carb tuning issue

[kuhlou]

Autolite 1100 non-scv carb, with internal vac ports blocked off, vac off manifold, 18 in at idle, idle 700 rpm, idles fairly smooth, choke disabled. Hooked to original non loado dizzy. Getting ready for the DSI conversion (have power steering on stang, so DSII does not fit), then a hw5200 install.

My issue, engine idles well and draws good vac once warmed up. I put auto trans in gear, still idles fine. Touch accelerator to pull away from stop, and engine sputters and dies. If I feather the throttle, it will still sputter a bit, but will get underway. Accelerating is smooth, and running at cruising speed and rpms is smooth while under load. Cruising at very slight throttle, say a slight downhill, will cause the stutter to occur. If cruising at low rpm's and pull to stop, will idle smooth at stop, but will stall when pulling away if I don't feather the throttle. However, if cruising under load and coast to stop or brake for stop, will pull away from the stop very smooth.

I'm new at this carb adjustment stuff. Once the vac leaks were corralled and idle was reasonably smooth, set the idle to 700 rpm, screwed the air screw in until engine stumbled, and backed out a turn. Initial timing with no vacuum connected is at 6*, do not notice any pinging when accelerating at that timing. It had been at 12*, and pinging, so I backed it off. Pertronix is intalled in non loado dizzy.

Could I get some advice on how to attack the "stumble" when pulling away from a stop, and at low rpm cruise? Thanks.

 

[BIGREDRASA]

First off, I never had any luck using manifold vac, instead of ported vac. In any event, did you disconnect/plug the vac advance line when setting the idle? With manifold vac engaged at idle, you'll suddenly retard the timing when you stomp on the loud pedal. Not my cup of tea.

There are check valves that prevent the sudden retarding in that event. That could help.

 

[Bort62]

Conversely, I've always had the best luck running vacuum advance off of manifold vacuum.

Both are correct, it just depends on how you set it all up.

Try increasing your accelerator pump shot.

 

[kuhlou]

thanks for the responses. Questions:
I set the initial timing with vac disconnected/plugged. But, set idle with vac hooked up. Do I need to set the idle before hooking up the vac to the dizzy? Thanks.

Bort, haven't messed with acc pump before. Is the screw the adjustment? And, screw in or out to increase acc pump shot? Change adjustment how much, 1/4 turn, 1/2 turn?

Tony please help me understand your comment on the timing. Due to dizzy vac hooked to manifold, when tromping the throttle, manifold vac drops, thus retards the timing?

Thanks for both points of view, I'll keep fiddlin' and post again.

 

[fordconvert]

Tony please help me understand your comment on the timing. Due to dizzy vac hooked to manifold, when tromping the throttle, manifold vac drops, thus retards the timing?

Same thing would happen if you were on ported vac, when you stomp the throttle there is no vacuum anywhere so it dont matter where you hook up. The difference is on manifold vac at ilde / closed throttle you will have full vacuum advance so there could be a period where its retarding but I have never found that to be an issue. If you have a problem in that area I would suspect and go after other issues before I worried about that one.

 

[kuhlou]

thanks tj, my trouble on the timing/vac is that I just don't understand the workings. Does the vac to dizzy allow timing to advance at idle/hi vacuum, or does the timing advance at lower vacuum. Also, as rpm's rise, the dizzy spins faster, does timing advance as rpm's rise for this reason. Sorry, brand new to this and trying to understand. Thanks!

 

[BIGREDRASA]

The traditional dizzy, Non-Load-O, has a vacuum diaphragm to advance the timing under high vacuum, and centrifugal weights to advance as speed increases. This is an attempt to provide the proper advance under varying loads and speeds. For instance, climbing a steep hill at moderate speed in high gear, vacuum drops, and timing retards to avoid pinging. As you cruise at higher speeds at part throttle, you get max advance from both sources. That is part of what's called the "Advance Curve." On pre-catalytic engines, specially MOPARs, carb and dizzy adjustments were nerve-wracking.

The "Ported Vacuum" port on the carb provides no vacuum at Idle (Throttle closed), then progressively provides close to manifold vac as the throttle opens. My experience was that using manifold vac would either make my engine ping at part-throttle take off. It also gave a very high idle speed, which can be ususally be adjusted with throttle and mix screws. On late '60s and most '70s Ford engines, there was a temp controlled valve to switch over to manifold vac when the engine got hot. This was intended to raise idle speed, and improve cooling.

If you use the carb's ported vac port, timing will never retard below what it is at idle. (Unless you use a dizzy with dual diaphragms. Most people disconnect the retard diaphragm.)

 

[fordconvert]

From a logical thinking standpoint if you think of it as a vacuum retard its easier to understand. The mechanical advance works independent of the vacuum. The mechanical is with the weights and springs and works based strictly on RPM. More rpm more advance. Its varied by the springs and size and shape of the weights. When the throttle is closed vacuum tends to be high so it pulls advance when the engine can handle it. When the throttle opens or the engine is under heavy load the vacuum drops and the vacuum portion of the advance which retards the timing which happens to work out because under load the engine cant handle as much timing. Thats why things like tractors and boats dont have the vacuum advance because they tend to be under constant load.

 

[kuhlou]

ok, thanks for the help fellows. Now to go back and tinker some more. The explanations help a lot. just so I understand, set initial timing and idle speed with no vac connected?

 

[Harte3]

What Bort62 said. Try it both ways to see what it likes. I can run either way with my present carb with Timed or full Vac off the carb base but it idles better with full Vac and I get 18+ hg and a very smooth idle. Other than at idle it runs out without any discernible difference.

 

[BIGREDRASA]

One thing I haven't seen is, which tranny do you have? It makes a difference. Either way, set timing with vac plugged. Set Idle with vac hooked up, if you're using manifold vac.

 

[kuhlou]

One thing I haven't seen is, which tranny do you have? It makes a difference. Either way, set timing with vac plugged. Set Idle with vac hooked up, if you're using manifold vac.

c4 trans, guess I need to get that in my description. What would be the difference? Thanks.

 

[BIGREDRASA]

For one thing, we take off differently with stick & auto. Timing for manual is 6*, and auto is 12* BTDC (Before Top Dead Center).

 

[kuhlou]

I have timing set to 6*, rather than 12*. I backed off from 12 to 6, due to pinging problems under load, going up a hill, etc. Would giving a bit more advance help with the "stumbling" problem when leaving a stop? Thanks.

 

[BIGREDRASA]

Absolutely. You can turn the adjustment inside the vacuuum diaphragm to reduce the pinging under heavy load.

I set mine by starting up a hill in 2nd gear, advance until it pings, then back off 1/4 turn at a time.

 

[kuhlou]

have not adjusted the vac advance before. Understand it is inside the vac nipple? Turn the adjustment clockwise for more, or less advance? Assume a hex key does the vac advance adjusting? thanks.

 

[kuhlou]

set initial timing back to 12*, idle and low rpm cruise improved a great deal, very little surging. I do get some pinging going uphill, when the tranny shifts from 2nd into drive. Now is the time to adjust the vac canister? Turn the adjustment in which direction to back off timing? Also, when I hook up vacuum line after setting initial advance, the timing mark disappears. I assume that's because the vac advance has kicked in. To know how much advance is beyond initial, do I need to put more marks on the balancer? What is the correct method for doing so? Thanks.

 

[BIGREDRASA]

As I said before, I've always used the ported vacuum from the carb, just as the factory did. When you do that, timing should be unchanged on hooking up the line to the dizzy, unless theres something wrong with the carb.

On this type of dizzy,

the vacuum advance is done by adding shims to reduce advance (increase vacuum needed for advance), and removing increase (reducing vacuum needed).

 

[kuhlou]

ah! I left the vac on the manifold. I'll try swiching it to the carb port. What I'd expect to see is 12* at idle, then advancing as rpm's move up? To measure advance at higher rpm's, are marks added to the balancer? If so, what method is used to determine where the additional marks are placed? I read a little on this in another post, but did not understand. Thanks.

 

[BIGREDRASA]

I added marks, but there are fancy timing lights "Dial-Back" I think, that let your read more.

To check mechanical advance, leave the vacuum disconnected, so only centrifugal advance is working. To adjust/check vacuum advance, have idle below 1000 RPM, and use a vacuum pump to increase the vacuum and record the amount of advance. If push comes to shove, you could retard initial timing to TDC, so you can read up to 14* BTDC off the damper. You're just using the engine to spin the dizzy. Remember, engine speed is twice dizzy speed (1300 = 650).