What's going on at Camp David?
might wonder what I've been up to recently, by the way, camp is a place not
an adjective! After all the excitement in the build up to Goodwood
track day, life has been a bit quieter of late.
Engine idle woes
The engine continues to run great off idle but had refused to tick over without a very high idle screw setting (the throttle screw, not mixture). It was much better at a more advanced setting on the distributor but then I got tons of pre-ignition on fairly light acceleration so this was no good.
On the overrun the engine would just keep going (like a stuck throttle) and only settle back to 1500 then 1000 revs after several seconds which made it a pig to drive in traffic, especially as in a queue it would sink lower and lower on the revs trying to stall.
Taking off the air cleaner and with the throttle closed you could still see the petrol dripping out of the main nozzle, delivering too much fuel and making the engine keep running - presumably when it did settle down in revs it then ran too rich and wanted to stall.
On the Edelbrock carb (1405), which incidentally I just noticed has 'Weber' cast into the body, the vertical rods operate in the main jets in two positions. On idle the rod is supposed to be down with the fat section in the jet thus reducing fuel, and on acceleration the rod rises up and the narrow section moves into the jet giving much more fuel delivery. The rod is held up by a step-up spring and operated by the level of vacuum in the intake manifold, which at idle is higher and pulls the rod down against the spring pressure.
Edelbrock do say in the manual that overlapping cams (and open manifolds) reduce manifold vacuum and this results in unnecessarily high idle speed screw setting to achieve idle. Lighter springs may help the idle by allowing less vacuum to pull the rod down. Well I ordered a kit from Summit Racing and three weeks later it arrived (I realised I could have got it from Real Steel but they didn't have some other parts I wanted). Summit should also have been cheaper but when you allow for carriage, tax and a minimum order fee they aren't.
5 minutes later I had tried even the lightest spring but it made no difference :(
I then decided that maybe the carburettor being leant forward by the engine angle and manifold angle was preventing good action on the rods. Particularly as I had noticed that with the engine was fast idling (out of gear) the revs rose considerable (500rpm?) when I braked!
So I cut an old carburettor spacer diagonally and filled the various gaps with Chemical Metal and filed it down flat to create a wedge. (I would have milled it but I didn't have access to my usual milling machine). I bought from Summit a special insulating gasket (about 1/4 thick) which is quite soft and fits between the manifold and the new wedge.
Result, I can see less than ever out of the rear window, and the glass must be just a few mm from the air cleaner now, but the carb is at last horizontal. So did it solve the idle problem - NO! No difference at all.
Vacuum advance Distributor
Now, I had bought a new Mallory Electronic pure mechanical dizzy for the engine rebuild as the considered wisdom is that vacuum advance is for wimps and real men use mechanical distributors ;). I also thought vacuum advance was just a fuel saving device, well we don't need that on a GT40 do we ;)
Thinking on the problem I needed more advance at idle without this affecting the advance under heavy load - right, exactly the reason for the vacuum advance function! Put your foot down and the vacuum disappears. So I had the no-cost solution all along, fit the dual point Mallory that I had on the old engine.
I had thought of this before but was reluctant as it means removing the rear bulkhead (again!) and trying to get the distributor in and timed - well they can be a sod to seat down with the cam gear and oil pump drive shaft - but when you realise the amount of freedom you have in timing by turning the body around the rotor its really quite simple job.
So I fitted it last Monday evening in about 1 hour, in time for the September club night on the Tuesday. I set up the timing at 10 deg BTDC without the vacuum connected and then hooked up the vacuum pipe, it then moved to about 30 BTDC and idled beautifully - I had to turn the idle screw at least one whole turn out.
Took it out for test run, no pinking either - problem solved. I still have to put the springs back but hey it works now.
|Reliability of a Mallory dual point
Another reason for my reluctance to use the old distributor is that its the only thing to let me down on the car so far. A condensor went and within a short distance the points were so badly pitted it wouldn't run at all hardly. Also its all those fiddly point adjustments to make! I just wanted an electronic system. But I figure using the MSD 6AL takes all the work from the points/condensor setup as it is just acting as a low power switch to the box and should last ages in tune. I'll keep you posted!!
One thing, its transformed the enjoyment of using the car. No running on, no throttle off lag and nice tick-over in traffic - and the performance seem unaffected. It just goes like stink with the new engine, spinning up towards 6K without any power drop off.