Detonation under hard acceleration

I did that last night and haven't driven it sents, I'll probably do that later on today.

 

Well I've turned it counter clockwise and I'm now at 2 turns and got less detonation but still a little. on my way home today I added some gumout complete fuel system cleaner when I filled it up with 91 octain and on the way home I didn't notice any more detonation but I think it's still a little to early to say for sure... So what exactly does that screw do? does it just affect the vacuum advance level? Also I noticed that it doesn't detonate at full throttle it was only doing it at just below mid to abit above mid... although sents the detonation is decreasing it's becoming abit difficult to differentiate between detonation and the other annoying rattles that I just can't get rid of... anyway. thanks.

 

the vacuum advance works pretty much as its name implies. first it recives what is called ported vacuum. that is where the port for the vacuum signal is located just above the the throttle blades in the carb. that means when the throttle is closed there is no vacuum signal to the vac. advance. as soon as you open the throttle, the vacuum advance is exposed to manifold vacuum. the greater the vacuum signal the more the timeing is advanced. that screw limits the amount of advance that it can give. when you floor it the manifold vacuum drops and the amount of advance decreases. i hope that answers you question as to what exactly the vacuum screw does.

 

the difference between 89 and 91 is at most 15 cents...on a full 16 gallon tank thats only $2.40 max per refill, its more, but on a $40 fill-up its not too bad

 

well it detonated abit again today... granted I was going about 90... it's alot better but it's still there. my current tank of gas has some fuel system cleaner in it so I don't know if I should'nt say much till it's almost gone giving the cleaner some time to work. so load + moderate/heavy acceleration = detonation, although I did floor it in the higher gears and the detonation went away... so I'm woundering if I should go ahead and turn that screw again or just wait for and see if my fuel cleaner fixes the problem. thanks.

 

ok I know it's been awhile but just to be clear the screw only affects MAXIMUM vacuum advance correct? not off of idle. Thank you

 

the screw does have an effect just off idle. as soon as those throttle blades start to open the manifold vacuum is exposed to the vacuum advance. the screw limits the max that the vacuum can advance the timeing. if you floor it there will be no vacuum to advance the timeing. if you lightly tip into the throttle there will be a good amout of vacuum to advance the throttle. i hope that helps.

 

It does, I was hoping it would be just max... I have noticed it takes a little bit more throttle to get the car moving from a stop but I guess thats a resonable sacrifice to prevent detonation... I might have to go another 1/4, 1/2 or so turn to completely elliminate it.

 

thats becase its not giving it as much advance as it used to.

 

shop manual

A shop manual will give you the correct procedure for setting the vacumn advance. You can always drop back the initial 1 or 2* and the add a little back in through the vacumn advance. It really just gradually adds advance to transition from the initial to the point where the centrifical kicks in. Its suppose to save gas too.

 

The vacuum advance reacts to manifold vacuum. Even with ported vacuum the vacuum advance has maximum advance as soon as the throttle opens the port. When you accelerate, the vacuum drops, and then as you cruise it goes back up. You can have it connected to a manifold port or to the ported vacuum signal on the carb - it will make no difference in the operation of the car other then make it easier to get a good idle and higher vacuum advance under deceleration - both of which increase fuel mileage. With ported vacuum the vacuum advance completely disappears as soon as the throttle is closed.
Detonation or spark knock is not associated with vacuum advance because it occurs under load and there is little or no vacuum advance under load. You are going to have to retard your initial timing or modify the distributor for less mechanical advance.

 

when Bryant and I did my distributor's vacuum advance I turned it counterclockwise and the timing actually advanced (not supposed to) but then we decided to turn it 2 more turns in the same direction since we couldn't turn it clockwise any more and the timing advance decreased which fixed it. So I actually turned it counterclockwise 4 turns.