Of Course, New engine problem. Fuel delivery?

Hey again guys,
Okay after driving and tuning and adjusting and driving again I still can't seem to get this right and I'm assuming I'll have to get a set of metering rods, but here goes.... I have the timing curve pretty good now after changing springs and weights to help it advance more gradually through the RPMs instead of advance too quickly as it was before. But since the engine's been in the car it seems to be so powerless unless the secondaries are opened. At all throttle levels before opening the secondaries it seems as if it's struggling.
It has that "worn out old 305" feel to it, but when the gas is depressed enough that the secondaries open it's like unleashing a beast and it throws you back in the seat. When it's hot it hesitates and stumbles a little bit upon immediate firm acceleration. The carb is only about 10 months old and was used for only about 2 of those months. A clean fuel filter was always used and it was never driven without an air filter. It feels to me as if there's a vacuum leak, but have found none... I checked all hoses and vacuum accessories with a vacuum pump. It's a 600 CFM Edelbrock 1406 Carb on a 1969 freshly rebuilt 4 bolt main Camaro 350. Edelbrock offers a kit with 3 different metering rods and jets. Should I try ordering the kit and richening up the fuel mixture ratio?

Hey again guys,
Okay after driving and tuning and adjusting and driving again I still can't seem to get this right and I'm assuming I'll have to get a set of metering rods, but here goes.... I have the timing curve pretty good now after changing springs and weights to help it advance more gradually through the RPMs instead of advance too quickly as it was before. But since the engine's been in the car it seems to be so powerless unless the secondaries are opened. At all throttle levels before opening the secondaries it seems as if it's struggling.
It has that "worn out old 305" feel to it, but when the gas is depressed enough that the secondaries open it's like unleashing a beast and it throws you back in the seat. When it's hot it hesitates and stumbles a little bit upon immediate firm acceleration. The carb is only about 10 months old and was used for only about 2 of those months. A clean fuel filter was always used and it was never driven without an air filter. It feels to me as if there's a vacuum leak, but have found none... I checked all hoses and vacuum accessories with a vacuum pump. It's a 600 CFM Edelbrock 1406 Carb on a 1969 freshly rebuilt 4 bolt main Camaro 350. Edelbrock offers a kit with 3 different metering rods and jets. Should I try ordering the kit and richening up the fuel mixture ratio?

Is your intake spread bore or square bore? If it's spread bore, are you using the proper plate/adapter sandwiched between a pair of gaskets? If not, there's your vacuum leak.

Check the normal stuff - look for cracked hoses, disconnected hoses, bad brake booster, that kind of thing. Also keep in mind that an engine that is not broken in will not have full power output - it takes horsepower and torque to seat in the parts. It will also run hotter than normal and you may smell burned oil in the exhaust; this is normal for an engine that is not yet broken in. I hope you're not using synthetic oil or a synthetic blend of any kind!

Also, check your timing. Since your engine does not have any sort of computer control, and it's not factory, there's a little playing around involved here. First, set your static timing (!with the vacuum hose pulled and plugged!) to about 8 BTC. This of course assumes that the harmonic balancer matches the timing tab on the timing cover. If not, all bets are off. In that case, stop right there, and obtain the proper timing cover and/or harmonic balancer for the engine before proceeding. So, the balancer matches the tab, right? OK. Set your static engine ignition timing to about 8 degrees before top center. Again, do this without any vacuum on the vacuum advance diaphragm, this is critical! Set the timing with the engine running between 600 and 800 RPM, no more. You don't want to be measuring centrifugal advance. You don't want to be measuring any advance. This is the same reason you pull the CCC harness from the distributor on a computer car. Once the static timing is set to 8 degrees BTC, take the car out on a spin and see if that improves performance. Chances are, it will.


If you're checking the timing with the vacuum line hooked up, you're checking it with vacuum advance and in this case, if you set the timing to 8^ BTC, you could actually be running at 4^ ATC! This of course, as you learned from your near nuclear header meltdown, is far from an optimal solution - power wise.

Okay, new, but even worse of a problem... MAJOR DETONATION. It seemed to come on suddenly. At light accelleration, and even gentle cruising I get the piston "ping"... badly. I don't want to ruin the engine. The only way I can keep the engine from pinging is to gun it. Anytime it's past about 1/2 throttle it stops pinging, but it's horrible on gas and beats the motor too much for me to be happy due to the fact that it's still in it's break-in period. I'm running 93 octane, and have the ignition timing retarded to 5 degrees BTDC as opposed to the 8 its supposed to be at. It practically detonates at a loaded idle. Any ideas? I've got others stumped. They all say it's usually visa versa where it detonates at higher throttle levels, or all through the band evenly, but never at low RPMs, and at a light throttle.

After giving some thought, I may have answered my own question here. Higher combustion chamber temperatures can cause pre-ignition as can a hotter running engine. A lean mixture will cause hotter conditions, and detonation. A 2 core radiator with a high compression 350 probably can't cool the engine sufficiently enough even with a 160 degree thermostat (no temp guage yet, all I have to rely on is the idot light). Therefore all three of these things are probably the cause of my issues unless anyone here has a better explanation. Possible lean mixture, possible hot running, high compression engine even with 93 octane and retarded timing can cause pre-ignition and/or detonation. Or am I wrong?

well if it were me, i would try seeing if a friend has a Holley carb to try, at least that may rule out a carb problem. If it still performs the same way then work your way back from the carb. At least that way you will know if its even a carb problem or not, but thats just me.

Lemme know if it helps ya.
Brett B.
81 Pro-Street Bu

It's a good idea, I have a holly sitting on a shelf in my garage wrapped in plastic. I'll bolt it on and see what happens.

I can tell you from personal experience that 305 radiator you have ain't gonna cool that engine.

Part throttle ping is sometimes caused by a vacuum advance that's plugged into the wrong vacuum port -- when it's connected to a port below the throttle blades, it will provide more vacuum advance at idle and part throttle than at WOT (sounds like your issue) -- a port above the throttle blades will lave zero vacuum advance at idle and part throttle, and full vacuum advance at WOT.

Disconnect yours, and see if this helps anything -- if your ping woes go away, use a port above the throttle blades. Also you may want to consider an adjustable vacuum advance can -- most stock cans provide too much advance.

I can tell you from personal experience that 305 radiator you have ain't gonna cool that engine.


What should I go with... a 3 core, a 4 core?

SpinnerCee is correct, so look to that as well. Distributor vacuum should be hooked to a ported vacuum source, not intake manifold vacuum.

I'm running a 4-core in mine, but I also live in the bowels of Hell (aka Metropolitan Phoenix, Arizona).

SpinnerCee is correct, so look to that as well. Distributor vacuum should be hooked to a ported vacuum source, not intake manifold vacuum.

Really? I always thought you wanted full advance at idle, and under load it's retartd the timing when vacuum is lost to keep the engine from pinging. Wow, that would help incredibly if that's the cause of the premature ping! I thought it was caused by high temperatures due to the fact the engine's running hot and that it only does it when the engine is hot.

Now that I think about it, on my non-cc rochester, it has a vacuum port at the back of the 4 bbl on the top near the air filter. Is that what it was for?

But wait, now that I thought about it some more, lets say for instance you're climbing a hill, you give it more gas to get up it. That reduces intake vacuum, but increases carb vacuum, thus advancing the timing more if it was plugged in above the blades. But then advance some more when the RPMs go up because of the weights & springs. Thats a case when you'd want it to retard to avoid engine ping right? Or am I completely wrong?

To answer all of your questions -- it depends.

Normally, on a stock engine, with a stock non-performance cam -- vacuum advance is connected to manifold vacuum -- this helps fuel economy and part throttle response -- with a performance cam, that has a lot of duration, is advanced, and has low idle vacuum -- the vacuum advance kind of detunes the cam. Modified engines can probably do without vacuum advance, or ported vacuum. The additional advance that the vacuum advance gives is likely too much, especially with a high compression ratio, and a hot cam.

Think about this: When you have vacuum advance connected to manifold vacuum, you kind of undermine the fine tuning you did to the mechanical advance curve because they will tend to work opposite each other, giving you little net gain or loss. The vacuum advance will only be zero at or near WOT, so you will have too much advance everywhere else -- that's why retarding the static timing helped a little -- if you have a timing tape, you may be surprised at how much max advance is in the vacuum can -- 15-20 degrees is common! If you add that to what the centrifugal advance is giving, you get big numbers.

Also don't confuse the effect of load on vacuum with WOT -- load will only decrease manifold vacuum slightly -- it should only be zero at WOT -- think about your secondaries (if they're vacuum), you need to do more than just climb a hill or carry extra passengers to make them open. The maximum vacuum found above the throttle blades will ususally be less than and not quite the inverse of the max manifold vacuum because it starts "pressurized" to atmospheric with the throttle blades closed, so it has a different curve with respect to RPM than manifold vacuum. That means that part throttle and most normal driving conditions will probably have more vacuum advance with manifold vacuum than with vacuum above the throttle blades.

There are more things involved here too -- the ignition timing does effect vacuum signals, as does the idle mixture -- one thing to shoot for is max vacuum at idle using the two tunables @ idle -- youl can eventually find a sweet spot where the vacuum signal will respond to mixture and timing changes. This should be your baseline along with the idle speed -- remember, the vacuum advance must be disonnected and plugged when you do this.

* Just wondering: What cam do you have in your engine anyway, and what is the compression ratio, if you know it?

Wow, thanks for the information. In terms of the ignition timing I think the vacuum advance was throwing me off, because what you said makes sense. I want to try running it without the vacuum canister hooked up, but how would the proper way to set the ignition timing be without the vacuum advance? My timing plate only goes to 8 degrees BTDC. Should I just advance it until I get ping then retard it just slightly to get rid of it?
Just wondering: What cam do you have in your engine anyway, and what is the compression ratio, if you know it?
The cam is a mild one... a .3" lift at the lobe. The compression ratio is a little higher though. It's pushing about 10.75:1 compression with the flat topped pistons and 63cc 2.02 heads.

Oh and...
P.S. Another problem I've noticed is a stumble when I rev the engine in park warm or cold doesn't matter... What's that a sign of? When driving it's pretty responsive and there's no sign of hesitation.

I was looking for (advertized, 0.50") duration on the cam. Rocker ratio is 1.5:1? The compression ratio (10.75:1) is high for unleaded pump gas -- you may find that adding octane booster will help.

To properly set the static timing, you must disconnect the vacuum advance can and plug the port you took it from (to prevent a vacuum leak). The static timing setting should also come with an RPM that it should be set at (600-800 RPM?) -- 8 degrees BTDC is about all you should need -- at idle RPM there should be no centrifugal advance in yet. On that note, all the centrifugal advance should be "in" by 3000 or so RPM. After you set the static timing, reconnect the vacuum advance and set the curb idle speed. Here you can tweak the idle mixture for max idle vacuum (you need a gauge for this), and re-adjust the idle speed to spec -- make sure to never adjust the timing with the vacuum advance connected.

For your PS: I dont know if you've ever had a high compression engine with a hot cam, but they are not as docile as a stocker (this includes HiPo stockers) -- the hot cam makes this so, and the high compression does not really like to idle quietly -- expect her to "rump" and "cackle" at idle, but that's what you want right :) When you blip the throttle with no load, it's probably normal for it to gulp that first air (the cam's duration) -- it may also try to twist itself out of the car if you're not careful, LOL :)

Headers can also sound like pinging underhood, if you're not used to it -- they're much noisier than cast iron exhaust manifolds.

Headers can also sound like pinging underhood, if you're not used to it -- they're much noisier than cast iron exhaust manifolds.
I've noticed that, but for the most part I can tall the difference. If it's a light engine ping though, I have trouble knowing if it is a piston ping, or just the header "Ping". But what i was asking was how do I set the static ignition timing if I'm not going to use the vacuum advance at all? Wouldn't I have to advance it further than 8 degrees BTDC?