Jonas240z Posted November 3, 2004 Share Posted November 3, 2004 Hello there, here's the problem: I recently did an engine swap on my 240z. I put a rebuilt L28 with some mild cam, flat top pistons, header and 71' SU's that where recently rebuilt. The problem stated one day when i steppted on the gas (to see how the new engine felt) and once i reached about 3500 RPM it started misfiring, then i reached 4000RPM and it didn't want to go any further. So i checked the air filter (K&N) and it was like new, fuel filter on engine bay (clear) didn't show any debris or anything else, electric fuel pump works well, carbs are tuned, coil is almost new (PS60), spark plug wires are fairly new ( 1 yr. old or less [ TAYLOR 8mm]), spark plugs are bosch +4's (used on the L24 for a few months without a problem), Dizzy is the 240z one but with electronic ignition conversion (Pertronix or similar). What i did notice was that sparkplugs 5 & 6 were covered with carbon deposits, No. 4 was slightly covered with carbon and the other three were perfect. So i tuned my carbs, check the float levels, and still have the problem!! Every time i step on the gas i try to go past 3500-4000RPM it starts hesitating and it doesn't go past 4000 RPM under load. It does however go past 4000...if the car is parked or cruising in neutral! I'm now clue less...and don't know what's wrong. Please help me! any help will be appreciated! Thanks! Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 3, 2004 Share Posted November 3, 2004 Sounds to me like it could be an ignition timing problem, or ONE carb has something amiss... Are you running stock timing settings and a vac. advance? Could be the vacum line is shot, or the diaphram has ripped.. Could try advancing the dizzy.. Im running mine REALLY advanced (lowest idle I can get is 900 or so) not using the vac advance, but it pulls hard up to 5500rpm. When I got the car running this past summer, I had the timing set close to factory spec and had simmilar issues. Almost seams like a badly adjusted/sticking carb though, if those three cyls are getting black. When it idles/revs in nutral does it run well? IE; does it strugle/pop/spit black soot/ect? You say the carbs are tuned.. are they balanced properly? (it'll run unbalenced, but not well at higher rpm.. trust me ) Also check to make sure the choke isn't sticking on.. and are you sure there is the same amount of damping oil in them? Quote Link to comment Share on other sites More sharing options...
Jonas240z Posted November 3, 2004 Author Share Posted November 3, 2004 I'm running the same dizzy that was on the L24 but i retarted it a little bit...cuz it was advanced all the way when it was on the L24 (ran without a problem), so i'm going to advance it all the way to see if it makes a difference. The carbs are balanced, same amount of oil on dampers, same mixture setting. Datsunlover you say you experienced something similar, so i'll try advancing it tomorrow! Anyone else experienced something similar??? Thanks for the help! Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 3, 2004 Share Posted November 3, 2004 Oh.. something I just thought of.. you say the dizzy WAS advanced all the way?!? Could it be that when this engine was re-built, the timing marks were not lined up properly by the builder? Like, a tooth out maybe.. I've seen it happen on other engines, and they run, but need to have the dizzy fully advanced, and ussually you cant get any revs out of them. Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted November 3, 2004 Share Posted November 3, 2004 I blame the distributor, but only three cylinders sounds like a trick... Try swapping the carbs front to back and see if your problem moves. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 3, 2004 Share Posted November 3, 2004 I once chased a problem like this for a few weeks only to finally figure out that I had SEVERAL bad spark plugs. How often does that happen? This Z had been sitting for several years and I just bought it from the original owner. Same basic thing, wouldn't go past about 4000 rpm. I tried distributors, coils, spark plug wires, you name it, and it was the spark plugs... They looked BRAND NEW, so I didn't even think to try them... Definitely sounds like an ignition thing to me. In reading the original post again I see you've got some Bosch+4 plugs. I would only use Bosch plugs on a European car. IME Bosch plugs work great on Volvos and Porsches. My experience with Bosch on jap cars has been less than stellar. When I worked at a Volvo dealer they referred to NGK's as NoGoodKind. They couldn't believe that I would use crappy NGKs on my autox car. I know this sounds like superstitious nonsense but there is something to it. Stick with jap plugs for jap cars, Euro plugs for Euro cars. Try some basic NGKs. It's cheap and might just be the problem. Quote Link to comment Share on other sites More sharing options...
Jonas240z Posted November 3, 2004 Author Share Posted November 3, 2004 The Bosch +4's that i'm using on my L28 were also on the L24 ( so they're used) and i had no problem getting to 7000rpm redline on this old tired L24 engine. Before this i had NGK's "V power", that were gapped to .44, and they actually worked worse than the Bosch! So i was just going to adjust the gap back down...but i accidentally dropped a plug on the ground and it broke the insulator (UPS!!)...that's why i'm using the Bosch, which did make a little difference. Other than that i'm still clue less...but thanks for all the help! Quote Link to comment Share on other sites More sharing options...
Hugh Posted November 4, 2004 Share Posted November 4, 2004 Funny, the tach on my 240z with L28 won't read past 4000rpm, however it revs well past it. When I get to 4000, the tach needle studders and falls down to the 2000-3000 range. I have to read it in both directions to figure out where I'm revving to Quote Link to comment Share on other sites More sharing options...
Jonas240z Posted November 5, 2004 Author Share Posted November 5, 2004 I still can't figure out what is wrong with the car. I checked and tuned the carbs again and nothing, i advanced the timing and nothing!! I just don't know what else could be wrong...maybe something internally....what if the cam is advanced a little bit, or the valve lash is wrong, could that be the problem?? The thing is that when i go for a test drive... and step on the gas WOT...once it reaches 4000rpm to 4500rpm it first gear!! it does not want to go any further, as if it ran out of gas or power!! It feels like it's missing or there's something blocking or restricting the car from going any faster. I really don't wanna take it to a specialist cuz i'm broke at the moment....but i just don't know what else to do ...I need some help really bad! Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 5, 2004 Share Posted November 5, 2004 Umm.. are you sure the throttle cable is opening the carbs up all the way at WOT? If not, it may be the reason it's hitting a 'wall' at 4500.. not enough air flow.. Quote Link to comment Share on other sites More sharing options...
Guest tony78_280z Posted November 29, 2004 Share Posted November 29, 2004 I'm betting on carb to win. An engine will rev easier if there is no load particularly when dealing with carbs and vaccum. I think you are leaning out at higher RPMS. As you add performance parts to your car (including high performance exhaust ) you need to adjust the jetting of your carb. Adding a cam and flat tops would count. I know nothing about tuning SU carbs. I don't know how hard it is to change the jets. You say it is in tune, but it doesn't sound like it's tuned to the RPM range that you want. Why have two carbs that must be kept in tune? Why not just get a bigger 4 barrel carb? I know there are manfiolds for 4 barrels out there (I've seen them for sale on the net). I thought about going carbed, headered and rebuilt L28 in my 280z too. The more I thought about it the more I wanted though. I think I went too far. Another thing... possibly... maybe... perhaps... what electric fuel pump are you using. IS IT possible that your electric fuel pump designed for a 2.4L motor can't deliver enough fuel for the dual carbed hidghly modified 2.8L motor? Can you increase your fuel pressure just a tad? I've run into stranger problems. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 29, 2004 Share Posted November 29, 2004 I'm running SU's on my 75 280 with no prob.. got a generic electric pump that puts out around 5 psi, and that's plenty for SU's. Hmm.. actually, now that I think of it.. My car behaved VERY similar to this when I first started driving it this year; Carbs were leaning out because the return line I used was too big (I think.. its all I can say it was cause I put 240z fuel lines on and it's fine.) and basicaly, the car would run off the float bowls. As they went dry, the engine stumbled and stoped at about 4K rpm just like you describe.. Can you drive around below say.. 3000 rpm with no problems? (I could) but a good pull through 1'st and into 2nd will cause it to stumble? Maybe your floats are sticking, or your return line is too large.. You can try what I did maybe.. (at first, just to test) I just pinched off the return line (that I 'built' from scraps of old 280z line) with a small pair of vice grips, and voila! Car ran awsome, and pulled right to redline. I drove around like this for a few days and then I got a proper feed/return loop from a 240z. Quote Link to comment Share on other sites More sharing options...
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