Took my 240z again back to my mechanical engineer. Tested the car and it seems to accelerate fine. The problem is in the deceleration. When coming to a stop sign, slowing to make a u turn or traffic light the car tends to lower the revolutions from 1,100 to 600 and tends to stall but immediately goes back to 1,100 RPM. THis is not normal, something is wrong here. After a couple of tests the needle now reads empty on the tank and the car is now shutting down completely even though there is still fuel present in the fuel filter (mine is a glass fuel filter which can show any impurity in the line). The engineer did tuned up the car and changed all the spark plugs in front of me.


This car has a New Datsun L24 motor with crank and rods from a L26, special oversized 84mm flat top black coated pistons from D.L. Potter Engineering with E88 Head and Far 311 New Race Cam and 2 original SU Hitachi round top carburetors rebuilt


Do you guys think i can solve this issue by myself or should i take the car back to him again? We are thinking on replacing this Far 311 race came with the Original Stock cam so i can get a smooth idle and use my car as a daily driver. THe spark plugs are getting dark in traffic jams due to the race cam.

I want to solve the stalling and irregular RPM drop when decelerating first and then swap the cam. Can anyone advise me on how to tackle this problem?

 

Just checked and I dont see any signs of rust or impurities in the glass filter. I even checked all the spark plugs and they are burning well. I also sprayed carburetor clear to all the hoses end in the car in order to check any vacuum leak with the car parked on idle and the RPM did not change. Do you think i should also adjust the mixture idle Wheel underneath each of both SU CARBS? The problem is when i reléase the gas pedal i.e. if i speed up real hard on 1st gear and then reléase the pedal the RPM drops to 500 and tends to stall but pick up back again to 1,100RPM. Should i play with the idle mixture wheels underneath each of both carbs? maybe give it a Little bit more counterclockwise?


Im really fed up with this situation!

 

Does this happen when you simply rev up the motor while not moving? Every time you've described the symptom, it has been while the car itself is in motion and decelerating. If it doesn't happen with a simple rev up, hold and let off the gas quickly, I'd start to suspect something that happens when a decelerating force is applied to the car. What's loose and moving forward?

Are all your electrical connections locked down tight? Start with the battery as it's heavy and can move around.

Could there be sloshing gas in the fuel tank. Have you had any work done? Is it an original tank? Are the baffles in place?

Are you braking during this event? Could you have a vacuum leak in your brake booster system that only shows up when you hit the brakes?

 

It does not happen when the car is in neutral parked. It only happens when i drive the car. As stated before, shift reverse from my garage, shift 1st gear and remove my foot from the accelarator pedal RPM then drops to 500 and tends to stall but slowly rises up to 1,100RPM. Really weird. It did not do this before. I dont want to start messing with the idle mixture wheels in the bottom of the car as it does speed perfectly.

Please advise.

 

OK so it is certainly motion related.

You didn't say if you're using the brakes or not so we haven't eliminated the vacuum booster.

You did say you're backing out and then going forward and then slowing. It could still be fuel sloshing, maybe you're floats are set too low in the bowls and there's no "reserve" in there to handle a temporary starvation.

Could still be electrical as well.

Is it a hard deceleration, just coasting or something in-between? Are you pushing in the clutch pedal at the same time? I suspect you are but just checking.

BTW, if I don't respond for a while, I'm not ignoring you, I'll be away from the computer for quite a while. I hope at least you've got some things to consider...

 

The mechanic recently replaced the needles and seats to both carburetors. He also swapped the turn signal switch as the former one was damaged. I will check if any connections are not secured. Everything else is working properly. LEt me put it this way, I reléase the clutch and step on the accelarator real hard on 1st gear remove my foot as it comes down to a curve and the RPM drops to 500 tending to stall. At the deceleration momento is where the car tends to stall. Could the mixture be to lean? Do i need to move the idle mixture nut counterclockwise on both carbs a Little more?

 

OKAY. Yesterday i removed the air filter, caburetor domes, inner springs and sprayed carb cleaner to both of the 3 screw SU carburetors. I refilled both carburetors with 3 in one motor oil and retightened each and every screw in the air filter and carb domes. Finally i did moved each of the idle mixture nuts a Little bit counterclockwise. Result: The car is maintaning its RPM when decelerating or stopping at an intersection.

Could this have been the problem??

 

Well, certainly at least one of those things caused the problem. It's hard to tell which one. I can certainly see how the clean-up of the mechanism in the domes and replacing the oil might have helped as that mechanism, if dirty, might behave differently under deceleration.

Anyway, congratulations! You've fixed it yourself and avoided an expensive trip to the mechanic. Good job.