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09-17-2007, 12:01 PM
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CC Member
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Join Date: Apr 2000
Location: Leesburg, VA USA,
VA
Cobra Make, Engine: Contemporary, 427 FE Center Oiler w/ 48 IDA Webers
Posts: 238
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Not Ranked
A locked timing may be okay for racing where you are mostly wide open but I think that you would want variable timing for the street. I know Webers like a lot of timing. I'm not sure about your engine, but for my stroked 427 FE I run 17 deg initial with 21 degrees advance for a total of 38 degrees. The advance comes all in by 2800 rpm. I believe that this will help your popping issue as well.
I have been told that exhaust popping comes from an exhaust leak for sure, but that it is also worse when leaner than when rich. If you are going lean when you decel this will make the problem more pronounced.
A lean condition makes for a very poor wave front in the combustion chamber causing an inefficient burn and a fair amount of uburned gas to be left over. This unbured gas hits your hot headers and expoldes causing the pops. Your advanced timing when on the idle circuit (under hard decel with your foot off the pedal) will make the unburned gas issue even more pronounced.
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09-17-2007, 02:07 PM
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CC Member
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Join Date: Feb 2006
Location: Gore. New Zealand.,
SI
Cobra Make, Engine: DIY Coupe, F/T ,MkIV.
Posts: 808
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Not Ranked
Quote:
Originally Posted by dlampe
Jac Mac, the popping is coming out of both pipes. It really doesn't sound like it is at the intake. My A/F gauge is reading lean not rich during the popping. I am going to try some smaller correctors.
>>>> >>>>>My timing is set at 32 deg. locked at the dizzy. Should i bump that or or does it matter as far as the weber tuning goes?<<<<<< <<<<<
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UMMMM Deane This locked timing wont be helping in an over run or coast situation ( or even if you try to accelerate particularly if your down below 2500 RPM ). I must repeat that you are asking the carbs & motor to do something it doesnt want any part of !
Jac Mac
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09-17-2007, 02:51 PM
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CC Member
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Join Date: Jan 2003
Location: Atlanta,
GA
Cobra Make, Engine: CAV GT40 with 331 KC
Posts: 2,187
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Not Ranked
Shirley on the race track you would never be below 3- 3500 RPM?
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09-17-2007, 04:07 PM
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CC Member
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Join Date: Feb 2006
Location: Gore. New Zealand.,
SI
Cobra Make, Engine: DIY Coupe, F/T ,MkIV.
Posts: 808
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Not Ranked
Quote:
Originally Posted by PatBuckley
Shirley on the race track you would never be below 3- 3500 RPM?
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Yes Pat, but when I went back to Deanes build thread I noted that he has a dual fuel setup for pump & race fuels and this 'popping' condition he is trying to solve is apparently in the coast mode right down thru the RPM range. I get the impression that he is asking the motor to perform in 'road' mode while it wants to be a 'race' motor. You wont find any current computer controlled road car that has the ignition permanently locked at 32° and a 'fixed' fuel distribution device that can work to maximum performance under all conditions with both fuels.
Also with 40mm chokes the airflow will fall well short of the heads capabilities on this motor.
Jac Mac
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09-17-2007, 04:57 PM
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CC Member
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Join Date: May 2001
Location: California,
Ca
Cobra Make, Engine: NAF 289 Slabside Early Comp Car with 289 Webers and all the goodies. Cancelling the efforts of several Priuses
Posts: 6,592
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Not Ranked
Pat: Dean it is the thread originator, who the hell is Shirley? And why is SHE on the race track? Who's on second
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way
Last edited by Rick Parker; 09-17-2007 at 05:01 PM..
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09-17-2007, 05:01 PM
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CC Member
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Join Date: May 2001
Location: California,
Ca
Cobra Make, Engine: NAF 289 Slabside Early Comp Car with 289 Webers and all the goodies. Cancelling the efforts of several Priuses
Posts: 6,592
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Not Ranked
Dean: If you are running fixed ignition of 32 degrees on the street you'll knock the ring lands out of the pistons. I've seen it done. Be very careful.
__________________
Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way
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09-18-2007, 07:45 AM
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CC Member
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Join Date: May 2004
Location: centralia,
IL
Cobra Make, Engine: B&B cobra (sold), Hurricane HMS1002 (sold), Kirkham 289 FIA, (sold) RCR GT 40(sold) SPF GT40 2122(sold) Hurricane HMS2002, (sold) RCR SLC (sold) GTR on the way!
Posts: 1,288
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Not Ranked
I have my timing locked at 32 deg. That is correct, but only for the track. I have a switch to change ingnition systems to a computer that will advance the timing up a normal curve. I have another switch to retard total timing 3 degs. I was just trying to tune the race system. I will admit that I haven't tried running hard on the street timing system.
I did up my idle to 1000 rpms and I backed out my mixture screws to 7/8 out. The popping wasn't there. My engine will not idle as low as many due to the very light wieght rotating assembly. If the race idle is set at 1200 for the track, the idle falls below 900 when I flip the timing switch and go to the street system. I think that would be okay. I have ordered some 190 Air correctors and 180 mains just to see what happens. The popping has gotten a lot better, almost gone. I am still lean any time I am close to WOT.
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Run & Gun 2003 - 2013
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09-18-2007, 09:05 AM
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Administrator
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Join Date: Jan 1999
Location: Duvall,
Wa
Cobra Make, Engine: KMP286, Shelby 482, Webers, 593HP
Posts: 4,161
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Not Ranked
Quote:
Originally Posted by dlampe
I did up my idle to 1000 rpms and I backed out my mixture screws to 7/8 out. The popping wasn't there. .
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Are you running rich at idle now because of this?
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09-18-2007, 09:14 AM
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CC Member
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Join Date: Jul 2002
Location: Greenville,sc,
SC
Cobra Make, Engine: Kirkham 427 (KMP 266); CAV GT40
Posts: 1,464
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Not Ranked
the idle mixture screws almost always correct the popping problems. Dean, I would drop the air correctors first before I went up on the mains, this will affect the WOT the most and mid throttle the least.
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SPF Daytona coupe 055, Roush 427R
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09-23-2007, 07:00 PM
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CC Member
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Join Date: May 2004
Location: centralia,
IL
Cobra Make, Engine: B&B cobra (sold), Hurricane HMS1002 (sold), Kirkham 289 FIA, (sold) RCR GT 40(sold) SPF GT40 2122(sold) Hurricane HMS2002, (sold) RCR SLC (sold) GTR on the way!
Posts: 1,288
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Not Ranked
Well it is getting much better. However it is really lean from 3000 to 4500 under hard acceleration. It looks like the pump kicks in at 3000 and takes the A/F down until 3500 rpm's from a spike of 18 when the plates open to 14.5 then it looks like the pump shot is over and it goes back up to 18 then gradually back to 12.5 all the way to 6500rpms. It takes 5 seconds for this to happen from the throttle plate opening to get the A/F down to 12.5. It takes 2 seconds for the Pump shot, 1 second to climb back to 18 then 2 more seconds to come back down.
Here is how it was set up
70 idle jet
120 idle jet holder
170 main
210 air corrector
f16 tubes
.00 exhaust/ float bowl valve
40mm chokes
50 pump shot
380 ci 302 with 8.7 deck, Yates heads with huge CFM numbers.
cam - solid roller dur. I 244, E 247 lift 631, 666 110 LCA 106.5 LSA
It was way lean at the top end so I went to 190 air correctors and thats when it got the results that I described above. The car seems to run good but I need to fix the lean spot between 3000 and 4500. I was thinking about going to 180 mains and putting my 210 air correctors back in. Any other thoughts?
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High Maintenance Racing Team
Run & Gun 2003 - 2013
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09-30-2007, 12:58 PM
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CC Member
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Join Date: May 2001
Location: California,
Ca
Cobra Make, Engine: NAF 289 Slabside Early Comp Car with 289 Webers and all the goodies. Cancelling the efforts of several Priuses
Posts: 6,592
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Not Ranked
Dean: Who is the maker of these pullies? Part #s?
Rick
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way
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11-04-2007, 04:38 PM
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CC Member
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Join Date: May 2004
Location: centralia,
IL
Cobra Make, Engine: B&B cobra (sold), Hurricane HMS1002 (sold), Kirkham 289 FIA, (sold) RCR GT 40(sold) SPF GT40 2122(sold) Hurricane HMS2002, (sold) RCR SLC (sold) GTR on the way!
Posts: 1,288
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Not Ranked
Rick, I don't know who makes the pulley. Call my engine builder, Lumpy. Reference the Kirkham cobra and he will tell you. 618 664 4767
After plenty of testing i feel like I am getting closer. I switched to an F7 tube and that really helped. My current setup is represented in the first post. I do notice the engine stumbling at around 2500 rpms. Any ideas? I can't really tell if it is lean or rich at this point. It is not popping, just not running smoothly. I tried smaller mains, 65 to 55 and the car was barely drivable. It runs great above 3000. Is this still idle jet holder territory? Remember this is at a constant speed.
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Run & Gun 2003 - 2013
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