pinging suspect new manifold

[Gray-man]

Hi new to this & need help,fitted a Holley intake manifold to my 74 vette,alloy low rise no part No. visable may be underneath, car pings badly now,timing is spot on,balencer is good,replaced accel dizzy with GM HEI, pinged as soon as i fitted manifold,could the wrong one or a badly fitted one be the cause,would appreciate help i'm stumped

[Cameron 77C3]

G'day  Gray-man, welcome ;42

What type of manifold, ie dual plane, single plane, etc, etc.?  Part number would be very handy.
What type of carburettor and size,
What have you set the timing to and how did you set it?
What fuel are you using?
What type of engine is it (ie S/B, B/B standard, performance and so on)?
Did you fit dissy and manifold at the same time or had you been running the dissy OK before manifold change?
Last but not least, why did you change the manifold?

Cam

[gtc]

One of the causes of pinging is incorrect (too much) spark advance. Although you say that the timing is 'spot on', how do you know that the spark advance curve is correct for the new intake+carbie combination?

[TPI-383]

Has the GM HEI dizzy been regraphed to suit your engine combo or is it standard. If it is standard it could be pulling too much advance too early. Why did you remove the Accel dizzy,Was it a problem? Most aftermarket dizzy's have adjustable advance and can be tuned to almost any engine combo.I would be checking this first. You could try disconnecting your vac. advance from the dizzy and blanking the line to the carb. Take it for a drive and see if the prob is the same or improves.When does it ping? On most of the engines I have built I have had the dizzy regraphed and do not use vac. advance so timing is more consistant.I personally have not had much luck with vac. advance on performance engines.Where are you located,maybe I can recommend a place to help with regraphing dizzy.
Cam, wasn't there a post on how dizzy advance works on this site before.
Tim

[Cameron 77C3]

Originally posted by TPI-383
...........Cam, wasn't there a post on how dizzy advance works on this site before.
Tim

Yep

There is this thread http://www.nswcorvettes.com.au/forum/viewthread.php?tid=6499

And this thread http://www.nswcorvettes.com.au/forum/viewthread.php?tid=6435

And this thread http://www.nswcorvettes.com.au/forum/viewthread.php?tid=6500

[MADLT1]

if it was running OK before I would tip your out a tooth on the distributer when you wacked it back in. I know you say its bang on but I have never trusted timing marks (been brung undone with sliped or poorly remaufactured ballancers. make soure you are at actual TDC not jsut the ballancer and check were your at, then go form there.

[MADLT1]

I doubt the manifold would be the course of the problem, one thing to try though would be to check the mixture on the carby if he's positive the ignition timing is correct, one of the idle circuits may not be working.

may also be an air leak, has the engine got a high (ish) idle?

easy way to check if an engine is lean it to squirt a bit of good quality carby cleaner (I use the Nulon one) into the intake, if the revs increase then the engine is on the lean side

another trap it to use a non ported vacuum source for the vac advance, by this I mean it doesn't bleed off, just holds vacuum. try disconnecting it to see if this is a problem.

[MADLT1]

I was also thinking the vacuum is connect to manifold vacuum rather than spark ported vacuum. Manifold vacuum will advance on startup and retard under acceleration (rather than the other way around) causing massive pinging. Other than that I would think the dissy (the only other thing you changed) would be the problem, either wrong advance curve or not enough spark.    Manifold vac will advance a few degress at idle, but during acceleration both port and manifold vac behave the same. The curve is similar if not exactly the same.

If the vac is suspected in this case, well block it off and try it. If no better, then retard the dizzy a bit and give that a go. Only a couple of things it can be...1..to adavanced...2...running to lean under load ( power valve or main jets to small ). I suspect with the old manifold that the vac was not plumbed in correctly. Now it is, the timing needs adjusting.
_________________
See ya on the road or at a run somewhere.

[Gray-man]

Hi again,to answer your questions,Holley dual plane no part No. on top I think it was underneath,Carb is standard Rochester rebuilt by carb specialist,Timed with T/light 8deg then with vacuum guage then at 36deg with adv springs removed but you can advance or retard it all you want it still pings from 2grand on.fuel shell optimax with flash lube,Its the orriginal 350 standard carb rams horns bouble hump heads mild cam, it was fitted with a offenhauser 360 manifold when I got it lot of corrosion at water neck and carb mount had been milled down to fit under bonnet but to far as carb barley covered the hole but it ran fine. I pulled radiator support out to fix usual rust and decided to replace manifold that when it started to ping, had a mechanic look at it to day drove it pulled a plug out and said running way lean and that I have a vacuum leak probably let a gasket slip a little when fitting so I'm gonna take it back off and try again thank for your input Ill let you know how I get on

[MADLT1]

OK keep us posted:b2

[Gray-man]

Hi Guy's Just letting you know,I spoke to Mike George from Long Jetty Auto Engineers who told me my dual plane manifold was not suited to a rochester carb because the divider between the high & low sides dosn't come all the way to the top of the manifold about half an inch lower than the carb bolt on surface & this often will make a motor ping ,he had a second hand Weiand one that he sold me for a very fair price and said he would refund my money if it didn't fix it so I put it on & it works great problem solved I even took it back to show him & he timed it spot on for me so Im back on the road & happy

[77CVT]

Thanks for the update.  Sounds like he looked after you.

Doesn't the stock manifold have the "step" in the divider??