volition

I've come to a roadblock that I just can't seem to get past. The thing is running great and screams in the high rpms finally but..

It's knocking at around 1500 rpm when warm under light throttle. When its cold it doesn't happen either. The Vacuum advance is hooked up and it the pretty "conservative" VC1838 which adds 7 degrees (14 degrees crank) at 12" of vacuum. Timing is 14 base 35ish at 2900rpm. I've tried changing the springs to the super-stiff GM ones that barely allow any advance at all and the knock is still there. The only way it wont ping seems to be to back the initial way down to 8 or 10 but then the motor runs like a dog at higher RPM.

I'm currently using 1427 ( 098 ) jets and 1455 ( 073 x 042 ) rods. Springs are stock. The engine itself is a crate goodwrench 350 with a Performer 3701 (no EGR hooked up)

At this point I'm kind of at a crossroads whether this last problem is timing or a carb problem. As I understand it the knock is happening right where the carb should be transitioning from idle circuits to full primary.

TheSkunkWorks

If your vac adv is plumbed to manifold, have you tried moving it to port? This may or may not help, and I'm certain the manifold vacuum crowd will have a fit for my suggesting it, but worth checking out all the same IMHO. And, what's it do with no vac adv? There are a lot of variables here, but your mechanical advance might still be coming in too quickly for the octane/CR/cam combination.

In any event, it's best to address any ignition system issues before touching the carb. My $.02. I'm sure you'll get plenty of advice on this topic if you, well, stay tuned. (Sorry about the pun.)

volition

Well I decided I had better double check and get a new timing gun. Lord are things different.

Using this gun (much better craftsman dial back) my HEI is only giving me 16 degrees advance. It also seems to be giving full advance at only 1300 RPM. I think this may be the issue. Stay tuned while I play with some spring combos :P

7T1vette

Yes, I believe that might be the issue. Also, hooking the vacuum advance can up to "ported" vacuum won't help your problem at all. The only thing ported vacuum does is eliminate any vacuum advance at idle [only]. Once the throttle plates are cracked at all, that port goes to manifold vacuum...same as the manifold fitting.

ajrothm

I would try setting the initial to whatever it wants, 12-16*, whatever, then set the centrifugal to give you another 20* or so and just disconnect the vacuum advance all together and see if it still pings....remember when the vacuum advance is hooked up, you could easily be cruising at 40-50* advance at low load....stock low compression motors handle this fine....modded motors may or may not.

volition

Ok I think I got the timing down. Heavier springs it's now at 20* initial 36 advance and 50 cruise with vac. Not perfect but I can now really see where the problem is.

I'm feeling pretty certain at this point that the problem between 1500 and 2000 pm is a transition problem in the carb. Not only is there a ping but when I sit in the car and increase the speed slowly I can feel that the engine is running horrid in that RPM range and smooths out to smooth as silk above 2000 (this is all with the car in neutral in the driveway). Whats more is it's not a multiple cylinder ping it sounds like only 1 or 2 is doing it which is making me start to think the transition from idle to primary on one side is off or not working properly

OMF

Just to confirm its not a timing issue try disconnecting the vacuum advance (plugging the ports) and see if you still have that rough transition in the 1500-2000 RPM range.

bobbarry

An adjustable vacuum-advance can be adjusted not only for the total amount, but also for the rate of advance, which is usually the problem with part-throttle pinging.

volition

hah I didn't think of that. It's definitley still there with no vac advance.

Avette4me

May be way off, but you're not running 87 octane, are you?

bobbarry

First thing to do is make sure your carb is not jetted too lean for your combination at THAT rpm and throttle-opening. Those jets and rods you listed are for an Edelbrock Performer (Carter AFB) carb?

If it's pinging at CONSTANT part-throttle cruising at that rpm, first make sure you don't have the idle set so high that your throttle-plate is nearly closed so you're still on the transition-slot in those conditions and not fully pulling from the main jets.

If it runs well at high-rpm and your plugs don't show you're running lean there, then you've got your main-jet and thin-end of the metering rods in the ballpark.

I'd try richening up the part-throttle jetting with a rod that is thinner on the fat-end so that you richen up the mixture when the rod is down in the jet. I'd go with the rod that is as rich as possible at part-throttle (thinnest one on the fat end), then step back back up in thickness until the ping returns and then go back to the largest one that didn't ping.

You could also richen up the mixture by going with a larger jet, but that's also messing with your WOT mixture, which you shouldn't need to do if it's running as you describe. You can do it for tuning-purposes only to dial in the part-throttle mix, but then you've got to calculate out what the proper metering-rod size would be to replicate that jet-opening with your original jets.

Now if this pinging is not at cruising but at transitional rpms, so it's there as you're gently accelerating through that rpm but not when you're cruising at that rpm, then look at changing the power-valve spring so that it's raising the rod out of the jet sooner.

The Edelbrock tuning kit for these carbs gives you an assortment of metering rods, jets and springs for dialing this in.

OMF

I'm not sure what year or model of carb your using, but if its a Qjet, and has an APT (adjustable part throttle) try raising this a turn or two (unscrewing) so the "thick" part of the rod isn't as far into the main jet, this will allow slightly more fuel into the carb at transition

volition

That sounds about right. Going to try going one or two steps up on the springs.

I should have just tossed this thing and bought a new carb instead of putting all this work into a carb that sat for too long

volition

Good some confirmation! It seems to be what I need to do. I'm using a Edelbrock so it's a matter of changing the rod springs to change the part throttle setting like that.

Thanks