Vaccume advance question.

mmudge

mmudge

True Classic
I put a Yugo elect dizzy and module in my '74 X, 1300cc. The stock setup in my manual shows me the vac line goes into an in line delay valve, then hooks to a thermal switch on the head. The hose coming off the other side of the thermal switch goes to.....?

I pulled all the smog parts so I'm not sure where to hook the hose to.

The car starts and idles fine but when I let the clutch out and give the accelerator a little push to get going, it stumbles and acts lean. If I jam thru the primary circuit and go to the secondary it rips. So I have an issue in the primary transition circuit near as I can tell.

The jets look all weird. AC jets 195/200. Idle jets 50/60. The AC jets seem too big. I'll have to back into the carb to check my float height but I just rebuilt it, put all new fuel lines in it and had the gas tank boiled and sealed.

Workin' out the bugs......:D

Mudge
 
No vac connection on your set-up....

Hey Mudge,

This is an often asked question for something that is often incorrectly implemented.

74-78 32 DATR carbs, and 79-on Bosch X/Yugo ignitions are not to be integrated with a vacuum advance line, unless the carb has been modified.

In your case, block all unused vacuum ports with vacuum caps and do the same for the distributor vacuum tap.

You will have to set your ignition up with slightly higher than usual base advance so that base advance + mechanical = 30 (or so) degrees. No vacuum advance will be present. Be careful though, as too much base advance can make the car hard to crank.

For an explaination of the above, repeating much of what TonyK and others have posted before, but a good refresher for those without JimD-like info-digging skilz....

-------------

Points-based X and 128 distributors utilize a vacuum retard-based diaphragm. Vacuum signal on the distributor moves ignition timing backwards.

Bosch electronic ignitions utilize a vacuum advance-based diaphragm.
Vacuum signal on the distributor moves the timing forward.

Engines fitted with points-based ignitions had carburetors whose vacuum advance ports were UNDER the throttle butterflies. This means maximum vacuum signal occurs at idle and low-throttle openings (cruising). Think of the engine trying to draw air with butterflies closed. Where is the air (or vacuum) going to be drawn from? Any other opening under the throttle plates. When the carb moves to an open throttle, the vac signal is significantly reduced from that seen at idle because there is no restriction now from the butterflies. Engines with these set-ups used several solenoids, water-temp switches and transmission switches to override or block this vacuum signal based on different conditions. For example, hot restarts with an extra 10 degrees of advance at idle is difficult, so the head temp sensor thingy closes the vac port off above certain water temps. For emissions purposes, a switch in the trans would activate a solenoid that blocked the advance when the car was in 4th gear, etc.)

Engines with electronic ignition, both carb (79-80) and FI, get the vac signal ahead of the throttle plate(s). This provides minimum vac signal at idle and steady state low throttle opening, but max signal at wide open throttle.

So.... Hooking an under-the-throttle plate 32 DATR vac port to a Bosch ignition system vac advance will crank 10+ degrees of timing advance at idle, a little less at steady state, and a lot less under accel, which serves no positive effect. In best-case scenario you want to advance the timing while the engine is under load, especially when an accel pump adds a big squirt of extra fuel when you mash the pedal.

The only solution to get true vac advance in this scenario is to modify the 32 DATR carb to put a vacuum tap and orfice above the throttle plates.

If you can't do this, you are best served leaving no vac lines connected between the carb and distributor at all.

Hope this explain helped.

-M
 
Mike... Matt HAS the answer... he just wrote too much...

(Not that I ever do...)

"The only solution to get true vac advance in this scenario is to modify the 32 DATR carb to put a vacuum tap and orfice above the throttle plates."


This actually a simple process and having a functioning Vacuum advance for street use is a PLEASURE you do not want to be without.

There is a section in 1.0 on how to do this but you will need to pull and study the carb you currently have. Once the proper place is found, a simple hoe is drilled, a nub installed and all the others are corked. Works great.

Here's the link... but the photos are GONE! What a pain... If ya can't figure it out from what al has been written... drop me a line and I'll try to help further.

http://www.network54.com/Forum/1215...um+on+a+DMTR+carburetor-++Distributor+control-



 
Would all this explain why the car idles great but

stumbles off the line on the primary circuit and will not go anywhere unhless the secondary is engaged?

MM
 
It might, but....

Mudge said:
Would all this explain why the car idles great but stumbles off the line on the primary circuit and will not go anywhere unless the secondary is engaged?

MM
Click to expand...

It might, but what you're describing sounds to me like crud in the primary circuit main jet or emulsion tube.
 
I concur

Your symptoms sounds like a restricted port in the carb. due to dirt/buildup. But the vacuum advance mod. will definately benefit the drivability.
 
HAHAHA... Like I ADMITTED, Matt...

I use words like they were a dime a dozen!

I once had a College Professor suggest I use an "economy of words" when I submitted my papers. He obviously saw through my "Tell 'um, Tell 'um, Tell 'um... approach when he was looking for 500 word essays that just TOLD HIM once what he was looking for!

HA!

I call it "Thinking Out Loud"... and found that the only TRUE difference between Introverts and Extroverts was this one phenominon!

Hope you and yours are well...
 
Mike... I agree with Eric on this issue...

Initial throttle response is dependant on the A/F ratio and the idle circuit and lots of other issues... and the Vacuum Advance doesn't come into play until much later.

BUT... a leaky vacuum hose, gasket, clogged rods/tubes, or an unplugged numb could be contributing to the problem... We are getting into an area of so many variables now...

If the carb has not been thoroughly boiled out and cleaned lately, probably now is a good time. Then approach the secondary and vacuum advance issues...
 
My 1976 128 also has a "flat spot" off idle until it reaches full operating temperature. I simply assumed it was typical 1970's lean emissions jetting. (Any of us old enough to recall the dark days of disco will also remember how poor driveability was BFI - before fuel injection.)

I replaced the timing belt last winter and needed to re-set base ignition timing. My car had been upgraded to a Bosch EI unit by the PO, but still retains its stock 32 DATRA 1/100 carb. The distributor vacuum hose was routed to the PCV fitting on the secondary throttle shaft. Now I understand why...the installer was trying to use the WOT PCV vacuum to provide vacuum advance to the Bosch distributor!

I was advised by several experienced FIATisti to set initial timing somewhere between 10 and 15 degrees BTDC (mine's at about 12 deg.) and to leave the vacuum unit disconnected. Now it makes sense why the car runs better with the vacuum unit disconnected.
 
Welp... I useta think the same way...

... and the method you describe works well, and VERY well if one is in a performance mode all of the time.

With my age though... I'm kinda looking for more smoothness and subtle-ty in life I guess...

Anyway... a CORRECTLY operating vacuum advance system will KINDA slow or even RETARD the advance for a bit when you first open the throttle as what vacuum there was, goes away. As the engine speeds up, the spark retarded allows it to catch up, minimalizes PING and provided for and overall smooth transition, even with MANUAL operated secondaries.

(With the available 91 octane gas in CA, I found I could not set my static advance more than 7 - 8 degrees without experiencing pinging (and a big loss in power) on our mostly hot and sunny days...)

Now nice and smooth and pulls steadily. Yes... it took a while tweaking and tuning to achieve this... but I'm pleased with the results for street use. If I was racing, I'd do exactly what you recommend!
 
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