Working out bugs in engine -- Next obvious things to consider?

[FastWoman]

Hi all,

I'm still working out the bugs in my "new" '78 as free time arises. Er... Maybe I'm working out all sorts of vermin, as I just replaced the air filter, in whose housing I found the remains of a family of mice, but that's another story. The engine runs very well, overall, but I'm still not quite there.

I have new vacuum lines throughout. Timing is correct. Engine vacuum is very steady, but a couple/few PSI low. Then again, it also has 160K miles on it. I've been running B12 in the fuel to clean the injectors.

My remaining problems are:

1. After I've left the car sitting for 2 or more days and then have started it, it hesitates on start-up, often with a very mild/brief backfire within the first fraction of a second of running. I never have to crank the engine very long before it springs into action, but this is the pattern after I've left it sitting. If I start the engine after the better part of a day, or even 24 hr later, the start is rather uneventful. It bothers me very much to have any backfire with this design of airflow meter, so this is a priority for me to fix.

My two hypotheses are: (a) check valve out of the fuel pump is bad, allowing fuel to drain out of the fuel rail, and/or ( the injectors leak pressure and eventually allow air back into the fuel rail. Or possibly © the cold start injector sticks if it's been sitting too long. The backfire suggests to me that i'm running too lean on startup. The stuttering and hesitation suggests not all of my cylinders are firing, perhaps because I'm initially injecting air instead of fuel (which would also make conditions too lean).

The injectors seem to be original (i.e. 160K mi). Are they due for a change anyway? How about the cold start injector?

Any thoughts as to what "usually" happens and what are the most obvious things to do?

2. Although my idle is more-or-less smooth, the exhaust puffs sporadically as cylinders miss. This happens maybe twice per second on a random basis (i.e. not attributable to any particular cylinder). My plug cables read in the neighborhood of 30kohm and appear to be in good condition. Should I "absolutely" be using coil-core wires instead of the resistive core? NGK plugs? Or are my problems likely elsewhere? My engine vacuum is very steady, suggesting to me that I don't have sticky intake valves, but the motor probably needs a flush anyway (on my to-do list). Perhaps a worn/sloppy distributor?

Again, what are the most obvious/likely things I should consider?

Thanks for your advice!

Peace,

Sarah

[FastWoman]

Dunno... I have two general problems, possibly interrelated, that might be attributable to multiple causes. I was simply asking which are the more likely of these causes, as it seems better to throw money at the cheaper and/or more likely causes first. Do I have too many questions in one thread? Should I have posted in multiple threads? It seems unlikely that the dozens of Z owners who have read this thread would not have a clue about any of the questions I've asked. I'd be very grateful for any thoughts about any of these questions. (Responders need not answer *everything* to provide info that would be helpful/useful to me.) Again, I greatly appreciate any help anyone is willing to offer me.

Edited July 21, 2009 by FastWoman

[Unkle]

That mouse thing sounds interesting , maybe there's crap in the fuel delivery system . Vermin bring and excrete '' extras '' that probably aren't Datsun friendly . They chew on wires and insulation , rats doing the most damage. Remove and inspect items that normally would be sealed up , check blocked exaust / intakes , etc. Replace / check distributor cap etc. This site might be lonely right now for better advice because it's peak summer vacation time !

[FastWoman]

Hi Unkle,

Thanks for responding! The air filter wasn't breached, so I know nothing got into the intake. Distributor is clean, just corroded/stuck at 10 deg BTDC (another thread about vacuum advance -- distributor to be replaced today). I've been through the vacuum lines and wiring pretty thoroughly. I've not found anything chewed. No evidence of rats -- only a small mouse nest.

Overall the engine runs quite well (better, now that I've fixed some vacuum leaks). Startup is just a bit rough after the car has sat a few days, and I occasionally have a miss, resulting in an exhaust "puff."

[beebeecivic]

How is the Cold Start? I replaced the hose on the cold start because it was leaking at etc. When I replaced it i had problems because the way I tightened the clamp restricted the flow. After squeezing the hose gas squirted out , so I moved it back a little further and it starts right up!

[FastWoman]

I haven't tested the cold start yet, to be honest. I do need to replace the fuel line to the CS injector, so perhaps I can test it at that time -- by jumpering it to the battery and seeing if it passes air.

[FastWoman]

Update: I still haven't tested the cold start, but I flushed the engine and found it to be pretty gunky. Sticky valves? I then pulled the valve cover to adjust the valve lash. I found that the #6 exhaust was a bit tight and MAYBE could have prevented the valve from closing completely on some cycles, allowing exhaust gasses to backflow into the cylinder during the intake cycle. Just speculation, but that could have caused the backfire. Fingers crossed... I haven't experienced backfire since then, but then again I haven't started the engine cold more than a single time.

The exhaust still puffs a bit. I only put a dent in the engine's gum and varnish, so sticky valves are still a possibility. I think continued de-varnishing and de-gumming is the most likely solution.