stumble

[Ironmike]

OK. The guys here got me straightened out on my timing issues. Now I have a problem that's been there from the day I bought the car. It's a 71 Demon 340, with the 509 lift Purple cam.
I have a "bog" or stumble off idle. Even just sitting in nuetral if I goose it a bit there's a slight hesitation.
I've run into this on a Chevy and thought for sure I needed a bigger squirter(Holley 700DP), so I wasn't worried too much. Went from a 28 all the way to a 33 and it just got worse. The acc pump has the pink cam on it.
Holley tech support says it's NOT a fuel issue.
The car now runs very strong, starts well, idles at 900 RPM.....just fine other than this hesitation.
Anybody out there have any ideas? I'm gettin' sooo frustrated. I love the car, but I've never had so many issues with Chevy's. I'm new to Mopar and NEVER want to go back to GM.
The guy at Holley says "valve lash, distributor, could be anything". Lotta help....
Any advice would be greatly appreciated.

 

[340 8BBL]

try a different accel. pump cam ,maybe the orange one.make sure the pump drive linkage is adjusted correctly.what power valve are you running?

 

[ramcharger]

In neutral if you goose it, do you get a black cloud? Also, make sure that your pump squirts right when the throttle is touched. You may also need more initial timing, how much are you presently running?

 

[Ironmike]

Right now, set at 15 degrees initial @ 900 RPM. Holley tells me there's a 6.5 power valve in there from the factory. It's hard to see by myself, but I don't THINK there's a black cloud after the bog.
Pump is adjusted correctly. What kinda vacuum you fellas figure I should be pulling at idle, with the 508 cam? Forgive my stupidity, but this Purple shaft IS a hydraulic lifter cam, right?
I've only had this car 2 weeks. Nothing but GM before it, so some stuff is completely foreign to me.
After driving this 340 only a little bit, I can really FEEL the potential but it's just killing me that I can't get the thing right........yet.
The guy I bought it from did a beautiful job restoring the car, but he wasn't much of a "gearhead". I felt the stumble when I test drove it and figured no big deal. Now I'm losing sleep over it.

 

[EL5DEMON340]

that cam probably is only pulling around 4" of vacuum i woud go with a p/v around that size or lower, i would also go with even more timing like around 18 you'll have to pear back the total if its going over 35/36. not sure what manifold your running but maybe a 4-hole spacer would help if you can close the hood

 

[EL5DEMON340]

also your primary throttle plates might be open a bit to far to get that big cam to idle. (that is a very big cam for a 340, like 249 @.050) i would try to add more timing like i pointed out in the other post then your idle may raise up to 1,000 or more then turn the idle screw ccw to lower to 800. may also try to open secondary plates a hair, their is a tiny screw under the linkage

 

[340 8BBL]

im running 16 initial,36 total on my 340 with a little smaller cam.my vac is 10"at idle,about 8 in gear.i use 6.5 power valves.i agree with the previous post,more initial timing, your vac in gear and will dictate which power valve to use.is the stumble at part throttle,or when you wing it wide open?

 

[demon seed]

Are you running a single plane or dual plane manifold and yes it's a hydraulic cam.

Terry

 

[Ironmike]

It's a Weiand dual plane. Put the orange cam in along with the original 28 squirter and the bog is almost gone. Almost wouldn't feel it if I wasn't looking for it.
Found out my mechanical advance in the distrib is only giving me about 12 degrees of advance so it's now set at 15 degrees initial and only about 27 total, unless I plug in the vacuum advance. Then I'm at 36 total.
I've NEVER had to run a vac advance on any of my Chevy's(I know).
I'm guessing I need to muck around with the distrib?

Took a nice ride tonight and jumped on it pretty hard a few times. Running pretty good but maybe a bit weak on the bottom end. I'm gonna try more initial, before I mess with the distributor.
Keep the advice coming, if 'ya can. I need it bad!

 

[green1]

put a vacuum gauge on the manifold at idle, you want a powervalve 2-3 steps below that.

 

[wish4hemi]

Timing needs to be 36 without the vacuum advance. Vacuum advance is only for part throttle driveability and fuel mileage. I had a stumble problem on my car and it turned out to be that the idle mixture screws just needed adjusting. Went from coughing and sputtering on takeoff to tires burning all through the gears.

 

[340 8BBL]

with that much cam id really think you need more initial timing,like 17-18.and like mentioned earlier 36 total. you say low end is a little weak,how much stall/gear are you running?that grind has alot of duration at .50 (249) like mentioned in post #6.ive never used that grind before but i dont think it would make good power below 2500-2800 rpms.

 

[demon seed]

It's a Weiand dual plane. Put the orange cam in along with the original 28 squirter and the bog is almost gone. Almost wouldn't feel it if I wasn't looking for it.
Found out my mechanical advance in the distrib is only giving me about 12 degrees of advance so it's now set at 15 degrees initial and only about 27 total, unless I plug in the vacuum advance. Then I'm at 36 total.
I've NEVER had to run a vac advance on any of my Chevy's(I know).
I'm guessing I need to muck around with the distrib?

Took a nice ride tonight and jumped on it pretty hard a few times. Running pretty good but maybe a bit weak on the bottom end. I'm gonna try more initial, before I mess with the distributor.
Keep the advice coming, if 'ya can. I need it bad!

That distributor should give ya 18 deg of centrifugal advance unless someone's been mucking with the stop. Also check that the Centi. advance is not sticking and it's returning to the minumum stop, a little WD-40 maybe what it needs. With the vacuum disconnected and plugged, initial set at 15-17 and 18 of centi. you should be smoking. Also with that cam make sure that you are fully advanced at somewhere between 2200 and 2400 RPM. There are also much better dual plane intakes out there Edelbrock RPM Airgap comes to mind. Make her bark like a dog!!!!!!!!!!!!!!

Terry

 

[Ironmike]

Well. Seems like all you guys are pretty much giving me the same advice. 18 degrees initial with 36 total. Have to pop off the distrib cap and see what the heck is goin' on in there. I'm pretty sure the prev owner installed the stop and heaviest springs. That would explain a lot.
Right now I'm running a 3:23 rear(I know), but not for long. Searching around right now for the best price on a 4:10 center section. BUT...I do want to get the thing running right before I shell out 11 or 1200 bucks for a rear end.
My luck, I'd order one and still not be able to get the darn engine right.
Like I said before it's just KILLING me that this thing ain't quite right. I've never had this much trouble before. I'm sure I will again, someday.
I plan to spend all day Thursday(day off) if I have to, to get it DONE.
Thanks, guys.

 

[demon seed]

This is the first you've mentioned the rear gears and I'll tell right now that cam hates those gears and would be a lot happier with 3.91's or 4.10's. Im running 3.91's with a .509, single plane and a high stall converter. It's all in the combo and if one element is out of wack the whole combination will fall flat on it's face so to speak. I'm also assuming that you're running an 8 3/4" rear right.

Terry

 

[340sFastback]

You'll need more initial timing like these guys say. I'd try 20 degrees intial and see what happens. Just don't wind it up or drive it much until you pull that total back down. Disconnect the vacuum advance until you have it tuned then connect it back up to ported vacuum not manifold vacuum. You will never get rid of the off idle stumble if the primary transfer slots are over exposed. If they are over exposed you need to adjust the secondary throttle plate open some so that the primary throttle plate can be adjusted back down. If the car idles rich and burns your eyes when you stand behind it at idle then most likely the transfer slots are over exposed. You should be able to stand behind the car at idle and be able to breath comfortably.

 

[Ironmike]

Well, you guys helped me finally get the stumble out and running great.
Now set @ 18 degrees initial at around 1000 RPM, with 36 total. all in by 2500 RPM. Had to pull the distrib, take the advance "stop" out.....then experiment with springs. The light gold springs did the trick. No longer running the vac advance. Don't see how that can help.
It's idling at 900 RPM very consistant and really running strong. I DO need to play with the idle air screws a bit to get it perfect. Just not quite right in the transition yet. Even found a local station selling 110 octane racing fuel. Got 10 gallons in the garage, ready to go in as soon as the tank is almost empty.

Thanks to all who helped! Boy what a difference. Even sounds better.

You all were right about one thing though. That 3:23 gear has GOT to go. Not letting that engine really do what it can. Plan to call around tomorrow for prices on a new third member. 4:10's for sure.

 

[MopaR&D]

Well, you guys helped me finally get the stumble out and running great.
Now set @ 18 degrees initial at around 1000 RPM, with 36 total. all in by 2500 RPM. Had to pull the distrib, take the advance "stop" out.....then experiment with springs. The light gold springs did the trick. No longer running the vac advance. Don't see how that can help.
It's idling at 900 RPM very consistant and really running strong. I DO need to play with the idle air screws a bit to get it perfect. Just not quite right in the transition yet. Even found a local station selling 110 octane racing fuel. Got 10 gallons in the garage, ready to go in as soon as the tank is almost empty.

Thanks to all who helped! Boy what a difference. Even sounds better.

You all were right about one thing though. That 3:23 gear has GOT to go. Not letting that engine really do what it can. Plan to call around tomorrow for prices on a new third member. 4:10's for sure.

Don't disconnect the vacuum advance unless this is going to be a strip-only car. The vacuum advance increases gas mileage when cruising by dialing in more advance when the engine is revving but not under much load. You won't lose anything performance-wise by leaving it connected and working correctly. Disconnecting it is just a waste of 110-octane race gas ($$$).