Thread: 3 dueces

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A buddy-o-mine has a 31 Roadster with a sbc and 3 dueces. I timed it and set the dwell for him a week or so ago. I went back over there lastnight to try and fine tune the carbs, and although he is pickled tink about the way it runs, I don't think it's right. I tried tying the other two in for secondaries(mechanical linkage), and when running it and trying them it fell on it's face. What gives? I also noticed that the two outer carbs have idle mixture screws, I thought they are only secondary and son't need the idle mix screws. How should these guys be tuned? I've got good vacuum readings on the gauge, and it doesn't sound bad, just isn't quite right to me, but I've been told my head is crooked too, so it may just be me. All info is greatly appreciated.

If you have any friends who are mechanics on multi-carb jap bikes, see if they have a set of Sync tubes you can borrow. They work great for multi-carb tuning.

Are the accelerator pumps on the end carbs working? Do they have them?

Sync tubes? Don't know anyone off the bat. As far as I can tell the two end carbs do have accelerator pumps, and by way of how the car acted when they were opened up I can only ASSUME they're workin, but we know how that goes. The car does run fairly well on the one carb, but I'm thinking the other 2 are presenting opportunity for vacuum leaks, cause the min. air bleed screw is run almost all the way in. I've tuned many carbs, but never a 3 duece, is there any way I can adjust them without having to buy a tool that may never get used again? Am I looking in the right direction with my thinking, or spinning my wheels aimlessly? Like I said, he's tickled with the way it runs, I am the one that's not completely satisfied, and I know he'd like to have all three working together, and not just have the look of the setup.

take the front and rear accelerator pumps loose. thats to much fuel at one time.

Disconnect the accelerator pumps on the end carbs and screw the idle mixture screws right in and the idle speed screws right out. You may have to go a size bigger on the jets on these two carbs too, before it will run right. The end carbs shouldn't be open at all at highway speed.

I assume your dealing with Rochester carbs ?? If so the only way I know of to make em right is purchase the kit from either Speedwaymotors.com or vintagespeed.com it's gonna set ya back about $350 bucks, kit consists of linkage,gaskets,jets,power valve block offs for front & rear carbs, a pair of base plates without adjusters for the front & rear carbs and instructions....

OK, first take a methodical approach, to insure that it's not a non-carburetor related issue. A couple of things to check first are:

The mechanical advance is working and your getting around 32 degrees or so total advance at around 22-2400RPM.

You do not have any vacuumed leaks.

The end carbs are getting fuel.

Unless you are running all three carbs synchronized (all three always opening the same amount at the same time) using the sync tube is a waste of time. If the manifold is running 3 3 bolt Stromberg carbs you may find you do want to run them synced due to their low CFM rating, but it sounds like you want them progressive.

DO NOT DISABLE THE END CARB ACCELERATOR PUMPS (this gets real ugly with an instant extremely lean condition usually followed by a major back fire (and sometime fire) through the carbs.)

Usually it's difficult to get a decent idle with the end carbs having idle circuits. If you've got it to idle decent you may not want to make any idle circuit changes. I cover how to modify the carbs to delete the idle circuits in a couple of the posts below along with building linkage. You do not need to spend $350.

If you problem stems from the end carbs being too rich, you MAY need to disable the power valve, again a simple procedure that's cover in the posts listed.

There is also a possibility that your linkage set up is not opening the end carbs enough and may need some fine tuning.

Let us know what carbs your running and what size the engine is and what mods have been made to it.

Check the posts below it should give you a fair amount of info on carb mods, sizes linkage etc.

http://www.clubhotrod.com/forums/sho...threadid=13948 (Page 2 covers Carb Mods)

http://www.clubhotrod.com/forums/sho...threadid=12288

http://www.clubhotrod.com/forums/sho...&threadid=9262

http://www.clubhotrod.com/forums/sho...&threadid=7704

http://www.clubhotrod.com/forums/sho...&threadid=7436


The pictures of the linkage I posted previously apparently went away when the site crashed a while back.

This is the linkage on my stroker ford FE

And the linkage on the Hemi

One way I adjusted the carbs with out sync tubes was to use a piece of rubber tubing. I stick the tube in my ear and laid the other end on top of the carb, first the center than the front than the back adjust each carb until they all sound the same. You may have to do this a few times but it does work.

I appologize for not sharing the make of the carbs. They're Rochesters, sitting attop a mildly built 350 Chevy. I didn't build the engine or the carbs, everything is still almost off the stand rebuilt. The engine has been run before I started looking at it. I went over to check it out because it was real rough, # 5 & 7 were crossed, and the engine didn't have a timing tab on it(oversight?) and the dwell was of. I set the dwell, and guessed at the timing first, then he went and got a tab, and now the timing is 36 total timing. The linkage is set up with the center hooked to the front with one rod and another rod going to the rear from the front. The piece going to the front from the center slides through a guide and there's an adjustable "stop" to tie them all together when you want the others to come in . Hard to explain without a pic. Thank you for the info, I'll read it thoroughly before I go back over there. I'll keep you'uns posted on the progress.

Great, keep us posted on the progress. You linkage is set up like I have on my Ford.

Make sure the linkage is set up further from the throttle shaft on the center carb than it is on the end carb (this determines the speed and the amount of opening on the end carbs).