Misfiring engine

[PeterW]

Help please before I go mad !. After a full rebuild of my 1933 Austin 7 RP , when starting up the engine misfires . It will run with misfires , and run better at higher revs with the occasional misfire . However , when in gear and under load , it constantly misfires and hardly moves without cutting out . I've replaced the coil , replaced the condenser , replaced the leads , and replaced the plugs . The points , rotor arm , and distrbutor cap are in good condition and the spark acrooss the points is excellent....blue and fat ! . The carburettor has been overhauled , jets are clear and the choke and float operating correctly . I'm using premium petrol for temporary start up  before adding an additive . Before the restoration started ( mainly body and interior ), the engine was working perfectly without problems . The rebuild has taken three years .
Any suggestions would be very welcome as I'm nearly demented . Thanks to all in advance 

["Slack Alice" Simon]

How much play is there in the distributor bushes?

The distributor can have earth return problems, can you swap dizzies?

[Bob Kneale]

I know it may sound stupid - and obvious, but have you got the firing order right 1-3-4-2?
I once did 80+ miles in my ruby after a hurried rebuild, and found out the following morning what I must have done!!
Excessively wide tappet clearances can cause this too. Don't ask me how I know....happened just post moreton

Hope you get to the cause whilst remaining sane. Good luck

Bob

[John Cornforth]

Hi PeterW

The symptoms you describe are consistent with weak ignition or fouled spark plugs

Although there should be some residual spark at the points when you "flick" them, the condenser is there to minimise this. If it really is blue and fat, it suggests the condenser isn't doing its job so I suggest you try another and/or check its connections. A blue fat spark from the far end of the king lead when it is removed from the dizzy cap and held a few mm away from earthed metal is what you should be getting when you flick the points.

I have often found that points tarnish during layup, even new ones. Careful cleaning with a fine file, followed by a squirt of switch cleaner or a piece of thin card soaked in petrol will get rid of any corrosion dust you have removed. Let this evaporate a bit before starting.

[Austin in the Shed]

Have you swapped the rotor arm and cap to check.Is the fuel pump delivering fuel.?

[Ian Williams]

If the engine has sat for three years you may find that, depending upon storage conditions, the valves are no longer sealing properly due to corrosion on the seats. Check compressions first to eliminate this from the equation, if they are low or inconsistent regrind the valves. I would actually always recommend you do this when recommissioning an engine as any drop in compression will noticeably affect performance. As others have said it sounds much like an ignition problem consistent with fouled plugs or weak spark, however a weak mixture can also cause misfiring. What do the plugs look like when you remove them, are the black and sooty, black and oily, a greyish brown, or whitish grey? Are they all the same or or is one or more cylinders inconsistent with the others? You should get a good indication from the plug colour/colours where the problem lies.

[flywheel1935]

Check for crud in the fuel pump/carb etc.
Mine did the same after a long layup (covid). and clean points with oil stone/wet and dry. good luck.