No oomph

[e-richard]

Jane (1937 Opal Tourer) has just completed a full bottom up engine rebuild. Rebore, replace every nut, bolt, stud and gasket. Looks beautiful.

But she just won't go properly. Compression OK, starts fine, turns over sweetly, firing neatly on all 4 cylinders. Take her on the road and seems OK to start with, 2nd gear and we're gradually moving forward. 3rd gear and we seem to have reached a limit. Any slight incline or change up to 4th and its reached its limit. Just won't go any faster. Probably doing about 15-20mph (Speedo not working). Then we get backfiring from Carb., and a we build up a snake of traffic behind us.

Does the above description of behaviour suggest anythingh to any of you experts out there?

So far, we have checked

• Plugs all OK
  
• Carb has been tested, adjusted all OK
  
• Running with new Accuspark electronic distributor
  
• New coil
  
• All valves seem smooth and tick over sweetly.
  

What next?

[Hugh Barnes]

Timing?

[Alan]

It’s too retarded

[John Cornforth]

Hi

Some more possibilities:

Engine very tight. Unlikely as it would affect idle and you would notice on the starting handle.
Exhaust system blocked. Rare but not impossible.
Intake air blockage. With no air filter and such simple manifolding this is almost unheard of.
Accuspark not advancing timing with revs. I believe 'bob weight' style advance is still used internally. A strobe light and marks on the camshaft pulley is the way to check. Insufficient advance is unlikely to cause backfiring though.
Low tension supply to Accuspark insufficient. Check Ignition switch, wiring and grounding, including distributor to engine block.
Fuel flow insufficient

From the details given, I think restricted fuel supply the most likely, especially if the tank and pipes have been unused during the overhaul. Can you blow back down the pump input pipe into the tank and feel airflow ? In desparation can you try a temporary gravity feed with a small (e.g. lawnmower) tank ?

As usual, I am prepared to be proved wrong !

[e-richard]

Brilliant replies. Thanks all. We'll now gradually work through ALL of these and try to report back on the magic solution.

[John Cornforth]

Hi again

One other thought. Did you fit suppressor caps to the plugs or use resistive HT cables ? As with any electronic unit, the Accuspark might be being upset by the high levels of ignition transients ('hash') radiated from unsuppressed copper HT cables.

["Slack Alice" Simon]

One tooth out on the timing gears?

[Mike Costigan]

Agree, Simon, that was my first thought. Alternatively, the backfiring could be an air-leak.

[e-richard]

Mike, an air leak - where?
I could just hunt..... 
but what is your thinking?

The timing tooth sounds very plausible, but maybe multiple problems ?

[Mike Costigan]

Carburettor flange most likely, but possibly manifold to block joint. Dribble a little oil round the joint and see if it gets sucked in.