BBC Timing & Carb adjust ... Lars?
09-13-2013, 06:25 PM
#1
Drifting
Thread Starter
BBC Timing & Carb adjust ... Lars?
Some advice needed as I've not much experience with a BBC (timing), all SBC. Anyway, helping out a buddy who just put in a 505 BBC in a 1970 Monte Carlo. Car has a TH400 and 3.73 rear end.
Dart Block, aluminum heads, dual plane, believe suppose to be around 10.3:1. Advertised as 620HP. Cam specs are as follows;
Howards Hydraulic Roller Cam
110 Lobe seperation - 106 Intake centerline
247 @ 50 Intake .612 lift
255 @ 50 exhaust .612 lift
Quick Fuel Carb. Either 830 or 850, can't remember.
Took my vacuum gauge over there today in an effort to dial it in a bit. He was having some idling and dieseling issues. Found the bowls were too high and fuel pressure was also too high so I adjusted. Hooked up the gauge to full manifold at the bottom of the carb. Showed 5-7in. with a unsteady bounce due to the cam. Wrote down the idle mixture settings which were closed in the front block and 1 turn out on the rear block. Set all 4 to 1 1/2 turns out and tried to adjust for best vacuum. Found the front being closed was the best idle (surprised ?), and rear I couldn't really get much of a difference on the gauge. Due to uncertainty I put it back to the original settings. All was done with idle around 900-1000 as that's the lowest I could comfortable go.
Timing was at 15-16 initial with 32-33 all in around 2500 rpm. Has a vacuum advance but has not been hooked up. I ended up setting it at 18 initial with 34 total. Seemed to really like 30 intial, but I did not have the time or parts to mess with the curve so it was not so high total!? ...When I hooked up the vacuum advance (full manifold) it raised to 55+ under throttle. It is not adjustable so I left it unhooked.
Car seemed to run fine, but want to tune it as best as possible for him. Smells rich at idle. He also does not have enough vacuum for brakes and is going to have to get a booster. Hoping maybe with more initial timing and reducing the idle (not sure if it is over riding the idle circuit) that he may get some more vacuum out of it?
Advice?
Dart Block, aluminum heads, dual plane, believe suppose to be around 10.3:1. Advertised as 620HP. Cam specs are as follows;
Howards Hydraulic Roller Cam
110 Lobe seperation - 106 Intake centerline
247 @ 50 Intake .612 lift
255 @ 50 exhaust .612 lift
Quick Fuel Carb. Either 830 or 850, can't remember.
Took my vacuum gauge over there today in an effort to dial it in a bit. He was having some idling and dieseling issues. Found the bowls were too high and fuel pressure was also too high so I adjusted. Hooked up the gauge to full manifold at the bottom of the carb. Showed 5-7in. with a unsteady bounce due to the cam. Wrote down the idle mixture settings which were closed in the front block and 1 turn out on the rear block. Set all 4 to 1 1/2 turns out and tried to adjust for best vacuum. Found the front being closed was the best idle (surprised ?), and rear I couldn't really get much of a difference on the gauge. Due to uncertainty I put it back to the original settings. All was done with idle around 900-1000 as that's the lowest I could comfortable go.
Timing was at 15-16 initial with 32-33 all in around 2500 rpm. Has a vacuum advance but has not been hooked up. I ended up setting it at 18 initial with 34 total. Seemed to really like 30 intial, but I did not have the time or parts to mess with the curve so it was not so high total!? ...When I hooked up the vacuum advance (full manifold) it raised to 55+ under throttle. It is not adjustable so I left it unhooked.
Car seemed to run fine, but want to tune it as best as possible for him. Smells rich at idle. He also does not have enough vacuum for brakes and is going to have to get a booster. Hoping maybe with more initial timing and reducing the idle (not sure if it is over riding the idle circuit) that he may get some more vacuum out of it?
Advice?
Last edited by Ibanez540r; 09-13-2013 at 10:48 PM.
09-13-2013, 06:46 PM
#2
Le Mans Master
Power valve? Throttle blade position over the transition slots? (both pri & sec) Sounds like the mechanical advance needs some work on the stop so it'll make 36* @ 3000.
09-13-2013, 09:00 PM
#3
Le Mans Master
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If it will run with the idle mixture screws all the way in, it's getting fuel from somewhere. Power valve and transition slots are the usual suspects, like Tim says. The vacuum advance can sounds a little squirrelly, how much advance does it give at idle, if any?
Scott
Last edited by scottyp99; 09-13-2013 at 09:05 PM.
09-13-2013, 09:29 PM
#4
Drifting
Thread Starter
Vacuum advance is not providing any advance at idle, just under throttle.
Power valve makes sense. Probably due to the low vacuum, not being blown. That is my assumption. Anyone know what valve they come with? It is a brand new carb, he did say he had a pretty good backfire on the first few starts, but I thought since the 90s, power valves are protected from backfire.
Thanks for the thoughts thus far.
Does the 5-7in. of vacuum sound accurate for the combination? I didn't think he would be so low. Anyway, I'm not sure when he plans on putting on the vacuum booster, but if not soon, what do you think for a power valve, 3.5?
Power valve makes sense. Probably due to the low vacuum, not being blown. That is my assumption. Anyone know what valve they come with? It is a brand new carb, he did say he had a pretty good backfire on the first few starts, but I thought since the 90s, power valves are protected from backfire.
Thanks for the thoughts thus far.
Does the 5-7in. of vacuum sound accurate for the combination? I didn't think he would be so low. Anyway, I'm not sure when he plans on putting on the vacuum booster, but if not soon, what do you think for a power valve, 3.5?
09-13-2013, 09:45 PM
#5
Race Director
It will need 30 dgr advance at idle. Hook the vacuum advance to full manifold vacuum.
Power valve adds fuel to main circuit, not the idle circuit. Idle vacuum is irrelevant to the power valve unless it's pulling fuel from the mains. Look at the boosters when it's idling. If they're dripping it's pulling from the mains. Adjust butterflies to get it on the idle circuit. If idle is too low with butterflies adjusted properly you'll have to drill them.
Power valve adds fuel to main circuit, not the idle circuit. Idle vacuum is irrelevant to the power valve unless it's pulling fuel from the mains. Look at the boosters when it's idling. If they're dripping it's pulling from the mains. Adjust butterflies to get it on the idle circuit. If idle is too low with butterflies adjusted properly you'll have to drill them.
09-14-2013, 01:21 AM
#6
Race Director
This spring I helped a friend who had a very similar situation, including a pig rich idle and low idle vacuum. I eventually got him squared away. The details wouldn't help you since he had an unknown roller cam in a 350 smallblock with Vortec heads. He also had a Quick Fuel carb and, once I identified which one it was, their tech guy was able to tell me what power valve, jets, and air bleeds it came with. I can't remember the fine details but he did give me the recommended way for adjusting the 4 corner idle and, although in our case it didn't help, he also gave me info on playing with the air bleeds. It may help to give them a call. I'd throw as much initial timing at it as it will take and adjust total from there. We did pick up some idle vacuum that way but those Vortecs don't like much total timing.