Team Camaro Tech banner

1 - 17 of 17 Posts

·
Registered
Joined
·
209 Posts
Discussion Starter #1
Hey guys -

I have a really strange engine surge and I would very much appreciate some help diagnosing. The surge occurs at lower RPM with the car in gear and it feels like I don't know how to drive a car with a manual trans. Sort of that bucking movement if you can imagine that. The problem is relatively new and I can't think of any major changes to the engine that may have caused it. Anyway, here are a few specs on the car/motor:

540 big block w Dart Heads
Relatively mild cam
Quick Fuel 850 carb
HEI distributor

I have checked for vacuum leaks and can't find any, mixture and float levels seem fine on the carb, the vacuum advance is working, plugs look ok and basic other tests. What else would you recommend I try? I'm a little stumped. Thanks all.
 

·
Registered
Joined
·
1,194 Posts
Have you tried adjusting the throttle linkage with a stiffer spring? Might make the gas pedal a little less sensitive and smooth out the action at lower speeds, reducing driver induced oscillation.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #3
Have you tried adjusting the throttle linkage with a stiffer spring? Might make the gas pedal a little less sensitive and smooth out the action at lower speeds, reducing driver induced oscillation.
I like the thought but I currently have two throttle return springs on the car. I run them on all of my cars as a safety feature in case one breaks. I will check to see if they have weakened though.
 

·
Registered
Joined
·
684 Posts
Possibly too lean or too much timing.
 

·
Registered
Joined
·
8,349 Posts
An HEI adds in a whole lot of vac advance. At light throttle if the advance is actually pulling in AND you're getting an accel shot it can rich misfire. To test for this, try pulling the hose off the vac can and plugging it and see if the situation is better or worse.
 

·
Registered
Joined
·
13,337 Posts
And check the sec butterfly height... ie mixture and idle speed screws in spec....1 1/2 turns +/- 1/2 turn

Combine that with the excessive VA degrees in HEIs as Steiner describes above...and check the total( intial +cent) is in the 3000 /3200 rpms 32 to 34 degs......you may also have springs too light...or way too heavy

Generally what u describe is a couple of issues combined not a single issue.. so remove one may 'fix' but also means something else will still not be right.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #7
Thanks guys. I will try this out in the morning and report back. I appreciate it.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #8
Alright guys... update. I have made a few more adjustments and the problem is the same/worse.

I adjusted the float levels. The primaries were a bit low but nothing major. I emailed back and forth with Quik fuel and Holley who felt the the primary jet sizes were too small and suggested going up two sizes. Those have now gone from 76 to 78 (84 secondaries). I am also starting to have a lower idle after driving for a while. When the car is cold, I have it set at about 900-950 but after driving half an hour or so, it falls to 650. That said, I am mostly concerned with the surge. Any other ideas guys??
 

·
Registered
Joined
·
13,337 Posts
Set the secondaries, / timing as previously mentioned?
Graphed the cent curve? are the cent springs too light and bouncing... is the VA vacuum range incorrect?
And if so then the messing with jets etc will most properly have been a total waste time...

Got to get/ KNOW the basics are right BEFORE messing around with jets etc.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #10
Timing hasn't change but I have checked it. VA is working properly. Springs also feel right.

Jet sizing wasn't my first thought but that's what the carb guys recommended so I figured I would try it since it's an easy change.
 

·
Registered
Joined
·
2,777 Posts
I had an annoying surge in a 540 (Brodix block) at lower cruising speeds. I thought she was lean,..changed primary jets, looked for vacuum leaks, etc. No help.

Tried (temporarily) a different carb known to run great on a 496. No help.

I adjusted the timing up and down. No help. Changed the advance springs in the distributor. No help.

Changed the distributor from an MSD 8360 to a Petronix equivalent. Problem completely solved.

FWIW

PS,..not sure what block you're running, but in my Brodix aluminum block, I had to get distributors with a slip collar so it could properly engage the cam gear.
 

·
Registered
Joined
·
13,337 Posts
VA is working properly. Springs also feel right.
So u have put a timing light on the engine, noted the idle advance is stable (no VA attached) and noted at what deg and rpms its all in at... If its below around 2600/2700, its a good possibility one of the springs is too light ....anyway such a fast advance is more suitable for very high compression on high octane fuels only

And u have established at what vaccuum the VA starts and finished at? and this doesnt relate to the engine vaccuums at which the surge takes place
Right?
u have done all that and u KNOW its "RIGHT"
 

·
Registered
Joined
·
209 Posts
Discussion Starter #13
I had an annoying surge in a 540 (Brodix block) at lower cruising speeds. I thought she was lean,..changed primary jets, looked for vacuum leaks, etc. No help.

Tried (temporarily) a different carb known to run great on a 496. No help.

I adjusted the timing up and down. No help. Changed the advance springs in the distributor. No help.

Changed the distributor from an MSD 8360 to a Petronix equivalent. Problem completely solved.

FWIW

PS,..not sure what block you're running, but in my Brodix aluminum block, I had to get distributors with a slip collar so it could properly engage the cam gear.
Thanks for the info. I will look into this. Do you have any idea what the difference is between MSD and Pertronix?
 

·
Registered
Joined
·
209 Posts
Discussion Starter #14
So u have put a timing light on the engine, noted the idle advance is stable (no VA attached) and noted at what deg and rpms its all in at... If its below around 2600/2700, its a good possibility one of the springs is too light ....anyway such a fast advance is more suitable for very high compression on high octane fuels only

And u have established at what vaccuum the VA starts and finished at? and this doesnt relate to the engine vaccuums at which the surge takes place
Right?
u have done all that and u KNOW its "RIGHT"
yes sir - i've enlisted the help of my father who owned an auto repair shop for over 40 years and this is one of the first things we tested.
 

·
Registered
Joined
·
209 Posts
Discussion Starter #16
Do you have an engine mount loose or broken and the engine twisting up tugging on the throttle lever?
Nope. Motor mounts are good and the throttle linkage inspected ok. It didn't hang up anywhere when we examined it.
 
1 - 17 of 17 Posts
Top