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Need advice from the engine wizards [message #281789]

Thu, 09 July 2015 22:44

kerry pinkerton

Messages: 2565
Registered: July 2012
Location: Harvest, Al

Karma: 15

Senior Member

My 403 has a problem. I had a burned valve and low vacuum. Found the bad valve and had the head done by a local shop. Put the engine together last week and the motor has issues:

- Low vacuum at idle 10 inches of mercury.
- A random miss that you can hear in the carb and at the exhaust pipe. Very noticeable at 2500 RPM.
- Motor acts lean. Slight improvement in the miss if the choke plate is closed a bit.

Compression runs from 200 to 188 PSI. Pretty dang good.

My car buddy neighbor has been working with me on it. He's more knowledgeable than I am and I'm OK...not great but OK. I've built a few engines and done my own tuning for 50 years.

Our first effort was to find the vacuum leak. We did all the spray test and nothing. Pinched off all the vacuum lines...no change. Finally decided the leak had to be in the intake manifold or a cracked manifold. Pulled the manifold and noticed it was an EGR model. Plugged the holes in the intake runners with brass pipe plugs. No change. Thinking the intake was cracked, I found an aluminum factory manifold and got new gaskets. Some work to drill and tap a couple things but it fit fine and was much, much easier to handle. We used silicone on the gaskets and got a good squish. The crossovers are blocked with stainless plates and the Mr. Gasket gaskets are cut out for the plates and a good bit of silicone holds them in place while the intake was set. We're confident the gaskets are good. We used modeling clay to verify the manifold fit the heads and also checked for straightness on the mating edges. I had high hopes this would solve the problem...no change. I am running a brand new Patterson carb and swapped it out for my old original carb...no change. Changed the distributor cap, coil, module and even the pickup coil...no change.

At this point we were both stumped and slept on it. Today, we verified the harmonic balancer (which is relatively new) is correct. The timing is set to 11 degrees. Vacuum was ported but we discovered it ran a bit better with a little more advance so we're currently on manifold vacuum and switched back to the new Patterson carb...no change. Pulling the spark plug wires one at a time from the distributor results in a noticeable change in sound and drop in RPM Getting a good spark about an inch long as the end of the wire gets close to the distributor cap. Vacuum doesn't change much. Today we removed ALL the vacuum lines, replaces the brass fittings with new one and plugged all the holes....no change. Made sure the spark plug wires are separated. Currently vacuum is about 12-13 and the needle swings rapidly 10-15 at idle. Observed the same readings on three separate vacuum gauges.

I talked to Jim Hupy and he said that if we can hear it in the exhaust (you can feel the flutter too), it is an exhaust valve or spring. We had previously pulled the valve covers and observed the valves working. While we did not measure them, they appear to be operating normally and consistently. Jim suggests a leak down test. Tomorrow I will borrow or buy one and perform the test. I will probably also swap out the Patterson distributor for my original, plug wires and all just to eliminate things.

We've been grasping at straws and eliminating possibilities for the last 10 days and the straws left to grasp at are getting pretty sparce. ANY ideas and suggestions are welcome and encouraged. As it stands now the coach is not running well enough to go the the Dixielanders rally in a couple weeks and the boss is getting frustrated also.

Kerry Pinkerton - North Alabama Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L