Rough Idle, RPM drop and stalling at intersections

[jalexquijano]

I have a 1972 240z Datsun with the following description:

 

- Recently Remanufactured Hitachi 3 screws SU CARBS, Scheider 274F Cam with new lash pads, retainer springs and NIssan OEM Rocker Arms. THe engine was overhauled by DATSUN LLC with cranks and rods from a L26, special oversized84 mm  pistons from DL POTTER ENGINEERING, Pertronix ignitor 1786 and 3.0 ohms coil, MSA Fan Shroud, Champion 3 row radiator.

 

The problem is that whenever the car warms up and i get in the middle of a traffic jam, the idle starts to become rough and suddenly drops to 500 RPM. The NGK BP6ES spark plugs start to foul and eventually the car shuts down. This car is supposed to be driveable and mantain its idle even under any kind of heat and traffic conditions. Well, it is not. I changed the plugs like 6 times, spark plug cables, distributor cap and rotor, checked for leaks with carb cleaner and NOTHING!!!

 

I have taken this car to 3 mechanics and none of them have been able to sort this rough idle and stalling issue!! Ignition timing is set at 11-12 BTDC.

 

Could i have a valve burned or warped? I also had to open exhaust valve on cylinder 4 to .013 as whenever you pulled out the spark plug cable there was no change in idle on that cylinder.

 

Hoping someone can guide me properly!!! I wouldnt like to spend 2,500 dollars in another Datsun LLC engine. Looking forward to some good guidance.

[datzenmike]

Recently Remanufactured Hitachi 3 screws SU CARBS

Are the SU tops rounded or flat? Flattops (from the later L26) are notorious for being difficult to tune. In fact I have never heard anything good about them.

 

 

Pertronix ignitor 1786 and 3.0 ohms coil,

 

Not a fan of these, as all they do is get rid of the points. A later EI distributor not only gets rid of the points but allows the use of a 0.75-1.0 ohm coil that draws more current and provides more spark. I doubt this is your problem but nice hot spark is always good for idle.

 

 

Could i have a valve burned or warped? I also had to open exhaust valve on cylinder 4 to .013 as whenever you pulled out the spark plug cable there was no change in idle on that cylinder.

Have a compression test done. Low compression could be a tight, burnt or a bent valve, a bad head gasket or a broken piston.

 

If compression is good, but still no difference when plug wire removed, could be no spark through that wire. Bad wire, broken distributor cap.

[jalexquijano]

Nice advice!! Sorry for not getting back to you earlier. Car is still at the mechanic and he still needs to test drive. Regarding your queries, i hereby respond as follows:

 

1. Carbs are Round Top 3 screws recently remanufuctured by Ztherapy. So the problem is not the carbs.

 

2. I am using the original single point Hitachi upgraded to pertronix electronic ignitor 1761 with 3.0 0hms pertronix ignition. Also think this is not the problem. Although if i pull the spark plug cable from cylinder 4 the rpm drop is not as noticeable as in the other 5 cylinders. I am using NGK blue wires and NGK BP6ES spark plugs gapped at .031.

 

3. Compression done was done. Results were the following:

 

Cyl 1: 180 PSI

 

Cyl 2: 180 PSI

 

Cyl 3: 185 PSI

 

Cyl 4: 180 PSI

 

Cyl 5: 180 PSI

 

Cyl 6: 175 PSI

 

The difference in rpm on cylinder 4 is minimal compared to the other 5 cylinders.

 

The issue is that the car heats up in traffic and the spark plugs get fouled and suddenly the car shuts down.

[datzenmike]

Compression is great!

 

Switch the #4 wire with any other wire that reaches, and see if the drop in RPM moves to the other cylinder. If it moves that wire is bad.

 

 

You're saying all the plugs foul??? Foul as in become carboned up? Are you turning the chokes off when warmed up???

 

If YES... what fuel pump are you using? Mechanical, electric? Have the fuel pressure checked. It should be no more than 3.5 or the pressure may be over powering the needle valve and flooding the carbs. See what Z Therapy recommends for pressure.

[Draker]

And make sure the carbs are adjusted correctly. 

[jalexquijano]

Airtex 8012S electric fuel pump which is pumping 4 psi and not overflooding the bowls at the moment! the rpm does not drop with any of the other wires. It remains the same. Plugs start to carbón foul when the car warms up and hits a traffic light or gets stucked in traffic. Idle becomes unstable and suddenly engine shuts off once the plugs are really fouled. If i should replace this pump with another one which one then?

[datzenmike]

Test the pressure first. Maybe it's faulty. I would try for 3 PSI.

 

Chokes are off??

[jalexquijano]

Chokes are off!!

 

Mechanic tested the car this morning. Got trapped in traffic temperature raised to 3/4, rough idle and car stalled. Isnt this a heat soak vapor problem? It doesnt happen with the car parked under a garage roof while at idle.

 

Will i need to wrap the fuel lines in asbestos?

[datzenmike]

If this was the cause, all Z cars would do it... they don't. Change thermostat.... they don't last forever.

 

Is the fan shroud still on the rad?

You have a proper anti freeze/coolant mix?

Rad is clear of bugs and dirt?

Is lower rad hose soft when squeezed?

Remove rad cap and look inside... is there mineral scale build up?

Is the heater working? or have the hoses been bypass connected?

[jalexquijano]

i have a champion 3 row radiator with an MSA Fan Shroud installed! The Temperature here in Panama is 33 degrees Celsius wth 80 per cent humidity. I havent flushed the radiator or cleaned it inside. will test the lower hose. DOes the heat affect the idle and consequently make the car stumble and stall in traffic??

[datsunaholic]

If it was heat soak of the fuel lines (vapor lock) the plugs would be white from an ultra-lean condition.  It still sounds more like it's flooding, BUT you could have an ignition problem instead (losing spark).  When you mean fouled, you mean wet with fuel or black and covered in soot?   Need to test for spark when it dies and won't restart, but it has to be in the non-running condition for it to be any value.  It's possible the coil is overheating.

 

Electric fuel pumps combined with no return path are notorious for forcing the needle valves in the carb bowls open and flooding the carbs.  You won't notice an external leak, though, unless you left the pump on with the engine off and it finally would overflow.  Not sure where on an SU that overflow would happen.

[jalexquijano]

Well i will try to explain this better. It seems as i am not making myself clear. The car accelerates fine even though some very slight hesitation if you dont step on the gas pedal properly. Car gets into a traffic light or traffic jam. After 2 minutes, idle starts to become rough and after 7 to 8 minutes the RPM starts to drop from 950 to 500 and rise again to a point where it just stalls in the middle of the Street. To avoid this i need to pull out the choke and drive with the choke on. Once back at the garage, if i pull the plugs they all come out black and sooty but not wet. THe heater is not working as i had to remove the heater box to install a Vintage Air Gen II mini evaporator and needed the space. The heater hoses were bypassed.

 

Should i purchase another Pertronix 3.0 ohms coil? Guys at ztherapy told me not to open the bowls of the carburetors when i received them on January. I have kept it that way.

 

Will i need a better pump than the Airtex 8012S?? I read in another fórum that guys who had this problem wrapped the fuel lines in reflective heater tape.

[datzenmike]

Driving with the choke on will cause rich running and cause the black plugs... so not really a symptom. 

 

Check the fuel pressure. SUs are sensitive to too much pressure. If in touch with Z Therapy ask them for a suggestion on what pressure is needed.

 

Changing a coil, that may not be a problem, is ill advised. Do you still have the original factory ballast resistor being used with that 3 ohm coil?

[jalexquijano]

FSM says 4.3 máximum psi pressure. I am at 4 psi. So i should be okay! I still have the original ballast resistor. Should i connect the 3.0 pertronix coil to it instead of bypassing it with a jumper cable?

[datsunaholic]

OK, your added info changes things.  If closing the choke makes it run rather than due, that starts to sound like a pretty major vacuum leak.  One that happens when it heats up.  With no airflow (from not moving), everything under the hood gets hot so it's possible something is opening up that isn't supposed to, so one thing I would check is make sure the carb-to-manifold bolts and the manifold bolts themselves are tight, because that's a good place for a vacuum leak to happen.  And I do hope you have the thick phenolic spacers between the carbs and intake (with gaskets on each side); I've actually seen folks omit them and the carbs heatsoak badly.  Plus most SU setups had a heat shield between the exhaust and intake manifolds, you have that?

 

And check to make sure the brake booster is working.  I've seen the diaphram go bad causing a vacuum leak, though it's generally not heat related (would happen all the time), but having the brakes on in traffic would change the amount the booster "leaks" of it was leaking.

[jalexquijano]

i meant pulling the choke when the idle starts to get rough to avoid stalling in traffic. i do have the spacer between the carbs and intake on! I still need to know if in order to avoid the 3.0 pertronix coil from heating up so badly i need to connect it to the ballast resistor and not bypass it! I am using a Pertronix 40511 coil with 1761 ignition module.

 

Do you also think i should forget about wrapping the fuel rails with heater reflective tape? Thermaflex tape? Which Brand and type if so??

[datzenmike]

 

 

Do you have the proper factory air filter housing??? They have a  hot idle compensator inside that corrects an abnormal rich condition during very hot idle conditions. It's a pair of small air bleeds that allow added air into the intake to correct this..

[jalexquijano]

I have the original air filter and housing. I need to know if i need to bypass or not the 3.0 0hms pertronix coil? Mechanic is flushing the radiator and if it needs to be fixed, it will be done.

[datzenmike]

I don't know anything about after market parts. The best parts are what came with the car... these I trust above all others. I can only assume the Pertronics coil and module work alright together as per the installation instructions. I would guess the ballast should not be used with the Pertronics 3 ohm coil.

 

The stock coil has a resistance of 1 to 1.5 ohms. The ballast doubles this to about 3 ohms. If you should go back to the stock coil be sure to use the ballast resistor.  

[datsunaholic]

 

 I still need to know if in order to avoid the 3.0 pertronix coil from heating up so badly i need to connect it to the ballast resistor and not bypass it! I am using a Pertronix 40511 coil with 1761 ignition module.

 

 

The coil isn't your problem.  It was just one of the things to check when you didn't give all your info.  If pulling the choke allows it to keep running, you still have spark.

 

 

 

i meant pulling the choke when the idle starts to get rough to avoid stalling in traffic.

 

That's what I understood.  Pulling the choke (that's closing it, open is when it's pushed in, where it SHOULD be once the engine is warmed up).  If it won't run with the choke open, it's going lean, probably from a vacuum leak, and closing the choke makes it rich.  But apparently too rich if the plugs get sooty, but with the choke shut with a hot engine, it IS too rich.

 

Now, there is 2 different issues that could happen, but I'm assuming it won't run at all once it starts misfiring unless the choke is closed.  If it will run but not IDLE with the choke open, that's a different problem, an idle circuit issue.  But I don't think that's it.

 

What I would do is look for vacuum leaks.  Spray carb cleaner at key points of the carbs and manifolds (anywhere they're a gasket junction) with the engine running- you hear the engine idle change, you've found a leak.

 

Or check plug color after it dies.  But leave the choke open!  Closing the choke makes it rich and masks the problem.  With new or clean plugs (you do know you can clean them with carb cleaner and a wire brush, the carbon comes right off)  run it till it dies  (drive it around for 20-30 minutes without traffic, then park in the driveway and pretend you're in traffic.  Let it die), THEN check the plugs.    You will likely find that 3 plugs will be different color than the other 3, which would indicate which carb has gone lean (that's why the idle drops and gets rough, because half the cylinders are misfiring) but if it's 1 or 2 plugs that's more an indication of a intake runner leak.  It could be as simple as loose nuts/bolts.

 

 

Do you also think i should forget about wrapping the fuel rails with heater reflective tape? Thermaflex tape? Which Brand and type if so??

 

If it runs with the choke closed it's not likely fuel line heat related (vapor lock). 

[jalexquijano]

Okay. Im not going back to the stock coil. In view of that i will leave the jumper wire on the ballast resistor thus bypassing it. I also do not think this is the problem. I already bought a roll of heat shield exhaust wrap and have told the mechanic to raise the car in the elevator before masking the fuel rail and exhaust to avoid percolation or "Vapor Lock". Engine Bay wont look nice but if it solves the rough idle and stalling problem i can setlle. Anyway, how much will it cost me to get a Datsun Mechanic from California to check my car and solve all these issues? Mechanics in Panama, Central America just know how to change or swap parts and not diagnose the car properly.

 

I prefer paying an expert than spending money on not needed parts!

[datzenmike]

As Doug says, if it runs otherwise normal but doesn't idle it's unlikely the ignition spark.

 

Do you have the proper factory air filter housing??? They have a  hot idle compensator inside that corrects an abnormal rich condition during very hot idle conditions. It's a pair of small air bleeds that allow added air into the intake to correct this..

 

Seems like this would correct for extreme hot idle problems.

 

Another is to connect a large hose to the air horn on the air cleaner and draw fresh cooler air from in front of the radiator.

 

Wrapping the exhaust is ok, but try insulating the entire length of the fuel line under the hood where it's warmest. You have an electric pump? shorten the fuel line at the engine end.... less exposure to heat. 

[jalexquijano]

I have an airtex 8012s electric fuel pump! Which is pumping 4 psi at the moment! Any better fuel pump than this one? No fuel filter before the pump just one glass filter at the engine bay. So its not necessary to bring a Mechanic from the US to fix my car? How come some cars do not have the fuel rails covered with heat wrap and they dont run rough or stall at intersections or Street lights?

[datzenmike]

Doesn't the early Z car have both a mechanical and an electric fuel pump???

 

How do you know it's pumping 4 PSI without a gauge? If you don't have a gauge, have the pressure tested to be sure. If a little high you can put a fuel pressure regulator in line to lower it.

 

I don't know of any Datsun that had heat wrap around the fuel rail. Most had a return line to the tank. The return line has a very small hole in it as a restriction so some pressure could build on the carb side. This would slowly recirculate cool fuel from the tank past the carb. It would also allow vaporized gas bubbles from heat soak to be pushed out and away from the carbs and once shut off there would be no residual pressure in the fuel line to the carbs. Can you post a picture of the front of your engine??? You may have a return line that's not working.

[datsunaholic]

Heat wrap shouldn't be necessary.  Some Z-car guys swear by it, others swear at it.  If the fuel system is working properly it isn't needed. 

 

Return lines weren't used until '75 on the 4-cyl (L20B) cars, but the Z-car I don't know.  Some posts on the Z-car forums I read say they do, or at least the late 240Zs do.  If the return line wasn't hooked up or it was plugged, that could certainly cause a vapor lock issue.  I did note that some folks state that the small hole for the return line is critical, if it's plugged off or worse opened up the carbs will not be happy.

 

I read where some folks attributed a vapor lock issue to be the fuel lines back at the tank.  From the exhaust heat building up under the car in traffic or really hot roads.  I find that somewhat unlikely, but it certainly is possible.