The Stovebolt Site Forums The Shop Area Engine Shop Roch B Carb Jets

I am tweaking this new rebuilt 235 and found sooty spark plugs and some dark splotches in the exhaust, so I want to tweak the carb a bit. The Jet has a 56 stamped on it. The numbered drill bit that will go through this stamped 56 Jet is a #53 drill bit.

Mike has a really nice chart that is in decimal, but with the above being the case on four different jets, I am confused as to what to go by. The 52 was too lean, the 54 was almost right, but I had to choke it just a hair too much. I am confused as to how Rochester numbered these Jets! Is there a conversion chart somewhere that converts Rochesters decimal system to ours? Are my chinese numbered drill bits that far off?

It might have been better if jets were NEVER stamped!!! The reason, people drill them oversize. Best advice......NEVER believe what is stamped on a jet!

If these are genuine Rochester jets, Rochester would have stamped the size in thousanths of an inch. Thus a "56" would have originally been 0.056 inch.

A number 53 drill bit is, by definition, 0.0595 inch. Therefore, a 53 drill bit should not go through an unmolested 56 jet. Your "56" jet is at least a 59.

You will note your numbered drill index has gaps. These can be partially filled in by purchasing fractional metric bits.

Jon.

but I had to choke it just a hair too much

Main jet size and choke operation are only distantly related.

I am testing using a test engine on a stand where all things are exactly equal. All of the test equipment can be hooked up permanently. So if I change the Jet and find I cannot maintain the exact RPM with the new Jet as I did with the old one, and Choking makes a difference, I know which way to go.

Is there a better way to test jets? I am new at all this so probably. Please advise!

On my 216, I installed an early Rochester that I rebuilt. The truck had a lot more power than usual. After a trip I pulled the plugs and they were all very black. I discovered that the main jet was stamped 58. I installed a 51 and it was a bit lean but much closer to what it should have been.

Did you try leaning the mixture screw from an 1/16th turn to a 1/4th turn before messing with jets?

Why are you choking to compensate when choke is for starting?

Why are you complaining the engine is running rich then putting in larger jets?

Yes, it was running way too rich. I compensated the OTHER way to ensure that I have some means of telling if I am on target. If you have to choke it, it's too lean. Then I work from there. The mixture screw doesn't do enough to put it in the ballpark so I am WAY too lean. Thats where I am now, but I am in the middle of resolving it.

Most people dont realize you can get Jets to cure those sooty plugs and give better performance. I sure didn't realize you can get jets in every decimal value. So I am tweaking it now. You just can't rely on the number on the Jet. The very good reason for this is that over 60 years, mechanics and novices alike have had the opportunity to drill them out to suit whatever condition they had at the time.

Another issue to complicate matters is bore size. I noticed on the Mikes Carbs chart that the 261 takes a significantly bigger Jet. So, must be that a larger cylinder takes more fuel. With a .080 overbore (246 cu in.) maybe this is why the listed Jet is way too small.

The difference is between .056 (235) and .065 (261).

There are other factors that can cause rich or lean conditions. The float level is critical. It can be tailored a little to compensate bogging or running rich. Also if the vacuum passage to the power valve is blocked it will cause a extreme rich condition. If the power valve is stuck or bent it can cause a lean condition.

The carb riser block I got from one of our vendors has notches in it for that George. But they are in the wrong place. Then if you don't notch it with a Dremel or something, you are relying on the gasket that does have the notch in the right place. I just marked the block using the gasket and notched the riser block to ensure there is no future blockage. The rest is accomplished during a very thorough rebuild of the carb. I have a .067 jet coming to replace the .070 that's in there, and that should remove the soot problems on the plugs. I also ordered the lower 11 steps so I can fine tune this. Thanks for your post!

The 261 takes a significantly larger jet, because the carburetor used on the 261 is a significantly larger carburetor than the one used on the 235.

Jon.

Jon it is SUPPOSED to be bigger, but just TRY to find an honest to goodness 261 carb. If you know where there is one, I am very interested. Over the years, the replacement carbs from a 235 are installed and they have all evaporated over time.

I see what you mean though.. the chart WOULD take that into consideration and that may very well be why the Jets are .065.

My recent experience with a 235 bored to 246 with a .070 Jet is although .070 is a bit large, the .063 Jet really IS too small. I will see soon if the .067 is a better fit.

Learning alot about Jets Jon! It's amazing what a difference they make. What to do and what not to do! Thanks!

Awesome Dave. Thanks.. Problem still exists though. At least now I can see the difference, so if I am at a swap meet and find the elusive, really impossible to find, 261 genuine carb, I have something to go by. Whenever I hear about one for sale on Ebay, I am even skeptical. I wonder how hard they really are to find. Dave those pics are really valuable for our craft.

I will take some pictures, and post, of the throttle plate.
The 261 butterfly is so large the throat is almost out to the power valve port hole.

The 292 used a very large "B" up through 1967.

ANYone having a real genuine 261 Roch B, I have a very worthwhile project that is begging for the proper Carb. Thanks again! But I gave up on that years ago. They are as rare as just about anything you can think of. Tons of 261's out there, but a very high percentage with 235 carbs.

235-261 throttle body comparison added to link above.

Thanks for posting those comparison photos Dave, that's the
kind if stuff we need to make informed decisions abouit what
it is that we have on the bench in front of us.
Anyone that's gone to battle with the Rochester 'B' probably
has a box full of them in various forms of disassembly
in box's somewhere in the shop. There were so many subtle
differences in them that the parts can easily get mixed up.
And as has been brought up before, the rebuilders don't
often get it right either and just about every 'B' that I've
gotten my hands on has been rebuilt, who knows by who and
how many times.
Nice to have some solid documentation to add to the reference file.

Denny Graham
Sandwich, IL

Another good thread.