Happy Holidays to all C3'ers

Are the idle mixture settings done with the engine running and in neutral??
JJ78

 

I'm not saying I'm right, but that's how I did mine. It worked out well, but it did take a bit of time to get the engine running smoothly and both sides sounding equal! Car runs nicely and sails through the emissions tests! I don't use any fancy gear - I was brought up tuning SU carbs by ear with a "listening tube"!😱

 

I also use a vacuum gauge. Jerry

 

+ 1 on the vacuum gauge

First pull out a few spark plugs and take photos of the electrode ends. After a couple hundred miles, look at the same plugs and compare appearances. What you don't want to see is added sooty buildup which would indicate it's running rich. Running lean would likely show itself as pinging.

 

Jerry tx for the tip
I have a vac guage but not sure how to use it or what to look for?
Perhaps you can share some more tips about it?
JJ78

 

With a warmed engine and the choke off, shut the engine off, connect the gauge. Carefully screw in the mixture screws until they bottom out. Be gentle. Back them out about three quarters each. Start the engine. Back out the screws one at a time until you reach the highest vacuum reading. Do the same for the other screw. Check each side again. That should do it. Jerry

 

Tx so much Jerry and I have one more ?
I have a 78 with the usual Qjet so can you tell where I need to attach the vac gauge line to without causing a vac leak??
JJ78

 

Any port that is manifold vacuum. I use a line off the fitting between the back of the carb and the distributer. Jerry

 

I do one additional step and that is to average the two screws from bottom. Once the highest vac readings are attained, with the engine off, count the number of turns (complete plus fractional) for each and then unscrew them the average for both counts. For example, if one screw is 2.5 turns to bottom and the other is 3, unscrew both to 2.75.

 

There's some interesting info on how to "read" spark plug condition here:

https://fboignition.com/articles/sparkplugreading

And here:
https://fboignition.com/articles/sparkplugchart

 

I've got a couple questions from the first link. What do others think of these 4 areas and what they indicate.




How can the spark be optimally jumping from the ground strap to the electrode from the 90° elbow. Why do we set the gap between the end of the strap and the electrode? Doing this doesn't change the elbow distance much at all.

I agree with this, but this plug doesn't look too bad.

I've long thought that the whiteness of the center porcelain is an indicator of how appropriate the plug's heat range. Never heard that the threads indicate anything in this regard.

Again, not aware the white porcelain indicates timing. A coolant leak if bright white, fuel quality and characteristics if there's red/brown coloration, deposits if too cold.

Any comments on the webpage or my thoughts?

 

It’s been a while since I actually read the webpage i linked to! But having now re-read it following Barkingrats comments, I have a few thoughts.

Re Point 1: It’s not saying that the spark jumps “early”, but that the colour of the electrode changes along its length based on the plugs actual heat (not heat range). So if it is getting too hot, or not hot enough, the colour change is not at the optimal point on the electrode, ie more or less on the apex of the angle.

Point 2: If any of the plugs in our ’78 looked as oily as this one, I think I’d be worried! 😀 A bit too oily for me I think?

Point 3: Back in the late 60s, when I was running my Austin Cooper, I used the info from an old English driver’s handbook (for a Humber Hawk!) as a guide for setting timing and mixture. That gives info on ‘reading the plugs and does in fact include mention of how the “top” of the body (thread ) should look. I’ve always worked on “a light carbon deposit, grey brown in colour”. But I don’t see it as a separate guide from the electrode insulation (porcelain) colour, using the two together as an indicator of timing and mixture settings.

Point 4: Both timing and mixture will have an effect on the plug condition. Over advanced timing will show very similar symptoms as a weak mixture. I work towards a light straw colour for the insulation. Too white is an indication of weak mixture or advanced timing. Advanced timing can of course be “cured” by a cooler plug, depending on how advanced you want to go. Or, retard the timing and/or richen up the mixture. My thoughts are that generally, the mixture in our Corvettes will be OK-ish, if set up well, so then the timing is the item to be adjusted.

I also like to periodically check how the exhaust tail-pipes look inside. I like them to have a light greyish colour, with little or no “soot”. That gives me an idea of the “running” mixture.

I’m not saying I’m right - just how I’ve always interpreted plugs.🙄 If I’m honest, it was probably more critical on the small Cooper engine, which needed to be spot-on to give the best performance and soon showed its displeasure at incorrect settings! The old Corvette just rumbles on…..😄

 

If you still have it, the emissions sticker in the engine compartment along with the factory service manual indicates the mixture screws and idle speed are adjusted in Drive with an automatic. Wheels have to be chocked, parking brake applied, along with remembering to not manually opening the throttle.

 

I'm acquainted with a gent whose business includes tuning true track-only carbureted race engines; as a matter of course, he routinely cuts plugs' entire threaded segment away; so that he closely inspects-reads the entire combustion-side of insulator. His carbs often win meaningful titles.

 

Point 1: Ah, I see now that I was misinterpreting what it was saying. Thanks for pointing that out.
Point 2: I agree the threads are very oily and not acceptable. I believe the article is still talking about the ground strap and it looking overly oily. It only looks to be slightly wet with the strap showing through which is why I thought it wasn't horrible looking.

If I were to "read" this plug, I'd say the heat range of the plug is appropriate given the porcelain color and the fuel mix seems appropriate given the lack of sooty buildup, but the engine has an oil problem given the threads.

As I understand it, the heat range of the plug has nothing to do with spark intensity or heat of combustion; it's all about heat transfer from the plug's exposure to combustion temps away to the head material. Because of this I'm unsure how timing effects can be addressed through the plugs.

Don't mean to hi-jack the thread, but maybe this sideways deeper dive adds to understanding.

 

I'll second the highjack apologies! Just a quick reply - it's bed time here! I think I'm with you. To my mind, if the plug is showing signs of poorly adjusted mixture or timing, I'd sort that out, not change the plugs to suit. But I've read a few articles that include changing to a hotter/cooler plug as one of the options to cure a problem indicated by the plug. Maybe in a non stock engine set-up, or race engine?