Hello folks, i'm setting up carbs for my Chevy 261 engine, 2 Zenith 228s to be exact, and i have some setup questions. Little background info: Both are chinese 228 replicas. one was bought as a Chevy 261 replacement and the other as a Willys, but being 228s, differences should be jetting, venturi size and bolt pattern on the base, but when i changed the base out on the smaller (Willys) one, i noticed both had the same jets and small Venturies basically they are the same carb with different bases. I decided to change both to the stock Chevy size venturi, way larger than the one on them. but now i'm thinking i may have to change the jets as well to avoid running lean, is that right, Or should i do it the other way around and revert them to the small venturi? I imagine that way it wouldn't be that big of a performance upgrade over the single large venturi one, but i am a total beginner when it comes to carburetor setup, so what do i know...
You are doubling the amount of mixture to the engine. How much different is the jet size to the stock carb? Hook them up straight with no progressive linkage, make sure the vacuum and mechanical advances are working correctly, just don't look, actually use a timing light to check their operation. Any change in the camshaft? Headers? If not, the 2x1 setup will work great.
I haven't actually compared the jets to the stock carb, because the only one i have is half assembled, and who knows who has tinkered with that carb before. I will look up for the sizing, but knowing how often people around here (Brazil) share this kind of stuff on the internet, I'd be better off hunting a NOS carb to open and check the jets... As for the engine, it is all stock other than the log manifold with 2 carbs and headers (yet to be built) Sent from my ASUS_X00DDA using The H.A.M.B. mobile app
Ok, so it seems i need to go after a jet kit for a stock 261 then. I looked up and found the sizes for that one, but i'll have to look up what i have, as the people that made my carbs figured that no ones reads the numbers on them, so they didn't put any ... On a side note, on the Willys 161 and Chevy 261, the jets are all different between them, but both have the same CR (7.8:1), could that be the reason they got away with using the same jetting on both carbs despite the 100ci difference between the engines? (both carbs have been tested on their respective engines and i never noticed any problems, but then, i have only ran the 261 on the bench, never under load/driving)
If you have access to test equipment, possibly hook up an O2 sensor in your exhaust pipe just below the exhaust manifold to tell if you are lean or rich on the mixture? Yes, the old timers are capable of coming close, but this would get you "right-on" the proper mixture right away.
Breaking a long-standing personal rule about answering questions on the knockoff carbs, but since you are not located in the USA, here goes: Do some research about various applications that used the Zenith 228 carbs. I would suggest maybe those from a 240 International, or a 248 G.M.C. would be a good choice. Once you have made your choice, locate a matched by tag number pair of the genuine Zenith original carbs, and rebuild them. Echoing the post by jimmysix, unless you or a very good friend has built an identical set-up, ALWAYS build the carbs to stock configuration. Now you have a repeatable baseline from which to tune if tuning is necessary. Stock carburetors are calibrated specifically for an engine by carburetion engineers. Most enthusiasts consider fuel main jet size. Some also consider venturii size. Carburetor engineers consider, in addition to venturii and fuel main jet size: idle fuel jets, idle air bleed, idle restrictor, idle bypass, main air bleed, pump jet size, economizer jet size, and economizer activation value (generally, but not always, vacuum). If the enthusiast selects the carburetor(s) wisely, then the original carburetion engineers did most of the work for him/her. Final word of advice, check the second line in my signature block Jon.
Carbking is correct. I disagree with Both Jimmy six and Rick’s Garage. We are dealing with fuel/ air ratio, ideally 14.7:1 to best ignite and burn our charge. Compression ratio has nothing to do with it. Those carb engineers picked a Venturi size based on the engine displacement and rpm range and a main jet to approximate 14.7:1 as best possible throughout the range of throttle opening, thus air flow. Those carbs used singularly or as duals or whatever, will run just fine with stock jets on any engine up until the point that your engine’s breathing is being restricted by the size of the venturi’s. They have no idea what engine they are hung on and don’t care. Just don’t hang four 97’s on a Crosley. I see Jimmy six’s post is now deleted and Rick’ Garage statement would be correct if he meant you were splitting the mixture in half’s through each carburetor. If you doubled the mixture the engine wouldn’t run at all.
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