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Discussion Starter #1
My BBC has a new MSD 8360 Ready to run in it. I had it curved to 20* and set initial to 14*. VA can is set to add 14* @ 12 inches of vacuum.
She accelerates really smooth but on hard deceleration I get backfiring through the carb.
I have a new Holley Ultra street 780 cfm that I have not dialed in yet.
Anyone experience this and what was your solution.

Thanks
Louie
 

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Sounds like an exhaust leak, is that a possibility?
 

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Discussion Starter #3
Exhaust leak possible but not likely. I have new header gaskets on and I see no soot around the flanges. Downstream looks ok to.
 

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Typically a pop or miss at throttle or WOT or at cruise even is a tune issue, if you get a 'pop pop pop pop' on no throttle deceleration its an exhaust leak, at the head or collector or somewhere along the line.

Is that what it is, say throttle up to X rpm and then let off the gas and let the motor drag it down, you get 'pop pop pop' ?
 

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Discussion Starter #5
Typically a pop or miss at throttle or WOT or at cruise even is a tune issue, if you get a 'pop pop pop pop' on no throttle deceleration its an exhaust leak, at the head or collector or somewhere along the line.

Is that what it is, say throttle up to X rpm and then let off the gas and let the motor drag it down, you get 'pop pop pop' ?


Yes, that's exactly it. Accelerate to 45k rpm. Let off pedal and pop pop pop. I'm tinkering now. Just adjusting secondary idle screws for best vac. 14 inches vac. Best I can do. I just blipped the throttle to stabilize idle and got a backfire through carb.
 

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It's not always an exhaust leak. Back in the day guys would put on an exhaust designed to rap or pop when decelerating, usually glasspacks and long tail pipes...

Also keep in mind when you run up the rpm a bit and take your foot off the gas it is going to lean things out on the deceleration and the rapping and popping could be caused by that which might be normal for your build.

Blipping the throttle causing backfiring through the carb might be indicative of the need for additional carb tuning and it's probably not associated with the deceleration rapping...
 

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Discussion Starter #9
Alright flat, cut me some slack. I forgot the decimal point. (Man, you guys are tuff crowd)

I just increased the 14* initial to 16* and now my high speed "slight" surge is back. I was hoping it was too much advance that was causing the surge and I think it was. Since changing it has no effect on the backfiring issue I'll probably back it off to 14* again. I also hooked up the vacuum gauge and was able to tweak it up from 13.2 to 14.8 @ 900.00 rpm. (decimal point added for flats) But this did not solve the backfiring problem.

So I have 36* total timing, a strong 14.8 inches of vacuum with a fairly nice idle, (Cam isn't too outrageous) and backfiring through the carb when I yank the throttle with my hand in the driveway and back firing when decelerating while driving her from 4 thousand 500 rpm (4th gear)

This engine has just over 600 miles on it. A fresh rebuild. I bought the carb and dizzie because I wanted everything to come together with as little guess work as possible.

I now have to get into the area of engine tuning reserved for the most hardcore and diligent. Playing with jets, power valves, idle bleeds, drilling holes into butterflies yada yada sounds fun but if you don't know what the cause and effect are of modifying them it's a very steep learning curve.

If anyone can ponder this and put down a down and dirty " do these things, in this order" list I will name my first son after you. If its a girl, so be it. Nobody said it would be easy south of the Yukon.
 

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I'm not the greatest with these issues so take this with a grain of salt. The pop pop pop sounds like detonation while the intake valve is open. To much timing or not enough?
 

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Basics.. to have a backfire thru the carb u need and ign, flame / spark to do so.
This means that a Ht lead mixed up, or a inlet valve leaking/ not closing on a firing cylinder back into the valley cover/runners.

Pop pop on deceleration thru the exhaust is a very lean tuned carb.. to lean

drilling holes into butterflies
That is an old school OBSOLETE practice....If have to do that then either the butterflies are not aligned in the bores, or the secondaries are not set correctly or have worn butterfly bushes, bad or wrong PVC valve... or the carb is just too damn big for the application.....and its not uncommon doing so on modern carbs to end up with a deceleration pop pop in the exhaust.

To establish your total....lock the dizzie/ no VA...fire up the engine and increase timing to 36.. do a rolling timed distance.. hitting the 1st mark around 3000 rpms... a steep hill works good
Drop the timing in 2 deg increaments till u have a sudden drop off in power, the increase 3 degs... that will be your total
you should have AFRs between 11.5 and 12.5 max....

u basically do the same with the locked dizzie to establish your best stable idle (NOT best idle) with an AFR in low 15s...
 

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I agree. Sounds like a lean condition. I would start by checking for a vacuum leak and possibly pull the plugs to see if they're white. Once you confirmed you're getting enough gas then I would play with the timing.
 

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Discussion Starter #13
Ok, problem solved.
Thought I'd follow up on my solution in case someone has a similar problem.

I did a test and tune. Basically had my timing light and tools in the back seat and found a good stretch of road to drive with little traffic. Truck weigh stations along the way that are closed are an awesome resource for pulling over and doing this.

Steps had some pretty good posts that I read through, and combined with the tip off that the carb might be too lean led me to increase jet size by 3. I was at 71 on the primary's. Now 74. I new that too much timing might be an issue but to isolate the problem I test drove with jet change only. It helped but I still had some backfire. I Backed off timing to 14* initial. No change. Maybe worse. I then turned the secondary idle screws out 1/4 turn on all 4 and then decided to increase timing to 18*. It worked. Extremely strong acceleration. Much improved. No popping or backfire on acceleration or deceleration.

So it turned out my BBC needed 18* initial, 20* mechanical (all in @ 3000 rpm) and 14* @ 12 hg while using 74 primary jets ended up being the solution.

What surprised me most was that very big changes in how the engine ran occurred from very small adjustments. Just 2 degrees in initial timing and a couple steps up in jets made all the difference.

Thx to all who posted to help me out.
 

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Cool, glad you got it.
I was just reading through and based on your recent description I was thinking it was lean on the idle circuit maybe, I think what you did to fix it was your idle screw adjustment. If it acts fat at cruise or part throttle I would take some jet back out and see if its still gone, I doubt the jet change did it but maybe. Jet change might have helped performance though if it needed it.

Either way, glad you got it. :thumbsup:
 

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my BBC needed 18* initial, 20* mechanical
Thats 38 degs .. ne petrol vechile with inter4combustion engine will require any more than 36 degs , over the last 100 yrs plus... rather the opposite on modern unleaded fuel forumula and lower octanes...

you have simply patched a problem not fixed it, and patching WILL kick u in the butt later on.
With the lean burn u have a slower burn, which increasing the advance will cover up.. patch

I was at 71 on the primary's. Now 74. I new that too much timing might be an issue but to isolate the problem I test drove with jet change only. It helped
you confirmned the lean issue here
I then turned the secondary idle screws out 1/4 turn on all 4
The idle circuits have very little significant difference on mixtures over walking pace in top gear.. u still have a jetting issue.. The higher speed air being still sucked past the venturis metered by jets, at very high engine vaccuums.
And if have a non egr engine which runs VA on manifold vaccuum so intial +Va= idle u now have a n idle of 18+14 =32 and overall 52 10 deg above a max of 40/42 if dialing in without a knock sensor and data logger.

ANY mild street cam will only require a idle advance of 14 to 16 degs MAX stronger cam 16 to 18, wild/ track 18 to 22 degs for best stable (not best) idle.

you will find under WoT getting up around the 4K rpms that engine wil be leaning out, melting pistons and stuff.. espec if lean AND runing extra advance to fire the lean condition and slower burn.

There is only 1 way to dial in
Get the timing into ball park..
idle around 14 degs
Total around 34
all up under 42.
THEN get the mixture correct
Idle circuit 14.8 to low 15s
WoT 11.5 to 12.5 max
Cruise mid 15s

THEN
do the timing again with locked dizzie as I descibe above.. establish the actual idle/ WOT advances... then adjust the cent and VA advances to suit keeping the all up under 40/42 degs , unless have a knock sensor...
There are no short cuts, roundabout way or patch ups ... these result in engines having so called part failures ....
This is the 21 st century, things like data loggers are cheap compared to a engine build now....20th century unless one had very deep pockets all we had where COx meters to tune with, and they where damn expensive.
(mine has been tucked away under a bench for many yrs now... obsolete).
 

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A vacuum leak somewhere will also cause the symptoms you've described.
What he said.

Vacuum signal is highest on deceleration and that's where lean-out from a leak will present itself.

If not, you could have a carb that's too small or too lean in general causing elevated exhaust temps.
 
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