Manifold Pressure Takeoff

garrycol · Post by **garrycol** » Sat May 04, 2019 2:09 am

I am confused as to where I should take the manifold air pressure from for my MJ.

I have a Rover V8 with the original twin carbs fitted to the manifold. I currently have one port at the base of each carb, one used for the LPG primer and the second for the old dizzy advance (currently has a vacuum gauge attached), it has a large port in manifold runner to cylinder 8 for the brake booster and a smaller port in the manifold runner to cylinder 2 for the vacuum that operates the 4wd system.

So where should I take vacuum for the MJ system?

Thanks

Garry

garrycol · Post by **garrycol** » Tue May 07, 2019 12:27 am

Any thoughts on this issue? Are the carb ports stable enough to provide MAP or should I piggy back on the 4wd port in No2 cylinder manifold runner?

Thanks

Garry

garrycol · Post by **garrycol** » Thu May 09, 2019 12:44 am

No ideas - ok the 4wd port is used to only change the position of a solenoid so 99.9% of the time does nothing. So in the absence of advice, I will piggy back off this port.

Alex B · Post by **Alex B** » Thu May 09, 2019 4:20 pm

That way you get a very uneven vacuum. You need to pick a place where the underpressure is more even. Do the carbs have a point under the butterflies you can tap into? You need a point that is not blanked off when the butterflies er in the idle position.

Alex

garrycol · Post by **garrycol** » Fri May 10, 2019 3:01 am

Thanks Alex - as I indicated each carb has a port at the bottom of the carb - one is used to provide LPG when the LPG primer is used and the port on the other carb was used to provide vacuum to the old distributor.

They are both downstream from the carb butterfly so I guess do not vacuum when the butterflys are closed.

Why would vacuum be uneven down in the manifold itself?

Cheers

Garry

Alex B · Post by **Alex B** » Fri May 10, 2019 9:46 am

I may have misinterpreted the way you described the setup. The vacuum down the manifold will be fine as long as the suction is even. From what you wrote I got the idea that the vacuum point would be near one cylinder and then you get suction pulses. The suction should be even. Does each carb supply four cylinders with fuel or do both carbs dump the fuel in a big plenum that feeds all 8 cylinders?

Alex

garrycol · Post by **garrycol** » Fri May 10, 2019 1:28 pm

Hi Alex - yes the vacuum on the manifold runners for cylinder 1 (4wd system) and cylinder 8 (brake booster) are right at the end near the actual cylinder head furthest away from the carbs.

Each carb only feeds 4 cylinders and they are not interconnected.