I have a 3x2 Rochester set-up on my car 283 SBC. The car took a dump on me on the way to the Jalopy Showdwon last month. I still haven't got it back right yet. Changed the Distbutor(57 Vette 2 point) That helped but it still won't idle. It chugs pretty hard to start when it's hot( won't hardly start at all when it's cold). It kind of "climps" then finally starts. We checked just about everything. My main question is, if I uncover the center carb the idle drops to nothing. If I cover the top of the filter with my hand it jumps back up?? I have ordered new throttle shafts for them. Hopefully I'll get them today. The car will idle fine then jumps around all over the place. It ran like a dream then all Hell broke loose. ( is it a vacume leak?) I also am running an electric fuel pump. Im going to install a regulator this week. What pressure should I be running. 3.5?? Is this the problem? Thanks in advance, Todd
Are the end carbs' throttle plates 100% closed at and idle? If not that is a huge vacuum leak. Check to may sure an accelerator pump didn't come apart inside and hang one end carb partially open at idle. Had this happen on my MAN A FRE, it ran very weird and the throttle pedal felt funny. Fuel pressure should be about 4.5 lbs. Marty McF.
Yes, if idle speed/quality improves. Your hand is reducing the amount of air entering the carb thereby increasing the fuel ratio slightly. Old mechanics trick I learned from...old mechanics. I had reused a GM pump shaft and replaced the seal only, the damn shaft separated at the metal from plastic part and jammed the throttle partially open, may have happen to you but not sure. Marty McF.
I just swapped out my 2GC due to a cam and intake manifold change. If you place your hand over top of the carb and find that the idle speed picks up, you have a lean condition. If everything was working fine with your initial setup and this problem started, its possible that you could have a vacuum leak. When the car is at idle, you are on the idle circuit only, the accel pump does not come into play. Let us know how things work out, will try to assist as best I can on this one.
I agree with the lean condition diagnosis, either caused by a vacum leak or some internal blockage in the carb (dirt in the fuel bowl or idle circuits?) You can hunt down vacum leaks with a propane torch if you have one. Just barly open the torch valve and then slowly make your way around pointing the torch at the base of the carbs, around the intake, any vacum plumbing. If you hit a spot and the engine speed picks up then you've found your vacum leak.
I mentioned the accel pump because if it hangs up (came apart inside carb as one recently did to me) it could hang the throttle plates open on the end carbs at idle creating a vacuum leak. Marty McF.
One other thing that could be the problem. With the butterfly on the carb closed, you will notice a small retangular slot and next to it is a round hole. Both of these are your idle and off idle circuit. They do have a habit of clogging and giving you the conditions that your are having. You can either spray this throughly with carb cleaner or use compress air. This is a good place to start and or a suspect vacuum leak. Here is a link to a book that i have. On page 16 there is a picture of the curb idle and off idle vents that are next to the butterfly that could be plugged. http://books.google.ca/books?id=DwU...=X&oi=book_result&ct=result&resnum=1#PPA16,M1
WOW. Thanks to all. I sprayed 2+2 around the carbs. It picked up around all 3 throttle shafts. I still don't believe this is my ONLY problem, but could be. They were bad when I put them in last year. I'mm thinking diirt somewhere. I read somewhere on here to take out the bronze filter in the center carb. Is that a good or bad idea? I am running a settlement bowl sett-up.
I would leave the filter in and this is part of the setup. If the three shafts are giving you a vacuum leak, that is possible. But for the car to all of a sudden give you the problem as mentioned I think that some type of blockage on the main center carb would be the first place to look.
I don't run them, they are simple to use but can cause a bigger flow restriction than an inline filter. If it is the throttle shafts leaking here is a company, http://www.supercooltools.com/html/ZA01/index.html, who sells a tool and bushings to seal up the shafts which work great. Replace the throttle plate screws (you have to cut/grind the original ones to get the crimp off) with stainless counter-sunk screws and lock-tite and stake the bottoms to prevent them from coming out. Marty McF.
When one has idle issues with a 3 x 2 setup; the first step is to isolate the center carb. This is done by putting blockoff plates under both ends. Once this is done, the end carbs do not effect the problem. Tune the center carb. Once the center carb is properly tuned; remove one of the blockoff plates (from either end carb); and restart the engine. If you have genuine end carbs with the THICK throttle plates, and they have been properly rebuilt, THE IDLE QUALITY WILL NOT CHANGE! If the idle quality changes, repair this end carb. When there is no difference in idle quality when the first end carb is introduced, in a like manner, introduce the second end. Jon.
If you have genuine end carbs with the THICK throttle plates, and they have been properly rebuilt, Jon.[/QUOTE] Yes to the first part. Properly rebuilt? I thought so.
OK I'm getting closer. I changed out the center shaft. It was worn pretty bad. I changed out the pump with an original leather one. The car idles fine runs great. But... when I rev the engine it takes about 5 seconds for the engine to come back down to idle. If I rev it up to say, 2000 rpms it will drop back down to 1500 quick but gradually get back to idle at 900. I still have not put in my fuel pressure regulator. Im thinking maybe im pushing feul through the secondaries. Im going to try that next, then go to blocking the end carbs off and see where the troubles lay. Any thoughts?
You should be able to look and see if the end carbs are flooding. With no regulator I wouldn't be surprised. I run mine at about 3.5 lbs. Once I got 'em set up correctly I haven't had to mess with them at all in three years. At idle, put your hand over the secondary carbs, one at a time, and see if anything changes. If it does, then your butterflies are not closing all the way. Until that's fixed it'll never run right. If no change then your problem is with the primary carb.
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