Mid Range RPM Hesitation

[landmizzle]

Hi all,

I have a '72 240Z with the original SU carbs and Pertronix ignition. Car is freshly tuned with valves adjusted, new plugs, cap, rotor, and carbs adjusted.

Idles great and drives around town and on the freeway fine. However, when I rapidly accelerate the car begins to hesitate at about 4500 RPM. It did this before the tune-up also if memory serves correct (the tune-up was probably six months or more ago. I don't drive the car daily).

Any ideas? My thoughts are maybe my A/F ratio isn't quite right or I need to go to a different viscosity SU oil which is currently 20 weight.

Revving the engine while the car is not in motion does not seem to illicit the hesitation/stumbling symptom.

[Dobber]

http://zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601d.htm

This link may help.

[d240zx2]

Based on what you wrote, it MAY be a problem with the mechanical weight in your distributor that's causing the problem. If that weight isn't free-swinging or if the spring is broken, you could experience what you're describing.

[landmizzle]

Just to add; if I slowly accelerate up to 4.5K RPM and beyond, the hesitation does not occur

Interesting idea on the distributor, I'll look into that.

Thanks for the replies so far folks!

[landmizzle]

So, I can't tell for sure if the mechanical weights move freely. I tried to remove the breaker plate from the distributor but one of the screws doesn't want to let loose and I'm afraid of mashing the head.

However, putting a timing light on the engine, I can see that the engine advances when I blip the throttle so, I'm assuming that means the weights are moving appropriately. Is that correct?

I was searching through this forum and I've seen more than one person state that with the Pertronix ignition, their car was only revving to 4 or 5K. I wonder if that's the problem here? As mentioned, my car revs freely to 7K with no load, but when on the road and accelerating hard, she hesitates badly at 4.5k or so. I have a point set in the glove box, so I can switch over.

The other idea gleaned is to check the float level.

Based on what you wrote, it MAY be a problem with the mechanical weight in your distributor that's causing the problem. If that weight isn't free-swinging or if the spring is broken, you could experience what you're describing.

[madkaw]

I'd check your float level. Richen it up a bit on the floats and see what that does.

[black gold man]

Here is a good read.

From: pharvey@mipos3.intel.com (Paul Harvey)

Newsgroups: rec.autos.tech

Subject: SU/Hitachi variable-venturi carbs theory of operation

Date: 12 Feb 92 19:28:41 GMT

"These carbs are very simple and very reliable relative to multiple-

jet/air-bleed type fixed venturi carbs. They have only one jet!

So how do they work?

Through a sliding piston arrangement, they maintain the same air velocity

through the venturi, independent of air flow, by varying the size of the

venturi. So as the air flow increases, the piston moves up and the size

of the venturi increases and the air velocity remains the same!

Now, if the velocity through the venturi is constant, that means that the

vacume on the fuel bowl is constant, (Bornoulli's law), and that means

you can exactly meter the correct amount of fuel into the air. A sliding

needle is attached to the piston and it is machined with a slight taper

so that as the piston moves up, more fuel is metered out of of the jet

port so that the air-fuel ratio is kept constant, independent of air flow.

The port is threaded, and a dial turns it so it can be moved up or down

relative to the needle so the idle mixture can be adjusted. Note this

also affects the mixture at all speeds.

An engine needs fuel enrichment during acceleration, so this is done

by a one-way oil damper on the piston. The oil damper delays the piston

on its up-stroke, so in this case the velocity through the venturi is

higher than normal and more fuel is sucked out of the jet port than normal,

which gives you enrichment! The thickness of the oil changes the duration

of the enrichment period as well as the ratio of enrighment.

That's it!

Compare this to a modern Weber-type carb sometime. I'll let someone else do that!

Sure, there are a few negatives!

This type of carb is not easy to modify relative to a Weber-type which

can me modified in ten-thousand different ways. The only things that can be

varied are the jet and needle valve, the idle mixture, and the damper oil!

Of course, this means if you modify your engine, you probably will not

need to modify your carb to compensate.

The machining of the piston needs to be precise, so some British manufactured

carbs are a problem. Of course, Weber-type carbs need more precise machining!

You do not get the instant throttle response of a good Weber setup, but

acceleration is limited by the speed at which the engine can push the car, but

most people like fast throttle response anyway!

Air leaks around the throttle bushings cause them to perform even worse than

Weber-types with the same problem.

And this carb never really made it through the emissions era, there

are some emissions versions, (73-74 Datsun Z), that mostly are junk and

cannot be maintained.

But there is nothing like the woosh sound of these carbs, and just imagine

how much fun you can have asking people what type of oil to put in them!"

http://yarchive.net/car/carb_oil.html

http://yarchive.net/car/index.html

[d240zx2]

#5. I have the Pertronix and am one that's never experienced any sort of bogging or hesitation at any rpm. It just goes and goes.

[landmizzle]

Yeah, I'm thinking that it's a long shot. Actually, I think I tried switching to points some time ago, and the hesitation remained. It's a fairly easy swap though, so I might try it.

The more I think about it, the more I'm inclined to think it's the float level as some have suggested.

[Bruce Palmer]

Again, what do we suppose has happened to cause the float levels to change?

If you are basically happy with how the car runs which would be determined by the fuel nozzle settings in each carb, you should be able to pull the domes and pistons and look down in the fuel nozzles and see the static fuel level within a 1/16" of the top of the nozzle. If the gas is puddling up on top of the nozzles you'll be fat and if it's way down in the nozzle you'll be lean.

I vote distributor.....

[landmizzle]

Hi all,

This is driving me nuts!

BTW, I should have listened to Bruce and left the floats alone. I messed with them and really had a hard time getting the level back to where they should be.

So, I've had the distributor rebuilt, survived my attempts at adjusting the float bowls. Still the car wants to hesitate hard at around 4.5 K RPM.

I mentioned before that I could slowly accelerate to 4.5 K RPM before, but that doesn't seem to be the case. She bogs down regardless how fast I accelerate.

It literally feels like she's running on one carb or half the cylinder are not getting spark while in that rpm range. BTW, I can continue to accelerate through 5 K and beyond but there just isn't any power.

Any suggestions?

[Gary in NJ]

What's your fuel pressure? You could be draining the float bowls on hard acceleration.

It could also be as simple as a clogged fuel filter or the last-chance filters that attach to the fuel bowls.

Edited October 29, 2011 by Gary in NJ