The Stovebolt Site Forums The Shop Area Engine Shop Weber 36/32 Jetting?

I want to convert my 67 250ci in my GMC to a Weber 36/32.

I bought the carb adapter and I have a couple of carbs and was wondering what others are using for jetting ?

I would like to know the same thing. My situation as follows:
1) at 5400 ft elevation
2) Chevy 235 cu in
3) Weber 32/36 installed a couple weeks ago ( actually an EMPI, a china knock-off from Tom Langdon bought several years ago)
4) mechanical fuel pump 5.5 psi, fuel flow in gph?
5) Running very rich, plugs black, AFR reading 10
6) truck does run/drive smooth, have had it to 60MPH, but due to rich condition have not driven it much
7)Jets out of the box;
Idle Prim/Sec = 75/60
Main Prim/Sec = 145/150
Air Restrictor Prim/Sec = 160/170
Emulsion Tube Prim/Sec = F50/F50
8) Am now trying to figure out what jets to buy.
Have you got your carb dialed in?

James, are you even able to purchase different jets for your Chinese Weber? Most knockoffs I'm aware of do not have parts that interchange with the original American versions.

A Weber 32/36 carb is meant for a 130 cubic inch engine ( or there about ). You should be running 2 of them for the correct cfm. I run 2 Weber 32/36 carbs setup on a 261 and it runs great. I will tell you the Webers are very fussy about clean fuel. Very small particles can plug the jets and cause what you are describing especially the idle jets. In the past I have seen jet kits for the Webers, but you need to look into a dual carb setup or get a carb to fit your 235 cubic inch engine to have the correct cfm.

You have about half the CFM you need, so you're going to be fighting a losing battle. That being said, the over rich condition is probably due to fuel pressure, not jetting. Webers only need about 2 PSI of fuel pressure, and if you're running an original type mechanical fuel pump it can go to 4-5 PSI or more, depending on the spring the manufacturer chose to use in the pump. The overpressure pushes the needle valve in the float chamber off its seat and causes the carb to run one of two ways- - - -rich or REALLY RICH! If you insist on running the wrong carburetor, install a good adjustable fuel pressure regulator.
Jerry

I am reading specifically that Weber jets fit the EMPI. I have a 30 micron sintered bronze fuel filter after the pump.
The tech page at TopEndPerformance (racetep.com) claims 4psi is perfect and they sell an appropriate electric fuel pump ($69 and no regulator, no return line required, for <150HP, 4 and 6 cyl, street). So, I have a source for that issue if just re-jetting is insufficient for my not a race truck. Also, I am running the Langdon HEI, 7 deg BTDC, ported vacuum working fine, new plugs, very good compression, and non-ethanol gas in a near new fuel tank and pump. I am reading that people have installed this carb on a 258 cu in AMC engine and are happy with the result on and off the highway. So, there's that, as well as youtubers doing the convert on a 235 and happy with the result. And the OP only asked what jets people are using, nothing else.

Update: My jetting is now as follows: (1) both emulsion tubes are F50, (2) Air jets are Primary=195, Secondary=215, (3) Main Jets are P=115, S=120, (4) Idle Jets are P=55, S=45. I am still at 5.5 PSI fuel pressure with just the mechanical pump and 30 micron filter. AFR typically ranges between 13.5 and 14.5. Plugs are no longer black. I am using the E3.52 plugs recommended at devestechnet.com. Also, my engine is bored 0.030 over. Nice smooth operation throughout the RPM range. This is a successful upgrade. The jets from Pegasus Racing fit the EMPI Weber 32/36 perfect. I recommend (1) stay away from the cheap Amazon knock-offs, (2) install an AFR, (3) start with the Idle jets, getting them so the Idle Mixture screw actually has an effect, (4) be patient as you iterate through jets to get a solution. It took me 14 iterations and numerous jets. I can return jets I am not using. My Autometer AFR is digital. The analog gauges seem to have reliability issues. Changing the Idle Jets does not require any disassembly. Changing the others requires removing the carb top. Due to difficulty handling it, I abandoned the teenie tiny C-clip securing the choke rod to the choke arm. I use a wire wrapped into the slot where the C-clip goes. The wire I use is out of a twist tie available in the Safeway produce department. No failures in all my test drives. Changing the Main Jets requires emptying the bowl. I used my hand vacuum pump to suck out the gas into a receptacle. A further note on AFR readings; (1) when pulling hard up a hill or accelerating hard, the reading can drop to 12, (2) when letting off the accelerator pedal and letting the engine slow the vehicle, the reading can increase to 16. I suspect this is normal for a carburated engine, and fuel injection/electronic ignition/computer control configurations corrects these situations.

I will now address how I analyzed the CFM claims. First, the Rochester B for a 235 has a bore of 1 and 9/16 inches, giving a bore cross sectional area of 1.92 square inches at WOT. For the Weber 32/36 at WOT, the total bore cross sectional area is 1.24+1.58=2.82 square inches. The Weber presents a nearly 47% increase in bore cross sectional area at WOT. So, my gut feel was that the Weber 32/36 will rate a higher CFM than the Rochester B, and the number I came across for the Weber 32/36 is 230 CFM at pressure drop 1.5" Hg, which I guess is for WOT. Second, the standard equation for an engine CFM = (cu in displacement) (Max RPM) (Volumetric Efficiency) / 3456. Using my engine displacement of 239, Max RPM of 3200, and a VE of 0.75 gives a required CFM of 166. Now, with the Weber installed, when I am at 60 MPH, the RPM is 2400, and I have pedal remaining for more, and in fact at 65 MPH I still have pedal available for more. So, I think the Weber 32/36 provides plenty of CFM for the 235. And my acceleration with the Weber is as good as it ever was if not better.

And just one more thing. I have read that the Rochester B needle valve can hold back up to 10 PSI fuel pressure. I have read that the Weber 32/36 needle valve can hold back up to 8 PSI. My 5.5 PSI is well below 8. So, I decided there was no reason to think I was flooding the carb with my mechanical fuel pump, and my AFR readings after re-jetting back that up. Also, the way the fuel pump is designed, once a pressure of 5.5 PSI is reached on the output side of the pump, it will not keep pumping more gas out. The needle valve must let gas through before the pump can push more gas into the line. So, the fuel pressure will never be higher than 5.5 PSI, which is exactly what the fuel pressure gauge shows. The mechanical fuel pump was never the issue for my initial rich condition. Reducing the size of the idle and main jets, and increasing the size of the air jets solved my Weber 32/36 issues for my engine at my altitude.