Canadianz Posted December 28, 2006 Share Posted December 28, 2006 hey guys, i need some advice on what might be the problem with my 71 240. at the moment it has an l26 with roundtop su carbs, which seem to have been cursed. I got the car in august and it ran fine up until about october, when we had a problem with the choke sticking on the rear carb. we fixed that, and the car ran fine for a week and a half, until it gradually seemed to be losing power, but was still driveable. it got to the point that one day it just had no power whatsoever and had a nasty backfire out the carbs. it was acting as if the front carb was doing all the work, and the rear was just sucking air. so, we began checking out the problem, but we really havent made much progress... we did a compression test on the engine, and it showed 80psi across the board, which seems low.. but it was consistant so i dont think theres a problem with the actual block. the carbs were our first suspect since theyre 35 years old, so they got sent to Whiteheads in Toronto, where they were rebuilt because the needles and seats were worn right out. so, we got them back and on the car, but theres still no change in how the car runs. the carbs should be fine, they were flowtested and everything, but they havent made a difference. so does anyone have any ideas what might be the problem? the next thing we'll check out is the vacuum advance on the dizzy, if that is stuck/inoperable could that cause the problem? Any help with this is greatly appreciated, this is my first time actually working with a twin carb setup, so i feel like im just taking shots in the dark -Rob Quote Link to comment Share on other sites More sharing options...
280zwitha383 Posted December 28, 2006 Share Posted December 28, 2006 It's not your vacuum advance. Have you hooked up a fuel pressure gauge? "Backfiring" out of the carbs can be a sign of running lean and your other symptoms seem fitting. I'm not sure, but is the front carb first to get fuel? I would guess fuel pump. Quote Link to comment Share on other sites More sharing options...
280zwitha383 Posted December 28, 2006 Share Posted December 28, 2006 Also, 80psi is VERY low, IIRC 130 is about the minimum recommended. However, this is probably not your problem. Quote Link to comment Share on other sites More sharing options...
Canadianz Posted December 28, 2006 Author Share Posted December 28, 2006 At the moment the car is in at the shop, we found a mechanic thats experienced with tuning the old su carb'd jaguars, so ill ask him about the fuel pressure tomorrow. It still has the old mechanical type pump, i did take it off and clean it out thinking maybe one of the diaphragms had torn, but it seemed in good shape, and still worked well, so i put it back on and discounted it, but it never hurts to double-check. thanks for the input! from what ive read 80 psi is pretty low, but its strange because as far as we know its a stock engine, and the pressure is consistant for each cylinder, so it seems a little odd, because if the engine was in bad shape youd expect variations in the readings.. i also should have added that it can be tuned to idle alright, but it will backfire if you try to speed the engine up, and its worse if you do it quickly. thanks for the input 383, its appreciated! -Rob Quote Link to comment Share on other sites More sharing options...
280zwitha383 Posted December 28, 2006 Share Posted December 28, 2006 It could also be the fuel filter but you would have had to run it pretty low on gas to clog it up that bad. Let us know what the mechanic says. What you're describing is not a matter of tuning. It is a matter of failure. DUN DUN DUN!! Quote Link to comment Share on other sites More sharing options...
Canadianz Posted December 28, 2006 Author Share Posted December 28, 2006 Ok, so i just got off the phone with the mechanic, he said fuel pressure is good and the bowls are filling up fine.. before we took it to the shop we replaced the vacuum lines and put a brand new fuel filter in, so thats good for sure. he said they had made some progress with it, it will idle smoothly and respond to throttle better, but its still not 100% soo at least theyre getting somewhere with it.. and if it is a failure, lets hope its a small one Quote Link to comment Share on other sites More sharing options...
sonomaz Posted December 29, 2006 Share Posted December 29, 2006 You need to get back to basics. Hard to believe but the SU's are pretty good carbs when set up right, and you should not suspect them unless you have a smoking gun. Unfortunatley back fires out the carbs are not indicative of carb problems. It indicates electrical or mechanical problem. Before anything check your fuel filter. If you can not breath easily through it, replace it. This is a problem area bacause it does not cause issues except at wide open or half open throttle conditions. The low cylinder pressure concerns me even if it is consistant. Did you check the compression with the throttle wide open? If you did, I would expect pressures in the range of 150-175 for a healthy engine. If you did them with the throttle closed they should still be up around 100-125. Before you do anything more check your valve lash. For a hot engine I think the valve lash is 0.010 for intake and 0.012 for the exhaust. Check your manule. Afterwords, check your compresion again. For electrical just change everything! I mean everything, because it is just easiest. It kills me the time people spend to track down a bad spark plug wire when the whole set costs $25 at Kragen. Cap, rotor, points, condenser, wires, plugs, change it all. Then check the vacuum and mechanical advance on your distributor. Slip the rotor on the distributor shaft and try to twist it. If it moves freely (there is spring resistance to overcome) it is probably working fine. Suck on the vauum advance lint to the distributor if the points plate moves it also is probably working fine. Start it up and set the timing between 5 and 10 degrees advanced. If you have done all this and still have problems, you still cannot blame your carbs until you do a few things more. Disconnect every vacuum line from your intake manifold and plug the ports. Caution, do not mistake the bowl vent lines for vacuum lines. These must stay open. If it runs great you have a vacuum leak. Check all the lines you just disconnected. If you are still haveing problems, NOW is the time to dig into your carbs. Two reasons 1) you have eliminated everything else, 2) you have done everything you need to before you tune your carbs. Ready? Shut it down and remove the air cleaner. Set up your tuning tach/dwell meter (your dash tach does not have the low RPM resolution for this job). Reach under each carb to the jet nut. Bend the stop tang out of the way or remove it completly. Screw the adjusting nut (it is not six sided, it has a real strange shape to match the stop tang that limits adjustability, an old smog thing) all the way up until it stops. Count carefully and back it out two turns. Repeat this on the other carb. Now, find the idle stop screw and back it off until it does not touch the throttle. Advance it forward until it just touches plus two more turns. Repeat on the other carb. Start the engine and run until hot. Place the flow meter over the front carb (you do have a flow meter, don't you? No? Get one from Edelbrock) and set the dial so the meter float is at the middle line. Place the meter over the back carb and adjust the idle stop to put the meter float at the middle line. Check both carbs to make sure they are even. Check your tach. Turn both idle screws the same amount until the engine is idling at 800 RPM. Your doning great! Now turn both neddle adjusting screws a quarter turn at a time OUT. Do this until the engine slows slightly (should be 1 turn max). Now go IN past the smooth idle part just until the engine slows again and return just until you ar at the smooth, fastest idle. Now check your idle speed and reset to 800 RPM again. Hang on, you are almost finished. At the top of the intake manifold neer the throttle linkage should be a screw the can push on a flat tab on the linkage. If it is missing do not be surprised the car is 30+ years old. Turn that screw until the engine is running at 2000 RPM. Set your flow meter on the front carb again and put the float on the middle line. On the connecting throttle shaft there are two screws. The one near the front sets the smog idle speed, forget about it. The back screw sets the high speed balance between the two carbs adjust it until both carbs are flowing evenly. Congratulations YOU, have just tuned a 240Z. You should either have a smooth running car or have found a serious problem along the way. Let us know what happens. Paul Quote Link to comment Share on other sites More sharing options...
MrFancypants Posted December 29, 2006 Share Posted December 29, 2006 May not be your problem, but I just spent a lot of time trying to get my Z running again and the main symptom was backfiring through the carbs. Turns out I jacked up my ignition by wiring the cylinders incorrectly Replace all the plugs, wires, dist cap and rotor ($50 total from AutoZone). When you wire in the plugs, 9 o'clock position on the distributor goes to cylinder #1 (closest to radiator). Then counterclockwise around the distributor, wire the remaining cylinders in with the proper firing order (1-5-3-6-2-4). My FSM described the firing order and ccw rotation, but mentioned nothing about starting at 9 o'clock. Thanks to whoever posted that in the archives -- worth a sticky!! Good luck! - Greg - Quote Link to comment Share on other sites More sharing options...
Canadianz Posted December 29, 2006 Author Share Posted December 29, 2006 Awesome, thank you guys! this should really help a lot, i really wasnt sure where to start with this problem, but this helps out alot! thankyou guys, ill post back if i learn anything more about this problem -Rob Quote Link to comment Share on other sites More sharing options...
datsunlover Posted December 31, 2006 Share Posted December 31, 2006 I will vouch for the SU carbs too, I love them. Simple and easy to work with, and they work well, despite being 35 year old technology! About the problem, I had something similar happen with my 75 280 (running 72 SU carbs) and it turned out to be a vacume leak in the intake. Turns out in my haste to get it going again, I just tied a line in a knot to plug a fitting. (instead of doing it right and putting in a pipe plug.. ) and it blew off as I pulled into a driveway. Of course, I didn't think anything of it as it was idlling fine. As soon as I tried to take off though, BANG pop burble... WTF?!?!? Easy fix on mine (when I found it) but I'm wondering if you have a leak AT the head/intake manifold? If it still runs, spray short bursts of wd40 at the intake (where it meets the head) working along slowly and listen to the idle. If it idles up/down in reaction to where you spray the wd40, there's an air leak. The 80psi though, that is not good.. Quote Link to comment Share on other sites More sharing options...
z bomb Posted February 12, 2007 Share Posted February 12, 2007 Backfiring out of the carb is a sign of a lean mixure, ignition misfire, and Ignition timing, I think retarded timing does that, but not 100%. If you still have points, get rid of them. Between the dwell and not having your mixture/air right, these problems will keep presenting themselves. To tune your carbs- Get the tool to measure air, set that first. Then get a mini-propane tank, with an adjustable momentary switch (push is on, let go is off). Put hose on switch, stick in air inlet of carb. The RPMs should rise by about 100 when you hit that switch. Anything past 100rpm the carb is too lean, if it bogs down it's rich. Match both carbs to get the same RPM out of both. Another thing is the piston in the carb. If the seal around that piston is bad, it will not rise or hold any throttle (vacuum) well. Quote Link to comment Share on other sites More sharing options...
Canadianz Posted February 12, 2007 Author Share Posted February 12, 2007 We're thinking along the same lines, z bomb! Right now im waiting for a 280zx dizzy to arrive, I believe the problem lies in electrical now.. First of all, the caps do not fit very tightly on the dizzy, and they wiggle a bunch, (both the old one and the new one I bought) and this affects how the motor runs, although holding it on tightly doesn't make a large difference. And number two, I had planned on swapping the dizzy out anyways, and if its causing trouble now, why not eliminate it and start from the basics.. I also plan to check the resistance of the wires to check for corrosion/cracks and replace the grounds in the engine bay.. Now all i need to do is get the ambition to go out there and work on it! winter here is good for sitting inside where its nice and warm and brainstorming, but not so much for wrenching! Quote Link to comment Share on other sites More sharing options...
z bomb Posted February 13, 2007 Share Posted February 13, 2007 Hard to get access to but... Know anybody with an ignition oscilloscope? Somebody I know of but never met can readthe patterns and tell you if it's valve guide seals, just by the ignition pattern. Quote Link to comment Share on other sites More sharing options...
badjuju Posted February 22, 2007 Share Posted February 22, 2007 update on this, Canadianz? Quote Link to comment Share on other sites More sharing options...
Canadianz Posted February 22, 2007 Author Share Posted February 22, 2007 Things are going slowly right now, just waiting for a 280zx dizzy to arrive. Its hard to pin down.. but i'm pretty sure its an electrical failure somewhere. I've borrowed a dwell tester, so im going to check the points with that, i have heard that the points style distributors have problems with the shafts not spinning perfectly straight, causing ignition timing between cylinders to get silly. so i guess ill see where that goes. I'll keep you guys posted! Quote Link to comment Share on other sites More sharing options...
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