Puzzle under the Hood

[ryanonthevedder]

I have read many posts and have a lot of success with my 1973 240Z, until now. I consider myself to be a pretty good mechanic and did all of the work on my car myself (with a little help from the body guru Randy), but to the problem at hand...

The car sat for 15 years before I got to it, and I did a fairly extensive freshening up on it. I did the carbs (flat tops), re & re on the engine, high lift cam, ported head (N42). The ign is original points, but received all new parts. Then all the new metal.

The car worked great for 2.5 months then one day a problem arose. I drove the car to work in the morning, but when I went for lunch I started the car and let it idle for a minute. It started fine but then developed a little miss. I thought maybe the carbs picked something up ( I did a lot of work to get them working well/reliably) so I took it out for a burn. The problem got worse as I went and it seemed to me that the car had developed an electrical fault. It drove like someone was switching the key on and off... Once I got back to work it died and would not restart.

It has lots of spark (new coil and condenser last week)

Points OK

TDC verified on the dampener

Cam Timing Checked OK

Tank drained & new fuel (last week)

Distributor drive shaft OK

No vacuum leaks

Front carb re & re and seemed OK, both seem to be working OK

I have hot wired the coil direct from the battery to eliminate electrical/resistor problems

Compression was 145 psi after break in and is the same today

The only way the car will start and run is if I advance the timing to 30 or 40 degrees. When I get it running I cant retard it past 20 degrees or it sputters and dies.

I am at a loss... any insight would be appreciated

Cheers

Ryan

[steve91tt]

Have you checked fuel pressure? Do the front three plugs look the same as the back three. Sometimes when one carb acts up the plugs will show differences front to back.

[tlorber]

Yeah, I say check your main fuel filter, and banjo bolt screens at the fuel bowls.

[steve91tt]

Good idea on checking the carb screens. If the fuel pressue at the carbs checks out then you can rule out the entire fuel system before the carbs (tank, pump(s), filter(s), lines, etc.).

[LeonV]

I'm with them, fuel comes to mind for me, as well. Check all filters and replace as necessary. You can put in a fuel pressure gauge in-line for diagnostics.

[5thhorsemann]

Also, when (if) you use a gauge to check pressure at the carbs, pinch off the returne line and note the difference between open and closed. If the pressure is low that is.

[ryanonthevedder]

Thanks for the replies. I hard wired the ignition today as well as tested the fuel pressure. I ran power from the batt to the resistor to the coil to the distributor. Spark is as good and consistent as always. The fuel pressure was 3-4 psi while running and 6 psi with the return blocked off. The float levels were good on the sight glasses. I did not check the banjo bolt screens because I don't have them. They were too deteriorated to put them back in. When I took apart the front carb after the problem arose I did not find anything in the float bowl that was out of the ordinary. I have also tried spraying carb cleaner in and around the intake and carbs in search of vacuum leaks and/or lack of fuel with no great results.

I have done a lot of work to the carbs to get them working well, but I am beginning to think they may have failed in some cruel and obscure way.... stupid flat tops.

[Captain Obvious]

I'm going to buck the trend... I don't think you're looking for a fuel related issue here. Especially with what you described below:

It drove like someone was switching the key on and off... Once I got back to work it died and would not restart.

The only way the car will start and run is if I advance the timing to 30 or 40 degrees. When I get it running I cant retard it past 20 degrees or it sputters and dies.

I agree with your initial inclination and believe that you've got an electrical issue of some kind. Intermittent something, maybe heat related? Coil? Condenser? EGR valve hanging wide open when it shouldn't?

Also, are you sure about those timing numbers? If those are your "at idle" numbers, then you've got something out of whack.

I have done a lot of work to the carbs to get them working well, but I am beginning to think they may have failed in some cruel and obscure way.... stupid flat tops.

Haha! That's Datsun's way of punishing the non-believers.

[ryanonthevedder]

Thanks Capt. there are a couple of things I need to double check. The egr seemed to be working OK but honestly I did not remove it completely.

I ve got a line on a electronic distributor and I think I am going to get rid of the flat tops. I ve poured my heart into those things but like any bad relationship, this one might be over... sob.

Thanks for the reply!

[Diseazd]

I agree with Captain.....it sounds electrical to me. If you replaced the coil and condenser, go back to what you had. also check the short ground wire that attaches to the points.....sometimes it gets pinched by the distributor cap if not tucked away. Look carefully at the rotor button and distributor cap for hairline cracks.

[Carl Beck]

Vaccum or mechanical advance in the old distrubutor - maybe. Do not change two things at once. If you get a new distributor put it in first - see what happens. Then change the carb's if you like.

I always keep a spare - "known good" distributor for things like this. It is usually one of the first things I try.

FWIW,

Carl B.

[Diseazd]

The only way the car will start and run is if I advance the timing to 30 or 40 degrees. When I get it running I cant retard it past 20 degrees or it sputters and dies. Quote

It may not be the timing that's making it run.......a slight twist of the distributor may be making a loose wire connect. Just a thought.