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vacuum leak

Posted: Wed Mar 11, 2015 2:58 pm
by mikeabn
My recently purchased M38A1 runs a little rough and my mechanic friend seems to think we may have a vacuum leak. It has a fording kit stack. When he holds hand over intake, the rpms increase which he says usually indicates a vacuum leak.
There is supposed to be a vent line from the air intake hose to the distributor that is missing but it would appear this is to give free air to the distributor when fording. There is also a short vent pipe from the top of carb to the side of the air intake hose. This was missing but has been put in place without changing anything.
The valve off the side of the fuel pump is there but open at the end There is no vacuum lines set up for wipers and the fitting for this line that feed around to the distributor is capped.
Any suggestions as where the look? The seller of the jeep has done an extensive job of new tub, tank, tires, wiring harness, canvass etc and says the carb was rebuilt so I would think at least the diaphragm, etc are all fresh.
Thanks for helping a newbie
Mikeabn

Re: vacuum leak

Posted: Wed Mar 11, 2015 3:21 pm
by artificer
I know nothing about fording kits on this model. But it should be easy to establish where any vacuum leak could be occuring, isolate & block off temporarily.
That is what to do to establish where the possble leak may be occuring rather tha relying on the prognosis that choking the engine fixes things so it must be a vacuum leak.

Your buddy may be right BUT altering air flow by holding a hand over the carb throat, always enriches the mixture & if there is enough air to support combustion, RPM's will increase.

By hooking in a vacuum gauge @ any point in the manifold below the carb will let you know if there is a vacuum leak.
Have you tried re-tightening the manifold to engine block?

Re: vacuum leak

Posted: Thu Mar 19, 2015 2:20 pm
by mikeabn
We disconnected lines and got 20 in reading at manifold. Tightened carb mounting and found line had a cross-thread. Fixed that and improved significantly.
Took a reading at the vacuum pump at bottom of fuel pump and only had 10 in. It appears the vacuum pump could use a rebuild. Are there kits for just this or does it require or suggest a total rebuild of fuel pump along with vacuum pump. Fuel pressure seems great