850 A Series Misfire

[haaallloooom]

Hi all

I am after some inspiration for a chronic misfire on an A-Series motor (single Hs4 carb) although I imagine the principles apply to any carb'd & coil ignition motor.

It starts OK but after 30 seconds or so it develops a regular misfire. Disconnecting each plug lead in turn seems to make no difference.

The coil, points, condenser, distributor cap and plugs and leads are all new (and all except the plugs & leads are from Distributor Doctor so tested).

Compression across the cylinders is relatively consistent (95-110psi) so hopefully this rule out head gasket or valve issues.

Is the only remaining option a blocked jet or similar? All help/ inspiration gratefully received!

Many thanks

[kevins]

I'd start by confirming the ignition is OK, one of those lights that goes in the plug lead or a timing light on each spark plug, if that's ok check for air leaks on the induction side or a blocked exhaust and valve clearances, after that take a look at the carb and fuel delivery.

[vitesseefi]

Not blocked jet as one carb feeds all and SUs don’t really block. Are you running this with choke, does the misfire start when you put the choke in? Does pulling the choke out help? Does the misfire go as the revs are increased?

If yes to above, possibly inlet manifold air leak, perhaps at the head face (could affect two cylinders more than the other two because shared ports.

Beyond that, new unfortunately doesn't mean perfect for the ignition parts and if you have one of those cheap electronic ignitions fitted they can do very strange things, especially if inadvertently fed reduced voltage via the ballast resistor (not always obvious)

Good luck!

[haaallloooom]

Thanks all,

Dad had the same thought at the weekend about the choke issue and air leak is now favourite.

Watch this space!!!