Edelbrock carb tuning question

[ian_uk1975]

I just started reading the manual and, in the part where it talks about setting idle mixture, it talks about looking for a 20rpm difference in the idle speed. How the hell are you supposed to know if your rpm has changed by 20rpm?! That sort of difference would not be audible (50-100rpm is probably the audible limit) and cammed American V8s aren't exactly the smoothest idling engines around, so the idle has a tendency to jump around (certainly +/- 20rpm) under normal operation anyway.

Cheers,
Ian.

[chedz]

On a rover you can hear the engine note change very slightly as it approaches too lean, its certainly audible. Perhaps give it a go and you might pick up the sound that the engine is starting to become less smooth and the revs do drop a little, more of a note change than a measurable drop in revs. Then turn the screws out a little (manual specifies) until it runs smooth again.

That's what old sidecardood showed me to do anyway.

G

[ianhunter]

Ian

I was having exactly the same problem, so I bought a mulitmeter with a digital tach readout at Stoneleigh.

Connected it up and the figures on the lcd screen jump about all over the place - can't use it at all. Is it becasue I have breakerless ignition?

Help!

Ian

[Sidecarbod]

I'd try open the screws up to about 2 +1/2 turns then slowly turn one of them in at some point the revs will just start to drop, (the engine may speed up a bit before slowing down if 2+1/2 is too rich) you can just about hear the engine slowing down. I'd then open the screw about 1/8th of a turn. You then need to do the same with the other screw. (Get the engine up to temp first)

After that I'd kill the engine and screw each one fully home counting the number of turns. Write this number down as you will forget! (then re-set them).

The above method seems to get the A/F ratio to about 13.5:1 which is a CO reading of about 2.5%.

My 4.6 runs at 2 turns to get 13.5:1. I think Grahams 3.5 will need to be around 1+7/8 or 1+3/4 turns to get the same reading. His car is on 2 turns at the mo (unless he's changed it and it was running at 12.5:1) You can easily see the difference 1/8th of a turn makes using a lambda probe.

The "proper" method would be by CO readings but the above method is not bad.

I'm in the process of setting my 4.6 lump up using a lambda probe and the settngs that appear to be near the mark are way off the standard settings. (Secondaries are now at 104, up from 94. They might even have to go to 107!). I've ended up "thinning" the needles that I'm running by 1 thou on the cruise step).

Its a long process and has cost about 150 quid in fuel! (Power runs down my test road are burning it at a huge rate! ).

Regards,

Pete

[plod]

Hi Ian,

Have you thought about taking it onto a rolling road for such fine tunin tasks?

We had a chap based down here that a group of us went to. It was a sort of special arrangement but cost £30 or so for 30 mins of his time

[ian_uk1975]

Originally Posted by plod

Hi Ian,

Have you thought about taking it onto a rolling road for such fine tunin tasks?

We had a chap based down here that a group of us went to. It was a sort of special arrangement but cost £30 or so for 30 mins of his time

Hi Paul,

If I could save the hassle and get it done professionally for £30, I'd jump at it! Unfortunaltey, finding anyone that's familiar with 4-barrel Yank carbs is difficult enough. Finding someone that IS familiar and doesn't charge an arm and a leg is even more difficult.

Cheers,
Ian.

[craigh]

familiarise yourself with the jet changes etc and when you take it you can do the adjustments yourself,thats what i intend doing if the rolling road operator near me is clueless

[TINKA]

Originally Posted by craigh

familiarise yourself with the jet changes etc and when you take it you can do the adjustments yourself,thats what i intend doing if the rolling road operator near me is clueless

I had my original Edelbrock set up on a rolling road and he informed me you more or less never need to change the jets, it is normally only the metering rods that need to be changed. If he is any good you will know more or less exactly which to change to as long as you use the scale in the manual supplied with the carburettor.