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Old May 30th, 2013, 10:47 PM   #1
Washt
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Default Crappy mechanic needs some terrible advice

Ha! thanks for checking this out......I'm hoping you folks can give me some insight. I have an old Datsun Jeep with the original engine and running gear. It has a 3.9 straight six. I've been slowly working through this truck for 4 years or so. I don't have a lot of time but have been plugging away on it. The engine and truck sat for a long time without any attention, the last time it was current with tabs was 1979. Anyway, I've made good progress slowly getting everything straightened out from electrical to engine. I had the head redone, new distro, clean carb, new clutch, tranny overhaul, electronic ignition too, etc, etc. over the weekend I finally got to take it out for a short spin on real roads....quiet, gravel roads far from anyone, anything. Anyway, it starts and runs well. But getting it out on the roadie a different story. It backfires like crazy...mostly out of the exhaust. Its crazy because it will hardly even climb hills it spits so much out the tailpipe. I'm sure most of you will think that it is the timing but it's currently set dead on. It has a vacuum advance that works fine. I even fidgeted around with the timing to see if ANYTHING would change the backfire..... No distro adjustment gets rid or it. This engine has timing gears and I haven't checked them yet. I changed carbs with the same result. I even changed the distro reinstalling the original Hitachi....no change. My dad is elderly but ran a couple service stations and he tried to sort it out. We removed plug wires in order starting at 1 working through to 6. And here is where things got a little weird. If you remove 1 it wants to die and sputters. But if you remove 2-5 no change while idling in the driveway....none. But if you remove 6 it's the same as 1. We actually pulled off 2-5 and sprayed a little starting fluid down the carb and it started and idled fine with only 1 & 6 plugged in.??????? I'm not too concerned about that really maybe 1 & 6 offset each other enough to allow for that scenario. Anyway, my compression is good 110-120 on all six cylinders. New, hotter plugs. I do have spark to all 6 plugs...... I checked the old' fashioned way!!!!

I never pulled the oil pan off of it.... Maybe I should have. The engine really wants to run right but doesnt. My dad was really impressed with how nicely it idles. I can't imagine the timing gears getting out of sync but maybe. I know nothing about the history of the truck but like all 4WD trucks they usually get beat to death so maybe something is amiss. Anyway, if you have any ideas please respond I'd love to hear them. You know in this whole process I have been very patient....but now I'm running out of answers and really have no idea do what to try next....anyway, thanks.
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Old May 30th, 2013, 11:53 PM   #2
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Why would you need to hit it with starting fluid to run on 1 and 6 if nothing changes when you pull 2,3,4, and 5? Seems like it would still run on the carb.

I know nothing about a datsun six, but....
If you have compression, gas, and the spark is on time I would look at the camshaft for flat lobes. If the valves are not opening much it may idle okay but die under a load. Usually backfiring through the carb.
And you are correct, I suspect the timimg is off.
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Old May 31st, 2013, 12:36 AM   #3
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2 things stand out to me, had the head done and electronic ignition. Its possible like previous post said that something isn't correct when you put the head on and the valves aren't opening fully or something in the new ignition isn't correct.

Or it might not be getting enough fuel for a number of reasons.
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Old May 31st, 2013, 01:12 AM   #4
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I would assume that #1 and #6 are very close to 180* apart in the firing order and are the only cylinders doing any work. You stated there is spark at all 6 plugs, but is there fuel at all 6 (are 2-5 wet after attempting to start the engine)?

If you have fuel at all cylinders, my next item to investigate would be the valve adjustment on cylinders 2-5. Do you know what type of lifters the engine uses? If it has solid lifters then move right past valve adjustment. Since you have compression at all cylinders the valves have to be closing. If you have hydraulic lifters it would be possible for the valves to close while cranking the engine to perform a compression check since the lifters might not pump up with the low oil pressure. Once the engine produces more oil pressure while running the lifters become firmer and can actually hold the valves open some. For hydraulic lifter setups the valve train needs to be adjusted with the engine running. Before starting it again, back the adjustment off on the rockers for 2-5 and see what happens.

Went through this with a Ford 302 with aftermarket roller rockers, it ran very much like you describe until I added shims to the rocker pedestal.

Pics of engine or a model number might help to diagnose.
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Old May 31st, 2013, 10:31 AM   #5
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Welcome,

btw a Jeep is a jeep, a datsun is a datsun

there is no Datsun Jeep, carry on :)
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Old May 31st, 2013, 10:52 AM   #6
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Timing order in wrong direction? Pull cap and check direction of rotor.
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Old May 31st, 2013, 10:59 AM   #7
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Quote:
Originally Posted by Sandals View Post
Welcome,

btw a Jeep is a jeep, a datsun is a datsun

there is no Datsun Jeep, carry on :)
thank you!
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Old May 31st, 2013, 12:27 PM   #8
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Quote:
Originally Posted by Sandals View Post
Welcome,

btw a Jeep is a jeep, a datsun is a datsun

there is no Datsun Jeep, carry on :)

Correcto... assuming an old Datsun Patrol? Plenty of other places in the world call this body style a jeep (lower case J), but people are a little sensitive around here...



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Old May 31st, 2013, 04:23 PM   #9
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Thank you for the replies. You've given me something's to think about. I'll let you know what happens. Thx again!!!
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Old May 31st, 2013, 05:21 PM   #10
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My guess is the firing order is off. If you run the wires going the opposite of the rotor 1 and 6 would be the only two firing at the right time.

With the info this is my best guess at the correct firing order.
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