Carb diffuser

[Joe Gopan]

I trust its purpose. It results in a more efficient fuel-air flow thru the manifold. Barney Roos was a good engineer, don't you think?

[Bill M]

If the early version had no diffuser I would think the evaluating authority the QMC would only have directed that the diffuser be added after specific testing revealed shortcomings with the original design. I am reluctant to relegate this change to the snake oil salesmen.
I don't believe the QMC and Ordnance departments worked like that. Evidence to the contrary?
Bill.

[Wessels]

About when was the diffuser introduced on GPW's and MB's? Thanks in advance!

[bazza46]

I believe its main advantage was that the thick asbestos gasket insulated the carby from the hot iron manifold, preventing percolation. Other makers, both before and since have used insulators of plastic, phenolic and bakelite to do the same thing.
When was it discontinued? Was it original equipment for the whole series of the Carter carbureted motors, through the CJ series?

[Chuck Lutz]

The early (no diffuser) and late (with diffuser) gasket is about the same thickness.
Nabholtz says: August 1942 @ MB170307 "Added diffuser to carb gasket"
I did not see any mention of it being eliminated

[Joe Gopan]

It was primarily used on the MB/GPW, CJ-2A, CJ-3A, and M-38 installations for the 4-134 engine only, the F-134 used in 50's Willys Trucks and Wagons, M-38A1,and CJ-5 Universal Jeeps had the intake manifold integral with the cylinder head with the carburetor mounted on the head, a totally different and more efficient induction system that did not use a diffuser.
One can claim that the diffuser is not necessary is "snake oil"or does nor work, that is best left for the engineers who designed the military and civilian Jeep engine, as they know best.
Another little L-134 carburetor tidbit or FYI is that the L-Head CJ Jeeps once came equipped with a restrictor plate thatwas installed under the carburetor to be left installed during the new Jeep initial break in period.

[Mark Tombleson]

Settle down guys... let me help you all get this thread back on track.

Here is the PRF. It appears I got this on a G503 thread back in 2002 from Todd Paisley. Sadly those threads are not available any more.



As most PRFs this one is a late issue... 8th to be exact dated 7/18/45 to include the last contract, for the MB-NB.

So, it shows it starts under "ALL" with a * at MB-152108 and includes all standard and specialty MBs from then on.

It also says A-923 is superseded by A-6354.

It was made by the Detroit Gasket MFG. CO.

In looking at the List of Parts (BOM) for the MZ-2 (dated 7-10-44) it states that A-6357 includes the following part:

A-6354 - Intake Manifold Diffuser (Carter Carb. Corp. #186-16)(Furnished to gasket MFG. for assy.)

[Joe Gopan]

No part number for the staples?
The NOS in box A-6357 Diffusers I have as spares are listed on the box as "Part of A-1537" which is a Valve Grind Gasket Set.

[HunterL]

Please tell!
....the diffuser makes little to no difference....there are some bad re-pro diffusers with copper rivets....you refer to Baney Roos of Studebaker fame & we are now have 2 pages of waffle regarding something answered on first respose to the questioner.
?

Hard to disagree!

How are the re-pros from fitzpatrick? i just ordered one (before i read this) and was curious about the rivets.

Thanks,
Hunter

[tamnalan]

I use Ron's parts kits (including the diffuser) and they're fine.

[HunterL]

I use Ron's parts kits (including the diffuser) and they're fine.

Thanks

[danover321]

Drill or punch out the copper rivets and install. It's just that simple folks! I have removed the rivets on all reproduction diffusers I have installed and not one has ever leaked air between the carb and intake manifold.

[70th Division]

Hello,

This is an interesting older post about the diffuser carb base gaskets.

I have an August, 1942 GPW that was running and idling great.
I replaced the 3 carb gaskets as they were leaking, and also replaced the original non diffuser base gasket.
The new base gasket has the difuser, and the 2 rivets.

The carb doesn't leak any more, except some weeping from the ends of the lower horizontal cross shaft,
but the idle seems a little sketchy, a little to revved up, and I am thinking
that perhaps I should remove the rivets in the diffuser gasket to see if it changes anything.

Did the introduction of the diffuser type gasket correspond with any changes in the carb system, or pcv system ?
There is a date listed on the document, for when the use of them would be appropriate.

I will see what develops once I remove the rivets from the diffuser gasket.



On another subject, on patching up my original wiring harness on the GPW, I have found zero color coded wires,
all the wires have a light tannish color, even when the old loom is removed and new loom placed over the wires.
The original headlight ground wires were the same, I replaced them with the "nos" ones that are available, with the color coding, and rubber insulating ends on the eye hooks.
Did early GPWs use all one color wires for their wire harnesses ?


Best Regards,
Ray

[Chuck Lutz]

Ray...in late August 1942 when the diffuser was added at Willys on MB-170307,there were no changes in the crankcase ventilation system as of yet. The change from the down-draft system took place months later. The Carter 539S remained the same after the original short lived "K1" style in the first few thousand MBs rolled out. The change from the pancake air cleaner to the Oakes Type I with the decal instructions did occur in that timeframe and the change to the Type II with the tacked on plate with instructions on it I believe occurred right around August as well.

Now to that carb...if you find that you can't get the jeep to idle at 600 RPMs, then air leakage is most likely suspect here...
The rivets may be holding the carb/manifold apart just enough for air leakage to occur or worn throttle shaft holes in the flange itself may allow air into the system as well. Using Brakekleen with the little red plastic nozzle will allow you to spray the throttle shaft holes first and then if you do not note a change in the engine RPMs, try squirting it where the carb/gasket/manifold meet. However, before doing anything I suggest you might just try to tighten the nuts holding the carb to the manifold first to see if you can eliminate any possible leakage that way.

I believe that over time, you can tighten those nuts as the repeated heating of the gasket may allow enough shrinkage to allow air to get in so I won't use the gaskets with the rivets, but I will use the EARLY style with the diffuser but without the rivets.

[Klaas]

On another subject, on patching up my original wiring harness on the GPW, I have found zero color coded wires,
all the wires have a light tannish color, even when the old loom is removed and new loom placed over the wires.
The original headlight ground wires were the same, I replaced them with the "nos" ones that are available, with the color coding, and rubber insulating ends on the eye hooks.
Did early GPWs use all one color wires for their wire harnesses ?


Best Regards,
Ray

Hi Ray,
On GPW17331 the original wiring was color coded fabric covered rubber wire which for the most part had faded to a light sand color; only on the loom under the tub the original colours were still clearly visible.
I did replace with a new wiring set for safety's sake (and used some of the original wires in good condition for my front indicators)