Priya

Depends on what you think I need 52 degrees of all in advance for. If you think I need it for maximum performance from my motor I agree with you. If you think I need it to improve my idle quality which is the only thing I was after anyway then I disagree with you.

If you are so certain centrifugal advance that doesn't even start to come in at idle speed improves idle quality how about you provide me with a step by step rational explanation as to how that works instead of just giving me this "I'm smart, accept my counterintuitive claim without questioning me" line.

I'm not concerned with wringing every possible horsepower out of my stock engine, all I'm after is the idle quality I think my stock motor should provide and I got that by turning out the idle mixture screws 8 turns. I told Lars I was highly confident I had already gotten all the improvement in idle quality timing advance could provide me and he took offense at that but I was right about that whereas he was right that my idle mixture was too lean when I thought it was too rich.

I'm not afraid to give credit where credit is due and admit when I'm wrong - how about you?

And the total timing figures I gave in my comment #39 were not including vacuum advance so there is little difference between what I have and what you're advocating anyway.

Shark Racer

I agree with Carter and Kevin, for what it's worth.

Timing advance isn't just about performance; it's about complete combustion and efficiency. Kevin, Lars and many others on the forum have spent thousands of hours studying engine performance and design. Discounting what they say for a few minutes of experiment is not the wisest path.

A poor idle air fuel mixture is going to be a greater contributor to idle quality vs a few more degrees of advance, that's absolutely true. The difference in quality may not even be perceivable to most; I know when I first got into this hobby I couldn't tell from listening to an engine whether it was lean or rich. Tricks like what Lars told you about CO (it's odorless) and that lean misses tend to contribute more to a stinky exhaust than a slightly rich mixture come with time and experience. It takes a *very* rich mixture for a car to start misfiring, but you can be just a little leaner than stoich and end up with a miss.

As for the 52* advance all-in, it's very good advice. The 36* recommended spec is at WOT. This is because at WOT you are both inducing load and getting the primary metering rods onto the richest part of the needle. These two factors demand less advance. A richer mixture burns more quickly, so you don't need as much advance for complete combustion.

Now when you're cruising down the road at 2000, 2500 RPM you'll need more advance because (hopefully) the engine is commanding a near stoichiometric (if not slightly leaner) A/F ratio. Your load as dropped so you can run more advance and the leaner mixture will also burn more efficiently with it. This will give you better throttle response, better fuel economy and better cooling; all for a few bucks.

Now that you've played with centrifugal advance, there's a decent chance that the vacuum advance unit you're running is providing too much advance. You should check your timing at cruise RPM with vacuum advance connected to make sure you're around that 52* number.

Good luck.

63mako

Timing affects carb operation. If you are all in @ 36 Mech and initial you will increase from the 12 base setting your at now. Increasing base timing will affect your needed carb settings. This is why, after you asked for advise, every knowledgeable poster told you the same thing, get your timing curve set up properly before any carb adjustments. If your mechanical is correctly set up to be "all in" 36 degrees @ 2500 to 3000 RPM it will be adding a few degrees at idle. This combined with higher initial will affect the needed adjustment of the idle screws, the point you are arguing with Lars, me and others responding to your thread trying to help you. If you chose to ignore and argue with this advise why even ask in the first place? Higher idle timing will noticeably improve idle quality, throttle response, fuel mileage, cruise efficiency and run cooler as Shark Racer pointed out if it is all set up right and the tuning is done in the correct order. Little differences and fine tuning are what make the difference between a decent running engine and a great performing engine stock or modified.

Priya

My initial bad idle was due to the base timing being set to 2 BTDC and a centrifugal advance mechanism that was behaving erratically at idle thus preventing me from setting base timing to a steady 12 BTDC. I had already sorted that out by the time I posted this thread and noticed a big improvement in idle quality and was certain there was no further improvement in idle quality to be had from worrying about the mechanical advance in particular and I see no reason to believe I was wrong about that.


I checked the base timing with a rubber band on the centrifugal weights and it was at 12 BTDC @ 900 rpm. I checked the base timing with the lightest spring in and no rubber band and it was still at 12 BTDC @900 rpm although the total timing (not including vacuum advance) was aprox 32 @ 3000 and 34 at 4000. I'm quite content with that timing curve, I am not interested in living on the edge and squeezing every last horsepower out of my stock motor.

So, yes I can believe that in some situations the centrifugal advance can raise timing above base at idle rpm but that is not true in my case. It could well be that if I had adjusted my mechanical advance to give 36 degrees at 2500 rpm that it would have increased the base timing beyond 12 at idle but as I said my goal was not to optimize timing for best performance but just to get a good idle and only 12 at idle was necessary for that.

Lars did help me, he solved my problem by encouraging me to open my idle mixture screws 6-8 turns which sounded pretty out there to me at first, but that resolved the problem without any need to worry further about base timing or mechanical advance than what I had already done.

So, it was well worthwhile for me to ask for advice on this topic even though I didn't agree with all of it - that's why I should have asked in the first place, it got the problem solved and I wouldn't have figured it out on my own without the help I got from forum members.

Contrary to what you seem to think I'm not ungrateful for the help I got, I'm thrilled to death - it was well worthwhile. I just don't agree with everything I was told and no one has explained to me how I'm wrong in a manner I can understand. I'm open to being wrong and admitting it, but I'm not going to just go along with something I don't see based on faith alone.

Haggisbash

Hi Priya, it seems you are placing a lot of emphasis on your base timing whereas the gurus all say to concentrate on total timing (less vacuum advance) then let the base timing fall where it may and the agreed sweet spot amongst the gurus is 36 deg total (vac disconnected)before 3000 rpm. Your current settings are a big improvement over the original ones you had and will give an acceptable idle/low speed running when vac advance is added but you are leaving some performance in the garage if not running 36 deg. Cost you nothing to try 36 deg and you can always retard with a few seconds work if it doesn't work for you.
FWIW my car came 'tarded but not as much as yours and Lars info and settings have it running better than it ever has. Interestingly the local mechanic reckons I should only use 34 deg max but I digress. The main thing is to forget about figures in the likes of Haynes manuals etc they are smog era figures for conforming to smog rules not for performance.

Priya

Nice to hear from you Haggisbash. I understand the approach of concentrating on total timing (less vacuum advance) and letting base timing fall where it may. I agree I'm probably leaving some performance on the table but I'm okay with that. My primary concern was to get the car idling the way I thought it should and if I got any extra performance out of improving the timing curve that was just bonus (not necessary, but bonus).

The other thing is that the numbers I got for my timing curve are estimates based on marks I made at 24 and 36 degrees on the harmonic balancer calculated using a piece of string wrapped around the harmonic balancer. Because its unlikely the string was precisely in the middle of the harmonic balancer at all points it probably was longer than the actual circumference and so my estimates were likely on the low side and the actual centrifugal advance numbers higher than the numbers I gave. So, I'd rather stay below my estimated 36 degrees total before 3000 rpm to be on the safe side.

Haggisbash

Yup, its safer if a little retarded rather than over advanced. If you could borrow a dial back timing light it would only take a few minutes to establish what the exact figure is.

scottyp99

Do you guys realize that you are arguing with a woman? You can't win arguments with women. Just sayin', is all.

Scott

7t2vette

Priya

Of course you can win an argument with me. You just have to be right and able to explain how you're right.

63mako

Did that in post 43. Didn't work.

Priya

I'll have to re-read that post. Don't want to give you short shrift.

Priya

Well, 63mako I re-read your #43 and I didn't dispute anything you said there. I think you're not reading what I wrote.

63mako

Yes Dear.

Priya

63Mako, how about I come pick you up and take you for ice cream? Would you like that?

OMF

We do understand your concern about not wanting to wring all the horsepower out of a stock motor.
What we are trying to advocate to you, is by bringing the total timing up to 36 degrees, you can increase the EFFICIENCY of your engine throughout the operating range from idle on up, including the time when you take your foot off the gas pedal and are just coasting.
This includes getting better gas milage, lower emmisions, and a cooler running engine.
Some other things you get by increasing efficiency are better drivability, and more power.

I hope this helps you understand why we are suggesting to use total timing @ 36 degrees rather than just setting a base timing setting.

Priya

I understand that. I'm just very satisfied with where things are now on my car and not interested in making any more effort to squeeze that last little bit out of it, at least not at this time.

Once again, thanks everyone for their help, I couldn't have done it without you.

71scgc

Why wouldn't you want to wring every last bit of horsepower out of your engine? I thought that was the mission of us all!!

If you checked your total advance and it came up at say 48*, to get to 52* you would need to add 4* to your initial advance. Doing this would affect your timing at idle, perhaps then improving your idle quality.

Come to forum, solicit advice. Proceed to tell posters how they're wrong.
Becoming habitual...

Carter

Priya

Carter, I love those steel cities grey Corvettes, how about posting a picture or two?

TopGunn

I'm going to a friends house this weekend to try helping him with this same rich condition and just ran across this thread.
His is a 69 carb, does anyone know if these carbs tend to run lean as Lars described and would require the same adjustment he recommended?
Obviously we're going to verify timing and advance before fooling with anything.

Thanks