67 Coronet RT 440 HP surging

[Rene Niolet]

Hi,
Engine surges at cruise??
It doesn’t appear to surge accelerating or with a load on it but cruising down the road you definitely feel it.
Don’t have history of the engine but fuel system is all replaced except for fuel cap.
New plugs, plug wires, distributor cap, rotor and electronic pick up in distributor.
Any ideas???
Thanks in advance,
Rene

 

[wagonman]

Back out the idle mixture screws about a half turn and see if it helps. Sounds like its a bit lean on the transition circuit.

 

[R413]

what carb is on there? Type and cfm?
what ignition box?
What coil?
Where is the timing?
Is the fuel old or has the car been run alot and fuel is good?

I knew a guy that put a 500CFM carb on a 440hp to get good mileage. Then wondered why it went 17.50 in the 1/4 mile. These people are out there running free among us.

 

[Cranky]

Lean is what came to my feeble mind too. At what rpm/mph is it surging?

 

[Cranky]

what carb is on there? Type and cfm?
what ignition box?
What coil?
Where is the timing?
Is the fuel old or has the car been run alot and fuel is good?

I knew a guy that put a 500CFM carb on a 440hp to get good mileage. Then wondered why it went 17.50 in the 1/4 mile. These people are out there running free among us.

500 is kinda small for a 440 lol. My old room mate put a 500 2 barrel on his 289 65 Mudstain with 389 gears and 4 speed and went faster than it did with the 600 that he had on it but that doesn't mean a whole lot. The 600 could have been junk....

 

[F4R/T]

Check and see if it’s when your vacuum advance is coming in or out or both back and forth.

 

[turbine68rt]

Back out the idle mixture screws about a half turn and see if it helps. Sounds like its a bit lean on the transition circuit.

The idle mixture screws are used to obtain the best quality idle characteristics, vac reading, etc. Once the throttle is moved off-idle they're taken out of the equation.

 

[wagonman]

The idle mixture screws are used to obtain the best quality idle characteristics, vac reading, etc. Once the throttle is moved off-idle they're taken out of the equation.

My 68 with a 3310 Holley surged at 35mph ever so slightly, backed the screws out 1/4 turn and surge is gone. OP can try it and if it dont work he can return it to the original setting easy. I think that when itis just off curb idle and transitioning it is still on the idle slots a bit.

 

[AR67GTX]

Check and see if it’s when your vacuum advance is coming in or out or both back and forth.

I second. It sounds like it could be too much vacuum advance at highway cruise causing ignition too far ahead of TDC. I have an older Corvette I had this problem with too much vacuum advance - some call the sensation trailer hitching. It feels like a constant light miss. Give it the gas - which causes your vacuum to drop out and back the vacuum advance back - and the motor will pull strong and smooth. Easy to check to see if this is your issue, after driving the car to note the issue, pull over and plug the vacuum advance hose and see if it stops. If so, you will need to come up with some way to limit how much vacuum advance your vacuum can contributes. I built a stop for the one in my Corvette and limited vacuum advance to 8 crank degrees to fix it.

 

[66Satellite47]

500 is kinda small for a 440 lol. My old room mate put a 500 2 barrel on his 289 65 Mudstain with 389 gears and 4 speed and went faster than it did with the 600 that he had on it but that doesn't mean a whole lot. The 600 could have been junk....

A 500 2 BBL worked fine on my 440.... but it also had a 500 in front of it and one behind it! LOL

 

[Cranky]

The idle mixture screws are used to obtain the best quality idle characteristics, vac reading, etc. Once the throttle is moved off-idle they're taken out of the equation.

The idle circuit stays around for a bit longer than that and when you think about how much the throttle blades are open at low speed cruise.....well, it's not much.

 

[turbine68rt]

The idle circuit stays around for a bit longer than that and when you think about how much the throttle blades are open at low speed cruise.....well, it's not much.

https://www.badasscars.com/index.cf...ct_id=105/category_id=60/mode=prod/prd105.htm

 

[Cranky]

There's lots of issues that could be causing it but it doesn't hurt to do the easy stuff first to see if it makes a difference......

 

[Cranky]

https://www.badasscars.com/index.cf...ct_id=105/category_id=60/mode=prod/prd105.htm

Sorry but I don't buy that train of thought......and many told me an engine has no use for 50 degrees of timing too. So, I still stick to my guns to at least try it and see if it makes a difference. Btw, I've had the 'displeasure' of tuning on two engines that liked 50 degrees total of timing!! And yes, I checked the first one with two other timing lights to make sure mine wasn't lying. There's always that exception that makes you scratch yer head....or ***.

 

[Rene Niolet]

Thank you all for the great info.
I’ll try the idle mixture first and then plug the vacuum tomorrow.
1.Quick Fuel Slayer 750CFM.
2.Mopar electronic conversion w/ Mopar controller.
3.Good 93 octane no ethanol.
4.Coil for resistor
5.Timing at idle vacuum disconnected is 8/10 BTDC I believe.

 

[Mackman]

Just solved that issue with my 67 R/T 440 HP. Too much vacuum advance. I had to drop from 24 degrees vac. advance at the crank to 15 degrees at the crank.

 

[Rene Niolet]

I removed vacuum advance line and plugged it.
Drove car no help!!
Reattached vacuum line and adjusted both idle mixture screws and seemed to help a little.
Driving at 40/50 mph I’m getting intermediate vibration/surge.
Driving me crazy

 

[Rene Niolet]

Intermittent not intermediate

 

[AR67GTX]

Well, the other thought is I’ve had a couple instances of a bad plug wire (the 66 Corvette again) and in a newer car, a weak/bad coil pack. Both missed intermittently at light throttle in high gear at low to medium speed cruise - but in both cases, flooring the throttle and downshifting the engines would rev out just fine. Something to do with system voltage and load/rpm on the engine I think. Might check your plug wires.

 

[Rene Niolet]

New plug wires with ceramic boots.