I replaced the Thermoquad on my 74 Jensen Interceptor (replacement 77 Newporter 440 engine) with a Street Demon 750. I am getting lean surge at freeway speeds, around town is fine. When climbing a hill on the freeway I can get out of the surge by mashing the throttle for a little while. I have 14 inches of vac at idle, about what it ever had. Should I swap distributors from the 5 degree intial advance stocker to a Mopar Perf I bought 20 years ago and up the advance to 10 degrees plus before starting on the Street Demon calibration?

If it is lean surging at a steady higher cruise RPM I would increase the jetting a small amount(.002 to no more than .004) first I would also work on the distributor later to set the curve up so you have between 10 to 18 BTDC at idle and no more than 36 total revved up wth all the mechancal advance kicked in IHTHs

wouldn't hurt to note the in hg that the surge starts/ends. No surge when the TQ was on there correct? & no other changes ex the carb correct? I would richen up the carb mains a fair amount to stop the surging then dial in the dist sub systems spot on then final dial in the carb (with an LM1) if available

I ran 15 initial on my stock 76 440. Ran 20 with an aftermarket cam. 5 on a low cr smogger sounds awful low.

What Daytona said, bump the initial up to 15 & you'll see a dramatic improvement. you'll (then) need to alter the slots to get the total in line but I'd try it as for a start to get things rolling (plus richen the carb as needed). bottom line you want no pinging. Was the only change preceding the lean surge the carb swap? I wouldn't think the street demon would be lean but I dont know that carb. More info needed. Holler back

Thanks, guys. Yes, no other changes besides the carb swap.
OK fix surge first and then timing.
I am thinking I will put a weaker spring in the carb piston because that is easy from the top of the carb. If that doesn't help, do the jets.
I will start at 10-15 degrees with the Mopar Perf distributor and see how it goes. There are a lot of hills around here.

Follow Cab's advice.
Lean surge at cruise means main jetting is a little small. This could be because the engine has a weaker signal.
My example, a carb that worked very well on a squarebore 4-hole intake became quite lean when mounted on a spreadbore intake with an open adapter. Open adapter cuts down the signal or pulse strength, pulls less fuel through the jets, results in lean condition.

After you get carb right start on the ignition. In my experience a low compression engine feels much livelier with a quick curve. But don't try to fix a carb problem with ignition.

R.

True, but spark advance affects fueling. Fueling does not affect advance.

OK, I went one needle richer and went richer on the primary (89 to 91) and secondary jets(81 to 84) with no effect on the test drive at 70 mph climbing two hills. So I guess it is basicaly a fuel delivery problem? I have 120 gph Edelbrock electric back by the muffler with a heat shield and a stock Carter mechanical. I have the vapor separator plumbed in. 3/8 line up just before the mechanical pump, but 5/16 line into the pump and after the pump. I have observed that the return line takes away about 2 PSI, maybe I should take the separator out of the system?? The small pressure gauge at the carb is hard to read, but averages 6 psi.

yes but what is the psi at the freewway RPM/VAC when it is surging? And it was fine with the prior carb, & it was a freeway cruise (not exactly a WOT demand). I'd still say it's a carb issue (lean) or psi issue right at the carb (filter/orifice) that's restricting flow/making it lean if the carb internals needle/seat/floats etc are letting it flow enough which wouldn't normally be a problem unless some system is acting up inside the carb

I just bypassed the vapor separator and made the line and hose size 3/8 from tank to carb. If that fails the test drive, I'll put the gauge back on with long hose and go for another drive while watching the gage. Good thing my hood opens from the rear.