Engine hesitates and back fires while driving

[Guest Anselcop]

I have a 1949 Silver Streak with the flathead 6 engine.  It sat for a few years so I had the carburetor rebuilt, new plugs, plug wires, distributor cap and rotor.  I drove it 10 miles into town and it ran perfectly.  Stopped for dinner and about a mile into the drive home, the engine started to hesitate and and miss causing some backfires out the tailpipe.  I thought the car was starving for gas, but with it backfiring I know it was getting fuel and seemed like an electrical problem.  The car is still the original 6 volt with points and condensor.  I did not change the points, condensor or coil.....could one of those cause the engine to cut out and miss if they are old and worn?  

 

Dan

[Grimy]

It's time to look inside the distributor.  At least look at the points and clean them with brake cleaner or other NON-flammable solvent.  Point gap is 0.022.  While you're there, look at primary wires for chafing or bare spots.  Perhaps clean the connections. If you have a friend with a Sun machine, test the condenser--or replace it.  Dwell angle is 31-37 degrees, which trumps the point gap.  The point gap is a mechanical expression of the dwell angle on a UNworn distributor cam.  Way back when, while the earth was still cooling and I was in college, I had a 90K-mile 1950 Pontiac 6 that would not achieve that dwell range with points set at .022--I had to set the points at 0.014 to get in the dwell range due to worn lobes on the distributor cam.

 

One other possibility is the coil.  They tend to die when warm, as insulation fails.  A 6-volt VW coil will do.

 

A third possibility is fuel in a reawakened Sleeping Beauty.  Did you clean the tank?  Change or add a fuel filter?  But I'll still bet on electrical.

[Guest Anselcop]

I do have a new set of points and condenser I will replace this weekend.  I do know the wires from the coil connections to the distributor are original (only 6,700 miles on the car) and may need to be replaced.  The gas tank was drained and fresh fuel put in with a new fuel filter just before gas goes into the glass sediment bowl.  THANK you for the guidance.....I will keep you posted.

 

 

[Grimy]

Please do keep us posted--and please post some photos,  I'd love to see such a low-mileage Pontiac!

 

Other items to change if you haven't done so already: flexible fuel hose just before fuel pump, and brake hoses. And fully flush the brakes, at very least.  And pack the front wheel bearings. 

 

As I've often recommended here, suggest you install the world's cheapest and best coolant filter (ankle-high women's stockings, $6 for 20 at the drug store) in the upper tank of the radiator.  Drain off a gallon or so of coolant, remove upper radiator hose (you've changed the radiator and heater hoses, right?--much to do on a Sleeping Beauty). use a screwdriver or ratchet handle or other blunt object to push the toe of the stocking into the top tank, then fold the open end over the neck, reinstall hose--so that all coolant passes through the stocking.  The idea is that debris in the block, loosened by repeated heating-cooling cycles, will be caught in the stocking rather than clogging the radiator.  At 300 miles or so, drain off a gallon of coolant for re-use, remove the stocking and wash it under a faucet and re-install.  The amount of crud in the stocking will give you an idea of the frequency of the next "filter" cleaning.  Whenever you've used a chemical flush, REPLACE the stocking.

[Guest Anselcop]

I purchased the car from an estate sale.  The elderly gentleman owned the car since 1953 and his family got the car after he died 8 years or so ago.  I did a complete brake job, with all new master and wheel cylinders and lines, rebuilt fuel and water pump along with a very detailed examination of all the other mechanical parts.  The car had a paint re-spray in the mid-80's but other than that it is very original down to the interior and headliner.....and the original bill of sale from 1949.

[Grimy]

Very nice car!  Upholstery looks perfect.  Happy to hear you went thru all the brakes.

 

BTW on the points: If a capsule of distributor cam lube comes with the points, be sure to use it.  If not, ignition grease or even vaseline, but a little dab will do ya.

[Guest Anselcop]

Update:  I ended up installing a pertronix electronic ignition kit since all the wires in the distributor were in very poor shape.  But, after 10 miles the car still has a strong misfire and hesitates.....so I don't think the problem is with the ignition system.  Is it possible that after the engine heats up it vapor locks??  If I let the car coast at idle for 5 seconds and then start accelerating it runs fine again for about 20 seconds or so until it starts acting up again.  The fuel pump appears to have been replaced in the past 10 years but that does not mean the diaphragm is old and weak.  But again, it runs very nice up until the engine reaches operating temp of 190 degrees.  Any other things I can check?

[Bloo]

Is your heat riser stuck? It needs to be open hot. I don't know exactly what that looks like on a 1949, but on 99% of all cars its "weight down" (as far as it goes) with the engine hot. Look on the exhaust manifold directly below the carburetor. On my 1936 for example, the weight points straight up when cold, and rotates down almost 90 degrees, sticking nearly straight in toward the block when hot.

 

Some, (maybe all?) Pontiac 6's have a steel tube pressed into the intake manifold that separates exhaust from intake. It looks more or less like a piece of exhaust tubing. It is right under the carburetor. The fuel-air from the carburetor flows through the tube, and there is exhaust (for heat) around the outside of it. If it rusts out, exhaust flows back into the intake, causing the engine to run horrible. Maybe someone else can say for sure if the tube exists on a 1949. If you have the carburetor off for any reason, look down in there below where the carburetor mounts for rust holes.

[Guest Anselcop]

I did pull the exhaust manifold off to install new gaskets and the heat riser was stuck in the open position, but I will double check again.  And I know the steel tube between the  intake and exhaust was in very good condition when I installed everything together again.  The goofy thing is the car runs absolutely beautiful for the first 10 minutes; good power and very smooth so I'm pretty sure the steel tube is intact.  I may try an electric fuel pump near the gas tank to see if that will keep the fuel moving?  But with the backfire out the tail pipe it still seems like an ignition issue 

[Bloo]

Well. the pertronix should have taken care of any points issues. If its ignition I guess it could be the coil. They do go bad, but honestly not often. Maybe take the distributor cap and wires off (as an assembly so nothing gets crossed) and hold the cap up to a bright light and look for carbon tracks.

 

Take a multimeter and check the spark plug wires for resistance. They should all be really low resistance if they are solid core wires. If they are resistance wires, they should be a few Kohm, but all about the same with the longer wires being a little higher.

 

Measure from the plug end of the wire all the way back to the brass or aluminum contact on the inside of the distributor cap. If you cant get a good connection at the spark plug end (it usually turns out this way), use an old non-resistor spark plug at the spark plug end of the wire and touch your probe to the center terminal of the plug. Wiggle the connection if you have to.

 

If you find an open wire, replace the rotor too. Speaking of the rotor, look at the center of it, It will be one of 2 ways. It could be just brass. If it is, there should be a spring-loaded carbon brush in the cap. make sure there is. If the rotor has instead a thin piece of springy metal in the center, then there will be a corresponding contact in the middle of the cap, but probably not spring loaded. The important thing is that it is gonna touch when assembled and not leave a big gap in there. When you look you will see what I mean.

 

Thats about it for ignition. You could take a multimeter and check voltage on the positive side of the coil, and then check it again when it is screwing up. Or you could use a test light and see if it looks about the same during the cold and warm phase. The idea here is to see that the coil is getting enough voltage when things go bad

 

I would be really extremely surprised if its the fuel pump. A cold engine needs more gas than a hot one.

 

Honestly, all of that is grasping at straws considering the symptoms. I'll bet something is up with that heat riser and its boiling the carb.

 

 

[Bloo]

Wait a minute... How does the pertronix ground? Just through the breaker plate? There should have been a ground wire, a little short piece of special extra-bendable wire, grounding the breaker plate to the distributor case. Is it still there? and unbroken? I suspect the pretronix probably needs it as much as the points did. An intermittent connection there will get you a lot of banging in the exhaust.

[Guest Anselcop]

I believe the pertronix does not need a ground strap since I believe it grounds through the small screw that holds the mounting plate to the breaker plate.  But I can put the small ground strap back on.....I think it went from where the vacuum advance plate fastens to the breaker plate to the mounting plate.  The coil is also new when I put in the pertronix.

I will confirm the heat riser is open.   Is it possible when I had the carburetor rebuilt it is set way to lean?  I was told to run the car with the choke half on and see if that makes a difference?

[billorn]

Ive tried Pertronics on several 6 volt cars and never had good results. I think there system is made for 12 volts but it will technically work on some 6 volt systems so they advertise it that way but some cars just dont like it. But since its acting exactly the same way as before its probly not the problem. I would check coil and see what happens and maybe the cap and rotor. A cracked cap can make all kinds of trouble thats hard to find. If that doesnt help I would install the new points and condenser you have and get it set up right just to eliminate that possible problem with the pertronics. Solving problems like this usually just comes down to checking one thing at a time and eliminating things that it could be then moving on to the next one in the chain.

[Guest Anselcop]

Here are a few pictures of the heat riser position.  The one picture is the position of the riser on the car; and it is seized in that position.  The other picture is of spare intake and exhaust manifold I have with the riser placed in the same position as the one on the car.  It sure looks like it is in the open position but I have been wrong before.

 

Dan

[Bloo]

5 hours ago, Anselcop said:

I believe the pertronix does not need a ground strap since I believe it grounds through the small screw that holds the mounting plate to the breaker plate.  But I can put the small ground strap back on.....I think it went from where the vacuum advance plate fastens to the breaker plate to the mounting plate.  The coil is also new when I put in the pertronix.

I will confirm the heat riser is open.   Is it possible when I had the carburetor rebuilt it is set way to lean?  I was told to run the car with the choke half on and see if that makes a difference?

 

Points will not ground reliably without that wire. The symptom is generally rough running and a bunch of banging in the exhaust, exploded muffler, etc.

 

If the Pertronix is grounding through the breaker plate, it needs the wire.

 

Regarding the carb: If running with the choke partway on fixes it, there is likely trouble in the carb.

 

[Bloo]

3 hours ago, Anselcop said:

Here are a few pictures of the heat riser position.  The one picture is the position of the riser on the car; and it is seized in that position.  The other picture is of spare intake and exhaust manifold I have with the riser placed in the same position as the one on the car.  It sure looks like it is in the open position but I have been wrong before.

 

That heat riser looks like it had been apart recently and welded? Its stuck? Why don't you take it apart and fix it? On one hand I cant imagine it causing your issue if it's stuck open. On the other hand, the car is never going to really run right with it stuck, so its probably time to fix it.

 

The heat riser is different than mine, so I cant say for sure whether it is open. I will say that the pictures appear to contradict each other. The rusty one does indeed look open (plate tipped in toward the engine). On my Pontiac tipped in like that is "open" and directs most of the exhaust down the side of the plate and out the manifold outlet. If the plate is tipped the other way "closed", out away from the engine, some of the exhaust is directed up into the chamber under the carb, then back down, past the other side of the heat riser plate, and out the outlet.

 

The pictures appear to contradict each other, as the clean manifold's weight looks like a closed position to me. All the weight seems to be up high, and should drop in one direction or the other as the spring relaxes, probably toward the engine on a Pontiac 6. Heat risers do exist that work backwards, and jack the weight up when they open, but that is really uncommon. I think I saw it on a Jeep.

[Guest Anselcop]

Good catch.....I went back out to the rusty old manifold and when I put the channel lock pliers on the counter weight it spun on the shaft and the flap inside the exhaust manifold stayed in the same open position.  So that is why the counter weight was up and the flap was in the open position.  I'll take off the exaust pipe so I can see inside of the exhaust manifold on my car to verify it is in the open position.  I initially took the exhaust manifold off last winter to weld a crack shut.  I guess I should have taken the extra time to repair the heat riser when it was off.  I'll keep you posted on my progress.

 

Dan

[Guest Anselcop]

I pulled the exhaust manifold and the heat riser was about 75% open or allowing hot exhaust to flow around the intake manifold.  I split the intake and exhaust and closed the flapper  so it will not allow any hot exhaust to enter the intake.  It was rusted so I had to use pipe wrench to move it into the correct position.  I have it installed again but did not get a chance to test drive it yet.....hopefully that was the issue.

 

The first picture is the position when I removed the manifold and the second picture is with it closing off the intake.

 

Dan

[Guest Anselcop]

I took the car for a 15 mile ride in 90 degree temps and it ran perfect!!  I'll go on a longer ride this weekend but I think you guys might have solved the problem......THANKS

[DABJR]

I have a 51 Chieftain.looks like I will have to take my manifold apart and fix mine ,  it  runs crappy when I rev it up.And was making a weird noise from that area..Almost a clanking noise.Is there replacement heat risers? I'm kinda afraid of rusty manifold bolts  and the repair with my skills.Can I take off the carb and look and see the flapper? It look's like it is down in there a ways.The counter weight  does not move by hand.Never heard this car run cause it's new to me,  but I think it just got stuck or rusted out lately because my friend said it ran ok two years ago.Any help  with disassembly would be appreciated.

[Bloo]

Don't be in a big rush to take it apart.

 

There are 2 things to look at here, with regards to the heat riser:

 

1) The stuff in this thread, in other words the thermal spring and flapper.

 

If the shaft moves freely, and the flapper hasn't broken off, you wont have to take it apart. When cold, the flapper should be held so the exhaust goes up by the carb, then back down. The weight will be way up high, as high as it can go. The spring will have about a half turn of tension or so holding it this way. As the spring relaxes with heat, the weight should fall almost 90 degrees toward the engine block. Thats really all there is to it. If you can accomplish that without taking it apart, you don't have to take it apart.

 

2) The pipe that heats the fuel.

 

On most or maybe all flathead Pontiacs, the heat riser is actually heating a big piece of pipe. You would think that those manifolds are only cast iron. Not quite.

 

There is a piece of sheetmetal pipe, like exhaust tubing, under the carb. It has an inner diameter about like the carb throat, and is vertical, directly underneath the carb. The air/fuel from the carb flows through the pipe on its way to the intake valves. There is exhaust on the outside of the pipe. This is what the heat riser flapper blows exhaust on.

 

If this pipe leaks, the car will run horrible, because exhaust will leak into the intake manifold. If you take the carb off and look down the hole in the manifold, you are looking down the center of this pipe. Look for holes.

 

You cant see the flapper from the carb hole, because the flapper is in the exhaust.

 

Let us know how it goes.

 

 

Edited September 23, 2017 by Bloo (see edit history)