engine flat spot/ misfire

[rah59]

Hi all

hope you can help. I have a 64 fhc 3.8 fitted with a lumitron optical ignition. The car ticks over and will rev happily under no load on the road it revs and behaves well up to 3500 however it misfires upto 4000 (more hesitation) it wont go above 4000

i have just replaced the HT leads to no effect, the plugs are NGK and are fine,i have a new standard dist cap and rotor but different to whats fitted, i suspect that was part of the lumitron kit (no detail in history file

any pointers at what to look for

thanks

robin

[caveman]

Hi Robin,

I had very similar symptoms a few years back with my 1965 4.2 fhc with luminition electronic ignition that was fitted during restoration. It misfired when warm at 3500rpm and above and only under load (driving).

As I had a spare new Lucas rotor arm in my toolkit (as well as a Lucas sports coil, condenser and points), I fitted it and the misfire improved but did not completely cure it. I removed the electronic ignition and re-instated new points and condenser and the engine ran perfectly and it continues to!
I checked my original rotor arm off the car and later found that it was tracking to earth when hot through the centre mount of the distributor.

I hope this helps in some way.

Regards

Steve

[rah59]

Thanks for your reply, i had replaced the HT leads to no avail and looked at the rotor arm. This appeared ok but i suppose you cannot tell at the voltages present,being a conversion the distributor is a 45D6 so the rotor and dist cap aer different to the standard And the ones i bought. Had come to the conclusion that a123 might be the answer but will try the cap and rotor

thanks for your help

rob

[Heuer]

Have you checked the air filter? Please try running the car with open carb inlets and report back.

[rah59]

Hi David

the air filter is ok the carb su's all operate in sync when the air filter assembly is removed with all pistons risong and falling.

I have taken the car of the road now for unrelated issues once these are resolved i wo will sort this out and report back


regards

Robin

[rah59]

Just got car back following investigation into fault, i was advised that the cam timing was out allowing inlet and exhaust valves to be open at same time. This resulting in a back fire? At first i thought this unlikely until i thought about the end of the exhaust stroke and the begining of the intake strok. I suppose the fault would get worse as piston speed increases

robin

[christopher storey]

The inlet and exhaust valves are open at the same time , and indeed they should be. The cam timing is 15/57/57/15 , so that the inlet opens 15 crankshaft degrees before TDC as the piston comes up on the exhaust stroke , whereas the exhaust valve does not close until 15 crankshaft degrees after TDC on the exhaust stroke . This 30 degree period when both valves are open together is called overlap , and is there to promote scavenging, whereby the inertia of the exhaust gas travelling into the exhaust port creates a suction which helps to pull the fresh charge through the inlet port . Virtually every engine has some overlap, and some have a great deal more than 30 degrees .