Its been a long time since ive been stumped, but this one has me. Ive been helping out cliffordb to get his car straightened out and running right. Its a 76 with stock throttle body. The p.o. had the diz advance hooked up to the vacuum port on the plenum meant for the decel valve. wtf? Removing the hose from diz and plugging results in backfiring, and dieing. It idles fine when attached, but has a bad hesitation at throttle opening. What the heck am I missing here? all components have passed testing. Valves, dwell, and timing have all been verified. No vacuum leaks. Tps adjusted.

When you commented that it idles fine when attached, are you saying that you have now attached the disti to the throttle body? If so, do you have it on the port that is zero vaccuum with the throttle fully closed, or the port below the throttle plate?

Sounds like you have to re-install the Dizzy correctly. You need to verify if the Dizzy lines up with cylinder 1 at TDC cylinder 1. Some people will line up the dizzy with how they time the car instead of line up the dizzy and then time the car.

Also, you can't mix up the hoses as the inlets are different sizes. Small vacuum connection to small vacuum connection and larger vacuum connection to larger vacuum connection. The retard and advbance connections on the Dizzy and throttle body are different sizes so you can't get them mixed up.

The car seems to be doing much better today, although there is still a hesitation and slight shutter under load.

For reference:

(Note: The PO had the blow by line hooked up to the air cleaner which is where it still is. The line below the plenum with the larger port is blocked off as shown above)

The MPS is hooked up to the plenum as it should, but not pictured.
The large hose to the AAR is currently plugged and I'm holding it out of the way in this picture.
The short line from the Diz to the Plenum is hooked up.
The longer line that goes from the Diz to the TB is also hooked up.

The 76 should not have the advance connected at all. Retard only. The 73 style 2 port T.B. Was re-introduced but the port that used to go to dizzy advance is now connected to the EGR valve (that is if it is still there). Advance port on dizzy is left open to atmosphere like a 74. Connecting the advance on late model will result in detonation at higher speeds in a trailing throttle condition.

Check for proper distributor for 76, mechanical advance function in dizzy, and that initial timing is correct.

per this Bowlsby diagram I see that both vac lines from the disti are routed to the throttle body, none to the plenum. I'm fairly certain that the advance line catches the port just above the throttle plate, and the retard line is below the throttle plate, maybe your current config is throwing off the timing...

http://bowlsby.net/914/Classic/zTN_FI_Hose...Late1974-75.jpg

Mark, and Frank, thanks for the response, but you need to put on youre reading caps. This is a stock 76 throttle body. It has no vacuum advance port. The vacuum advance hose is connected to a port on the plenum.

Pancake, 75/76 had a two port tb with one hooked to egr? Ive seen dozens of late models. I have three, and have 4 that I maintain. I have never seen this. Maybe a california thing. I have also added the advance port with no ill effects or detonation.

I'm well aware its setup wrong, and know how it should be. I understand the off idle hesitation is because its already advanced. My question is, why it runs with advance having constant vacuum, and not when disconnected?

Btw,diz advance and retard are both operational.

Thanks

With the vac adv hooked up to the plenum you have the adv in (aprox 10 degress) so unhooking it screws up the timing.
Unhook it and plug the hose, re-time the engine using only the mech adv to 28 degrees @ 3500 rpms.

If the engine still acts funny after doing this look at the mech adv weights inside as they might be all gummy/sticky with old grease throwing off the adv timing.

We actually did that. Dis was dismantled, cleaned, and relubed before reassembly. Timing was set both ways with the same result.

I feel for you on this one. I have a bug that I couldn't get to run but when I took it to my mechanic, he worked miracles. I never touch it now but I'm always curious and just itching to tinker.

As I was rebuilding my dizzy a few days ago, I noticed that when I test the advance before installing the points the plate moves freely. After mounting the points, the plate wouldn't move. I found that the small gap that is on the centers the advance ( i think it is a stopper for retard or adv. ) was touching the back of the points. I used a needle noise to move it back a bit. Something to look at .

I'll check that Paul, thanks.

I hope I Explained it right. Let me know if you need a pic .

Your explaination was perfect Paul.

Forgive me if you know all this.
I have a 76 and in theory it has a single hose from the throttle to retard on the dizzy. Advance on dizzy is a short tube to air (keeps mice out? ). As I understand it the port for retard is to retard spark at idle and then as the throttle opens the vacuum retard goes away, allowing more advance. The port on the throttle body is just behind the butterfly. The plan was to reduce curb idle NOX emissions by retarding the spark, but that causes poor running as well.

The fact that you need the advance hooked to vacuum tells me that the timing wants that advance - but on a 76 vacuum advance isn't supposed to be there. I think the problem may lie with too much retard at idle. I seem to remember that the instructions for timing in the manual(s) I used were ambiguous. Normally you set base timing with the hoses off the dizzy. I am not certain that is the case with the 914. I think it might work to pull off the advance, plug the vacuum leak, and while leaving the retard hose in place set the timing per instructions at idle and at 3500. That would give you better timing at idle and may solve the problem.

I should have taken notes because I don't have that system anymore - I put on an earlier throttle body and now run both ports hooked up as in the earlier models. My experience was very positive - increased torque and smoother curve.

From what I know with other engines of the era, the vacuum retard wasn't really doing anyone any favors, and can be dispensed with. The CAT saved the automobile. At least here in the US.

Worn, You do understand correctly about the retard only theory, and the advantages of the back date on the throttle body, I have added the advance port on the late body and it does enhance the transition from idle. The 914 is timed with both disconnected and plugged. You have given me an idea where to go next. I have actually leaned toward mis-matched components due to the fact the ecu is a reman unit and has no #s. Who know what it was set up for.

Post your results when you nail it.

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