69 L36 With Elusive Miss

[Michael C.]

Earlier today I asked for help regarding the vacuum advance hose connection location. I eventually found an AIM drawing that seemed to confirm that my connection was correct as one of three hoses on a three port fitting on the intake manifold. A photo from a fellow member provided further confirmation.

I raised the question because a repair shop is having trouble finding the cause of a consistent miss that causes as much as a 200 rpm swing. This miss occurs at idle all the way to 3000 rpm and higher. When the hose from the vacuum advance to the intake fitting is removed and the intake side plugged, the miss "almost" disappears. The ports for the power brakes booster and vacuum tank were also blocked with no discernible effect.

The shop replaced two defective plug wires and also replaced all plugs, distributor cap, rotor and points. The Rochester MV4 was also "rebuilt." The timing supposedly set at 4 deg BTDC. I don't think they've tested the centrifugal advance because the miss is present at all rpms. Before taking the car to the shop I tested the vacuum advance itself with my Mity-Vac and it does operate and doesn't leak down. I don't know that the shop has tested vacuum advance timing. Doesn't seem like this would be relevant for a miss at all rpms.

I'm using non-ethanol 91 octane, which is the best I can buy locally. No additive.

In addition to PB, the car has PS and A/C (currently removed). The A.I.R. is also removed and the vacuum port on the carb is plugged.

Any suggestions will be appreciated.

Thanks,
Mike

[Dan P.]

Re: 69 L36 With Elusive Miss

A miss… not sure about the swing in RPM, but the 427's of that era were very prone to bad valve springs - they break easily. They're hard to spot, and and the result is a pesky miss. If the springs are original, there's a good chance you have one or more that are broken.

I had a consistent miss w/my 66 L36 and found 2 broken springs. I changed them out myself w/a complete new set, and the car (56k original miles) runs like a sewing machine.

Good luck!

[Greg L.]

Re: 69 L36 With Elusive Miss

I'm not sure about the rpm "swing" either but when I bought my 69 L36 back in the 80's it had a miss too. I had a heck of a time finding it until my Dad said that it was probably a valve spring. Sure enough an exhaust spring was broke and the dampener was the only thing keeping things together. I replaced the spring and all was good for about a year and then the same thing happened...another broken spring. This time I changed them all and it's been good ever since. It has about 70K miles on it now so about 60ish when the first spring broke.

[Michael C.]

Re: 69 L36 With Elusive Miss

Sounds like we need to check the valve springs. Ugh!

Thanks

[Edward J.]

Re: 69 L36 With Elusive Miss

Mike, I would make a quick check to see if you have a vacuum leak some where. hose, base gasket, or just a lean condition with the carburetor. a easy check would be is to warm up engine so the choke is wide open, Then take some carburetor cleaner and spray a small amount down the throat of carb while the engine is idling, this will enrich the mixture, and if the engine idle does smooth out you have something to look for (lean condition), I had a friend last year that purchased a rebuilt correct numbers carb. for his 69 BB that had problems from the get go. things like stuck metering rods are common, float level set to low, Idle circuit passages plugged.Another thing is the mechanical advance that is stuck may be masking a problem because setting the timing to spec.s with a mechanical advance that maybe stuck in full advance will result in the engines timing retarded. causing the engine to not have a power loss.

[Greg L.]

Re: 69 L36 With Elusive Miss

Hey Mike I just re-read your post.

When you say a "200 rpm swing" do you mean that when it's missing that the rpm drops 200 rpm and then when the miss stops it picks back up? For example it's running at 800 rpm, starts missing and drops to 600 and when it stops missing it goes back up to 800? If this is the case then I really doubt it's valve springs, possibly sticky valves but not a broken spring.

My miss was mostly during hard acceleration or by quickly opening the throttle at idle and it was actually "spitting' back out through the carb while it was missing. It's amazing what a simple broken exhaust valve spring can do.

[Michael C.]

Re: 69 L36 With Elusive Miss

Greg, thanks for revisiting.

Your description is about right - down a couple hundred (or so), then back up. Down and up within a second or two. No pause during these cycles. Very irregular. I don't remember it missing during hard acceleration or quick throttle opening. I'll see if I can get more details or more clear description when I can get to the car tomorrow.