Motorcraft 4350 OE jet sizes, need help

[Beach_Dude]

Original jets are in the 4350 and the same as the technical service manual of 197.

However, I'm wanting to richen the cruising use a little because of overheating and white plugs.

Manual says No. 202 for the 6tha4c, probably California, but No. 197 for the standard 6tha4. I'm confused because everything is in hundredths and thousandths; example of .074 to 078.

What are these sizes to order larger size jets?

[tgreese]

The 4350 is a metering rod carb, and it's possible the holes in the jets are ca 200 thou. The metering rod taper blocks most of jet orifice. Parts book does not break out that part at all - part of the carb body. All the difference between the M4, A4 and A4C is in the carb body or absence of an idle stop solenoid with a manual transmission. I don't know these carbs at all, but I wonder if you can bend the lifting arm so that the needles are higher at a given throttle opening. I'd start with the TSM and any rebuild instructions you might have.

[Beach_Dude]

All parts diagrams are the same. What I'm gather the difference in tuning and parts is:

6THA4 - Auto Trans
6MTA4 - Manual Trans
6THA4C - Auto Trans, California

The logic I'm using is after reading the technical service manual, all three of those are the same carb with different tunings.

I've upgraded my ignition system and the plugs look like they've been running pretty hot at cruising high speeds. Only methods to Rich it out is to change the jets.

Regardless, anyone have knowledge on the No. 197 and 202 that the TSM is referencing in diameter?

[tgreese]

Hot rodders would modify the jets themselves back in the day. You can drill out the jets, or fill with solder and redrill. The only references I found online are for conventional Motorcraft jets for the 2100 and 4100 carbs; nothing for the 4350. The Quadrajet is more like the 4350 in this respect, and there are replacement needles that change the Quadrajet mixture, not jets as such. The parts book spec's the same needle for all three of those carbs, thus the difference must be in the parts that come with the carb body. Not to belabor the point, but everything but the carb body has the same part number for all three of those carb numbers.

I speculate that you're not going to find any modifying parts for those carbs. The numbers may only be useful to identify one carburetor body over another by the jet markings.

You can richen the mixture some by raising the float level. The increased hydrostatic (petrostatic?) pressure will send more fuel through the existing opening. May be enough to cure your over-lean condition.

[Beach_Dude]

I don't have to modify my existing jets since there's a world of parts out there .. I just need to know what the conversion or representation of the 'No. 197' and 'No. 202' are in the TSM and the stamped numbers on our stock and OE jets are.

My jets are stamped '197' which matches the TSM.

Here's a link with a TON of jets


Does anyone know what the standard/factory jet size is for the 4350?

[tgreese]

You don't understand how a metering rod carburetor works.

[Beach_Dude]

Okay... So I don't understand how a carburetor works.

So, for those that know how carburetors operate, what the OE jet size equivalent for the 6THA4 setup on the Motorcraft 4350 that can be purchased today?

Here's the TSM:

[tgreese]

You can see that the 4350 does not work like the other Motorcraft carburetors. The shape of the tapered metering rod determines how much fuel enters the jet.

I kinda doubt you will find much support for the 4350. As an OEM carb, there's no requirement that any speed parts - or information - be made available. It also has limited application, many fewer than the 21xx or 4100. I recall some of the big Fords with the 460 also got the 4350 - you could ask on maybe a Lincoln forums? Jeep only used it a few years, 75-77 and maybe 78 (I have no book for that year).

Also guessing if you have the A4C and the 202 jet you already have the larger of the two possible, and the California version compensates for some other California addition, like more EGR gas? Are you running an EGR valve? More exhaust gas will lower the combustion temperature and could compensate for an excessively lean mixture.

Did you look at whether you can bend the lift arm? That and the float level are obvious ways to richen the mixture some.

[Beach_Dude]

Tried it yester, regarding changing the float. Bent the tang to just above the maximum float level. Took it on the freeway, and overheating.

I'm just going to order six to seven pairs and return the remainder, giving the retailer a heads up in advance .

Whoever uses the 4350, shout out your jet sizes if you know!

[Beach_Dude]

The stock No. 197 jets seems to be .069

Ordering 0.071 and 0.073

Trying the 71s first. Will keep ya updated!

[FSJunkie]

First of all, I just have to say that white plugs don't necessarily mean it's running lean. It could just mean it needs colder spark plugs. Lean engines run poorly. A carburetor that is lean at part throttle will bog or backfire. You know when they are lean. They run bad when they are lean. An engine that otherwise runs good and strong but makes white plugs just needs colder plugs. Same thing with an engine that runs great but just runs hot.

Over-advanced ignition timing will turn plugs white and make the engine run hot. It's really common on 360's because people crank up the ignition timing thinking the OE timing is some kind of smog spec killing their power when it's not. Then they start frying spark plugs, piston ring lands, and chase fancy aluminum radiators and electric fans trying to keep their engine cool when the real issue isn't a cooling issue, it's an owner/operator issue.

That all being said...

You are tuning your 4350 exactly right. Pick the OE jet size and step up in .002 in. increments. You can also look into different metering rods if you can find them. Don't mess with the float level. Either tune with jets and rods or don't tune at all. The float level doesn;t affect the fuel mixture very much but it sure can screw up other things. Especially on the 4350.

[Beach_Dude]

Oh My God!!! It runs so much better! The amount of power is freaking amazing in difference!

One major thing that I did was shorten the accelerator linkage since it misses WOT by about 3/4". So, I placed a spacer in there.

One thing that I've noticed now is that at WOT is detonation. It's not the timing because at the same RPM, if I release the throttle by just a bit from WOT, the detonation stops.

So, it has to be that the secondaries aren't dropping enough fuel, closing the flaps by enough to richen it up, am I correct?

Here's the crazy part, the TSM shows the secondary jets to be .144. How does that work!?!