backfire (1 Viewer)

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My FJ40 has been backfiring for over a year now, it is stuck at my parents house. Here's a summary of what has happened so far:

Truck ran fine, but then had problems starting. I replaced the fuel filter (filter was full of debris) and all was well for another few weeks. I only drove it short distances but it ran fine. Shortly thereafter it began backfiring when under load. I drove it to Yakima, 150 miles from where I live, over the mountains. It stumbled but it made it. I tried to check the timing but the light would not come on, tested it on 2 other cars and it worked fine. In that last 12 months I've done this:

Removed and rebuilt the weber carb - no change
Installed new fuel pump and replaced all the rubber fuel lines, cleaned out the hard line from pump to carb - no change
Replaced weber with trollhole carb - no change
Removed stock distributor/crane fireball ignition that was all screwed up, just floating around loosely inside the distributor with new DUI HEI unit - no change
New plugs
New fuel filter
Lashed valves, followed instructions I found here somewhere but added .002 since I can't get the engine hot - no change

Today I tried to remove the thermostat housing, because the temp sending unit shows I'm overheating in only a few seconds, but the bolts are frozen. The timing light worked sporadically with the old distributor in and I set it as close to the bb as I could at idle. When I installed the DUI I was hoping the light would work, but no difference there and I don't understand why my timing light won't work on my FJ40 when it works on 2 other cars I've tried it on. I also noticed it will start and run decently when cold for about 30 seconds to 1 minute, then it starts to get rough, backfire and die. Could the thermostat cause this, can I overheat in only 30 seconds? I also noticed it was making a high pitched squeek while idling, not constant but every once in a while, I thought this might be the water pump but when I removed the fan and belt the pump nose didn't feel like it had too much play in it. So my question is, what haven't I thought of? What do I do next? From going over all this again, I'm wondering if I should hook up a small gas can in front of my pump to rule out fuel delivery, let me know what you think, I'm running out of ideas.
 

kurtnkegger

Running Topless and Dirty
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Your timing light might not work because you have a broken plug wire. You didn't say if you changed your wires, including the high tension wire. When you changed your plugs, what color were they? Are you back firing through the exhaust pipe or the carb?
 
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Not sure about all your different symptoms, but for what its worth, I just dumped my crane elec. ignition. For about 2years I've had weird things goin on on my stock '75, stumble off idle, hard starting when hot, sometimes hard starting when cold, sometimes it would just die and start right back up, but difficult. finally it just got so unreliable, I was arfraid to drive it. I took it to and old geezer who restored old vehicles. He checked everything on it and asked me if I had the old set of points, because he suspected the elec. ignition. He put the old points and condensor back on and it's never run better. I went ahead and bought a new set of points, cap, rotor, and condensor so I'll install all that at the next tuneup. Good luck:cheers:
 
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2 sets of plug wires, got the first from ccot for the stock style cap, then had to buy a set of hei wires locally for the DUI. I bought a set of accel HEI v8 wires, too long but I can't see them causing any other problems. The old plugs looked good to me when I pulled them, they had some soot but nothing serious, no built up oil, no damaged electrodes. New plugs have a bunch of soot, but it's only run cold and mostly with the choke on so I think that's why. It backfires through the carb, sometimes spraying fuel everywhere, sometimes you can see a flash through the carb when it backfires. It will run with the choke off, and sounds fine, but only for a short time, as soon as it warms up at all it starts backfiring. My best guess right now is the thermostat is stuck shut and the head/plugs are getting hot and igniting fuel/air as it enters the cylinder before the valve can close. I'm just not sure that should happen in less than a minute of running and I had no luck getting the thermostat housing off to check if the thermostat is working.

Also, my timing light is a cheap craftsman dial back, could that be a problem with HEI. I was just reading that multi spark ignitions can cause problems with timing lights, which would explain why the old ignition didn't work right with the light, but isn't HEI a single spark at all RPM?
 
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kurtnkegger

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If it will run only with the choke pulled, I would suspect a vacuum leak. I think a backfire from the carb to also be a lean condition tell. I would start by looking for vacuum leaks first before trying to rebuild any more components. I'm sure someone with a lot more knowledge will chime in, just throwing in my two cents.
 
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HEI is like any other ignition when it comes to the spark end of it. If the timing light works sporadically, then your ignition is also sporadic. Fix that first and see if the problem goes away.
 
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What could cause the intermittent firing, bad ground? Is ground the spark plug to the head? The distributor isn't grounded is it? I'll check all my grounds next time I go back to work on it, could something else cause the issue, bad ignition solenoid? Right now I have +12V going to the HEI from the ignition solenoid, and plug wires to my plugs. Thanks for the help, I think you must be right about the timing light, I must still have a problem with spark.
 
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Hang in there Helmut..you'll get it figured out. I have been fighting with mine as well. Mine had spark to all the plugs which allowed it to idle but the spark was super weak. If you tried to go off idle it was done. Turned out to be the coil

Here's a link from the Chevelle site I'm in that covers a lot of info on HEI's including how to check the coil etc. It will be GM specific but there might be something of use for you. Hope this helps

Technical Reference #5

BTW Where are you located?? Sounds like you live on the west side of the mtns. I'm in Maple Valley
 
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The HEI solves the coil, distributor, points problems. If backfiring out the carb, you are too lean, probably a vacuum leak. If backfiring out the tailpipe then you are getting unburned fuel in the exhaust, too rich. Have you checked compression? Maybe a leaking valve.
 
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Haven't checked compression, just read some instructions that said I have to get the engine warm before checking, not sure I can do that. Unburned fuel in the exhaust and a timing light not working would make sense, but I haven't noticed it backfire out of the exhaust, only out the carb. How do I check for a vacuum leak, I think the only vacuum line from my intake goes to the brake master cylinder, I capped off the accessory port on the carb, if it's leaking how do you tell where the leak is coming from? If the valves were leaking would it run ok when cold?
 
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spray starting fluid around the base of the carb and around the edges of the intake manifold. If the idle picks up there's your leak. For vacuum lines I like to take vise-grips and pinch the vacuum line closed somewhere close to the carb, again idle will pick up. Hope this helps
 
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Intermittent firing could be anything in the circuit, so ignition switch, wire to distributor, signal pickup in distributor, ignitor in distributor, wires, ground. The good news is that the pickup and ignitor for HEI are cheap and you need to carry a spare set anyway.
 

spotcruiser

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Hey Helmut,

How've you been? I'm kind of busy doing an engine install right now, but I could spare a couple of hours to help you out some weekend. I'm over in Bremerton. Hit me with a PM if you want some local help.

Todd.
 
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Thanks for the offer Todd, unfortunately the cruiser is stranded in Yakima which is about 2.5 hours away, probably 3 for you. If things get desperate I might have to offer free beer to get additional help, but I'm not quite there yet.
 

spotcruiser

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I don't have time at this moment, but I'll try to throw you a list of things to look at later on (tonight, tomorrow, or this weekend).

A few of the guys in the group have trailers and might be willing to haul it over to this side for you. I have a tow bar if you want to borrow that to bring the truck over. That is, if you even have room for it over here and want to bring it over.

Todd.

Thanks for the offer Todd, unfortunately the cruiser is stranded in Yakima which is about 2.5 hours away, probably 3 for you. If things get desperate I might have to offer free beer to get additional help, but I'm not quite there yet.
 

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