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385 SBC Running Rich

  
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385 SBC Running Rich

 
Bart McDonald Bart McDonald
New User | Posts: 2 | Joined: 11/13
Posted: 11/06/13
12:24 PM

I have a 385 i have built an put in my 79 vette.Has older 195 afr heads 74cc have been milled slightly to clean up. the pistons are pt#H600p. flat top with 2 valve relief.piston sits .003 in hole.has 5.7 rods with arp bolts,scat crank comp xs274-10 cam.specs-dur @.050 236 242 lobe sep 110 lift .501 .510.springs to match 1.5 roller rocker,performer rpm new holley 750 mech secondary 4779c.running th 350 tci break away conv,3.55 gear.msd street fire ignition.problem is running very rich,I've set carb with vaccume gauge jetted it down to .069 in prim .076 in sec.Ive also put curve kit in dist.Am i just way off with this combination or am I missing something car is used mainly on street occasional track,any help would greatly be appreciated.very frustrated.  thanks  

Dave632 Dave632
Addict | Posts: 2220 | Joined: 07/08
Posted: 11/06/13
03:50 PM

Here is what I have had troubles with many times when putting in a long duration camshaft and is probably, (about 99% sure), your trouble. You have to open up the idle speed adjustment a lot to get the motor to idle. Then the idle transfer slots are being uncovered and dumping too much fuel into the motor at idle and low speeds. This will foul plugs all the time. Some high end carbs, such as the newer BG carbs have an adjustment for this under the air cleaner hold down stud. For other carbs you have to drill holes in the throttle plates.
I had to install huge 3/8" holes in all 4 plates on my King Demon, (early BG carb that did not have the adjustment), in the last 632 as it ran a very radical cam but the plugs looked great after the process. I went up in size by small increments and it took about 20 hole size adjustment to get it right on my carb but this was on an all out race motor. This closed the throttle plate down to where the transfer slot was no longer uncovering the slot more than .020 and it cured the trouble.
Before I learned about this problem I was constantly cleaning plugs and changing them. After I hardly had to touch them.
You will have to do the throttle plate modification or get rid of the Holley carb and get one of the new BG carbs with the adjustment under the air cleaner stud.  
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wayne712222 wayne712222
User | Posts: 198 | Joined: 10/13
Posted: 11/06/13
07:12 PM

i have some images that will help describe this issue...

and.. dave... on massive motors... do any of your race carbs have something like a PCV passage or a power brake passage..  that you could put a street elbow into .. a short nipple.. and a fine adjust valve... so you could create your own IDLE AIR BYPASS.. save you from needing to drill the carb plates.. or even if you have any kind of spacers... if you had an air passage or even a bypass screw thru the side.. so you could adjust a filtered air bleed to get your idle down in the idle passage only .. wow... that could change the world...


back to this fellows issues...


Idletransitionslottuning Zpsae2722d8


this might be where you are  operating the engine at.. up into the idle transition slots  where you are running way to rich at idle..
Holley Off Idle Zps85779e58



below is where your carb should be feeding fuel from when you are at idle..

notice the only fuel is coming out of the idle circuit that is controlled/limited by the idle mixture screw..
Holley Atidle Zpsbdbaea76  

Bart McDonald Bart McDonald
New User | Posts: 2 | Joined: 11/13
Posted: 11/07/13
02:39 PM

Thanks alot their was set screw under carb that help the idle transfer slots.Its a lot better,I will run it  a little see how it does.may just end up changing to different carb if this hasnt solved the problem.  

skyeking skyeking
Addict | Posts: 2738 | Joined: 08/09
Posted: 11/07/13
08:15 PM

Good to to see a pair of old codgers getting their act together.
 Welcome Bart...  
skyeking