My 66 Starfire idles well, accelerates well even under hard acceleration. In short everything seems OK but. At cruising speed it surges. Fresh tune up, checked all vacuum lines, etc. It's a TH400. any ideas out there?

 

Disconnect and plug the vacuum advance and take it for a ride.

 

Did it surge before the "fresh tune-up" as well?

 

I'll try unplugging the vacuum advance, but it does respond when unplugging it during timing check. Yes it was surging before, that's why I tried tuning it up figured that would fix it. Not a serious surge but definitely noticeable. Thanks for the help.

 

If the vac advance is connected to manifold vacuum you’ll have raise the idle speed some.

 

I’ve gotta make a couple of assumptions here which may be wrong: 1) you have a qjet; 2) you have either a working points or working HEI ignition; 3) your compression cylinder to cylinder is on target - meaning no dead holes and variance of up to 10% or so hole to hole.

I run my ‘66 S/F (with later more or less stock 455) on manifold vacuum. It’s probably smart to be sure even after the tune up that your plugs & plug wires are working correctly.

One thing to try is to test ported vs. manifold vacuum to see what works best for your car, but surging may be related to an incorrect air/fuel ratio at part throttle cruise.

I chase my A/F ratios using oxygen sensors in the exhaust stream to prove or disprove that the mixture is right at idle, part throttle cruise and WOT. Check out Innovative Solutions for wideband oxygen sensors. Not required, but very helpful.

Part throttle on qjets is a little more difficult to turn than idle & WOT since, if you have a qjet, you have to combine the right jets/rods and _critically_ the power piston spring which meters gas at part throttle.

If it’s surging, I believe that means the part throttle metering is probably a bit lean, meaning you need a stronger (spring-ier), power piston spring.

You can get a selection from Cliff Ruggles website or maybe try qjets.com. The springs are not expensive, but you’ll need a few (5-6) qjet gaskets for testing - get 5-6 each of the airhorn gasket and carb-to-intake gasket. Those will allow you to test springs/jets/rods as you learn.

Hope that helps. I’m no carb whisperer, but I do have. ‘66 S/F which runs pretty well for the past 25 years or so. Great cars. Just ran it 80 miles or so today, pure delight on the freeway at 65-70 mph.

Cheers
Chris

 

Thanks for the replies. Your assumptions are correct. It's a healthy stock 425 and runs very strong. I did replace the Q jet over the winter with a rebuilt from I five. Supposed to be tuned correctly for this engine. But the more I check things the more sense it makes that your on the right track. I'm no "carb whisperer" either but will probably try your fixes. I really like keeping my Starfire all stock but wish Q jets were more user friendly. I agree, the Starfire is a great car and a joy to drive.

 

Tried moving vacuum from port to manifold, no change. Ordered gaskets and springs, have a couple different jet sizes. Will try this when I get the parts and post the outcome. Again, thanks to all that replied

 

Did you try disconnecting the vac advance all together and taking a drive?

 

Plug wires, did you change them when you did the tuneup? Just had this happen to me. Found out the hard but proven way, touched the wires when checking the oil, nice jolt. Now I know why my car was surging.... Lost in the fifties ...Tedd

 

Does it have plenty of power? If so, more than likely it’s not surging due to insufficient fuel.

Rig up a way to mechanically close the choke just a little bit. The idea is to richen up the mixture. Take it for another drive. If the surging is gone, the engine might be a little on the lean side.

 

In case it helps my 455 carbs are late model (75 &up) 800 cfm units. They’re set up as follows17057553 reconditioned in 2017.
Float 9/32”,
Primary jets & rods 74/49 (single taper)
APT 2.5 turns out
M secondary hanger, DF secondary rods. Idle screws at 2 turns out. Throttle plate holes 5/64”. Throttle plate notched to permit lower hole use. No phenolic cup in place of the anerobic altitude thing. Acts like more float bowl

Qjets are good in part because they’re very sensitive to part throttle air/fuel mixture jetting (and related “rodding”). The third part of jets & rods in a qjet is the power piston spring, which, if it’s too soft will cause the car to run lean. If you have a spare spring on hand, get some gaskets and test it.

Chris

 

The plug wires were new last year. Vacuum advance appears to be working properly. The car has amazing power at wide open throttle. Since this started after changing the carb I'm leaning toward spring and or jets on primary side.

 

Per OldCutlass about disconnecting the vac advance. I too had a light throttle cruise surge at freeway speeds. I found that when the timing was all in I had about 50 to 52 deg total timing. When I restricted total timing to around 45 deg. the surging went away. I made a limiter from a piece of hacksaw blade to limit vac adv to about 11 deg. I've had no issue since.

 

The vacuum advance choices we have today and related gas choices we have today are not as good as we would have liked.

I’m glad you arrived at a solution. There are a lot of ways to skin the timing cat, I tried various vacuum cans and got close to perfect timing but what solved it for me was the Bluetooth HEI ignitions from progression ignition. More or less, it removes the constraints presented by vacuum cans, springs and weights from long ago in favor of digitally controlled timing you direct.

I’m not sure how long they’ll last or if the software might one day be an orphan but for now I’m happy.

Happy motoring
Chris

 

The parts arrived so I opened the carb and found a light spring, #73 jets and best as I can tell by measuring B43 rods. Put in the medium spring, left jets and rods the same and reassembled. Fixed the problem at cruising speeds but still surging when pulling a hill at lower speed. I believe that's a low vacuum situation meaning it might need the heavier spring?

 

I'm not dismissing responses about vacuum advance and timing issues but since this started after carb change decided to head there first.

 

It's fixed. Left spring at a medium, put in 71 jets and 39 rods. Runs beautifully. Hard to belive carbs are that sensitive that a couple thousandths here and there. Thanks everyone for the help.