View Full Version : Cylinder 1 2 misfiring
06-02-2008, 09:19 PM
I have a `75 FJ40 with a stock engine except for a Mallory distributor and a Webber carb. I have replaced the points, cap, rotor, plugs and checked the wires and coil as per the repair manual. The compression is great and I verified the timing is correct.
I failed an emission test due to high hydrocarbons and I think this is due to misfire. I can feel the truck misfire and when I pull off plug #1 and #2 the engine slows down a little but not much compared to when I pull off plug #3, 4, 5, or 6. I verified that all the plugs are getting good spark. I thought maybe there was a vacuum leak forward of the carburetor (that feeds #1 and #2) but I can' find one. Anyone have any ideas as to why my engine might be misfiring? I also suspect that all cylinders are misfiring and just more noticeable on #1 and #2.
06-02-2008, 09:30 PM
You might try IH8MUD.com too; you'll find a much broader group with ideas and suggestions.
Off the top of my head - did you notice the "misfire" before taking it to emissions? Do you remember when it started, or more important, what may have changed that would have caused it to happen? My buddy replaced his spark plugs and then started running rough and throwing codes. Turned out he smashed the electrode on the spark plug as he was inserting it so it wasn't sparking on 1 cylinder. Did you replace the points correctly, verify the gap is correct? Is it getting sufficient strength spark? Correct plugs for the engine? Just some things to think about...
06-02-2008, 09:45 PM
Do you have your HC numbers? How far off from the limit are you? To get my FJ40 to pass, I ended up taking it to a shop that had the emissions equipment and having them make the adjustments to get me through. In my case, they had to bump the idle up to 1200 rpm, which was pretty high as the limit was 1300. Even with the high idle, I barely squeaked by.
06-02-2008, 09:54 PM
Thanks for the quick replies,
Yep, checked the point gap and using the specified NGK spark plugs. I have verified the gap is correct (multiple times so no smashed electrodes) and the spark looks good and strong.
My HC reading was 1800 and the state of Oregon only allows 300 :-(
06-03-2008, 07:58 AM
Is this the first time you've tried to get it emission tested? Maybe the Weber is throwing it off (dumping too much fuel into the intake)? I don't know squat about Webers...
When I had a Weber on my first Cruiser, a 1975 FJ40, I installed a fuel pressure regulator to reduce the amount of fuel going to the carb. It looked like this:
06-07-2008, 12:06 PM
Troubleshooting 101 - Process of elimination and logic.
(1) ASSUMING that 1 & 2 are misfiring, and 4-6 are working correctly we can probably eliminate carb malfunction. If the carb were not working properly, then all the cylinders would be having issues.
(2) You say that you are getting good spark at 1 & 2, so we can probably eliminate wires, distributor cap/rotor, and plugs. What was the color and appearance of those two plugs, and how do they compare to the other plugs in the good cylinders? That can be a major clue. The appearance of plugs can tell you a LOT about what's going on in there.
(3) If 4-6 are firing well, then we can probably also eliminate timing, and distributor shaft wobble. Also, if there were a crack in the distributor cap, or even carbon tracking, it would not affect adjacent cylinders, due to firing order.
So we have to ask ourselves the question: What makes 1 & 2 different from 4-6? Maybe the valves in those cylinders aren't adjusted properly? BUT, it seems awfully coincidental that those 2 adjacent cylinders would be the only ones with improperly adjusted valves.
Since we know that an F/2F intake manifold has adjacent mounting at 1/2, 3/4, and at 5/6, it might be worth investigating to see if there is a vacuum leak at the intake manifold at 1/2. If the nuts were loose, or if there were a flaw in the gasket or mating surface there, it would only affect 1 & 2, and would cause misfiring in those two cylinders. It's easy enough (compared to some other jobs) to simply check the torque on the nuts that hold those manifold ears to the head. (Proper torque on the M12 2F manifold nuts is 33 foot pounds). There is also the time-honored tradition of spraying carb cleaner around suspected vacuum leak areas while the engine is running and listen for a change in idle. And of course, if those two plugs were white or very light compared to the other four, that would be a dead giveaway.
06-26-2014, 08:42 PM
Good call Rzeppa. I've been scratching my head trying to figure out why the two cylinders 5 and 6 have been misfiring on my '71 FJ40. The power brakes get their vacuum off the section of intake manifold the supplies 5 & 6, and apparently the brake booster has a leak. I pulled the hose off, plugged the hole, and the thing started running smooth immediately.
Thanks all, for the good thread!
06-26-2014, 11:08 PM
Wow - 6 year time travel! Glad it helped someone. I had a leaky booster in my '76 many years ago that I ignored and it cost me a valve job when #5 exhaust got too hot from the lean condition and disintegrated.
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