Low RPM stumble

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DartVadar

DartVadar

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Recently my 360 has been having an issue at low Rpms. It idles fine at 1000, but it occasionally pops and surges when I quickly tap the gas. While cruising it hesitates and surges and it has also popped out the carb a few times up until about 2000rpms. Past that it runs just as strong as it ever has and has no hesitation or misfires. Fuel is getting to the carb and the accelerator pump is working great.

The things I've recently changed are an MSD ignition (made sure all grounds were good) but could I have low voltage to the box? New fuel lines and sender unit, new fuel filter. I also got a new distributor, and the timing set with vacuum advance not hooked up is about 13 initial and 34 total. Could I possibly need more initial timing? I also can't remember what the plug gaps are, but I know they aren't to MSD spec.

The engine is a 360, kb Pistons, stock stroke, lunati solid roller 245@.050 and 578 lift, Holley 650w vacuum secondaries, eddy heads, headers and so on.
 
Is 1K as low as it will idle? I would suspect a vacuum leak.
 
It will go a bit lower 850-900 but once it get put into gear it revs a bit lower and wants to die (needs a different converter I know). And a vacuum leak would make sense, I'll take a look at all of the lines. Also I didn't hook up the vacuum advance yet as I didn't have enough line, should I hook that up and see what happens?
 
I don't think the vacuum advance will affect it at idle. It should not.....unless you have it connected to a manifold vacuum source, which I don't recommend on a street car. Some people run manifold vacuum on the vacuum advance, bit it's my personal opinion not to.
 
I was going to hook it up to the vacuum port on the carb.

And I took a look at the carb, the secondary float level seems good, but the primary one seems low. When I took the sight plug out of the primary one, no fuel came out, and I could barely see any in there, and when I opened the secondary sight I could see a slight trickle come out. Could this be the issue? Under cruise when it's only using the primary it runs out of fuel, and when I get on it and the secondaries are used it gets enough fuel to run better? Because a lean condition would cause popping right?
 
I don't know. If it's low, it certainly needs to be right. I always adjust mine so fuel barely trickles out when you bump the fender.
 
I adjusted the float level, so when I bump the car a bit trickles out. But that didn't change anything.

I'm thinking its timing related because that's the only thing I've changed. I got the distributor re-curved and just put it in. I will re-check the timing, and if that doesn't work I'm outa ideas lol
 
Set the initial up to 20, you will see some nice changes, don't no what carb but you may need a larger squirter in the pump.
 
Tekslk may be onto something...

13 initial with a 245 at .050 camshaft, good luck with that! :)

Probably needs 20-24 minimum for initial to get the carb under control. It will need the mechanical advance limited.
 
"Recurved" is a suspect for idle problems also.
Do you know what springs were used, and if the total timing was limited in it?
 
I'll bump up the initial and see how it likes it. Would not enough timing cause the slight stumble I'm having? So more initial should improve the low end power? Because it's seems like it doesn't have that much down low, but once the Rpms get up it breaks the tires loose going pretty much any speed.
 
TrailBeast said:
"Recurved" is a suspect for idle problems also.
Do you know what springs were used, and if the total timing was limited in it?
Click to expand...

I'm not too sure what exactly he did to it (good friends with the shop) he told me to try it out and if it didn't work well to bring it back so He can crush it lol. He said it was the most frustrating one he has ever done because somebody previously meddled with it.
 
crackedback said:
Tekslk may be onto something...

13 initial with a 245 at .050 camshaft, good luck with that! :)

Probably needs 20-24 minimum for initial to get the carb under control. It will need the mechanical advance limited.
Click to expand...

No doubt. I am running 26 initial w/ closer to 40 total on my 340 with less cam. That is where it ran best on dyno and no detonation issues, so there she stays
 
TrailBeast said:
"Recurved" is a suspect for idle problems also.
Do you know what springs were used, and if the total timing was limited in it?
Click to expand...


I like to "map" my distributor curves. It's easy to do with an advanced timing light. Start at idle and go every 250 RPM until 3000 RPM.

Then I can graph them in Excel and compare one to the other and see what the different spring combo's do...
 
DartVadar said:
I'll bump up the initial and see how it likes it. Would not enough timing cause the slight stumble I'm having? So more initial should improve the low end power? Because it's seems like it doesn't have that much down low, but once the Rpms get up it breaks the tires loose going pretty much any speed.
Click to expand...

Low initial timing could cause both, yes.
 
When you take the throttle past idle the vacuum port to vacuum advance opens, providing needed advance at light loads. If the vacuum advance dash pot or hose leaks, there will be a lean and retarded timing ... conditions for hesitation and pop.
 
Just a very late update on the problem. Have only been able to drive the car recently. The problem is getting better but still having issues I believe.

I verified the timing marks on the balancer and added a timing tape (360 balancer diameter is 7.25 inches right??? Had a hard time measuring it). The timing is set at about 22 initial and 35 all in. The car idles fine and when I quickly step on it the car has no hesitation and goes really good unlike before.

The issue I am having now is when im cruising it has a stumble still. It only happens in third gear at around 1300 rpms with moderate throttle, and past 1500-1600 there is no stumble. I do know the rear gears are severely mis-matched for the cam (2.76!!!) I am putting 4.10s in really soon. The cam really only makes power past 2400-2500 RPM so is it possible that it is lugging at that low rpm? I am also worried about it potentially detonating, I havent heard any I dont think, and looking at the plugs I dont see any specs that would mean detonation. This problem gets worse the more timing I take away.
 
Accelerator pump needs adjusting.
 
It doesn't happen when I quickly get on it though, it happens when I slowly press the gas. Could that still be accelerator pump related then? It's also weird that it gets worse the more timing I take out.
 
Missing the check ball - valve in the accelerator pump circuit in one end or the other.... or it is dirty.
 
Using my rule of thumb, the target idle-timing needs to be about 22* to 27*;starter kick-back being the limiting factor.
On Holley type carbs,the accelerator pump freeplay needs to be readjusted after nearly every idle speed adjustment.
With the new idle-timing, it may be possible to close the primaries to get a better transfer port sync-up. Put a vacuum gauge on the spark port. Crank the primaries closed until the sparkport reads less than 2 to 4 inches vacuum. Then set the idle speed with the secondary cracking screw. Reset your idle mixture screws for best idle quality. Reset the accelerator pump freeplay(s). That 245cam should be ok with the standard A-pump, but all the checks need to be in there, and with 2.76s will need a pretty good shot to get going at low revs.

2.76s with a 245 cam! Are you serious? You cant really expect to tune that very well.Wait til the 4.10s are in there and a hi-stall. Everything will magically come together!
 
AJ/FormS said:
Using my rule of thumb, the target idle-timing needs to be about 22* to 27*;starter kick-back being the limiting factor.
On Holley type carbs,the accelerator pump freeplay needs to be readjusted after nearly every idle speed adjustment.
With the new idle-timing, it may be possible to close the primaries to get a better transfer port sync-up. Put a vacuum gauge on the spark port. Crank the primaries closed until the sparkport reads less than 2 to 4 inches vacuum. Then set the idle speed with the secondary cracking screw. Reset your idle mixture screws for best idle quality. Reset the accelerator pump freeplay(s). That cam should be ok with the standard A-pump, but all the checks need to be in there, and with 2.76s will need a pretty good shot to get going at low revs.

2.76s with a 245 cam! Are you serious? You cant really expect to tune that very well.Wait til the 4.10s are in there and a hi-stall. Everything will magically come together!
Click to expand...

I'll maybe add some timing to it, I just don't want to worry about detonation. I have adjusted the idle speed without checking the accelerator pump. I've recently noticed that my a/f ratio at idle jumps around a bit and steadies out once the Rpms increase a little bit, I've adjusted the idle mixture too. Could this be part of my problem?

I know! The 2.76 gears were just in the car when I got it and I haven't changed them out yet, so the 4.10s will make a difference!
 
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