'77 L20 advanced a tooth? Confirmation needed, please.

1972-1979 Datsun 620 forums. All 620-specific topics and discussion can be found here.
Chzyrider
Posts: 1
Joined: Fri Apr 06, 2012 10:22 pm
Location: Salem, Or.

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Hi all, new here to this site and new to Datsuns, but not new to wrenchin' on my own cars. I could use a little assistance with some Datsun expertise and confirmation on what I've been able to research before further digging into this truck. I'm working on a friend's recently purchased 1977 Datsun 620 pickup with L20 engine that had been stripped, cannibalized and monkey-rigged by the previous owner(s) so I have no idea what has or has not been done to it other than screwing it up.

This is either a challenging repair project or a nightmare, I'm not sure which yet.

Here's the story... please bear with me, there's pictures along the way. ;)
A couple months ago I had went with my friend to look at buying this truck and there were two knuckleheads that reminded me of the tv characters 'Darrel and his other brother Darrel' messing with the carb and fuel pump trying to get it going. I was surprised they didn't blow themselves up while I was there. Well, a week or so later my friend tells me that he bought the truck. It was apparently barely running enough at the time to drive it home. He tried a little more oblivious fiddling with the carb (attempting to reduce the high idle speed and the clouds of exhaust smoke), then it wasn't running at all when he calls me shortly afterwards to tow it over to my place to help him figure it out.

First, I see the choke and anti-dieseling solenoid power wires were disconnected without any dangling power source seen to reconnect them. And there were a few open vacuum leaks from the missing air cleaner housing and most all other emissions parts. We removed the remaining EGR bypass body and made a block-off plate for the intake manifold reusing the old gasket (as I found that this gasket is only available by dealership special order or in full rebuilt sets). I started to rebuild the carb and discover missing springs and a lot of grey/white powdery stuff inside it....hmm.

So my friend found a '76 replacement carb, but it's a slightly different model of the Hitachi. The old carb only had a flat plate with a spring behind it where the BCDD would be and the replacement carb has a BCDD that does not have a vacuum or solenoid connection. So anyway, I rebuild that carb with new gaskets and set all the bench adjustments according the books. I'm still not positive which exact version of carb this truck is suppose to have with Hitachi's lack of hard-stamped ID numbers.

Old carb
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Replacement carb
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We put the replacement carb on and try to start it but it would act flooded with the occasional 1-2 second start-up then dying again. I inspected the spark plugs and they were blackened, and wet from fuel. We cleaned them and tried again but same thing and I checked for spark at cyl. No.1 and saw a weak spark, but it also has a weak battery. So we wait for another day as it charges again and we both have time to work on it.
Meanwhile, I had cleaned the rusted connections at the coil and discover the frayed fusible link off the pos. battery cable had been poorly bypassed so I replaced that. Then I manually inspected the timing and I discover the point gap was about twice what it calls for. After rotating the engine to TDC with a chain-wrench, I see the rotor is pointing ahead of No.1 position relative to the cap.

Close enough to be called TDC for this purpose, +/- a degree or so.
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This image shows the rotor position at TDC ahead of the No.1 location (yellow dot). Through a couple computer programs, it measures to be approximately 16 degrees ahead of the yellow dot in the image.
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In my online research for carb info, I recalled seeing images of the oil pump shaft positioning with the tab offset frontward at 11:25 position.
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So I lifted the distributor out and took a photo to see if it was pointing where it should be relative to these other images. This is the shaft position on this truck at TDC. I did not remove the lower housing with the two bolts seen here.
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With a little photoshop blending and marking the lines where the tab is, I now had an overlayed comparison image. I used another program to calculate the angle of difference and discover that the shaft tab (RED lines) is appaearing to be approximately 14 degrees ahead of the clean example (GREEN lines) that is supposed show where the shaft is to be aligned.

Blended image showing difference in shaft position at TDC. Yellow is centerline between bolt holes.
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The relative comparison of these images makes me suspect that whomever previously worked on this truck had put the shaft in 1 tooth advanced therefore why it ran like crap when it did. Can anyone confirm this timing issue before I pull the oil pump and move it over a tooth for a 'try-it-n-see' thing?

I also found a small mention in my research that the Datsun timing sets allow for 4 degree adjustments... Can someone explain the 3 positions mentioned? Being unfamiliar with Datsuns, a wild thought is perhaps the previous owner was trying to make up for a worn chain/gear set and overshot the correction?

Also, other than one hand drawn diagram found online for California models, I've had a hard time determining which vacuum lead is supposed to go to the vac advance diaphram on the the distributor. Since this thing was so messed up and cannot be sure of being correct to being with, I have no real reference for what hose goes where to determine correct vacuum routing. I'm suspecting it's the front one of the two coming off the passenger side base.

Old carb before removal. Note the open vac on EGR passage and monkey-rigged throttle return spring.
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Any assistance, hint, tips, or other helpful information would be greatly appreciated to get this truck running again and out of my garage so my friend can drive to/from work again and I can get back to other projects that I've had to put on hold for what I thought would only be a simple carb repair. We'll be working on it again in a few days as schedules allow and hoping for some replies here before we do.

Thanks!


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PEZi
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Car: Mitsubishi Lancer Evolution IX Mitsubishi Racing Edition
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Not a complete confirmation, but it looks like you're spot on... good luck, man.


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