1/15/2020 Update: Had a great experience talking to Ron over the phone at the Daytona Carburetors company. Sounds like a nice small outfit. Very knowledgeable gentleman. He took his time to listen to my technical questions and answered them clearly and calmly. It sounds like he's been working with carbs and in the automotive industry for a long time. What I learned. One is that originally GM ran the Zenith carburetors on these engines, (im not sure if he was referring to the 261's or stovebolt engines in general) and that these are a direct copy of those zeniths. These had the 32mm venturi on them, which was meant for these engines. I was happy to hear that and that they were OEM equipment on these Chevy's, that makes us purist a bit happier. I mentioned that I wasn't getting the high end performance I was getting before with my Rochester, he mentioned that perhaps my air cleaner wasn't allowing enough air to enter, or I simply hadn't richened the high speed jet enough and that I should first find out how many turns I had in fact turned out, by starting from the start (turning in and bottoming out the needle, its a very fine adjustment as I was told). I found out I was 6 3/4 turns out. I went 1/2 turn out more as instructed to see if that helps and to keep going out in half turn increments to see if I notice an improvement. He also suggested I test-run my truck without the air cleaner to see if that improves it. Another thing I learned is that the carb kit comes with a plastic red adapter to accommodate for larger air-cleaner base plate openings. He mentioned the diameter I believe it was 3 1/2 in. for the earlier type but they changed in 1960 I believe he said. I didn't quite understand how its supposed to work. Another thing I learned: I mentioned that I was looking for an adapter fitting (male to male) that would go from my glass bowl fuel filter to my UN2 carb. The one I had that worked on my Rochester was 1/8th to 1/8th mpt. but the inlet for the UN2 is a metric thread. So at no extra cost, (since I had ordered the carb from them) he said they would send me the 1/8th to metric fitting to my address so I can run my glass fuel filter again. Great customer service in my book. He wants me to follow up after I try the adjustments. Very content with my experience and am looking forward to dealing with them again in the future. p.s he also suggested I run only 87 octane fuel because it has a higher boiling or percolating point which is 180-185 degrees Fahrenheit while 93 octane is approximately 40 degrees lower around 145 degrees Fahrenheit. Good to know.
Last edited by ClassicsR4Me; Fri Jan 17 2020 11:22 PM.
Classic - with all due respect, your last post is historically inaccurate.
Chevrolet DID use Zenith as original equipment..............................from 1912 to 1925 on a 4 cylinder 171 CID engine. These were updraft carburetors.
Chevrolet DID use Zenith as original equipment on their 6 cylinder of 299 CID......................in 1912 ONLY. This also was an updraft carburetor.
The last Chevrolet to use Zenith as O.E. was 1925.
Zenith did offer a replacement carburetor for the stovebolt 216.
Zenith did NOT offer a replacement carburetor for either the stovebolt 235 or 261
Zenith did offer a number of "universal replacement carburetors" of different venturii; and the customer was responsible for picking the correct one for his/her application.
GMC DID use Zenith as original equipment on many different size engines, and we have been recommending these GMC versions for years.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify If you truly believe "one size fits all" try walking a mile in your spouse's shoes! [image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Thanks for clearing that up Jon, I must have misunderstood Ron, as we spoke I was jotting down what he was saying. I have heard though that the Zenith carburetor is the better carb to have for our engines, and this is a pretty much direct copy. Do you know if perhaps they came as standard equipment for GMC trucks, because I did write that they came as standard equipment on GM products. That might be where the discrepancy lies.
Last edited by ClassicsR4Me; Thu Jan 16 2020 12:49 AM.
Back in the early 1950's, Dad used a big Zenith 2-barrel carb designed for a gasoline engine Mack truck on a flathead Ford dirt track engine. That carb had removable venturis, which he put in a lathe and bored out until the restriction was barely there, just enough to develop a little vacuum to pull the fuel from the float bowl. Then he drilled out the main jets to the point they would break off if they were tightened too much. Why all this modifying? He was running a mixture of alcohol and Hydrogen Peroxide as fuel, and the engine needed a LOT of it to run right. 8 laps per gallon on a 1/4 mile track was about average, running a 300 cubic inch engine. That single carb routinely outran competitors running three Stromberg 97 2 barrels. All the mixture tuning was done on a chassis dyno owned by one of his sponsors, one that was normally used for road tractors. Mixture adjustments were done by changing the high speed air bleeds, meaning that to get a richer mixture, a smaller air bleed was used. Jerry
The murder victim was drowned in a bathtub full of Rice Krispies and milk. The coroner blamed the crime on a cereal killer!
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
1/17/2020 Update: Took my truck out on another 30 mile round trip stint. Runs beautifully with the UN2 Carb. I tried the two things that were suggested to me, one, run the engine without the air cleaner to see if it was too restricted, two, I also backed of the fast idle screw a bit more. I was originally at 6 3/4 ill say 7 to keep it rounded. so I went back in whole turns to notice the difference I kept going to about 11 turns out and then notices a bit of fuel coming from the adjuster screw, I guess I went too far, so I backed it back in. I can say now with confidence that this carb has the same or just about near the same power as my Rochester in the top end. The idle is better for sure, and the low end torque feels as good or better. That coupled with the fact that its a new carb gives gives me confidence in my purchase. Now just to wait and see how it keeps up as time passes. Also, we'll see how much gas mileage it gives me, the same, less, or more. So far so good.
Last edited by ClassicsR4Me; Fri Jan 17 2020 11:22 PM.
Hi Jerry, Ive been honest since the beginning, im not a scientist or a professional mechanic. Just your weekend home mechanic that loves classic Chevys'. Nobody had tried this carburetor, so I thought I would check it out and document my experience, like anybody else would do at home. If you get something good out of it, then that's great. Im not recommending anybody buy this carb or not, just an unbiased test from a regular guy.
Yes, thanks for posting. Your experience and reporting will help others who are curious and want to read how it worked for a real purchaser and not some fabricated online review written by a company employee.
Last edited by tom moore; Sat Jan 18 2020 07:46 AM.