408 breaking up

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Cudapete

Cudapete

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I have a blueprint engines 408 stroker in my 68 dart. Its has 10.1 comp, edelbrock aluminum heads, comp roller rockers, a comp hyd cam and a air gap intake, The trans is a coan and has a coan 3500 converter and 3.55 gears with a summit brand electronic box with rev limiter and a pro comp distributor. EVERYTHING is new on the car Except the carb (was all rebuilt and tuned but bought from a friend used)and it has about 1000 break in miles on it. Just recently I took it to the track and pulled a 12.66 @ 106, which is respectable BUT it breaks up and pops through the exhaust at 5800, I fixed a couple of vacuum leaks and replaced the base of my holley cause I found the shaft was worn and whistling Its much better but still has the issue. I was thinking the ign box or maybe the valves arent set right?? Anyone have any ideas before I go tearing this thing apart. Thanks Pete
 
Pete,

I have no experience with the Pro-comp distributors, but I have found a distributor phasing problem with my Mopar Performance unit. Check your cap and rotor and see if the arcing is happening way to one side of the cap terminals.

My next guess would be valve springs. Are you using Comp's springs or the the Eddie springs? What are the specs on your cam?
 
Here's something else to throw in the mix after you've checked the other suggestions individually: you didn't say what type or cfm carb is on your engine.
Since it's a high-speed problem, and sounds like a lean condition, maybe you don't have enough carb. Years ago I ran into the same type of problem on a stout 340. The terminology of the day was "shooting ducks" as it was a rapid popping through the exhaust at high rpm's. The engine ended up needing a 850 double-pumper to keep it well fed and happy.
 
It sounds lean though the cfm rating is not the problem but is a easy fix since with bigger carburetors comes the more radical metering blocks and such.

In nascar they sometimes run 390 cfm mech carbs and make 750+hp

If you give everyone here the carb modle# cfm, jetting, power valve/s it will help us improve the quality of advice you'll receive.

If it's a 750 dbl pumper try 73 primary and 80-82 secondary with matched power valve in front, plugged rear pv.
If you are running 2 pv's then square up the jets front and back, make them the same, and pv's should be the same as each other when running 2.

Sounds like lean jetting though.
 
1wild&crazyguy said:
It sounds lean though the cfm rating is not the problem but is a easy fix since with bigger carburetors comes the more radical metering blocks and such.

In nascar they sometimes run 390 cfm mech carbs and make 750+hp

If you give everyone here the carb modle# cfm, jetting, power valve/s it will help us improve the quality of advice you'll receive.

If it's a 750 dbl pumper try 73 primary and 80-82 secondary with matched power valve in front, plugged rear pv.
If you are running 2 pv's then square up the jets front and back, make them the same, and pv's should be the same as each other when running 2.

Sounds like lean jetting though.
Click to expand...

Please explain the nascar thing to me,im intrigued on how thats possible...

An i hope cudapete,that your dart gets sorted,sounds like
she should be haulin way harder than 106 mph trap speed.
 
What cam is installed? A 268H isn't going to run up there anyways... a 295hl or 305H would.

Fuel system details? The one area where people tend to not have enough umph.

Where is the rev limiter set. If it's at or near the 6000 range, that could be your issue. Turn the limiter at least 700-800 above your desired shift point. They have a tendency to start pulling spark earlier than the actual set rev limit point.
 
I believe that motor has the 280H in it or something close to those specs. Its not a lot of cam for a 408.
 
nemesis said:
Please explain the nascar thing to me,im intrigued on how thats possible...

An i hope cudapete,that your dart gets sorted,sounds like
she should be haulin way harder than 106 mph trap speed.
Click to expand...

I don't build nascar engines and I don't wanna hyjack this thread, I'll just say this.
If a carbs mixture was spot on for the weather but was too small, cfm wise, it would be come a rev limiter but not the sense of popping and or sputtering, instead it just wouldn't ''rev'' anymore.

google Restrictor plate racing.

no biggy,
Just trying to give an idea of the diff between running too lean and running out of air. btw fuel pressure is another place to look. :smile:
 
Cudapete said:
I have a blueprint engines 408 stroker in my 68 dart. Its has 10.1 comp, edelbrock aluminum heads, comp roller rockers, a comp hyd cam and a air gap intake, The trans is a coan and has a coan 3500 converter and 3.55 gears with a summit brand electronic box with rev limiter and a pro comp distributor. EVERYTHING is new on the car Except the carb (was all rebuilt and tuned but bought from a friend used)and it has about 1000 break in miles on it. Just recently I took it to the track and pulled a 12.66 @ 106, which is respectable BUT it breaks up and pops through the exhaust at 5800, I fixed a couple of vacuum leaks and replaced the base of my holley cause I found the shaft was worn and whistling Its much better but still has the issue. I was thinking the ign box or maybe the valves arent set right?? Anyone have any ideas before I go tearing this thing apart. Thanks Pete
Click to expand...

Mine was doing the exact same thing and I changed the coil and then it would dial up. I always try the simple stuff first.
 
AdamR said:
I believe that motor has the 280H in it or something close to those specs. Its not a lot of cam for a 408.
Click to expand...

Isnt' that cam something like 230/230 and 480 lift?
That cam barely turns 5600-5800 in a 340/360. This car is feeding 15-20% more cubes

I bet the car would run better shifting at about 5000 and running it through the lights.
 
Thanks for all the advice guys. To answer all the questions here are the specs right from the blue print site. also my carb is a Holley 4150 series race carb #hly-0-9379 in the summit catalog , my fuel system consists of a 1/2 " fuel line from the tank to a holley electric fuel pump then 1/2" line to a holley mechanical pump to a regulator(set to 6psi) then to the carb. The dyno sheet said 470 horse @ 5800 and 510 lbft torque @ 4100. I've always built my own motors but time was a factor and for a $500 difference I got a warranty and when its up theres room to make more power! I planned on checking all the basic easy stuff first but thought i'd pick everybodies brains before diving in.thanks again! Pete
Block:
Seasoned 360 block
Square and parallel decked
Align honed main bearing bore
Cylinders honed on computer controlled machine to within .0002 straightness and roundness
Cylinders are sonic tested for thickness

Rotating Assembly:
New SCAT cast iron crankshaft
Keith Black forged pistons
SCAT H-beam rods with 150,000 psi ARP bolts
Hastings Moly rings
Balanced rotating assembly
Melling high volume oil pump
Flat tappet hydraulic lifter camshaft
Heavy duty double roller timing set
Comp Cams Pro Magnum roller rockers

Cylinder Heads:
EDELBROCK Aluminum Performer RPM heads 1.437 diameter valve springs
Hardened retainers and springs
2.02 swirl polished intake valves
1.60 swirl polished exhaust valves
Hardened push rods
EDELBROCK RPM Air-Gap Intake
Mopar Performance polished aluminum valve covers
Brass freeze plugs
Dyno tested – and shipped with results
Critical Specs:
Camshaft:
.545 Int / .545 Exh & 241 Int / 2467Exh duration @ .050 - 110 degree lobe sep.
Compression Ratio:
10 to 1
Ignition Timing:
13 to 16 Degree Initial / 34 Degree Total
 
I was incorrct on the camshaft size its
.545 Int / .545 Exh & 241 Int / 247 Exh duration @ .050 - 110 degree lobe sep.
 
Cudapete said:
Thanks for all the advice guys. To answer all the questions here are the specs right from the blue print site. also my carb is a Holley 4150 series race carb #hly-0-9379 in the summit catalog , my fuel system consists of a 1/2 " fuel line from the tank to a holley electric fuel pump then 1/2" line to a holley mechanical pump to a regulator(set to 6psi) then to the carb. The dyno sheet said 470 horse @ 5800 and 510 lbft torque @ 4100. I've always built my own motors but time was a factor and for a $500 difference I got a warranty and when its up theres room to make more power! I planned on checking all the basic easy stuff first but thought i'd pick everybodies brains before diving in.thanks again! Pete
Block:
Seasoned 360 block
Square and parallel decked
Align honed main bearing bore
Cylinders honed on computer controlled machine to within .0002 straightness and roundness
Cylinders are sonic tested for thickness

Rotating Assembly:
New SCAT cast iron crankshaft
Keith Black forged pistons
SCAT H-beam rods with 150,000 psi ARP bolts
Hastings Moly rings
Balanced rotating assembly
Melling high volume oil pump
Flat tappet hydraulic lifter camshaft
Heavy duty double roller timing set
Comp Cams Pro Magnum roller rockers

Cylinder Heads:
EDELBROCK Aluminum Performer RPM heads 1.437 diameter valve springs
Hardened retainers and springs
2.02 swirl polished intake valves
1.60 swirl polished exhaust valves
Hardened push rods
EDELBROCK RPM Air-Gap Intake
Mopar Performance polished aluminum valve covers
Brass freeze plugs
Dyno tested – and shipped with results
Critical Specs:
Camshaft:
.545 Int / .545 Exh & 241 Int / 2467Exh duration @ .050 - 110 degree lobe sep.
Compression Ratio:
10 to 1
Ignition Timing:
13 to 16 Degree Initial / 34 Degree Total
Click to expand...

whew!
Thats better. I was hearing 280*-.480
so do you know what the jetting is?

make sure you actually have 6psi after the regulator, hope it's not a holley reg.
 
I only plan on running this cam until the warranty runs out then I plan on some extensive port work, a victor or M-1 single plane manifold, and a much larger hughes solid cam, and some 3.91's. I know this thing isnt making power up there and I'm shifting it more like 5500 right now but even if I rev it past makin power it still should'nt break up right? The regulator is a summit brand one (which come to think of it is probably a holley) what is the correct pressure for a holley and what reg do you guys recommend? As far as the carb my buddy set it up to match the motor as a favor he's got an 11 sec coronet so I"d hope he knows what hes doin anyway I guess I dont know much about the carb I'll have to get a hold of him.
 
Turn the fuel pressure to 7.5-8 and see what happens. If it's not blowing by the needle and seat, run it. Run as much fuel pressure as the carb can stand.

Make sure the floats are set correctly.

If you have the clear sight plugs, set the level near the top of the window and go run it. It will probably blubber a bit down low. If it craps out again, it's likely not a fuel delivery issue.
 
That sounds easy enough, I'm gonna pull the plugs too just to see how they're lookin. If nothin else a good excuse to make a couple passes down the local back road!
 
Did it pop or crap out on the dyno?

If it's making power to 5800, no sense in shifting lower than that if it will pull there. It's always a test to find what the combo likes best.
 
I do not have the faith that they test every engine. I think they build one, tune it, get the numbers, and run in the clones after. The 106 number means it's way down on power. It should pull clean to about 5500 where the carb is out of steam. So, if the ignition is stable and the voltages at the coil are good, Try to borrow a larger carb and go race again. at least an 850 4150 series, an HP 950 or Avenger 870 if possible.
 
Are 5 and 7 plug wires near each other? I know of a couple of people that had high rpm misses that came in about that rpm due to 5 and 7 touching in spots....

Just a thought.
 
I've heard of that before never gave it a thought thats anothe reasy one to try. I 'm leaning more towards a electrical issue for now as i've got a miss even cruising @ 30 40 mph @2000 rpms. The more I think about it problem was not there before I installed the box.Last year on my relatively stock 340 this same carb went over 6000 rpms no sweat, so I'm gonna take the box out of the equation and see what happens. Still interested in other ideas though. Thanks PETE
 
if thats not it i'm thinkin of tryin another distributor. Forgot to mention that plugs looked good no spark jump in the cap either.
 
So good news kinda bypassed the box and the car pulled hard to 6500 now I've got a small hesitation (surging?) @ like 2200 around 30mph tomorrow gonna try a different coil and possibly a distributor if that doesnt cure it .
 
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