Hello -

My 327/300 L75 in stock form oscillates about 100 rpm at idle in random fashion. If I set the idle at 700rpm the car will idle up to 800 and back to 700 but not in a regular pattern. You can feel the random "bumpty bump" as the car sits at idle. I am guessing the bumpty is something causing the engine to stutter and the bump is the engine catching itself.

I changed the plugs and it seemed to help a little but not completely. The engine runs strong otherwise. No problems. Any ideas to eliminate the rough running? I should probably change out the dist cap. Points worn? Fuel flow issue with the Carter carb? Thanks in advance for the help

 

Have you done a complete tune up? Use a vacuum gauge when you do. Have you checked for a vacuum leak? Dennis

 

Sounds like an vacuum leak to me also. But u cant rule out carb or dist either.

Vac leak check with some carb cleaner spray. U can usually see dist cap/rotor erosion/arcing wear visually. Points worn? Usually u can just treat them with a file and set the dwell - just make sure your dwell meter has a good battery, dont ask how i know this.

Carb? Of course check/change the fuel filter. The Carter AFB is fairly rugged with few faults. But any carb can have worn inlet needle and seat or leaking float. Also cars rarely driven have a lot of fuel varnish sediment build up as the fuel bowls dry out. Those AFB style carbs are fairly cheap and Edelbrock sells their own so a tool box carb just for comparision shouldnt be too expensive. Just a known good spare carb to bolt on for testing comparison is fairly quick and helpful.

Distributor can have sticking weights a bad vacuum advance can (that tiny diaphragm inside wont last forever) or even shaft wear with high use/age. Funny but i found my advance weights were completely frozen which stopped any mech advance but in your case the weights could be sticking or the springs could be broken. Again a tool box distributor can be a quick help and the 1 wire HEI with the coil in the cap are cheap to buy and fast to hook up if needed.

Hope this helps ya some.

 

All good advise. Only thing I can add is to check spark plug wires.

 

If your car is staying near factory idle, distributor weights shouldn't be involved. You can always rubber band them closed as a test. Most likely vacuum. Check your firing order too.

One other issue if you get to 'chasing' the perfect idle you can get your carb so out-of-whack that your throttle plates are too far open and the transfer slot is over-exposed and you'll never get the idle correct and mixture screws are ineffectual. You can back the idle speed screw out and adjust your mixture screws 1-1/2 turns out from lightly seated and start over.... Definitely use a vacuum gauge and note its readings carefully: http://www.secondchancegarage.com/public/186.cfm

The animations at the bottom of the above web page should help. A vacuum gauge is one of the best engine diagnostic tools the DIY fella can have.

 

Truer words are rarely spoken and it amazes me how many guys who work on their cars don't own one or know how to use it, including myself until I received this tip years ago. A vacuum gauge tells all, the mechanical equivalent of a medical stethoscope and thermometer.

Dan

 

I think a new vacuum gauge is in my near future! Thank you for all the advice. I will let everyone know what I find. I too think it is a vacuum leak. I recall on a 70 I owned years ago it had a similar problem and it was a leaky check valve in the network of vacuum hoses on a C3.

I see three vacuum hoses on my L75. 1) The PB booster is connected into the manifold. I am checking the connection at the intake fitting to be sure it is properly clamped. The hose is a little short so it doesn't reach the intake manifold (short by about 1 inch). Thinking of disconnecting the hose and capping the fitting to see if that is the culprit.

2) The vacuum canister on the distributor is connected to the carb in the front. Visually it looks good and I checked the fitting at the carb. Save further investigation for later if needed.

3) The carb from the back side is connected to the long (black) cylinder tube with the chrome cap in the front end of the intake manifold. The chrome cap has a cork gasket and the cap will spin all the way around....so I have to stop tightening it at its maximum point of resistance. Not sure if this is the way the cap is supposed to be or should it tighten completely like a lid on a jar?

I'll be getting vacuum gauge should anything that I have currently checked etc doesn't solve the problem.

Thank you all.

 

Well.....with my new $25 vacuum gauge hooked up and having previewed a number of YouTube "How to" videos I fired up the L75. I also thought it would be wise to check the timing so I had my timing light also hooked up.

The vacuum needle was shaky which I have heard could be from a cheap vacuum gauge. But the needle also oscillated a mg or two of pressure in both directions. So something was not right. What seemed ok was the vacuum change to a "snap throttle" (things you hear on You Tube!) as the vacuum first dove towards zero and then back above the initial reading for an instant when the throttle closes and then back to its normal state.....in my case about 18mgs.

So.....thought I should check the timing and I was surprised that the timing was WAY beyond the 8 degrees advanced mark. As I adjusted the distributor I also had to adjust the idle screw and settled on about 12 degrees advanced. I adjusted the idle air screws and again the idle setting all the while watching the vacuum gauge which now was steady at about 17 mgs. No more up and down. Engine sounds great and after a short drive I adjusted the idle to 850 rpms and listened to an L75 that now sounds like a smooth L75.

How did the timing get so advanced? My own fault from advancing it several months ago without a light thinking I would hear the pre-detonation pinging f it was too advanced. Never had any pinging but I must have been far too aggressive in my adjustment. Hate causing my own problems!!!

Thank you all again for your help. I learned a lot about engine vacuum as a diagnostic tool.

 

I hope you had the vacuum line to the distributor disconnected and plugged when you checked/set the timing - that's the way it MUST be done, or you'll think the timing is way over-advanced.

 

Congrats on learning your way around a vacuum gauge. Timing "by ear" is a matter of experience and you can't always hear light detonation, you can cause yourself some grief going that route.

Timing light is far and away the best approach...

 

After you set your base timing correctly, per post #9, then you should post a short video of your vacuum gauge with the engine idling.

There's a better than even chance that there's no problem with your engine, whatsoever. If you have no drivability problems, then I wouldn't worry about it.

Also, make sure that the advance weights are free, clean, not binding, and VERY lightly lubricated with machine oil or white lithium grease.

 

I know that the dist line needs to be plugged but I DIDN'T REMEMBER to do that! Thanks John.

My guess is that since the engine ran so well at where the timing is currently set that the "truth" lies between the prior "WAY" advanced setting and the new (more retarded) setting I locked in today.

Tomorrow is another day.

 

If you reduce the efficiency of the engine by retarding the initial timing the engine will want more fuel and the carburetor throttle blades will need to be opened a little further causing a richer mixture and better fuel distribution at idle.

The engine will also create more heat in the cylinders because the piston has longer to compress and gas the mixture and the combustion will be better.