340 hesitation/ slow stumble with light pedal input

[culp71]

I turn to your collective wisdom again....

1971 340 w Edel 1406.
it was running very well other then a timing issue that turned out to be with a distributor where timing would erratically jump way to far advance and gave me a slight random ping, I changed distributor and the ping was instantly gone and timing rock steady. I set around 30 all in.

since I noticed the engine will slightly stumble and or hesitate, when accelerating slowly , worse when turning and going up hill after a stop light or sign. the engine reacts to the initial pedal input for a few hundred rpms, then will stumble or hesitate, if I let of the pedal and then press it again still lightly it smooth's out and engine rev will increase smoothly like usual. runs perfect other then this. I tried adjusting the IMS screws but neither setting it richer or leaner seems to help this issue. it affects the idle speed but very slightly with this odd hesitation, sometimes I stop then go to take off it fine and the next stop it will stumble. I also have tried setting the pump to all three settings but this too didn't seem to change. could this be timing related? should I advance it further to 36 would this effect this acceleration? I hesitate to do so as I fear the ping again. i did not have this issue when the timing was advancing too far, but the carb had to be re-adjusted as once i corrected the timing issue the engine would barely run. it only does this weird hesitation when I am easing into the throttle.

I currently checked the IMS which I started at 5 turns out and it was way too rich seem to get better when I turned them in last I checked they are only out about 3/4 turn is this a normal setting for these carbs.

all other rpms and pedal input reacts perfectly, but if engine is allowed to drop to idle for a little bit, it hesitates.

id hate to think it but any way that timing issue caused valve damage etc that could contribute to this issue?

also I do not have vacuum advance connected, should I? would this affect this?

thanks

[Shane Kelley]

A couple things can cause this but most times it's not enough initial timing. 

Some guys like this and some don't. But the way I run my 340's is the timing locked at 35 degrees. Meaning that's what it is on fire up and under any condition. It gives me instant throttle response. Some possible down sides are if you don't have a strong starter you could have a issue with hard cranking when the motor is hot. I run a factory mini starter and have no issues. You may have a issue getting the motor idled down depending on different variables do to the higher initial timing at low rpm. I don't have any issues here either but they are possibilities.  Food for thought.

What pushed me into this was the stumble you talk about. When I would put light enough springs to get the timing to advance quick enough they wouldn't be strong enough to pull the weights back all the way. So when I pulled up to a stop light the car would idle high and I would have to blip the throttle a couple times to get it to settle down.   

[culp71]

I will advance the timing slightly.

I did also notice when I would let off throttle and come up to a stop, the idle rpms would sometimes hang high around 1200, for a few seconds, then drop down to normal 850 or so? what would cause this?

[Shane Kelley]

Could be carb linkage not fully returning. You can try a stiffer return spring and see if that changes anything. You might also be having the issue of the weights in the distributor not fully returning. Are you running a vacuum advance?

[7212Mopar]

May also be a bad throttle cable. I replaced mine with one from Roseville that has the correct length and that solved my problem.

[73_Cuda_4_Me]

You stated that you changed the dizzy... what did you replace it with?

If it's a stock Mopar distributor, and you are running a cam, it may be that the mechanical advance is right on the brink of advancing the timing...

When coming to an idle, the advance weights are still slightly advanced, causing a little higher rpm, which eventually settles down back to normal idle.

Without running vacuum advance, if you set all-in at 30, and you are using a stock dizzy, then your initial is probably around 0, or TDC...

If you have a cam, this will make idling and accel erratic, and you will have a hard time adjusting the carb... you may even be running with the throttle plates open past the idle bleed slots, so that the carb is pulling fuel through the mains, causing it to be rich no matter what you do with the IMS's...

I have a 70 340 engine with a cam and same carb, and I set the timing at idle first... advance for best idle, and lower idle speed screw as needed to get down below 950, and keep going between timing and carb idle screw until there isn't any improvement... that is the timing the cam likes best. Mine likes 18 initial at idle...

Then try tweaking the IMS's... you should find about 1-1/2 turns each gives best idle speed, but use idle adjusting screw to keep rpm below 900 when adjusting IMS's, so that you aren't causing mechanical advance to throw another variable into the mix...

I was getting 5 to 10 degrees mechanical advance bounce on my stock dizzy with my idle at 950... timing light would show the mark bouncing around that much...

Once you find out what initial timing is that the cam likes, then you can dial the total amount of mechanical that gets added by using a limiter plate (available from 4secondsflat.com or making your own, like I did) so that you end up with around 36 all in...

After that, we can show you how to wake that 1406 Eddie up to get rid of the bog at mid-range throttle when you accelerate... it's set from the factory way too lean! I bumped my jets and rods up to the same size as the 1405 has from the factory, which is about 4 stages richer than the 1406 is, and boy, what a difference it made on my setup!

[Chryco Psycho]

Have you ever chaecjed the float levels ? they are never right as delivered .

[chargerdon]

I agree with Cuda_for_me method of setting the initial timing, and then using a limiter for the mechanical to get to 34-36 degree total at higher RPMS.   

However, on my 408 with Lunati 703 cam this turned out to be right around 18-20 BTDC for the initial timing.   Trouble was at 20 BTDC it got to be difficult to crank when hot.     SO, i used my vacuum advance into the MANIFOLD side of the carburater...  using an allen wrench on a mopar vacuum advance you can turn it down so that its maximum is around 6 degrees of added timing.   Knowing that i then set the initial with the vacuum connected to the 18-20 degree point.    Since it is on the Manifold side, it quickly goes away once the throttle is opened so that it doesn't affect the all in of 34-36.   

[71383bee]

Agree with a lot posted here. You have to get your initial dialed in to get a proper tune on the carb.

It's very odd to for any mopar engine to ping with total set at 30. 34 to 36 total is typical from a slant 6 to a Hemi. What throws things off at this point could be too much vacuum advance.

Whenever I setup for the first time I always set total at 34 at 3k rpm and idle it down to get an idea of where initial is. On most stock or reman units I'm lucky to see 12 or higher at initial meaning there is way too much mechanical in it.   That means the dizzy needs to be opened up. I typically aim for 18 initial. Usually with 2 degrees of that range seems to work.

The trick with using VA on the manifold is a good one to try. My buddies 383 won't even idle if it's not connected that way. In his case though I feel it's a crutch to makeup for the fact that his mechanical is out of whack and he does not want to open it up. It actually runs real well that way! 


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[73_Cuda_4_Me]

Great minds think alike...! I am using vacuum advance also, and I ran mine through the stock OSAC Valve, because it has a 3 second delay in pulling the vacuum advance on.... starts like a dream, then advances for better idle after it's running, but drops immediately when vacuum drops while adding throttle... only comes back on when at stable cruise... engine runs cooler and gets better mileage when cruising...

[culp71]

thanks to all, once again.
It seems a little more initial (dis-rotation) timing was all it needed.

I advance the initial timing from current 13 to 16 and its 99.9 percent better. total is 36. no vac connected.
I think ill even advance it slightly more the next chance I get.
mech. advance is 24 in dis which will plan on limiting so as to not pass 36 total.   I re-set carb IMS's to 1 1/2 and turned in until it idled really well, and rpms now don't hang up few a few second like before. idles around 850 and no hesitation or stumble now!

I did already check float height and changed out the jets and rods in carb and throughout the entire rpm range this carb is a beast now.
thanks!

[RUNCHARGER]

Ha, ha: The cars really get running well towards the end of the season as we tweak them better and better.

[71383bee]

Great to hear!

Timing is everything baby! 


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[culp71]

I advanced the timing again, closer to 18 initial timing, all the hesitation/ stumble is completely gone now!
idles around 900 nicely. and no ping at WOT. starts and restarts hot or cold very nicely.
I noticed that while the driver side IMS screw on the carb effects the idle speed and quality the passenger side IMS screw seems to do nothing. closed all the way or backed out 4 turns makes no change. 
suggestions to correct this?
and still surprised that with both IMS's all the way in the engine idles just fine... thoughts?

[dodj]

Ha, ha: The cars really get running well towards the end of the season as we tweak them better and better.

Ain't that the truth.... lol