250 Six. Rough Idle. Stalls When Accelerated

[Roger Frazee]

I purchased a '68 Camaro 250 six last weekend, and drove it 300 miles to my home without a problem, except for a rough idle. The next day I added a bottle of Lucas fuel treatment to a fresh tank of gas. Suddenly, shortly thereafter, it developed a problem.

The car idles rough like before, and will rev up when I press the gas, but will immediately die unless I let off the gas. I cannot keep the engine running while holding the gas pedal down, even a little.

I took the carb (Rochester Monojet) apart, cleanded it and installed a rebuild kit. The problem is no better and no worse.

Any help or advice is greatly appredciated.

[Rusty_OToole]

How long was the car sitting? Did you check for bad gas? Old stale gas can gum up your motor carburetor and fuel system.

Sounds like fuel starvation. Was the carb full of dirt? Did you change the fuel filter? Should be one under the hood. They did not come with a filter under the car but you could check if someone added one. Could also check if the fuel pump is pumping enough fuel.

When I buy a car like that my first move is to do a compression test and a full tuneup. Of course if the compression is bad a tuneup won't help much but you can usually get them to run halfway decent

[Roger Frazee]

The carb was fairly dirty when I took it apart, but is clean as a whistle now. I ran two tanks of gas through the car before she started to misbehave, so I'm confident that the gas is fresh. I did replace the filter that is in the base of the carburetor. The old one looked clean, though.

Your thought about the fuel pump is interesting. I'll look into that next.

Thanks,

Roger

[Rusty_OToole]

You can test the fuel pump by disconnecting the gas line at the carb and sticking it in an empty 2 liter pop bottle or tin can. Have someone turn the key. The pump should pump a cup of fuel while the engine turns over 5 or 6 times. If it just dribbles out there is something wrong.Be careful to disconnect the ignition coil you do not want to risk a fire.

Here is a tricky one. When an old car loses fuel pressure sometimes it is caused by a leak in the fuel line allowing the pump to suck air. A tiny rust hole or deteriorated rubber line can do it. If the steel line rusts through it is usually where the line goes through a hole in the frame or in a support clip so it is hard to spot.

[Guest carlnut50]

Hi, These engines never idled as smooth as most engines. The fuel distribution is too uneven for them to idle smooth. Also, they never were very economical to operate either. However, they are rugged. Make sure the intake manifold is not loose and leaking at the gasket. The 3/8" bolts tend to work loose over time. You can usually hear a whistle at a gasket leak especially if you listen with a 2 or 3 foot piece of garden hose to your best ear and the other end searching for the vac leak at the manifold gasket or the carburator base gasket.

Make sure the accelerator pump is ok or the engine will go lean when you open it up. Also, check the distributor shaft for wear which causes uneven running. These engines were bad about loose dist shafts. This causes the points cam to orbit and fire uneven. Some years did not have an upper bushing at all and didn't hold up very well. Don't forget a compression check too.

I worked on these cars at a Chevy dealership when they were everyday transportation and the stuff I have told you was just what you did. Thanks.

[Larry Schramm]

Just a thought, but did you check the EGR valve for correct operation?

[Rusty_OToole]

Don't think a 68 Camaro has an EGR valve just a PCV valve. I think carlnut may have it, old six cylinder engines tend to vibrate the carb bolts loose and sometimes the manifold bolts.

[Roger Frazee]

Here's the latest update. I replaced the fuel pump (replacing a 43 year old fuel pump seemed like a good idea, regardless). That did not help. I disconnected the fuel line, connected one end of the hose to the new fuel pump inlet, and put the other end in a gas can. That did not help either, but it eliminated the possibility of a clogged fuel line. I checked the mechanical advance and vacuum advance with a timing light. Both checked out good. I checked for slop in the distributor shaft. No slop. I checked the compression. 130 pounds on all cylinders. The carburetor mounting nuts are tight. All vacuum hoses are plugged, but I still have the problem. Also, I cannot feel any suction at the air cleaner intake. Can it be anything other than a leaking intake manifold?

[Guest carlnut50]

If you believe one or more cylinders are missing or almost missing at idle, You can check how balanced it is firing by letting it idle and remove one plug wire at a time FROM THE DIST CAP. This way, it is very easy to find a weak or dead cylinder because when you pull the wire, little or no change will occur. Hope this helps.

[Guest WEB 38]

Do you have a vaccum gage? see how many inches of vac. that you have at idle strive for the highest that you can get by adjusting your idle screws. If you have the motor minder type it has a scale on it which will tell you a lot. Bill WEB 38

[Guest WEB 38]

Also check that all your spark plug wires are in the right order. Bill WEB 38

[Rusty_OToole]

Did you check the manifold bolts for tightness? I mean where the manifold bolts to the head?

You can spray some WD40 around suspect joints, the engine will speed up if it sucks WD40 into the leak.

[Roger Frazee]

The manifold bolts were tight. I pulled the manifold yesterday and laid a straightedge accross the flange surface. It has a high spot on the center intake flange. I am taking it to the machine shop today to have it milled. Thanks to all for your input. I'll let you know if this fixes the problem.

[Guest 47jag]

You say you've checked the timing with a light but in my experience the 230/250 cube engines the vibration damper shears the rubber bonding and the pulley moves thereby retarding the timing mark. One solution is to 'power time' the engine. With a warm engine, loosen the distributor slightly, rev. the engine to about a steady 2000 RPM (use the fast idle cam) and move the distributor slowly in the direction of the advance unit. The speed should increase slightly. At the best speed tighten the distributor and road test to ensure the engine doesn't ping. Check again with the timing light and if the timing mark is way out replace the damper.

Art

[Ron of Chicago]

If I am reading this correctly you are not getting any power under load.

How is the integrity of the exhaust? You can check for an exhaust blockage with the vacumm gauge. The vacumm should drop and then fairly quickly rise back to the high end. With a blockage in the exhaust the vacumm will recover to the high end very slowly or not at all.

Ron

[Roger Frazee]

Thanks, everyone, for your help. Those who suggested that the problem was in the carburetor, even after a thourough cleaning, were right. I finally purchased and installed a remanufactured Rochester Monojet, and the problem has gone away. As a bonus, I now have a new fuel pump, pcv valve, vacuum hoses, manifold gasket, and freshly- milled manifolds. Now -- I'm off to enjoy my Camaro!

[Guest 1hooligan]

I did not notice if you replaced (or added a inline fuel filter) the I have two 68 camaros, one just like yours. They are of the age, when the gas tank tends to have a lot of stuff (ie crap)

in them. I always place a second filter at the tank. It really fixes the problem before it get to the fuel pump and carb. Secondly gas tanks are cheap for them, under $150. including shipping when you pocket book alows, replace it. Pat

[Roger Frazee]

Pat, I have an inline fuel filter ready to install, but was hesitant because I didn't know if it was OK to put a filter between the tank and the fuel pump. I was afraid putting a filter on the suction side of the pump might cause problems. If not, I will install the filter tomorrow. I agree that a new fuel tank needs to be on the parts list.

[Guest 1hooligan]

No problem installing the filter!! The 230 uses gas sparingly, and the minute amout of resistance the filter causes will not effect it at all. I have purchased several tanks from these people. thier current listing on ebay for the 68 tank is #170634741299. Canadian made, good quality. Have fun. Mine is an automatic, with a bench seat, no air. She runs along well at 70 on the freeway (75 here in AZ.)

[rocketraider]

Torpedo, you have my utmost respect for sticking with this problem and making the Six run right. The vast majority of Camaro people would have yanked it and stuffed a crate 350 in its place.

That is why I will walk right past any modded or even a stock big-block Camaro at any show I visit, but will stop and look at a six-cylinder one at length.

To look at most Camaros at shows, you would think Chevrolet only built big-blocks or Z28. Running joke here was there were more Z28 in this city than the factory ever produced.

[Roger Frazee]

Thanks Rocketraider. The fact that this car is preserved, unrestored, and unaltered is what attracted me to it. The six-banger adds a little novelty.

One of the members of our region owns a '63 Impala SS with a 250 six. Talk about a survivor!