Hesitation Problem Driving Me Nuts

[slepr1]

I have this hesitation that I can't seem to fix. I have a 850 Holley Dbl. Pumper on top of a M1 intake. Since I changed from a 750 Vac.Sec. Holley with a Edel. Performer 440, the hesitation has been there. I had the timing set up, distibuter checked, smaller jets, and still it persists. The hesitation does get much less when the engine is warmed up and driven for awhile (10 minutes). But within the first 10 minutes the hesitation is so bad it sounds like crap. The only thing I changed to get this problem is the carb, intake, and coil. Any help would be great.

[cuda66273]

Try this and get back to me..

Bring the idle down until it tries to stall...now TAP both pumps ..just a small shot of fuel...

Did it stall or pick up RPM?

[Mills]

Your Edelbrock Performer had heat cross over in the intake - where as the M-1 (if single plane) doesn't. 10 minutes sounds like your engine just needs to heat up. Do you have the choke hooked up.

My 440 will not run when cold - even on a summer morning here in Canada - I have to keep my foot on the gas and let it warm up, as I have removed the choke plate from the carb and have no heat cross over in the intake.

When your idling, put your hand over the primaries - if it speeds up - your lean - which is causing the hesitation. Maybe you can play with the accel. pump cams to give you more fuel.

[slepr1]

Originally posted by cuda66273
Try this and get back to me..

Bring the idle down until it tries to stall...now TAP both pumps ..just a small shot of fuel...

Did it stall or pick up RPM?

Will do. My Cordoba is in the paint shop getting a real costly paint job and should be done in a few weeks hopefully. When I get it back I'll try and revisit and post my answer. Thanks.

[mtrv8n]

You're not getting enough pump enrichment off idle. The M1 intake needs about 15 minutes to truly warm up, (got one on my crate..). In 90 degree weather, It's not so bad, but when it's cold, it really need time to heat up(which keeps the fuel from puddling and not getting to the cylinders.).

Also, the 850 is not getting as much signal as the 750, hurting it's response to your foot.

[slepr1]

Originally posted by mtrv8n
You're not getting enough pump enrichment off idle. The M1 intake needs about 15 minutes to truly warm up, (got one on my crate..). In 90 degree weather, It's not so bad, but when it's cold, it really need time to heat up(which keeps the fuel from puddling and not getting to the cylinders.).
Also, the 850 is not getting as much signal as the 750, hurting it's response to your foot.

Hey mtrv8n,
I don't wanna sound like an ignoramous, but I hafta ask you line by line...
1. What is pump enrichment off idle? (is it air/fuel mix?)

2. <<Also, the 850 is not getting as much signal as the 750, hurting it's response to your foot.
>> Hu? Signal? Foot? Please explain it to me like I'm 8 yrs. old.


The car is garaged where the temp is around 75 degree. Outside, the temp is a hot 85-95 degree. When your M1 is warmed up, do you still get a slight hesitation like mine?
Thanks a bunch.

[Doug Wilson]

A double pumper has two sets of two accelerator pump outlets - a pair on the primaries and a pair on the secondaries. They are located in the center of the carb, at the top of the air horn (not the choke housing, below that). Pump enrichment off idle refers to the fuel that is squirted into the throttle bores when the throttle pedal is depressed.

An 850 flows more air/fuel than a 750, because it has bigger throttle bores. At the same gas pedal depression, the air through the venturis will be flowing more slowly. The little thingies in the center of the throttle bores, which are round, sticking into the throttle bores are called boosters - these provide the signal to the rest of the carb, telling it how much fuel to provide. If the air is moving slowly and the vacuum signal is low, it can produce a hesitation.

It is best to run vacuum secondaries on a street driven car because they are easier and more forgiving about tuning.

Did I make this clear to you, or did I just confuse you more??

[slepr1]

Thanks Doug...well explained. One question though...

<<If the air is moving slowly and the vacuum signal is low, it can produce a hesitation.

"Vacuum signal is low"...please explain.

Thanks Bunch.

[pishta]

Let me try...you ever see an airbrush? The compressed air flows out, over the end of a narrow tube, the vacuum the air flowing over the tube causes, draws up the paint and it finally hits the air stream, getting the paint out to the target. Blow hard over a thin straw in a Coke bottle, and itll draw up the coke until you are spraying it everywhere. OK now try it with a big bore straw.
it takes alot more lung to draw up the Coke because the vacuum is over a larger area. Conversly, your motor only produces a finite amount of vacuum, lets say 15 for instance. You put this through a small opening (a 750 venturi, or 1 inch, again an instance) and itll produce alot of suction (a thin straw). Put the same amount of vacuum through a 1 1/2 inch hole (lets say this is the 850, a big bore straw) and itll produce alot less suction. This equates to a low or weak vacuum signal, the inability to draw fuel out of the booster!

Put a 500 CFM carb on a 440 and itll idle great and pull from 500 RPM, but itll run out of carb real fast. Great on a truck or Imperial where high Rpm's are not needed, but where low end grunt is.

Hmmm, I might have made things worse!

[slepr1]

Oh, now I get it! Let see, if I blow hard enough through a big airbrush, coke will come out.


Kidding aside, I get the picture. Just needed a little prodding. I have a rebuilt 750 Vac Sec. sitting on the bench, maybe I'll test n' tune. ( I really like the punch of that 850 D.P. though. ) Luckly there is a dyno nearby that charges 70CN$ (~45USD) an hour to use his dyno. Just drive up on it and find the best combo.

[cuda66273]

Send me a ticket and I'll come up and tune that baby for ya'll, Eh?

Do they still have the The Tigercats in Hamilton?

[slepr1]

Originally posted by cuda66273
Send me a ticket and I'll come up and tune that baby for ya'll, Eh?

Do they still have the The Tigercats in Hamilton?

Funny...ending a sentence with ya'll is as American as apple pie eh!

Tigercats...every year there's talk about them moving. I don't follow it too much except when I hear us taxpayers have to pay for them to stay.

Some really cheap flights in September coming up I hear, I'll have my mopars a-waiting.

[hemivaliant]

A carb's booster and idle circuit works because of the "venturi effect". Just like pishta explained.


HV

[Doug Wilson]

In physics, the "venturi effect" is actually called the "Bournelli Principle", after the scientist who first discovered/observed/defined it. This is the the same physical characteristic that creates lift on a bird's wing or aircraft wing which allows them to fly. Basically, it says that when a moving fluid passes through a constriction, its' speed increases and the pressure decreases. The top surface of an aircraft wing is longer than the bottom surface, so the speed of the airflow increases, creating a low pressure area over the top surface. The low pressure draws the wing toward it, which lifts the aircraft.

Another effect that this principle defines, is that, when the pressure drops, so does the temperature of the fluid passing through or over the venturi. This explains carburetor icing and why aircraft wing icing is so dangerous. On the aircraft wing, the ice will form where the low pressure is - the top surface of the wing - as the ice forms, it destroys the venturi shape of the top of the wing and the lift goes away. No lift - no fly. Aircraft crashes.

There are mathematical formulae wfich precisely define how to measure this difference, and they are pretty easy to use to calculate this.

In a carb, there are at least two places where this happens - in the venturi, and in the boosters, which are inside the venturis. The big hole - the venturis - pass more air and have a smaller amount of low pressure than the boosters, because of their size (cross sectional area). Boosters have a larger amount of low pessure (vacuum), so they provide a stronger "signal" to the fuel in the float bowls, at a lower air velocity. This is why they are there - to provide a "boost" in the fuel flow during the transition from idle to cruise. Both the large hole and the small hole are actually venturis of different sizes.

A carb with mechanical secondaries provides fuel and air based on the driver's foot position on the gas pedal. A carb with vacuum secondaries provides fuel and air (through the secondaries) based on the engine's DEMAND, and is easier to tune and smoother when going from part throttle to wide open throttle

[Doug Wilson]

Anyway, to describe what created your hesitation in terms of what happened: You went from a smaller, vacuum secondary carb which had a good deal of velocity through it no matter what your pedal position, to a larger, mechanical secondary carb with much lower velocity through it. That improves power at higher RPM and reduces torque at lower RPM. It also reduces responsiveness - there is your hesitation.

These are the same things that happen when going from unported to ported heads. Low end is sacrificed to create better breathing on the top RPM range.

How did I do guys??? Clear as mud??