Soup Posted September 13, 2011 Share Posted September 13, 2011 Hey hybridz folk, I recently picked up a S30 off a fellow member on here and have been putting on the miles while the weather holds up here in the PNW. It's my first carb'ed car and I'm not sure whether this is proper or not so I'll ask for your advice. The L28 is currently running triple 44mm. Mikuni carbs and a 3" straightpipe. I hit the search already - which is why I will refrain from using the term 'backfire' Upon deceleration, the exhaust would pop and bang all the way back down till about 1500 RPM. If I am in the higher RPMs, the bangs would be quite loud. I had my father trail me and after each bang there would be light grey smoke coming from the exhaust. I don't know what the culprit is - whether the carbs are running too rich or lean. The 3" straightpipe exhaust may be potentially amplifying the pops. I came from a R32 Skyline before so pops and afterfire from the exhaust were pretty normal... Attached are a couple pictures of the plugs and of the carbs. All 6 plugs. #1 = closest to firewall http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plugset.jpg Plug 1 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug1.jpg Plug 2 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug2.jpg Plug 3 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug3.jpg Plug 4 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug4.jpg Plug 5 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug5.jpg Plug 6 http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Plug6.jpg Mikunis http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=Mikunis.jpg http://s6.photobucket.com/albums/y248/SoberPhotography/310Z/?action=view¤t=DSC_5000.jpg Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 13, 2011 Share Posted September 13, 2011 Backfiring out the exhaust....is simply unburned hydrocarbons in the pipe from drop-throttle. Not enough O2 to support combustion. But it's in that hot pipe...and it mixes with O2 in the pipe reaching the upper or lower explosion limit, and BOOM! it lights off quickly. Actually if you work it right what you get instead is a BURN in the pipe, and not an explosion. Then you can actually see the blue flame front go back up the pipe. If you are back on the throttle before it gets to the lel limits, no bang. Take a paint can, pop a hole in the top and fill it with propane. Light the hole. It will burn externally for a second then go inside....get quiet, then BANG! Blow the lid clean off. That's what you're getting. By 1500 everything in the exhaust is burnt. Put A.I.R. on it, or a throttle dashpot or snubber to hold the plates off-idle for 3 seconds after total lift-throttle and it will greatly alleviate this issue as well. Drops emissions precipitously at the same time...but emissions controls add weight and couldn't have any other purpose than sapping power from a performance engine, so it's probably best you don't apply them in this instance. Quote Link to comment Share on other sites More sharing options...
josh817 Posted September 14, 2011 Share Posted September 14, 2011 (edited) Same happens to me, I wouldn't worry about it EXCEPT that it gobs up fuel economy. Maybe it's different on Mikunis but on my Webers when I let the engine brake the car, it wreaked of fuel fumes and exhaust and I got the loud pops like you have. Fuel economy took a dump and after that I stopped engine braking down hills. Probably because off throttle at high RPM's you are drawing a ton of vacuum and going rich from the idle mixture. Horrible coming straight from the header but with a pipe to the back going into a muffler not so bad. Don't worry about it. Post a video so I can listen to the pops, strictly for diagnosing purposes. Edited September 14, 2011 by josh817 Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 15, 2011 Share Posted September 15, 2011 "Probably because off throttle at high RPM's you are drawing a ton of vacuum and going rich from the idle mixture." Not 'probably' that is exactly what is happening! The high vacuum is put on one port and effectively siphons gas at a high rate from the idle circuit. This is one area that Megasquirt guys play with extensively and are rewarded with PHENOMENAL gains in fuel economy. Tailoring the 'decel fuel cut' parameters to keep the engine from injecting fuel when in effect it needn't even be running is a terrible waste of fuel. It's why every EFI system in OEM applications uses it, even pre-emissions era! After emissions were factored in, the decel fuel cut became the primary method for decreasing HC Emissions in Rolling-Road Emissions checks. Simple fuel cut and dashpot on the throttle does wonders for emissions reduction, increases fuel economy, and doesn't effect performance one whit! But "they're emissions components, they aren't needed on a performance vehicle"... Quote Link to comment Share on other sites More sharing options...
Soup Posted September 15, 2011 Author Share Posted September 15, 2011 Great news! That would explain my current 16 MPG with a 'conservative' foot. I will definitely post up a video for 'diagnosing' purposes once the weather clears up. Another question, not sure if related, but when I romp on the throttle, the engine sometimes bogs and sputters for a split second before it fires right back normally again. This only happens if I quickly romp on the accelerator. Usually more prevalent after sitting in traffic for a while but I've had instances where I couldn't heel-toe downshift quickly into 2nd since the engine was sputtering instead of revving up to match. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted September 15, 2011 Share Posted September 15, 2011 (edited) I need to upgrade my MS firmware...I can't do "real" decel fuel cut with my version. It only cuts fuel when it sees negative TPSdot instead of off-throttle decel. I cleaned up the popping, instead by assuring about 13.5:1 AFR during maximum vacuum. Leaner, it pops and richer, it pops. Edited September 15, 2011 by cygnusx1 Quote Link to comment Share on other sites More sharing options...
josh817 Posted September 15, 2011 Share Posted September 15, 2011 (edited) Great news! That would explain my current 16 MPG with a 'conservative' foot. I will definitely post up a video for 'diagnosing' purposes once the weather clears up. Another question, not sure if related, but when I romp on the throttle, the engine sometimes bogs and sputters for a split second before it fires right back normally again. This only happens if I quickly romp on the accelerator. Usually more prevalent after sitting in traffic for a while but I've had instances where I couldn't heel-toe downshift quickly into 2nd since the engine was sputtering instead of revving up to match. We have the exact same problems! I never bothered trying to figure mine out but here is a video of my Z... yours does exactly the same yah? Jump to 1:45 to skip the bullshit. At 2:30 I showed what you are describing. I had it pretty bad on my 521 pickup that runs a DGV Weber. That disappeared when I hooked up the vacuum advance however on the Z, it still occurs. http://www.youtube.com/watch?v=uM05EdPeG8U Edited September 15, 2011 by josh817 Quote Link to comment Share on other sites More sharing options...
Soup Posted September 15, 2011 Author Share Posted September 15, 2011 ^ Your video is private Quote Link to comment Share on other sites More sharing options...
josh817 Posted September 15, 2011 Share Posted September 15, 2011 (edited) Woops can you not see it? I'll change it. I do that so people don't sneak a peak at my retarded videos. EDIT: Alright now try it haha Forgot to mention that I have an exhaust leak on like... 4 of 6 cylinders... haha Contributes to the popping I'd imagine from oxygen seeping into the system. Open headers or having a short exhaust pipe would do it too though. http://www.facebook.com/v/10150111324138136 Tell me if that link works... Anyway you can hear it bogging there too. Once you get the RPM's up past a certain point you don't have it bogging, right? Same happens to me as you can see. Is there a link now? lol Edited September 16, 2011 by josh817 Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 15, 2011 Share Posted September 15, 2011 there is a link? Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted September 15, 2011 Share Posted September 15, 2011 (edited) Here is what I have learned, and I think it trips up many people trying to combat this. If you don't/can't completely cut the fuel off during decel, then don't simply adjust it lean during decel, as this seems to make it worse. When my carbs popped on decel after finding the best-lean-idle, I added a touch of fuel at idle and the popping all but disappeared. I applied this to my MS tuning and it had the same effect. My firmware currently does not have fuel cut on overrun. Edited September 15, 2011 by cygnusx1 Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted September 18, 2011 Share Posted September 18, 2011 I would also look into the carburetors to see if you can see fuel dripping out of the main jet while idling. This would indicate too high a float level or a stuck float. Quote Link to comment Share on other sites More sharing options...
Soup Posted September 20, 2011 Author Share Posted September 20, 2011 Still in the process of rigging up a proper camera mount... OCD issues Quote Link to comment Share on other sites More sharing options...
josh817 Posted September 20, 2011 Share Posted September 20, 2011 Still in the process of rigging up a proper camera mount... OCD issues Packaging tape... tape camera to inside windshield or on headrest of chair, you're set. :icon14: Quote Link to comment Share on other sites More sharing options...
rayaapp2 Posted September 20, 2011 Share Posted September 20, 2011 Emulate the AB valve on a stock Datsun. Dump enough air into the intake on decel so that it doesnt pull fuel from the carbs. Ultra high vacuum situation = decel Setup a large vacuum switch valve to dump filtered air directly into the intake manifold(bypass the carbs). This will only work if you have a central balance tube or individual ports on each runner. Personally I used a stock AB valve on my triple Mikuni manifold through a central plenum to each runner. The brake booster was run off the same plenum. You have to be able to completely lean out the mixture so that it does not draw in any more fuel. It will help a lot with your fuel economy if you spend a lot of time in gear on decel. If you do not decel in gear not so much. Tony's Dash Pot also works(and was implemented on the stock engine as well!) A combo of both would work best! GL! Quote Link to comment Share on other sites More sharing options...
Soup Posted September 22, 2011 Author Share Posted September 22, 2011 Video will have to wait. Car died tonight on the way home. Complete loss of power as it coasted to a halt. Managed to pull over and fuel pump primed only when the lights were on, I'm thinking either the battery is toast or the alternator bit the dust. I'll be throwing in another battery and testing the old one to see what's going on. Any ideas on how to test the alternator? I would hate to replace both willy nilly. Quote Link to comment Share on other sites More sharing options...
Leon Posted September 22, 2011 Share Posted September 22, 2011 All tests are described in the FSM (Factory Service Manual), which can be downloaded from Xenon's site. Quote Link to comment Share on other sites More sharing options...
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