Low speed surge

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Ricks70Duster340

Ricks70Duster340

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I have a 408/727 in my Duster with 3.23 gears. The carb is a 750 VS (80508-10). The engine performs well at mid & upper RPM's but at low speeds (around 30 MPH and 1500 rpm or so) has a noticeable surge. Am using the stock 6.5 PV. Am wondering if the IFR needs to be opened? Never done that before.
 
Have you checked your vacuum at idle with dist vacuum plugged?? You should have 13 inches of vacuum for a 6.5 PV.
 
Last edited:
You said
Ricks70Duster340 said:
At idle, I'm at around 7.
Click to expand...
low idle vacuum can be related to;
1) a big cam,
2) too-tight valve-lash
3) late idle-timing
4) vacuum leaks
But you are NOT at idle...... you said;
Ricks70Duster340 said:
at low speeds (around 30 MPH and 1500 rpm or so) has a noticeable surge.
Click to expand...
So what is your cruise vacuum at 1500/30mph?

By this time, the vacuum should be high enough that the PV is closed.
Make sure that;
your Vcan is working, and not hunting,
your low-rpm ignition timing is stable and not bouncing around,
your coolant temp is up to ~180,
your bowl-vent is not being blasted by the fan.
Then, if the above is all good;
My guess is that your idle mixture screws are in too far, probably due to an improperly adjusted Transfer-Slot exposure.
Here's what I would try;
Rev it up to ~1800rpm and set it on the nearest fast idle cam. Now readjust the mixture screws for highest rpm. Then, on a Holley-type carb, richen it up about 1/8th turn. Kick the fast idle off. This should get you a mixture-screw adjustment of near to 3/4 to 1.0 turn out. If so then;
go roadtest it.
If the engine does not idle decent at this setting, it is probably the Transfer slot that will need to be reset, and then; the idle speed brought back into line with ignition timing. That is to say, at idle, your engine is idling on a combination of mixture-screw fuel and transfer-slot fuel. Since we set the cruise mixture with the mixture screws, we can't move them..... so you will have to reset the idle-mixture with some combination of Transfer slot exposure underneath the primary throttle blades, and or Secondary A/F if the carb has a 4-corner idle-system. To take some of the guesswork out of this, set the TransferSlot exposure to just noticeably taller than wide; some say .040 tall; then re-adjust the secondary side (if you have a 4-corner-idle carb).
If nothing seems to work, and your engine has solid-lifters, reset the lash just a lil looser, and start over.
If you do not have a 4-corner idle, then most likely the engine will want some idle-air bypass.
 
7" of idle vac becomes a can of worms. If this carb has a choke, try this: engine idling, slowly close the choke blade until the engine note changes. Back off the blade a tad & tie/secure in place for a test drive. If surge has gone, it is too lean. But why?

With only 7" of vac, I would expect waaaay too much T slot exposed, which upsets the idle/cruise transition. Also, assuming the low vac is just due to a big cam, the engine is going to need a TON of idle timing for best idle & off idle response. Maybe over 40*. 12-15* is certainly not going to cut it.
 
AJ/FormS said:
You said

low idle vacuum can be related to;
1) a big cam,
2) too-tight valve-lash
3) late idle-timing
4) vacuum leaks
But you are NOT at idle...... you said;

So what is your cruise vacuum at 1500/30mph?

By this time, the vacuum should be high enough that the PV is closed.
Make sure that;
your Vcan is working, and not hunting,
your low-rpm ignition timing is stable and not bouncing around,
your coolant temp is up to ~180,
your bowl-vent is not being blasted by the fan.
Then, if the above is all good;
My guess is that your idle mixture screws are in too far, probably due to an improperly adjusted Transfer-Slot exposure.
Here's what I would try;
Rev it up to ~1800rpm and set it on the nearest fast idle cam. Now readjust the mixture screws for highest rpm. Then, on a Holley-type carb, richen it up about 1/8th turn. Kick the fast idle off. This should get you a mixture-screw adjustment of near to 3/4 to 1.0 turn out. If so then;
go roadtest it.
If the engine does not idle decent at this setting, it is probably the Transfer slot that will need to be reset, and then; the idle speed brought back into line with ignition timing. That is to say, at idle, your engine is idling on a combination of mixture-screw fuel and transfer-slot fuel. Since we set the cruise mixture with the mixture screws, we can't move them..... so you will have to reset the idle-mixture with some combination of Transfer slot exposure underneath the primary throttle blades, and or Secondary A/F if the carb has a 4-corner idle-system. To take some of the guesswork out of this, set the TransferSlot exposure to just noticeably taller than wide; some say .040 tall; then re-adjust the secondary side (if you have a 4-corner-idle carb).
If nothing seems to work, and your engine has solid-lifters, reset the lash just a lil looser, and start over.
If you do not have a 4-corner idle, then most likely the engine will want some idle-air bypass.
Click to expand...
Hey there AJ, good to hear from you.

I'm not using vacuum assist with the current setup. I pulled the carb and set to expose more of the transfer slot. Idle speed went way up so had to bring it back down. Running the engine at 2000 rpm I adjusted the idle mixture screws and got the best with them at 2 3/4 out. Previous to that the screws were 2 1/2 out to get best idle speed. Carb is a 3310 style with only front mixture screws. Engine has hydraulic roller lifters. On a whim, I installed another carb I have (new Holley ultra 750 double pumper). Idle mixture screws on that only need 3/4 turn out to get best RPM. No low speed surging with that one either. I much prefer the vacuum secondaries so want the VS carb to work. Initial timing seems best at 17 degrees. Centrifugal advance starts to come in around 1200. When I adjust the idle speed screw to increase the engine speed (up to 2000 for the idle mixture screw adjustment) the engine really revvs up fast as I hit 1200, so thinking the engine likes that timing coming in!
 
Ricks70Duster340 said:
I have a 408/727 in my Duster with 3.23 gears. The carb is a 750 VS (80508-10). The engine performs well at mid & upper RPM's but at low speeds (around 30 MPH and 1500 rpm or so) has a noticeable surge. Am using the stock 6.5 PV. Am wondering if the IFR needs to be opened? Never done that before.
Click to expand...
Stick some wires in the PIABs
 
I have a thought, why not just run the Ultra? 1/2 throttle or less, the engine doesn't know the difference. Maybe if the DP blows the tire off down low with your 408, but mileage or drivability shouldn't be much different.
 
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