Tri Power Carb Questions on my 496 Build
10-04-2009, 01:54 PM
#1
Racer
Thread Starter
Tri Power Carb Questions on my 496 Build
Have posted elsewhere the progress on my 496 tri power build but need some expert opinions on what is going on with the fuel curve.
1. The idle mixture screws have very little effect on the idle and it is very lean even backed all the way out. I have always heard the idle circuit is rich on this center carb so wonder what is going on. Cam is only 244/253 solid so it really shouldn't be a problem. How much effect does the fixed idle circuit in the end carbs have. I put in jet conversion plates and I am wondering if the holes drilled in these plates are smaller for the bypass idle fuel that feeds the fixed orifices in the base plate?
2. The carbs were flowed and modified by AED and ran a very flat fuel curve at all depression drops when finished. But on the motor the midrange is very fat 10.5-11.3 AF and then gradually leans to mid twelves by 6000 RPM. I know I am giving up some serious midrange peak torque with this curve. Any ideas on how to correct this?
3. Has anyone ran any type of carb spacer under these carbs and if so what were the results. They are all two hole design and I would have to think an open spacer giving more manifold volume would help top end.
4. Alternatively, has anyone milled down the divider plate in the manifold or eliminated the separate holes to achieve more plenum volume and top end.
Mike
1. The idle mixture screws have very little effect on the idle and it is very lean even backed all the way out. I have always heard the idle circuit is rich on this center carb so wonder what is going on. Cam is only 244/253 solid so it really shouldn't be a problem. How much effect does the fixed idle circuit in the end carbs have. I put in jet conversion plates and I am wondering if the holes drilled in these plates are smaller for the bypass idle fuel that feeds the fixed orifices in the base plate?
2. The carbs were flowed and modified by AED and ran a very flat fuel curve at all depression drops when finished. But on the motor the midrange is very fat 10.5-11.3 AF and then gradually leans to mid twelves by 6000 RPM. I know I am giving up some serious midrange peak torque with this curve. Any ideas on how to correct this?
3. Has anyone ran any type of carb spacer under these carbs and if so what were the results. They are all two hole design and I would have to think an open spacer giving more manifold volume would help top end.
4. Alternatively, has anyone milled down the divider plate in the manifold or eliminated the separate holes to achieve more plenum volume and top end.
Mike
Last edited by Irish69427; 10-04-2009 at 02:03 PM. Reason: mispelling
10-04-2009, 02:54 PM
#2
Race Director
the idle fuel flow is directly proportional to the vacuum.
higher speed=higher vacuum=more fuel(to a point)
what is current vacuum, idle speed? base timing? vac advance? total advance at idle?
higher speed=higher vacuum=more fuel(to a point)
what is current vacuum, idle speed? base timing? vac advance? total advance at idle?
10-04-2009, 07:09 PM
#3
my 468 has the same carbs mine are stock all but the AED plates which i also have mine has 77 on the outers and the stock 71's on the center...... however after my last dyno session im getting some STS baffles here is my motor dyno
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
10-05-2009, 05:35 PM
#5
Racer
Thread Starter
Vaccum is about 15 inches although it is tough to get it to idle because of the very lean mixture.
10-05-2009, 05:39 PM
#6
Racer
Thread Starter
my 468 has the same carbs mine are stock all but the AED plates which i also have mine has 77 on the outers and the stock 71's on the center...... however after my last dyno session im getting some STS baffles here is my motor dyno
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
Your air fuel mixture curve looks almost the exact opposite of mine with yours being a little leaner everywhere. Your lean in the midrange and then go fat. Mine does the exact opposite. Are those 1/2" spacers? Do they fit under a stock 69 hood and what did they do for power?
10-05-2009, 05:41 PM
#7
Racer
Thread Starter
Was looking through a mopar magazine (yes I know it is a moral violation) but I noticed company called promax that is expert in 6 pack carbs on mopars. I know these are very similar. They have a modified base plate for the end carbs that allows idle mixture adjustment while the carbs are on the motor. Anyone ever use these or have experience with them?
10-05-2009, 05:54 PM
#8
Race Director
does it have a PCV valve?
.
as a test, try simulating a vac advance:
1. advance base 10-15
2. reduce idle speed screw
3. try adj. mixture
4. dont drive! it is an idle test.
does this work? it should.
if yes, then it needs vac adv.
.
as a test, try simulating a vac advance:
1. advance base 10-15
2. reduce idle speed screw
3. try adj. mixture
4. dont drive! it is an idle test.
does this work? it should.
if yes, then it needs vac adv.
10-05-2009, 09:18 PM
#9
mine has no vac advance either..... yes the nitrous plates fit under the stock hood and yes they are 1/2 inch thick.... i know the a/f went from leaner to richer on the engien dyno but on the wheel dyno it was straight across the board needs a bit more fuel.
10-06-2009, 09:23 AM
#10
Race Director
is to plug the pcv and try to idle down to 13" or ~ 700 rpm.
Last edited by Matt Gruber; 10-06-2009 at 09:30 AM.
10-08-2009, 08:10 AM
#11
Racer
Thread Starter
Did the spacers have any impact on power or did you not run the engine without them as a baseline to know whether they helped or not? I can put them on and take them off when I get back on the dyno but if you have some data great. Nice custom linkage. Any bogs with that set up given the lack of accelerator pumps?
Last edited by Irish69427; 10-08-2009 at 08:15 AM.
10-08-2009, 10:09 AM
#12
Le Mans Master
my 468 has the same carbs mine are stock all but the AED plates which i also have mine has 77 on the outers and the stock 71's on the center...... however after my last dyno session im getting some STS baffles here is my motor dyno
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
and the wheel dyno. the overlay is with and with out the air cleaner top
the torque dropping off and hp laying down is from the crappy baffles
Dude you are losing 42% of your power by the time it gets to the wheels....something is not right there. Granted you have a TH400 and a 9"..... Those side pipes must be DESTROYING your power.... I could see 30% parasitic loss but....that should still leave you at 400+rwhp...
I would go back to an under car exhaust.....side pipes just suck.
Sorry to the OP about the threadjack. I don't know **** about Tri-powers but I agree you are way fat in the midrange. Are you using a wideband datalogger or getting this info off of a chasis dyno? If on the chasis dyno, are they putting the oxygen sensor in the collector or a tailpipe adapter? I have seen screwy sh*t happen with the tailpipe adapters, one of which is an extremely lean idle and low speed readings....too much room for error with tail pipe sniffers....(exhaust leaks, reversion etc)
10-08-2009, 10:18 AM
#13
Team Owner
Are you certain that the carbs can open completely via the throttle linkage/cable and the accelerator pedal? Testing them manually under the hood is OK, but it doesn't assure that your hardware is operating the same way with your foot. Perhaps its a transition thing at WOT between the middle carb and the outer ones....
10-08-2009, 03:47 PM
#15
Racer
Member Since: Aug 2005
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you have 2 orifices in the metering plates. Only one is the jets, the other is the idle circuit. Increase it a few thousanths. There are other plates available from holley with both orifices increased if you want a guide.
PS. I am jealous that you beat me to get it up and going.
PS. I am jealous that you beat me to get it up and going.
10-11-2009, 09:04 AM
#16
Racer
Thread Starter
you have 2 orifices in the metering plates. Only one is the jets, the other is the idle circuit. Increase it a few thousanths. There are other plates available from holley with both orifices increased if you want a guide.
PS. I am jealous that you beat me to get it up and going.
PS. I am jealous that you beat me to get it up and going.
Hope you get yours on the dyno soon
10-11-2009, 09:05 AM
#17
Racer
Thread Starter
Are you certain that the carbs can open completely via the throttle linkage/cable and the accelerator pedal? Testing them manually under the hood is OK, but it doesn't assure that your hardware is operating the same way with your foot. Perhaps its a transition thing at WOT between the middle carb and the outer ones....
10-19-2009, 10:21 PM
#18
BYW i took mine back apart to rejet... i had 71's in all three when i took it to the chassi dyno it was a bit lean 13.2ish now i went to 73's on the outer two and im a steady 12.7 all the way.
