Diagnosing possible carb issue

[RoadZog]

I've been enjoying the heck out of my first roadster now for a few weeks (69' 2000), but inevitably just had my first breakdown. She was running fine, though fuel was getting low, then suddenly while coasting out of gear seemed to lose two cylinders all of a sudden. Pulled over, refilled the tank, but could only get started on full choke and still sounds like only two cylinders. The fuel pump seems to be working, and it doesn't seem flooded.

I don't pretend to be a mechanic, but I do want to learn. What are first steps to diagnose the problem (aside from just taking it to a real mechanic)? Does it sound like a problem with a float? The suction chamber seems a little low on oil, but otherwise clean.

Thanks

[Linda]

I would check the fuel filter first in case junk has clogged it. I keep VW filters on hand, small and cheap and changed frequently for optimal fuel. Maybe blow back on the fuel line where it connects to the filter, you should hear some bubbling, if not you might have a clog there. If all good, then can check fuel delivery to the carbs by disconnecting the line in, put hose in a bottle away from heat, crank a few seconds and see if fuel shows up. Carbs might be clogged at the fuel screens inside each carb. Carb cleaning in order if suspected.
Can check floats to see if one is a sinker. Check all hoses and connections on the fuel route. So much for fuel issues.
Electrical issues- spark plug wires, distributor cap, coil capacity. Is the spark good and getting to where it needs to go? Points, pigtail wire in distributor, condenser- new or old? Maybe suspect..
More electrical- check the starter harness connectors at starter and other end, might be marginal, dirty connections.
Check fuses, ditto.
Mental note to clean all grounds and connections for a project no matter what it turns out to be
Linda

[fj20spl311]

The needle and seat might be stuck on one of the carbs.

Review the Wiki page by Keith Williams on this site.

SU tune

[Gregs672000]

If it's running on two then it's not likely spark unless there's physical damage to the distributor cap inside. You can check for spark a couple ways... pull the plug wire, put a screw driver in the end and while someone is cranking the motor hold the insulated end of the driver to allow the metal to jump the gap to the valve cover; or you could spray that carb throat with starting fluid and see if it fires. If so, then you know you have spark but no fuel. Linda mentioned most of what you need to check for fuel, but if it's isolated to a particular carb focus your attention there.

[Linda]

Also watch out for corn gas.... it gets gummy quick, making things stick , like the fuel valves as Phil mentiioned
Linda

[notoptoy]

Check your linkage, just make sure one of the dogbones hasn’t come off.

[spl310]

Check that both carb pistons still move ok

[RoadZog]

Hi folks - thanks for the helpful suggestions.

So my local mechanic took a look and agrees my issue is with the carbs, but he's not really familiar with these cars so he's suggesting having them rebuilt. We've ruled out a sunk float, and there's plenty of fuel in the bowl. I'm going to give everything a good cleaning, though it honestly doesn't look too bad to start.

Linda mentioned checking the filters in the carb. Am I correct in assuming those are located in the top of fuel bowl (pic attached)? The bolt on the inlet is giving me some trouble, and I'm trying to be gentle with it so I don't damage the float or inlet, but is this the right place to be looking for a clog? That's still my best working theory...

20200923_080019_compress10.jpg

[spl310]

Remount the float lid to the carburetor and use a socket to break the banjo bolt loose.

[Linda]

Yes the bolt is on tight. Once off, preserve the washers or replace if bad. The little screens inside can be dirty or disintegrating. They surround the bolt you are taking off. Replace via vendors as needed.
I would suggest you check/ clean/replace the fuel valves also. You get to them from the bottom of the lid. Tap out the pin for the hinge and remove. Remove valves with a small socket and don’t lose the parts. Examine the tip, it might be rubber and shot or metal and scored. Replace if so. Note washer(s) also. There might be little screen bits in the valves....not helpful...clean with carb spray.
Rebuild kits available from vendors and Ebay online. https://www.ebay.com/itm/DATSUN-1600-20 ... Sw2gxY2UbT
Fuel bowl lid gasket you can make yourself or get included in kit. Must have good gaskets there also.
The floats can be deceptive if brass, leaking just enough to be difficult to detect. Or repaired badly. Just get the plastic floats for safety. Oops , I see you have a 2000, so disregard the float info, that is for a 1600
Not too hard and once done should be good for a long time.
Lots of threads on this, so good to review
Linda

[RoadZog]

Thanks Linda - really appreciate the advice.

I disassembled and cleaned the fuel bowl and everything seemed to be in good shape. The valve seemed ok. No deterioration or obvious clogs in the banjo bolt filter rubber floats good and gaskets are decent. I refilled both suction chambers with synthetic ATF. Both pistons move smoothly, but the rear carb requires noticeably more force to move up than the front.

My efforts do seem to have had some impact. It will stay running now, but only with the choke fully open, though I haven't really run it enough to warm up. It's revving a little smoother, but still seems bogged down. Both times I ran it after cleaning I got backfire out the exhaust that freaked me out, especially the second time. Gonna let it sit for a bit.

So, could the sticky suction piston be making it run too rich, or is the problem likely somewhere else?

[Linda]

Try placing the dome on the carb that sticks and tightening the screws a little at time while checking that the suction piston moves up and clunks down when you release the little pin that lifts the piston. If it does not lift and clunk down easily you may have a bent needle which hangs up on the nozzle it goes into.Check the needle by putting it into a drill and see if it wobbles. If it does you can straighten it with a towel and some pressure on the needle as you run the drill. Hard to describe but it is in the ZTherapy CD for their carbs.
You may also have carbon in the groove on the carb piston, hanging it up. Be sure you did not mix up the pistons....
The nozzles need to be centered also which usually never needs to be done so try everything else first.
Nozzles must be set to correct height. I like the “ magic number” method described in the Tech Wiki for setting the nozzles. Get a digital caliper and measure the depth of the nozzle on each carb, they should be the same and around .087, at least to start with.
Float height is important too, so check that per Wiki....I only know the 1600.
Idle screws also for an idle around 650 I believe, affect the throttle plate at the back of the carb. You do not want any vacuum leaks which can happen when the bushings are worn.
Getting everything clean, with good gaskets at intake, fuel bowls, good hoses etc always a good starting point. You are making progress.
Also sticky chokes can mess things up, be sure chokes are off and linkeage is not hung up.
Dog bones should be long enough per Wiki
Not too many parts but they have to be right. Of course rebuilt carbs fix all this stuff which is why they are so great, but many things you can do before you go that route.
Check back in with what you find!
Linda
Be sure to put a long length of fuel hose on the metal overflow tubes so any gas goes below the car NOT on the exhaust..