popping at high rpm

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racyroy1

racyroy1

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Hi all, I've been building a 340 scamp for the track and I finally got it out on the track a couple of weeks ago and i went through some I guess you'd call them bugs that had to be worked out. One was that the motor was missing on the top end, well i jetted it up and that took care of the missing issue. I then had the issue of running out of motor in the last 400 ft or so. I found out that the distributor clamp was loose and the timing had changed dramaticly, i got that fixed at home. I then took the car to the track yesterday and with the proper timing in it at 14 initial, 20 mechanical, 34 total. The first pass was good but still a little short on motor at the top end so i jetted up to 73 primary and 83 secondary and i picked up another 2 tenths. Keeping this in mind i'm shifting at 5400 with no miss whatsoever. Well on the next run, at the burnout I mistakenly didn't get the tranny back in first, so I left the line in first gear and it shifted into second way to soon. Well I reacted by shifting back into 1st instead of just writing off the pass, well when I did this it reved what had to have been around 7000. Right after this the motor developed a miss/pop/backfire around the 4800-5000 range. I backed off the timing and it was still there. I removed the valve cover and did a quick visual and found no obvious signs of damage. While sitting there and just running the motor up i can go to 7000 without a miss. I checked the coil and it was hot, but no hot enough to where you couldn't grab it and hold on. All electrical connections were tight, checked the cap/rotor and there seemed to be no issue there either. I can't see the fuel pressure due to the guage is under the hood.

My thoughts are either the fuel pump got hurt, (it's a manual pumper 80 gph I believe) or the elec box is done.

now here's what I have in the motor and electronics

340- stock bottom end, 10.1 comp,
mildly polished and ported j heads, gasket matched intake and exhaust.
stock rocker assembly
comp springs, retainers and keepers to go with cam
competion cam single pattern 484 lift [email protected] if i'm not mistaken
RPM air gap
old 750 holley, (doesn't even have power valve protection) but was cleaned and professional checked out
FBO recurved distributor, coil and box
904 trans with stage three manual shift kit

I'm going to pull the rocker assembly off and check push rods and make sure no springs are broke

Can ya'll please direct me in the right repair direction

thank you in advance

Kevin.

PS. IF i've missed any info that would help, please ask and i'll fill in the blanks.
 
I agree rocker gear and springs are a good place to start. I doubt you hurt the pump. Remember they only pump at half engine speed because it's cam driven. If you haven't already, pull the plugs and check for a closed gap or broken electrode. It's been run lean because you picked up 2 tenths by jetting up. Not likely, but easy to check. If your timing didn't change after this then i doubt you hurt the chain or distributor shaft. Was the valvetrain noisier after this happened? Even a stock bottom end and valvetrain can take a quick buzz to 7,000. I've done it, even a few times on purpose....lol. I'll keep thinking:-k and post if i think of anything else.

As a side note, what rpm were you "running out of engine" at on the top end? That combo should pull to 6K with no problem.

P.S. This is probably your cam.....
http://www.summitracing.com/parts/CCA-20-232-4/
 
OldmanRick said:
I agree rocker gear and springs are a good place to start. I doubt you hurt the pump. Remember they only pump at half engine speed because it's cam driven. If you haven't already, pull the plugs and check for a closed gap or broken electrode. It's been run lean because you picked up 2 tenths by jetting up. Not likely, but easy to check. If your timing didn't change after this then i doubt you hurt the chain or distributor shaft. Was the valvetrain noisier after this happened? Even a stock bottom end and valvetrain can take a quick buzz to 7,000. I've done it, even a few times on purpose....lol. I'll keep thinking:-k and post if i think of anything else.

As a side note, what rpm were you "running out of engine" at on the top end? That combo should pull to 6K with no problem.

P.S. This is probably your cam.....
http://www.summitracing.com/parts/CCA-20-232-4/
Click to expand...

thanks for the response rick


the valve train seemed the same, and like I said under no load i can run it up to 6-7 grand and she won't miss a beat but on the track a whole nudder story.
As for the rpm that i was running out of power it had to be in the high 4's but i'm not really sure due to the fact that it was a night run and the tack light wasn't working. But like I said I believe i got that problem whipped.

Well i checked the rocker assembly and it's fine, i pulled the plugs and they are all tan and in one piece. Tomorrow i'm going to take the carb off and check all jets just to make sure that in an off chance that one of the jets got plugged with who knows what. I checked all valve springs and rolled the push rods, nothing seems to be bent or broken.
I've got an extra ignition box and coil that i'll install and try test and tune this wednesday.
 
sounds like your running out of fuel...

what is your delivery and intake system comprise of?
 
Sounds like your still mechanically sound so if the carb check and box doesn't help mopar kid might be on to something with the fuel delivery? Good luck, i'll stay tuned to see how it goes.
 
805moparkid said:
sounds like your running out of fuel...

what is your delivery and intake system comprise of?
Click to expand...

hello kid, I just got done reading your whole thread, didnt' mean to start another thread about almost the same thing, but i thought mine was slightly different. To answer your question. I have a 8 gallon cell running through a high flo summit filter and then through the factory lines up to the pump which was a 120gph mechanical.
This is the /6 factory lines which i thought would be way too small but i have a fellow racer running a low 12 360 scamp with the same lines on a 850 holley. And it never had a problem until i dropped it down a gear as explained above. But as I'm typing and thinking (quite a feat i tell you) I did jet up quite a bit and that could explain why it's doing it now and not before under the smaller jets. but in the same breath I ran one other pass with no problems at all. Don't get me wrong I think i'll be installing some 1/2 line real soon
 
CHEATHAM said:
somthing as simple as fresh plugs may fix it and maybe tighten the gap on them some
Click to expand...

it could come to that, I liked the idea on moparkids thread where to test the dist and plugs is mentioned just to see.
 
the more i think about this, (without typing) the more "running out of fuel) makes sense, I'll hit the test and tune this weekend after I jet down a bit and see if the popping/misfire goes away. If so then i totally misjudged the fuel system and I'll remedy it ASAP
 
I think I suggested the test idea but it is not a perfect test, because ignition is more difficult at full load. One reason is the dynamic compression is less unloaded because the air limitation by the throttle. The more compression, or more the fuel mixture is off the harder for ignition. The higher the RPM the less time is available to charge the ignition coil, and a weaker ignition when needed the most.

At 6,000 RPM the time between cylinder events is only 2.5mS. In that time the coil needs to be charged and there also needs to be time for the ignition event, it takes some time. The coil charge time or dwell is related to the voltage level of the supply, the inductance and resistance of the ignition circuit. To get the best performance requires a good supply voltage, low impedance coil. Ignition systems that do not have active dwell control (points or OEM mopar box) require a ballast resistor, this is a disadvantage. A mopar chome box helps but it requires the application to be racing, or there may be reliability issues of box and coil on the street. The GM HEI ignition actively controls the dwell, it does not require the ballast, so a shorter dwell is possible for more high RPM spark energy.

To get serious about diagnosing ignitions requires a scope and special voltage and current probes.
With the number of variables at play it is sometimes necessary to be a scientist and detective to sort things out.
 
KitCarlson said:
I think I suggested the test idea but it is not a perfect test, because ignition is more difficult at full load. One reason is the dynamic compression is less unloaded because the air limitation by the throttle. The more compression, or more the fuel mixture is off the harder for ignition. The higher the RPM the less time is available to charge the ignition coil, and a weaker ignition when needed the most.

At 6,000 RPM the time between cylinder events is only 2.5mS. In that time the coil needs to be charged and there also needs to be time for the ignition event, it takes some time. The coil charge time or dwell is related to the voltage level of the supply, the inductance and resistance of the ignition circuit. To get the best performance requires a good supply voltage, low impedance coil. Ignition systems that do not have active dwell control (points or OEM mopar box) require a ballast resistor, this is a disadvantage. A mopar chome box helps but it requires the application to be racing, or there may be reliability issues of box and coil on the street. The GM HEI ignition actively controls the dwell, it does not require the ballast, so a shorter dwell is possible for more high RPM spark energy.

To get serious about diagnosing ignitions requires a scope and special voltage and current probes.
With the number of variables at play it is sometimes necessary to be a scientist and detective to sort things out.
Click to expand...

agreed, The FBO electronic box was said to be good to 8000 rpm, not saying it might not have an issue now, and the coil seems to be a stock coil. I have a spare box, just off the shelf at autozone, and a blaster II coil that I'll take along with me to the track this coming fri/sat. I'll work with the fuel and the spark delivery. I'd even thought about bringing a fuel gauge into the car to watch the pressure to see if i'm dropping off.
 
I'll guess something bent/broken in valvetrain.

Any of the plug gaps closed up?

5/16 line can feed that OK, a 3/8 line would be better. Suck air through a 5/16 line and then do the same with a 3/8 or 1/2 line. The mechanical fuel pump does the same.

As a baseline, try to jet change 2 sizes at a time. Only time I would go more is if I had an air/Fuel meter that displayed a really rich/lean condition.
 
crackedback said:
I'll guess something bent/broken in valvetrain.
Click to expand...

anytime i over rev an engine...i run a compression test to see if a valve might have kiss a piston.....plus check for bend pushrods or broken valve spring.....

if you floated the valves it would be easy to kiss a piston...only takes a few minutes to check..
 
racyroy1 said:
hello kid, I just got done reading your whole thread, didnt' mean to start another thread about almost the same thing, but i thought mine was slightly different. To answer your question. I have a 8 gallon cell running through a high flo summit filter and then through the factory lines up to the pump which was a 120gph mechanical.
This is the /6 factory lines which i thought would be way too small but i have a fellow racer running a low 12 360 scamp with the same lines on a 850 holley. And it never had a problem until i dropped it down a gear as explained above. But as I'm typing and thinking (quite a feat i tell you) I did jet up quite a bit and that could explain why it's doing it now and not before under the smaller jets. but in the same breath I ran one other pass with no problems at all. Don't get me wrong I think i'll be installing some 1/2 line real soon
Click to expand...

hey no worries... im still hunting down my issue lol... as for mine i think its leaning out but im having a hell of a time proving it...
 
crackedback said:
I'll guess something bent/broken in valvetrain.

Any of the plug gaps closed up?

5/16 line can feed that OK, a 3/8 line would be better. Suck air through a 5/16 line and then do the same with a 3/8 or 1/2 line. The mechanical fuel pump does the same.

As a baseline, try to jet change 2 sizes at a time. Only time I would go more is if I had an air/Fuel meter that displayed a really rich/lean condition.
Click to expand...

I'm with ya cracked, I change jets 3 sizes as per spec in my holley tune book
but 2 wouldn't hurt. I have a air/fuel wide band gauge in the car it's burning about 13.1-2 throught out the quarter.
And no I pulled all plugs, they're tan and still gapped correctly

70aarcuda said:
anytime i over rev an engine...i run a compression test to see if a valve might have kiss a piston.....plus check for bend pushrods or broken valve spring.....

if you floated the valves it would be easy to kiss a piston...only takes a few minutes to check..
Click to expand...

did the pushrod check, alls good. checked all springs, without removing and putting them on a compressor they looked good. Compression test is next. After the 1st popping run I put a heat gun to the cylinders and they all read just about the same with the front 2 (1-2 cyl) running about 30 deg cooler. due to having the coolest water.

805moparkid said:
hey no worries... im still hunting down my issue lol... as for mine i think its leaning out but im having a hell of a time proving it...
Click to expand...
I'm crossing my fingers for ya.
 
wellllllll, checked the compression,,,,, 150 psi in every cylinder. So I'M convinced the motor is good. So i'm gonna focus on the coil and box unless another thought comes up
 
Well, i installed a blaster 2 coil and headed to the track last night and today, the motor never skipped a beat and with some jetting changes I'm knocking on the 12's yead i know it should do 12's all day, well i'm getting there, I'm learning as I go
 
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