147 engine misfire/problem?

[147 Franklin Airman]

I am hopeful that someone can pin point my problem.

The engine starts fine and idles great, but when both cold and hot the engine appears to miss a beat and feels like a misfire, still pulls well but not as well as she could.

These are the checks that I have done and their results.

Compression on all cylinders is between 60-68 psi

Leak down test reveals leakage at about 30% in all cylinders into the crank case. No leakage through the valves. (probably because its all going into the crank case)

The plugs are burning very rich resulting and a lot of black soot out the exhaust but she never smokes.

I have tried advancing the timing whilst underload but no improvement, just makes it worse.

The valves were replaced with ones from a Massey Ferguson tractor back in the 1970's

The pistons were also changed and I note that they now only have 3 rings and not the 4 as on the Franklin drawings.

The carburetor float level is correct and she has new economiser parts. Jets have not been changed.

Rocker valve clearances seem to keep changing and I now check and adjust them every 2 weeks. I run her every weekend.

Valve cages have been levelled off and rockers sitting nicely on the valve stems. Push rods rattle a little but due to replace with the new ones I got from the club if I can get them to fit.

I have installed a conductive discharge unit to protect the points. All ignition components are new except the distributor body. Plugs are NGK AB6.

No inlet leakage, tested with water on joints.

Is she being choked by the rich fuel mixture?

[odyssey]

Knowing the mixture is too rich should be your first priority. Plugs should not be black & sooty, but rather a very light tan color on the porcelain. What kind of carburetor is on the car? Original U-3?

If the float level is correct, there could be an internal problem with the carb, causing it to run rich. I don't suppose it has an adjustable main jet?

tom

[147 Franklin Airman]

Hi Tom

Thank you for your suggestions. I am currently looking into the carb and trying to find more details on the U3. Main jet is not adjustable.

Paul has been helping me solve a lot of the issues with the car and my problem with the misfire has been narrowed down to inductive crossfiring with the HT leads. Simply taking the HT leads out of the metal loom and feeding them separately through the exhaust manifold resulted in amarked improvement. Oh and I have changed all the plugs back to Champion D16 after finding another 2 of the NGK not firing when in the cylinders.

It would appear that cars of this age do not like modern components without some form of adjustment elsewhere or without understanding the basics of how they were engineered in the first place.

I have realised that in order to work on these cars, one must put on the other hat and almost ignore modern practices and think like they did back in the 1930's.

All credit must go to Paul for his patience and knowledge in helping me to resolve the many problems that I have had.

Now all I need is an instant fix with my rich mixture but we are working on that.

Martin

[odyssey]

I agree 100%. Make them right and they will run as they did- which was quite good. I have removed lots of improvements on cars- some of which I installed 25 years ago when I thought I had to try to make everything better, or before I had learned how to restore difficult components.

I am especially a supporter of original ignition and fuel delivery systems (except for the V12)

We each learn our own way at our own pace however. I try not to tell people what they can and cannot do, but rather try to share some of my experiences and things I found while having fun with these cars. There may be better ways to make things run, but there is no 'right' way to enjoy the hobby.

Tom