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73-87 Chevy _ GMC Trucks => Performance => Topic started by: septantrionalis on December 07, 2018, 11:27:55 PM

Title: Engine Hesitation
Post by: septantrionalis 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
Title: Re: Engine Hesitation
Post by: blazer74 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
Title: Re: Engine Hesitation
Post by: blazer74 on December 08, 2018, 05:42:41 PM
Super nice by the way.
Title: Re: Engine Hesitation
Post by: bd 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?
Title: Re: Engine Hesitation
Post by: 75gmck25 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
Title: Re: Engine Hesitation
Post by: septantrionalis 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.
Title: Re: Engine Hesitation
Post by: bd on December 10, 2018, 07:07:05 PM
Vacuum or mechanical secondaries?
Title: Re: Engine Hesitation
Post by: septantrionalis on December 10, 2018, 07:58:50 PM
Vacuum or mechanical secondaries?

Its a Holley Street Avenger 670CFM which looks to have Vacuum Secondaries.
Title: Re: Engine Hesitation
Post by: bd 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.
Title: Re: Engine Hesitation
Post by: septantrionalis 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.
Title: Re: Engine Hesitation
Post by: JohnnyPopper on December 20, 2018, 05:40:54 PM
Holley Carbs don't like snow... neither do raped apes.

Glad you found it, good job!
Title: Re: Engine Hesitation
Post by: septantrionalis 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.
Title: Re: Engine Hesitation
Post by: JohnnyPopper 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.
Title: Re: Engine Hesitation
Post by: septantrionalis 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.
Title: Re: Engine Hesitation
Post by: blazer74 on January 02, 2019, 07:32:40 PM
I’d look at the choke first. If it’s not set correctly this will happen.
Title: Re: Engine Hesitation
Post by: septantrionalis on January 02, 2019, 09:22:55 PM
Ignore me, problem solved.
Title: Re: Engine Hesitation
Post by: septantrionalis on January 02, 2019, 10:33:07 PM
So I just went out to look at how the choke was adjusted.  I wanted to see where the center pointer was in relation to the index marks.  It was actually pointing to the front of the truck!  As a reference point, the index marks are at the top.  So the choke cap was almost 45 degrees clockwise away from the actual index marks.  I'm actually hoping it was 45 degrees clockwise instead of 315 degree counterclockwise because I turned it 45 degree counterclockwise to get it pointing to the top.  The other option would have been to turn it 315 degrees counter clockwise to get it to the top.  I also hope this didn't screw up my choke.

Anyway, I set the center point to the top, directly in the center of the index marks and drove the truck around.  The hesitation is completely gone!

I noticed, however, that the truck is idling at around 2000 RPM, which is really high.  I can manually force the fast idle cam to disengage and my RPMs drop to 1000, which is what I want.  Once I hit the gas, it engages again causing the idle RPM to go back up to 2000 RPM.  Any idea as to why the fast idle cam is engaging when I hit the gas and not disengaging later?  Its cold outside, but I feel like my truck is warmed up at this point.  My engine temperature gauge reads 180 degrees.

Here's a video.  You can see the fast idle cam and choke engaging the second I give it gas.  At this point, the engine is pretty warmed up.

https://www.youtube.com/watch?v=LBKmWAghszs
Title: Re: Engine Hesitation
Post by: MrFiveOh on January 03, 2019, 01:29:53 AM
you need to adjust your choke more. check the tension on the choke by taping it with your finger, you should feel slight tension (do this prior to a cold start) It may have too much advance choke causing the idle cam to get caught.
Title: Re: Engine Hesitation
Post by: VileZambonie on January 03, 2019, 04:07:56 AM
Make sure you have 12 volts at the choke coil when the engine is running.
Title: Re: Engine Hesitation
Post by: septantrionalis on January 03, 2019, 08:53:22 AM
Dang, false alarm.  The center point and index markers aren't correct on my carburetor.  With the truck cold and off, I gave the carb just a little bit of throttle.  I then turned the choke cap clockwise until the choke was open.  I then turned the choke cap counter clockwise until the choke was closed.  According the the instructions, this is where the choke cap should be set -- and this is where it was set before I messed with it.  I guess the hesitation went away because the RPMs were so high.  So I'm back to square one.

In summary: I'm seeing a hesitation in the lower to mid throttle range that goes away when the truck is warm.

What I've done:
Engine Timing: Validated that its spot the manufacturers recommended setting.
Jets: Set according to what Holley recommends for my altitude.
Accelerator pump nozzle: I tried .025, .031, .035, .037.  The higher number seems better for me, though I still get the hesitation.

What is left to try:
A less resistant vacuum secondary spring
Choke: Different adjustments
Title: Re: Engine Hesitation
Post by: JohnnyPopper on January 03, 2019, 05:04:25 PM
I agree with Blazer, go there first.

Classic symptoms of an ineffective choke.

Title: Re: Engine Hesitation
Post by: septantrionalis on January 03, 2019, 07:46:29 PM
ok, I ended up rotating the choke cap about a quarter of an inch counter clock wise from where it initially was.  I'll drive it for a few days and see how it does and continue adjusting it.
Title: Re: Engine Hesitation
Post by: blazer74 on January 04, 2019, 05:59:01 PM
https://m.youtube.com/watch?v=OoW9pWZmtBI
Title: Re: Engine Hesitation
Post by: JohnnyPopper on January 05, 2019, 07:25:06 AM
Sounds like you're getting close, especially if when everything is at operating temp you are at the idle speed you want.

Rule of thumb: with everything hot, and 12 volts to the coil, loosen the choke clamp ring, rotate the choke both directions, stopping with the choke plate straight up. That should be the end of it.

When you cold start it, depress the pedal slightly to allow the choke cam to step up, and it should stay there with fast idle till it reaches operating temp.

We're going to miss you... ;)
Title: Re: Engine Hesitation
Post by: septantrionalis on January 06, 2019, 09:37:27 PM
Just an update.  I think I finally got it.  The solution was actually a mix of issues. 

I adjusted the choke cap and it fixed the hesitation issue when I took off at a stop light.  However, when gunned it while turning a corner, the engine would start to sputter.  I decided to go down two springs on the secondary springs from uncolored to yellow, and that actually solved that problem.  Not sure why, but whatever.  There was still a very very small blip in the power if I launched very quickly towards the bottom end (if the engine wasn't warmed up).  If I launched mildly or slowly, I didn't feel it.  It wasn't anything that caused the truck to stall or anything, but it was definitely noticeable.  Upping my idle solved that.  So in the end, this is everything I had to do :

Accelerator pump cam: Orange -> Blue
Accelerator pump nozzle: 0.031 -> 0.025
Choke: Turned 3 notches counter clockwise.
Secondary springs: uncolored -> White.



Title: Re: Engine Hesitation
Post by: JohnnyPopper on January 07, 2019, 04:12:38 PM
Nice work! Glad you stuck with it.

I noticed you mentioned a blip when it wasn't warmed up... These beasts like to operate in a narrow temperature range starting around 180 degrees. When you are below that, expect blips and farts and sputter. Especially in Cold Country!

Now, can we just know the meaning of your name. Please?  :)

Title: Re: Engine Hesitation
Post by: septantrionalis on January 07, 2019, 10:30:23 PM
Now, can we just know the meaning of your name. Please?  :)

haha.  I was tired of trying to find usernames that weren't taken.  I wanted something unique but easy to remember without numbers.  I'm into astronomy, so I took the star name "Torcularis Septentrionalis" and modified it.  I just took the Septentrionalis portion of it and changed the second "e" to an "a". 
Title: Re: Engine Hesitation
Post by: JohnnyPopper on January 07, 2019, 11:59:14 PM
Ah hah! I now see what you did there!

The wife just shared stats on UY Scuti, check out that beast! What kind of planets and moons must orbit that monster?