Carb Tuning Help - I hope!
02-17-2002, 01:12 PM
#1
Le Mans Master
Thread Starter
Carb Tuning Help - I hope!
I replaced the Road Demon carb on my project cars engine with a new Holley Avenger (670cfm). The Raod Demon bit the bullett for some reason.
I've adjusted the valves (with the engine running - messy messy - glad I didn't paint yet I'd be freakin') and set the timing (around 14BTDC). It's idling great and seems to run pretty decent but I haven't had it on the road yet.
The plugs are darker then they should be (rich). Actually they are pretty much black but dry. The thing that bothers me a little is if I punch it good and then let off on it, as it gets down near idle and blowing out some back pressure I see a little bit of blueish/blackish smoke out of the exhaust. It wasn't doing this before so I don't think it's an internal problem. (New gaskets and umbrella valve stem seals). Could this also be from the carb? It doesn't do it idling or when you punch it.
Another carb challenged question - I assume to lean it down I have to buy jets that are a lower number. There's probably something like 70's in there now. Any suggestions on what size jet to use? (350, about 300hp, HEI Mallory Distributor, block hugger headers with 2.5" dual exhaust (Borla) with no cats).
Thanks...
ps: For those who may recall, my head gasket leak problem is all fixed and trying to move on to the next level now.
[Modified by 78Vette-SA, 12:13 PM 2/17/2002]
I've adjusted the valves (with the engine running - messy messy - glad I didn't paint yet I'd be freakin') and set the timing (around 14BTDC). It's idling great and seems to run pretty decent but I haven't had it on the road yet.
The plugs are darker then they should be (rich). Actually they are pretty much black but dry. The thing that bothers me a little is if I punch it good and then let off on it, as it gets down near idle and blowing out some back pressure I see a little bit of blueish/blackish smoke out of the exhaust. It wasn't doing this before so I don't think it's an internal problem. (New gaskets and umbrella valve stem seals). Could this also be from the carb? It doesn't do it idling or when you punch it.
Another carb challenged question - I assume to lean it down I have to buy jets that are a lower number. There's probably something like 70's in there now. Any suggestions on what size jet to use? (350, about 300hp, HEI Mallory Distributor, block hugger headers with 2.5" dual exhaust (Borla) with no cats).
Thanks...
ps: For those who may recall, my head gasket leak problem is all fixed and trying to move on to the next level now.
[Modified by 78Vette-SA, 12:13 PM 2/17/2002]
02-18-2002, 07:30 PM
#2
Race Director
Re: Carb Tuning Help - I hope! (78Vette-SA)
If you haven't had it on the road yet, I assume you're pretty much idling and revving. Your idle circuit might be too rich. (Does the idle speed up if you remove a vacuum cap?)
Leaner jets won't help if your idle circuit is rich. But with a Holley you have a couple or three options to get more air into the mix if you're overly rich at idle. I have no personal experience with any of these procedures so this is all hearsay. I'm relying on folks with firsthand knowledge to chime in and assist. Here goes:
1. Adjust the secondary throttle plates (butterflies) so they're cracked open SLIGHTLY at idle to allow more air in. This is a trial-and-error thing and you can always put it back if it doesn't work.
2. You can drill a couple of 1/16" or so holes in the primary throttle plates (butterflies) on the front side. Again, trial-and-error. Start small and sneak up on it. Screw this up and you'll be buying new throttle plates.
3. You can use a pin vice and drill out the idle air bleeds. Again, it's a trial-and-error thing. Screw this up and you'll be buying a new main body from Holley or Proform. At least.
After you get your idle circuit sorted out you get to start playing with shooters, pump cams, secondary springs (or linkages) and all the rest of it. Jetting should, IMHO, be the LAST thing you think about when tuning any carburetor.
I dunno. Hope this helps a little.
Leaner jets won't help if your idle circuit is rich. But with a Holley you have a couple or three options to get more air into the mix if you're overly rich at idle. I have no personal experience with any of these procedures so this is all hearsay. I'm relying on folks with firsthand knowledge to chime in and assist. Here goes:
1. Adjust the secondary throttle plates (butterflies) so they're cracked open SLIGHTLY at idle to allow more air in. This is a trial-and-error thing and you can always put it back if it doesn't work.
2. You can drill a couple of 1/16" or so holes in the primary throttle plates (butterflies) on the front side. Again, trial-and-error. Start small and sneak up on it. Screw this up and you'll be buying new throttle plates.
3. You can use a pin vice and drill out the idle air bleeds. Again, it's a trial-and-error thing. Screw this up and you'll be buying a new main body from Holley or Proform. At least.
After you get your idle circuit sorted out you get to start playing with shooters, pump cams, secondary springs (or linkages) and all the rest of it. Jetting should, IMHO, be the LAST thing you think about when tuning any carburetor.
I dunno. Hope this helps a little.
02-19-2002, 08:27 PM
#3
Le Mans Master
Thread Starter
Re: Carb Tuning Help - I hope! (The Dude)
Thanks for the suggestions. I'm getting ready to take it out on the road for some test drives. I'll play around with the idle speed screws and also have to re-adjust the valves and timing. Little by little I'll probably get it all sorted out.
Thanks again.
Thanks again.
