Attempted to tune SU Carburetors but still backfiring on intake

[Mark Maras]

 Anything over 4 psi will overwhelm the needles & seats. 2.5-4 psi is OK.

 What color were the plugs the last time you pulled them? In fact I'd like to see a pic of them & a description of what it was running like when you shut it off. 

 There are still other things to confirm. I've read the many threads about your build but refresh my memory, what have you done to ensure the basic tune-up procedures were done properly? Such as valve timing check, valve adj, ign. timing, & vacuum leaks. You will be chasing your tail trying to find the problem in the fuel system if any of of the basics are wrong.

 So, give us more info, pics.of the plugs, & any changes to the carbs you have made except mixture & normal tuning.

[jalexquijano]

Thanks so much Mark. Give me a couple of days and ill get back to you with photos of the plugs. The last time the Porsche mechanic adjusted the carbs, he did a good work. However, after a couple of weeks of using the car and getting stuck in traffic, plugs 4 and 5 started to get black.

[Mark Maras]

 If #s 4 & 5 were getting black but not #6, I would suspect there is an electrical problem in 4 & 5. Check the rotor, cap, plug wires & plugs. The front carb. feeds #s 1, 2 & 3. The back carb #s 4, 5 & 6. There is a balance-equalizer tube between them, but it has a limited effect on plug color. 

 There is another possibility however. It could also be that the rear carb is set rich & there is an air leak into the #6 cylinder, maybe an intake manifold leak. That scenario would give you the same symptom. Black #4 & 5 plugs but not #6.

[jalexquijano]

Thanks Mark. I have reschedule the appointment for this Friday and will advise him of this situation. The spark plugs are NGK BP6ES, the Spark Plug wires are new such as the rotor and cap. Do both of the carbs need to have the exact amount of turns on the mixture adjustement knob? ie 4 turns on the front, 4 on the rear?

[Jarvo2]

If you can't get your hand on the Z-Therapy DVD, check out YouTube for tuning SU Carbs.  This video is for an MG, but its the same concept for our Datsuns. My guess is that your carbs are not your issue, most likely 40+ year old engine that needs to be fully rebuilt.

 

http://youtu.be/4nvGLgO6pj0?list=PLFD71AE1E7D0C1B1D

 

Edited October 28, 2015 by Jarvo2

[psdenno]

My 72´ 240z is still not getting the correct tune up! I have tried almost everything but its still backfiring at the intake on both carburetors. 

 

Car is still sputtering and backfiring on the intake. I step on the gas, the car is not running smoothly and both carbs exploding on their intake.

 

WHat could be the cause of this intake backfire? Cant believe i cannot manage to tune the carbs properly

 

Looking at what you describe and then referring to page ET-10 of the factory manual P/N 99999-20015 (undated)  for the L24 Series engine Allow me to quote word for word what the factory has to say on the topic (Some earlier posts have mentioned some of the same points). 

 

Troubles

ABNORMAL COMBUSTION (backfire, after fire, run-on, etc.)

 

Possible Causes

Improper ignition timing

Improper heat range of spark plugs

Damaged carburetor or manifold gasket (back fire or after fire)

Defective carburetor jet

Improper function of the float

Uneven idling

 

Corrective Action

Adjust the ignition timing.

Use specified spark plugs.

Replace damaged carb or manifold gasket.

Dismantle the carburetor and check it.

Adjust the float level and check the needle valve.

Adjust idle.

 

I've owned and used this factory manual since 1971 and it has always served the purpose.  If you haven't downloaded a copy of the factory manuals, do so.  If you can't find the factory recommended carb adjustment procedures, let me know and I can walk you through the process.

 

Perhaps, more importantly, find a mechanic familiar with SU carbs who can make the adjustments and has a flow meter and other diagnostic tools.  Did the car ever run right after you had the carbs rebuilt and installed?

 

Dennis

[jalexquijano]

 If #s 4 & 5 were getting black but not #6, I would suspect there is an electrical problem in 4 & 5. Check the rotor, cap, plug wires & plugs. The front carb. feeds #s 1, 2 & 3. The back carb #s 4, 5 & 6. There is a balance-equalizer tube between them, but it has a limited effect on plug color. 

 There is another possibility however. It could also be that the rear carb is set rich & there is an air leak into the #6 cylinder, maybe an intake manifold leak. That scenario would give you the same symptom. Black #4 & 5 plugs but not #6.

 

Okay Mark.

 

The car was not receiving enough fuel. The mechanic replaced the Airtex 8016S electric fuel pump (2.5 to 4 psi) with a 8012S model (5 to 9 PSI). He did commented that cylinder 4 is not working properly as it is burning too black and that maybe he will need to adjust the valves. I just hope the valve is not burned or warped! Anyway i bought a set of NGK BP6ES spark plugs and this was the result after i raced the car for 5 to 6 minutes this Sunday. Spark plug 4 is still getting dark while 5 and 6 seem okay.

 

Do you recommend purchasing a better electric pump? I think this one is okay for the moment.

[Stanley]

Here's a link from September: http://www.classiczcars.com/topic/52317-1972-fsm/page-1

The very first file uploaded is section EF which has the tune-up procedure for SU's starting on page EF-21. The whole section is about 50 pages so easy to put it on a thumb drive and take it to the printer. Then you can have it sitting on the fender while you do the tune-up.

By the way, the FSM lists maximum fuel pressure to be about 4.3 psi. If it's much over that, the pressure could overpower the needle valves, causing fuel to overflow from the float bowl vents. It might be a good idea to monitor the vents (by running hoses from the vents to a vented catch can, or some other method) so you'll know if they overflow.

I don't think it's a fuel pressure problem, unless it tested low (under 3.2 psi). Don't want to argue with a Porsche mechanic, though. He should know. My stock mechanical fuel pump puts out 3.9 to 4.0 psi (with no return line, and filter by tank only).

Edited November 1, 2015 by Stanley

[Mark Maras]

 I think that much fuel pressure, without a regulator, will soon over power the needles & seats filling the bowls to overflow. Keep in mind, those float bowls sit over the hot exhaust manifold. 4 psi should be the MAXIMUM pressure. 

 IMO There is no maybe in getting #4 and all the others adjusted & burning properly. Honestly I would change direction temporarily and leave the carbs alone. Fix everything else and make sure it is right. These are things that you can do and should learn if you are able, however

your mechanic should have taken care of that before tackling perfectly good carbs. I question whether your mech. is ignorant, incompetent, or milking you for money. Your choice.IMO, As long as he doesn't quite fix it, he gets paid for yet another attempt. The only way you will know is to try to find a better mechanic or learn to do it yourself.

 I doubt very much if you need a new pump. You could test it yourself & know for sure.

[jalexquijano]

I have been monitoring any over spill and have not noticed any spill yet. My mechanic says the valves on cylinder 4 need adjustment. Lets see if once they are adjusted, i can manage to tune it correctly and all plugs burn equal.

[siteunseen]

If you have the stock air filter you'll smell overflow before you see it, the overflow hoses go to the filter housing.  As long as you aren't smelling a strong fuel smell you're probably okay.

[Chickenman]

OP. Have you done a compression test on this motor yet. May have been in one of your other threads, but there's too many to search.

 

If #4 is low it could be your issue. Post up compression figures please.