Mallory Advance Curve Kit

[conrod]

Does anyone have one of these sets of springs (or the remnants of one!) that are now surplus to requirements, or know of a UK source (Mallory kit number 29014, for the YH/YT advance system)?

I'm getting some mild pinking at around 2500 revs, which suggests to me I need a softer spring. It currently has the purple (delayed action) spring plus a grey spring, which according to the advance curve charts on the Mallory website gives 20 degrees of advance at 2500 rpm. If I were to replace the grey spring, I would get 18 degrees at 2500 rpm with a brown spring, or 17 degrees with an orange spring, so experimenting with these looks to be worth a try.

What a job it was getting to the springs - the HEI is a doddle in comparison! Then, when I was putting everything back together, I managed to twist the head off one of the brass retaining screws for the breakerless ignition plate and, in full accordance with Sod's Law, it's a non-metric thread, so I don't have a spare.

As a result, the Cob's now stuck in the garage, and to add to the frustration, it's perfect Cobra driving weather!

Oh well, I might as well spend the rest of the weekend doing all the jobs that have gone onto the back burner since the Dax arrived!

Anyway, if anyone has some spare Mallory springs (and indeed a spare brass screw!), please let me know.

[keane]

Alan, I have a kit in the cupboard for my own Distributor, I wont be needing it for some time, if you are desperate, you are welcome to it and sort out my replacement later, I'm in Poole, Mob 07717201018, call if its of any help. Dave

[TINKA]

Conrod have to disagree with your diagnosis on the pinking issue Pinking is normally due to too much advance in the timing so in fact what you are after is a slightly stronger spring to reduce the timing advance at that RPM and let it come in at slightly high revs.

[conrod]

Tinka,

You're absolutely right - finger trouble on my part. I'm having a bad day! In fact, after the post I finally managed to find my spare HEI dizzy springs and replaced the grey spring in the Mallory with a stronger one from the HEI set. I also managed to find a steel bolt that I then cut down to size as a temporary replacement for the brass one I sheared.

By the time I'd managed to get the dizzy back together, all I could do was to fire the engine up to check it started OK - which it did. So tomorrow will be road test time. I suspect the spring I've inserted is a little too strong though - but if it stops the pinking I'll know I'm on the right track.

Dave - thanks for your offer, and I'll give you a call. At least I can now tell you all the disassembly steps required to get to the springs, which you won't find in the instructions (well, not in the PDF version on their website anyway).

I looked at Summit Racing's site, and the kit was coming out at around $40 plus postage from the US, then duty and VAT on top I guess. I also found a UK supplier, but at £38 plus postage - for nine tiny springs and some plastic strips! And ultimately all I will need is one spring

Ho hum!

Thanks for your replies guys.

[wilf]

Conrod - do you have vacuum advance on your distributor?

If so there's your likely problem, rather than the advance curve.

[conrod]

Wilf,

I do indeed, and I had been keen to retain this for fuel economy reasons. However, I have now disconnected the vacuum advance. Explanation follows!

The carburation and timing had been professionally set up using a RR dyno a while ago. The HEI dizzy was replaced with the Mallory, and the mechanical advance tuned for maximum power delivery. As a result, it has a steep advance curve - giving 38 degrees mechanical advance before 3000 rpm (I haven't measured this exactly yet as I'm still waiting for my timing light to arrive from the States).

It appeared that the vacuum advance (set up to give an additional 8 degrees) was coming in and over-advancing, thus causing the pinking at around 2500 rpm. But I couldn't understand why it was doing this under acceleration, as the inlet manifold vacuum should drop, and the vacuum advance should therefore go to zero.

However, I recently fitted a vacuum gauge, and all was revealed. My engine (a 383 SBC stroker) maintains a very high inlet manifold vacuum. When cruising, the vacuum is between 17 and 22 inches. Even on a steady incline at 2300 rpm (70 mph in 5th) the vacuum is around 12 to 14 inches! So as vacuum advance mechanisms are designed to operate above around 10 degrees, there was my problem - effectively the vacuum advance was operating at all times except WOT.

What I had been doing previously was retarding the mechanical advance by a few degrees to enable the vacuum advance (on near its minimum setting) to operate. But having diagnosed the problem, it occurred to me that by so doing I was sacrificing power under acceleration to achieve fuel economy when cruising - but given the steep mechanical advance curve, I would be pretty near maximum advance at 2500 rpm anyway.

So yesterday I removed the vacuum pipe, plugged the carb port, and then performed an experiment. I filled the tank until I could see fuel in the filler pipe, drove 100 miles cruising in 5th at 70 mph, then refilled to the same point. The result - 22.5 mpg, which is exactly what I had under these conditions with the vacuum advance connected. Ergo - it's redundant!

For information, I have a Crane camshaft with the following spec:

Intake Duration at 050 inch Lift: 228
Exhaust Duration at 050 inch Lift: 228
Duration at .050 inch Lift: 228 in / 228 ex
Advertised Intake Duration: 284
Advertised Exhaust Duration: 284
Advertised Duration: 284 in / 284 ex
Lobe Separation (degrees): 112

The compression ratios are 10.5:1, and the carb is a Holley 670 cfm 4150 with vacuum secondaries.

Anyway, the upshot of all this is that I now have the best of both worlds - a car that goes like a rocket when "playing", but that delivers 22 mpg when cruising on long trips - and no pinking.

Happy days!

I now have a Summit Racing digital CD ignition box (MSD 6AL equivalent - but at only $160) on order to see whether that will further improve performance and/or economy.

So thanks for the posts - and especially to Dave (Keane) for the "loan" of the Mallory advance spring kit. Let me know when you have your replacement and I'll settle up with you!

[keane]

Alan, only too pleased to help, 22.5 mpg! I'll be happy with half of that, enjoy the weather, I'll give you a call once I replace it. Dave

[wilf]

Alan - now that you have established the vacuum conditions for your engine, if you are running a holley, then you might think about checking the power valve vaccuum rating, to ensure you have the best rating foir your particular engine.

[conrod]

Wilf,

Not a bad idea. I think I can rest assured the power valve won't be opening too early!

Ooh - just noticed the "Topic Review" screen when you reply to a post. Is that new? - dead handy Rob!