Author Topic: Engine Hesitation  (Read 1335 times)

Offline septantrionalis

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Engine Hesitation
« on: December 07, 2018, 11:27:55 PM »
I just got my engine installed in my truck.  Its a 383 Stroker Engine from West Coast Engines.  Its running great with one exception -- hesitation.  Its hard to explain, but the hesitation doesn't occur when I slightly press the gas pedal.  In fact, it runs great when the gas pedal is slightly pressed.  Once I give it some more gas, the truck starts to stutter and spur but then recovers after a few seconds.  The warmer the engine gets, the less hesitation there is.

Here's some videos I took of me giving the truck some gas.  I'm barely giving the truck gas as the while truck going the opposite direction passes me.  At 4 seconds, I try and gun it and it sputters.  I continue gunning it and the engine recovers at around 6 seconds:
https://www.youtube.com/watch?v=xx1kR-XywCc

Another video.  This one, I am giving the truck light gas and its running fine.  At around 2 seconds, I try to give it more gas and it sputters.  You can hear it at around 3 seconds.  I continue to have the gas pedal pressed and it recovers at around the 4 second mark:
https://www.youtube.com/watch?v=WaSJ728HtSk

West Coast Engines had the engine dyno'd and tuned in Washington State. The carburetor is a Holley 670CFM Street Avenger.

I've taken it to two shops and they both messed with the carb.  All the did was adjust the primary jets up/down and replace the power valve.  It still sputtered.  All the shops did was complain that the Holley is a hard carburetor to tune -- that I should go with Edelbrock.  This seems like a cop out.

I called West Coast Engines and they told me to check the timing.  It should be "32 degrees at 2500 RPM for total advanced".  I had a shop check that and its spot on.  Do I need to adjust the timing from those numbers when living in a higher altitude city?

Any other ideas as to why I am getting that initial hesitation?

Just for fun, here's a walk around of my truck:
https://www.youtube.com/watch?v=G7_7xzyRMN0

Offline blazer74

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Re: Engine Hesitation
« Reply #1 on: December 08, 2018, 05:42:09 PM »
Accelerator pump probably if float level, Timing and power valve are correct.
Too much or not enough pump shot can cause a stumble.
Holly’s have different cams for the linkage to change pump shot.

Here’s Holley’s link.

https://forums.holley.com/entry.php?428-How-To-Cure-Carburetor-Stumble

Offline blazer74

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Re: Engine Hesitation
« Reply #2 on: December 08, 2018, 05:42:41 PM »
Super nice by the way.

Online bd

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Re: Engine Hesitation
« Reply #3 on: December 08, 2018, 07:27:53 PM »
With the engine off remove the air cleaner and look down the primary throttle bores using a bright flashlight.  Operate the throttle by hand while you watch for two strong streams of fuel emanating from the accelerator pump nozzles.  What did you see occur?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 75gmck25

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Re: Engine Hesitation
« Reply #4 on: December 09, 2018, 12:14:33 PM »
I'm a lot more familiar with Edelbrocks and Quadrajets, but it sounds like its an accelerator pump problem.

At idle and during moderate acceleration a carburetor runs off its primary jets, supplemented by a small accelerator pump shot to bridge it into the cruise mode.  It sounds like this works fine on your truck.

When you accelerate harder you have a sudden inrush of air and you need a quick feed of fuel to keep up during the primary side transition to cruise/power mode, keeping the mixture good until the secondaries open.  It looks like you aren't getting enough fuel to cover that transition.  This sounds like either an accelerator pump problem, or secondaries that are opening up too quickly and giving it a really big gulp of air.

Bruce

Offline septantrionalis

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Re: Engine Hesitation
« Reply #5 on: December 10, 2018, 03:03:46 PM »
With the engine off remove the air cleaner and look down the primary throttle bores using a bright flashlight.  Operate the throttle by hand while you watch for two strong streams of fuel emanating from the accelerator pump nozzles.  What did you see occur?

Thanks for the help guys.  I tried the accelerator pump nozzle.  Currently, the truck has a 31.  I was going to go up to a 34, but the guy at the shop told me I should go down.  I tried a 25 but I feel like the hesitation is worse.  I went up to a 35 (he didn't have a 34) and it does feel a little bit better, though there is still some hesitation.  If I slowly give it gas, it doesn't hesitate.  If I quickly give it gas, it does.  I may try a larger nozzle.

bd, to answer your question, this is what I see.  Fuel came out of the accelerator pump nozzle.  With a 25, it was a thin stream.  With the 35, it was a larger stream.  The stream came out the instant I manually operated the throttle and continued to stream fuel as I continued to press the throttle.  I assume the top plate is the choke plate?  That stayed open.  The throttle plate moved slowly from a closed position to an open position.  Eventually, vapor came out of the carb which prevented me from seeing inside.

If this is confusing, I can post a video.

Online bd

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Re: Engine Hesitation
« Reply #6 on: December 10, 2018, 07:07:05 PM »
Vacuum or mechanical secondaries?
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline septantrionalis

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Re: Engine Hesitation
« Reply #7 on: December 10, 2018, 07:58:50 PM »
Vacuum or mechanical secondaries?

Its a Holley Street Avenger 670CFM which looks to have Vacuum Secondaries.

Online bd

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Re: Engine Hesitation
« Reply #8 on: December 10, 2018, 08:19:31 PM »
Temporarily wire the secondary closed to see if there is any effect on symptoms.  If the secondaries are opening you will notice a loss of power by wiring them closed.  The question is whether acceleration bogging disappears.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline septantrionalis

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Re: Engine Hesitation
« Reply #9 on: December 19, 2018, 02:18:41 PM »
O.K., I think I finally have it dialed in.  I went from the original 31 to 35 on the accelerator pump nozzle.  There was still a slight hesitation though it definitely got better.  I moved up to a 37.  The hesitation exists when the engine is cold.  Once its warmed up, it runs like a raped ape.  I think I'll just leave it like that.

Offline JohnnyPopper

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Re: Engine Hesitation
« Reply #10 on: December 20, 2018, 05:40:54 PM »
Holley Carbs don't like snow... neither do raped apes.

Glad you found it, good job!

Offline septantrionalis

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Re: Engine Hesitation
« Reply #11 on: January 02, 2019, 10:02:31 AM »
Just an update... I feel like I haven't solved the problem 100%.  When the truck is completely warmed up the problem goes away.  However, when the truck is cold/warm, or when the weather is cold, the truck still has that hesitation.  I noticed it this morning when driving to work (its about 11 degrees out).  In fact, it's almost a safety issue.  I tried to pull out of my neighborhood this morning into oncoming traffic.  The truck accelerated perfectly initially then almost stalled a second afterward.  It's almost like the transition from the primary to the secondaries is causing the problem.

I think bd is on the right path with the secondaries.  I am going to try and install a secondary spring with less resistance.
« Last Edit: January 02, 2019, 10:11:59 AM by septantrionalis »

Offline JohnnyPopper

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Re: Engine Hesitation
« Reply #12 on: January 02, 2019, 01:22:12 PM »
Where is the choke in all of this? It's supposed to keep you out of stuff like this... Is is working?

In BRRRRR cold like that it probably needs to be adjusted to a more advanced or positive setting.

Offline septantrionalis

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Re: Engine Hesitation
« Reply #13 on: January 02, 2019, 03:47:10 PM »
Where is the choke in all of this? It's supposed to keep you out of stuff like this... Is is working?

In BRRRRR cold like that it probably needs to be adjusted to a more advanced or positive setting.

That was next on my list.  If the secondary springs dont do anything, I'll try adjusting the choke.  I've never adjusted the choke, so I'll have to research how to do that.

Offline blazer74

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Re: Engine Hesitation
« Reply #14 on: January 02, 2019, 07:32:40 PM »
I’d look at the choke first. If it’s not set correctly this will happen.