Danger,Will Roberson,Danger!

[voyager]

40 mile run ends with near disaster! My exhaust
leak might be from the intake manifold..is this
possible? I removed the air breather to discover
burned wires and the plastic wrap around them.
I had painted the intake manifold a few months
ago. When I had it running in the driveway I had
noticed a little soot around the " plate with
two small pipes coming out running to the carb "
This plate looked slightly curved upon inspection.
Meaning that it doesnt sit flat on the manifold.
I had put permatex gold on there before putting
it back together...big mistake. Looks like fire,
heat shot out from under and cooked the wiring
harness. BUMMER. I seperated the wires. Most are
ok. A couple had the insulation burned off. Now
my questions.
Anyone painted there intake manifold? Did the
paint burn off on both sides of the carb? My
paint did. Looks like a really hot spot there!
Right in the center of the manifold.I had the
heads off and reconditioned a few months ago.
Did I do something wrong? Could my timing be
off causing such a mess?
Help. Thanks,
Robert Teed 74 Painted Desert

 

[voyager]

Thanks for the great info Rick. Well said. You
put my mind at ease. I know the wires burned
from the choke stove plate leaking. Thats kind of
good news. They were next to the valve cover in
the factory wirewrap. I'll repair them tommorow.
The rest of the area around the carb is just
heat damage. So much for my nice paint job. Wish
I had closed off those heat ports!
Thanks,
Rob Teed 74 Painted Desert Lansing,Mi
( Still heading to the Keys in 3 Days )

> -----Original Message-----
> From: owner-gmcmotorhome
> [mailtowner-gmcmotorhome]On Behalf Of
> RickStapls
> Sent: Wednesday, May 12, 1999 1:48 AM
> To: gmcmotorhome
> Subject: Re: GMC: Danger,Will Roberson,Danger!
>
>

>
> > Now
> > my questions.
> > Anyone painted there intake manifold? Did the
> > paint burn off on both sides of the carb? My
> > paint did. Looks like a really hot spot there!
> > Right in the center of the manifold.I had the
> > heads off and reconditioned a few months ago.
> > Did I do something wrong? Could my timing be
> > off causing such a mess?
>
> Rob,
> You are simply experiencing the effect of the exhaust
> crossover passage,
> although maybe yours is a little hyperactive, with a bad choke stove.
>
> As we all know, the exhaust crossover (aka Heat Riser Passage) is a
> passage running beneath the carburetor in the intake manifold,
> carrying hot
> exhaust gasses from one side of the engine to the other. This
> creates a "hot
> spot" directly below the carburetor, whose purpose is to ensure complete
> vaporisation of gasoline in the fuel-air mixture. This is especially
> important during warm-up, allowing the choke to be turned off sooner,
> reducing cylinder wash-down and pollution. GM refers to it as the "Early
> Fuel Evaporation" system.
>
> The flow of exhaust is encouraged by the heat-riser valve, (GM: "EFE
> valve") mounted at the end of one exhaust manifold between manifold and
> exhaust pipe. This thermostaticaly-controlled butterfly valve blocks the
> flow of exhaust, forcing it to back up and flow through the
> crossover to the
> other exhaust manifold. It's supposed to open as it warms up,
> allowing free
> flow of exhaust, but they are prone to sticking. (A stuck heat
> riser valve
> causes reduced power and overheating, BTW...)
>
> Finally, since we have this very hot passage just under the
> carburetor,
> manufacturers use it to provide heat to the automatic choke.
> Sounds like you
> may have a leak in this "choke stove" system, although even
> without a leak
> the passage gets hot enough to burn the paint off the intake
> manifold in that
> area. Of course no wires or hoses should be allowed to rest on
> the intake
> manifold on either side of the carburetor.
>
> IMHO, this crossover is a candidate for removal in our coaches. Our
> engines work so hard, things get REALLY hot around the carb, and I don't
> think we need the additional heat. Besides, we have the
> thermostatically-controlled air cleaner to preheat the air BEFORE the
> carburetor, reducing the need for post-heating. Mondello sells a
> couple of
> steel stuffing blocks that may be inserted in the crossover
> passage to block
> it off safely. To complete the job, one must remove the heat riser valve
> (good, as it eliminates possibility of exhaust
> restriction/overheat from that
> source), and replace the OEM choke with an electrically-heated one. I
> haven't done this YET on my coach, but it's definitely on my to-do list.
>
> HTH.
>
> Rick Staples
> '75 Eleganza
> Louisville, CO
>
>
>

 

[lorry clavelli]

> ..... I removed the air breather to discover
> burned wires and the plastic wrap around them.
> I had painted the intake manifold a few months
> ago. When I had it running in the driveway I had
> noticed a little soot around the " plate with
> two small pipes coming out running to the carb "
> ........ Looks like fire, heat shot out from under and cooked the
> wiring
> harness.

A friend of mine had the same problem with a mid 70's Eldorado. It
caused delayed upshifts in the transmission from 1st - 2nd and 2nd -
3rd. The 3rd - 4th shift wouldn't occur until about 70MPH. It seems
that two of the burned wires in this harness attached to the switch on
the carb that controls the trans shift points. This may be the same for
the GMC and might be something to check in cases of shift problems.

Lorry
GMC Wannabe

 

[thomas g. warner]

Mine does not get that hot, but you are seeing exhaust gas recirculation
which is normal. I would replace the choke pot cover so that it is sealed.
Since you had the cylinder heads redone, it would have been a good time to
seal the exhaust port channel to the intake manifold.

>40 mile run ends with near disaster! My exhaust
> leak might be from the intake manifold..is this
> possible? I removed the air breather to discover
> burned wires and the plastic wrap around them.
> I had painted the intake manifold a few months
> ago. When I had it running in the driveway I had
> noticed a little soot around the " plate with
> two small pipes coming out running to the carb "
> This plate looked slightly curved upon inspection.
> Meaning that it doesnt sit flat on the manifold.
> I had put permatex gold on there before putting
> it back together...big mistake. Looks like fire,
> heat shot out from under and cooked the wiring
> harness. BUMMER. I seperated the wires. Most are
> ok. A couple had the insulation burned off. Now
> my questions.
> Anyone painted there intake manifold? Did the
> paint burn off on both sides of the carb? My
> paint did. Looks like a really hot spot there!
> Right in the center of the manifold.I had the
> heads off and reconditioned a few months ago.
> Did I do something wrong? Could my timing be
> off causing such a mess?
> Help. Thanks,
> Robert Teed 74 Painted Desert
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"

 

[zachary zehnacker]

Rick,

We haven't really found a good way to block the crossover yet. The
Edelbrock intakes now come with the steel blocks you mention. They
recommend they be used on any engines that see long periods of nearly WOT
and heavy loading. They recommend drilling a small hole through them to
allow some crossover on street applications. We put them in to the heads
and had them fitting very tight. After 20,000 miles, they had actually
loosened up and were actually wearing themselves down by bouncing around
inside the head. Luckily that had not gotten small enough to get into the
cylinder. Apparently they vibrate or something and eventually wear just
enough away to start moving around. Then they really start to wear down.
On the Olds mailing list, it was mentioned that these could be tack welded
into place. It is supposed to keep them from vibrating and bouncing
around. We didn't try it though.

Mondello also sells some sort of zinc alloy which is melted down and
poured into the head to fill it. I had heard good things about this
method, but while looking on the Olds mailing list, I found one report of
someone who had used the zinc alloy method with very bad results. They
took their intake off 2 years later and none of it was left in the heads.
It all had to melt and go through the exhaust valve and out the exhaust.

Zak

> IMHO, this crossover is a candidate for removal in our coaches. Our
>engines work so hard, things get REALLY hot around the carb, and I don't
>think we need the additional heat. Besides, we have the
>thermostatically-controlled air cleaner to preheat the air BEFORE the
>carburetor, reducing the need for post-heating. Mondello sells a couple of
>steel stuffing blocks that may be inserted in the crossover passage to block
>it off safely. To complete the job, one must remove the heat riser valve
>(good, as it eliminates possibility of exhaust restriction/overheat from
that
>source), and replace the OEM choke with an electrically-heated one. I
>haven't done this YET on my coach, but it's definitely on my to-do list.
>
> HTH.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>
>
>