Last Saturday, my engine stumbled, and ran poor for about one minute. Then I drove it 300 miles more with no problem -- in fact it was running its best ever, very strong.

Then on Tuesday it acted up on the way to work. On the way back it got worse. It idled fine, and revved strong in nuetral, but when trying to go it stumbled and barely moved. Oddly, I discovered if I floored it, then it ran OK. Which didn't seem like a fuel filter problem.

Anyways, I think it was the fuel pump. My gauge said only 1.5 pounds, and the minimum is 2.6. So I took the pump off and found some crud in it. I cleaned it up and put it back together and it seems to be running fine now!

Quote:

I had a similar problem a while back. Turned out to be crud in the fuel line from the tank side. I blew it out with an air hose at 100 lbs of pressure and all kinds of mung came out. It's been a year and half or so now since I performed that operation, and it's been running well ever since.

OK, so it wasn't the fuel pump. The problem has re-occured from time to time. Mostly after the engine warms up. I can still drive it but sometimes it gets pretty bad -- a kind of jerking, stumbling unless I floor it.
* idles fine
* jerks at part throtle
* fine at full throttle

This weekend I got a vacuum test tool:


MityVac Tool

You can test all the vacuum parts (motors and diaphragms) without the engine running. Squeeze the handle a few times and it makes 25+ inches vacuum.

Bingo! Bad distributor vacuum advance. Bad choke unloader. Also I found the hose on the intake was very hard and was cracked at the fitting.

That's three vacuum leaks on my engine ... which for the most part was running well. Until recently.

jeez, you got onto that that problem quickly. finding three vacum leaks is not an easy thing to do.

Nice job.

Lol so many vacuum lines! I have 1 to the vacuum advance.

I get funny problems like spark plug wires that got wet and burn up that make me insane, so it's always going to be something. Simpler systems just have more obscure problems... remember my oil pump disaster.

need to stop being poor so i can get tires and drive the datsun again.

I went through a similar problem last race weekend. After the breakfast stop on the way to deep river the car could barley move. It would idle fine, and run at full throttle but not run on the lower half of the primary. If you fully opened the primary enough gas would be delivered to run the engine but no gas was delivered at less than half throttle in the primary bore.

This issue was traced down to a partially plugged primary main jet . With high vacuum on the primary vacuum booster the gas would pull through the jet. At part throttle no gas was delivered via the primary main jet.

After removing the jet covers to drain the float bowl and shooting some carb cleaner down the vent tubes and down the air corrector jets to flush out the float bowl the car is running better than ever.

Some crud in the bottom of the float bowl could be causing your intermittent problem. Removing the main jet covers is a nice easy to clean the float bowl without having to remove the carburetor. For obvious safety reasons, only drain the carb when the engine is cool...

Thanks Gary, I will be doing that next.

Here's a TIP: it's a good idea to test drive -- after changing a bunch of engine parts -- before you drive to work the next day.

After fixing these leaks, fast idle shot up to 2,500 rpm, and after the choke finally came off, curb idle dropped to 400 rpm. So fixing the leaks had a big effect -- and now it stops running (no dieseling) when I turn the key.

However there is still more wrong, as I cannot lean out the idle mixture properly (it runs OK fully turned clockwise), and it still jerks/bucks at part throttle, different than before. (Now rpms drop way down, I give it pedal and it jerks, then goes). I tested this evening for vacuum leaks around the gaskets, but that all looks good. I'll be cleaning the carb jets & air passages next.

Whenever I get a new, pre loved car, I run an ongoing maintainance program on it that involves replacing all 'service' rubber parts in the engine bay. It might not be possible to do it all at once, but as long as a steady trickle of these service components keep flowing in, it's all good.
I bought a roll of vacuum hose to replace any & all vacuum lines in the car, & I also replace engine bay fuel lines, radiator hoses, heater hoses, radiator cap, & fan belt.

This policy also includes automatic replacement of the distributor cap, rotor button, leads & plugs. I have replaced the points with a Pertronix unit in my 1200 so ALL of these potential problem areas are now in a KNOWN condition.
Naturally anything else needing attention is addressed straight up, like alternator, water pump, radiator, thermostat, etc, etc.

This way I can be assured that these components should give me a high degree of reliability for a long time, .... & trouble free motoring is what it's all about for me.

Yes, good idea. Back in the day -- when I was single -- I did a lot of preventative maintainence. Since I depended on my 1200 for my livelihood (delivery job), inspected the car from head to toe every month.

Nowadays I give it a cursory look occasionally, but pretty much wait for a problem to occur. See I've had this 1200 for four years, and it has never stranded me. It's been my daily driver for most of that time.

This carb/fuel problem is the worst problem I've had with it, yet it hasn't died on the side of the road, what a great car these 1200s can be.

After adjusting the idles and choke, it is running better now. Last week's jerking/bucking problem is gone, and now there is a different one, just barely off-idle. It's as if the accelerator pump isn't working quite right. Definitely different than before. It may be the new pump I fitted on Monday. I had a new one, so threw it on while I had the top off the carb. Maybe that was a mistake.

Anyways, I cannot recommend this tool enough -- it will find vacuum leaks in a few seconds, or confirm that the parts are OK:


vacuum gun

Gary_P I followed your advice and it seems to have fixed it.

Last thursday it ran so badly I thought it wouldn't make it home. In the morning I removed the bottom jet covers and sprayed some cleaner down the air horn vents.

After that it immediately ran good, and has been good through four commutes now.

Thanks for the tip!