Mopar Muscle Association UK

Thanks for this Dave. I'll try to get this done during next weekend.

There is a company making secondary plates that take normal jets. However these are very expensive.

Dave McB had a secondary metering block conversion for Holley 600, might that be more help than a plate for tuning?

Also, a thought on the idle rpm, is it a stock tach? Remember the erratic readings mine gives me

My 600 Holley has a secondary plate that has jets. I can't remember the price, but it was from Real Steel. ?25?? Less jets.

I didn't have my head on right.

You can of course just fit the secondary metering block from a 4150 series Holley instead. The plates with the jet conversion are for six-packs like mine where you do not have the space for a metering block.

Sorry about that.

Come to think of it I have a secondary metering block you can have if you need it.

May do - I'll see what things look like after the next (hopefully less dramatic) test drive.
It is an aftermarket tach - I'll check that too.

Just a quick update. There was no vacuum on the port at the top of the metering block (hope thats the dizzy port) at idle, but plenty by about 1000 rpm. I found the adjustment for the secondaries (very clever mr holley putting it underneath so you have to take the carb off to adjust it) and screwed it in about 3/4 turn. The idle speed went up, so I adjusted it back down again. The idle mixture screws seem much more responsive now - they never seemed to do much before. The dizzy vacuum doesn't seem any different though...

The rev counter seems ok, within about 100 rpm of the other one I checked. Out of interest, I can adjust the idle speed down to almost nothing (maybe 300 rpm or less) and it still picks up ok.

I'll do the road test asap.

There should be no (or negligable) vacuum at the dizzy port on idle.

That's right. If you let more air in the engine speeds up. But it allows you to close the primary throttle more so that you loose any vacuum at idle found on that dizzy port.

That port 'sees' the manifold vacuum when the primary throttle blades are open slightly. The reason you need to loose that vacuum at idle is because with the throttle blades open slightly the idle mixture circuits do not work very well or at all.

If the mixture screws are working better now chances are the throttle was open just slightly before.

The very least rpm you should set your idle speed to is 600rpm. Even if it pulls down lower in gear (get a higher stall converter for chrisksake!) you should set your idle mixture at that. Either go for around 800-850mv on your sensor or tune for max manifold pressure with your vacuum gauge.

Get those set right first before you do your other tests.

When I say the engine will pick up from 300 rpm, I just meant it will rev in neutral without any problem - not that it will drive away...

I very much doubt that all the cylinders are firing at engine speeds less than 500rpm.

Results at last. Having taken along my trusty assistant to write things down we got a fair number of readings with (a) primaries only, (b) secondaries reconnected as per normal.
The graphic shows the results.
The two upper lines (blue and yellow) show the mixture (higher is richer). Yellow is with secondaries disconnected and it appears to then be running very slightly richer. (I think the curve is pulled down by the rogue point at 3200rpm). In other words reconnecting the secondaries makes it run slightly leaner?
The lower curves (green and purple) show the vacuum readings. The higher (green) curve is with the secondaries disconnected.
I couldn't get the revs any higher as it was rush hour on the A20.
So... I can get Excel to draw graphs, but can anyone more usefully interpret the above?

bump

Why do you keep putting the secondaries back on? Leave then off until you get to the bottom of this.

Well there is no statistical difference between your two lines below 3500rpm. But I would expect that as I doubt the secondaries would open below that rpm.

Note how the purple vacuum line follows the shape of your mixture line.

Why it is getting leaner as rpm rises is the problem. I just don't get it. You should have a straight line.

I seriously suspect it is either a vacuum leak (my best guess) or something to do with your sensor or its placement.

Look at it this way. The ratio between the air going down the carb and the fuel through the jet is FIXED. It cannot get richer or leaner unless fuel or air is added somewhere else.

So I say you have a vacuum leak and air is getting sucked in.