Surging at part throttle

I bought a 62 Skylark convertible sight unseen off of Ebay last year. I just got it home and drove it for the first time last night. The car surges and bucks at small throttle openings/steady state driving at any speed between up to 60 mph. Car starts and idles fine when cold but frequently dies when the clutch is depressed when warmed up. I have to tickle the throttle to keep it running at idle when warm. The car accelerates and runs fine at bigger throttle openings - when running up through the gears from stop to 50 mph for instance. The trouble starts when cruising at steady state/part throttle.

I had a shop put in Florida put in a new coil, points, condenser, distributor cap, rotor, plug wires, spark plugs, fuel pump, and fuel filter before having the car shipped up to Minnesota so those parts (in theory) shouldn't be the issue. They also set the dwell and timing.

Car is mostly original as far as I can tell but it does have an non-stock Edelbrock 4 barrel carburetor. I am not sure which model it is. Picture is attached. It has a single exhaust but it is LOUD so I suspect it is less restrictive than stock.

I suspect the vacuum advance is NOT working properly - maybe the vacuum diaphragm has a hole in it? I checked the vacuum hose from the carb this morning and it is pulling a vacuum. I suppose it could be a carb issue as well.

Any advice on where to start before I tear into this thing?

 

It sounds like it's running lean.

Could be a blown vacuum advance, leaking carb base gasket, leaking intake gasket, a simple disconnected vacuum hose or missing rubber plug.

Look for an obvious thing like a missing plug or unhooked hose. If none found get the engine running at idle and with carb cleaner spray small amounts around the carb base and intake gasket to see if the idle changes. Unhook the vacuum advance and plug the vacuum line to see if it's the vacuum advance.

 

Stacy,

It is very likely that the carb is lean at cruise. It's an aftermarket carb with unknown calibration/condition.

Assuming there are no other vacuum leaks...

I would do this to isolate the problem... remove the choke coil so that you can have manual contol over the choke flap over the primaries.

Then use a bit of duct tape, and tape the choke flap partially closed.. play around with it to find what it wants. By doing this, your forcing the carb mixture rich.

If the problem goes away, then you have isolated the source. Assuming everything is clean and in good working order, then your looking at changing the primary metering rods, to fatten up the mixture.

You would start by taking the top off the carb, and verifying that no foreign material is present in the bowl.. if there is, then a rebuild kit and good soaking is in order as a first step here. Many guys would do that anyway, with this type of issue, but that carb looks pretty new. See what it looks like inside, often times in cars that sit a long time, the fuel evaporates outs, and leaves behind a scale type residue. When fuel is applied again to it, that residue breaks up and goes everywhere.

JW

 

I just built a nailhead and it surged from 1600 to 2000 .As long as i was in the gas it ran well and had good Idle.I just this morning set the total by marking the balancer to 34 BTDC and the surge is gone .Right now I have the light springs/Bushing in the Distributor however I am going to change them and test to bring it in a little later .Also changing the plug wires helped also

 

Its a 215 engine. I had to take a Buick hiatus for a few hours while doing family stuff this afternoon but am back at it now. I have a vacuum gage and will start there followed by Jim's trick of richening it up by simulating a choke.

 

The vacuum advance diaphragm holds a vacuum so that is working properly.

I disconnected the electric choke and put a couple of strips of duct tape across the choke plate on the carb. Just enough to bridge the gap between the air horn an the now stationary choke plate. The engine seemed happiest running with that amount of choke. I ran it up and down the street a couple of times after warming it up and the surging is gone. I can run at a steady 50 mph without surging. The car kind of bogs a little in the mid-range when accelerating (too rich?) but seems to run fine at light throttle openings and steady state speed. I took the duct tape off and the surging is back. So maybe Jim is on to something.

I forgot to mention in the original post - the shop in Florida also drained the fuel tank. So the fuel in the car (about 1/4 tank) should be no more than 3 weeks old. However the car sat for at least 4 or 5 months with old fuel in it before I had it worked on and had sat for who-knows-how-long with who-knows-what-kind of fuel in it so I am guessing the carb is somehow gummed up. I don't know if the shop road tested the car before they called it quits so this problem may have existed in spite of all the other things they did to the car.

 

He's on to something IF you are not masking a vacuum leak from somewhere else. It was a simple diagnostic check to prove the engine is running lean, it does not tell you why it's running lean.

 

OK so I checked for vacuum leaks around the base of the carb using carb cleaner. Couldn't find anything obvious there. I then took the top of carb off and cleaned everything I could get to with carb cleaner and then sucked all of the gunk out of the float bowls. This wasn't a bad place to start because the fuel in the float bowls was the color and consistency of swamp water. I bolted everything back together and went for a ride and no change - same surging as before. I was able to figure out that the car has a Edelbrock 1406 Performer 600 CFM carburetor.

One thing I can say for sure is that getting old sucks. I can't see tiny carburetor-sized parts without bifocals. I can't see see through the bifocal part of my glasses unless I hold my head up a certain way. The hood on the '62 doesn't open that much so I as a result I am bent over straining to see and MY FREAKING NECK HURTS.

I have done all the old car stuff I can do for one day, I will start over at the top of the list tomorrow, probably with timing and double checking for vacuum leaks. Thanks for all the help...

 

Out of curiosity, or to cover all basis, it didn't get a tank of E-85 did it?

 

I would suggest changing fuel filter .you might not even have one .my edelbrock carb needed an inline.

 

Edelbrock 1406 carbs are famous for being lean out of the box. Even their website discusses the dreaded lean surge. Go buy a pair of 7347 metering rods and some pink step up springs and you'll be much, much happier with it. You may just want to buy the tuning kit. It takes five minutes to change both rods and springs.

 

The fuel in the car shouldn't be E85 - it came from Florida, not Iowa! The original style inline filter in the glass bowl was replaced. I also checked the filter sock in the tank - it crumbled apart in my hand as soon as I touched it so it wasn't doing much good. I ordered a new one. Fuel cleanliness may not be causing my problem but it isn't helping the carb stay healthy.

I also ordered a rebuild kit and tuning kit for the Edelbrock carburetor. I did some reading online and richening up the mixture at cruise looks pretty straight forward and I will try that when I get the parts.

I also went to NAPA this morning to buy a timing light since I haven't owned one since about 1982. The guy behind the parts counter looked at me like I was an idiot for trying to buy such a thing. So I ordered one online from Summit Racing that should be here in a couple of days. Will post more details after my after I take a whack at this problem again.

This whole experience kind of forced me to get acquainted with the car in a hurry. Its pretty solid throughout but it is definitely a 20 footer. The paint looks like its from Earl Scheib. Underbody is solid and rust free. Except for the carb its remarkably original. Once I get the thing running properly I am going to enjoy driving it in the summer without having to worry about getting rock chips and nicks. I enjoy driving my other Buicks but am kind of paranoid about getting damage from road debris and other drivers.

 

Isn't a 1406 way too big a carb for a 215? Won't that in itself will make it harder to tune?

 

The 500 cfm carb is what is recommended for the 215. The 600 is to big. I've been going through a ton of problems on my 215 build with getting it running right. I've been on a standstill for a week or so waiting on a waterpump. But I am planning changing the edelbrock out for a 390 holley.

 

Probably is too big but it's what came on the car. The metering rods are easy enough to change on the Edelbrock so i will give that a try before figuring out plan B.

 

Even if you cure the surging problem, won't that large carb (for the engine) result in poor throttle response because of slow air velocity through the venturis.

 

I rebuilt the carb last night. It contained lots of sediment and contamination so that may have been part of the problem. I also changed out the metering rods for a slightly richer mixture at part throttle cruise. I also got a new pickup sock for the fuel tank. I will install everything and see how it runs if it stops raining long enough.