This Does Not Sound Good ....

[wilf]

........bit gay tho'

[maxGD059]

...no more insane than you marching around the house obsessively measuring the temperature of stuff with your new toy though Wilf

[wilf]

I am better now.

Still "special", but better.

[conrod]

Well, here's the latest.

I've now ruled out the PCV valve and pipe, and the brake servo and pipe. The spray technique has left me none the wiser - same for the modified stethoscope. Aaaarrrrgggghhhhhh!!!!!

However, I conveniently have a very experienced mechanic living opposite me. He had a listen, and concluded pretty quickly that the sound was coming from the carb itself. He then removed the link rod between the secondary and primary throttle spindles, so we could open the secondaries independently at idle. This revealed that as soon as the secondary butterflies open, the noise stops. The conclusion at that stage was therefore that there must be a vacuum leak on the secondary side of the carb.

I took the carb off again, and was able to tweak the screws securing the base plate to the body. But this made no difference. I've tried removing the 7 and 8 plug leads (and indeed all of them subsequently) one by one - but this made no difference to the noise. The next thing I'm going to try is to replace the new carb base gasket with the old one, carefully smeared with Wellseal both sides (which I'm allowing to "go off" before I fit it) to see if that makes any difference - which would then indicate a warped carb base or manifold flange.

Change of plan! I stopped for a tea break, and decided to have one more listen before I removed the carb again. Moving my head rapidly backwards and forwards over the dizzy now points the finger of suspicion in that direction - which was my original "prime suspect" a couple of weeks ago.

So I've whipped off the dizzy, and I now realise that the grease I applied before was to the plate that revolves to allow the vacuum advance to operate. Buried underneath the mechanical advance plate is presumably an upper bearing (it's a Mallory Comp SS Pro BTW). It looks as though to gain access to it I need to remove the roll-pin that secures the worm drive gear at the bottom (marking its position relative to the top end of the shaft first as the worm gear tooth offset is different in the two possible (i.e. 180 degree apart) positions on reassembly. I should then be able to remove the shaft and mech advance plate from the top to gain access to the bearing.

Before I do this though, I'd like to find a way to spin the dizzy shaft with an electric drill to see if the top bearing then starts screeching. Time for another cup of tea .......

As Wilf said, you have to love these cars ........

[wilf]

If only you had a Ford engine......................

[conrod]

Originally Posted by wilf

If only you had a Ford engine......................

............ my problems would be so much worse .......

I attacked a drill bit socket with an angle grinder to cut a slot to engage with the key at the end of the dizzy shaft, then spun the shaft with an electric drill. No noise - nowt.

I then refitted the ignition module plate and noticed that a) the ignition module pickup was very close to the reluctor (less than 10 thou - recommended 10 to 20 thou) and b) that the coil assembly is loose on the pickup - i.e. it can move along it. I'm wondering now whether the coil was so near to the end of the pickup that it was rubbing against the reluctor.

The only way to tell is to put it all back and try it. However, it makes sense to glue the coil firmly in position - but the question is, where? It probably isn't critical, but if anyone has a Mallory dizzy with a breakerless ignition module, could you whip the cap off and let me know how much the pickup shaft protudes from the end of the coil? The possible range is 0 to 2.5mm. I'm going to set it at 1mm for now and see how it goes - it probably makes sense to try this before I attempt to dismantle the shaft to get at the top bearing!

[conrod]

Right - red herring. Dizzy back on and no difference. Sound definitely appears to be coming from the dizzy (that's the rear for me Wilf ) end.

It's back to the air/vacuum leak theory. I think I'm at the stage now where I need to remove the inlet manifold. I wish I'd planned ahead and ordered a new gasket today .....

The curious thing is that my vacuum gauge (which is plumbed into the rear half of the inlet manifold) is showing normal idle vacuum, and the engine idles fine without any hunting - surely a vacuum leak which results in such a loud noise would cause a weak mixture which would be evident at idle?

[TINKA]

Sorry have I missed something here you said you moved the secondary butterflies and the noise goes, so why have you now ruled this out

[conrod]

Originally Posted by TINKA

Sorry have I missed something here you said you moved the secondary butterflies and the noise goes, so why have you now ruled this out

I got sidetracked having convinced myself the noise was coming from the dizzy! False hopes I guess - this is starting to become a pain! But I'm a reformed character now, so back to the logical, methodical approach

The noise goes if I blip the throttle or if I open the secondaries - so opening either the primary or secondary butterflies causes the noise to momentarily stop. I haven't as yet tried fitting the base gasket with the layer of Wellseal both sides - this is probably worth a try before I move on the the inlet manifold. I guess there could also be a leak between the carb throttle baseplate and the main body?

BTW, I removed the top from the "quick change" vac secondary spring housing and placed my mouth over it and sucked (oo'er missus!). The secondary butterfly opened, so the diaphragm is obviously free from tears/leaks.

[conrod]

Right - as I popped the car back in the garage (I've had enough for today!) I watched the vacuum gauge as I revved the engine. As I blip the throttle and the vacuum goes below around 10" Hg the noise stops. It increases in pitch in direct proportion to the rising vacuum when I release the accelerator. The noise increases in pitch to a maximum frequency, which coincides with the maximum vacuum as displayed on the gauge. Tickover vacuum is around 14-16" Hg.

The take-off point for the vacuum gauge is directly from the nearside rear of the inlet manifold.

So this has to be vacuum related. But the vacuum as displayed on the gauge looks normal - if there's a leak (and by the sound of it a large one), why isn't the vacuum reading lower, and why is the tickover still nice and even? In the past when I've forgotten to reattach the vac advance pipe, it's been obvious from an uneven tickover and stalling.

I'm starting to wonder whether rather than there being a leak, something in the vacuum path is vibrating. Could it be an airway in the carb? The carb is a 670 cfm 4150 "Street Avenger" BTW.

But I'm starting to run out of ideas now!