change jets or increase carb size

82 GT · 06-28-2004, 10:24 PM

I'm wondering if I should change my jet/rod sizes on my Carter 625 carb or just go to a 750 holley.
I think I'm running a little lean because my plugs have no carbon deposits....at all and are very clean.
Is this a lean condition indication? I also feel that the engine doesn't pull as hard as it could at higher rpms.

crazypete · 06-29-2004, 08:05 AM

You could attack the problem scientifically by inserting an oxygen sensor attached to an A/F guage. Thats how I tune my holley. In all honesty, my plugs always look white even when I am running super rich, so I would heartily recommend a sensor instead of the plug reading method.

A 625 cfm seems a bit low in my opinion but I dont think its really hurting you that much. If the car runs well and doesnt overheat, then I dont think there is much of a problem..heck, you are saving yourself some gas. What jets/powervalve/shooter/cam are you using? I am using 68/70's,8.5 pv, .28 shooter and the base cam on my 670 street avenger vacuum secondary to run my little 302.

I love holleys for their adjustability. I will never fuel inject again. I also had a lean situation that was affecting idle and I alleviated this by spray carb cleaner into the fuelbowl and letting the carb clean itself from the inside.

Good luck!

420nitro · 06-29-2004, 08:32 AM

I would recommend a 750 carb with adjustable air bleeds and Idle Feed Restrictors. (the screw in type.)
Either a 750 hp series Holley or a 750 Speed Demon, Mighty Demon (depending on modification level)

Reading plugs is not that hard to do.....you just need to know what you are looking for. Basically you are looking for a slight tan color on the porcelin of the plug all the way inside the plug. The best way I have seen this described is by seperating the porcelin into 3 sections. That part that most people look at (or top part) is of no importance to a Performance tune. This is from your idleing circuit or idle tune up. The middle part is from the intermediate circuit and the bottom or very bottom, inside part of the plug is the tune up area where you need to look. Generally requires a Light and magnafying glass. This is by far a very short description on how to read a plug but gets you a basic understand on whats going on.
Oh yea one more thing....you can only properly read your plug when you shut it down from a WOT position under full load. Idling on the return road will mess up the reading.

LX5liter · 06-29-2004, 07:24 PM

Ditch the Carter and get a Holley. Just my opinion.

mustangII460 · 06-29-2004, 07:30 PM

This is from the poor mans perspective.

Keep the carter and change jets size. I think your close in the cfm just not the fuel amount.

Or if you have to get a holley go with a 650-700 cfm. and re-jet it.

and,,, Id stay with vacum secondaries on the automatic.

06-28-2004, 10:24 PM	#1
82 GT Import Slayer Join Date: May 1999 Posts: 2,241	change jets or increase carb size I'm wondering if I should change my jet/rod sizes on my Carter 625 carb or just go to a 750 holley. I think I'm running a little lean because my plugs have no carbon deposits....at all and are very clean. Is this a lean condition indication? I also feel that the engine doesn't pull as hard as it could at higher rpms. __________________ '82 GT 351W (.060)Comp Cams 274* Extreme Energy cam, ported & polished heads w/ 1.94/1.60 valves 10.3:1 flat top pistons,stealth intake, Mallory dizzy,Holley 750dp carb, BBK shorties,Flowmaster exhaust,C-4 with 3700 stall converter, B&M pro shifter,8.8 rear, 4:10's, subframes, electric fan, powermaster alternator, 4 core radiator.

06-29-2004, 08:05 AM	#2
crazypete Registered Member Join Date: Sep 2002 Location: Boston Posts: 592	You could attack the problem scientifically by inserting an oxygen sensor attached to an A/F guage. Thats how I tune my holley. In all honesty, my plugs always look white even when I am running super rich, so I would heartily recommend a sensor instead of the plug reading method. A 625 cfm seems a bit low in my opinion but I dont think its really hurting you that much. If the car runs well and doesnt overheat, then I dont think there is much of a problem..heck, you are saving yourself some gas. What jets/powervalve/shooter/cam are you using? I am using 68/70's,8.5 pv, .28 shooter and the base cam on my 670 street avenger vacuum secondary to run my little 302. I love holleys for their adjustability. I will never fuel inject again. I also had a lean situation that was affecting idle and I alleviated this by spray carb cleaner into the fuelbowl and letting the carb clean itself from the inside. Good luck! __________________ 91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior

06-29-2004, 07:24 PM	#4
LX5liter dude5l Join Date: May 2001 Location: Grande Prairie, Alberta, Canada Posts: 603	Ditch the Carter and get a Holley. Just my opinion. __________________ Good luck! Brad R. 1992 L.X. 5.0 coupe MODS: A few little bolt ons New Best time ...11.60 @ 128 mph @ 2000+ ft altitude. Rice Haters Club member #39 Users rides #2542

06-29-2004, 07:30 PM	#5
mustangII460 Factoy Five Roadster Join Date: May 2000 Location: Sevier Co,Tennessee Posts: 1,681	This is from the poor mans perspective. Keep the carter and change jets size. I think your close in the cfm just not the fuel amount. Or if you have to get a holley go with a 650-700 cfm. and re-jet it. and,,, Id stay with vacum secondaries on the automatic. __________________ Frank

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06-29-2004, 08:32 AM	#3
420nitro Registered Member Join Date: Nov 2003 Location: Rockford Posts: 596	I would recommend a 750 carb with adjustable air bleeds and Idle Feed Restrictors. (the screw in type.) Either a 750 hp series Holley or a 750 Speed Demon, Mighty Demon (depending on modification level) Reading plugs is not that hard to do.....you just need to know what you are looking for. Basically you are looking for a slight tan color on the porcelin of the plug all the way inside the plug. The best way I have seen this described is by seperating the porcelin into 3 sections. That part that most people look at (or top part) is of no importance to a Performance tune. This is from your idleing circuit or idle tune up. The middle part is from the intermediate circuit and the bottom or very bottom, inside part of the plug is the tune up area where you need to look. Generally requires a Light and magnafying glass. This is by far a very short description on how to read a plug but gets you a basic understand on whats going on. Oh yea one more thing....you can only properly read your plug when you shut it down from a WOT position under full load. Idling on the return road will mess up the reading.