SOOR, sooner or later: Questions about fuel percolating

D

dan rarick

New member
Jul 26, 2005
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Can any of you out there help with a new problem for me (but I gather=20
that it is "old hat for GMC's in general)? I heard fuel percolating=20
when I shut the coach off after a short test drive. The coach only=20
reached normal operating temperature, I live close to sea level, and it=20
is only 85=BA outside. This is absolutely the first time for me to=20
experience the weak power at low rpm and to hear this percolation on my=20
motorhome.

Today I replaced my fuel pump because the old (OEM) fuel pump preferred=20
to not go highway speeds, as opposed to now. On my short test drive It=20
ran much better in the higher rpm's and under loads (hills), but a=20
brand new problem occurred: weak acceleration in the very low rpm's=20
(but good acceleration on the slightly higher rpm's.

Now, I want to know if this slight percolation is just a forerunner of=20
what's going to simply upgrade to a crippling vapor lock, since I=20
understand it is likely a mild vapor lock now?

I have some winter fuel in the coach because I haven't used it since=20
March. The manual says this can add to the vapor lock problem. =20
Especially since there is water in the fuel. I found this out when=20
working with the generator.

Do you folks experience this and just keep going with your plans to=20
leave on a trip? Or do you consider this mild vapor lock manageable,=20
and just drive in the cool of the morning?

Dan Rarick
'75 Glenbrook
Springfield, OR

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Bob,

Thank you for responding, I appreciate your help, I heard it right in=20
the top of the carburetor. I just recently put in a new fuel filter=20
(that's what led me to replace the fuel pump), I'll check the=20
centrifugal advance in a minute. By the way, do you bring that up=20
because of the hesitant spot on slow (10 mph) speed acceleration? ..=20
just wondering if that's where the spark advances on those speeds as=20
opposed to the vacuum advance.. does that come in on higher load? =20
..just hoping to learn more.

Dan Rarick
'75 Glenbrook
Springfield, OR

>
>
> "" I heard fuel percolating
> when I shut the coach off after a short test drive. The coach only
> reached normal operating temperature, I live close to sea level, and it
> is only 85=BA outside. This is absolutely the first time for me to
> experience the weak power at low rpm and to hear this percolation on my
> motorhome.""
>
> Dan--where or how do you hear fuel perculating? The winter fuel may be=20
> a problem depending on when and where you got it so it would be a good=20
> idea to run it through. You probably should check the carb fuel filter=20
> and replace it--if it is dirty, a good fuel pump should be able to=20
> bypass it on it's spring, but it is worth doing. I suspect you may=20
> have a stuck centrifugal advance in the distributor after sitting.=20
> Remove the cap and see if you can rotate the rotor 10 or so=20
> degrees--if not it is stuck. I assume you have checked the vacuum=20
> lines around the carb for cracks or leaks. How does it idle? I'm sure=20
> others will have more thoughts.
>
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
>
> ---
> To unsubscribe send a blank email to=20
> mailto:unsubscribe-gmclist
>

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Oh, I forgot to respond to the question of idle: it idles real steady=20
now, but before the fuel pump change it was a wavering idle at times.

Dan

>
>
> "" I heard fuel percolating
> when I shut the coach off after a short test drive. The coach only
> reached normal operating temperature, I live close to sea level, and it
> is only 85=BA outside. This is absolutely the first time for me to
> experience the weak power at low rpm and to hear this percolation on my
> motorhome.""
>
> Dan--where or how do you hear fuel perculating? The winter fuel may be=20
> a problem depending on when and where you got it so it would be a good=20
> idea to run it through. You probably should check the carb fuel filter=20
> and replace it--if it is dirty, a good fuel pump should be able to=20
> bypass it on it's spring, but it is worth doing. I suspect you may=20
> have a stuck centrifugal advance in the distributor after sitting.=20
> Remove the cap and see if you can rotate the rotor 10 or so=20
> degrees--if not it is stuck. I assume you have checked the vacuum=20
> lines around the carb for cracks or leaks. How does it idle? I'm sure=20
> others will have more thoughts.
>
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
>
> ---
> To unsubscribe send a blank email to=20
> mailto:unsubscribe-gmclist
>

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Arch,

Thanks for asking, no they aren't yet. I know it was boiling in the
carburetor, the only thing I'm kind of curious about regarding that, is
do you need to have any cracks in the manifold to get the symptom of
boiling? Or is that just for free, no cracks required?

Just interesting,
Dan

> In a message dated 7/19/2006 9:38:59 P.M. Central Daylight Time,

>
>> I heard it right in the top of the carburetor.
>
> Dan
>
> Do you have the crossovers blocked? If not what you heard was the fuel
> in the carb boiling. JMHO
>
> Take Care
> Arch
>
>
> ---
> To unsubscribe send a blank email to
> mailto:unsubscribe-gmclist
>

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In a message dated 7/19/2006 9:38:59 P.M. Central Daylight Time,

>I heard it right in the top of the carburetor.

Dan

Do you have the crossovers blocked? If not what you heard was the fuel
in the carb boiling. JMHO

Take Care
Arch


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"" By the way, do you bring that up
because of the hesitant spot on slow (10 mph) speed acceleration? ..
just wondering if that's where the spark advances on those speeds as
opposed to the vacuum advance.. does that come in on higher load?
..just hoping to learn more.""

Hi Dan--I mentioned the centrifugal advance since mine stuck in the advanced position and it took me a while to figure out why I had a hot start problem. They can also stick in the retarded position which will show up as unresponsive feel at slow to mid speeds as well as a hotter running engine. I don't think you should see much percolation in the carb at 85 degrees unless it sits and heat soaks for a while. These coaches should run just fine without the heat crossover blocked (even though I think it's a good idea). Even if the fuel percolates after sitting, it should clear up and run fine after less than a minute. It does sound like a lean condition at part throttle which could be due to a stuck power valve, low float level, or a cracked intake. I still think it may be best to run that old fuel out of there first.

--
Bob de Kruyff
78 Eleganza
Chandler, AZ

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In a message dated 7/19/2006 9:59:26 P.M. Central Daylight Time,

>Thanks for asking, no they aren't yet. I know it was boiling in the
>carburetor, the only thing I'm kind of curious about regarding that, is
>do you need to have any cracks in the manifold to get the symptom of
>boiling?

Dan

No you do not need cracks in the manifold to boil the gas in the carb.
The cross overs can do it all by them selves. LOL

Take Care
Arch


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Bob,

I took it on another test drive after checking that the centrifugal
advance is quite fluid and with good spring action. This last test
drive was very enlightening: cleared up the hesitation on slow speed
acceleration, found no more symptoms of that slow type. I guess it
just might have taken the carburetor some miles and cooler driving
conditions until the float set just right. I'm very surprised, I've
never experienced such a (lengthy) transition to proper operation after
changing a fuel pump on any other vehicles I've owned, but then they
aren't 12,000 pound antiques with plumbing. I guess I could have
experienced something getting pushed through the fuel filter or a carb
jet earlier, don't know. I also might have had a temporary sticking of
the mechanical advance, would that problem always have hard restarting
of a warm motor?

Dan

>
>
> "" By the way, do you bring that up
> because of the hesitant spot on slow (10 mph) speed acceleration? ..
> just wondering if that's where the spark advances on those speeds as
> opposed to the vacuum advance.. does that come in on higher load?
> ..just hoping to learn more.""
>
> Hi Dan--I mentioned the centrifugal advance since mine stuck in the
> advanced position and it took me a while to figure out why I had a hot
> start problem. They can also stick in the retarded position which will
> show up as unresponsive feel at slow to mid speeds as well as a hotter
> running engine. I don't think you should see much percolation in the
> carb at 85 degrees unless it sits and heat soaks for a while. These
> coaches should run just fine without the heat crossover blocked (even
> though I think it's a good idea). Even if the fuel percolates after
> sitting, it should clear up and run fine after less than a minute. It
> does sound like a lean condition at part throttle which could be due
> to a stuck power valve, low float level, or a cracked intake. I still
> think it may be best to run that old fuel out of there first.
>
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
>
> ---
> To unsubscribe send a blank email to
> mailto:unsubscribe-gmclist
>

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"" I also might have had a temporary sticking of
the mechanical advance, would that problem always have hard restarting
of a warm motor?""

Hi Dan--glad things are looking up. I doubt that you would have temporary sticking of the centrifugal advance--my experience has been that they are stuck or they aren't. However, it's not unusual for them to get stuck after sitting. If the timing is too advanced while cranking, you will have a real hot crank problem. I first resorted to the typical battery stuff including cable sizes and connections. I fooled with it on and off for a year and did make some progress. but when hot it would still crank slowly--I even rebuilt the starter. Later, when I was on another quest to reduce pinging, I discovered the mechanical advance stuck n full advance. I had to remove the distributor to fix it. After that, the engine cranked like it has never done before--I guess all of the other stuff I did was not in vain :)
I suspect your issue was fuel related. I sure would love to attnd the rally, but just came back from a 4000 mile trip and I'm broke!

--
Bob de Kruyff
78 Eleganza
Chandler, AZ

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Dan,
A vacuum guage should tell you about any possible vacuum leaks. A cracked
manifold usually will show up more when the manifold is hot and the crack
widens. At least it did on mine. HTH,
Al Scott
75 PB
Dixon, CA


> Arch,
> Thanks for asking, no they aren't yet. I know it was boiling in the
> carburetor, the only thing I'm kind of curious about regarding that, is
> do you need to have any cracks in the manifold to get the symptom of
> boiling? Or is that just for free, no cracks required?

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