MG Car Repair: 70MBG Acceleration Issue, vacuum leak, vacuum advance

Question
QUESTION: Have a serious bog down when acceleration off of idle. Original SU and Distributor. Replaced with solid state ignition, no help. Put inspection hole in cap. Advance starts to move at 6"hg smoothly through 20" with mini vac tester. vacuum very erratic and does not pick up reliably when connected to carb. Not sure it is lack of vacuum that causes the bogging or bogging causing the lack of vacuum when blip the engine and really bad under load. Had to resort a tone of advance just to attempt to remove the bog. Won't idle below 600 - 700, really rough and stalls.

Not sure where to start with this vacuum advance if that is even the problem.

Thanks for the help.

ANSWER: Hi Ray,
When I had a car that was not right no matter what the symptoms were, the first thing necessary was to determine if the engine was good. That is done by a compression test. Good compression means it will have good vacuum. Ignition timing MUST never be altered to try to make an engine run right. Ignition timing is a fixed figure that can only be altered by highly modifying the engine itself. You can test the vacuum advance (which you have already done and it is correct (if you have a 25D4 distributor.) You still need to check the centrifugal advance. (Zero below 600 RPM on up to 22 deg at 2200 RPM) Set the timing at 10 deg BTDC static and leave it alone.
A bog off idle is more often a carburetor problem. (too lean) Lean because of no oil in the top pots, lean jet settings or a vacuum leak. All three of these can give you poor idle plus a bog off idle.
Also check valve clearance as a tight intake valve can cause this also.
Let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: Thanks Howard,
Was a smoker and did a rebuild that stopped it about 3 years ago. It was the easiest thing to clouch out for kids but never the same afer the rebuild. Compression is all fine.
Will check valves, check and make sure not lean and will put timing at 10 BTDC until I(we) figure this out.

Does act like starving for gas. If I pull choke out a little, no bog at any speed and can really feel the engine torque on throttle change at any speed.

Pots are full. I have always used Marvel Mystery Oil.
I need to check centrifugal advance with timing light without vacuum advance. Mechanically when I release the rotor it springs back about 80%. Springs a little stretched. That is a total of 22degrees I should be looking for from just the centrifugal at 2200?

Do you think it could also be floats?
Maybe time to tighten up maifolds since my engine job was so long ago. It does seam to be getting gradually worse over time.

Will let you know what the next round of tests show.  

Thanks again  

Answer
Yes, 22 deg just from centrifugal at 2200 RPM. Float level should be checked and run a test of the float chamber venting by removing both vent hoses. Marvel oil works ok so next is set the jets so a lift of the side pin gets the correct engine response.
Howard