87 FJ mystery issue update

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'87 FJ mystery issue update:

Replaced coils to no effect. Plugs show heavy black soot coating. Bike runs perfectly when cold. As the bike continues to run and warm up, the idle begins to die down. This seems to happen faster if the bike is actively being ridden versus being left to idle. Eventually, the bike will not idle, and runs poorly: poor running and stumbling at low-med rpm, and won't rev past ~5000. Requires significant throttle input. Ignition module is functioning properly: swapping with another unit had no effect. Unplugging the vac advance line on the ignition and inducing a vacuum leak increased the idle temporarily, but died back down after a minute or so.

I should also add that this is a sudden problem. The bike has run fantastically since I got it. This just showed up suddenly one day.

%d comments
  • You've said that the choke moves freely but have you checked that it's actually returning fully to the off position.

  • Yep. It has full free travel.

  • Even a millimeter makes a difference.

  • Well, is there a measurement? Should it butt up against a hard stop or something in the off position? It's the same as it has always been as far as I can tell.

  • Carb choke plungers should seat fully, just because the choke knob is returning to the closed position doesn't mean the plungers on the carbs are. Only way to check us by lifting the tank. My FJ displays those exact symptoms if I forget to close the choke fully after riding off.

  • I've done all my checking on the carbs themselves. I've had that tank on and off so many times now that I'm going to consider it my new workout routine!

  • Urf. This is majorly frustrating. Literally the only thing that makes sense is choke problems. Ignition could cause it too, but I've been through the major components and they're all good. Battery is good, coils are now brand new for $300, pickup coils are good.

  • Silly idea, but try detaching the choke cable on the carbs and choke it by hand.

  • Soot normally indicates that it's running rich which..speaks of a carb problem. If it runs fine when cold that says something about the choke setup. My 3XW ran fine and then one day, decided to piss fuel all over the road the minute I started it up. CV carbs are generally fine but they aren't perfect, I'd say strip them, check that all jets are clear, float heights are right and nothing is sticking. Make sure the choke is doing its job, if it runs good when cold that means that there's something wrong in the transition to being warm. Another option is to check the carb diaphrams for pin hole leaks. From the sounds of it it isn't electrical or it wouldn't run at all.

  • I'm thinking about running an external fuel tap and holding the choke open by hand. The cable has slack when the knob is in.

  • Have you ruled out a non venting gas cap?

  • Soot on the plugs is normal , wet fouled would say rich mixture.

  • Just went back and reread your first post,have you checked the vacuume

  • To the petcock? And that it pumps fuel .no vacuum , bad pump ,won't run long.

  • Rob, Gas cap open/closed makes no difference. The plugs don't have a little soot, they're completely coated with dark black soot, thick layer. Yeah, it has good vacuum to the petcock. Plus, wouldn't running out of gas to the carbs make the plugs ash white, not jet black?

  • New plugs, went back to standard type instead of iridiums. No change in behavior.

  • still wondering .... have u replaced/bypassed the fuel filter ..... if so check the fuel pump and fuel pump relay..... don't go for difficult start easy first fuel supply then ignition .... I had similar and it was fuel filter except the time I had problems with the fuel light then I could only runon reserve

  • It's a petcock screen only, no in-line on the 1TX. No fuel pump either. Gravity feed to the carbs.

  • I will need to have a beer on this or it could be the CDI

  • Let me know when you rebuild the carbs :).

  • Checked the box. Swapped it with another and it made no difference. Literally done everything short of rebuilding the carbs. The reason I'm hesitant to think that it's the carbs is that it's affecting every cyclinder the same. The likelihood that the same thing went wrong in each of the four carbs at the same time is extremely slim. Choke checks out. Nothing is out of place on the arm, and it moves freely through the full range of motion.

  • Low speed and idol not a good test for a plug read. Fj s have a week spark ,your going to get soot on them, fouled would to the point of not fireing you would see wet with fuel.If it were lean like tuning out of fuel,at low speed or idle this would not clean a plug with soot coating.You need to try tuning the eng on a temporary fuel tank of some sort and keep the vacuum line the fj tank to check the petcock pump to see its pumping fuel dont know if you tried that?

  • Been running it around the neighborhood up to 40mph, 2500-5000+ rpm short pulls at WOT as of two test runs ago. Going to check the petcock tomorrow. Not entirely sure how the weird electronic/vacuum hybrid works. I'm used to a little metal switch on a 1972 R5.

  • But you can add the pump and filter if you rejet the carbs to 3CV spec.

  • Have u considered the carb piston diaphragms

  • Still not a good test for plug read.Not sure about the petcock on a 87 but the vacuum part just acts to open or close the petcock, wires probably for reserve switch does yours have reserve bypass swltch, if so could be problem ?if I remember right reserve opens the petcock to access the last bit of fuel? Some one will know better than me on this?

  • Randy, hoping to avoid that if possible, but it may be necessary. Dave, would they all go out at the same time, or would one affect all four carbs?

  • No, I think you're right. I'm wondering if whatever vacuum actuating mechanism might be cracked or broken. I used a vacuum pump and had to constantly pump for it to maintain vacuum pressure.

  • Together the FJ group will figure this out ,let us know if the petcock is opening and clossing with vacuum ,id use engine vacuum to test this not a vacuum pump.the 84-85 have the a vacuum petcock and a prim manual operated switch ,prim just opened fuel flow all the time.

  • This thing just has the vacuum. I'd rather have a manual switch!

  • You can get parts to fix your petcock if it tests bad or there are after market manual petcocks available if you wanted to go that way.

  • If it turns out bad, I'll definitely convert it. I was looking into kits earlier.

  • Sam, if you have black spark plugs you have a fuel issue since you have determine it is not spark. Have you reached out to Randy @ RPM yet to see what he says?

  • Don't know if one from 84-85 has the same bolt pattern as 87 ,some years are different may be cheaper than after market.