The Stovebolt Site Forums The Shop Area Engine Shop Rochester B Jet size

I have a 1960 235 with a 1 barrel Rochester B carb. I just put HEI distributor and it runs awesome. It has been about 500 miles and I checked the plugs after setting them at 50 thou and they are running very lean. Should I put in a larger jet for it to run a little richer? I believe I have a 56 should I go to a 6o?

Thanks for any help I am also running AC 45S plug.

Mike

I have a Langdon style HEI in my '58 235, I have the plugs gapped at .045". My plugs are burning just right with a .058" jet in the Rochester B.

As Craig said, I had the best luck using a 58 jet...at your altitude it should be good. Currently I am using a Carter YF3211S carburetor (the one designed for your model 235) and an HEI that is a clone of a GM coil-in-cap model and it is doing fine. If for any reason you are using Pertronix products, please see the post I made in the Electrical Bay...might be information worth knowing. Good luck.

Have a good look at the jet. My carb was running lean* and I found the jet partially plugged with some white stuff - looked like calcium. Cleaned it out with appropriate size welding tip cleaner and it runs better now.

*I knew the carb was running lean as it smoothed out when I pulled the choke out.

Great info guy's thank you so much, I will check the the jet for blockage and size. you folks are a blessing.

I have had to pull choke out a few times also. I will check out the jet. would a .060 be to large for the 235 ? Thanks

Get a pin vise and a set of wire drills- - - -#61 to #80 will handle most carburetor jets, along with the usual #1 to #60 drill set that's a lot more common. Always remember that a very small change in jet diameter can make HUGE difference in mixture!
Jerry

60 should be OK. Read this post. Unfortunately, George passed away earlier this year.

https://www.stovebolt.com/ubbthread.../re-hesitates-bad-rochester-b#Post790897

Thank you Jerry Please forgive my ignorance what are wire drills #61 to #80 I have a couple extra Jets and I enjoy fixing my own stuff.

That was a great forum read just the info I needed. Sorry to hear about George :-(

Thank you

There are three types of drill bits- - - -fractional, letter, and number drills. Number drills start at a #1 which is slightly smaller than 1/4" and most sets go to a #60, which is smaller than 1/16". Then the really tiny ones come in a separate set from 61 to 80. An 80 is extremely small diameter. Most of the smaller bits need to be driven by hand with a tool called a "pin vise", or with a very low powered drill motor like a Dremel tool.

https://www.templateroller.com/template/154268/drill-size-chart.html

Jerry

Another option to the sets mentioned by Jerry IF YOU USE THEM FREQUENTLY (read "expensive").

There is a set of drill bits designed for the furnace repair industry called "orifice bits". Typically, these range from a number 40 (0.073) to a number 80 (0.0135) which cover virtually all sizes necessary in most carburetor work other than carburetors with metering rods. Each of these drill bits has a "handle" (the drill bit is pressed into a roughly 2 inch length of 1/4 inch hex brass); so no pin vise is necessary; these may be used as is.

I know the above description sounds vague, but if interested, use your favorite search engine to see a picture.

I wore out several pin vises before my Dad, who used to work with natural gas, mentioned the orifice drills to me.

Whichever way you go, DON'T BUY THE CHEAPEST SET YOU CAN FIND. Carburetor jets are produced from a brass that is comparatively hard to minimize erosion from the gasoline. The cheap bits will simply not cut the brass.

Jon

May I play devil's advocate here. Besides being a B carb, what else can you tell us about the carb? is it a 1960 or something different? Does it still have the tag on it? Are you certain it has a 56 jet in it?

What makes you think you are running lean? The HEI can fire plugs hotter and longer than a points distributor. So you are not necessarily looking at the tip of the plug but rather looking at the ceramic when reading them.

Regarding drilling jets. I have rarely done this, preferring to get the size I wanted from the carb source; though I realize that's not always possible anymore. I have read that aside from orfice size, the shape of the jet's inlet affects the fuel flow.

Recently I've dealt with Amal and Mikuni motorcycle carbs, whose jets are stamped with the number of cc per unit of time (Minute?) so an idle jet stamped 20 flows 20 cc and a main jet stamped 160 flows 160 cc. Needless to say, the #20 idle jets plug up with silt that gets by the filters or whatever and have to be cleaned out. I use a gas-welding tip-cleaner and/or a dose of spray carb cleaner.

There are tiny orfices in automotive carbs too, particularly in the idle and transfer circuits within the carb body, and emulsion passageways. I don't remember about Rochester Bs but some enulsion jets have a whole raft of tiny radial holes and a very tiny orfice at the end. When these plug up, off-throttle hesitation is often the result and part-throttle leanness also results. The residue from ethanol mix gasoline plays the devil with these.

I have also read on various blogs I follow that one can't read plugs anymore like in the days before no-lead and ethanol laced gasoline. 'Tis said that the insulators will always be white because it was the lead that created the color. I don't know what to believe but I've not been successful at reading the plugs on my two Zenith 228 carbs on my stovebolt. Plugs are always white. I've seen lots of color on my Norton motorcycle engines that burn no-ethanol premium. I'd like to hear any opinions on this subject. But I don't want to hijack the thread either.

Back when I had access to a 4-gas exhaust analyzer and a voltage converter that allowed me to use it on road tests, (Thank you, public school system!) I was able to confirm that reading plug color was a very poor way to evaluate fuel mixture. One thing for sure- - - - -if a plug is black and sooty, you're not running rich- - - -it's more like REALLY RICH! An insulator that's blistered, or glassy-looking is a sign of a VERY lean condition. Anything in between is difficult to evaluate.
Jerry

That's exactly what I've found, Jerry. Gone are the days of looking at the plugs to get a good idea and I don't believe that is widely understood. Texas is changing/has changed the way exhaust gas is analyzed now. I've been hoping to find one of the old ones but they're long gone. Not that they were all that accurate but they would give an idea of relative rich/lean.

Jon, there are usually a few on Ebay. The trick on later model vehicles is getting a gas sample upstream of the catalytic converter. It usually requires that a bung be welded into the exhaust pipe ahead of the converter, and plugged when not in use. Then you'll need a pretty good length of heat-resistant flex tubing because of the temperature of the exhaust gas. I used my 4 gas machine during dyno runs on racing engines- - - -sampling the header collector exit gases. Quite an education!
Jerry

I've looked at those, Jerry. A friend here warned me off of them, though. He said you have to be really careful or you'll get one with either a bad sensor, bad battery (which are costly to replace I've heard) and one other thing I can't offhand recall. He said unless you're dealing with a seller who knows how to test them and guarantees them, it could be a bad deal.

Oh for a portable four-gas analyzer.

The best I could do, if I'd just get out the torch and do it, would be to weld a couple of 3/8-pipe bungs on the down pipes and wire up the Isspro pyrometer left over from my 855 Cummins days. Then I could keep track of the exhaust temperature and that would tell me both if I was working the engine too hard and if the Mixture was too lean.

Thank you Craig. I took my Carb of today took out the jet it was 57 thousands Drilled it out to.062 runns like a champ I will check spark plugs in 3-4 hundred miles and see what she is burning like.

Thanks to everyone's input

Thank you so much Jerry your a blessing.

Mike

Hello Mike.

The Carb is not a 1960 the best I could tell. I have the the 1960 auto choke carb that came with it. It looks like a car carb and air cleaner. but tag number 7019000. This carb looks like my 1955 1st series I restored in the 90's. I took it apart today it was stamped #57. The reason It seemed to me to be running lean it just didn't feel right when accelerating, even after I adjusted the valves even though it ran a lot smother. Before the HEI the plugs looked like they should in color. So I did some reading, and thought I should look at the plugs.

I drilled the jet out to.062 and it is so much better. I checked the listing of carbs for jet sizes from 50-62 on Mike Carburetors website for the 235 and chose .062 1/16 drill bit.

Any input is always welcome

Mike