4 replies to this topic

[Mini30BVA]

Hello all,

I've got a 998cc with flat top pistons, stage one kit and a 12g940 head. Everything is in pretty good condition, but I have a problem with low rpm bog. I know this is a common problem with the 12g940 head as the valves are bigger meaning air velocity is lower in lower rpms.

Is there any way to improve the low rpm bog? I've got an AAU needle in my HS4 carb, a 59d4 distributor with the advance set to what is suggested in the Haynes manual.

Is there anything I need to change or alter?

Thank you!

[sonscar]

When you say low bog what do you mean?flat spot exiting roundabout at 1500 rpm in third?slow to exit junctions in second?just asking Steve..

[whistler]

Maybe try changing the damper oil to one slightly thicker.

[Mini30BVA]

Sonscar - It happens whenever I'm pulling away from stand still in the 800 to 2000 rpm range. Takes a bit to get going until about 2000rpm then its off like a rocket.

[Spider]

The issue with the 12G940 on smaller engines is the Inlet ports are too big, as a result, at lower engine speeds, the flow rate through the ports is slow and so tends to change the state of the inlet charge from being atomised to a raw fuel and air state. You can make the car more drivable but selecting a needle that is considerably richer at these speeds, but it's not the best for engine life. Sorry, I don't have a needle recommendation here.