I'm running a 3x2 setup on my 56 olds that uses the center carb for most driving. I set it up this summer but was not totally satisfied with the results. Now that the weather is much cooler, it is running much better. I'm thinking this is showing me the main jets are too big. Any thoughts on this. Carb is a Holly 94. Thanks Chuck

 

If you want to adjust it to the specific O2 level, you can get O2 sensors that can be hooked up to a gauge, and mount the gauge in the car. You would get a direct readout of mixture. You can get a bung, and weld it to the exhaust (at least 18" from the end, and on top (within 45deg). I wish I had the availability to put an O2 sensor on cars I had back in the 70s.

 

OP,
Fred is right, if you want to get it right without a lot of trial and error. The 02 sensor in the exhaust stream gives you real time feedback on just how close the carb is to stochiometric optimal.

Once you get there for idle, part throttle and WOT conditions, you can move onto whether that perfect theoretical ratio is what’s best for your car.

The other alternative is basically iterate carb settings using feel, time and parts.

Cheers
Chris

 

Thanks for your input on the O2 sensors. At this point I'm not in a position to install the sensors. I was just looking for a confirmation on my deduction about going to smaller jets. I have smaller jets and being a pretty old, retired guy, I can do some tinkering in the engine compartment easier than I can install the sensors. Chuck

 

Interestingly enough I tried using the AI mechanic on the Hagerty website. Using the same info I provided earlier the reply explained in great detail why the jets were too large and even recommended what size to use. Supposedly the answer was vetted by a knowledgeable human. Just saying.

 

You can always remove your spark plugs and see what they tell you. I'm no expert, but I'd start here over throwing parts at an engine suggested by an AI.

https://www.jalopyjournal.com/forum/...reading.61897/

 

Olds 64, Thank you for the link on sparkplug readings. I have tinkered with these carbs off and on for many years, trying to sort out the numerous issues that they had when I bought them; sticking throttle plates, float setting due to the front rake on my car, sticking float valves, missing springs under the accelerator pumps, unblocked port holes, just to mention some., I believe I have now located the remaining problem with the main jets. The sparkplugs are, and have always been "sooty" in appearance, no doubt that it's running rich from day one. Now with all the other issues seemly resolved it seems pretty easy to say the jets are too big. It takes about 4 hours to remove the carb linkage, the carbs, block the gas lines, install blocking plates on the #1 & 3 carbs and put everything back together for a test drive and then reassemble. All three carbs must be removed in order to access the center " working" carb. Gaining knowledge on 70+ year old carbs, and what actually works is not as easy as some of the online videos suggest. Just thought I would share some background. Thanks again for your help. Chuck

 

If you're set on changing the jets in carburetors, you can get a kit of Holley jets.

https://www.summitracing.com/parts/sum-210230

I don't know if your Holley 3x2 setup is standard or progressive. You'd probably have to get at least 2 kits to properly tune your carburetors. That'd be a significant investment to change your carburetors without the aid of an O2 sensor.

 

Olds64. Again thank you for the link to Summit. I find it will be more cost effective to order from Vintage speed. The cost there is less than 10 bucks each with shipping. I have 6 on order now of .047 and I will work my way up from there if necessary. Thanks, Chuck

 

Mike's carbs references using different power valves on tri power setups. Maybe a stainless shim base gasket to prevent heat absorption in the summer.

https://www.carburetor-parts.com/hol...BOtls96i8WAII8

 

Keep us posted!

 

Shiftbyyear: Thanks for the input. I have experimented with a number of different power valves. I'm currently using a 4.5. On a tri-power with a vacuum reading of about 19, a # 5 is a good starting place. One of the problems I have had is that every time you get a backfire, it blows the valve. And with all the carb problems I had, referenced in an earlier reply, I went through several. Always the question, did the new valve cause the backfire that doomed it? At this point I have not considered a stainless gasket but will take it under consideration. When I change the jets in the spring, I will post an update. I'm too old to be working in an unheated garage. : ) Merry Christmas to all from Chuck