Hesitation When Accelerating

[saki302]

One idea- if you have a cheap go-pro camera, tape a mount in the engine bay, and take video of it during a drive- keep the time on it accurate, and you can sync it with what's going on during the drive.

I have a crappy go pro 1 I can use for stuff like this.

-Dave

[zzmac]

Thanks Patrick!

[zzmac]

Thanks everyone!

[Grubby]

Everyone is focused on secondaries. It could easily be a transition circuit. A tip in stumble is usually caused by idle air bleeds.

It would have to be WOT or near it to be the secondaries.

Start at the beginning and tune the carb by the book.

John

[patrickt]

Originally Posted by patrickt

It could also be a clog on the air bleeds over on the secondary side or any other transitioning orifice that's involved.

John, are you reading these posts carefully?

[Gaz64]

Originally Posted by Grubby

Everyone is focused on secondaries. It could easily be a transition circuit. A tip in stumble is usually caused by idle air bleeds.

It would have to be WOT or near it to be the secondaries.

Start at the beginning and tune the carb by the book.

John

Originally Posted by patrickt

John, are you reading these posts carefully?

Originally Posted by zzmac

Okay, so I closed down the secondaries and took it for a test drive. No bog.

One step closer. :-)

So seems like the engine runs fine on only the primary side, so a bog sounds like the secondary spring is too light.

[Moriarty]

Originally Posted by eschaider

Out of the box, Holleys will almost always come with a 6.5 power valve. That means the power valve opens at 6.5 inches of idle vacuum. If your car idles at 6 or 7 inches of vacuum that means your power valve is open at idle creating a rich idle. You want your power valve to open at roughly ½ your idle vacuum. An engine with 6 inches of idle vacuum wants a pwervalve with a 3 inch opening value.

This was exactly the info I needed for the similar issue I had been chasing. Switched out the 6.5 for a 4.5, readjusted the transfer slot square and viola!

Thanks,
Aaron

[zzmac]

Ok, I finally got the car back to the shop to further investigate. In order to eliminate some possibilities he checked the timing and then took a look at my MSD and replaced the bushing in there with a thicker one. Still had hesitation.
Then he increased the timing. Still had hesitation.
Then we tried a stronger spring in carb. Still had hesitation.
Then we tried a lighter spring in carb. Still had hesitation.
Then we ran out of time but he's now confident it is the carb (not ignition or timing).

When I take it back next time he's going to try a different carb to see if it disappears. If so, we'll rebuild the carb again or buy a new one.

[eschaider]

zzmac, I'd bet you dollars to donuts when your carb was rebuilt they put in new power valves just to cover all the replaceable componentry bases. I'll also bet they put in 6.5 or higher vacuum point opening power valve. If they did, that means your power valve(s) are open at idle.

To correct the rich idle mixture caused by the wrong power valves it becomes necessary to lean out other fuel delivery circuits which is why your engine stumbles. Stumbling is almost always (99% of the time?) inadequate fuel delivery in the tip in / off idle behavior of the engine.

Take another look at post #13. Your fix could easily be as simple as installing a 3.5 or 2.5 power valve and correcting your idle mixture. Just for grins why not measure your idle vacuum. I would bet you are down in the 6 to 7 inches of vacuum range at idle. If you are, you need a power valve that is essentially ½ that number.


Ed

[Timinator]

Put in a 50cc Pump. I had to use one of those to get rid of my hesitation in a built 351 Cleveland motor.

[eschaider]

Anytime you are experiencing a throttle response issue in a 7 liter or thereabouts sized engine with a single four barrel carburetor in the mid to high 700 cfm range your problem is almost never inadequate accelerator pump shot size. The problem is almost always low speed or tip in fueling delivery through the powervalve fuel enrichment circuit.

In the old days (middle to late 60's) before low vacuum point (2.5 or 3.5) power valves were readily available the fix was to block the power valve fuel delivery circuit with a powervalve plug and go to the largest accelerator pump available (at the time that was one step down from the 50cc REO pump) and start adjusting accelerator pump nozzles to produce the required fuel flow during the off idle fueling transition period until the vacuum signal at the boosters was adequate to pull the fuel needed through the main jets.

The only thing different today is the low vacuum powervalves are widely available and the large capacity 50cc REO accelerator pump diagphrams are too. Both approaches work. The correct powervalve and powervalve metering is the more flexible solution to the problem.

Your chosen powervalve should open at ½ your idle vacuum or lower. If you idle at 12 inches of vacuum or higher you can use a 6.5 power valve. The 6.5 means it opens at 6.5 inches of vacuum.

If your idle vacuum is 7 to 12 inches of vacuum then you need a a 6.5 powervalve at the upper end of that range and a 3.5 powervalve at the lower end of the range. If you idle vacuum is in the less than 6 inches category, you only have two choices — the 2.5 and 3.5 inch opening power valves.


Ed

[zzmac]

I'll check that out and report back. Thanks.

[zzmac]

The powervalve was not replaced during the carb rebuild.

They are recommending replacing the Holley 770 with a Holley 650 double pumper (0-4777C or0-4777S or a 0-4777SAE).

Now to find one that won't break the bank.

[Moriarty]

What’s the rating on the power valve that’s in there?
What’s your vacuum at idle?

I’m pretty sure I have a 0-4777C (although it has the electric choke) installed. I like the dichromate finish myself, and mechanical secondaries.

[eschaider]

Originally Posted by zzmac

The powervalve was not replaced during the carb rebuild.

They are recommending replacing the Holley 770 with a Holley 650 double pumper (0-4777C or0-4777S or a 0-4777SAE).

Now to find one that won't break the bank.

That just costs you additional money for someone that doesn't know how to tune a craburetor. Don't do it, you don't need to.

What vacuum does the power valve you currently use open at and what vacuum does your engine idle at?



Ed


p.s. Holley power valve numbers will look like 125-XX where the XX postscript in the part number indicates the vacuum opening point. There can also be additional postscripts following the XX that might include the letters "QFT". the QFT component of the part number stands for Quick Fuel Technology. A simple power valve part number of 125-65 would be a powervalve that opens at 6.5 inches of vacuum.

You will also find single stage, dual stage, high flow and six digit part numbers. A part number of 240025 would be a power valve for a Demon with a 2.5 inch vacuum opening point.

Click here for a listing of currently available Holley power valves => Holley Powervalves Holley offers a total of 63 different choices for you.

[patrickt]

Originally Posted by zzmac

They are recommending replacing the Holley 770 with a Holley 650 double pumper (0-4777C or0-4777S or a 0-4777SAE).

When I was in high school back in the 70's and one of us had what we thought was a carb problem, but couldn't figure it out, we would switch out our carb for a buddy's carb that was known to be running fine. Fifty years later we do the same thing at my local Cobra club. In a lot of ways, we're still back in high school. And every now and then a particularly frustrated owner, after bolting on someone else's carb and having the car run beautifully, will offer to buy that $400 carb for a grand or two. Seriously though, that's such an easy test that I'd like to see you do it before you even spent two bucks on a power valve.

[eschaider]

Patrick is providing some very good (and easy to do) guidance for you ZZ. Whatever you do, do not spend the $600, or so, to replace your current 770 carb with a smaller one. In addition to being the wrong thing to do, that will make your wallet $600 or so lighter, your engine less powerful and your car less fun to drive. As long as you are using a Holley you might want to consider getting some additional knowledge about the carburetor, how it works and how to tune it. You're going to be living with it for a while and knowing how to operate it sure beats not.


Ed

[jhv48]

Replacing the carb with a mechanical secondary Holley is a great idea even if you can get the Street Avenger to perform properly. Vacuum secondary carbs are great for heavier cars but not ours. Even Holley recommends a mechanical secondary carb for cars as light as ours.
I, too, had a roush engine with that street avenger carb. Car performed ok, but when I replaced it with a mechanical secondary carb, it was like a light switch was turned on. Woke that engine right up.
Good luck.

[zzmac]

The guy who checked out my car is a respected engine builder and in the time we had he determined that the problem 100% lies within the carb. So even if I had access to another carb to try out (I don't), all I'm more than likely going to find is that it's my carb and then I'm back to square one. Rebuild again or replace. The powervalve was never changed so it's size should be a moot point as it worked fine before the rebuild,

For another rebuild the mechanic is an hour away, I'd have to leave the car all day and if another rebuild doesn't work it'll be more time and money out the window. AND FOR THOSE REASONS..... I think I'm just going to bite the bullet and get a new carb and install it myself. :-)

He recommended a Holley 0-4777 650cfm double pumper which has mechanical secondaries and I'll probably go with an electric choke again.

I was just on the Holley website and their simple calculator recommends a 750cfm double pumper. For street driving and the occasional spirited driving do you think the 650cfm is more than enough for this engine (Roush 402R) or is the 750cfm a better choice? Thx!

[jhv48]

Do what Holley tells you to.
And if you live in a warmer climate, you might want to ditch the electric choke as well.