BBC teardown and diagnosis

malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
Been working on this for a while now...
This has got me confused.
Pulled the 482BBC out of my wagon, couldn't get it to stop backfiring, especially when cold. Never had a miss or studder.
Basic build spec, MK6 BBC STD bore and 4.25 crank, FT pistons +3cc valve relief, 113cc chambers puts CR ~9:1
280TFS heads, 224/234° .050" .630/.640"(1.8 rockers) 114lsa, RPM intake, 650cfm carb, HEI ignition.
Tube headers with 1-7/8 primary and. 3" collector.
(Yes I know the carb is small, also not plan on buy another carb when plan is to put MPFI and a 1000-1250 Throttle body on the engine)

Did everything thing I could with engine in the car, compression test had holes #2/#3 at 150psi everyone else at 160psi. Technically that's in spec, but have less than ~50hrs of run time it's a point of interest.

Fast fwd, pull heads, i find the cause of the backfire exhaust for #2 #3 don't have good seals and have signs of blow bye through the valve seat interference area..

What I didn't expect to find was the following.. and has me and few others confused

4 holes that look roughly normal
2 look almost new (#2/#6) with #2 having more carbon build up than #6
#1/#5/#7 round out the middle. I'm carbon build up. Plugs are black, but I set the carb up very much on the rich side.

decks on both the block and heads are good, cam isn't flat, no signs of blown HG or cracks for coolant to be washing the cylinder down, exhaust never smelled like coolant either.

Any ideas to why the clean holes?

I've got a fresh set of OEM MK6 heads, that I had done as a set of spares.

Part of me wants to see what TFS's warranty is about.
 

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Last edited:
Chevy1925

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Somethings not allowing a good swirled mixture down into each hole. Not uncommon on a carb intake. 2 & 3 being lean from leaky exhaust valves can cause those two cylinders issue. If you have standard firing order, it’s possible the backfire could have been playing havoc on the intake of cyl 6 since it comes after 3. It’s not that the backfire happened, it’s more that flow gets screwed up from it.

I’d get it back together, new plugs and take it for a wot spin. Then bust out the magnifying glass and look down at the base of the porcelain in the spark plug to see what the fuel ring looks like. See if they all look about the same or if the offending cylinders are still leaner than the others.
 
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malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
Chevy1925 said:
Somethings not allowing a good swirled mixture down into each hole. Not uncommon on a carb intake. 2 & 3 being lean from leaky exhaust valves can cause those two cylinders issue. If you have standard firing order, it’s possible the backfire could have been playing havoc on the intake of cyl 6 since it comes after 3. It’s not that the backfire happened, it’s more that flow gets screwed up from it.

I’d get it back together, new plugs and take it for a wot spin. Then bust out the magnifying glass and look down at the base of the porcelain in the spark plug to see what the fuel ring looks like. See if they all look about the same or if the offending cylinders are still leaner than the others.
Click to expand...

Yeah carb is notorious for rich/lean ports and BBC long/short runners don't help anything.. just I've never seen one with this stark of a difference..

I could get it to backfire through the intake, though very low rpm and a lot of throttle to do it..
The disruption of the air flow makes sense as you've described it.

I'm going to put the fresh OEM heads on so I make the meet. Get a fresh valve job on the TFS heads.
 
J

jlawles2

Well-known member
Jan 28, 2010
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Danbury, TX
That's an odd one for sure. If the head and the piston looked the same, I'd agree dead hole(s). Wonder if the carb is so small it's starving those 2 cylinders. Note that hole 3 and 2 are evenly spaced, 3 between them in firing order.

By chance did you temp gun the exhaust when it was running?

Just watched a Steve Morris video where they actually cut the plug to see the bottom of the porcelain better. He made a comment that MSD did not like resistor plugs. Don't know what spark system you are running, but might be worth a shot to see if it's something similar.
 
Chevy1925

Chevy1925

don't know sh!t about IFS
Staff member
Oct 21, 2009
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Phoenix Az
Need to be careful with non-resisted plugs. Ok for a full race car, issue for a car that runs any digital stuff. They give off a bunch of rf and screws with electronics
 
malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
I've always heard that, about non-resisted plugs... Never found any, nor have I ran into any problems with digital stuff. Maybe if I got into higher end ignition systems that problem will arise.

Plug wires are MSD SC 8.5mm with sub 1ohm per foot resistance . I've ran both MSD 6AL with pro billet distributor and blaster SS coil and stock AC Delco HEI with upgrade coil.

Plugs I ran in the TFS heads are XP3923 gapped at .045" starting out haven't gone bigger gap because of the valve sealing and back firing issue.
Recommended starting plug from TFS is XP3924.. no gap specified.

With the iron all my iron heads I run/ran same ignition but with AC Delco 42TS equivalent plugs with .060" gap, I have ran them without a ground strap as well.
 
malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
NC-smokinlmm said:
I’d take a closer look at heads and gaskets. That looks like they have been steam cleaned.
Click to expand...
I took a straight edge to both the block and the TFS heads, I couldn't get a .002" feeler guage under it, I did focus on the 2/6 holes,
Then head gaskets is Fel-pro 9502PT which I have been using for my Gen 6 blocks builds going back to '18.
I mic-d the thickness coming up with .044-.046 on the used gaskets. Could there be a twist in the head, possibility, but my equipment isn't that accurate enough to find a sub .002" twist. Other hand that's well within factory specs as well.. so.. IDK

The TFS heads are bagged and waiting to go to the machine shop probably end of summer.

Side note TFS chambers are 113cc 0279 chambers are 110 stock and have been decks .007 so probably 108-109cc now.

Put the fresh set of OEM 0279 heads on, with some comp mag roller tip 1.72 rockers... The 1.8 wouldn't clear the VC.
#3 did 150psi everyone else did 170psi.

Played with rocker lash settings they seam to be hydro locking And not allowing the valves to close right.. I pulled the lifters for quick inspection nothing obvious.

Im going move the lifter to another hole see if the low compression follows. FWIW...Lifters are COMP Cams High Energy Hydraulic Roller Lifters #900-16
 
malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
Harshman said:
Are you sure you don't have an ignition problem? Bad cap, wires crossed up. Those clean pistons look washed clean like the plug wasn't firing or at the wrong time.
Click to expand...
Everything was new when I assembled the engine first time, original thought I hooked the plug wires up wrong.. first thing I checked.. after pulling the heads and valves for said holes found blow-by between the valve and valve seat.


gmc502 said:
how much lash after zero are you going?
what intake are you running?
Click to expand...
1/8-1/4 turn on these comp lifters anymore they bottom out the plunger and don't pump. I actually watch the plunger move to set lash. And I have the partner valve open when setting lash.. example intake open, set exhaust,

I didn't give the part number in the first post when I mentioned the intake it's a Edelbrock performer RPM #7161
 
J

jlawles2

Well-known member
Jan 28, 2010
1,062
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48
Danbury, TX
Is the engine using coolant? If it's not loosing coolant, steam cleaning the cylinders is out. If it is using coolant, have the block water jacket vacuum checked to ensure there is not porosity in the casting, an internal crack, or a pin hole in the machining between the water jacket and the intake runners.

It's extremely odd that it's on opposing banks.

You could try lapping the head to the block to check for twist by coloring either the block or head, then use lapping compound and look for the color to be left behind. Use small figure 8 motion to do the lapping check.
 
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malibu795

malibu795

misspeelleerr
Apr 28, 2007
8,298
610
113
42
in the buckeye state
No I haven't, pretty much set on dropping the TFS heads off at the machine, it never used coolant.

The runners on the intake are split up as 2/3/5/8 and 1/4/6/7 on a dual plane intake..
About to find out if the low compression will follow lifters
 
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