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Discussion Starter · #1 ·
My problem is back firing thru carbs when i rev past 3500rpms. I have timing set at 34 total and i have pulleys to run only 5lbs of boost. Gauge says im only at 1 lb of boost when it back fires. I have power valve plugs on all sides of carbs. I have 92 jets all the way around and 31 and 25 squirters. I have a boost timing master set to take out 2 degrees of timing per pound. This is what i have.
427bbc 8.7 compression
345cc dart iron eagles.
850 dp holleys.
I have a feeling its to much jet or timing not like 34. i ran the car with power valves in front only and had no issues,but when i put plugs in front it started acting up. Also sometimes i get a big puff of smoke out the headers if it dont back fire thru carbs. Thankz for any comments
 

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Discussion Starter · #3 ·
Thxs guys lets see if this makes any sence to you. I ran the car without the blower and ran 12.1 [email protected] so i know everything is mechanically okay. When i started to set up the blower i had 6.5pvalves in front only with the same 92 jets all the way around. I knew i was going to have issues with plugs in it onLy. so i wanted to set up the motor and blower in good running order. So around the house it ran fine until i put those front primary plugs in. If i cant fix the issue then i will take it to the local blower shop and have them refrence the carbs for 150.00. I dont trust my self to start drilling holes into my carbs no matter how easy it looks. I have seen two ways to do it. Blower is weiand 8-71,cold plugs,
I notice that it got a lil better when the air bleeds are open all the way. Im sure its not all the answers you want. I cant go below 34 in timing without car having a really hard time starting the cam is a nitrous cam with 714/715 lift.
 

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Discussion Starter · #4 ·
I seem to fix the back firing thru carbs problem. I changed the timing up a lil to 36 that helped somewhat then changed the squirters back to almost stock at 31 all around then changed the gap from 38 to 25 on plugs on my -9 ngk plugs. Now only issue i have is massive amount of black smoke out the headers when i floor it and all my plugs are all black. I have 92 size jets all the way around on the 850's cause i have no power valves and airbleeds are open all the way. I wonder if it is normal to see no differance in idle if bleeds are in or out with the 8-71 I am not sure how far to go down on jet sizes maybe 87 or 85's. Any advise on bleeds and jet sizes.
 

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If I'm reading this correctly your running the same jets as you were before the blower but you now have a plugged PV circut? That would most definitely imply you are running lean. Tuning a carb on a blown BBC can be a risky proposition, you really should have referenced PV's if you're driving on the street and you should also have it set up but a guy that is good at tuning FOR THE STREET. The difference between street tuning and race tuning is most racers couldn't give 2 ****s about the PV circuit- they just block it like you have. On the street your PV circuit is the most important circuit and is usually misunderstood.

The holley was designed so that your jets are for part throttle/cruise tuning and your PV circut is for WOT tuning. You can block your PV and tune your jets for WOT, but that leaves you with poor part throttle fuel metering.

I hate to be the bearer of bad news but along with boost referencing it sounds like you have a long way to go on tuning your carbs.
 

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Do you have a lean rich guage on the car. You can use a standard o2 sensor and cheap lean rich guage to at the very least give you a warning its running lean. Basicly for blown carb builds it should always read rich sep at idle. If your foots in it and it does not read full rich then let off the gas and up the jets. even if it reads on the rich side that may not be enough gas to keep a street motor happy. Wide band or egt would be better for racing but atleast some type of indicator in the car if it goes lean. Also an engine saver if your pump starts to get weak once its all tuned up and running for a while.

IF you have the full jet kit with a set of jets in each size go up one or two size larger than you think you need and dirve it then pull back the jets one at a time. Just to make sure its always rich during test runs and beginning tuning.
This isn't a bad idea at all but BE AWARE that running too lean can show up as a rich reading and running too rich can show up as a lean reading as it'll cause all sorts of misfiring conditions that you may not detect at rpm.
 

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Discussion Starter · #7 ·
Thank you guys for all the info, i will soak it all in you have all been a great help and im sure this thread will help others also. Next i will move timing to 38 and try the 86 jets all the way around.
 

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Discussion Starter · #8 ·
Just an update. Ran the car it improved to [email protected] im sure it has a lot more in her. But do to a wrong choice in gear ratio i kept hitting my 6400rpm rev limiter in 3rd gear. Going to a 4.30 now from 4.88 in 1/4 mile. I changed the jets to 85 all four sides. Pv is still pluged, runs really rich and puff of black smoke when i take off. Changed gap to 38. So next just gonna get carbs boost refrenced at a shop here and put hotter plugs in it. Need to read up on how to check vacuum on my .714 solid roller cam
 

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Discussion Starter · #11 ·
My 60ft is 1.66 and running a 31/14/15 hoosier. I dont want to run my 427 bbc no more than 6200 if possible. From what ive been told the 8-71 starts making to much heat and slows car down. Something ive noticed is that it doesnt make 5lbs boost until im at 6000rpms. I wonder why this is? I was hoping i would be getting into boost much sooner.
 

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why not shift at 6200 RPM and let it rev to w/e in high gear.
427 is fairly short stroke.
Blower might need a gear change.
1.66 60 foot times,car should be in the tens,what part of the track is it laying down?
Post a video of the first 10 second pass.
ironing the turdles out can sometimes be a frustrating process,its supposed to be a fun process,,, keep on it
 

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Discussion Starter · #13 ·
Update changed some stuff and got close to the 10's i wanted. Ran [email protected] with my 6200rpm traps i was looking for. Went from 4.88 to 4.30 and went with a Hughes converter and boost referenced the holleys. Im happy finally. Only 5lbs boost
 
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