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Thread: Idles fine, drives a bit then bogges down hard.

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    Default Idles fine, drives a bit then bogges down hard.

    260z
    So i've been reading around and have been able to take care of a few issues that came up since i bought the car (about 5 days ago)

    On the ride back from where i picked it up it ran fine for about 80 or so miles. Then it just started to bogg down and even back fired a bit.

    I'd cruse at 60 or so and if i tried to accelerate it would stumble and chug like i was running out of gas.
    So i limped off the side of the road and thus started my fuel issues.

    To sum up the events that followed...
    The car stopped getting fuel and i determined that the electric fuel pump was to blame.
    I first tried to bypass the electric pump because i read that i could run on just the mech to get home but i still wasn't really getting anywhere as it drove for a few yards then bogged and backfired and limped.

    So i got a tow the rest of the way and replaced the electric pump with an "off the shelf" aftermarket one from advance auto the next day. Car gets fuel now.
    I also replaced the fuel filter before the mechanical pump. The electric pump came with a new inline filter too.
    Idles fine and revs fine without a load on the engine.

    I was able to go for a short ride, short because as soon as i turned out of my development and started on my way the car wouldn't get up past 40 or so. So again i limped off to the side and turned around. This time to get moving i needed to rev pretty high and mess with the clutch a little to get moving. When i stopped in front of my house it was still idling pretty well. There seems to be air bubbles coming from the filter before the mechanical pump.

    So where i'm at right now is i think i need to drop the tank and clean that out. I came to this conclusion because when the electric pump was bypassed i've read that it should still run ok off the mechanical pump. I'm not sure how accurate that is though.

    I may look into seeing if the mechanical pump is failing first but i doubt it. I'll also need to see about why there are air bubbles showing up in the fuel filter before the mech pump.
    I also think i have the '73 SU carbs.

    If anyone has any suggestions, that'd be great. I'm mainly posting this to sort of catalog my symptoms and hopefully a solution for anyone that may need it.

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    Welcome to Zville, We're going to need more info about your ignition & fuel system to pin this down. The first thing to do to eliminate the obvious is a good tune-up including valve adj. & carb tune. The mechanical f. pump will supply enough fuel for SUs you should install a f. filter in front of the f. tank. 2 types of SU carbs. factory 74s had flat top carbs. (the suction chambers shoulders are 90 degree rather than rounded.) Most flat top SUs have been swapped for the earlier round tops. A description of the spark plug color & type. Does it have good spark to the plugs? Bubbles in the filter, tighten all clamps & fittings. I don't think that will affect the float level in the carbs unless it's excessive.
    My gut instinct tells me this is a fuel starvation issue. Check ALL f. filters & f. lines. I don't know if flat tops have a filter at the carb f. inlet but they do have a fuel level window. Keep us posted of your progress.

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    Sounds like fuel starvation.

    Are you running the original 260 carbs (often referred to as "Flat Tops" because of the shape)? Because if so, as Mark mentioned, they include a sight glass in the sides of the carb bodies that you can use to verify that the carbs are getting fuel.

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    No they're not flat tops. I'll attach a pic of them. From what i've read they're the '72 SU carbs but i don't know for sure.
    Click image for larger version. 

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    I'm fairly certain it's fuel starvation too but the fuel system is so simple i can't imagine it's anything other then the fuel tank clogging up.
    Although since you've mentioned it i'll have to check out cleaning up the carbs. I'm new to carb stuff, more or less. Had a klx 250 and cleaned that carb out so shouldn't be too intimidating. Just need to source the right replacement parts/gaskets/whatevers.

    I'll check the plugs and start with seeing what comes out of the tank and work from there.

    For the record, there is a filter before the electric pump (between the tank and pump) and one before the mechanical pump. Both new, but the one before the electric pump isn't see through so i can't really tell if it's clogging. Gonna try and replace that too. I like the see through ones.
    Last edited by SheWantsTheZ; 07-26-2014 at 03:45 PM. Reason: i meant '72 su carbs

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    A great source for Su carbs, parts & knowledge is ZTherapy. Their DVD "Just SUs". will answer every question that you have. SUs are easy carbs to work on & understand.
    The inlet fuel fitting on the float bowl has a filter in it. When you pull the float chamber tops to verify the fuel level, the old gaskets will "Pringle" and be difficult or impossible to re-use, Be careful after pulling the suction chamber and piston. The jet needle is easily bent. Store it carefully.
    I set my pistons on the cowl with the jet needles down in the louvres.

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    Just a small update, had alot to do so i only just now got to get a wrench on the car.

    So i dropped the tank, took a while mostly because all the hose clamps had the head of the screw/bolt facing in the most inconvenient position possible.

    Came out fine and to my surprise it's pretty clean! The top part of the main tank is corroded though so i think i'll still go ahead and clean/seal it. Side note, i'm worried about the baffle and this little tube inside the tank getting clogged but apparently if i blow in it a little while the bulk of the sealant is draining out i should be ok.

    HOWEVER!
    While i was draining and dropping the tank a thought occurred to me. The choke seemed a little wonky so i took a better look in the engine bay and noticed that the choke on the rear carb actuates smoothly and returns on it's own while the front carb is sluggish and doesn't even move to the same degree as the rear carb. so looks like i'll learn how to adjust the choke once the tank goes back in...

    I like how almost all the bolts had no problem coming out, though. On my 2001 impreza I've snapped off more bolt heads then i care to mention.
    Last edited by SheWantsTheZ; 08-07-2014 at 03:35 PM.

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    Just an update. I dropped the fuel tank and cleaned it out with muriatic acid, coated it with some tank sealer and put that all back in.
    Also dissembled and cleaned the carbs while i was waiting for the sealer to dry.
    There was actually very little rust in the tank, i was surprised! But some stuff did gunk up the filters so i replaced the two inline filters i have in place and cleaned out everything in the fuel lines from the tank to the carbs.

    Car starts and runs fine now but i'm working on learning how to adjust them.

    Moral of the story, clean your carbs!

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