Surging

[moparmucelli]

hi guys I had a problem with surging a while back and I got a lot of input on it and I was messing around with the vacuum advance I had it in manifold vacuum and I plugged it into portal vacuum and it seems to run better why is that?

 

[Caliscamp72]

Manifold vacuum sources allow vacuum to advance at idle with throttle plates closed AND as they open .
Ported vacuum sources give no vaccum signal at idle with plates closed and only after they open enuff to expose the "port" .

 

[Caliscamp72]

Manifold vacuum would be giving you full VAC advance at idle . Part throttle cruise is mainly where vac advance "works" by design.

 

[moparmucelli]

Oh ok I see but why was my car surging with the manifold vacuum?

 

[RustyRatRod]

It was probably pulling too much timing in with manifold vacuum. You need to re-adjust the timing since you changed to the ported vacuum side.

 

[BigBlockMopar]

Ported vacuum possibly hides the issue.
*Usually* surging is a lack of fuel.

 

[Caliscamp72]

Was the engine surging at idle ? Like rpm hunting ..... Or are you saying it surged at cruise ? Like you physically feel an extra load coming and going while at speed ( as if you were lightly applying brakes or like when an air conditioner kicks on and off on a smaller engine)???

 

[moparmucelli]

Only at cruising speeds

 

[moparmucelli]

Yea rpm hunting

 

[BigBlockMopar]

Install an AFR gauge.
Best tool for tuning a carb.

 

[RustyRatRod]

Surging at highway speed with OBD cars is almost always a vacuum issue. No reason it would be any different for non computerized cars.

 

[moparmucelli]

Ok I will look into it

 

[383Duster]

Blown carb gasket is possible..

 

[moparmucelli]

Ok

 

[crackedback]

Surging at cruise is usually too much timing or a lean condition.

 

[moparmucelli]

Ok thanks

 

[mcmlvii]

Surging at cruise is usually too much timing.....

This was the problem I had, as my 383 car was surging at highway speeds and light throttle. I disconnected the vacuum advance and the problem went away, so I took the distributor apart and found the replacement vacuum canister I was using was marked 13 (distributor) degrees, so it was putting in 26 (crankshaft) degrees advance at high vacuum. The canister arm is sometimes (usually?) stamped with the degrees of distributor timing it will impose at high vacuum but you need to double that number to get "crankshaft" degrees - mine was stamped 13, thus 26 crankshaft degrees. I wrapped some wire around the canister arm to limit travel and it fixed the surge problem.

The original vacuum canister on the car had a hole in the diaphragm but the arm was stamped 9.5, so it would put in 19 crankshaft degrees of timing at full vacuum. I didn't measure the timing advance after the wire modification but figure it is about in line with the original canister now.

 

[AJ/FormS]

Once steady-state cruising, it shouldn't make a difference where the can is getting signal. At that point total available is total available.

However I have seen some combos not pull the same vacuum from the two sources, and if the can was stiffly sprung, the two sources could be pulling different totals

 

[halifaxhops]

See if that vac canister is adjustable with a 3/32 allen wrench