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Thread: car stumble and backfire when accelerated!

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    Default car stumble and backfire when accelerated!

    Hey! I'm new here, just wondering if anyone can help? After pumping 89 unleaded fuel and adjust the timing, my car stumble and backfire when i accelerated. Before car was running on 91 premium fuel and it ran fine just a little lag sometime. Modded on car are: 44 mikuni, mild ported head, bigger cams shaft, header, 3 inch straight pipe, electric fuel pump and regulator set at 3.8 to 4 psi. One more thing some how the alternator wouldn't charge the battery so i crank the car multiple times and i could see fuel coming out on one of the mikuni. Any help well be appreciated.

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    Looks like your first post- welcome to Classic Z Car.
    You might want to fill out your signature on your profile that briefly explains what year and model Z you have which will help us help you.
    Have you tested your charging system is putting 13+ volts to your battery? Sounds like your battery is getting weak which will causes issues with performance.
    Maybe you have a float sticking on your Mikuni's? Sometimes a little tap on the carb in the right spot might loosen things up.
    Some history on the car? Like carbs been rebuilt recently?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    It's a 1971 240z, I'm new to the Datsun world, if you or anyone could tell me how to check the charging system? I could felt the fuel pump getting weak it could be the battery getting low but fuel pressure still show 3.8-4 psi. Next thing i'll want to do is drain out the fuel and refuel with 91 premium to see if that helps. oh.. and i just install new spark plugs but that didn't help.
    Last edited by Cali71z; 04-17-2012 at 10:54 PM.

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    Don't drain the gas- thats not it.
    Sounds like your new to wrenching on cars too. Put a voltmeter on the battery and measure volts while the car is running and not running.
    What did the spark plugs look like?
    A weak charging system might cause a weak spark but still run your pump.
    What do you know abou the engine- we need some history here to help diagnose your issue.
    Try to give specific information and as much as you can.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Quote Originally Posted by Cali71z View Post
    It's a 1971 240z, I'm new to the Datsun world, if you or anyone could tell me how to check the charging system? I could felt the fuel pump getting weak it could be the battery getting low but fuel pressure still show 3.8-4 psi. Next thing i'll want to do is drain out the fuel and refuel with 91 premium to see if that helps. oh.. and i just install new spark plugs but that didn't help.
    The factory service manual (FSM) will tell you exactly how to check your charging system (provided the car is stock). How do you get a FSM? Well, hardcopies pop up on eBay from time-to-time. You can also download them from XenonS30 (link in signature). The full 71 manual isn't available there, but the 72 FSM will work for testing procedures for your charging system. Look for the Engine Electrical section.
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    Try undoing your timing adjustment. Putt it back where it was. Also make sure your throttle linkage didn't get bumped which could cause the carbs to be out of synch.

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    Thanks guys! Let me go work on the charging system and timing, and i'll update!

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    Quote Originally Posted by Cali71z View Post
    Next thing i'll want to do is drain out the fuel and refuel with 91 premium to see if that helps. oh.. and i just install new spark plugs but that didn't help.
    There's not THAT much difference between 89 and 91. The problem is almost certainly something else (unless you bought bad gas, of course). FAIW, I've been running my '78 on 91 octane since I've gotten it, but I've downgraded to 89 lately, owing to price. The verdict? It likes 89 better -- more pep.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    He would have to retard the timing to run on 89 octane. A stock L28 runs a lower compression ratio than the early L24. Given the symptoms, it sounds like it may be leaning out when he steps on the gas. If he was running SUs I would say that his damper oil was too thin, but I don't know much of anything about Mikunis.

    I do know that my car pings like crazy on the interstate with anything below 92 octane gas, but will otherwise run OK with 91 octane. I have never tried 89...
    '71 240Z, Because any fool can drive fast in a straight line.

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    Seems like he screwed up the timing adjustment. There is no difference between 89 and 91, unless you get pinging with 89! Otherwise, there is zero difference in power. I switched to 89 from 91 on my L24 a while ago, just as an experiment. I've heard no pinging and felt no loss in performance. The only difference is that 89 will be more prone to "dieseling" after a hard run on a hot day.
    2/74 260Z

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    Ok! I adjust my timing back to stock and I use a voltmeter to test the battery after it was charge from the auto part store it show 12.5 volt and when I hook up the battery and start the car, it show 11.2 volt. I thing the volt regulator is bad? But can't located the volt regulator to test it. Previous owner had multiple wiring scatter around the car! (The battery came for my Honda so its smaller! My original battery is still charging.)

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    The voltage regulator should be on the passenger side fender in the engine bay near the alternator. That said, the PO might have changed to an internally regulated alternator and removed the voltage regulator.
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    Ok! I check my alternator and its a 12v 60a alternator! I believe it's a 280z or 280zx alternator? The previous owner Bridges the connector where it suppose to plug into the voltage regulator! He jump two wire for the light and one wire on the connector back down. I don't know if that's right? I don't see any wire coming back from the alternator to the battery! The only wire that's connect to the battery is the big red wire from battery positive terminal to the starter motor. Any one have the charging system diagram to convert external to internal voltage regulator for 1971 240z?

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    It sounds like a PO did a swap for a later model alternator. Here is a link to the alternator swap for a 240Z: http://www.zhome.com/ZCMnL/tech/280Alt.html.
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    Thanks! I'll try that converting!

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    OK! I have fix the issue! It was the alternator! The previous owner wire it wrong and it did't charge my battery so the car was only running on battery with nothing charging it. With that issue! My MSD ignition set didn't have enough juice to run properly. Thanks to all ya help! Now if anyone can tell me where's the idle screw for the Mikunis? so i can do my timing or can i just have someone rev it to 3000rpm?

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    Quote Originally Posted by Cali71z View Post
    Now if anyone can tell me where's the idle screw for the Mikunis? so i can do my timing or can i just have someone rev it to 3000rpm?
    Have somebody hold your RPM's at 3000. If you are running triples you really need to get a wideband to know where you are. Lots of recent posts on this topic.
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
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    Thanks for the info! but i would like to adjust my idle, it's setting at 1200-1300 rpm. Any one help!
    Last edited by Cali71z; 04-22-2012 at 02:35 PM.

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    OK! i adjust my timing at 3000 rpm with 35 degree and at idle it's showing 20-22 degree is that right? I think i did it right? Anyone help?

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    If the idle speed is still to high the base timing will seem too high because it is still advanced some. Did you get the idle speed lowered properly?

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    Thanks! I forgot to check the idle speed! yup i believe it set around 1200-1300! I still couldn't figure out how to adjust the idle! Any help well be appreciate! But i'll do more read and hope i figure it out some day!

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    Odds are your carbs each have an idle speed screw that adjusts each carb independently of each other. You will need to resync the carbs carefully. Which from the sounds of it you may not know how to do. You need to find an article on setting mikuni carbs up for the first time and do that. It will require patience and a flowmeter.
    1971 240Z HLS30-38691
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    Thanks! I have been reading and have fix my issue! Car is running strong now!

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    Quote Originally Posted by Cali71z View Post
    Thanks! I have been reading and have fix my issue! Car is running strong now!
    How did you fix it, it would be nice to know.
    2/74 260Z

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    I fixed, reconnect my 60 amp alt. and clean the flow bow and found that one of the seals on the flow was bad so change that and the car run fine! With no problem just need a tune to run better!

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