of idle and much smoother through the rpm range. A crisper feel throughout. Easier to drive the car this way because it doesn't hit you at one rpm like a supercharger kicking in.

Jets are now 72/73 on a 750 DP Holley 4179. I had an 850cfm secondary metering block on it which uses a power valve. That was set at 2.5in-hg, or in other words would open only at WOT after the engine was ready. I couldn't get the timing right on the opening since I was tuning on the street and this type of tuning can only be done on a race track since this is flat out death wish type of tuning and totally unsafe and irresponsible to do on the street. Also well over 130mph to tune it too.

But the secondary power valve IS an alternative to the large secondary jets but is complicated to handle.

What you could do is look at mechanical (no vacuum) advance at 1000 rpm, 2000, 3000, 4000 5000. That would be interesting, as I said, mine was way of, had to adjust the initial timing 10 degrees and the curve built into the ICE ignition had to be chosen very different. High rpm advance didn't change, but 1000-3000 changed a lot and really woke up the low rpm.

Goes without saying, but be careful, too much advance causes detonation, causes a new boat anchor.

Clevelands are very sensitive to fuel quality. The high octane gas necessary to run a fast, all in by 2,500rpm, advance curve is long gone.
Try it if you like but it will ping like the end of the world is here and commonly detonate so badly that the engine will shut off.
If you want to run 106 octane race gas, then fine, it will work great, but on the street you are going to have to slow the curve down and be all in by 5,000rpm instead of 2,500.
This is even true in my experience if you have a low compression Cleveland.