The Stovebolt Site Forums The Parking Lot Tons 'o Fun Detonation on acceleration while cruising

I'm having detonation on acceleration issues with my 1950 3804. It has a 1956 235 bored .040 over freshly broken in rebuild with 2000 miles. I went 2 sizes up on the main jet but there's a lot of carbon in the exhaust and it still detonates. Still have the 5:14 rear. Maybe RPM's too high? Rochester NOS Model B early 235 carb #7002051. I have a Carter 2100 NOS carb and a NOS Stromberg BXVD-3 #380286 I can use if they are better carbs. Any ideas?

My first guess is timing.
Drive it to homecoming in April and we'll get it sorted out.
Sweet has the magic touch and adjustable timing light to make it easy.

Carbon in the exhaust- - - - -too rich. Lean it out at least halfway back to the original mixture. Find a LONG hill where you can run full throttle at normal cruising speed for 1/2 mile or more, without overspeeding the engine and listen for detonation- - - -"spark knock"- - - -whatever term you choose to use. It sounds like BB's rattling around in a tin can. Retard the timing a degree or two and repeat the uphill pull. Keep doing this until the rattle goes away, or at least can be stopped by easing up on the throttle position a little. That's what the "octane selector" was originally designed to do- - - -allow a driver to tweak the timing a little to compensate for differences in altitude and/or differences in the grade of gasoline from tank to tank.
Jerry

Need a bit more info. This assumes that you have set your base timing to factory specs.

What did you have done with the heads?

Did they deck the block?

Do you know what your compression ratio is?

Do you have a temp gauge and where is it running?

Where is your base ignition timing set?

If the heads were milled and/or the block was decked, then it raised the compression. If you're above 9.5:1 you'll need to run mid grade or premium gasoline.

You might want to check the amount of mechanical advance in the distributor. If it advances too fast too far, it can give you problems.

Base timing at 0 degrees TDC

848 head with hardened valve seats for unleaded

don't know compression ratio or if head was milled or block decked, shop owner retired so no way to find out

temp gauge running at 180, 4 psi radiator cap, factory optional 5 blade heavy duty fan

running ethanol free 92 octane fuel

IIRC the distributor is a NOS heavy duty truck option but I'll check part number

The detonation happens between cruising and opening up to WOT, mainly just as I'm pressing the pedal down. Maybe accelerator pump adjustment?

Hopefully I can make it to Homecoming. I'm planning on it

Retard the total timing two degrees and you'll cure the problem. I'd change the centrifugal advance setting, but I've been tinkering with distributors for over 45 years and I know how to do it. Telling you how on the forum would be somewhere between difficult and impossible.
Jerry

What Jerry said... Hey Jerry, just have him take a little off them weights with the bench grinder! LOL!

Actually, it involves welding up the slots the weights move in a little, and re-curving the centrifugal advance- - - -used to do it on the round track cars all the time, but I had a Sun distributor tester and a delay-flash timing light.
Jerry

Sounds like the vacuum advance is advancing too much. Try running it with the vacuum adv. tube unhooked. If that is the case you can probably limit it with a machine screw in the slot. If you have the vacuum advance on manifold vacuum you could change it to ported vacuum.