Short answer,
a compression test, even if it has to be done 'cold', is universally accepted as _the best method_ of checking the condition of an engine.

Slightly less than 150 psi for an 8.5:1 engine 'cold' with wide open throttle, closer to 175 psi for a 9.0:1 engine.

ken

I guess you will have checked this too but have you cleaned out the carb, making sure all the jets are clean, no crap in the bowls and the inlet filter is not full of little bits of old fuel?

Tev
simply swapping the whole engine is a bit of an extreme as its likley to be something simple , and there is no guarantee that the new engine wont need the same amount of problem solving to get it running. stick with the one youve got running and solve the small problem- or asses how bad it is with a compression test.

here is the stages id go through to get an engine running.

Tappets 0.25mm
New- as in out of the packet new not ones that have been used for a bit and left lying about points and condenser( or points assisted gizmo)
Points gapped at 0.45mm
Plugs, new and gapped at 0.65mm
Timed to static using the peg 8* A line on the flywheel at *8 and the 7th tooth round marked for next bit.
Fresh petrol

the 2-4-6-8 rule
And then start it with the fan off and the lid off the points box.

Then, using auto motive multi meter set dwell (points gap) to indicated 53* (4cyl 4 stroke) attach strobe and with meter still attached and set to rpm run the engine up to 3000rpm and check advance and set to 2nd flywheel mark watch for scatter.
Switch off engine and if there was minimal scatter re assemble all the other stuff, if there was scatter take cam off re fit 180* and repeat, still scatter find out which side is too high and dunt the cam nose.
Check for oil leaks.
Then id make sure there were no leaks on the intake by spraying round the carb and the manifold with flamable liquid if all ok id go ahead and tune the carb.
Once all that's done its quite warm so i do a comp test and write the results on the cowling anything that has the comp more than 5psi difference side to side or much (10psi) below Kens figures then id look at getting new rings for later. Id check the breather with a manomiter as well and either re furb an old one or get a replacement on the shopping list too.

You can generally tell a bit about the state of an unknown engine by a few tell-tales is it oily, if the internals of the points box are dirty, the rockers and inside of the covers dirty? if so its going to need breather and or rings at least.

Cant stress how important the new condenser is, Every year local farmer gets D*lly out the shed and comes to me ( in his c2) with it not starting , this year it started got to the mot station and back before the condensor failed. Its lucky if it does 600miles each year and the condenser always fails I put a Velleman kit on it last week so it should be good to go next spring.

Neilsthebass all good but a carb wont affect one cyl more than the other unless there is an additional leak on one side

Sean

I always change the condenser when changing points!

Yes, brand new condenser, fresh from ecas. I even tried disconnecting that and wiring up the old one and it still wouldn't go.

Points set to 0.4, got a friend to check it to.

Didnt open up the rocker covers but they were correct 100 miles ago or so

Plugs again nearly new and correct gap (as in done less than 100 miles or so), both look fine.

Had the timing double checked by Spanners a little while back... he seems to know what hes doing

I will probably go through and try doing all of the above again - alas I dont know what else to do!

I think I am going to have to try to find someone I can borrow a compression tester from - still, I doubt it wont start due to low compression - thats not generly the sort of thing that just suddenly stops it working(?)

If you did tappets 100 miles ago, maybe the locknuts weren't fully tight when it went back together.

You can use mine tev.

Either means you make a trip to Stratford if urgent or will bring to next WASPS.

True, though from memory they hadnt moved in the 50 miles since I had checked them befor. Still, its worth another look

then id be looking at Gingerbloke and Neils suggestions of an inlet leak on one side and a carb that is marginal.
Spray or dribble something combustable -petrol wd 40 ezistart etc around the carb base and around the manifold joint if it increases in tickover then there is a leak at that point.
And the down and dirty way to clean out the carb is to get it running at fast tickover and place the flat of your hand over the mouth of the carb lifting off before it stalls completely, this sucks the petrol up through all the drillings flushing them out.