Guest D_Weisner Posted December 2, 2006 Share Posted December 2, 2006 Hey, ive got a '72 240Z with (presumably) an L24. It was my dads SCCA Pro Rally car and im looking to rebuild it and race it myself. When he last drove it he said it was backfiring through the carburetors (three Weber DCOE44's). I'm thinking it could be something off with the timing or possibly a bad valve spring(s). The carburetors need rebuilt anyways but if you guys know anythin I wouldn't know of, could you help me out? Im somewhat new to the whole engine rebuilding scene. I've got some technical experience (i swapped engines for my CRX) so i can take some jargon. I'll see if I can get some pics of it (the Z) on her sometime. Thanks a bunch. Quote Link to comment Share on other sites More sharing options...
Austin240Z Posted December 3, 2006 Share Posted December 3, 2006 Gonna be a problem with an intake valve... Could be a flat valve, or a valve that's stuck or damaged... But my guess is that it's most likely gonna be something like incorrect valve clearance keeping an intake vavle from closing, or valve float due to weak springs. Regardless of what it is, you're gonna need to take the valve cover off and inspect everything, My guess is that it may just be incorrect clearance, and in that case all you need to do adjust the clerances. If it's something like a stuck vavle, or a flat vavle, you'll probably have to take the head off and try to figure out which vavle it is. Then depending on what's wrong, you may need to talk to a machine shop and/or your local part's store. Good luck to you! Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 3, 2006 Share Posted December 3, 2006 Alright, thanks a lot. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted December 3, 2006 Share Posted December 3, 2006 Im my experience the majority of all backfiring through the carb issues are ignition timing related. A little bit of dirt in the dist cap can cause crossfiring between cylinders. I suggest you start simple. Basic tune up, new cap, rotor, points, plugs, wires etc. Rebuild the carbs if you know that is needed. If you still have the backfire problem, then do a compression/leakdown test. Should tell you the status of the rings and valves. Quote Link to comment Share on other sites More sharing options...
Austin240Z Posted December 5, 2006 Share Posted December 5, 2006 Im my experience the majority of all backfiring through the carb issues are ignition timing related. A little bit of dirt in the dist cap can cause crossfiring between cylinders. I suggest you start simple. Basic tune up, new cap, rotor, points, plugs, wires etc. Rebuild the carbs if you know that is needed. If you still have the backfire problem, then do a compression/leakdown test. Should tell you the status of the rings and valves. That's also true. I'm actually helping a guy with a civic out tonight who's also backfiring through the intake, ironically enough... In his case, when we first got the car started a week ago, before the backfiring problem, I could hear the valves tapping, which makes me think his case is probably valve float. If your old Z isnt having other problems, starting out with a tune up is a great idea, but i would still suggest pulling the vavle cover and checking out the condition of everything on the head anyway. Especially since it's an older motor. Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 8, 2006 Share Posted December 8, 2006 All right, took the valvecover off and found that, indeed, one of the springs was broken. I was really surprised to see how clean it was underneath. I half expected it to be fairly dirty under there. Its the inner spring on the Number 1 bore. I'm thinkin it'll be fun to remove it all. Not sure what else need work to get it running, but in theory this should be a fairly easy fix right? Quote Link to comment Share on other sites More sharing options...
olie05 Posted December 8, 2006 Share Posted December 8, 2006 for the average person it shouldn't take more than 2 hours to get the head off. Quote Link to comment Share on other sites More sharing options...
z bomb Posted December 10, 2006 Share Posted December 10, 2006 if the car has been sitting a while get the points replaced if any, and check the weights in the distributor. they slide in a groove and that was rusty on mine. just some white lithium grease should do. and just as a sidenote- watch it for hondas backfiring through the carb... damn rotor freespins on the dizzy in those sometimes... Quote Link to comment Share on other sites More sharing options...
Austin240Z Posted December 10, 2006 Share Posted December 10, 2006 All right, took the valvecover off and found that, indeed, one of the springs was broken. I was really surprised to see how clean it was underneath. I half expected it to be fairly dirty under there. Its the inner spring on the Number 1 bore. I'm thinkin it'll be fun to remove it all. Not sure what else need work to get it running, but in theory this should be a fairly easy fix right?It's not too hard, I've only been working on cars for about two years, and rebuilt my first head a few months ago. Putting my new motor in next week actually. If you're gonna replace a spring and have the head off, I'd suggest taking all the valves out, replacing all the springs(since one is already broken), the valve seals, and go ahead and lap the valves. Just DONT MIX THEM UP. If you take them out, PUT THEM BACK IN THE SAME PLACE. When you put everything back together, adjust the valve clearances. Get a feeler guage, the clearances are: .08 for the intake .10 for the exhaust I cant think of anything else to suggest at the moment, though i know theres more to mention... Hopefully other people will chime in with some good suggestions to help you out as well. If you have a digital camera or anything I'd love to see how your engine's looking, and once again, good luck to you! Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 10, 2006 Share Posted December 10, 2006 Im going to take some pics tomorrow after I get the Samurai's fuel pump on (I need something to drive). Looked at the cam and im pretty sure that it and possibly the springs are NISMO parts. Not sure about the spraybar tho. Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 11, 2006 Share Posted December 11, 2006 Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 11, 2006 Share Posted December 11, 2006 All the pics i have taken so far. Some in better detail than others. You can kinda see the inner spring pokin out of the outer spring (4th row, last pic). And yes the other car under the cover is a Z, its the second of three we have. Along with it's other Datsun/Nissan buddies in our backyard, including a 510, Fairlady 2000, and an original Silvia (CSP311). Quote Link to comment Share on other sites More sharing options...
bubbleguinea Posted December 11, 2006 Share Posted December 11, 2006 OMG! what kind of lash retainers are those?? what kind of metal are those??? they are NICE! Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 11, 2006 Share Posted December 11, 2006 I think they're a NISMO part but not positive. Quote Link to comment Share on other sites More sharing options...
OlderThanMe Posted December 11, 2006 Share Posted December 11, 2006 nice!!! NASCAR fuel cell...that thing must have cost a bundle... looks like the car has been curbed from the underside of the passenger floorboard. My car has similar damage but is only bashed up about 1-1.5" That looks like a really bumpy cam too!!! Sweet setup you have there!!! Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 11, 2006 Share Posted December 11, 2006 Thanks, yeah the cell is also FIA cert. I wonder if NISMO still has parts available. Anyone know about that. I remember my dad went to a dealership a while back and ordered a windshield and lo and behold they actually got him one. Quote Link to comment Share on other sites More sharing options...
Austin240Z Posted December 12, 2006 Share Posted December 12, 2006 not bad man Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted December 12, 2006 Share Posted December 12, 2006 If you only have one spring gone bad, then wouldn't the back firing be limited to that one bore of the one carb? Quote Link to comment Share on other sites More sharing options...
Guest D_Weisner Posted December 12, 2006 Share Posted December 12, 2006 Yeah, im sure my dad forgot that little detail when he initially told me about the car. Quote Link to comment Share on other sites More sharing options...
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