Lister D - Carb blow back

Hi all, I’m new to the forum but not entirely new to stationary engines. My lister d brings me here!!

Im in the process of recommissioning a lister D that hasn’t been ran for 10 years +(I haven’t ever seen it run). I have had to replace the magneto for a known good one (I think it’s a good one) and have checked/re-times as well as various other things before cranking. Whilst cranking over there is clearly pressure coming back through the carb, almost like it’s not timed correctly, it’s pressurising the petrol tank. I can’t imagine this is normal so am sounding this out here to see what advice I get before taking next steps. The pressure isn’t constant which, to me, doesn’t indicate leaking valves but can’t be sure. When the blow back happens a fine mist of petrol comes back through a hole on the side of the carb.
Any help or encouragement will be most appreciated.
Thanks
José

I cant believe its pressurising the petrol tank as the bleed off at the top of the bowl would allow the fuel/air out at this point and if the fuel bowl is full the float would stop the fuel from the tank, how are you timing the magneto, I time mine with the points about to open with the flywheel 50mm/2" before top dead centre

Maryalice
.

I agree with what maryalice says.
It may also be worth checking that the inlet valve has the correct clearance.

I should also have said:- check that the intake valve is not sticking, and is closing properly.

Hi all, thanks for your prompt responses. I'm glad to say that I now have a running engine!! I advanced the timing by a further 1/2", now sparking 2" before TDC. The blow back was being caused by ignition just as inlet valve was opening ;-).
The fuel tank wasn't being pressurised, I should have paid more attention as its a leak from around the filler that only appeared whilst I was cranking.

Briggsy,
You ought to check the timing again. If the spark occurs just as the inlet valve is opening, it means that it is one crank revolution out, i.e. firing close to tdc valve overlap position instead of: close to tdc end of compression stroke.
This does not apply if the mag. runs at crankshaft speed.