Carb Bolt Pattern-225 Vs 231

Is the bolt pattern for the carburetor on the intake manifold the same on a 225 and231 odd fire?

Both used the 2G I thought so, I figured they'd be the same.

 

At least my 75 OF231 used the same 2G

 

There's two different bolt patterns. The 225 uses the small one, I don't recall which one is on the 231.
Easy way to tell the difference is by the carb throat diameter. 2 5/8" or 3 1/16"

 

Ok. So there are 2 possibilities.

I bought a 2100 and the adapter's bolt pattern is too small for my 231 intake. Working with the seller to try to figure it out and need correct info. I'm ignorant of there being more than 1 bolt pattern. I'll pass it along to him.

He's being very accommodating so far.

I think the Mr. Gasket adapter plate that was under the Holley 2 bbl on this 231 intake might work but, I kinda think it's junky and the gasket needs replaced anyway.

Keep the info coming, anyone who can chime in.

edit: Found this.

Rochester Two-Jet

So, yea, 2 different bolt patterns. Looks like it changed to a larger pattern after the V6 tooling went back to Buick. My Holley adapter may work but, it's kinda junky and not nearly as nice as the one supplied by the carb vendor.

Rich, maybe your 231 has an earlier 225 intake on it.

 

Home now, the pattern on my manifold measures exactly 2"x3-3/4"

 

I am curious as to what the 2100 bolt pattern is. Also what is the adapter pattern you have?

 

The 2100 and 2150 have a Ford/Holley 2V pattern, like half of a Holley 4V.




This is completely different from the small/large Rochester 2V pattern, which is much more compact.

 

I have a two barrel Holley, small Rochester, large Rochester, so I can check all that. Just not understanding how wheelie adapter is two small. My 225 pattern is 1 7/8 x 3 1/4. And the larger Rochester 2 x 3 3/4. Of course the Holley and Autolite are 3 3/4 x 5 1/8. Just unsure of what the two barrel adapter is that wheelie has. I would hate to buy one and have the same issue. And the stock 231 even is like nothing else at 3 1/4 x 3 5/8.

 

I have a Holley "smogeurator" that uses a gasket very similar to the one posted on my 231. I was told it is from the early to mid 80's. The engine is a 1987.

 

Here is a Holley version that appears to work for a Holley 2300 or Motorcraft/Autolite 2100 2150 and the small pattern a 225 would have.
Mr. Gasket 1937MRG Carburetor Adapter Kit - Large Holley 2-Barrel to Small Rochester 4-bolt - 2-Barrel
So it appears the 231 Odd fire is a large bolt pattern just like Rich noted.

 

Yes. My OF 231 is the large pattern which matches Rich's dimensions of 2" x 3.75". Measured it earlier. Sp the Mr. Gasket adapter I had under my Holley 2 bbl would work but, I don't like it. I'll see what he sends me as a replacement for the adapter he sent me ( which is for the 225 manifold/small Rochester pattern).

Only other issue I see is that the Holley has a vacuum port at the back for the PCV. Not sure where the PCV plugs into the 2100. Gotta look at the instructions I guess. He did say he can provide the correct adapter either with or without a vacuum port. Maybe I should ask for the one with the port and use it for the PCV.

I'll try to add some pics soon.

 

Here's a comparison of the two sizes of 2Jet, side by side.

 

This appears to work for just about any combination.
Mr. Gasket Carburetor Adapter Kits 1933

 

Here's a few pics.

Stock OF 231 intake.

Gaskets were my first indication that something wasn't going to match up. New adapter to intake gasket over the old one.

Old adapter that was under the 2 bbl Holley. With it's slotted holes to mate to the intake, I think this would work with the 2100 but, I want to see what he sends me to replace this.

New, but incorrect, adapter. Much nicer piece IMO.

Intake manifold bolt pattern on the incorrect, for a 231, adapter. I believe it would however be correct for a 225.

Bolt pattern for the Autolite 2100 on the new adapter. Didn't think to compare this to the bolt pattern on the old adapter that the Holley mated to. I'll check that later.

 

Would it be simpler to use a dauntless intake?

 

Perhaps, Rich. Or it may be 6 of one and half dozen of the other. An adapter is required either way and the correct adapter is en route.

I am also ignorant of and changes/improvements to the flow of a 231 intake over the 225 intake. Maybe there are. Maybe they aren't. I lean to "there aren't."

A bit more work swapping intakes. No biggie but, I'm short on time. And finally, I don't think I have a 225 intake in the parts stash anymore. I might. But I don't think so. Not that this isn't easily remedied but would have to wait for another time.

I need to dial some stuff in after this carb goes on. Check/set the timing for sure. I had very low vacuum when I checked it last summer but could find no source of a vacuum leak. Guessing my timing is off. Hoping the timing is off and it's not some other issue. That is again another topic.

I wish I were a better mechanic. I can muddle my way through putting things together. Better at taking them apart. Not so great at understanding the various systems and diagnosing them.

 

New adapter arrived yesterday. I thought I asked for one with a port for the PCV but, that's not what showed up. Maybe I didn't ask. In any case, the Mr. Gasket adapter that was under the Holley is what I think I will use. Meaning that the Holley and 2100 are the same bolt pattern. Just not feeling the new adapter he sent.

So, this leaves me the PCV issue. Can anyone tell me (Beeg Dummy) what to plug the valve into. Manifold vacuum I know, right? But, where do I source that on the 231 OF intake. I see a couple of plugged holes but they are all pretty small for the PCV I think and look to be in one runner in the manifold, not a place where the gasses would be equally distributed throughout the intake.

I do have a breather cap on the valve cover so, environmental concerns aside, would this make the PCV unnecessary, in terms of crankcase ventilation alone?

2 small plugs. Maybe one of these could be used for putting the vacuum gauge on the engine for diagnosing engine issues? Just a barb fitting and cap it off when not in use?

This large plug, beside the choke cable laying there.

Non of these enter the coolant passages do they?

 

Judging by the ports between the 2 intake bores for the carb, it looks like they Siamese into that block off plate area at back of block? I would assume the 231OF has a smog pump assembly on it in factory applications? possibly that was a CA thing, but one of my 225OF has a smog pump on it in its factory CJ application. If in fact that block off plate at rear is for emissions control and leads to those ports in intake, I’m sure you could pull it, drill it and tap it for your PCV line if need be.

Edit, to answer your question about PCV valve, that plug you are pointing out, should go directly to your intake runner. I don’t see why it wouldn’t work for a PCV tie in location.

 

That plate is where the EGR valve should be. My 74-83 Midsize Pontiac manual, which has some OF 231 info in it, does show smog equipment on the engine in it's original configuration. All that stuff was gone when I got the engine many moons ago.

 

So it could be drilled/tapped for the proper male/male connector to run the PCV?