Hesitation still present!

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JelmerE12-E28

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Today I've worked on the carbs again. One of the acceleration nozzles wasn't squirting enough so I swapped the pumps of the front and rear carburetor. This didn't change it so the pumps (with new skirts) are fine.
Next I took the carburetor section so that I could take the check ball out that gives acces to the acceleration pump. The ball wasn't sealing when I blew air from the pump side thru it so I took a check ball from another carburetor. This one was closing when blowing from the pump side so I put it in the carb section and bolted it back to the rest of the carb. Next I tried the acceleration pumps and now the have the same resistance and squirt equaly! Problem solved?!?

I took it for a spin and quess what! The hesitation is still present!!

I running out of ideas of what this could be. Could it be ignition related?
I have an electronic ignition but all vacuum lines are hooked up like factory. I also set my ignition at factory specs, with the ball centered at the flywheel inspection hole.

Any suggestion?
 
Are you sure you have no air leaks ?

Which carbs are you running ?

Mine (32/35 DGEV) hesitated as well out of idle due to two sources. I had air leaks from the where the carb meets the intake manifold, as well as the primary throttle shaft on one of the carbs. I didnt find the leaks until I emptied an entire can of carb cleaner around the carb trying to find it. You (I) need to spray it precisely, but heavily to find the leak.
 
I does it all the time, doesn't matter how much gas I give. Even in neutral and blipping the throttle a little it hesitates.

Only when the choke is on it doesn't hesitate. But the choke cuts out pretty soon and I can't find that either.

My carbs are the Zeniths which I rebuild, all new gaskets, seals and o-rings.
 
Couple things to check:

If it runs better with the choke on, the stumble condition is masked because the mixture is richer while engaged. Once the choke pulls off the mixture will lean out and if hesitation returns then this suggests a lean condition from vacuum leaks (most likely). Gas from a small propane torch or starting fluid carefully aimed to all possible locations will cause an idle fluctuation if a leak is present. If no leaks then the rich running is masking something else. Are both chokes fully opening when the engine is warm?

Are your secondary barrels closed and adjusted properly? If one or both is slightly open at idle you will have problems as you state. This can be checked with the synchronmeter over the barrels at idle, very little if any air should draw at idle from the secondaries. There is a screw to adjust this to spec with a feeler gauge or drill bit of proper diameter, basically just barely off fully closed to prevent binding. Squirting a bunch of carb cleaner into the barrel should pool at the butterfly and not drain out immediately. Also the vacuum mechanisms can leak.

Did you time the correct ball in the flywheel window at the correct rpm? It should be the "Z" ball not the "0" ball. Verify that your timing mark the crank pully lines up to the cam chain cover mark at TDC (may need to be rotated twice for TDC) the same time the "0" ball is in the flywheel window. Your distributor rotor should also line up near the mark on the distributor housing under #1 plug.

If ok there try pulling and plugging the vacuum lines at the distributor and see if that affects change either way. While there I would leave off the retard line (and plug it), that was an emmissions feature that will only make your engine run hotter at idle.

Cap, rotor, coil, and plugs all new?
 
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Is there slop in your accelerator pump linkage?

If it was my car, I'd have a timing marks out front where I can see them move, one at TDC and another at 36° BTDC. I'd start by disconnecting the VA and setting the mechanical timing to 36° BTDC at 3500 rpm or so. Next I'd put a vacuum pump like a Mighty-vac on the VA can and see how much vacuum it takes to pull it all in, hopefully somewhere around 12inhg. Make sure it doesn't leak.
Hook the VA can to full manifold vacuum and set your idle speed to whatever you want to live with. At this point you need enough manifold vacuum to pull your VA can all in.
Set your idle mixture for max efficiency, and adjust the throttle stop to keep your idle speed where you want it.
As mentioned before, if it works better with the choke engaged, you are probably operating on the lean side. Try enriching the idle mix to see if the off-idle response improves. In fuel/air mixtures the consequences of being a little rich are much less than the consequences of being a little lean.
 
Having spent months trying to fix the same symptom, I like Steve's methodical approach here:

1. Confirm no vacuum leaks
2. Confirm secondary is closed at idle
3. Confirm no binding in the linkage
4. Confirm timing

One nights work, no fancy tools, eliminates the easy possible causes.
 
I also "had" a hesitation with my stock Zeniths after warm up. As I always do, start projects on a whim, I went out to check on a small coolant leak in the area of the water pump, then I noticed a little fuel smell, a dirty area under the carbs, a little dampness, blah blah. I have two Weber 38/38's sitting on the shelf so I started taking my stock carbs off to do the swap, taking care of that pesky hesitation at the same time. 8-) Since its not a straight swap its still apart and unfinished, although I'm making progress.

The reason for the long intro is because when I got the 38's out the first thing I did was tape some sandpaper to my table saw top and started sanding the bottom of the carb mounting surfaces. As with every time I do this, the corners sand first, showing a nice new shine, then it slowly sands more and more toward the middle until the entire surface is shiny and smooth. If the gaskets can't fill the gap made by the bow in the bottom of the carb you'll have a vacuum leak. And, mentioned already by a few others, the throttle shafts are another high probability of leakage.

Checking for vacuum leaks is something everyone should do, regardless of a hesitation or not.

Good luck!
Dan
 
+1 when I rebuilt my Zeniths I wet sanded all sections flat and true. The achilles heel of these carbs is that they warped and were prematurely swapped for Webers without figuring this out.

dang said:
I also "had" a hesitation with my stock Zeniths after warm up. As I always do, start projects on a whim, I went out to check on a small coolant leak in the area of the water pump, then I noticed a little fuel smell, a dirty area under the carbs, a little dampness, blah blah. I have two Weber 38/38's sitting on the shelf so I started taking my stock carbs off to do the swap, taking care of that pesky hesitation at the same time. 8-) Since its not a straight swap its still apart and unfinished, although I'm making progress.

The reason for the long intro is because when I got the 38's out the first thing I did was tape some sandpaper to my table saw top and started sanding the bottom of the carb mounting surfaces. As with every time I do this, the corners sand first, showing a nice new shine, then it slowly sands more and more toward the middle until the entire surface is shiny and smooth. If the gaskets can't fill the gap made by the bow in the bottom of the carb you'll have a vacuum leak. And, mentioned already by a few others, the throttle shafts are another high probability of leakage.

Checking for vacuum leaks is something everyone should do, regardless of a hesitation or not.

Good luck!
Dan
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Thanks for all the reply's! I really appriciate it.

I'm using the stock Zenith Inat carbs which I disambled, ultrasone cleaned, put new gaskets (the right ones!), o-rings and diagraphs in. The throttle shafts looked and feeled very solid and free of play.

The stumble/hestitation is when you drive of, after taking a turn, blipping the throttle, after taking a speed bump etc.
I found out that it does it when you let of the gas completly like you do in the above situations, because when your driving with, let's say 1/4 throttle and floor it, it responds good and without a stumble/hesitation.

This is were I'm at right now:

  • My choke is fully open when warm
  • Idle is at 900 rpm (but sometimes it is at a higher level)
  • Cap, rotor, coil (new) wires and plugs are in good condition
  • Valves are adjusted at factory setting
  • No slack at the acceleration pump linkage
  • New secondary diaphragms
Now that I think about the issue, when I was balancing/synchronizing the carbs with the 2 caps and vacuum meters the engine didn't respond to good when turning the idle mixture screws, I could turn them alot before anything happend. Also, the engine vibrates when idling, like it runs on 5 cilinders.


So this is what I should start with:



  • Block vacuum retard line at carburetor end
  • Check for vacuum leaks around carbs
  • Check secondary throttles if closed/adjusted (what are the specs?)
  • Check ignition timing again (Z ball)
Anything more I should check?


I'm making a list that I'm going to check on the carburetors.
 
Textbook indication of air leak-having to open the mixture screws way past spec to get any change. IIRC the mixture screws should not need to be out more than 2 turns or so. Vibration is likely from the 2 carbs not drawing the same air, or the secondaries not being closed at idle, or poor/lean/rich mixture at idle, or any combination of the above.

The secondaries should be adjusted so they are "closed but not binding" according to the manual.

A simple check for warpage would be to remove the top covers and put them on a flat surface (I used granite counter top) to see if any light shines underneath.


JelmerE12-E28 said:
Now that I think about the issue, when I was balancing/synchronizing the carbs with the 2 caps and vacuum meters the engine didn't respond to good when turning the idle mixture screws, I could turn them alot before anything happend. Also, the engine vibrates when idling, like it runs on 5 cilinders.
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Another manual has the secondary gap at .05mm measured from the bottom side, so since it is on the car just get them as closed as you can without sticking and still opening freely when the vacuum valves activate it.

And also check for manifold leaks
 
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Stevehose said:
Another manual has the secondary gap at .05mm measured from the bottom side, so since it is on the car just get them as closed as you can without sticking and still opening freely when the vacuum valves activate it.
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And then check the flow through the secondary after gapping, right ? Aiming for zero flow at idle ?

Do Zeniths suffer from leakage around the throttle shafts ? If so, maybe smear some bearing grease around that area as a temporary check on sealing off any possible leaks ?
 
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I saw a mechanic wave the end of an unlit propane torch around the carbs and hoses looking for a vacuum leak. The engine RPMs increased when the propane was sucked in through the vacuum leak. You should be very careful if you try this and have a fire extingusher next to you.
 
Yes check flow after gapping, looking for smallest flow possible without binding shut.

carbs are going to leak past the throttle shafts some, as long as it is minimal and doesn't have an effect on mixture, or at least it can be compensated for in the lean best idle adjustment.


David said:
And then check the flow through the secondary after gapping, right ? Aiming for zero flow at idle ?

Do Zeniths suffer from leakage around the throttle shafts ? If so, maybe smear some bearing grease around that area as a temporary check on sealing off any possible leaks ?
Click to expand...
 
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