RedLeader289

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I still have a hesitation upon sudden acceleration. Can't figure it out.

GMPP 385 Fastburn Crate Engine, Holley 750 DP.

Some background: About a 5 weeks ago I noticed that the car would stumble/bog if you tried to get into it too hard while going down the road. It would be fine while idling in park.

Changed out the stock GM ignition coil (it tested bad both on the bench and at the store) with a MSD blaster coil. -Hesitation GONE. I could stomp it all day while in park or goose it in gear and it'd be fine, but it would drop RPM's going down the highway and still bog/stumble if you tried to push it past 3000 RPMs.

Resistance tested the pick-up coil and it tested right in the middle of the range it was supposed to test, so perfect (as far as I could tell).

Car died on me while driving home from that excursion, which imediately led me to the ICM. (I know that a bad coil can fry them). Swapped it out and the stumble/bog with higher revs was gone.....but the hesitation from idle was back (hitting head against wall).

I tuned the carb - it's fine.

I noticed she was cranking really slow (a couple degrees off time....somehow???) so I checked the timing and yes, it was somewhere in between 4 and 6 degrees off. Fixed that. Timing is SPOT ON now and appears to be holding. I think I knocked the distributor or something in the middle of swapping the ICM/Coil/etc, not too worried about that.

Still have the hesitation upon sudden acceleration. You goose it (anywhere from 1/4 to full throttle) and it'll act like it didn't get any fuel, then VROOM! it catches right up and pulls off after about a second or two.

Oh, I also put new spark plugs in, because WHY NOT?

This engine barely has 15,000 miles on it, same for the carb. This just recently started happening, prior to 5 weeks ago the car ran perfect, started perfect, and was quite the menace on the pavement. Now, I'm at a loss, no clue what's up.

I'm just going to drive it for a couple days and see if it goes away on it's own before I spend anymore money, make sure the timing doesn't change. Maybe something is just gummed up in the carb (i'll shoot some cleaner in there).

Could it be the pick-up coil? Has anyone had one test out ok but actually be bad? I'm not past just buying a whole new MSD distributor and saying screw it.

Thanks,

RedLeader289

RedLeader289

iTrader: (2)

Bump.

UltRoadWarrior9

Are you sure the carb is right? You mentioned you thought that 'hesitation on sudden acceleration' felt like lack of fuel. Been awhile since I messed with a carb, but I remember having same problem when I was screwing with my Rochester main plunger spring, which affects how fast the fuel is delivered when pressing the throttle... I wish I had my Rochester book here so I could get the terminology correct, but it's at work. I still kept it heh. Hope you get it figured out, but these instances are great teachers. You never forget when you figure it out on your own.

sofakingdom

iTrader: (1)

Tried the "Holley Tuning" sticky over on the Carb forum yet?

RedLeader289

iTrader: (2)

Yup...twice. I took a look at the accelerator pump nozzles last night, the primaries have a 28, which seems awful small. I'm thinking of picking up a 31 and a 35 (since their so cheap) and see if either makes a difference. Because it pulls like a beast after the hesitation, no backfiring, missing, anything. I know the pump is good (I can watch it squirt) so my current thinking is the extra fuel just isn't getting there quick enough (hence the larger nozzle).

But...but...I'm still hesitant because it hasn't always done this. This car has been running/driving fine for years. Which raises the question "if the nozzle was too small...wouldn't it have been noticeable, oh, say 7 years ago?".

UltRoadWarrior9

That is strange... Is it possible the combustion chambers are now slightly larger due to normal wear and tear from compression/combustion and now it's at that threshold where all the cylinders need just a tad bit more fuel?

RedLeader289

iTrader: (2)

Maybe? it's worth a shot, and it's cheap. I'll update later whenever I get to it.

RedLeader289

iTrader: (2)

Took the carb off yesterday to get a better look at things. The primary accelerator pump was shot (diaphragm leaking/not in good shape). I'll be replacing the primary AND secondary diaphragms with Holley part# 135-10, which is made for modern fuels (the original clearly wasn't).

I'll update after the parts come in and I get her back together!!

RedLeader289

iTrader: (2)

Alrighty, update: new pump diaphragms in (holley part # 135-10) and I readjusted the accelerator pump arms (to still have .015" left at WOT).

drumroll........

I now have fuel in my primary nozzle! but it doesn't squirt out. If I take the nozzle off, and work the throttle, fuel will shoot up like it's supposed to, but with the nozzle on I get nothing.

I took the nozzle back off and checked it to make sure it was clean, it is, passages are clear.

Is it possible to overtighten it so that fuel can't make it past the screw? That's the only thing I could think of that I did. I was gonna put it back on tonight and see.

RedLeader289

iTrader: (2)

New thread since this is now soley a carb issue:

https://www.thirdgen.org/forums/carb...ml#post5969129

Ferburdz

Try changing and adjusting your tps with a volt reader. If you have the sensor positioned correctly the volt output should be .54 with the key in on, but not started. If this doesn’t fix your issue you may need to take a look at your fuel lines for leaks, a clogged or leaking fuel pressure hose that runs just behind your rear wheels could also be the culprit. My memory is a little foggy as to where exactly it’s located but it’s clustered up with the rest of the fuel lines running up your gas tank and into your fuel pump. If it’s not a bad sensor or throttle tune (downshift cable, linkage, bolt adjustments etc)