Mid Range RPM Hesitation

[Zedyone_kenobi]

Well I am still in the same boat as mizzle. Although my problem is popping from the exhasut at about, you guessed it 4500 rpm. Mizzle I hope you get your floats perfect, good luck.

I have not touched my floats since I got my carbs from Z therapy. They ran great then, so my issue must be something else. But what I have no freaking idea anymore.

[CW240Z]

I had this same problem with the SUs on my 72. I had a completely new fuel system, new cap, rotor, and good plugs. Carbs and dizzy were dialed in correctly and timing was perfect. However, under load the car felt bogged down when I quickly accelerated, but ran great when in neutral or slower acceleration. After ruling out all options, I pulled the plugs for the heck of it to check them. Turns out I had a bunch of fouled plugs which were causing poor accelerating. After putting in new plugs the car smoked the back tires off and accelerated like a dream. It's probably a dumb suggestion, but check the plugs, I didn't see that you mentioned anything about them in your description.

Chase

[Duffman]

I agree with BeermanPete, seems fuel related. Under load, sounds like not getting enough fuel to carbs. Will be interesting to see what the issue is!

[madkaw]

Steve,

When you were messing with your floats did you blow thru the float inlet while you moved the float around. To see where your needle and seat are really closing off the fuel you can hang the float lid in it's normal position and put your mouth on the inlet and blow on it. While your blowing air thru the--- -cycle the float up and down to see if your needle and seat are working properly. It should smoothly shut off the flow of air as you shove the float upwards, and then smoothly let air thru as you release the float. Make sure that is all working properly and take note where the air flow cuts off and where the flaot is at.

Steve

Edited November 21, 2011 by madkaw

[landmizzle]

Hi all,

Still at this....

At this point, I've gotten very good with adjusting the float level bowels. I do not believe the issue has to do with a low fuel level in the bowls. Nor is it the "last chance" filters in the lids of the float bowl as air and gas flow freely through them.

At this point I've;

Replaced the distributor

Replaced fuel pump.

Replaced coil

Adjusted fuel level to specs. in bowls.

Performed fresh tune-up, plugs, adjusted valves, cap and coil.

Pertronix is currently installed. (switching to points does not change anything)

I have NOT actually checked fuel flow with a gauge as I don't have one. May be my next desperate act will be to purchase one.

However, I may have stumbled on a clue, maybe. I mentioned earlier that the mechanical advance appears to work because when I blip the throttle I can see the timing advance with a timing light. This is still true. I'm wondering though if there is a problem with the vacuum advance. The car drives the same whether I run with the vacuum line attached to the dizzy or not. So, I'm thinking the vacuum advance is not kicking in. Any thoughts on this?

Vacuum at the dizzy line is 6" Hg. Vacuum at the center vacuum port on the intake is about 11-12" Hg. Are these numbers normal?

I maybe totally off here with the vacuum idea, but it seems reasonable. I agree it still could be fuel flow issue, but with the new pump, I think it's less likely.

BTW, had my old Z mechanic ( he's retired now), look at the car. He put a compressor on the fuel line from the engine bay going back to the tank and air flowed cleanly as we could hear bubbling coming from the tank. He checked the dizzy, needles for straightness, fuel level in the nozzles, and fuel filter and noticed nothing amiss. I didn't want to abuse his courtesy and so we left it at that.

Any ideas?

[landmizzle]

Hi Steve,

Yes, the float inlets appear to work fine.

[mlc240z]

Didn't Arne run into this same problem? Believe it wound up being Pertronix dwell or something along those lines.

I could definitely be wrong, tho.

[landmizzle]

Hi,

I know some people have reported issues with the Pertronix, but I've ruled that out in that I get the same symptoms even when I run points.

Didn't Arne run into this same problem? Believe it wound up being Pertronix dwell or something along those lines.

I could definitely be wrong, tho.

[landmizzle]

Hi Blue,

I pinched off the fuel return line for about a minute and noticed no difference in how the engine was running at idle. What should I be looking for?

I did pull off the rubber hose from the steel return-line and fuel was squirting out at what appeared to be a constant pressure.

Could be fuel delivery problem. What is fuel pressure?

You can try pinching off the return fuel line for a quick and dirty test to see if it makes a difference.

I also had a points car that went weaker at high RPM's and this seemed like a fuel issue. After checking everything it turned out to be the distributor, however your checks above seem to have ruled that out.

[beermanpete]

You really need to measure the fuel pressure and flow rate while the engine is running. The factory service manual has a good description of the method and the only "special" tools you need are a pressure gauge, a 1 quart measuring cup, and a stop watch. Your vacuum gauge should be able to measure the fuel pressure, most will.

[landmizzle]

Hi all,

For all you imploring me to measure the fuel flow rate and pressure, you'll be happy to know that mission is accomplished. Now, I'm not sure how accurate the measurements are. Here's why. The the rubber fuel lines are 5/16" but the nozzle on the pressure gauge I have is smaller, maybe 1/4". So I had to use a "T" adapter that would accommodate the different sizes.

Rear carb had 3.4 psi while the front was running 3 psi. I believe specs call for 3.41-4.27 psi. So I'm on the low side (especially the front) assuming the my readings are accurate. I'm a little concerned that the readings are being affected by the T as the readings went down on the front when I used a T with a larger ID opening for the connecting fuel lines.

I went to measure the fuel flow next by measuring the flow from the individual carb lines. The readings averaged out to be 2.5 pints/min rear and 2 pints/min front. Looking at the FSM and it called for 3 3/8 quarts/min! Or 1600cc per min.

I then measured the flow from directly past the fuel pump and averaged 3 pts/min. Still low!

I then realized that there has to be a mistake in the FSM. 1600cc does NOT equal 3 3/8 quarts. It equals about 3 3/8 Pints!

Even with correct conversion, I'm a little shy of the proper flow from the fuel pump. If the measurement is supposed to be from just before the carb, than I'm really low.

I'm a little surprised that the flow and PSI are not meeting specs as the fuel pump is brand new.

Measuring from the fuel pump would rule out any obstructions between the pump and the carburetors (as far as fuel flow), but I'm not sure if that's where it's supposed to be done.

Thoughts?

I'll probably pickup a new pump locally if I can, but I might try putting my old pump back on just to see the results

I'm also curious if the length of lines might be having an effect on the measurements. For example the length from the T to the gauge is probably 12"(this is also the thinner 1/4" line). The connecting fuel lines to the T probably span 8" to 10". Of course these shouldn't really affect the fuel flow reading which is low also.

[landmizzle]

Hi all,

Just for comparison, I put the old fuel pump on and got the following readings;

Front= 2-3 fluctuating psi Rear= 3.1-3.4 fluctuating psi Note: both would stabilize by giving slight input to throttle shaft.

So, all-in-all, slightly lower readings than the new pump.

BTW, with either fuel pump, pinching the return line either removes the fluctuating reading or slightly raises PSI (<.5).

I then put the gauge directly on the output nipple of the fuel pump and got a steady 3.5 psi.

BTW, answered my own question regarding the tube length coming off "T"; I shortened them and saw no change.