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Help with engine suddenly not running properly


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This is my "daily driver," and currently my ONLY transportation, so being down is particularly crappy.


1968 520, J13, garage kept.  It's been running fine for a very long time (I don't recall any issues in at least the last year or more).  Last time I drove it was Tuesday.  Today (Thursday), I started it up and backed it out, then drove down the fairly steep hill of my long driveway, so no accelerator needed. When I got to the street, I started off in 1st and immediately noticed the truck felt very "heavy" under the accelerator.  I stabbed the clutch and revved the engine, which _seemed_ to rev fine.  I got out, popped the hood, and checked the fuel filter and bowl.  Both were full (I had replaced the fuel filter about a month ago during routine maintenance... so figured I'd start there), and the fuel level through the glass face on the carb bowls was fine.  It's 100° outside, so I decided just to turn around and go home rather than get stranded somewhere.  Getting back up my driveway was a significant challenge.  I had to fiddle the throttle and slide the clutch the whole way up.  In other words, it simply wasn't making power.


I'm old and tired, and don't have much stamina, so after a rest, I went out and pulled off the top of the air cleaner, and reached through the driver window and started it with key only.  It started flawlessly.  So, my first though was that the issue was just under load (so, possibly a vacuum leak that suddenly sprung.  Nope.  The vac line to the distributor is metal, and fitted at both ends with factory threaded/compression fittings.  While running, I sprayed carb cleaner at both ends, as well as at the base of the carb, and didn't get a reaction from the engine from the extra fuel.  So, I feel like vac leak isn't it.  


While running in the garage, I played with the throttle... it definitely will die if I hold the throttle steady, high, low, or otherwise.  But it will idle.  I checked down the throats and only see ONE spray from the accelerator pump on the inboard venturi.  Should both sides be spraying?  Could it be that something (suddenly) clogged one side and it won't hold throttle because it's running out of fuel?  Any ideas?


I'm without additional transportation, and have very limited stamina, so I want to try to avoid "try this, try that" ideas that aren't well reasoned.  


Thanks in advance for any assistance you can provide,



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Could be something stuck in the carb. A cheap and easy thing to try is to pull the air cleaner, start it. Put your hand over the top of the carb to pull vacuum. It dies or come close to it. This might pull whatever is stuck and clear the jet.


Worth a shot.. cheap and easy. 


it does sound like a blocked jet or something though.

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Worth a shot.. cheap and easy. 



Cheap and easy are always good with me.  I just went out and gave it a try.  Started truck with key only.  Idles fine.  Used a junk towel to help block off the carb throat as I revved it hard.  I did that 4-5 times, letting it return to idle, then testing again to see if it ran better with regular throttle.  On the last time, it did!  I'm going to take another break and rest up, then go out and button the air cleaner up and try driving it around the block.  My biggest fear is if it fails at the end of my driveway, it would take 20 strong young'uns to push it back up my driveway.


If something (like an ant) was stuck, where do you think it was stuck?  I did look down into the venturis on the last run, and still only the inboard side was squirting from accelerator pump... maybe that's all these things have (???)


Here's to hoping your idea worked.  Thanks!  Will update the post with success or any new info if not.

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The ant was stuck right in one of the holes in the venturi. The engine would idle ok, but when I would spray the pressure washer it would almost die. This was strange cause the pressure washer was almost brand new. It just sat in my shed over the winter and I pulled it out in the spring and it was broken. So I tore into the carb thinking maybe the fuel gummed up (even though I put fuel treatment in it). Found the ant, cleared it, back to normal.


Best of luck.

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The idle circuit is totally separate from the primary barrel. Wouldn't take much for some sediment to drop down into the primary jet partly blocking it. It will idle and rev up without a load but as soon as you try to drive it, it can't supply enough gas for the air and it bogs down.


I imagine the J13 Hitachi carb is like the L series and there are two cover plugs just under the float chamber window. I also imaging the J13 carb is simple and has not pollution equipment and hoses around it in the way of removing it? This would be the best way to get at it. A 12 mm box end will take the plug out and then grind down the shoulders of a bladed screwdriver so it will fit up inside to unscrew the jet. 


I had a 318 that had a mosquito stuck in the jet and my old 521 would do this if you hit a large bump. You have to shur the engine off and count to 10 then restart. When I got time I took it apart and there was a small flake of casting caught in the jet.


Try revving it way up and quickly cover the carb air inlet with a rag and your hand to stall it. The high vacuum just might suck the blockage through.

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Looks like Draker's suggestion (also mentioned by datzenmike) solved the problem... fingers crossed.  I drove the truck about 15 miles today, and it seemed to have plenty of pep (well, it's a J13, so that's relative)... and, it may be placebo, but it sort of seemed a tiny bit better than before.  Maybe whatever grunge is going on in there was building up in the arteries over time, and then a larger piece of debris finally gave it a stroke.  Maybe the fuel filter I mentioned in the OP that I replaced a month or so ago had some crust that got jostled loose when I replaced it.  I didn't dissect it when I removed it... just tossed it and replaced with a new one, so I'll never really know.


Thanks to all the input on this. I don't get out very much, but when I do need to run an errand or go to a Dr. appointment, I need to feel confident the truck will get me there and back.  I've got another old Chevy truck that can sometimes serve as backup, but it's kind of torn down at the moment, trying to get some deferred maintenance issues resolved.

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