SU Tuning Trouble

[JonZ]

I'm having problems tuning my SUs. Any suggestions for what to do next?

 

It seems that they are OK or slightly rich at low RPM and low engine loads, but when I go for a spirited drive and check the plugs they look white and very lean. The engine also misses at high RPM - not a backfire out the exhaust or carbies, it just misfires and stutters.

 

Here's a more detailed overview:

Engine - completely standard L24, except for headers and a 2.25" exhaust. It is a little bit tired, but still has compression of 162,171,161,178,171,175

Ignition - timing and advance checked OK as per FSM.

Carbies - 3 screw SU's with standard N27 needles and 20W40 oil in the dampers.

 

How I tuned:

1. I set the float heights to 23mm below the top of the float bowl cover. I did this with a mark on the outside of the bowl and a clear tube from the outlet.

2. I adjusted the mixture nuts so that when the piston is lifted slightly, the RPM rises slightly then returns.

3. I set the idle adjustment screws so that there was even airflow at idle between the carbies.

 

Other things I've checked.

* Put a new kit through the carbies including new gaskets and new jets

* Checked for vacuum leaks and couldn't find anything major. The throttle bushes have a very small leak, but I think that's quite common.

* New high tension ignition leads fitted

 

Next step?

I was thinking of raising the float level to richen up the high RPM high load then adjusting the mixture nuts for lower engine load. Is this reasonable logic, am I missing anything???

 

[dat2kman]

You emptied and refilled the oil on the dashpots/dampeners, with correct qty and correct weight of oil?

[JonZ]

Yes, I believe so.

 

I disassembled and cleaned the carbies when I put the kit in them. I used 20W40 for damping oil as I couldn't find any straight 30Wt. I put slightly too much in and some overflowed into the carby while driving, but I've cleaned this up.

 

I've also noticed that if I rev to high RPM at half or 2/3 throttle, then the miss is worse. I guess the damping oil would have a bigger effect when RPM is changing faster (ie less effect at part throttle). So it is slightly worse when the damping oil is having less effect.

[theremm]

Do the idle adjusting screws to get the air flow right before doing the mixture.

 

I just did mine this afternoon, followed this tutorial. Carbs are perfect now

 

http://www.nicoclub.com/archives/2-write-ups-on-tuning-su-carbs.html

[gav240z]

What ignition do you have? Points / Electronic?

[dat2kman]

Yes, I believe so.

 

I disassembled and cleaned the carbies when I put the kit in them. I used 20W40 for damping oil as I couldn't find any straight 30Wt. I put slightly too much in and some overflowed into the carby while driving, but I've cleaned this up.

 

I've also noticed that if I rev to high RPM at half or 2/3 throttle, then the miss is worse. I guess the damping oil would have a bigger effect when RPM is changing faster (ie less effect at part throttle). So it is slightly worse when the damping oil is having less effect.

Incorrect!

Use 10 weight oil, Singer sewing machine oil is very good, refer to manual for correct cc amount in each.

 

They dont get hot enough for slow sluggish 40 w to heat up to be nice thin 20w!

Dashpots will be sluggish.

[JonZ]

I though if the oil was too heavy, I would create a rich mixture at WOT instead of the lean mixture i'm seeing. If the dashpot is slow to rise, then the venturi sucking the fuel will be stronger according to this. http://www.classiczcars.com/topic/45956-pneumatic-tool-oil-in-sus/page-2.

 

In any case, I think trying some lighter oil is a good idea. A lot of people do say they run sewing machine oil, so I'll give it a go next weekend. I'll let you know how that goes.

 

 

I have 12V points ignition. I did wonder if the miss was due to higher RPM scatter or incorrect dwell. The points look to be quite new and looking at the plugs I can see that they're definitely lean so I thought I'd deal with that first.

[gav240z]

I have 12V points ignition. I did wonder if the miss was due to higher RPM scatter or incorrect dwell. The points look to be quite new and looking at the plugs I can see that they're definitely lean so I thought I'd deal with that first.

 

I had a stutter or mis fire years ago in my 260z when I ran points. I did fix it, but I can't remember for the life of me what I changed to fix it. I might have swapped distributors (had a spare points distributor laying around).

 

I would try removing the distributor and check the shaft for play, check the rotor cap, rotor button and maybe replace the condensor also.

 

Does the vacuum advance work? Most of the diaphragms have shat themselves by now.  You can check by removing the cap and sucking on the vacuum hose, you should see the arm inside the distributor move to advance the assembly inside slightly.

[JonZ]

*Update*

 

Still not fixed... 

 

Use 10 weight oil, Singer sewing machine oil is very good, refer to manual for correct cc amount in each.

Just replaced the damper oil with some pneumatic tool oil that I had sitting around. It looked to be around the same consistency as water, so pretty thin, but no change to the misfire. Was worth a shot and probably can't hurt though. I also double checked what I had put in there and it was 10W30 - not 10W40...

 

Does the vacuum advance work? Most of the diaphragms have shat themselves by now.  You can check by removing the cap and sucking on the vacuum hose, you should see the arm inside the distributor move to advance the assembly inside slightly.

I've given it a suck and can seen the arm move inside the distributor. I think i also checked it when i checked the timing by removing the vac hose and seeing the timing reduce.

 

I would try removing the distributor and check the shaft for play, check the rotor cap, rotor button and maybe replace the condensor also.

I'll have a better look on the weekend. The rotor doesn't seem to have much play though and the cap, rotor and condenser look almost brand new.

 

Any more suggestions before I reluctantly change the float levels?

[neRok]

I had the same drama with my old one. I fixed the carbs up mint, did the timing etc, and it would idle beautiful but wouldn't drive (misfire etc like you mention). I sold it before fixing it, but spoke to the buyer many months later when I saw the car on gumtree again. He said he took it to mechanic and it turned out it was weak spark. I can't remember what he said was the exact problem, but I know I replaced the rotor and cap with a better one I had, and I put a new coil in it, so it must have been something else.

 

Presuming you problem is also spark, I would recommend a new coil as they are cheap. If you have already done the leads, I would cross that off the list. How new are your plugs? I would also look at all the earth wires between the engine and the battery and/or chassis. Ensure the wires and lugs are good condition, not covered in oil, etc. Replacing the wiring to the coil and dizzy may also help, similarly replacing the ballast resistor as it may be lowering the voltage to the coil too much.

[JonZ]

I'm bringing this topic up from the dead because the problem is fixed now. 

 

I replaced the coil but that didn't fix the problem, so i bit the bullet and took it to get the carbs tuned by Steve at Paul Pavlou Motors on a rolling road dyno. The choke cables were sticking a bit and they also reset the float levels and tweaked the ignition timing slightly. Problem fixed!!!!

 

97.6kW at the wheels. Nothing to set the world on fire but better than I was expecting for a stock L24 if I'm honest. Have taken it for a good drive and managed to wind it all the way out a couple of times. No misfire and it pulled really well. Stoked.

 

Would recommend Paul Pavlou motors to anyone looking to get carbs tuned in Adelaide. He did a great job and was a gentleman.