Author Topic: help me get my 6 pack 340 tuned!  (Read 5477 times)

Offline The4ork

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help me get my 6 pack 340 tuned!
« on: August 09, 2011 - 09:58:50 PM »
so right now it starts and runs and idles...

however, if i try to gas it in i nuetral it spits gas and backfires out of the center carb.

so far ive checked for vac leaks and found one but it didnt seem to help it at all. im not sure what to adjust on the carbs at all so i started with a timing adjustment, moved the distributor - and + and it still spits, so im convinced its not timing. checked all the plugs (new) and they are on the rich side alright but not too far out, a dark brown, borderline flat black - not wet tho.

im out of ideas as of right now. i know you have to remove the outboard to adjust the center carb. which im not looking foward too as their is a lot of linkages and junk attached to it :/

now what?




Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #1 on: August 09, 2011 - 10:09:25 PM »
i just was looking online for some help, there is really NO good websites i can find to help me... found some helpful pics tho.

the vac line that goes from the two diaphrams on the outboard carbs. mine are plugged. im not sure what this would do to the operation of entire setup, but im sure it is wrong and will affect it badly. however, i dont think its my primary problem, as the center carb i think is most likely the culprit right now.

here is a pic




oh and i cant really 'fix' it easily by adding a vac line... because the pressed in nipple on the aft carb has fallen out... its missing :(

Offline Chryco Psycho

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Re: help me get my 6 pack 340 tuned!
« Reply #2 on: August 09, 2011 - 11:59:20 PM »
there is a Holley tuning guide in the Archives , this basically relates to the 6 pack as long as you think of the 6 pack as a 4 bbl with 2 secondaries not 1
there are idle screw in the base plate of the end carbs as well that need to be adjusted , typically they are turned out 1/2 - 3/4 of a turn from bottom
the vacuum line to the outboard carbs should not be plugged this will prevent them from opening or possible they have rigged a mechanical linkage to override the vacuum , this will not work as the end carbs have no accelerator pump circuit which will cause it to go lean & backfire which is what you are experiencing .
 A model / hobby store may have brass tube to replace the missing nipple . there are 2 nipples on the center carb , one into the metering block higher up for the dist & a second one lower down for the secondaries .
Start with that & let me know what you find

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Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #3 on: August 10, 2011 - 02:00:22 AM »
ok i took the rear carb off so i can get at those screws, and also possibly drill/tap a nipple into the diaphram area - coulnt find a brass nipple but i found a small 1/8" brass ntp with a nipple so im gonna use that.

should have it all back together tomorrow hopefully :P

Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #4 on: August 10, 2011 - 02:09:10 AM »
took a quick vid with my phone... its idling high i know... just click the picture



here are some pics of the carbs



Offline Chryco Psycho

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Re: help me get my 6 pack 340 tuned!
« Reply #5 on: August 10, 2011 - 10:40:55 PM »
Well you have the wrong linkage , that is for the mechanical carbs , you should be able to make it work , the center carb should force the outboard carbs closed when the center carb closes , with the center carb open the outboards should be able to open on their own but not be forced open by the linkage .
 the fuel bowl on the back carb was damaged at some point & replaced with a rear bowl from a 4 bbl carb which is why the fuel enters from the wrong side , adjust the float level so fuel is lower than the sight plug hole
 You stumble would appear to be caused by the secondaries being forced open by the linkage but the out boards have no accelerator pump circuit
« Last Edit: August 10, 2011 - 10:45:15 PM by Chryco Psycho »

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Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #6 on: August 10, 2011 - 11:36:32 PM »
soooo i need to buy all new linkage? and that should fix my problem?

i took all 3 apart, cleaned them thoroughly, set the bowls exactly how they are suposed to be, set the outboard mixture screws 1/4" turn out and put it all back together with new vac lines, and a fuel pressure gauge (7psi) and it runs, but still spits and shoots fuel and an ocassional spark or two out the carbs!

Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #7 on: August 10, 2011 - 11:40:33 PM »


is that the linkage im suposed to have?

Offline Chryco Psycho

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Re: help me get my 6 pack 340 tuned!
« Reply #8 on: August 11, 2011 - 08:16:06 AM »
yes , your linkage should work as long as you set it up as I said so the center carb forces the outboards closed , just put the bushing on the rod behind the center carb link instead of in front .
 it back fires as you open the throttle because there is no fuel added as the outboards are opened so it goes lean  backfires , which is exactly what I expected when you said it was backfiring . he outboard carbs cannot be forced open without accelerator pumps , hey simply are not designed to work this way , the vacuum pulls them open when there is enough vacuum present to draw air through them & pull fuel into the airstream without added fuel 

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Offline Cuda54

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Re: help me get my 6 pack 340 tuned!
« Reply #9 on: August 11, 2011 - 04:19:37 PM »
It looks like someone tryed to make it a progressive linkage. Change it to the right rod linkage and it will work the way it is suposed to. It is no hard to make one the right way. You will play and play with it and it still will not be right till you get the linkage fixed. Fix the linkage and see a big differance in the way it will run. I have had a 340-6 AAR and now a 440-6 525hp 72 Cuda. I know how much the linkage makes it run correctlly. If you have the right linkage and have one rod adjusted wrong it will not run right. I know I have played with them to get it setup right after some one changed it for me. ( they were messing with me) And all it did was piss me off till i got it set back up right.

Offline Chryco Psycho

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Re: help me get my 6 pack 340 tuned!
« Reply #10 on: August 11, 2011 - 05:53:25 PM »
The arms on the outboard carbs may have been modified so try using the linkage you have first , as long as it will allow the end carbs to open when the middle carb is wide open & will force the Outboards fully closed at idle the linkage you have will work fine .
 the center carb has a slider link , & it pushes on the bushing on the rod connecting the two out board carbs , if you move the bushing behind the slider it will pull the outboard carbs closed but allow the outboard carbs to open on their own using vacuum .
« Last Edit: August 11, 2011 - 05:55:54 PM by Chryco Psycho »

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Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #11 on: August 11, 2011 - 08:38:31 PM »
ill make a video on how the linkage opens the 3 carbs and you can tell me if its right or not.


Offline Matt70

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Re: help me get my 6 pack 340 tuned!
« Reply #12 on: August 11, 2011 - 08:48:04 PM »
 :popcorn:  This is cool
People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.

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Offline 72rtchallenger

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Offline The4ork

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Re: help me get my 6 pack 340 tuned!
« Reply #14 on: August 11, 2011 - 09:16:42 PM »
ok i also found out this is the cam in the engine, and that it has 10:1 .040 pistons as well.

http://www.summitracing.com/parts/SUM-6901/