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More carb probs


Guest DatsuNoob

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Guest DatsuNoob

Was driving on the freeway for the 1st time ever today on a trip from Puyallup to Sammamish. Got about to the Ikea exit in Kent, when I felt a huge explosion come from under my hood. Sounded like a damn cannon went off. I quickly pulled off to the shoulder after looking in my rearview and seeing a cloud of black smoke. Popped the hood and saw that my electric choke had completely blown off the side of my carb. Not quite sure what may have caused this. Any similar experiences or ideas? Had to get towed home.

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hey, same exact thing happened with my old carb...scared the heck out of me...like a bomb going off...had to get towed less than 2 miles home. Then Hainz, the Datsun hero put in a Weber, just like my PL 410 has and no more problems...runs great! I will be looking for some better wheels one of these days, mine are horrid.

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Guest DatsuNoob

I did a fresh oil change and even added some Lucas, because I planned on doing quite a bit of freeway driving that day. I had my PCV port on the intake plugged off and the tube leading from the block was left open. I figured it would still allow crankcase blow-by vapors to escape that way, since alot of old cars used this same method. Not quite sure what may have caused my choke to blow off though. Improper valve timing perhaps? The only way my truck seems to run smoothly, is to advance my timing quite a bit. I think I got a dead hole. I'd like to find a better head to put on, because I'd hate to have to sell this truck. I like it, but at this point I dont have the money to keep pumping into it. I need reliable transportation to and from work, not a project vehicle.

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I'm sitting here staring at a Weber wondering how the heck you could blow the choke off. Nothing on the electic choke assembly comes in contact with fuel. How much came off? Just the plastic part, or the whole mounting bracket- or the whole side of the carb? I could see how the carb side blowing out could happen, but not just the choke. It would take one heck of a backfire or a flooded float chamber to do that though. And that probably would have set the engine compartment on fire.

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When all the fuel in the intake explodes back through the carb, it's VERY impressive. I've lost temporary hearing and mustache, eyebrow and nostril hair from this, and avoid looking down a carb when someone is cranking it over.

 

I suppose the choke plate could be slammed violently closed from this, it's certainly powerful enough. The choke shaft would be forced against any resistance from the choke unloader linkage and the heater assy. Maybe if it was loose it could fly off.

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On a Hitachi, that'd be true- the choke shaft goes directly to the heater coil. On a Weber it's a bunch of small linkages, and if the choke plates slammed shut and blew the plate rod out it would just take the air horn with it (and the filter assembly). On the 34 DGEV (which is virtually identical to a 32/36 except the bores) the choke heater is bolted wth 3 bolts to the side of the carb. Only thing I can think of is the whole carb exploded, either from a too-rich condition coupled with incorrect valve/ignition timing or the carb was flooding from too much inlet line pressure. I had that lovely ball of flame when I started work on my '76 4X4 which had no return line and an electric pump when I found it. Fuel would POUR out. The return line is so important with webers- without them you have to run a regulator and typically that's not a good thing with most pumps.

 

If it has a dead hole maybe an exhaust rocker fell off. That'd cause a MAJOR backfire through the intake (since the exhaust valve would never open) and would blow parts off.

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Guest DatsuNoob

There was no fire in the engine compartment, and funny enough, I was thinking how important it would be for me to invest in a fire extinguisher to place behind the seat as a precautionary measure for long trips, just before the choke exploded. I think it was from a combination of advanced timing on a crappy head, slightly rich air/fuel mixture, and possibly the screws on the choke may have loosened up from vibration. I try never to overtighten aluminum components. Anyone got a decent head for sale by the way?:D

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Guest DatsuNoob

I really dont want to sell the truck, it's alot of fun, and if I can find a 5 spd tranny for freeway driving, I wont have a reason to sell it. The header wont really be an issue either since all I have to do is unbolt the header flange and push it away toward the driver's side fender. It isnt welded up solid to the rest of the exhaust system. I'll do a compression check today if I can get that carb back together, to verify my prediction of needing a new head, but I'm pretty sure I do.

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You don't need the carb on it to do a compression check. The chances of it being the head are very low, though it could have a bad valve. I still think it could have dropped a rocker. I've had it happen twice. A compression check could indicate that- then just pull the valve cover.

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,,, I had my PCV port on the intake plugged off and the tube leading from the block was left open. I figured it would still allow crankcase blow-by vapors to escape that way, since alot of old cars used this same method. Not quite sure what may have caused my choke to blow off though....

 

Does the tube to the crank case go anywhere near the carb choke? Could this have been a 'mimi' crank case explosion directed up the tube pointed toward the carb? (it's a bit of a stretch I know)

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Guest DatsuNoob

No, the crankcase tube points kinda toward the firewall, like all 620's. I didnt mess with it's location, and the choke points toward the valve cover, kinda ass backwards from the 32/36. But like I said, the only way it seems to run well, is idling pretty high, and with the timing advanced. Otherwise throttle response is shitty, and I have to gas it pedal to the floor and it barely goes anywhere when timing is at the factory setting at 10* b.t.d.c. Running advanced, sometimes it'll diesel and blow a cloud of black smoke out of the carb when I turn the key off. I just got off the phone with my folks, and Mom's offered to sell me her 95 Corolla so I can have reliable transportation. Then I wont have to sell the truck, and I can do a KA swap like I've been wanting to do, and still be able to drive around. Then I wont have to nickel and dime my L motor trying to "polish my turd"(Icehouse :D). Just tired of scratching my head, trying to get her to run well when all it seems to do for me is give me trouble.

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When all the fuel in the intake explodes back through the carb, it's VERY impressive. I've lost temporary hearing and mustache, eyebrow and nostril hair from this, and avoid looking down a carb when someone is cranking it over.

 

LMAO ... cuz ive been there :P

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Yeah, the stupid (like me) do it once, the truly stupid do it a second time.(seen that) Usually happens after poring gas down it and trying to start it. Shotgun blast of sound immediately followed by a big orange rolling ball of flame that travels up the inside of the open hood and out. Your buddies mouths are moving and they are showing a lot of teeth, your kind of glad you can't hear the laughter.

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Guest DatsuNoob

Had to grind my adapter plate a little bit, and trim the thin part of the gasket between the primary and secondary butterfly valves. Now it idles smooth, and runs like a top. The primary was sticking a little, not fully clearing the plate, and getting a little too much gas. I also bought slightly longer elec choke bolts and fitted them with lock washers. Should be a good start. Got me home just fine with no problems. Still plan on finding a 5 speed trans before any more freeway travel.

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The first 720 was made in 1980 and was available with a factory 4X4. It was the last year that the venerable L20B was used. I have a 5 spd from an '80 4X4.

 

For one Cyber beer

 

If I gave Noob this transmission could he use it in his truck? Assuming he had the right drive shaft. (Doug, I know you know the answer)

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Sure it'll work... the divorced T-case 4X4 tranny was the same as the 2WD, except there wasn't a speedo output on most of them. The early ones had a block-off plate. It was the late, married transmissions in the 83-on 720 that were shorter. A LOT shorter.

 

As long as nOOb doesn't have a '73 620 the '80 tranny is a bolt-in, no mods deal.

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