Ahoysandy Posted June 21, 2017 Share Posted June 21, 2017 (edited) Hey guys,Just after a little insight on the issue as I'm just about out of ideas trying to solve this without replacing things from square one.My Z has come back from the exhaust shop after putting new headers on and has developed what feels like a rev limiter at 5kRpm. Now just before it went IN to the shop, i did change the plugs, oil, filter, fuel filter and coolant. Free revving, it will make it to 6.5kRpm but is hesitant around the 5k mark. Underload it struggles to surpass 5kRpm. You can also feel it stuttering (i believe misfire) at lower rpm when under load but nothing that holds it back significantly. When i pulled the NEW plugs, cylinder 2 & 3 had some mount of carbon buildup on the sides of the node and cylinder 4 has a large amount (entire plug black). This was after only 40km of sydney traffic (mostly idle lol) and 75km of low rpm highway cruising without noticing much. it only occurs when i give it some and ride the tacho up higher. Tracing this back, i checked all plugs whilst out on the valve cover, making good spark, then checked all leads are creating good spark. At the same time i removed the dizzy cap, cleaned all the contact points & rotor with some sandpaper.After this i went over the ignition coil, checking connections and spade connectors, cleaned the spades on the ignition module (280zx distributor & coil).I'm just about out of things to tackle on it. Car has a refurbished 280zx Distributor & coil, OEM electronic pump with mechanical block off plate, Malpassi FPR.My thoughts are along bad coil/module/fuel delivery now. Potential thought on tune as the headers are different, but misfiring low rpm confuses me. Idles perfect.Cheers . Edited June 21, 2017 by Ahoysandy Quote Link to comment Share on other sites More sharing options...
ScottyD Posted June 21, 2017 Share Posted June 21, 2017 (edited) Sorry. My mistake. Edited June 21, 2017 by ScottyD Quote Link to comment Share on other sites More sharing options...
Roady Posted June 21, 2017 Share Posted June 21, 2017 (edited) What kind of carbies are you running Does it do this in every gear (I.e. 2nd and 3rd). The reason I ask is I originally had SU's on the car and with no heat shielding on the fuel lines around the engine... when taking it on the track, after several laps when in 3rd it would bog down as you describe around 4K due to vapourisation. I could drop back to 2nd and it would rev all the way back to 6.5K no worries. This way I figured it wasn't electrical it was fuel related. Perhaps the new headers are creating more heat and a similar issue to what I had. That said I was also running the standard mechanical fuel pump at the time and less likely for you with an electric fuel pump I may be overthinking it as well. ? Edited June 21, 2017 by Roady Quote Link to comment Share on other sites More sharing options...
Linton Posted June 21, 2017 Share Posted June 21, 2017 the misfire i think would be the plugs they will need to be replaced, hard to bring them back when they get as you described, but the plugs are not what caused the problem, its most likely fuel delivery. Quote Link to comment Share on other sites More sharing options...
Ahoysandy Posted June 21, 2017 Author Share Posted June 21, 2017 (edited) I removed the Killswitch incase of a faulting switch, nope. worth a shot. What kind of carbies are you runningDoes it do this in every gear (I.e. 2nd and 3rd).The reason I ask is I originally had SU's on the car and with no heat shielding on the fuel lines around the engine... when taking it on the track, after several laps when in 3rd it would bog down as you describe around 4K due to vapourisation. I could drop back to 2nd and it would rev all the way back to 6.5K no worries. This way I figured it wasn't electrical it was fuel related.Perhaps the new headers are creating more heat and a similar issue to what I had. That said I was also running the standard mechanical fuel pump at the time and less likely for you with an electric fuel pump I may be overthinking it as well. They are triple 45mm Webers and happens in every gear at roughly the same point, the fuel filter appears clean still so possibility that the FPR diaphram has gone bad from what i've read. I've put my 3 previous plugs in place of cylinders 2-4 to see if this would rectify, but it still happens. Linton is most likely correct with fuel delivery. what i have taken out is the heat sheilding i had between the headers and carbies as it fouled on the new pipes, will need to re adjust and fit to see if this fixes. drove it around the block a few times after trying each change and the misfire feels to be getting worse lower in the Rpm, but that could just be me thinking about it more and more and looking for the issue (lol) Edited June 21, 2017 by Ahoysandy Quote Link to comment Share on other sites More sharing options...
1600dave Posted June 21, 2017 Share Posted June 21, 2017 Intake manifold air leak ? Manifold has to come off to fit headers. Just a thought........ Quote Link to comment Share on other sites More sharing options...
RLY240 Posted June 21, 2017 Share Posted June 21, 2017 Intake manifold air leak ? Manifold has to come off to fit headers. Just a thought........ +1 if the header flange isn't the same thickness as the inlet flange then the bolts don't pull down evenly and leaks are likely. Quote Link to comment Share on other sites More sharing options...
Howdy Posted June 21, 2017 Share Posted June 21, 2017 G'Day Ahoysandy. Do a full throttle pull to where it cuts, hold it there for a second or two, foot on clutch and shut it down immediately (don't lock your steering ) and roll to a stop with the engine off in neutral. Pull the plugs out and have a look, that will tell you if your too lean or which cylinder has a vacuum leak. They should all be roughly the same colour. If they are black after this it is probably due to the misfire and this is likely due to a weak or bad ignition system. If you changed the plugs the gap might be wider than before. If you have high compression, low coil output, crappy leads, or very lean mixtures you may have previously closed the gap to compensate - check your old plugs to compare the gaps. If you changed the fuel filter, are you getting enough fuel flow through the new filter? After the full throttle pull and shut down you could check each float chamber - all should be at the right level and not empty or very low. You can check the resistance of the coil - high and low side and compare to specs. Also the lead resistance, including your coil to dizzy lead. While you have the multimeter, check your earth resistances. I imagine you have already checked the timing and stability at idle and revved up? Sounds a little too coincidental that you replaced the headers (I'm assuming better system than before so engine will run a little leaner) and the plugs then suddenly have this issue. So my first guess would be leaner mixtures has tipped the possibly weak ignition system over the edge. Wider gapped plugs would make it even worse. Usually ignition modules just crap themselves so either working or not working. You mentioned a fuel regulator so I'm assuming you are using an electric pump so it might be worth checking the pressure which could be either high or low (maybe higher due to the new filter's extra flow and the return not flowing enough and flooding the float chambers). Also assuming you are running a return to the tank? Oh yep and I'm also assuming the exhaust shop didn't take your car on a joy ride AKA Ferris Bueller Day off and over revved it. If it is a possibility, check the harmonic balancer and bolt. A loose balancer can cause misfires. Ahoysandy 1 Quote Link to comment Share on other sites More sharing options...
HKSZ Posted June 21, 2017 Share Posted June 21, 2017 Are all the grub screws under the carbs still there? Lose one and the aux venturi can rotate, tune goes crazy. Quote Link to comment Share on other sites More sharing options...
Ahoysandy Posted June 21, 2017 Author Share Posted June 21, 2017 Man, there certainly is a copious amount of other things to check over! hahahFor now I've checked the plugs against my previous ones and they measure up the same gap, an i also went looking for any visible air leaks or blow out from the exhaust gasket. The new manifold did NOT have the same spacing as the intake manifold and i found 2 nuts on the lower side that were finger loose. So my bets at the moment are one intake leak. Working on spacing the manifolds today and redoing the gasket. Leaner mixtures is more than likely true as this is something i had thought of before having new headers put on, but i assumed as its still a 2.5 system, it shouldn't have varied enough to notice, but i'm not as experienced in tuning yet.If I piece it all back together and the leak didn't solve this, I'll whip out the multimeter tomorrow and get testing!I really appreciate the suggestions guys, has helped a lot with such quick response! Definitely keeping this thread in my notes for next time its goes bad on me hahah Quote Link to comment Share on other sites More sharing options...
Ahoysandy Posted June 22, 2017 Author Share Posted June 22, 2017 (edited) I tightened down the manifold, put spacers to match the manifolds and re did the gasket but no luck with that. Still misfires, i also put the Heat shielding back in to prevent any further vapour lock but no results there. @howdy I did the full throttle pull to check plugs and cylinders 2,3 & 4 came out BLACK. unfortunately, when i went to grab the multimeter to start probing, i found the wire to be ripped off. so new multimeter has to be acquired before i continue. Put a larger sized fuel filter in to see if this could help, nothing on that. I do believe it to be a lean mixture, resulting from the new headers having better flow or different backpressure. I'm considering spending the $100 on new leads just for peace of mind in the future too (these were replaced when the engine was built though). Edited June 23, 2017 by gav240z Quote Link to comment Share on other sites More sharing options...
Howdy Posted June 22, 2017 Share Posted June 22, 2017 Ahoysandy, what is your comp ratio, what plugs are you running and what gap? 2, 3 & 4 black - what colour are the others? Did you check, with a timing light, what the stability looks like when you rev it up? Is it all over the place and does the balancer look true? Also not sure how your fuel lines are set up, when they removed the manifold if they disconnected the lines did they reconnect the return line to the inlet? I don't suppose you are running a restrictor in your return line like the 260z has standard? Wait and measure your leads before lashing out on new ones, if they look OK, ends are good, no sharp kinks or damage and are clean - they are probably OK. Did you check the float chambers after the power pull? Quote Link to comment Share on other sites More sharing options...
Ahoysandy Posted June 24, 2017 Author Share Posted June 24, 2017 Got it Sorted!! right before the weekend too which is good Huge shoutout to James & Brett at MIA Engines, i gave them a buzz and they had suggested the weber air filter socks to be stuck up against the trumpets, thereby restricting flow.BINGO! Took those off to see how it ran. Like a dream!I will be putting the suggested air filters on from them and having it the once over in the near future.Thanks for the help on here guys, definitely learn't a few more things about keeping her running! Linton 1 Quote Link to comment Share on other sites More sharing options...
Cozza Posted June 24, 2017 Share Posted June 24, 2017 That's why they're on the Big Bucks. Good news! Quote Link to comment Share on other sites More sharing options...
Riceburner Posted June 24, 2017 Share Posted June 24, 2017 The filter socks have coil springs in them to stop this. Were the springs removed? Quote Link to comment Share on other sites More sharing options...
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