Slight acceleration hesitation driving me mad

[Cranky]

ICE=Internal Combustion Engine.....really? Still would like to know why Mopars like something that other engines do not like. Let's make it easy....383 Mopar vs a 383 Chevy. Yeah, bore and stroke is a bit different but both are still basically an air pump......

 

[RJRENTON]

ICE=Internal Combustion Engine.....really? Still would like to know why Mopars like something that other engines do not like. Let's make it easy....383 Mopar vs a 383 Chevy. Yeah, bore and stroke is a bit different but both are still basically an air pump......

ICE Nope....never heard that expression b4....but I'm sure every field of endeavor has a certain "language" or expressions unique to that endeavour. Perhaps you are familiar (or not) with the the 3rd, 5th and 7th order harmonics that can occur during an asymmetrical short circuit condition on a 138 kv electrical distribution network.
I sort of agree with your premise re air pump .... but not all air pumps are alike. Displacement is a major variable. But RPM is another variable. Would the Chevy 383 (presumably a stroked 350) have a higher piston speed (ft/min) at a given RPM than a standard stroke Mopar 383 (3 3/8") at the same RPM? IF that premis is correct, would the intake event have a higher velocity than the Mopar engine because of higher piston speed? Since velocity is key to uniform fuel distribution, the higher the velocity tends to minimize distribution issues. Intake manifold is another important consideration....single plane vs 180° two plane design. Carb type also influences mixture velocity AND fuel distribution....spread bore (Qjet) vs Carter vs Holley vs "Eddys" / Weber of the equal CFM rating.
In my endeavours with my Mopars, I've found that slightly richer idle and off idle transition helped my issues, whether you agree or not. Richer by how much? Richer enough to fix the problem...a quantitative number is empirically determined. Ideally, 14:1 A/F at idle (nominal) is difficult to maintain. True, Stiochiometric confitions that provide optimal best power operation ftom idle thru max RPM is almost impossible to achieve with carb and manifold designs of older cars, like my GTX, given today's ethanol blended fuels. The best I have achieved is by reading the plugs to get reasonably uniform color.
Just my opinion of course.
Bob Renton

 

[Cranky]

Piston speed isn't going to have whole heck of a lot of input at idle with pretty slow valve movement and that's what you/we/thread poster was talking about. Off idle hesitation. The 383 Mopar and the 383 Chevy are too close to each other that a simple adjustment wouldn't cure a mild off idle stumble if there was one when swapping the carb from one to the other. So you're going throw a bunch of electrical engineering crap at me because I used a simple abbreviation for the internal combustion engine? Man. Btw, I read across that abbreviation when I was a stooopid teen and it's been used quite often in the business/hobby/habit......

 

[miller]

So I can change the timing in the distributor and should change that to 10 degrees instead of using the timing light to advance it to ten degrees at the timing mark on front of the engine? Am I understanding that correctly?

Hmmm...kinda sorta. I don't know just how much of this stuff you know, okay?

These days, initial timing works around 12-14 degrees. You can't go by the setting in the manual, since back then...different gas. Have to tune for the gas, and the engine.

If you pull the distributor cap, you'll see the rotor. By hand, twist it, and it will turn a given number of degrees. Should move back, once you let go. That movement is the mechanical advance, set in the distributor, under the points plate.
Just saying, for the initial timing, 12 degrees BTDC (0), and getting full advance where it should be, that mechanical advance should be adjusted for it.
Stock distributor? Some are adjustable, some are not...but, with ways to fix it. Of course, the distributor needs to be working right, in the first place!

Yes, use a timing light to set initial timing, at the timing marks. But, you also need to know how much ADVANCE your getting, for it all to be right.

If your advance is not right, and going more than it should...could be why it was running like you said.

There's tons of info out there, on this stuff.

 

[HALIFAXHOPS]

By internals are you referring to points and condenser rotor etc? I’m not sure what else is in a disributor, I’m still kinda learning all this as I go.

Make sure the mechanical advance is not sticking and also that the vac advance is good. Mechanical is easy take the cap off and turn the rotor it should move freely and return to the starting position. Just fyi

 

[HALIFAXHOPS]

Lol Miller just posted it!

 

[RJRENTON]

Piston speed isn't going to have whole heck of a lot of input at idle with pretty slow valve movement and that's what you/we/thread poster was talking about. Off idle hesitation. The 383 Mopar and the 383 Chevy are too close to each other that a simple adjustment wouldn't cure a mild off idle stumble if there was one when swapping the carb from one to the other. So you're going throw a bunch of electrical engineering crap at me because I used a simple abbreviation for the internal combustion engine? Man. Btw, I read across that abbreviation when I was a stooopid teen and it's been used quite often in the business/hobby/habit......

Well, I guess we'll have to agree to disagree.
I'm not exactly sure what the differences or similarities between the the two referenced engines are.....yes, displacement is a common item....but are the other variables the ssme? Specifically compression ratio, and maybe cam profile. Especially the acceleration ramps built into the lobe profiles....not nore degrees or lift, just faster rates ....doesn't a cam profile with a large percentage of overlap run the risk of incoming dilution of the fuel charge? This would present as if there were excessive EGR (Exhaust Gas Recirculation) present, making the In coming fuel charge more difficult to ignite resulting in an off idle stumble? I'm sure you remember disconnecting or plugging the EGR system on vintage 1975 vehicles and how much better they ran. The purpose of EGR was to reduce the combustion chamber temp during combustion to lessen or eliminate the NOx formation by reducing chamber temp to less than 1500° F. In my experience, by richen the off idle transfer circuit slightly (enough to eliminate or make tolerable) the problem went away. I have no experience with 383 Chevy engines to form a parallel comment. The purpose of my comment was: my opinion as to what worked for me.....if you disagree.....fine..... if you accept my premise....fine. Adjusting the timing (rate and total) CAN and often does fix a multitude of problems.....but when it does not, one is forced to look elsewhere for a possible solution.
Bob Renton

 

[khryslerkid]

Yes it’s single points set up. I replaced the points and condenser last summer, and regapped it then, I haven’t double checked the gap yet. But watched the timing line advance smoothly while rpms went up under timing light. It feels like something along the accelerator pump maybe because once it is on the road at 55mpg I floor it and it just pulls and goes faster, it’s only from a dead stop that it hesitates. Everything I read and seen in videos from Holley it seems like I may need bigger size accelerator shooters, but everything is all original on the car so I wonder why it might need that or if it is even the issue.

Sounds like the Accelerator Pump to me too.

 

[ddpsnake]

nope the pump cam i still think thats the easy fix .

 

[BSB67]

If a slight hesitation at just off idle, richen the idle mixture, put more initial timing in it. This assumes that there isn't something else wrong that you're missing.

That thing does not need bigger squirters or pump cam

 

[Coronet-383]

Hmmm...kinda sorta. I don't know just how much of this stuff you know, okay?

These days, initial timing works around 12-14 degrees. You can't go by the setting in the manual, since back then...different gas. Have to tune for the gas, and the engine.

If you pull the distributor cap, you'll see the rotor. By hand, twist it, and it will turn a given number of degrees. Should move back, once you let go. That movement is the mechanical advance, set in the distributor, under the points plate.
Just saying, for the initial timing, 12 degrees BTDC (0), and getting full advance where it should be, that mechanical advance should be adjusted for it.
Stock distributor? Some are adjustable, some are not...but, with ways to fix it. Of course, the distributor needs to be working right, in the first place!

Yes, use a timing light to set initial timing, at the timing marks. But, you also need to know how much ADVANCE your getting, for it all to be right.

If your advance is not right, and going more than it should...could be why it was running like you said.

There's tons of info out there, on this stuff.

I don't know too much, sorry if that came off sarcastic. the strange thing is I didn't have or at least didn't notice the hesitation before I rebuilt the carb. everything I have read tells me its the accelerator pump, possibly needs larger shooters, cuz it has a new diaphragm, and the cam was moved to position #1 (engines that idle at 600-800 rpm?) but Im confused why it might need larger shooters for the accelerator pump if everything is stock, wouldn't I need bigger shooters if I put in a larger cam or something like that? like I said Im still pretty new and learning as I go. Ill have to research the adjustment youre speaking of and try that and see if it works. I appreciate your input and thanks for explaining it more.

 

[Coronet-383]

If a slight hesitation at just off idle, richen the idle mixture, put more initial timing in it. This assumes that there isn't something else wrong that you're missing.

That thing does not need bigger squirters or pump cam

I was told to start with the timing and make sure that is correct before moving on to carb or anything else. we spent a few hours adjusting the timing, taking it for a ride seeing if it helped or hurt, readjusting, test drive and repeat. I tried 10-12 degrees btdc and it seemed to perform best at the 7.5 degrees btdc. we hooked up a vacuum gauge and adjusted the mixture screws that way, the engine runs smooth at idle, doesn't diesel, starts right up, has no knocks or ticks or anything, driving at 55 mph it still has half the pedal left and puts u right back in ur seat. its just at a dead stop, there is a slight hesitation, then a surge like I slammed into the gas pedal quick. it seems like a fuel delivery issue but like you said why would it need larger squirters to correct the issue when it was fine before and everything is stock?

 

[BSB67]

Good grief. Take 3 minutes and turn the mixture screws out 1/2 turn and test. Then, if you touched the distributor at all, spend 5 minutes and put the timing at 15° and test.

We have no idea what you've actually done, but you need to figure what you did wrong and fix it. You're either too lean at idle and/or just off idle. It is the idle circuit/transition or the pump. A piece of dirt will screw either of these up.

If you've touched the distributor there are several things that you might have gotten wrong. Advance the timing to see if it helps, then figure where you went wrong.

If you "think" it's running better with less timing, your senses are likely wrong.

 

[miller]

I don't know too much, sorry if that came off sarcastic.

Not at all. It's the 'I don't know you, you don't know me' thing. Your willing to ask, that's a good thing. (Besides, there's times I get too dang long-winded!)

Yep, getting a little bog off the line, 'can' easily be carb related, just as the guys are saying. Though, especially for someone new to the game, it 'can' be caused by other issues. That's why I brought up my junk.
IF the distributor mechanical advance (long-winded again) is hanging up...that motor is gonna bog, until the advance comes in. Okay? If it has been some time, since the distributor has been 'serviced', it can happen. A little cleaning, a little lube...will fix it right up.

 

[Coronet-383]

Not at all. It's the 'I don't know you, you don't know me' thing. Your willing to ask, that's a good thing. (Besides, there's times I get too dang long-winded!)

Yep, getting a little bog off the line, 'can' easily be carb related, just as the guys are saying. Though, especially for someone new to the game, it 'can' be caused by other issues. That's why I brought up my junk.
IF the distributor mechanical advance (long-winded again) is hanging up...that motor is gonna bog, until the advance comes in. Okay? If it has been some time, since the distributor has been 'serviced', it can happen. A little cleaning, a little lube...will fix it right up.

Is that information in the service manual by any chance? How to service the distributor? I’m going to check that for sure as soon as I get a chance. Thanks for explaining and being patient with my lack of understanding.

 

[Coronet-383]

Good grief. Take 3 minutes and turn the mixture screws out 1/2 turn and test. Then, if you touched the distributor at all, spend 5 minutes and put the timing at 15° and test.

We have no idea what you've actually done, but you need to figure what you did wrong and fix it. You're either too lean at idle and/or just off idle. It is the idle circuit/transition or the pump. A piece of dirt will screw either of these up.

If you've touched the distributor there are several things that you might have gotten wrong. Advance the timing to see if it helps, then figure where you went wrong.

If you "think" it's running better with less timing, your senses are likely wrong.

Maybe i wasn’t clear, everything you have recommended to try, I have already done.

 

[threewood]

If a slight hesitation at just off idle, richen the idle mixture, put more initial timing in it. This assumes that there isn't something else wrong that you're missing.

That thing does not need bigger squirters or pump cam

Agree. I don't think I got the right quote though lol.

I'm no expert but an off idle hesitation would have me looking at the accelerator pump / cam. Engine off, looking down the barrel and you just start to move the throttle back you should see 2 streams of fuel going into the primaries. If it sputters or doesn't shoot immediately adjust or replace parts. It's a lean stumble because the transition circuit isn't adding in enough fuel once the throttles crack open.

 

[BSB67]

Maybe i wasn’t clear, everything you have recommended to try, I have already done.

You were clear. Ill try a little differently. It did not hesitate before. You touched a bunch of stuff. Now it hesitates. Despite what you think, you've screwed something up.

 

[Coronet-383]

Agree. I don't think I got the right quote though lol.

I'm no expert but an off idle hesitation would have me looking at the accelerator pump / cam. Engine off, looking down the barrel and you just start to move the throttle back you should see 2 streams of fuel going into the primaries. If it sputters or doesn't shoot immediately adjust or replace parts. It's a lean stumble because the transition circuit isn't adding in enough fuel once the throttles crack open.

That makes sense, I’m gonna replace the cam and check the shooters stream again and see if that helps. Thank you for the advice

 

[RJRENTON]

An additional point to consider: there is a small steel ball check valve at the bottom of the accelerator pump well. This ball check valve admits fuel to the accelerator pump well when the pump plunger travels up then seats to allow the pump plunger to push the fuel to the discharge check valve passage and shooters. MAYBE if this check valve is not seating, part of the pump plunger stroke will be waisted, allowing the fuel to be pushed back into the float chamber, resulting in a possible stumble. The rebuild kit furnishes a replacement ball and instructions how to form a new seat. Just my opinion of course.
Bob Renton