sputter/hesitation at highway speed

[Kosta M]

Hi,

I've had my 72 Cutlass S for about 6 months. For the first time yesterday I noticed it sputtering at highway speed (60-70mph). Seems like it's starving for fuel. Below 60mph everything's fine. When I try to accelerate past 60mph it starts jerking and wont go any faster. This is the first time I've experience it and have driven the car on the highway many times before at speeds above 80mph with no problems.

Engine is a numbers matching 350 2bbl and a TH-350 tranny and a non-posi highway geared rear end.

Fuel pump is a month old as well as air and fuel filters. I know the carb has issues (planning on a 4bbl swap over the winter).

Any ideas? Car runs great around town, no overheating, sputtering or anything. It's just at 60+mph and just started doing that yesterday.

[Brentman]

I'm grasping at straws, but what the hay.

Try changing the fuel filter out and see if that helps. if it does, then its likely the old one was clogged. Possible culprit is the gas you're putting in it, if your area has alot of ethanol blends.

I've been told that ethanol blends can loosen deposits in the gas tank and fuel lines. These deposites will then clog up the fuel filter. It takes awhile for things to get flushed back out again. So you're stuck swapping out filters periodically for awhile.

[Sgt Art]

You may want to check your ignition system, wires, plugs,points, dist cap and rotor. Sometimes a problem at speed that feels like it's fuel related is really electrical (ignition). I had a Corvette with a Mallory dual point dist that gave me fits. Points constantly burning out and it felt like a carb related problem. I finally fitted a Unlite system into it and no more problem. I recently replaced the point and condensor in my 72 Lincoln with a Pertronix system. It's really nice. Screws right into the old distributor and your ready to go. I also replaced the coil with the one Pertronix recommends.

[Kosta M]

Thanks guys.

So here's what I've done in the meantime to get ready for storage. I've replaced the plugs, wires, cap, rotor, points, condenser, pcv valve and put in a new fuel filter and air filter. Weird, now I have the opposite problem.

Car idles really rough, kind of sounds cool like I have a high lift cam but very low idle and very rough. Car is rough until approx 30mph. Car runs fine on the highway now, I hit 80mph to test it out with no problems.

I made sure I put the cap/rotor on in the same position, checked all the plugs for correct gap, now I'm at a loss. It seems to smell more like unburnt fuel now more than before.

Anyways, she's going into storage for the winter and as long as it can make the 3hr drive to the storage facility at hwy speed I'm fine until April.

Thanks for your suggestions.

[Sgt Art]

Okay, so basic tune up fixed your high speed problem, now we want a smooth idle. Do you have a dwell/tachometer? If not, they are pretty cheap and easy to use. The dwell part is used to make sure your points are gapped correctly. on an 8 cylinder engine, the each cylinder has at most 45 degrees of dwell (360 degrees/8) by this I mean the length of time the points are closed and the primary circuit is "holding back" the secondary circuit which is where the power is being made. You want about 30 degrees of dwell. The meter will tell you that and you can make adjustments with an Allen wrench through the little slide up door on the distributor cap. Let me stop and say if you already know all this, then fine, not talking down to you, but others may not. This is the first step in the tune up. For every degree of dwell you are off, timing is either advanced or retarded by 2 degrees. So if you have only 25 degrees of dwell, your timing could be advanced 10 degrees. Conversely, if you have 35 degrees of dwell, you are 10 degrees retarded and that will cause a low idle and crappy performance. So check that and see if your dwell is at 30 degrees (1 degree +/-). Now, comes the idle speed. So, switch over to the tachometer and see what the engine is idling at. I like an idle around 750 rpm in Park, which goes to 550 in drive (NOTE: SET THE PARKING BRAKE AND PROPERLY BLOCK THE FRONT TIRES - IF YOU ARE THE LEAST BIT WORRIED ABOUT THE BRAKE OR BLOCKS, PULL THE CAR RIGHT UP TO A TREE - A BIG TREE - BEFORE PUTTING IT IN DRIVE). With in trans in park, check your timing. I don't know the specs for your engine, but somewhere around 12 degrees before top dead center (BTDC) would be a start. You should remove the vacuum advance line from the distributor and plug it (golf tees work great for this) before checking the timing. If you have 30 degrees of dwell and about 12 degrees (or whatever your car's engine calls for) then your basic timing is set.

Once this all done, check the vacuum advance by reconnecting it. You should see a slight increase in rpm. This will tell you the vac advance is working. If not, make sure there is vacuum in the line. If there is, then the vacuum unit on the dist is probably defective and needs replacing, easy to do. You can also (with engine off) suck on the line with dist cap off and see if the vac advance unit moves that will confirm the canister is no good. Once you have established that timing is correct and vac advance is working, check the centrifugal advance. This the weights inside the distributor that sling outward as the engine rpm increases to compensate for it's faster speed by delivering the spark earlier. This is easy. Simply rev the engine with the timing light going. You'll see the timing mark move. If all the above is happening, you've done a proper electrical tune up.

Next, check for vacuum leaks around the carb. Using carb cleaner (that's what I use) spray a little along the seal between the intake and carb base. A sudden increase in rpm (watch the tach) will tell you there's a leak there and it's sucking in air. You can tighten the carb a little but don't go over board here. The torque spec for carb bolts to intake isn't much I think around 20 lbft. YOu can warp the throttle plate by over doing it. If the leak persists, you should get a new gasket. Also spray around where the intake mates to the heads and using a 9/16 box wrench, check the tightness of these bolts. Spray around any vacuum fittings to see if there are leaks there.

Based on your last post, I'm betting your timing is retarded either too much dwell or base timing is off. This means the spark is coming too late in the engine's cycle (after the piston has reached TDC) and unburned fuel is going right out the exhaust pipe. Also, power is down and your engine runs hotter.

[dbuff]

nice read sgt art