any 6 pak gurus out there?

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dust-u

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hoping someone can help me tune this gd sixpack. I have a 72 340 duster 340 plus 30.. comp cams 274h cam. here is where it gets interesting . the motor has 12.5 slugs . in trying to reduce the comp ratio it has cometic head gaskets .083. thick and a set of open chambered 318 heads with 188/160 valves. I cant remember what the cr actually is ? ANYWAY THE SIX PAK IS BRAND NEW.the only changes made were minimal. the center jets were changes out from 62s to 63s. the motor runs 8 in vacuum. and it has a 4.5 power valve. I have the initial timing at pretty much where it wants and @3000 36 degrees. the outboard carb idle screws are 1/8 out. I can get it to idle but clearly can tell it is rich. can anyone offer me a place to start. ? I really want to run the sixpak but in all honesty it ran great with a holley 770 street avenger and an ld340 intake.. beyond frustrated.please help?
 
Have you checked the float levels on all three carbs? That would be a place to start.
 
This set up runs and idles on all three carbs. 8" of vacuum can be a problem. I'd look for vacuum leaks, six packs have lots of places for this to happen. Next make sure ALL three float levels are correct. This is a Pain In The A$$ but must be correct. Also make sure the air bleds are open. Even though they are brand new those holes are very small. Outboard screws at 1/8 maybe 1/4 sounds good. Center carb I'd start with 1.5 turns out. Make sure both outboard carb linkages are opening and closing smooth and make sure they both close all the way. You only adjust the idle speed on the center carb once you get it close. Also you are choking that engine with small port 318 heads and 188 valves. That thing will need to breath. You just doubled it's CFM.
 
Six pack carbs will pull 1350 CFM. He had a 770. I guess not double but a bunch more.
 
8 in of vac with that cam = not enough initial timing is my guess.
 
Make sure your ignition has good spark. I found out years ago that no grounding strap can cause a weak spark which mimics carb problems and will run very rich as well. A long shot, but possible. Rod
 
What, You think they rated 2bbls differently "just for six paks"?? Nooo, they didn't. They are rated at 3"Hg each just as any 2bbl. The total rating of 1350 CFM is still at 3"Hg
depression, the conversion factor is 1.4142135 ( square root of {old depression/new depression}). The Sq. root of 3"/1.5"(std. 4bbl) <2>. 1350/1.4142135=954.5942 CFM.
You have 950 CFM available to maintain a 1.5"Hg depression in the intake manifold.
 
" I have the initial timing at pretty much where it wants and @3000 36 degrees"......that sounds more like total timing...
 
dust-u said:
hoping someone can help me tune this gd sixpack. I have a 72 340 duster 340 plus 30.. comp cams 274h cam. here is where it gets interesting . the motor has 12.5 slugs . in trying to reduce the comp ratio it has cometic head gaskets .083. thick and a set of open chambered 318 heads with 188/160 valves. I cant remember what the cr actually is ? ANYWAY THE SIX PAK IS BRAND NEW.the only changes made were minimal. the center jets were changes out from 62s to 63s. the motor runs 8 in vacuum. and it has a 4.5 power valve. I have the initial timing at pretty much where it wants and @3000 36 degrees. the outboard carb idle screws are 1/8 out. I can get it to idle but clearly can tell it is rich. can anyone offer me a place to start. ? I really want to run the sixpak but in all honesty it ran great with a holley 770 street avenger and an ld340 intake.. beyond frustrated.please help?
Click to expand...

I never liked the way the vacuum operated outboards feel while driving, so I have always run the Direct Connection mechanical carbs. They are much easier to tune and have no idle circuits in the outboards, a feature the factory used to keep the fuel from going stale in them if they are not opened for a long time.

Its difficult to get all six idle circuits perfectly balanced, especially if you are only guesstimating by counting the turns of the screws. Mechanical secondary carbs don't have that issue. You need to be able to turn the screws while the motor is running to find the spot where they induce lean misfire while turning them in, and then back them off 1/2 to 3/4 of a turn from that. It takes a few trips around the 6 screw adjusted circuits to get them balanced and correct.

Promax makes a nice baseplate to make it easier to adjust the idle circuits while the motor is running.

How did you determine your initial timing, and what is it currently set at?

Have you sprayed carb or brake cleaner around the manifold, vacuum lines and carbs to verify no vacuum leaks?

Invest in a wide band A/F meter and get it set-up in your car. It will make dialing in these carbs much easier.
 
thank you for all the replies.. I appreciate it. I have read the articles...I will look things over carefully and see how things go.i had expected a stronger vacuum signal from this cam. at least that's what the fella who sold me this combo said. it really doesn't matter where I try to set the timing 8 inches of vacuum is all I end up with.i will keep digging. failure is not an option.
 
Put a vacuum gauge on it with a long hose and tap it to the windshield, drive around and watch the vacuum. My guess is the PV is open most of the time, take a long piece of vacuum hose and put it in your ear and run it around the base of the carbs and if you have a vacuum leak you will hear it. Plug your PCV valve hose and see if the vacuum comes up, Ive had a bad PCV valve cause problems.
Her is a good read on CFM, It is not cut in stone
Carburetor Air Flow Ratings
 
Speaking of PCV valves, which one are you guys running?
 
1350/1.414 = 954ish

That's the cfm conversion from 2barrel to 4 barrel flow rates.

You can flow a lot of air if you like, but, if the manifold is not good (cough cough 6 pack cough) it won't make power it should. The manifold is a real bottleneck.
 
alright.. I will check for leaks, brian6, what makes you think the pv is open all the time? I will ck the power valve as well. this one has me stumped .my ta 340 six ran like a swiss watch.all my other 340s also ran well. this one is giving me a run for my money..i am determined to get it to run right.thanks for all the suggestions.
 
You have less cylinder head than a stock, low compression 340, and alot more induction. And with that low vacuum on a 274 cam, I'd say you have multiple tuning and mismatch combo problems.
 
When you open the throttle the vacuum drops if it goes below 4.5" the power valve opens, you really wont know unless you run it down the road with a gauge on it. If the PCV valves does not work right, spring broke, wedged open, stuck, they will leak to much vacuum. Just some ez things to check. It has 8" vacuum at idle?
Where are you checking this at?
 
yes, at idle . at the brake booster port on the manifold.. the booster is gone. converted to manual brakes.
 
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rep. that's not good news. this combo was purchased as a matched set, or so I am told.and was promised it would run like a scalded cat the heads have had some extensive porting, also so I am told. it does look like it..maybe mrmopartech will chime in and tell me how great this combo REALLY is.?
 
I just looked up the cam and it's not that big, you should have more vacuum than that.
 
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