The Stovebolt Site Forums The Shop Area The HiPo Shop quick question on 2x1 setup.......

i'm sure this has been covered but its not on the tech section and i couldn't find it searching..... anyway getting ready to fire up my 235 with a offy 2x1 and rochesters but not sure how to run the vacumn from the dist. advance. do i run it to one carb? both? if just one which one?

tks

Either one.

Big Ed,

The question of this thread dealt with the vacuum advance connection, not the choke.

Stuart

Some prefer to run the advance off the manifold port instead of the carb port. Stronger vacuum signal is what I was told was the reasoning behind running it there.

Bill,

Pulling the vacuum signal off the intake manifold will give you advance at idle which isn't required. The ported vacuum signal is blocked by the throttle plate at idle so you get no vacuum or advance.

The different locations result in different advance dynamics. I think this is how it works. Most applications call out ported vacuum for the advance can...I think.

Stuart

I'm with you Stuart. I wasn't really condoning running it off the manifold port, just noting that some do run it there. personally, i would run it to the ported vacuum off the carb as well. Or at least I would if I had a vacuum advance on mine .

ok guys i ended up hooking the vacuum up to the first carb (closest to the radiator) now the truck runs but weak. i noticed i'm getting a lot of vacuum/suction from the first carb and almost none from the 2nd carb. if i block the air from the first carb the engine dies, if i do it to the 2nd it doesn't skip a beat. i pulled plugs #1 and #6. #1 looks brown which is good, #6 looks super super black. so i'm guessing the 2 rear cylinders (#5, #6) are running way rich. the linkage i'm running is completely adjustable and home made but i dont think thats the problem. i've tried messing with the 2nd carbs mixture screw and it doesn't do much. i have carb 1's turned in till it almost wants to die then half a turn out. when driving it, it's lacking major power. it will move but seems to rev like crazy and go almost no where. i checked the brand new vacumn advance on the dist. and it working fine. the dist. timing mark is set to "0"

little history the carbs are rochesters from a 60's chevy truck. both have fresh kits in them, no clue what jets are in there. this is a 235, 040 over, with dual exhaust and the offy 2x1, cam and head are stock. motor and head are fresh rebuilt. when i was running single exhaust and carb the engine/truck ran awesome. only thing thats changed is the 2x1 and the duals

i'd appreciate any help or pointers you guys can give me!!!!

this is my set up by the way.....

home made linkage
http://i121.photobucket.com/albums/o235/marcelo54/fiddy2010.jpg

http://i121.photobucket.com/albums/o235/marcelo54/fiddy001.jpg

fuel line set up
http://i121.photobucket.com/albums/o235/marcelo54/fiddy006.jpg

http://i121.photobucket.com/albums/o235/marcelo54/fiddy014.jpg

http://i121.photobucket.com/albums/o235/marcelo54/fiddy009.jpg

If you switch the positions of the carbs and the symptoms reverse then you would know for sure that the carb guts are different and need to be gone thru. There were many different power pistons among other things inside that may be mismatched.

Good luck.

coolerthanethan,

That is a beautiful looking setup, a lot of attention to detail and your work is very well done. The linkage for the Offenhausers is a real tough gig as you have found out.

Have you disconnected the linkage between the carburetors and done a preliminary adjustment to each carburetor with the engine running? You should be able to do this and it will make tuning them together easier.

It's important to get the throttle plates closing completely, then to get them adjusted with the idle speed screw exactly the same. Try doing it one carb at a time with the linkage removed.

The small single bolt where the linkage attaches to each carburetor throttle arm is a real bugger. If it isn't completely tight and locked, it will move and then one carburetor will be down on the idle screw and the other won't.

Your pictures are excellent and the green engine is neat too, I like it!

Stuart

Stuart,

tks for the props and the tips. i havent tried tunning the carbs with the linkage disconnected so i'll try that next. and also make sure the t-plates are closing completely with or without the linkage attached. i used all nylon locking nuts everywhere on the linkage exactly so they wouldnt get loose. might not look it but once i set the screw that attach the little aluminum plates to the carb linkage they really dont move.

as for switching the carbs around i have 3 of these carbs which i rebuilt and the first one i put in position 1 ended up leaking bad so i swapped the 3rd one into that position. i haven't however swapped 1 and 2 around each other.

i'm guessing my lack of power is the rich condition. i went ahead and pulled all the plugs out and 1,2,3 look brownish while 4,5,6 are super black. i'll mess with it come monday again and report my findings. tks for the help guys!!!

1 more thing you might check, make sure the carbs are truly a matched pair. I ran into this problem with a pair of Carters. They were suppoed to be a matched pair, same part number and such. But it turned out one was actually for a different application, and was jetted differenty, along with other internal differences.

they're not a true matched pair. i know that but very close.....

well messed with them some more today. first i disconected the linkage and the truck was running as b4. i adjusted the mixture screws and the 4-6 cylinders were still running rich when i checked the plugs. so i swapped their positions and the one carb thats sucking alot of air still does it in the #2 position. i also had the vacumn advance from the carb disconnected from both carbs and both plugged. had no effect so thats ruled out. so i pulled both carbs and took them apart, internally they are identical, although the power piston on the carb that was sucking alot was hard to move up and down. so i swapped from the spare carb and ran some 1000 grit sandpaper through it and it moves freely now. i checked all the parts and everything is where it should be. finally i discovered the number of the jets the one that sucks alot has a 56 and the other one a 54. from my understanding there is no way to get these jets correct? i've heard of people soldering them shut then drilling new holes with tiny bits?!

anyways i threw them back together and put them back on the truck, now it wont even idle. i'm kinda fat up with these and thinking of just buying two trully matched rebuilt carbs, anyone know where to go for such?!

Bummer. Here's some more bits of information:

http://www.thecarburetorshop.com/RochestermultipleB.htm

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

There's also an article on multiple Rochesters on the tech tip page at inliners. Good luck.

yeah i've read through that site..... and even called them up, guy talked a bunch of smack about rochesters and then tried to sell me a couple of carter w1s at $590 each!!!!! they actually seem like a good choice from what i've been reading but umm $590?!?!?!

"when i was running single exhaust and carb the engine/truck ran awesome. only thing thats changed is the 2x1 and the duals

i'd appreciate any help or pointers you guys can give me!!!!"

Isn't the problem pretty obvious? You've got one too many carburetors!
Jerry

thanks for the clever and super helpfull reply!

It might be a lot of work, but put the single intake back on and run each carb till you get them just right. You might be able to do it on the dual intake with one hole blocked off. They will work if you keep at it. And yes in a pinch, you can solder up the jet holes and re-drill them. Use quality drill bits and a drill press to get a clean straight hole. The drill size will not always be the jet size so measure before you buy anything. And your jet size will be very close to the original size that came in the carb, with in 1 or 2 I would bet. Start lean, two sizes smaller and work up .001" at a time. You won't be able till much from idle time, it will take road time and seat of the pants feel to know for sure. Once you know the jet size, then you can shop for factory jets from junk piles.

A uni-syn ( http://www.speedwaymotors.com/Uni-Syn-Carb-Synchronizing-Tool,267.html ) gauge will be a great help. Also, don't bother with the linkage till you can get it to idle. Dual springs might be needed to insure each throttle blade is returning to the set screw. Place a thick solid gasket under one to check the other, its the easyest way to find the problem. Joe

LOL Your fondness for dual carbs and cats always cracks me up.



I'm told the jets in a 2G will fit so if you find some cores laying around you might find matching pair. I bought my carbs at a flaps and they were done by Canada's biggest? rebuilder. They were $90.00 Canadian w/$50.00 core and were good to go out of the box.

Curt, I'm not totally against dual carbs, or even triples- - - -if that's what the factory put on the engine to begin with! All through the 1960's, I made a pee pot full of money rebuilding and synchronizing the S-U carbs on MG's, Jags, Triumphs, etc. Then Datsun got the brilliant idea to put the same design carbs on their Z-series cars, the 240's and 260's. More big profits through most of the 70's! The 60's muscle car V-8's with the triple 2-barrels were fun and profitable too! Of course, a well set up 4-barrel could leave them all in the dust while they were spitting and backfiring, and burning the paint off the hood!

The problem with duals on stovebolts is that people put two of the same carburetor that was working well as a single setup, and expect them to work. Too much airflow, not enough venturi velocity, so the things go from WAAAAY too lean to super-rich as the power valves open up. There's a right way to make a dual-carb setup work, but nobody's interested in doing it that way- - - -they'd rather gripe and complain, and spend lots of time tinkering with a system that can only be mediocre, at the very best.
Jerry

Jerry, the same problem exists with most engines. Many enthusiasts try to buy carburetors to match the manifolds. When the manifolds were sold new; the guy that purchased a dual one barrel manifold for his stovebolt ALSO purchased a dual point distributor, a set of domed high compression pistons, a 3/4 or full race camshaft, headers, and a higher numerically rear end gear. Many don't even stop to look at factory examples. When Chevrolet offered the dual quad engines for the 283 the single quad engine used a carb of approximately 485 CFM. The dual quad engines got a bigger cam, better exhaust, yada, yada, yada BUT TWO CARBURETORS THAT WERE APPROXIMATELY 415 CFM! NOT 2 of the 485 CFM single units. Same is true with your sports car examples. The big Healy used a pair of H-6 carbs, but when they upgraded to 3 carbs, they used H-4's.

But as always:

The most expensive carb one will ever buy.....
is the wrong carb one tries to modify!

Jon.

Jon, the shop where I did my growing up was a Carter factory-authorized service center, so anyone who did anything to a carburetor there was a graduate of the Carter factory school. We tuned virtually all the hotrod Oldsmobiles in a 5-state area around Tennessee, and I'm sure you remember the growing pains the 4GC Rochester had when they first brought it out in 1953! Lots of the research and development for the 1954 modifications to that carb happened in a little 5-bay shop in Nashville, with my Dad and a Rochester field engineer bouncing ideas off each other. I was the kid hanging out in the corner of the shop trying to understand what they were talking about.

It's been a fun ride, fixing other people's screwups on carburetors for the next 40 years or so!
Jerry

Jerry,

Now 'coolerthanethan' is the kid hanging out at the corner (forum) trying to understand how to make his setup work. We have seen all your credentials, why don't you share some wisdom with him.

Stuart

Believe me, if there was any way to work with what he's got, I'd be the first one offering advice. He's fighting a losing battle. The first, and probably only thing to do is go to a much smaller carburetor. The Carter YF designed for a 216 might work, if he can get two of them jetted exactly the same, and synchronized properly. Jon can probably offer more exact advice along that line than I can, since I've been out of the carburetor end of things for several years, teaching computerized fuel injection.

If the manifold cool is using fits a Rochester, it would need adapters, or repositioned studs to bolt the YF on there. The stud spacing is different. He could also benefit from higher compression and a radical camshaft, like Jon mentioned.

I'm about to do some machining on a 235 stovebolt intake to eliminate the single carb mounting flange, and mount a small-venturi Rochester 2-barrel in its place, like the 265 and 283 V-8's used. That would be a much better option than duals from a performance standpoint, but it would lack the "cool" factor of two carbs. It would run a BUNCH better, though!
Jerry

Was I just lucky back in 1956 when I installed a McGurk dual carb manifold with two Stromberg BXOV-2 carburetors on my 1953 235? It ran nearly perfectly when I first fired it up, and after a few minutes of adjusting the carbs with a "synchronizing" gauge, it was perfect. My mother and father were amazed. Along with a Corvette six exhaust manifold, the 1/4 mile e.t. dropped from 19.3 seconds to 17.7. The HP calculators indicate a 29% increase in HP for these times.

This increase in HP is also consistent with the dyno tests by McGurk that were written up by Don Francisco in a 1953 Hot Rod Magazine article: Page 1 Page 2 Page 3 Page 4 (the dyno test data) Page 5 Page 6

Installing one more modern 2-barrel carburetor, such as the 2G that Jerry described, or Tom Langdon's Holley/Webber setups, might be just as good, if not better.

I also had a lot of fun (and earned a small pile of money) tuning and fixing the tri-power setups on the Chevy 348s, Oldsmobile J-2s, etc., while I was working my way through school back in the early sixties.

Stuart - you are absolutely correct about the kid hanging out at the corner, and why all of us try to answer questions on the forum.

And Jerry has shared his wisdom.

However, there are simply some issues that have no satisfactory solution other than change. The use of the wrong size carburetors fits this category.

A good analogy would be tires. Many of the old trucks use 16 inch wheels. Many race cars use 16 inch wheels. So theoretically, race tires could be put on the truck, or 8-plys could be put on a race car. While both could be driven, neither would perform to the users satisfaction. Trying to tune the wrong size carburetors would be like trying to tune the tires with different air pressure. As Jerry mentioned in his last post, it is fighting a losing battle.

I will slightly disagree with Jerry on absolute performance. We have consistantly bettered a single carb's performance, both in power and economy with well-chosen multiple units; although the power and economy gains are modest. But using the single carb is definately easier.

Multiples improve performance due to a better fill ratio for the manifold, thus the cylinders. Using the wrong size carbs changes the air velocity, and can hurt fill ratios, rather than improve them.

And Hoyt - the Stromberg BXOV-2's are the smaller size 2 carburetor. You started with the right setup.

Jon.

Isn't this fun! This is what hot rodding is all about. If you are like me, even though you know better, you still have to try......

Joe

Too many people are against multiple carbs on the Stovebolt simply because they don't understand what they are doing.
The carbs size have to match the demands of the engine and the carbs themself HAVE to match each other. They can not just be "close".
A 2x1 and a 3x1 setup will run just as well and even better than any single carb on any inline engine.
Just ask anybody who has followed Frank McGurk's advise by installing 2 or 3 of the Stromberg BXOV's.
I just wish that a person could still buy new BXOV's, to me Stromberg's are the only way to go. But again, THEY HAVE TO MATCH.

well i had the same problem many years ago with an 2x1 on my 261. i thought i tinkered i cussed even prayed in chruch.

then the light came on corvires used dual carbs and they worked just fine. so i put a pair of them on and it ran just great. good milage good low end.

I thought the carbs used on the Corvairs had a different bolt pattern on the bases. Seems like they were not evenly spaced from the throttle bore. I guess an adapter could be fabbed up. Maybe some were different, I'll be checking in the yards!

yes i had to make up ome adapters.

i think for my dual carb 270 gmc i am putting in my 46 i will try the staged 2 barrels.