Why does my 40 run fine with 15 degrees advance if 7 is spec?? (1 Viewer)

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Jul 25, 2019
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So decided I would "tune" my 40....got a variable advance timing light, found TDC line, painted it and started it to check timing. I set 7 degrees and the line was no where in sight. So I kept dialing it up and when I got to 15 the line was perfectly centered on pointer. Runs & idles OK with just a little pop every now and then in idle. I know, I know don't F with it if it's running OK, if it ain't broke don't fix it, whatever. The question is why does it run, accelerate fine with 15 degrees advance when the spec is 7. Don't really plan on messing with it unless there is some reason I need to. BTW no "ping" on hard acceleration in 4th gear that I can discern. Checked valve lash...seems OK. Haven't messed with carb, although I did notice idle mixture screw cover had been drilled out at some point....although I owned it for 33 years. Just curious...thanks.
 

dmaddox

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Good morning. A few things. First, timing is all relative and is influenced by several variables. Carb settings, rpm, altitude, etc.. Second, very rarely are your run-of-the-mill timing guns accurate, especially the variable sorts with the plastic dial on the back.

If your valves are set correctly, and you don't have any vacuum leaks, and your rpm's are around 650-700, and you've done the lean-drop method on your idle mix, AND you are at sea level or thereabouts.....there is a really good chance your timing gun is shooting at 7-10 degrees BTDC and not 15.

Note that pops out the exhaust is rich at idle, pops up through the carb are lean. (Again, assuming your valves are lashed correctly).

So, if all above is set, and you idle fine - with an occasional pop out the exhaust pipe, verify your idle mix (I would lean drop it, or start with 1/8 turn in and see how it behaves). Or advance it just a bit more, reset idle speed and recheck the behavior.

Also keep in mind the line is indeed TDC, the BB is 7, if I recall. I am at sea level and the BB right on the needle is 7 for me.

Cheers.
 
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Joined
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Sumter, SC
Thanks here's some more info...I'm in SC at 238' so basically sea level, all the smog junk is intact (who knows if it works or not )- except catalytic converter. It hasnt been tuned in over 20 years...no idle adjustment, no timing, no valves, no nothing. It ran great so I left it alone minus oil changes, put in a couple fuel pumps, direct wired idle solenoid, and couple other small things like hoses and belts. I've driven it all over the country NC to TX to AZ to CO to TX to SC...ran good from seal level to 5500'...never had a problem. So I decided I would get tune-up done, but couldn't find a mechanic who knew anything about it...if they can't plug in to computer then young guys don't seem to know the intricacies of tuning old style. Toyota place won't even take in old cars. Anyway I decided to do it myself and after reading info on Mud it didn't seem that hard. So got an Innova timing light with twist dial and checked timing that's where I got 15 degrees advance. ALso checked it on BB and got 8 degrees...makes sense...7 degrees + 8 degrees = 15 I was getting on TDC. I guess light could be off some but find it hard to believe it would be 50 to 100% in error. So what is the best sequence to make sure everything is right? From what dMaddox told me it is a "happy accident" it runs ok if it is really at 15. Basically just want to make sure everything is done right. Or if one of you guys who know this stuff is nearby come on by...steak and beer on me.
 

dmaddox

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For what it's worth, most of my 2F's over the years are happiest when the BB is in the bottom of the window, which consequently is about 14 degrees BTDC.

If the idle is smooth, be happy with it - and then chase down that pop. That is, if the pop is annoying enough to address.
 
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As @dmaddox said, when my '78 FJ40 was carbureted, I set the timing BB to 'just leaving the window of the casting on the passenger's side', which was supposed to be 5-7-ish degrees over stock advance.

At 7000' elevation, it ran just fine. I drove the truck to sea level and as I lost elevation on the trip, I noticed spark knock, and dialed back the timing to stock.

Signs of excessive advance typically include hard starting and spark knock. If your engine can tolerate more advance than normal, it's ok to run it that way. If you run higher octane fuel, you can also run more advance. I doubt your 'popping at idle' is an advance issue.

Steve
 
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Jul 25, 2019
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Sumter, SC
You‘re getting more advanced timing at TDC then on the BB?
No it's the same...I have an adjustable advance if the BB indicates 7 degrees of advance and it takes setting of 8 degrees to center BB for a total of 15 then that should be same as 15 on TDC from what I have read. Thanks for everybody's input...the more I learn the better. But probably should take a look at valve lash a little more often than 20 years :)
 
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Stanwood, WA
I’m not familiar with innova lights, but plenty experience with other knob style, dial back timing lights. One thing with the knob lights is that 0* isn’t always 0*. One nicer one I had, you could twist the knob backward past 0 about 5 degrees and that was the actual 0*.
that said, if it runs fine, no pinging, no hard starts and no run on after you shut it off, run it where it’s happy. My high performance big block Chevy is like 38*.... just keep adding advance until it pings, then back off a little. But that’s a whole different animal.
 

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