help- 318 running on 4 cylinders.

Car si a 66 Plymouth Poly 318 with an Auto. car has always been down on power but figured its a big car so its not fast. Well it started haign a low idle miss.. when in gear at a stoplight or basically just idling in gear the car runs a little rough. going faster seems to be okay and if I take it out of gear it seems better.

I decided to take a look today- Plugs are fine, wires are good, cap is good, changed the rotor. Points gapped good. I tried pulling spark plug wires at the cap while it was running. If I pull any wire for the drivers side of the engine there isnt a change. In fact I can pull all 4 wires on the drivers side and there is only a slight drop in the idle. If I pull any of the wires for the other side there is a noticeable drop in idle. If I pull two wires the engine gets very rough- 3 wires will near kill the engine.

So I pulled the valve cover- lash is still good and all the rockers seem to move good. Running a compression test- if cranking for 4 seconds I get about 125 psi for every cylinder- If I crank for 7 seconds it goes to 150 psi.

So what am I missing??? What can cause the cylinders on on one side to run weaker than the other side?

 

If you slide a screwdriver up the plug boot, or use a tester, are the plugs getting spark?

This is the kind of thing you'd hook a car up to a scope to diagnose, I think, and who does that anymore. I'm not getting any ideas how to diagnose that, everything I'm thinking to look over you can rule out by it being all one bank running weak.

Just a couple of dumb things that are easy to rule out:

Where is the flap valve for the exhaust crossover under the carb? I assume this has that. Check and see that it's not broken and stuck closed. If you can move the counterweight by hand, the spring is broken or loose.. wire it open and see how it runs. No other restrictions in the exhaust off this bank?

Also, check the oil. Sometimes if they get too low the lifters don't fill, the valves don't open or open wrong, and you get a misfire condition.

 

I agree with him. Almost has to be exhaust related, or a rocker arm issue. Lots of stuff can mess up 1/2 the engine, but those things would be firing order related, not a right bank/left bank issue.

I'd be looking at the heat riser in the exhaust manifold being stuck closed, or a bent/ collasped exhaust pipe on one side. Next thought would be a rocker arm shaft being loose, an intake gasket that has moved and is sucking air, or, I euppose a blown head gasket loosing compression, though you compression test does not inducate that.

While your in there, change the timing chain, if its not been done recently (last 10 years). A scrwed up carb can take out 1/2 the engine, but it would be 2 per side weak. A bent dist shaft, or worn cam lobes in the dist can mess up a good engine, but it would be pretty unusual to take out one bank or the other. Gene

 

Thanks for the help guys-

Timing chain is new- Intake gaskets are new and I already fought with a carb being too rich on one side causing 2 cylinders on each side of the engien to be rich but this is now all the cylinders on one side of the engine.

The exhaust thing is somethign I had been thinking of because the engine has a flapper in the passenger side exhaust pipe that I have no idea if it is open or closed because the arm is broke off- I was going to get new exhaust soon and check it when I pulled the old stuff off. The only problem is the flapper is on the passenger side and the side which doesnt change the idle when you pull the plug wires is the DRIVER SIDE.

I will see if I can pull the exhaust and check the flapper anyway and see what happens- I just gotta make sure I dont break the studs off as this is my daily driver.

Compression check seems to be good so I dont think its a bad head gasket.

 

Oh and yes the spark plugs are firing- I pulled each wire, plugged in a spare plug and grounded it against the engine and they all fire the same.

Also- No wet plugs and they are all a redish color.

 

firing order?

 

Is it dual exhaust? I once had a piece of metal from a corroded muffler get jammed sideways in the muffler baffles and block flow... took me weeks to figure it out.

 

firing order

 

If you have a plugged jet an engine will do a similar thing. OldWolf

 

I would agree, but it is one bank of cylinders and the poly 318 has a 180 degree intake manifold so a plugged jet would affect two cylinders on each bank

 

If you know it's sparking and the plugs come up dry and clean, you have to have an idle jetting issue/major intake leak.

 

Firing order is correct- I also have fought that issue before ;-)

Intake was good when I pulled it and I just replaced the gaskets and im sure they are torqued proper.

Exhaust is single with the drivers side crossing over to the passenger side then runnig back. There is a peice of flexible tubing from the drivers side to the passenger side- I will pull it off and see if thereis any restrictions anywhere.

ajmopar is right- The intake feeds 2 cyls on one side and 2 on the other. If it was a jet issue I would have issues on both sides. I can change the jets side to side and see if any change.

 

Okay I pulled the exhaust off tonight and there is no obstruction. I ran the engine with the exhaust off and the driver side seems to put out as much pressure as the other side, but doesnt put out as much heat.

So pressure tells me exhaust valves are opening enough, but less heat tells me maybe the intake valves may not be opening enough. Im thinking I may have a cam problem.

The rockers are all moving what looks like enough and the lash is correct but I am wondering if maybe they arent moving enough. I guess tomorrow I will pull both valve covers and measure the lift at the edge of the rocker and compare drivers side to passenger side...

At this point I cant think of anythign else that would cause half the engien to act like this...

Anyone else have ideas?

 

I would put the motor on TDC and make sure it is not the timing chain First.
You said you check the points, but your condensor might be the problem. Since you are checking the cam check your valve springs they may be broken.

 

Ill give it a try but I dont know how I will spray anything into the valley. No openings in the inake.

 

Intake manifold internal cracks? Maybe try another intake manifold. What's the vacuum gauge do?

 

Ok, start with the basics. You say it has good compression. You say you have spark, it's not misfiring or farting around. Just not firing on one bank. Cam is ok or comp would be weak. Fuel is ok? Start it and put your hand over the carb without the aircleaner and let it draw against your hand, like a choke half on. Does it pick up cyls? If it does, you have a lean carb problem or intake gaskets leaking sucking air. If it stays the same, you have a compression problem and your gauge is lying. Another thing, make sure the spark out at the plugs is blue. If it's pink the condenser is bad. Pink spark won't shock a fat girl off a tri-cycle, it's weak. It has to be one of the three, spark, fuel or comp. Lippy

 

Wow, that's exactly what my old truck is doing, exactly! My cure, buy a parts car with a good running, lower mileage motor, seriously. The one in it has more problems than I want to address and besides, I got it from a guy with a foot even heavier than mine so, I figure it's just worn out. Sorry I can't offer any help diagnostically but I can commiserate?

 

Now we are getting somewhere- Spark is defintiely pink. The points and condensor are new but maybe bad- Also the coild is a new accell coil with the ballast resistor removed.

I will replace the condensor again and check- But why would it only effect one bank.

 

Did you check to see if the engine drops off when you pull the plug wires while the exhaust was off?

I'm thinking that red spark plugs inducate a lot of rust in the gas, you may have a lot of junk inside the carb, and that junk could be floating around messing you up.

Exhaust temp differences may be as simple as one side running a little richer then the other. Are you checking the temp at the ends of ther manifolds on both sides? Checking one from a pipe and the other from the manifold will give you different temps.

I'm thinking you have several small issues, begining with the pink spark, then the rust in the fuel system, then the flex tubing on the exhaust. Everything is complicated by not having actual test equipment that can pinpoint the actual problem. A scope where you can short out each cylinder and watch a tach for known rpm change, and read the actual dwell and timing would probably solve your issues. Got any old time repair shops around your area that may still have a scope in the back room and still have someone that understands how they worked? Gene

 

lol!!!!

 

Is the distributor dirty or rusty inside. I have had black deposits that apparently made a path to ground when the charge was high enough. I statred changing the condensor but just cleaned it all first. The engine ran much better. That was in a 67 Dodge that would kill a condensor every few months.

 

Yes I pulled plug wires witht he exhaust off- No change on drivers side.

I changed the points, condensor, cap and rotor today- Spark is now white (slightly purple) and no change in the original problem.

I sprayed starting fluid around the intake, plugs and headgasket area- No change.

Choked the carb a bit and tried pullign plug wires- No change in original problem.

measured lift of the rockers from one side to the other- No diiference.

I did find one thing that didnt look good- To gap the points I rotate the distibuter until the points are at full lift. I noticed that the distibuter cam seems loose- When I twist the distributer one way all is good but if I turn it backwards, the cam does not move immediately- It moves after about a quarter inch of movement in the distributer body- I dont know how good/bad that is, but its still not something I would think would cause my problem.

I still dont understand how this could be a fuel issue if its effecting all the cylinders on the same side. I can pull the carb apart and check it out (Holley 2 barrel) but that just doesnt make sense to me.... Of course this problem isnt making sense.

I dont have cash to take the car somewhere- And I dont think many around here have classic car experience. I dont have a dwell meter, but dwell being off isnt going to nearly kill 4 cylinders on one side. I have a timing light- Car runs best timed at about 18-20 degrees... Even though spec is 10.

Im not a complete idiot but this has me stumped. Like I said earlier its my daily driver and I just need to get this damn thing fixed.

 

What does a vacuum gauge show?

 

I havent checked Vacuum yet- Since idle was choppy I assume the vac would be the same- Will test and report.

I will set up something to short the plug wires out and see what happens and report back.

-Joe-

 

Yes I have run a compression check-

If I crank the engine for 3 seconds I get about 120psi on each cylinder.

If I crank for 7 seconds I get 150psi on all cylinders.

Compression seems fine.

 

Did you ever change the plug wires? could be that when your pulling them out of the cap, the wires are shorting out to the next wire in the firing order. Any dark spots on the wires where they are against anything inducates a place where the spark is jumping through the wires. One or 2 weak plug wires would mess you up when you put the wires out. Gene

 

Plug wires are new- And when I pull them I am removing them at the distributer and plling them away from all other wires.

I tried shorting the plug wres as directed and it acts the same. I let the spark jump from the pin coming from the cap to a grounded wire and the spark still looks weak. Everythign in the ignition system is new- Should I try to regap the points to a bigger gap? They are at .016 now.

Vac guage shows flucuatign between 17 and 19hg when plugged into the manifold vac on the bottom of the Carb (not the vac advance but full manifold port) with the car idling.

 

Anyone else have any ideas? Im really stumped on this one.

Its still driveable and I drive it to work every day but I cant help thinking if its only really running on 4 cylinders that it will be alot more fun when all 8 are running right.

 

have found this>carbon gunk buildup on the valves closing off intake opening ...would not show on compression test ...if oil return holes not letting oil return out head or?seals guides ect