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Author
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Topic: Need tuning help-351c
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iamonewithjustin
Journeyman Posts: 97
From:
Registered: Nov 2004
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posted 04-17-2005 03:16 PM
   
Ok.. here's the story
My stock engine ran like crap.. I just found the problem when i was swapping intakes this morning.. worst vacuum leak in the history of man.. problem solved.New problem.. I swapped from stock intake and 2bbl carb to an edelbrock performer/750cfm carb. Throttle response is much improved, and it runs great.. as long as i do not open the secondaries.
The car accelerates better that it ever did before as long as i am very light on the gas. As soon as a open the secondaries it bogs (possibly misses) real bad. As soon as the secondaries open it stops acceleratins completely. (almost feels like it starts to slow down slightly) It acts like this over the entire range of secondary opening. If I floor it it will accelerate slowly (slower than at 25% throttle). The proble seems a little less bad a higher rpms but it's hard to tell. The carb was rebuilt after it came off a 455 pontiac. (the engine appeared to have sever performance parts) This leads me to believe the jetting or tuning may be way off because of the difference in engine size.
What do you guys think the problem is? What should I do to correct it? I do not have any way to measure rpm..
initial timing is 10 degrees.
the engine is a stock 351c from a 73 with a c6 tranny. The carb and intake are the only changes made.
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iamonewithjustin
Journeyman Posts: 97
From:
Registered: Nov 2004
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posted 04-17-2005 03:37 PM
also.. another quick issue. I have no vacuum lines attached to the carb. (all are plugged) The vacuum advance for the dizzy is pulling off the intake so are the other vacuum lines.. Is this OK or should I pull the vacuum for the dizzy off somewhere else?
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68F100
Gearhead Posts: 2542
From: Fort Madison, Iowa USA - United We Stand
Registered: Oct 99
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posted 04-17-2005 09:59 PM
Is it a vacuum secondary carb?
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iamonewithjustin
Journeyman Posts: 97
From:
Registered: Nov 2004
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posted 04-17-2005 10:09 PM
I am not completely sure but I think all the performer carbs are mechanical secondariesdoes anyone know for sure? [This message has been edited by iamonewithjustin (edited 04-17-2005).]
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Scott H
Gearhead Posts: 281
From: Chicago area
Registered: Mar 2005
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posted 04-17-2005 11:27 PM
If you run the vacuum for the distributor from the intake it will always have full vacuum and it will nullify any timing advance action you hoped to have. The sistributor line must come off a ported vacuum source on the carb that has nearly zero vacuum at idle and starts to come on strong at part throttle.
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Scott H
Gearhead Posts: 281
From: Chicago area
Registered: Mar 2005
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posted 04-17-2005 11:31 PM
Check the two vacuum ports on the front of the carb at the base. At idle one of them will have suction, the other one won't. Hook up the distribution to the one that doesn't.
Check your timing after you do this, chances are that the timing was always in an advance mode and now you may have to change it.
This may help your secondary bog problem. Other problem could be a clogged passage or lack of fuel in the secondary system of the carb.
Edelbrocks are mechanical secondaries in that they don't use a diaphragm to open the secondaries.
Your other vacuum accessories like the trans line and brake booster are fine running direct off the intake.
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iamonewithjustin
Journeyman Posts: 97
From:
Registered: Nov 2004
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posted 04-18-2005 12:44 PM
The timing was off before.. I finally got my distributor out and replaced it. Advance it now attached to ported vac on the carb.
The initial timing is set at 10* BTDC now.
The problem a little better but not much. There is still a huge flat spot over the range of the secondaries opening.
I'm not an expert carb tuner so I'm not really sure where to start. [This message has been edited by iamonewithjustin (edited 04-18-2005).]
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johnmustang
Gearhead Posts: 5939
From: British Columbia , Canada
Registered: Nov 2001
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posted 04-18-2005 01:06 PM
Just a thought, but do you think you may have to much carb for the car?------------------
JOHN
65 FASTBACK 2+2.....14.44 @ 107mph 1/4
87 TAURUS WAGON
03 F150 XLT SUPER CREW 4X4 SHORT BOX
Member:Vancouver Island Mustang Association
M&M #1710
65 FASTBACK
2003 F150 XLT SUPER CREW 4X4
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indyphil
Gearhead Posts: 2516
From: Senoia, G.A. USA
Registered: Jul 2002
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posted 04-18-2005 02:29 PM
The performer series carbs are a combination of mechanical and air operated secondaries. When you get to almost WOT it will open the secondaries a little mechanically, if the engine has any vacuum left it will draw air through the secondaries. There is a little "air door" which will open the secondaries more and more as the engine demands more flow - so in theory the secondaries open on demand after you crack them open with that last bit of the pedal motion. My car has a little bog too (289 with 600CFM e-brock carb) which I suspect is due to having too much carb for the engine, but once its revving its no problem. I dont know how to tune the jets for the secondarys and have not looked deeper since Im planning on putting headers on and didnt want to spend time tuning on something that might go away as I build the engine to flow more. In your case it sounds much more serious and you might have gotten a carb that someone had modified. Some folks mess with the air doors to try to get them open sooner but I dont know the details. this could also be a fuel flow issue that you never saw before with the 2 barrel system because it never had the demand placed on it. Lots of things to check like floats, filters, pump etc...------------------
'68 coupe, '66 289 C code
engine, edel 600cfm carb, performer intake, dual exhaust http://www.geocities.com/ottouk_77/68mustang.htm
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iamonewithjustin
Journeyman Posts: 97
From:
Registered: Nov 2004
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posted 04-18-2005 09:53 PM
It may be a little much carb for now, however I chose it with certain intentions in mind. Those being lots of modification in the future..cam, headers, 4v heads, 4.11 gears (after t-5 swap) etc.
I am looking for advice on tuning it to alleviate the problem as much as possible until then.
fuel system (pump and filter) is new.
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RonnieT
Gearhead Posts: 835
From: Port Allen, La. 70767
Registered: Jun 99
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posted 04-18-2005 10:32 PM
You can go to the Edelbrock site and view the owners manual for your carb. It goes through the tuning procedure and gives some troubleshooting info. Since the carb came off a 455 it may be jetted a little rich on the secondary side causing the bog.
http://www.edelbrock.com/automotive/index.html
------------------
Ronnie
69 mach1 351W-4V engineless at the time!
70 Torino GT 351C-4v with a "shaker"
Mustangs and More Member #23
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