annoying detonation - 4ac

**RobertoX** · 19-08-2006, 03:59 PM

For some unbeknownst reason my 4ac has this very annoying detonation rattle whenever the idle ignition timing is set to any more advance than 0deg. It occours between 3500-4500rpm under load and goes away as the revs pick up. with the timing set at 0 it pretty much goes away and it feels like any other 4ac

Just asking around a few people the following possible causes have come up

bad petrol: occours even with 98 (ultimate etc)

cambelt skipped a tooth: advanced it forward 1 and backwards 1 and it runs like a bucket of shite for both (as I thought it might)

rpm advance in distributor broken and coming on too early (ie broken a spring) : replaced distributor, problem remains, old one appears to be ok when I pulled it appart

so thats where I got up to this morning, does anybody have any ideas? Would the next best direction be to check the fuel side of things? Rich? Lean? I hate carbs
The plugs seem to be ok being a light tan sort of colour, the same colour that i have got from previous engines I think...

any help would be appreciated

Rob

**oldcorollas** · 19-08-2006, 04:57 PM

check the advance weight posts are not worn. i have seen ones that are half worn thru, and the weights were hitting the outide of the housing... and also caused significant pinging..

is your TDC mark actually TDC?

**jonra23** · 19-08-2006, 04:59 PM

Harmonic balancer slipped?

Turn engine over until just befroe coming up on timing marks, take number one spark plug out, stick a small screwdriver down spark plug hole as vertical as you can get, slowly wind over until screwdriver stops moving upward, mark balancer, keep turning over until screwdriver starts to move down, mark harmonic balancer, TDC is half way between those 2 marks.

Maybe to lean, too rich will not lead to detonation as burns slower.

regards
jon

**RobertoX** · 19-08-2006, 05:19 PM

check the advance weight posts are not worn. i have seen ones that are half worn thru, and the weights were hitting the outide of the housing... and also caused significant pinging..

they seem fine altough I haven't pulled the weights out, just the part that the points sit on. Also the fact that it happens exactly the same with 2 different distributors indicates that it isn't this. (I know though... stranger things have happend

)

is your TDC mark actually TDC

yup...
...confirmed by doing this:

Turn engine over until just befroe coming up on timing marks, take number one spark plug out, stick a small screwdriver down spark plug hole as vertical as you can get, slowly wind over until screwdriver stops moving upward, mark balancer, keep turning over until screwdriver starts to move down, mark harmonic balancer, TDC is half way between those 2 marks.

**jonra23** · 19-08-2006, 05:23 PM

Only under accelration or any time between 3 1/2 and 4 1/2?
Throttle pump in carby would be my next suggestion, with ENGINE OFF take air cleaner off look down top of carby and see if you are getting a good strong flow of fuel out of thottle squiters when pumping throttle.

regards
jon

**RobertoX** · 19-08-2006, 05:29 PM

only under acceleration yeah, and it gets really bad going uphill and that sort of thing...

I'll check out the accel pump

**RobertoX** · 19-08-2006, 06:11 PM

looks like it works fine, a steady stream of fuel goes in when you pump the throttle...

**oldcorollas** · 20-08-2006, 04:09 AM

that it happens most at 3500-4500 is expected if there is too much advance, mechanically, as that is the kind of nose of the advance curve (ie, all in).. and you would expect it to ping less up higher...

either too much advance (is your timing light lazy? remote possibility but...) how old is engine? eating oil? carbon caked on pistons/head? then carb..... might have to bite the bullet and clean it up a bit.... or just check the main jets are not blocked in any way....

it's either going to be too much advance.... or too much advance for the cylinder conditions/ mixture (but is not nirmal conditions)

**kingmick** · 20-08-2006, 10:58 AM

Prob an idea to have a look at the carbies as well. It might need a rebuild, if it hasnt been touched for years.

**RobertoX** · 20-08-2006, 12:23 PM

that it happens most at 3500-4500 is expected if there is too much advance, mechanically, as that is the kind of nose of the advance curve (ie, all in).. and you would expect it to ping less up higher...

Yeah, that is why I swapped out the dizzy, also when you say "advance weight posts" does this refer to the small posts that the springs are attached to? These seem to be fine.

The carb recently had some slight gunk in it that caused the choke to jam a bit but this has since been cleaned up... the jets may suffer the same thing I guess. Also, because the choke was stuck there may be excessive carbon build up in the cylinders, however the engine does not run-on at all which would probably be expected from this.

The general condition of the engine is not too hot, it eats oil (possibly from poor compression as there is a slight amount being blown into the airbox?).

I might go nuts with some carb cleaner and see if that helps and maybe even try a coke bottle oil catch can

I don't want to spend any money on this, as when uni finishes this year it's ge time again.

**oldcorollas** · 20-08-2006, 06:11 PM

the posts i mean are the ones the advance weights actually pivot on... they wear on the inner side, allowing the weights to pivot further out.. giving you more advance at all times...
then again, this should show up when you rev it with timing light anyway..

oh... stick on timing light, hold it at 4000rpm and see what the timing is? at least then you can rule out extra timing as cause...