Trouble when cruising

[PierreBelvedere]

Well, discovered this today.
When I hold the foot steady on the gas, it kind of...runs rough?
No hesitation, good idle, pulls great.
Just when I cruise (no matter the speed/rpm), it just runs rough.

Tried to adjust the timing, don´t have a timing light, so just by ear.
Good vacuum (around 14 at idle), and no vacuumleaks to my knowledge.
I suspect the timing, since if I retard it waaaaaay much, it runs smooth.
But the MPG is NOT fun or good.
Also, it has no power.

Well, so I messed with the timing some more.
Now it has WAY better power, and MPG.
But still, runs rough when keeping the foot still.

Any ideas?

 

[6packMIKE]

I kinda had the same problem while crusing.my car was stuttering for lack of a better word. I had to disconnect my vaccuum advance and increase the timing. Maybe u have the same issue. It definitely seems like a timing problem to me.

 

[godfatherofchry]

what car with what motor carb size will help.sounds like a lean prob.is it surging at cruise speed? or just stumbiling (missing).........Artie

 

[PierreBelvedere]

I kinda had the same problem while crusing.my car was stuttering for lack of a better word. I had to disconnect my vaccuum advance and increase the timing. Maybe u have the same issue. It definitely seems like a timing problem to me.

Yeah, sounds like that to me too.
I´ll try that, thanks!

what car with what motor carb size will help.sounds like a lean prob.is it surging at cruise speed? or just stumbiling (missing).........Artie

273 with the following recipe:
Bored over 030
1973 340 Camshaft
Weight adjusted pistons and connecting rods
Ported heads, and machined 0,5mm
Commando intake
600 Edelbrock carb

Just stumbling, at any speed over 20 when holding the foot still.
No stumbling at acceleration or deceleration.

 

[Secret Chimp]

I have/had the exact same problem in my car. I have a 318 with a 625 cfm Carter AVS (basis of an Eddy Thunder AVS), Summit 6900 camshaft. At lower speeds, say 30 mph or lower, I got that weird stuttery feeling if I was just barely touching the gas. More gas or simply cruising faster and the problem went away.

I got rid of most of the problem by adding some timing, but the basis of the problem according to many is that the idle circuits on these carbs can't get enough gas out when they're being used for low-speed cruise, so what you're getting is a constant lean stumble. If you go faster or increase the throttle opening you're transferring most of the load over to the primary circuit so the problem goes away. I've cracked open my idle mixture screws a half turn beyond where I get the most vacuum at idle, but it stops helping beyond that.

Supposedly a solution is to take the venturis out and drill out the idle restrictors by .002 or some very small number with a numbered drill bit to richen the circuit. If you do a Google search for "edelbrock light throttle stumble" or "carter light throttle stumble" you'll find some threads on other forums that discuss this solution. For right now I'm just living with it since I've improved it to the point where it isn't much of a bother.

 

[zdillinger]

Had a similar problem. A new accelerator pump fixed me right up. No problems with this after that new part.

 

[Secret Chimp]

Had a similar problem. A new accelerator pump fixed me right up. No problems with this after that new part.

That'll fix a studder/stumble off the line or when quickly changing speeds, sure, but the accelerator pump isn't doing anything at all on a steady-state cruise.

 

[zdillinger]

That'll fix a studder/stumble off the line or when quickly changing speeds, sure, but the accelerator pump isn't doing anything at all on a steady-state cruise.

True enough. All I know is that I had a part-throttle stumble and I posted a thread about it. Several guys said accel pump. I changed it, and its gone. Why, I don't know, but it fixed it.

 

[PierreBelvedere]

So, I disconnected the Vacuum Advance and plugged the line.
Gave it a little more timing, and the stumbling went away.
However, now it´s harder to start, and seems like it lacks in power?

Thinking that the Vacuum Advance...uhm...thingie on the distrubitor might be the problem?

 

[67 coronet]

is your vacuum advance hooked to ported vacuum?(runs backwards from normal vacuum)if so,make sure the vac advance woks smoothly inside the distrib.just operate it by had and make sure it is not hanging up.you need a vacuum hand pump to check the dashpot itself.

 

[Secret Chimp]

So, I disconnected the Vacuum Advance and plugged the line.
Gave it a little more timing, and the stumbling went away.
However, now it´s harder to start, and seems like it lacks in power?

Thinking that the Vacuum Advance...uhm...thingie on the distrubitor might be the problem?

Did you do all of those things at once before you drove it again?

It sounds like it may just be that the vacuum advance pot is coming in too soon.
If you disconnect the line from the pot and stick a small hex wrench in there (carefully, don't poke through the diaphragm) you may find there's an adjuster it fits into. If you have one, try turning it counterclockwise a half-turn at a time and see if the stumble feeling gets better.

 

[PierreBelvedere]

is your vacuum advance hooked to ported vacuum?(runs backwards from normal vacuum)if so,make sure the vac advance woks smoothly inside the distrib.just operate it by had and make sure it is not hanging up.you need a vacuum hand pump to check the dashpot itself.

Yepp, passenger side, ported vacuum.
Seems like everything is running smoothly, I´ll have to check again.

Did you do all of those things at once before you drove it again?

It sounds like it may just be that the vacuum advance pot is coming in too soon.
If you disconnect the line from the pot and stick a small hex wrench in there (carefully, don't poke through the diaphragm) you may find there's an adjuster it fits into. If you have one, try turning it counterclockwise a half-turn at a time and see if the stumble feeling gets better.

No, first I disconnected the vacuum line, it ran better.
Then I messed with the timing, it got WAY better.
Oh, sweet, I´ll check that tonight or tomorrow.

Thanks guys, this is why I love this message board.