Thanks. With those numbers, I would simply switch to 89 or 91 octane and likely your detonation will go away. 160 psi with 87 octane could be on the edge, IIRC. Gas is cheap compared to every other cost involved in a car...

You could also retard your cam by 3-4 deg and that will drop the numbers and make 87 OK.

Remind me, do you have an AFR gauge? That would unambiguously tell you if you are rich or lean. If you already have enough fuel to not ping, then changing jets will not help you. Of course, the timing curve has a big effect too. Not sure I have seen a mapped curve for your dist. Sorry, I am not up for wading through your previous posts at the moment.

Lou

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Home of Slant6-powered fun machines since 1988

I don't have an AFR gauge, but I could probably get one, and a glass plug, cheaper than taking the car to the shop (although I do have a good shop that works on race cars as well as daily drivers).

I reported my original distributor curve in this post. However, since then I've replaced the lighter of the two springs with a heavier one and I haven't checked the new curve.

Of note, I had approximately the same detonation problem before rebuilding the engine. At that time it was all stock. Premium fuel didn't help, replacing the exhaust system from manifold flange back didn't help, replacing the fuel filter didn't help, and the plugs were not running hot or lean at the time (see picture in my previous thread). I assumed the detonation was because of carbon buildup, but I can rule that out now.

Thanks for the link to the curve and measurements. Curve looks good on quick inspection. Personally, I would get an AFR gauge (weld bung about 12-24" downstream of the exit of the exhaust manifold), and before then or concurrently I would top the tank off with 91 gas after you are below 1/3 tank. You want to keep learning here with careful measurements, which it seems you have been doing. You will save yourself a ton of time with the gauge.

It is possible your carb has some metering problem that is not giving enough fuel at WOT or near WOT, since that is where the detonation is occurring. A 64 jet should be more than sufficient. At some point of "overjetting" the passages in the carb will no longer support more fuel and any jet increases won't do a thing. It will be very hard to correct WOT fuel ratio with plug readings unless you do dragstrip tests or long WOT pulls followed by immediate shutdown and plug pulling/reading. The AFR gauge gets beyond all (primitive) plug reading and shenanigans. AEM makes excellent units like this one: summitracing.com/parts/avm-30-0300?seid=srese1&gclid=EAIaIQobChMIt9CZzbOY6wIV04NaBR2B4w0EEAQYASABEgLeqfD_BwE

Lou

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Home of Slant6-powered fun machines since 1988

So that suggests to me that there was ether a carburetor metering problem, or an ignition problem?

I have no experience to back this up, but thinking out loud, a 64 jet ought to show up on the plugs as approaching rich.. so, somehow the fuel quantity has not increased with the jetting changes?

Today I went for a test drive with the EGR disconnected (I pulled the hose from the EGR valve and plugged the end of the hose). The detonation was not noticeably different. I also checked EGR function by manually applying vacuum to the valve while idling, and the idle dropped quite as it should. I'm pretty sure the EGR works properly and is not the cause of detonation.
I did the on-car float adjustment and I had to raise the float about 1/16 inch. I doubt the float level is the cause of detonation. I also noted that I have the 3 sets of air bleed holes, and the highest one is about 24-25/64" from the "roof" of the bowl cover (pic below). I'm not sure how to check the fuel level when the tube is out of the bowl.

Unfortunately since I put the carb back together the engine bogs down when I open the throttle. I assume that means the accelerator cup didn't go properly back into its cylinder.

You can verify proper accelerator pump function by looking down the carb throat and manually opening the throttle, you should see a stream of fuel being expressed into the Venturi. IF it does so and seems to be a strong stream of fuel, no dribbles from the beginning to end. If that is the case, then the stumble you are experiencing may be a rich stumble rather than a lean bog..?

Also have you checked that your air bleeds are open, & unobstructed by grit-gum-sediment? I see what appears to be some corrosion like build up on the accelerator pump rod. Not saying that to give you grief in any way, but if the carb has sat a while (as most of our carbs have) it may have some of that white stuff in the air bleed passages... Or in the idle circuit which does have an effect on overall air fuel ratio.

My background thinking here is If your idle circuit is lean,or obstructed it will effect the AFR. As I remember the idle circuit is generally 1-25 MPH, the transition circuit is 25-40mph and over 40 is the main system comping into play.. Generally speaking, and when cruising. At the drag race I am sure it is quite different.

The accelerator pump cup was torn. Fortunately I had a spare, and it now works as it should. However, raising the float level did not improve the detonation problem at WOT.

While I had the bowl cover off to replace the cup, I held the bleed tube up to the light and could see clearly through the bleed holes. Of course that doesn't rule out a blockage in the tube itself or in the port that allows air into the tube. Anyway, I believe my mixture is too lean, and I believe if the air bleed tube were clogged I'd be running rich?

Thanks for mentioning the idle circuit. I was not aware that the idle circuit continues to provide fuel at higher speed. But surely at WOT it's not a significant source of fuel? I say that because I only have detonation at WOT.

I took a look at a couple of the spark plugs again. The electrodes are now a bit more of a light tan.

Raising the float level can bring the main fuel enrichment system in earlier, whereas a low fuel level it will take longer for the main jet to draw fuel. I believe it can skew the overall AFR a bit towards the rich side.

As for the Air Bleeds, I can only guess as to how the carb would behave if they were plugged. I suspect the fuel would not emulsify very well, and have a rich mixture when it did flow. For the moment we will look elsewhere..

The idle circuit if you only have detonation at WOT should not be an issue. I will have to re-read what I read about the idle circuit being involved more than I realized as well, to be more clear, or to correct myself if I am mistaken.. but basically yes, the main system should be doing most of the work when the throttle blade is vertical.

Plugs look better.. tho I would be inclined to put a bigger jet in and see what happens.. both to see the plugs and to see how the engine responds...I personally would like to get the porcelain to become as tan as your ground strap appears... That may end up being a bit too rich, but better rich and happy than lean and put a hole in the piston.