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Old 10-14-2002, 05:48 PM
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Low manifold vacuum 1978 280SE

Hi guys: I have a California 1978 280SE W116.024, M110, K-Jetronic.
I have good compression, and it runs fine although the idle is very lumpy. Power is good.
Manifold vacuum at idle is only about 9 inches; even goosing the throttle and releasing does not bring it above 15 inches.

I have been unable to find an intake leak, using unlit propane torch or carb spray.

I have systematically disabled all the vacuum consumers that I can find - transmision modulator, ACC, distributor, door locks, power brakes; I have disabled the EGR, even remving and blocking its attachment to the intake manifold.

I am measuring vacuum both at the connection for the ACC/door locks, and I tapped into the line to the warmup compensator, they read the same.

I had to remove and replace the cams, had a flat lobe that was welded and reground (by a reputable cam grinding shop).

I aligned the cams properly, although looking at the marks, I thought I might be less than one link off from perfect - did not put in an offset woodruff key.

Valves are adjusted and fairly quiet. Exhaust is all new. As I said, power is good and mileage is about 18MPG in country driving.

I have done the pressure testing of the K-Jetronic system, everything is in order now that I installed a check valve at the fuel pump. Starts instantly hot or cold; fast idle warmup is good.

Just a lumpy idle and low vacuum. Any ideas?
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