uf240z Posted May 11, 2017 Share Posted May 11, 2017 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 Quote Link to comment Share on other sites More sharing options...
madkaw Posted May 11, 2017 Share Posted May 11, 2017 So it's just a sound or do you feel anything ? Quote Link to comment Share on other sites More sharing options...
madkaw Posted May 11, 2017 Share Posted May 11, 2017 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) Quote Link to comment Share on other sites More sharing options...
uf240z Posted May 11, 2017 Author Share Posted May 11, 2017 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. Quote Link to comment Share on other sites More sharing options...
30 ounce Posted May 11, 2017 Share Posted May 11, 2017 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. Quote Link to comment Share on other sites More sharing options...
skirkland1980 Posted May 11, 2017 Share Posted May 11, 2017 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. Quote Link to comment Share on other sites More sharing options...
uf240z Posted May 11, 2017 Author Share Posted May 11, 2017 What do you mean by cross firing and spark plug blow out? Quote Link to comment Share on other sites More sharing options...
madkaw Posted May 11, 2017 Share Posted May 11, 2017 Cross firing under the dizzy cap. Cracked cap , worn cap . With that kind of performance engine you need everything working at 100% Quote Link to comment Share on other sites More sharing options...
skirkland1980 Posted May 11, 2017 Share Posted May 11, 2017 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. Quote Link to comment Share on other sites More sharing options...
uf240z Posted May 11, 2017 Author Share Posted May 11, 2017 Thanks guys, I'll check the cap tonight and see what I find. Quote Link to comment Share on other sites More sharing options...
jasper Posted May 11, 2017 Share Posted May 11, 2017 (edited) Check valve lash. Or broken valve spring, or wiped cam lobe. Could be backfiring into intake manifold. Edited May 11, 2017 by jasper Quote Link to comment Share on other sites More sharing options...
uf240z Posted May 12, 2017 Author Share Posted May 12, 2017 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? Quote Link to comment Share on other sites More sharing options...
Chickenman Posted May 13, 2017 Share Posted May 13, 2017 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. Quote Link to comment Share on other sites More sharing options...
HuD 91gt Posted May 14, 2017 Share Posted May 14, 2017 Webers haha. Your cross reading too many threads too many problems to troubleshoot at once. Quote Link to comment Share on other sites More sharing options...
jonbill Posted May 14, 2017 Share Posted May 14, 2017 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 Quote Link to comment Share on other sites More sharing options...
munters Posted May 15, 2017 Share Posted May 15, 2017 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. Quote Link to comment Share on other sites More sharing options...
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