Le Mans tuning using vacuum gauges

Has any one ever used vacuum gauges to tune a big block vee twin. How on earth do you do it. EnglishA

On most big block engines there is an M6 screw in each manifold. Remove these and connect vacuum guage tubes.

Yes you need the adaptors that screw in. Something like this: http://www.mandp.co.uk/productinfo/525118/Workshop/Vacuum-Gauges/Davida/Vacuum-Gauges HTH

I just drilled a hole down the middle of some M6 bolts. Amazed at how much better Eldo runs now it’s done.I slackened the throttle cables, then equalised the idle stops, then tweaked the mixture, rechecked idle. Synchronized throttle cables last.

YOU will also need to clamp the tubes and be very careful on any throttle opening or you WILL pull the liquid through the manifold by far the best tubes are the newer ones with steel rods or guages. The old type using mercury can be fatal and the home made one with water equally so.

It is however well worth doing but you need to restrict the pipes or the power pulses will override the result

Sorry, wasn’t being very clear. I know how to get them connected up, but what are you supposed to do once they are? I realised they’re going to give you the best balanced carbs in the business, that’s fairly straightforward, but can you do the mixtures with them as well?Andy E

DOH!! Well no all you can do is make vacuum equal, for idle (throttle stops) and twistgrip (cable adjusters). EDIT: do you think you might have a problem with mixture then?
Mike H2014-04-05 15:15:51

as above to set mix you need a carbtune or similar

Balancing the carbs will merely get both cylinders to receive the same volume of air / fuel mix. Setting of the mixture is a different matter, a case of making sure all jets are clear and free flowing, the needles are both on the same notch and finally that the mixture screws are screwed out an equal distance.

Assuming you have the right jets and needles, the only mixture adjustment is the idle mixture. I set this by starting with the number of turns of the screw from the book, and then screw in until engine stumbles, screw out until it stumbles, and then set halfway between. Have a look at plugs when you’ve ridden and maybe fine tune. Guzziology has standard settings if you need them.

Warm up the engine fully.

Switch it off and connect the gauges up then start it up again.

Make sure you have some slack in both throttle cables so that you can be sure the throttle slides are bottoming onto their stops.

Adjust the throttle stop screws (idle speed screws) on the two carbs until you get equal readings on the gauges and a steady idle at 800 - 1000 rpm depending on your preference. Raise the throttle slide in the carb to decrease the vacuum reading and vice versa.

Use the twistgrip to open the throttles slightly and run the engine at around 2500 rpm. Adjust the throttle cable adjusters to achieve equal readings on the gauges then lock the adjusters checking that the reading doesn’t move (it will).

Finally check that there is still some slack in the throttle cables.