A wild guess: have you checked the valve timing? Lazy vacuum despite good compression might make me suspect that intake valves are late or early for the intake stroke. (I said it is a wild guess!)

Thanks for the replies! Scooperman, I tried to ignor the "Did you check this" part of your response, but I can't! Insert roll eyes imogy here...

But seriously, I have checked the compression and all four cylinders are between 200 and 180 psi. Like I said, I've checked everything on this engine, including the valve lift that came in right around the number you stated in your reply. Can't remember the head casting number at this time, but seem to remember it being a big valve unit. I've recently moved close to Mini Mania and don't have my shop set up yet, so it will be some time until I get back to the Mini.

Again, thanks for the replies!!!

OK an 1100 will more than likely have a 12G202 head. Next the 12g940 is not a smaller valve head to the 295 but a 1275 head.

I might not start with cam investigation.  I would pull the valve cover and see what head (casting number) is on there.  Should be a 12G295, 12G202 or 12G206, but a lot of 1100 builders go with the smaller-valved version of the 12G940, and those can need additional valve relief in the block and pistons, or metal removed from the valve face.  With the 12G940, if the head was flycut or a lumpy cam installed, they could have whacked a valve.  See https://www.minimania.com/Cylinder_Heads

 

So I would do a leakdown check, car in gear.  I would want to see less than 20% on all cylinders.  A lot less is good, 20% is driveable.

 

I think if the cam is unknown (no info from the builder) then you have to measure things yourself.  I dont see your cam listed here:

 

https://www.teglerizer.com/midgetstuff/spridgetcamspecs.htm

 

I have an old parts catalog listing the 12G726 as the stock 998/1098/1300 cam.  That would be spider drive, the pin drive would have been 12G165. 

 

12G726 specs:

Intake open 5 deg BTDC

Intake close 45 deg ABDC

Exhaust open 51 deg BBDC

Exhaust close 21 deg ATDC

In/Ex duration 230/252 deg

Valve lift .318"

3/8" wide lobes

 

A symmetric-profile cam with those durations has full lift at half duration; so full intake lift at 110 ATDC and full exhaust lift at 75ABDC.

A stock cam is likely to have max overlap close to TDC.

 

Dimly recall that later 1098s had a cam with 1/2" wide lobes but dont know the part number.

 

Measure valve lift on #1 with a dial indicator.  A couple of measurements should make it clear whether its a stock or mild cam, and tell you if it is correctly timed or off a tooth.  Finding TDC is easy, but getting accurate degree info needs a degree wheel on the crank and that is difficult with engine installed in a Mini.  You could count flywheel teeth through the clutch cover hole (fun!) or the starter hole (also fun!). 

 

edited - list of cylinder heads

Here is an article by Keith Calver about building an 1100 as he calls it. https://www.calverst.com/technical-info/engine-1098-initial-tuning-potential/

I'm trying to track down the problems with tuning what seems to be a "built" 1098 that I installed into my Mini to replace the twin cam until I can rebuild that. I have asked a few questions here about this arrangement in prior posts. Please, I'm not looking for advise on how to track down the issue(s). I have eliminated vacuum leaks, rebuilt the carb and distributor, traded SU needles until I can't even remember which ones I have and a bunch more. I guess I can't stop anyone from posting such comments, but rest assured I will do my best to ignore them.

It idles OK with an unsteady lamda of 1 (14.1:1 for those of us that use SAE units), but it is lean off idle at any RPM and only makes 7 to 9 in Hg vacuum regardless of engine speed. After having fixed everything that could be leaking or whatever, I'm beginning to think the camshaft is not right for a road car. In that light and seeing as the original camshaft for a 1098cc engine (8g2732 for an MG1100) is NLA, is there a suitable replacement available today? Let me know what it might be, please. Additionally, if you happen to know the standard cam lift and duration specs, please let me know them too.

Cheers, Craig