Towing with my '74 C-10

[egleaves]

I was going through the paperwork turned over to me by the dealer and found a specifications guide with the following excerpt:

"Set spark timing a 32 degrees BTDC @4000 rpm with the vacuum advance line to the distributor disconnected and plugged...(some other stuff).....This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve."

Does this sound like a good idea?  Have I been losing power by having the vacuum advance connected? The dealer connected the vacuum advance module to a port on the carb.  Today, I capped off both ends to see if there's a difference.  We just set up camp today, so I'll be driving around for a few days without the camper.

[Irish_Alley]

the vacuum advance should only be unplugged while setting it. if youre driving it it should be plugged into ported or manifold vacuum. im going to suggest manifold vacuum but this in my opinion wont fix youre problem

[Captain Swampy]

 Maybe your jetting is wrong for the elevation your at? It's possible the float might need adjusting. This is a stretch but, what about vapor lock? Where are your fuel lines routed. When you drop into 2nd and stand on it, you're using a higher volume of fuel and working the pump faster. If your starting to vapor lock, maybe that's flowing enough fuel to stop it for a while?

[blazer74]

You said backfire. Exhaust or intake? If you are at a higher alt than normal carb could be going lean at light to cruise throttle causing stumble or surging.

Vac can on dist blown would cause lack of power but shouldn't cause a stumble like you have.

74 points still?

Carb Quadrajet?

Mechanical advance is all part of the timing.  Base+mechanical+ vac advance.

32 & 4k might be a tad low but don't think it's your trouble.

[egleaves]

I found at least part of my problem. The cap and rotor were toasted!  A lot of the button was missing and some of the metal had actually transferred to the rotor tang. A hole was melted through the cap large enough that the button (what was left, anyway) fell out when I lifted the cap. See pics attached.

Installed new cap & rotor yesterday. Going to check and probably change the plugs today. I'm doing some research on the correct heat range for my application.

[egleaves]

Now that I'm somewhere i can post pics, here's what we're talking about.

Edit: I have no idea why the pics all posted upside down. 


Edit: Image orientation corrected - Admin

[egleaves]

I changed the plugs today and went with a slightly cooler plug.  The ones I took out didn't look bad....nice light brown color, but when I grabbed the boot on the #5 plug part of it disintegrated in my hand.  This boot was cooked also and must have been arcing to the exhaust manifold.  At times, I could swear I heard an arc, but could never find it.  Found it!

I went with cooler plugs in an attempt to reduce the overall resistance in the ignition system hoping to avoid future meltdowns.  I didn't bother checking the plug wire resistance because they're new, but I probably should anyway.

[Captain Swampy]

What plug? R44TSL?

[egleaves]

What plug? R44TSL?

MR44LTS is what was in it. I cross referenced MR43LTS to get the NKG equivalent. I forget the NKG number.

[Jim_Hensley]

  load on the truck shouldnt be over 3,740 , you can see these truck got 3.07 rear gear unless the optional ratio was selected.

[Captain Swampy]

 Sorry, screwed up the suffix. I just wanted to make sure you had the right length plug. Your vortec heads take longer threads,  3/4" I think, than your old heads with 1/2" long threads. I believe the lts means it's the long plug, and the short would be a ts. NGK's are my favorite.

[egleaves]

It's running much better since I replaced all of the components that melted.  Tomorrow is moving day again....we'll see how things go with the camper in tow.

[egleaves]

Sorry, screwed up the suffix. I just wanted to make sure you had the right length plug. Your vortec heads take longer threads,  3/4" I think, than your old heads with 1/2" long threads. I believe the lts means it's the long plug, and the short would be a ts. NGK's are my favorite.

Hah! And I screwed up, too!  NGK is correct.   

I did a side by side comparison before buying them to make sure the length was correct.  It all looks good.

[bd]

It's astonishing that the vehicle ran as well as it did, isn't it?  I've seen that occur a few times over the years.

I recommend that you replace the ignition coil, as well.  Once a spark begins jumping the wide gap between the coil center terminal and rotor contact for an extended period, the coil secondary insulation is often compromised by heat, excess voltage and carbon tracking.  Eventually the dielectric breaks down and 30+kv punches a hole through the coil insulation to the grounded coil frame or flashes internally to neighboring windings.

Do the spark plug wires have 90° boots at both ends?  The distributor boots generally are not the same material as the spark plug boots and will not cope with exhaust heat as well.  Make sure the wires were not installed backwards, end-for-end.  Additionally, smear a light coat of dielectric grease (e.g., GM 12345579, Permatex 81150, Loctite 37534, etc) on both metal terminal ends of every spark plug wire and inside the boots at the spark plug ends.

[egleaves]

bd, interesting point on the coil....it looked fine, but that sounds internal.  Sounds like a good excuse to get an MSD coil.   

I am quite surprised it ran very well at all.

The wires are 90 degree on both ends.  I noticed the markings on the replacement wire I bought indicating which end was which, but the other wires don't have them, however, one boot is noticeably thicker than the other.  I want to assume the thicker boot is meant to be next to the exhaust, and in this case that's how it was installed.  The material looks and feels the same, but there is lettering that says "DEED 13" on the thicker end and "DEED 22" on the other.