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dual carb trouble...
https://slantsix.org/forum/viewtopic.php?t=37078		 Page 1 of 2
Author:  slantsixlancer [ Sat Sep 12, 2009 7:20 am ]
Post subject:  dual carb trouble...
Installed a dual carb intake (offy) on my slant 6
2..1920 carbs..( jets 50)

not quite sure where to go from here..I have tried sync ing them but no luck getting it to idle..( It will around 8-1000 smoothly)
the back carb seems to pull way more air then the front one not sur why> I am going to swap them and see if its a front back thing or the carb itself

Has any one done this before..I have been seaching but havent found specs on stuff like
what jets work
what the idle setting should be
what timing works best
Where to set idle mixture..( I have heard any where from 2 1/2 out to only one out..( how do you tune for lean with 2 carbs anyway???)
does it matter which carb gets tuned first
Honestly I am looking for the whole detailed tuning prosses...as I am just winging it fer now... :shock:
Author:  Fopar [ Sat Sep 12, 2009 8:39 am ]
Post subject: 
You need to get both throttle plates set so that they are closed at the same time. Back off idle screws all the way set linkage so that it fits without making one or the other of the carbs open even slightly then use only one idle screw to set idle speed. You then need to set idle air screw on both carbs for max vaccum. Jetting will be what ever it takes to get the plugs to burn with the right color.

Hope this helps.
Author:  slantsixlancer [ Sat Sep 12, 2009 8:56 am ]
Post subject: 
that does help thank you...
One more thing does it matter if the dist vacume line is hooked up to dist...or should it be plugged at carb

I do have a sync tool.. (the one with the little bobber or ball)
but can you do this with a vac gauge too?
Author:  Fopar [ Sat Sep 12, 2009 9:29 am ]
Post subject: 
You might use a "T" fitting for the vaccum dist hose. The sync tool will work better, but can do with vaccum gauge if tool not available.
The most important thing is to have both throttle blades closing equally.
Author:  slantsixlancer [ Sat Sep 12, 2009 9:58 am ]
Post subject: 
so, to get the throttle plates to open and close in sync do you close them up fully then hook the linkage up on both carbs...
then set the ide screw (on only one carb)
then set the fuel air mix scew...

or unhook one carb linkage
set the idle screw on the hooked up carb
then set the unhooked link to match

(the second way is what offy says but
not sure if there would be a differance and not sure how to measure if the pre set linkage they supply is wrong)....hope that made sense..
Author:  Fopar [ Sat Sep 12, 2009 10:13 am ]
Post subject: 
so, to get the throttle plates to open and close in sync do you close them up fully then hook the linkage up on both carbs...
then set the ide screw (on only one carb)
then set the fuel air mix scew...

That is the way to do it
Author:  slantsixlancer [ Sat Sep 12, 2009 10:34 am ]
Post subject: 
nice ..easy enough...thank you
two car shows in my town today, hope to join the fun
thanks again...
Author:  james longhurst [ Sun Sep 13, 2009 8:11 am ]
Post subject: 
I always set mine with no linkages attached to the carbs. Just treated them like a two barrel (with two idle screws :shock: )
Go for the lowest throttle angle/leanest mixture settings that will give the strongest idle around 700 rpm. Then adjust and attach your linkages. Make sure to use some hi-temp grease to lube the rod pivots on that manifold to avoid binding.

-James
Author:  hantayo13 [ Sun Sep 13, 2009 9:33 am ]
Post subject: 
your big thing is to sync carbs to operate at same time...and then set idle mixture like you would with a two/four bbl carb...then idle speed,remember to do same amount turns on both or you end up out of sync again....a multi carb sync gauge will help


keep on roddin'
Author:  slantsixlancer [ Sun Sep 13, 2009 10:10 am ]
Post subject: 
still having a few minor problems
1. flat spot on take off
2. poor fuel
3.front carb just not want to reach as much vacume as back carb

does it matter on a slant which carb is tuned first, I did back first..am going to try front first this morning
and also going to time it a little advance...pretty much stock right now
I think I will go 8-10 advance
Author:  ceej [ Sun Sep 13, 2009 10:40 am ]
Post subject: 
Just a WAG, as I haven't run your set up, but I'd say your a couple of jet sizes too small with 50's.

Bump up to 51 or 52's and try again. Velocities are lower now, so your going to need bigger jets. The rear carb is on the hotter end of the engine. It may be able to run a tad leaner than the front three cylinders. Your seeing that in the vacuum.

By the time you get done, you may have a 52 in the front carb, and a 51 in the rear carb. Just have to play with it. Three digit jets are high precision. 51-3 is a tad leaner than a 52-1 and so forth. Not sure how tight you'll need to get it to make things right, but the carbs may need to be set up slightly different front to rear to make things work.

CJ
Author:  Dart270 [ Sun Sep 13, 2009 2:56 pm ]
Post subject: 
I would start with a 56 jet at least. Don't think I ever got mine to run well below 54s.

Lou
Author:  slantsixlancer [ Mon Sep 14, 2009 5:04 pm ]
Post subject: 
Update
Timing fixed hesatation I put it about about 8..

Will the jet sizes affect how much vac. I can get? ( I am fairly new at this)
also I know some one already said but...

Is that the only way to know which jets to use is by looking at the plugs???

Still tring to get it synced..Had a few spring throttle issues I didnt catch till It was time for bed :?
Author:  madmax/6 [ Mon Sep 14, 2009 6:06 pm ]
Post subject: 
I run 56,s in mine and it runs real close to perfect,20 mpg,Mark
Author:  james longhurst [ Mon Sep 14, 2009 6:11 pm ]
Post subject: 
Quote:
Update
Still tring to get it synced..Had a few spring throttle issues I didnt catch till It was time for bed :?
Yeah, that throttle shaft that comes with the Offy linkage kit is pretty weak. Too much spring and it will bind and flex.

-James
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