vacuum leak

1950 - 1968, M38, M38A1, M170, modifications, sugestions, and ideas, official MWO's and unofficial field mods.
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mikeabn
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vacuum leak

Post by mikeabn » Wed Mar 11, 2015 3:58 pm

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
1935 Chrysler AIrflow
1950 Packard
1952 M38A1
1953 studebaker
1956 DeSoto

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artificer
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Re: vacuum leak

Post by artificer » Wed Mar 11, 2015 4:21 pm

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?
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

mikeabn
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Location: Barrington, illinois

Re: vacuum leak

Post by mikeabn » Thu Mar 19, 2015 3:20 pm

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
1935 Chrysler AIrflow
1950 Packard
1952 M38A1
1953 studebaker
1956 DeSoto

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