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L30 stuttering/machine gunning issues during acceleration


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#1 uf240z

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Posted 2 weeks ago

Hey guys, I'm having some trouble getting my '72 240Z to accelerate without "machine gunning". I've searched all over the site and have not found suitable answers. It's a 3.0l race motor, 13:1 CR, triple Weber 45s, ported head that flows 227/155 CFM, 280zx distributor, summit racing ignition box(per Dave Rebello), new coil, carter fuel pump, Rebello road race cam grind. I had my timing set around 15, 35 advanced. I advanced and retarded it from there and the problem remained. As I accelerate, it doesn't have full power and makes stuttering/machine gun sound. It's not a mechanical sound, like a rod knock or broken rocker arm, but sounds more like ignition or fuel related. Also I am running ngkbr9 plugs and 110 leaded fuel. Any help or suggestions as to a cause would be greatly appreciated.
Thank you in advance, Andy

#2 madkaw

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Posted 2 weeks ago

So it's just a sound or do you feel anything ?
Steve,
9/71 240z. 2.4 -.40 over, balanced. E-88 Bigger SS valves, stage 2, 6x2 header, true dual exhaust. BW t-5 , STI LSD 3.90- triple Mikunis 40mm

#3 madkaw

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Posted 2 weeks ago

Re reading your post I would suspect ignition. You would need everything right for that motor . Plug wires , cap, rotor , coil ( I know it's new , but suspect for sure)
Steve,
9/71 240z. 2.4 -.40 over, balanced. E-88 Bigger SS valves, stage 2, 6x2 header, true dual exhaust. BW t-5 , STI LSD 3.90- triple Mikunis 40mm

#4 uf240z

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Posted 2 weeks ago

Thanks madkaw, I also am starting to suspect that it is ignition related. It all worked a couple months ago, but I've had fuel issues since then. (Rusty tank, clogged fuel filters, now that's all fixed) So now I guess wires, cap, rotor, and plugs need to be examined.

#5 30 ounce

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Posted 2 weeks ago

I had an issue with my MSD ignition box when I first wired it up. The trigger leads from the distributor aren't marked + or - and I had them hooked up backwards. If you are using those try switching the magnetic trigger leads around.

#6 skirkland1980

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Posted 2 weeks ago

Could be cross firing. I'm not sure about na applications but I'm also wondering if spark blowout is possible with such high compression.
:1983 280zx turbo, L28ET, custom cam grind, 96 saab 900 intercooler w/2.50" piping, 3" mandrel bent exhaust, megasquirt II v3.57, 540cc injectors, walbro gsl392 255lph fuel pump, aeromotive regulator, -6 supply lines, custom fuel rail, MSD ignition wires, MSD 8202 coil, homemade boost controller, Garrett TC4305 turbo, 70mm throttle body, Godzilla BOV, Turbonetics wastegate, custom intake, custom turbo header, Cooling Mist H2O/meth injection, 20psi boost, all stock body, stock interior and weld draglite wheels, TH350 trans 4000 stall with B&M 'street stick' shifter and hughes trans brake, nitto nt555R drag radials, best 1/8 mile of 7.53@ 98 mph

#7 uf240z

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Posted 2 weeks ago

What do you mean by cross firing and spark plug blow out?

#8 madkaw

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Posted 2 weeks ago

Cross firing under the dizzy cap. Cracked cap , worn cap . With that kind of performance engine you need everything working at 100%
Steve,
9/71 240z. 2.4 -.40 over, balanced. E-88 Bigger SS valves, stage 2, 6x2 header, true dual exhaust. BW t-5 , STI LSD 3.90- triple Mikunis 40mm

#9 skirkland1980

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Posted 2 weeks ago

With high energy ignitions "they say" it's possible for a wire to arc to another wire and cause the wrong cylinder to fire. And spark blowout is when there's too much cylinder pressure for the spark to jump the gap on the spark plug. So you tighten the gap a little. I had it on my L28et. At 15psi it would start missing. I closed the gap to .025" and cured it.
:1983 280zx turbo, L28ET, custom cam grind, 96 saab 900 intercooler w/2.50" piping, 3" mandrel bent exhaust, megasquirt II v3.57, 540cc injectors, walbro gsl392 255lph fuel pump, aeromotive regulator, -6 supply lines, custom fuel rail, MSD ignition wires, MSD 8202 coil, homemade boost controller, Garrett TC4305 turbo, 70mm throttle body, Godzilla BOV, Turbonetics wastegate, custom intake, custom turbo header, Cooling Mist H2O/meth injection, 20psi boost, all stock body, stock interior and weld draglite wheels, TH350 trans 4000 stall with B&M 'street stick' shifter and hughes trans brake, nitto nt555R drag radials, best 1/8 mile of 7.53@ 98 mph

#10 uf240z

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Posted 2 weeks ago

Thanks guys, I'll check the cap tonight and see what I find.

#11 jasper

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Posted 2 weeks ago

Check valve lash. Or broken valve spring, or wiped cam lobe. Could be backfiring into intake manifold.


Edited by jasper, 2 weeks ago.


#12 uf240z

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Posted 2 weeks ago

So, number 5 plug was wet fouled. I hope fuel and not oil. I'll check lash tomorrow (left my feeler gauge at work). Valvetrain looks healthy. Possibly a compromised plug wire?

#13 Chickenman

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Posted 2 weeks ago

So, number 5 plug was wet fouled. I hope fuel and not oil. I'll check lash tomorrow (left my feeler gauge at work). Valvetrain looks healthy. Possibly a compromised plug wire?

 

Could also be a faulty injector. Run some good injector cleaner though it. CRC One Tank Power renew is very good. Or better yet, send them out for ultrasonic cleaning, testing and calibration. InjectorRehab and Injectorman ( Motorman ) are two excellent  sources

 

You should also have local places that do Ultrasonic cleaning and testing of injectors. 



#14 HuD 91gt

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Posted 2 weeks ago

Webers haha. Your cross reading too many threads ;) too many problems to troubleshoot at once.

1971 240z - Balanced .040 over L28 w/ Ported P90 head - Colt c542.s cam, SU w/SM needles, MSII (ignition)- LS2 Coils Wasted Spark, MSA Premium Exhaust - 6-3-2 Header

 


#15 jonbill

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Posted 2 weeks ago

With plugs that cool, I think they'll foul pretty easy if you're not loading it, which could be the case if it's been off the road for a while.
I'd stick a new set of plugs in, gapped at 0.6mm

#16 munters

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Posted 2 weeks ago

Is it when slowly open throttle off idle coffing back throug the carbs? If so, then the coffing carb is to lean. I just fixed this 5 min ago.




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