Weak Valve Springs

[Sprocket]

G'day folks

 

My poor old 260Z isn't going very well. Actually, it's never gone very well since I inherited it.  It's basically a stock L26 engine. It starts and runs OK in rev ranges below about 5000 rpm, but then it hits a wall. It doesn't even get to where the peak power should be. I took it in to Andrew at Pro Automotive in Moonah (Tasmania) for a dyno tune. His diagnosis is weak valve springs. The SU carbies are a bit out of whack but there's not much point going any further with carb tuning while the valve springs are knackered.  At least the ignition timing is spot on!

 

So, I need to buy and fit a set of springs. There are plenty available on the interweb, from people such as Zcarsource, Zcardepot, Andy's Autosport, TheZstore, etc. but they're all in USA. I couldn't find any in Australia. Are there any local suppliers for Z valve springs, and any suppliers or makes of spring to avoid?

 

Cheers,

Cam

Tasmania

[dat2kman]

Got a set of H/D used valve springs here, can pack n post, $140 total

Plenty of life left in them!

Also sets of stock springs, still at original free heights

( weak/tired springs will be shorter heights)

[Sprocket]

Not keen on buying used valve springs. I've already got a set of those in the engine!

 

I don't have a lot of experience with cars, but I do heaps of work on motorcycles. I'm aware that some aftermarket motorcycle valve springs are rubbish and perform like they're made from coat-hanger wire. I just wanted to avoid the same trap with car part suppliers.

 

Free length is one indicator. Others are seat pressure, pressure at full lift, installed height, coil bind clearance, etc. I haven't lifted the cam cover yet so I dunno how my springs measure up to any of these specs.

 

I was hoping to source a set of good springs before I start spannering.  That way I can take the old springs out and fit the new ones straight away, so the car isn't out of action for too long (planning to do it with the head in place). The car is a bit of a problem when it doesn't go because it blocks access to my bikes in the garage.

[Sprocket]

G'day. It's been a while since I looked in here, and I found this old thread I started a few years ago and thought an update might be of interest to members. 

Long story short - I bought new valve springs and fitted them without removing the head. To hold the valves on their seats while removing the springs, I fed a length of soft 1/4" rope down the spark plug hole and turned the crank until the piston jammed the rope against the top of the combustion chamber.

However, the new valve springs made absolutely no difference to the engine's performance. The "expert" engine tuner I took it to had made a wrong diagnosis. Now at a loss as to what the problem could be, I figured I'd leave it as a job for another day. That was a couple of years ago. In the meantime, the car was still driveable, just not to its full potential.

Short story long - April 2020. It's the middle of the Coronavirus lockdown, so I’m looking for stuff to do.  The “job for another day” became the job for today. Time to do a bit more head scratching.

The head scratching goes like this:  It can only be a couple of things - fuel mixture leaning out (not getting enough fuel) or a breakdown of the spark at high revs. 

Fuel starvation could be poor pump capacity, but it has a mechanical pump as well as an electrical one.  The car belonged to my late brother Bob. Reading through his notes, he was under the impression that the electrical pump was a bit under-rated, so he fitted a mechanical pump as well.  With both pumps operating, it seems unlikely to be a starvation issue.  Sure enough, with the fuel line disconnected, it seems to gush petrol at a prodigious rate. 

I also took the carbs apart, cleaned, checked, re-balanced. All good there. Fitted new air filters. None of which made any difference.  

Cross fuel starvation off the list.

So, looking at the ignition system, it has an aftermarket electronic doohickey in the distributor that replaces the original points system. That’s a good thing I guess, and it seems to work because the car runs and the auto advance works (I checked it with a strobe).  Plugs are all new so no problem there either.  Coil looks new, so I guess someone has added it at some stage.  I checked the coil's primary resistance and it reads 3Ω on the meter.  That seems about average for a motorcycle (I'm a motorcycle fettler, not a car guy). Typically a motorcycle coil only fires once per revolution (usual bike setup is an ignition trigger on the crank), but what about a 6 cylinder car where it's firing 3 times as often? Checking the workshop manual specs, it says 0.5Ω.  Aha, we could be onto something.  I need to try a coil with the correct resistance, so I bought a 0.5Ω coil and installed it. The car would start but not run. Hmm ... more head scratching.

It would continue to run if I connected a jumper wire from the battery to the coil positive.  That got me thinking about the voltage getting to the coil via the ballast resistor. The ballast resistor measured 1.5Ω. That’s abought right for the 3Ω coil because the coil gets about 8V.  (Ohm's Law for series resistances - system voltage = 12V, of which 4V is lost through the resistor and 8V through the coil).

But putting a 0.5Ω coil in with the same 1.5Ω ballast resistor meant that most of the voltage drop was across the resistor rather than the coil. The coil was getting heaps of current due to the low overall resistance, but the voltage drop now becomes 9V across the resistor and only 3V across the coil, which is nowhere near enough to allow it to make a spark. It would start OK because the coil gets full battery voltage when the starter is cranking. As soon as the key turns back to the run position after starting, the ballast resistor comes into circuit, effectively shutting the coil down.

That's when the light bulb moment happened. The car originally had points/condenser setup and the electronic trigger is an aftermarket fitment, so the original coil and resistor spec may now be irrelevant. 

I rummaged through Bob’s paperwork and found a receipt dated 3/2/2015 from Performance Ignition Services in Nunawading for the supply and installation of a Pertronix ignitor kit. A bit of internet googling on Pertronix ignitors found the installation instructions for that unit. It revealed that the 3Ω coil is in fact correct, but it said the ballast resistor should be removed or bypassed.  Pennies are starting to drop.  Even with the 3Ω coil, it appears that 8V it was getting via the ballast resistor was insufficient to allow the coil to reach saturation when it was trying to cycle faster than 15,000Hz (3 cycles per revolution @ 5,000 rpm). The ballast resistor could be all that’s standing in the way of full ignition performance.  So I bypassed the resistor with a short piece of wire.  

I took the car for a test drive and it revs right through to the red line.  Problem solved … finally! 

All that farting around changing the bloody valve springs, and all it needed was a little bit of wire to short out the ballast resistor!  

You’d think a company called Performance Ignition Services would have got an ignition installation right, especially when the correct installation procedure was in the instructions that came with the Pertronix unit.  According to their invoice, along with the Pertronix unit and the new coil, they also supplied and fitted the problem ballast resistor (presumably to replace the original lower value resistor?). They also supplied and fitted a new condenser - another item that's not required with an electronic trigger. 

The take home lesson is never trust self-proclaimed experts. The professional ignition installer got it wrong. Then the engine tuner I took it to mis-diagnosed the problem that was created by the first guy, and led me up the garden path in the direction of valve springs.

Edited July 3, 2021 by Sprocket

[datsunrally]

That was an interesting read, thanks for posting. It's so satisfying to finally solve a puzzle like this, hope you can now enjoy many happy drives. 

One of the (few) upsides of lockdowns / COVID etc, is the amount of jobs completed on cars while tucked away at home.

Jeff