Engine Surge While Stopping

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Zelus

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Just got the 340 back from the shop. Had them re-install the intake manifold after I botched the job the first time, after replacing the rocker arms.

On the way home, I noticed that while coming to a stop, (while in neutral, foot on the brake only) the engine revs a good 500 RPM toward the last moments of stopping (highest deceleration), then calms back down to normal idle speed once stopped.

I should note I have MANUAL brakes.

Do I need to hunt a vacuum leak?

Thanks.
 
It's a manual. Sorry.

Floats should be fine, it's a fairly young carb.
But speaking of floats, could it be the idle mixture?
 
Zelus said:
It's a manual. Sorry.

Floats should be fine, it's a fairly young carb.
But speaking of floats, could it be the idle mixture?
Click to expand...

a bad mixture wouldnt change the rpms when coming to a stop. that is very weird, a new one on me. i would guess it is something in the carb. maybe the choke is doing something weird. i wouldnt think a vacuum leak would do that either. the only vacuum lines you should have is the PCV and the distributor advance
 
Hmmm... I'll go check that.

There IS a possibility something in the carb stuck, now that I think about it. She'd been sitting for 10 weeks until now...

It's also all fresh filters. And she's just had double oil changes to get the coolant out of the oil system.
 
Throttle linkage is fine.

Also, to answer your earlier statement: the engine has only a PCV (w/breather), and vacuum advance for vacuum lines. There is also a capped off power brake port (pft! why is that there?!).
 
Does it rev smoothly as you slowly open the throttle in neutral? My 69 Dart slant used to "run away" when it was idling lean (I think) because it wanted more timing advance and the centrifugal advance did that and made a "positive feedback loop". The Holley Commander 950 manual (search) explains that you must design a reverse slope in the timing advance curve to realize a stable idle. You can't do that, but similar should be designed into your mechanical fuel and spark controls, so some part might not be working perfectly (carb or distributor weights?).
 
I actually ended up increasing the advance from where they had it. Not sure if it was 20 degrees.

Regardless, I turned the idle screw back to lower the high-idle. Nothing. At full closed I'm idling at 900 RPM. Normally, it would be 0 RPM. Guessing a vacuum leak.

It SEEMS like spraying carb cleaner on the rear intake-runners causes it to idle faster, but just a little. Without a 2nd man to watch the tach, I'm going by ear.

Is any of this making sense?
 
Zelus said:
Throttle linkage is fine.

Also, to answer your earlier statement: the engine has only a PCV (w/breather), and vacuum advance for vacuum lines. There is also a capped off power brake port (pft! why is that there?!).
Click to expand...


for power brakes at some point. i would check that for leaks at the intake. do you have a power brake booster? i guess that wouldnt matter anyway.
are you sure the choke isnt just flapping around or something?
im just throwing thoughts out there lol
 
Thanks for the thoughts.

Her "happy place" seems to be adv. 30 degrees, at idle. That seems very high. I'm using this camshaft:
http://www.lunatipower.com/Product.aspx?id=1545&gid=287

What's my risk of tightening down the intake bolts? They ran a coolant pressure test and a head leak test. Both passed. It's an aluminum intake. One of the rear bolts tightened easily. The other's aren't. It's hot right now.

Thoughts?
 
Zelus said:
Thanks for the thoughts.

Her "happy place" seems to be adv. 30 degrees, at idle. That seems very high. I'm using this camshaft:
http://www.lunatipower.com/Product.aspx?id=1545&gid=287

What's my risk of tightening down the intake bolts? They ran a coolant pressure test and a head leak test. Both passed. It's an aluminum intake. One of the rear bolts tightened easily. The other's aren't. It's hot right now.

Thoughts?
Click to expand...

you can go through them again with a torque wrench at 35 pounds. but that wouldnt explain why its doing it only when you are slowing down, unless you are loosing vacuum at the distributer and the advance is sticking. unhook and plug the vacuum advance port and the distributor and take it for a spin
 
73AbodEE said:
you can go through them again with a torque wrench at 35 pounds. but that wouldnt explain why its doing it only when you are slowing down, unless you are loosing vacuum at the distributer and the advance is sticking. unhook and plug the vacuum advance port and the distributor and take it for a spin
Click to expand...

Been watching for a bit and i think your on to something with the vac advance. Also what carb are you running and did they have the distributor out during the intake replacement?
 
Carb: 600 CFM Edelbrock
Yes, distributor was out during the intake replacement. I came to check on things and installed the distributor myself while I was there.

Another discover: My secondaries aren't moving (while the engine is off). I can manually move the throttle from closed to TOW and nothing happens. If I stab them with a screwdriver on the backside, they open. If I stab them frontside, nothing. WTF is this? Aren't they supposed to move without vacuum?
 
maybe the secondary butterflies are getting hung up on the carb gasket or the linkage is binding up
 
Zelus said:
Carb: 600 CFM Edelbrock
Yes, distributor was out during the intake replacement. I came to check on things and installed the distributor myself while I was there.

Another discover: My secondaries aren't moving (while the engine is off). I can manually move the throttle from closed to TOW and nothing happens. If I stab them with a screwdriver on the backside, they open. If I stab them frontside, nothing. WTF is this? Aren't they supposed to move without vacuum?
Click to expand...

they should operate with the throttle. i have the same exact carb and mine open all the way at WOT.
 
73AbodEE said:
maybe the secondary butterflies are getting hung up on the carb gasket or the linkage is binding up
Click to expand...

I was thinking carb gasket also albeit the primary side. If they were hanging and the advance hose is ported it would be sending vac to the can. Might loosen all 4 mounting screws and disconnect the linkage to see if you have smooth movement then retighten.
 
Reason i was asking about the dizzy is if it's liking 30* without the vac hooked up something ain't right.
 
OldmanRick said:
I was thinking carb gasket also albeit the primary side. If they were hanging and the advance hose is ported it would be sending vac to the can. Might loosen all 4 mounting screws and disconnect the linkage to see if you have smooth movement then retighten.
Click to expand...

i would agree on popping off the carb and making sure everything is functioning properly . also make sure the advance is working smoothly
 
A stuck float would make it bog and die (drown it out).

Spray wd-40 around the intake ports at the head and the carb base and see if the engine speed changes. If it does, you have a vacuum leak there.

Just because you paid a shop to do it, don't mean it was done right...
 
krazykuda: I agree.

Upon further investigation: my Edelbrock 1406 Carb is definately not closing it's secondaries. They are staying open about 30-45 degrees.

What could be causing this?

The gas and air going into it has been very clean since it was installed in 2008-2009 (replaced the fuel tank once; air filter, and fuel filter, often).
 
Zelus said:
krazykuda: I agree.

Upon further investigation: my Edelbrock 1406 Carb is definately not closing it's secondaries. They are staying open about 30-45 degrees.

What could be causing this?

The gas and air going into it has been very clean since it was installed in 2008-2009 (replaced the fuel tank once; air filter, and fuel filter, often).
Click to expand...

Could be either the linkage hanging up or the carb gasket was put on wrong. I agree go ahead and pull the carb off and make sure everything is working the way it should, if it still sticks with the carb off you know the problem lies within the carb.
 
The carb gasket hasn't changed. The carb wasn't removed from the intake manifold during the transition. It would be hard for it to catch on that.

The 'closing action' of the secondaries seems to be locked up solid. I can't force it closed, however they are definately open, and unresponsive to any and all mechanical movement of the throttle.

I don't remember these being vacuum operated secondaries. I seem to remember them being mostly mechanical. Right now, they don't respond to any throttle input.
 
Alright, so apparently the Edelbrock 1406 has TWO sets of butterflies for the secondary: a lower an an upper. The lower set seems to be closing properly.

Going back to vac leaks. See if I can find anything.
 
i have edelbrock 1406 on my 80 ramcharger and 70 dart, both are mechanical secondaries and open when i move the linkage

agree with other loosen the carb mounting bolts and lift the carb up a lil to see if it frees up, also check the linkages for bent rods etc i had a large buddy lean on my air cleaner and my throttle stuck open when i gave it gas to fire up (pulled pedal up with my foot so no harm done)
 
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