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Old 10-29-2012, 05:32 PM
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blower guys running no power valves only sees track

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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Old 10-29-2012, 09:16 PM
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I would look at fuel system flow. Blockage? Spark plug gap. Distributor cap rotor phasing (the boost retard effects the phasing too) if it gets out of aceptable range, backfire crossfire. distributor cap/rotor carbon tracking (caused by this)

For some reason sometimes the mag pick up leads need to be reversed ( polarity) when a boost retard box is connected. (timing gets fubared up if mag pick up polarity is wrong.)
What was the previous jetting and powervalve channel restriction orifice size (behind the power valve in the metering block) what was the power valve vac rating?
When you ran sucessfully with the power valves.?

Why not just run the power valves but boost reference the carb main bodys? Its easy.
IMHO you are pretty much always better off using te power valves.

manifold vacuum at idle? boost pressure at max power?
vacuum created at the carb base by the blower @ WOT max rpm under full boost?

Blower size and pulley drive ratio?
Valve lash? basic engine compression test? (bent valve or other mechanical issue)
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Old 10-29-2012, 09:25 PM
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What jets did you have in in the primaries before you took the power valves out? Did you jet up when you took the power valves out?
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Old 10-30-2012, 12:09 AM
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Thxs guys lets see if this makes any sence to you. I ran the car without the blower and ran 12.1 sec@117. 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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Old 11-09-2012, 02:52 AM
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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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Old 11-09-2012, 09:10 AM
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If you had to open up the idle air bleeds so much it indicates the idle feed restriction is too big.
Small changes make a big difference. restore the idle air bleeds and add a V shaped bent piece of mig welder
wire (.025" to .035") to the idle feed restriction holes, in the metering block to lean the idle circuits on both carbs.

When you eliminate the PV and jet up to compensate, the idle circuit gets fatter. Yours is probabily now a bit too fat.
The mig wire in the idle feed restriction will tame it down and allow you to reinstall the idle air bleeds the carb came with.
You can fine tune the idle circuit from there. for blocked power valves and big primary jetting or
do the power valve boost reference modification so you can run power valves.

if the motor is hard to start with locked timing for a clean idle, install and use a simple ignition power interupt toggle switch. Be sure the rear support bracket is on your starter motor.

Plugs may be a bit too cold. This combined with a fubared idle circuit and not enough idle timing will surely
foul the spark plugs and it will misfire for sure with throttle when the plugs are fouled.

with only 5 psi boost you do not want or need a super cold spark plug heat range.

Timing should be 38deg+ at idle with the blower. Then employ the boost retard box to take timing out
as boost comes in. Blower motors like lots of spark timing at idle to stay clean (Especially with a big camshaft)

the throttes on all 8 barrels must be in the sweet spot at idle ( idle transfer slot exposure)
remove both carbs, flip them over and reset the pri and sec throttle idle position. .020"+/- evenly.
reinstall and give it all the idle timing it needs at idle.
Use a hotter -6, -7 NGK plug to get the idle set up correcty. ( idle feed restriction, idle air bleed, idle timing)

Last edited by F-BIRD'88; 11-09-2012 at 09:25 AM.
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Old 11-09-2012, 09:37 AM
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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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Old 11-09-2012, 10:32 AM
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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.
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Old 11-09-2012, 11:16 AM
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What is the model list number of your holley carbs? Correct metering blocks or mix and match stuff?
What are the jets, PVCR sizes, IFR sizes, idle air bleed sizes and hi speed air bleed sizes?

92 seems a bit big.. I would think 86-87 or 88 square would be a good start point
with plugged power valves.
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Old 11-09-2012, 11:22 AM
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Quote:
Originally Posted by hcompton View Post
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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Old 11-09-2012, 01:46 PM
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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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Old 02-17-2013, 10:08 PM
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Just an update. Ran the car it improved to 11.2@117 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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Old 02-18-2013, 12:07 AM
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Your M{H and ET are not too far off. The car will go a lot quicker than that Im thinking,,,,Have you talked to Pat at the carb shop?
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Old 02-18-2013, 12:13 AM
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questions:
why a 6400 rpm chip? I would think 7200?
What are your 60 foot times? are you using a 30 inch slick?
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Old 02-18-2013, 05:35 AM
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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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