Backfiring

[davidlive]

My project coach, which i'm trying to only keep running barely, the engine isn't running the best,
started backfiring though the carburetor, but the engine starts up easily ?
(it started doing this after i parked it from a 1 hour drive)
it has a newer distributor (Hei) , i suspect the timing is off, but the mark has fallen off the front of the engine, so idk how to check that, other
then that, i've pulled the cap to make sure there was no moisture under the cap, i did pull a spark plug and it was black, which would confirm the
incorrect timing.

I plan on replacing the plugs, wires and cap. would there be any suggestions at which brand/model to use ?
and any suggestions at where to look for the reason for the backfiring though the carb ?

Thanks David

--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[robert mueller]

David,

Here's the $16.00 fix:

Tiny URL: https://tinyurl.com/hnrlgvw

Full URL:

http://www.ebay.com/itm/68-74-OLDSMOBILE-442-CUTLASS-W-30-TIMING-COVER-TAB-POINTER-BRACKET-CLIP-350-455-/161246841691?hash=item258b0
f8b5b:g:ElMAAOSwf05XPH-Q&vxp=mtr

Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of David Morrison
Sent: Sunday, November 27, 2016 6:22 PM
To: gmclist
Subject: [GMCnet] Backfiring

My project coach, which i'm trying to only keep running barely, the engine isn't running the best, started backfiring though the
carburetor, but the engine starts up easily? (it started doing this after i parked it from a 1 hour drive) it has a newer
distributor (Hei) , i suspect the timing is off, but the mark has fallen off the front of the engine, so idk how to check that,
other then that, i've pulled the cap to make sure there was no moisture under the cap, i did pull a spark plug and it was black,
which would confirm the
incorrect timing.

I plan on replacing the plugs, wires and cap. would there be any suggestions at which brand/model to use ?
and any suggestions at where to look for the reason for the backfiring though the carb ?

Thanks David

 

[Ken B]

Recheck the firing order.

The firing order picture of the distributor cap in the GMC manual is wrong.

That little plastic indicator that Rob pointed out is readily available. Dick Paterson also had a bunch of them on his table at one of the GMCMI
rallies.

That said, you could mark where the distributor is now and slightly change it to see if it makes anything better.

Finally check the spark plug wire routing to see that there is plenty of separation between wires. Then run it in the dark, looking for a spark
jumping between wires or to ground.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

 

[chris tyler]

To clarify, is it backfiring at idle? Or when you rev it?
If its backfiring at idle its probably retarded. Bump the timing in smalll increments.
Also, where is your vaccuum advance hooked up to?
Check to make sure the advance weights are not stuck as well.
Last, if you are changing the plugs, it would be a good idea to do a compression check. An engine that has sat around may have rusted up the valve
seats and could also explain the problem.That may or may not improve with it running a while.

--
76 Glenbrook

 

[john r. lebetski]

Concentrate on the left rear cyls. Many GM engines gave been ruined by a 5-7 to 7-5 wire swap. Essy mistake to make.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First

 

[davidlive]

I will check the firing order, and I'm going to get that timing tab (thank you), but to clarify
the backfiring though the carb, is at idle and reeving the engine, and the engine ran ok before i parked it after i drove it for 1-2 hours when i
moved it from a house to storage, and i found the backfiring when i went to start it after about a month in storage.

Thanks David
--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[jim kanomata1]

The back firing through the carb. indicates it must be running lean.
Suggest you remove the top cover of the air cleaner and see if the choke is
on strong enough.
Also advance the timing without the marker by rotating the distributor
clockwise few degrees.

On Sun, Nov 27, 2016 at 10:01 AM, David Morrison
wrote:

> I will check the firing order, and I'm going to get that timing tab (thank
> you), but to clarify
> the backfiring though the carb, is at idle and reeving the engine, and the
> engine ran ok before i parked it after i drove it for 1-2 hours when i
> moved it from a house to storage, and i found the backfiring when i went
> to start it after about a month in storage.
>
> Thanks David
> --
> 1974 G-M-C Painted Desert #136
> "Oversteer is when you hit the wall with the rear of the car, understeer
> is when you hit the wall with the front of the car. Hp is how fast you hit
> the wall, and torque is how far you take the wall with you when you hit
> it."
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502

 

[sgltrac]

Could also be a fuel supply issue.

Sully
77 eleganza 2
Seattle

> The back firing through the carb. indicates it must be running lean.
> Suggest you remove the top cover of the air cleaner and see if the choke is
> on strong enough.
> Also advance the timing without the marker by rotating the distributor
> clockwise few degrees.
>
> On Sun, Nov 27, 2016 at 10:01 AM, David Morrison >

>
> > I will check the firing order, and I'm going to get that timing tab
> (thank
> > you), but to clarify
> > the backfiring though the carb, is at idle and reeving the engine, and
> the
> > engine ran ok before i parked it after i drove it for 1-2 hours when i
> > moved it from a house to storage, and i found the backfiring when i went
> > to start it after about a month in storage.
> >
> > Thanks David
> > --
> > 1974 G-M-C Painted Desert #136
> > "Oversteer is when you hit the wall with the rear of the car, understeer
> > is when you hit the wall with the front of the car. Hp is how fast you
> hit
> > the wall, and torque is how far you take the wall with you when you hit
> > it."
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Fremont,CA
> jimk
> http://www.appliedgmc.com
> 1-800-752-7502
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[hal stclair]

Not to try and cry wolf but with the quality of our present gas you might have a fuel induced problem. If you ran the engine with old gas prior to the
last trip you might have a valve that is stuck and a guide that is galled. I've seen it happen way too often with the lousy fuel we have to use. If
the ignition tips don't work a quick compression check should tell the tale.
Hal
--
1977 Royale 101348,

1977 Royale 101586, Diesel powered,

1974 Eagle Bus 45',w/slideout

 

[davidlive]

Firing order is correct, I don't think bad fuel is a problem as I don't have the fuel tanks installed I use a 5gallon gas can, only thoughts on the
fuel is its just 87octane

So far wire brushing the plugs, (I do plan on replacing later on) but cleaning them should help,

So far the result on the compression check is
Cylinder
1 - 125
3 - 130
5 - 130
7 - 135

So the left side is good, now to check out the right side

--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[sean kidd]

A sticking float, or a needle that is not seating will cause erratic behavior at idle, check to see if carb is flooding by shining a flashlight down
carb an look for fuel dripping in from Venturi or around vents.
--
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.

Colonial Travelers

 

[davidlive]

The right side
2 - 140
4 - 125
6 - 135
8 - 120

Cylinder 8 is the lowest at first I got a reading of 100but I rechecked it after the rest and I got a steady 120

--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[davidlive]

I made a video of it as well

http://i3.photobucket.com/albums/y92/DavidLive/1974%20GMC%20Motorhome/VID_20161127_132908878_zpstqq5ggun.mp4
--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[davidlive]

A couple pics down the throttle

http://s3.photobucket.com/user/DavidLive/media/1974%20GMC%20Motorhome/IMG_20161127_133530298_zpszxy99vru.jpg.html

http://s3.photobucket.com/user/DavidLive/media/1974%20GMC%20Motorhome/IMG_20161127_133540592_zpsta5zftx0.jpg.html
--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[charles boyd]

Sir, your pressure readings do not indicate a leaking valve issue. So probably electrical. The video indicates the backfire is not random but is
probably isolated to one cylinder. First check cc firing order. 18436572. Check inside the distributor cap for carbon tracks which will cause
crossfiring. Also start the engine in the dark and look for wires sparking and crossfiring. I have seen a foulded plug backfire.

> The right side
> 2 - 140
> 4 - 125
> 6 - 135
> 8 - 120
>
> Cylinder 8 is the lowest at first I got a reading of 100but I rechecked it after the rest and I got a steady 120

--
C. Boyd
76 Crestmont
East Tennessee

 

[davidlive]

After I read your post c Boyd, I pulled one spark wire at a time from the cap, and it appears to be cylinder 5 right next to the do wiring , when I
disconnect it stops backfiring but oh course misses
--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[john r. lebetski]

Triple check that wires 5 and 7 are not swapped.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First

 

[davidlive]

I've done that, and i replaced the cap and rotor, and it's still doing it, I started replacing the plugs, which the old ones were autolite 87's which
i believe are non-hei plugs gapped about 40 avg, and put in a/c delco hei plugs gapped at 60, the wires i got were crap from autozone, so i have to
exchange them, one wire kept falling off the cap, another wire was missing the crimp piece in the boot .... so i'll get another set tomorrow and
hopefully make it out in the evening to see if that's helped.

I have a gut feeling telling me that's not the issue, and i'm left confused at why the #5 would be backfiring, tomorrow i plan on checking the engine
ground and grounding the distributor directly too, maybe there's a issue there.

Thanks David
--
1974 G-M-C Painted Desert #136
"Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit
the wall, and torque is how far you take the wall with you when you hit it."

 

[Ken H.]

David,

Once more for everyone: Set GMC plugs to 0.038"-0.040". More and you'll
overstress the HEI module, possibly leading to premature failure.

As you've already found, it's difficult to identify problems like yours.
But if you've truly refined the problem to #5 cylinder, you're almost
there. You reported the compressions on the right cylinders, but not the
left -- what's #5 compression? I fully expect it to be low. And if so the
earlier hypothesis of a sticking valve is probably correct. Even if it's
not significantly low during the cranking-speed test, it may still stick at
idle, giving the intermittent problem you've seen. It may clear itself up,
but a dose of Marvel Mystery Oil may be needed too.

JMHO,

Ken H.

On Sun, Nov 27, 2016 at 10:07 PM, David Morrison
wrote:

> I've done that, and i replaced the cap and rotor, and it's still doing it,
> I started replacing the plugs, which the old ones were autolite 87's which
> i believe are non-hei plugs gapped about 40 avg, and put in a/c delco hei
> plugs gapped at 60, the wires i got were crap from autozone, so i have to
> exchange them, one wire kept falling off the cap, another wire was missing
> the crimp piece in the boot .... so i'll get another set tomorrow and
> hopefully make it out in the evening to see if that's helped.
>
> I have a gut feeling telling me that's not the issue, and i'm left
> confused at why the #5 would be backfiring, tomorrow i plan on checking the
> engine
> ground and grounding the distributor directly too, maybe there's a issue
> there.
>
>

 

[jim kanomata1]

Did you replace the coil recently?

> I've done that, and i replaced the cap and rotor, and it's still doing it,
> I started replacing the plugs, which the old ones were autolite 87's which
> i believe are non-hei plugs gapped about 40 avg, and put in a/c delco hei
> plugs gapped at 60, the wires i got were crap from autozone, so i have to
> exchange them, one wire kept falling off the cap, another wire was missing
> the crimp piece in the boot .... so i'll get another set tomorrow and
> hopefully make it out in the evening to see if that's helped.
>
> I have a gut feeling telling me that's not the issue, and i'm left
> confused at why the #5 would be backfiring, tomorrow i plan on checking the
> engine
> ground and grounding the distributor directly too, maybe there's a issue
> there.
>
>
> Thanks David
> --
> 1974 G-M-C Painted Desert #136
> "Oversteer is when you hit the wall with the rear of the car, understeer
> is when you hit the wall with the front of the car. Hp is how fast you hit
> the wall, and torque is how far you take the wall with you when you hit
> it."
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk
http://www.appliedgmc.com
1-800-752-7502