[DeTomaso] Carb hesitation on level ground at cruise
Asa Jay Laughton
asajay at asajay.com
Wed Jul 31 21:39:17 EDT 2024
You and others have postulated the same idea. I have carb off and I’ve already found two rubber port pugs with cracks all the way through. First thing to replace.
While the carb is off, I will “bench” adjust it.
👍
Asa Jay
Sent via radio waves
> On Jul 31, 2024, at 18:14, Mike Drew <mikeldrew at aol.com> wrote:
>
> AJ,
>
> What you describe sounds to me like lean surge (or vacuum leak).
>
> But I freely admit I know less than most about these particular things….
>
> Mike
>
> Sent from my iPad
>
>> On Jul 31, 2024, at 18:06, Asa Jay Laughton via DeTomaso <detomaso at server.detomasolist.com> wrote:
>>
>> It is -not- an acceleration stumble. Perhaps I did not describe it properly. It accelerates nicely without stumble. Once up to speed and backing off throttle to simply hold the speed limit on level ground, the engine begins to hesitate, surge, stumble. Push the throttle and it picks up nicely and accelerates. Lift off throttle it smooths out.
>>
>> Accelerating is smooth
>> Deceleration is smooth
>> Hold throttle at cruise and it begins stumbling.
>>
>> Dawg’s carb worked fine on his car. He wanted to bling up the engine with the eight stack EFI. You know Dawg. 😆
>>
>> I’m in the shop now looking at things prior to any new adjustments.
>> <image0.jpeg>
>>
>> Thank you,
>> Asa Jay
>>
>> Sent via radio waves
>>
>>>> On Jul 31, 2024, at 17:34, Larry Stock <larrys at panteraparts.com> wrote:
>>>
>>> If we want to attack the Carburetor approach first, here's how I would approach it.
>>> It doesn't sound like a jet size thing, more like an accelerator pump thing, since it's an acceleration stumble.
>>> I would start with the easy, bigger (4 sizes) shooter nozzles, see what that does then analyze what kind of accelerator pump cams are in it
>>>
>>> Did the Dawgs carburetor work well? Sounds like a 780 is too big to work all that well, especially with low end response.
>>> Our Carburetors start with a 650 base with down-leg boosters for good low end grunt, but our venturis will flow 735 to keep the high end alive.
>>> Todays ethanol blends tend to lean our AF ratio's, so generally we need to feed more than we used to. It's called progress
>>> Larry Stock
>>>
>>> On 7/31/24, 2:36 PM, "Asa Jay Laughton" <asajay at asajay.com <mailto:asajay at asajay.com>> wrote:
>>>
>>>
>>> Thanks Jeff, I’m going after the lean stumble theory first. I’ll let you all know what I find though it may be a few days. Wrenching on other folks Panteras is more of a hobby after I get off my day job. 😐
>>>
>>>
>>> I’d get a lot more hobby done if I didn’t have to work for a living.
>>>
>>>
>>> Asa Jay
>>>
>>>
>>> Sent via radio waves
>>>
>>>
>>>>> On Jul 31, 2024, at 14:33, Jeff Kimball <jgkrenton at comcast.net <mailto:jgkrenton at comcast.net>> wrote:
>>>>
>>>> AJ:
>>>>
>>>> RE: "Trouble is there is no AF gauge on it, and my portable can only be stuffed in one of four exhaust tips."
>>>>
>>>> Your portable AF sensor should work fine. There is only one carb and the exhaust in all four pipes should be identical. What it reads in one should be the same in all. Check several if you want, but I don't think you'll find much difference.
>>>>
>>>> Sounds like a lean stumble at cruise/high vacuum conditions to me and when the power-valve opens under load/low vacuum it provides enough fuel to get rid of the stumble.
>>>>
>>>> And.. FYI, I was just digging around in the programing for my Pro-M EFI and noted there are something like 1500 variables that can be messed with.
>>>>
>>>> Good luck.
>>>>
>>>> Jeff/2467 with twin turbos and EFI.
>>>>
>>>>
>>>>>> On 07/31/2024 2:17 PM PDT Larry Stock <larrys at panteraparts.com <mailto:larrys at panteraparts.com>> wrote:
>>>>>
>>>>>
>>>>> Take the on off Madd Dawg's Pantera, but don't tell him I told you to.
>>>>> Larry
>>>>>
>>>>> On 7/31/24, 1:53 PM, "DeTomaso on behalf of Asa Jay Laughton via DeTomaso" <detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com> <mailto:detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com>> on behalf of detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>>> wrote:
>>>>>
>>>>>
>>>>> I wish I did have a spare carb ready to bolt on. Best I’ve got is a Holley 850 collecting dust on a shelf awaiting a rebuild (someday).
>>>>>
>>>>>
>>>>> I don’t recall a vacuum advance on this distributor. But now I’m going to investigate more because it should if it’s got a carb, unless there is a crank trigger wheel for something else.
>>>>>
>>>>>
>>>>> Plenty of ideas to get me checking things again.
>>>>>
>>>>>
>>>>> Thank you.
>>>>> Asa Jay
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Sent via radio waves
>>>>>
>>>>>
>>>>>>> On Jul 31, 2024, at 07:55, Will Kooiman <will.kooiman at gmail.com <mailto:will.kooiman at gmail.com> <mailto:will.kooiman at gmail.com <mailto:will.kooiman at gmail.com>>> wrote:
>>>>>>
>>>>>> It actually sounds like the timing is off. Does it have vacuum advance?
>>>>>>
>>>>>> 1) If it has vacuum advance, try unplugging it to see if it improves.
>>>>>> 2) Can you try a known good carburetor?
>>>>>>
>>>>>>
>>>>>> On 7/31/24, 1:09 AM, "DeTomaso on behalf of Asa Jay Laughton via DeTomaso" <detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com> <mailto:detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com>> <mailto:detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com> <mailto:detomaso-bounces at server.detomasolist.com <mailto:detomaso-bounces at server.detomasolist.com>>> on behalf of detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>>>> wrote:
>>>>>>
>>>>>>
>>>>>> I have successfully (for the most part) tuned an Electromotive system. Still some things to learn there but it sure ran a lot better after I tuned it.
>>>>>>
>>>>>>
>>>>>> I agree, carbs are fairly simple but there are some complexities. I’m not so much lost as I may be leading you to believe.
>>>>>>
>>>>>>
>>>>>> Asa Jay
>>>>>>
>>>>>>
>>>>>> Sent via radio waves
>>>>>>
>>>>>>
>>>>>>> On Jul 30, 2024, at 20:55, Mike Drew <mikeldrew at aol.com <mailto:mikeldrew at aol.com> <mailto:mikeldrew at aol.com <mailto:mikeldrew at aol.com>> <mailto:mikeldrew at aol.com <mailto:mikeldrew at aol.com> <mailto:mikeldrew at aol.com <mailto:mikeldrew at aol.com>>>> wrote:
>>>>>>>
>>>>>>> AJ,
>>>>>>>
>>>>>>> If you are a bit lost with a carb I think you will be completely in the woods with injection. A carb is an extremely simple device by comparison.
>>>>>>>
>>>>>>> Smart people, chime in!
>>>>>>>
>>>>>>> Mike
>>>>>>>
>>>>>>>
>>>>>>> Sent from my iPad
>>>>>>>
>>>>>>>>> On Jul 30, 2024, at 20:45, Asa Jay Laughton via DeTomaso <detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>>>> wrote:
>>>>>>>>
>>>>>>>> Owner has asked if we could fix all problems with fuel injection. I’m not a fan of saying that because I know a well tuned carb can be just as good as well tuned EFI.
>>>>>>>>
>>>>>>>> If the owner persists, what is the collectives opinions on various EFI systems out there today?
>>>>>>>>
>>>>>>>> Thank you,
>>>>>>>> Asa Jay
>>>>>>>>
>>>>>>>> Sent via radio waves
>>>>>>>>
>>>>>>>>>> On Jul 30, 2024, at 20:37, Asa Jay Laughton via DeTomaso <detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com> <mailto:detomaso at server.detomasolist.com <mailto:detomaso at server.detomasolist.com>>>> wrote:
>>>>>>>>>
>>>>>>>>> Hi all,
>>>>>>>>> I'm working with a PIM 427 Hammer (stroked Windsor) in a Pantera. This has a Demon carb on it. I don't know what it has for jets. I know it runs a little rich.
>>>>>>>>>
>>>>>>>>> Here is my situation and in a bit of humility, am stumped at the moment. Sam and I just took it out for a long test drive and it has problems on leverlk ground, no load, cruising mode.
>>>>>>>>>
>>>>>>>>> It idles at about 800 rpm, a bit lumpy but starts right up cold, doesn't die, no choke (no choke on the Demon carb).
>>>>>>>>> It revs easily and pulls away from a stop sign beautifully.
>>>>>>>>> Under load, moderate throttle or foot to the floor, it runs great.
>>>>>>>>> Off throttle, engine braking, it runs fine, though it doesn't burbble like I would expect
>>>>>>>>> Uphill great, downhill not so much (that cruise position of the foot on the go-pedal)
>>>>>>>>> On level ground, 5th gear, holding cruise at 60 MPH at around 2,000 rpm, it hesitates, jerks, feels like it's misfiring.
>>>>>>>>> Same when in say 4th, holding cruise at a higher rpm but no load either with go-pedal down, or engine braking
>>>>>>>>> It does this hesitation at nearly any rpm from 1,500 to 3,500 only when holding cruise speed, not when accelerating (even mildly)
>>>>>>>>>
>>>>>>>>> I'm thinking the problem is the idle mixture and speed settings. I'm thinking the throttle plates might be closed wee bit too much and idle mixture is compensating. When the throttle is at off-idle, at cruise, the transfer ports aren't giving enough gas, or maybe it's air. Trouble is there is no AF gauge on it, and my portable can only be stuffed in one of four exhaust tips.
>>>>>>>>>
>>>>>>>>> <sigh> so I'm asking for the collectives opinions, I know there are some good ones out there. I figure I need to re-check timing, advance, and carb tune again.
>>>>>>>>>
>>>>>>>>> Thoughts?
>>>>>>>>> Thank you,
>>>>>>>>> Asa Jay
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Asa Jay Laughton - W7TSC, MSgt, USAFR, Retired
>>>>>>>>> Spokane County ARES-RACES-ACS EC/RO
>>>>>>>>> ******************************
>>>>>>>>> https://w7tsc.org <https://w7tsc.org> <https://w7tsc.org> <https://w7tsc.org>> <https://w7tsc.org> <https://w7tsc.org>> <https://w7tsc.org>> <https://w7tsc.org>>>
>>>>>>>>> https://www.teampanteraracing.com <https://www.teampanteraracing.com> <https://www.teampanteraracing.com> <https://www.teampanteraracing.com>> <https://www.teampanteraracing.com> <https://www.teampanteraracing.com>> <https://www.teampanteraracing.com>> <https://www.teampanteraracing.com>>>
>>>>>>>>>
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>>>>>>
>>>>>> _______________________________________________
>>>>>>
>>>>>>
>>>>>>
>>>>>>
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>>>>>> http://server.detomasolist.com/mailman/listinfo/detomaso <http://server.detomasolist.com/mailman/listinfo/detomaso> <http://server.detomasolist.com/mailman/listinfo/detomaso> <http://server.detomasolist.com/mailman/listinfo/detomaso>> <http://server.detomasolist.com/mailman/listinfo/detomaso> <http://server.detomasolist.com/mailman/listinfo/detomaso>> <http://server.detomasolist.com/mailman/listinfo/detomaso>> <http://server.detomasolist.com/mailman/listinfo/detomaso>>>
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-------------- next part --------------
You and others have postulated the same idea. I have carb off and I've
already found two rubber port pugs with cracks all the way through.
First thing to replace.
While the carb is off, I will "bench" adjust it.
Asa Jay
Sent via radio waves
On Jul 31, 2024, at 18:14, Mike Drew <mikeldrew at aol.com> wrote:
AJ,
What you describe sounds to me like lean surge (or vacuum leak).
But I freely admit I know less than most about these particular
things....
Mike
Sent from my iPad
On Jul 31, 2024, at 18:06, Asa Jay Laughton via DeTomaso
<detomaso at server.detomasolist.com> wrote:
It is -not- an acceleration stumble. Perhaps I did not describe it
properly. It accelerates nicely without stumble. Once up to speed
and backing off throttle to simply hold the speed limit on level
ground, the engine begins to hesitate, surge, stumble. Push the
throttle and it picks up nicely and accelerates. Lift off throttle
it smooths out.
Accelerating is smooth
Deceleration is smooth
Hold throttle at cruise and it begins stumbling.
Dawg's carb worked fine on his car. He wanted to bling up the
engine with the eight stack EFI. You know Dawg.
I'm in the shop now looking at things prior to any new adjustments.
<image0.jpeg>
Thank you,
Asa Jay
Sent via radio waves
On Jul 31, 2024, at 17:34, Larry Stock <larrys at panteraparts.com>
wrote:
If we want to attack the Carburetor approach first, here's how I
would approach it.
It doesn't sound like a jet size thing, more like an accelerator
pump thing, since it's an acceleration stumble.
I would start with the easy, bigger (4 sizes) shooter nozzles, see
what that does then analyze what kind of accelerator pump cams are
in it
Did the Dawgs carburetor work well? Sounds like a 780 is too big to
work all that well, especially with low end response.
Our Carburetors start with a 650 base with down-leg boosters for
good low end grunt, but our venturis will flow 735 to keep the high
end alive.
Todays ethanol blends tend to lean our AF ratio's, so generally we
need to feed more than we used to. It's called progress
Larry Stock
On 7/31/24, 2:36 PM, "Asa Jay Laughton" <asajay at asajay.com
<mailto:asajay at asajay.com>> wrote:
Thanks Jeff, I'm going after the lean stumble theory first. I'll let
you all know what I find though it may be a few days. Wrenching on
other folks Panteras is more of a hobby after I get off my day job.
I'd get a lot more hobby done if I didn't have to work for a living.
Asa Jay
Sent via radio waves
On Jul 31, 2024, at 14:33, Jeff Kimball <jgkrenton at comcast.net
<mailto:jgkrenton at comcast.net>> wrote:
AJ:
RE: "Trouble is there is no AF gauge on it, and my portable can only
be stuffed in one of four exhaust tips."
Your portable AF sensor should work fine. There is only one carb and
the exhaust in all four pipes should be identical. What it reads in
one should be the same in all. Check several if you want, but I
don't think you'll find much difference.
Sounds like a lean stumble at cruise/high vacuum conditions to me
and when the power-valve opens under load/low vacuum it provides
enough fuel to get rid of the stumble.
And.. FYI, I was just digging around in the programing for my Pro-M
EFI and noted there are something like 1500 variables that can be
messed with.
Good luck.
Jeff/2467 with twin turbos and EFI.
On 07/31/2024 2:17 PM PDT Larry Stock <larrys at panteraparts.com
<mailto:larrys at panteraparts.com>> wrote:
Take the on off Madd Dawg's Pantera, but don't tell him I told you
to.
Larry
On 7/31/24, 1:53 PM, "DeTomaso on behalf of Asa Jay Laughton via
DeTomaso" <detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>
<mailto:detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>> on behalf of
detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>>> wrote:
I wish I did have a spare carb ready to bolt on. Best I've got is a
Holley 850 collecting dust on a shelf awaiting a rebuild (someday).
I don't recall a vacuum advance on this distributor. But now I'm
going to investigate more because it should if it's got a carb,
unless there is a crank trigger wheel for something else.
Plenty of ideas to get me checking things again.
Thank you.
Asa Jay
Sent via radio waves
On Jul 31, 2024, at 07:55, Will Kooiman <will.kooiman at gmail.com
<mailto:will.kooiman at gmail.com> <mailto:will.kooiman at gmail.com
<mailto:will.kooiman at gmail.com>>> wrote:
It actually sounds like the timing is off. Does it have vacuum
advance?
1) If it has vacuum advance, try unplugging it to see if it
improves.
2) Can you try a known good carburetor?
On 7/31/24, 1:09 AM, "DeTomaso on behalf of Asa Jay Laughton via
DeTomaso" <detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>
<mailto:detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>>
<mailto:detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>
<mailto:detomaso-bounces at server.detomasolist.com
<mailto:detomaso-bounces at server.detomasolist.com>>> on behalf of
detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>>>> wrote:
I have successfully (for the most part) tuned an Electromotive
system. Still some things to learn there but it sure ran a lot
better after I tuned it.
I agree, carbs are fairly simple but there are some complexities.
I'm not so much lost as I may be leading you to believe.
Asa Jay
Sent via radio waves
On Jul 30, 2024, at 20:55, Mike Drew <mikeldrew at aol.com
<mailto:mikeldrew at aol.com> <mailto:mikeldrew at aol.com
<mailto:mikeldrew at aol.com>> <mailto:mikeldrew at aol.com
<mailto:mikeldrew at aol.com> <mailto:mikeldrew at aol.com
<mailto:mikeldrew at aol.com>>>> wrote:
AJ,
If you are a bit lost with a carb I think you will be completely in
the woods with injection. A carb is an extremely simple device by
comparison.
Smart people, chime in!
Mike
Sent from my iPad
On Jul 30, 2024, at 20:45, Asa Jay Laughton via DeTomaso
<detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>
<mailto:detomaso at server.detomasolist.com
<mailto:detomaso at server.detomasolist.com>>>> wrote:
Owner has asked if we could fix all problems with fuel injection.
I'm not a fan of saying that because I know a well tuned carb can be
just as good as well tuned EFI.
If the owner persists, what is the collectives opinions on various
EFI systems out there today?
Thank you,
Asa Jay
Sent via radio waves
On Jul 30, 2024, at 20:37, Asa Jay Laughton via DeTomaso
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<mailto:detomaso at server.detomasolist.com>>>> wrote:
Hi all,
I'm working with a PIM 427 Hammer (stroked Windsor) in a Pantera.
This has a Demon carb on it. I don't know what it has for jets. I
know it runs a little rich.
Here is my situation and in a bit of humility, am stumped at the
moment. Sam and I just took it out for a long test drive and it has
problems on leverlk ground, no load, cruising mode.
It idles at about 800 rpm, a bit lumpy but starts right up cold,
doesn't die, no choke (no choke on the Demon carb).
It revs easily and pulls away from a stop sign beautifully.
Under load, moderate throttle or foot to the floor, it runs great.
Off throttle, engine braking, it runs fine, though it doesn't
burbble like I would expect
Uphill great, downhill not so much (that cruise position of the foot
on the go-pedal)
On level ground, 5th gear, holding cruise at 60 MPH at around 2,000
rpm, it hesitates, jerks, feels like it's misfiring.
Same when in say 4th, holding cruise at a higher rpm but no load
either with go-pedal down, or engine braking
It does this hesitation at nearly any rpm from 1,500 to 3,500 only
when holding cruise speed, not when accelerating (even mildly)
I'm thinking the problem is the idle mixture and speed settings. I'm
thinking the throttle plates might be closed wee bit too much and
idle mixture is compensating. When the throttle is at off-idle, at
cruise, the transfer ports aren't giving enough gas, or maybe it's
air. Trouble is there is no AF gauge on it, and my portable can only
be stuffed in one of four exhaust tips.
<sigh> so I'm asking for the collectives opinions, I know there are
some good ones out there. I figure I need to re-check timing,
advance, and carb tune again.
Thoughts?
Thank you,
Asa Jay
--
Asa Jay Laughton - W7TSC, MSgt, USAFR, Retired
Spokane County ARES-RACES-ACS EC/RO
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