Vacuum leak has me stumped

[FJmonkey]

I have an 18" floor fan with a strong air flow that I use for cooling. I turn the front wheel and get the fan close to the front wheel and aim for the center of the headers. I can idle this way for 10 minutes and not over heat, more than enough time to sync the carbs. Get another fan if you feel your current one is not enough. You need some air on the engine at idle, you only need more if you run the RPMs higher.

[Pat Conlon]

Bozo, when you say ensure that the throttle slides are exactly the same height, what do you mean by throttle slides? 
I'm synchronizing the carbs using a set of vaccuum gauges. 

Would that have the same effect?  If so, then you were right.  Once the vaccuum increased and I was able to sync the carbs better, the cylinder temperatures are more consistent with each other, based on my IR thermometer.

The guys missed this, if you are using vacuum gauges to sync, then you are fine....

[tmkaos]

A long. long time ago, when I was a tech at an MG dealer, I was given a MGB to synch the carbs. Pulled out the factory manual and it instructed to use a piece of heater hose and listen to the carbs with the hose in each carb throat and adjust the linkage so they sounded the same.....

When I was discussing synching my carbs with my father, he mentioned synching his twin Weber set up on his Mini(?)  in England in the 60's in exactly this way..

[Flying Scotsman]

I had severall MGB's back in my youth.1972 1972 and a early 1974.My first car in back in 1979 was a 1972 MGB
Had twin SU carbs and to sync them I had a crapy vacume gauge you held over each carb one at a time.You had to see where the ball floated on one and then do the other so the ball floated at the same height.Pain in the ass.
I had lots of fun in those cars red line was 125 mph not bad for a 4 cyl 70's car.i always wanted to do a V8 swap just couldnt aford too.I did do a cam and a header.I have seen at least one MGB video with a Jaguar v12 stuffed in there not quite oem but it would work for me lol......

[Pat Conlon]

Don't forget to check your dash pot oil.....(I too grew up with MGA/MGBs....That's why I love my Miata)

[Flying Scotsman]

Shhh you dont want to start another oil thread 

[chacal]

I might have some time over Thanksgiving week to stop by and take a look.

Did you do the compression test with the throttle held wide open?
Have the valves been adjusted?

Who did you buy it from?  (Rich Simmons by chance?)
Where abouts in SW Ft. Worth?  I'm over near Allen.

DavidR.

David, if you do have some time over Thanksgiving week to stop by, that would be great.  I can't tell if I am having carb settings issues, or whether there is a deeper problem.  It seems like the carbs are synched fairly well now based on the vacuum gauge, but when I reduce the idle speed to bring the vacuum up to where the Haynes manual says it should be (25cmHg), the idle speed drops to around 500 RPMs, and the motor does not want to continue running at that speed without choke.  Increasing the idle speed quickly drops the vacuum to 10cmHg.  Should I worry less about the vacuum values and focus on the idle speed?  I still have the idle mixture screws set at 3 turns out.  At that setting, the RPMs drop pretty fast back to idle after I blip the throttle.  I'm wondering if maybe I need to go back and clean the carbs again in case my previous cleaning missed something in the idle circuits. 

Thank you

[chacal]

Update: I now have a fairly stable idle around 1000 rpms, and the vaccuum is around 25 cmHg and synched between the carbs.  It turns out there were two main issues: the carbs were set really rich, and I apparently can't use vaccuum gauges properly.  The floats were set too high, and my idle mixtures were too rich, which is why the idle kept slowing down and dying.  I think I was compensating by increasing the idle speed too much, which is why I kept having the revving issue.  Once I lowered the floats, I was able to see changes in the rpms when adjusting the idle mixture screws.  I now have the idle mixture screws set at 2 turns out from lightly set. 
As for the vaccuum gauge, I had the dampers set too high, which allowed me to synch the carbs, but did not allow me to see the peak vaccuum (yeah, trying to use tools I didn't fully understand).  So I was chasing a phantom vaccuum leak once the intake manifolds were resealed.  However, thanks to that chase, I now have clean, adjusted, and correctly timed valves; excellent valve stem seals (thanks Randy!); well-sealed intake manifolds; and rebuilt and cleaned carburetors.  Overall, I think the motorcycle is much happier.
I know I still have more tuning to do, but it is in rideable condition now, and I am looking forward to getting to know it better before I spend more time tuning it.  Thank you all for your suggestions since they helped a great deal while fixing issues and ruling out other problems!