Troubleshooting the new (to me) truck...

[shuttermutt]

A few weeks ago I picked up a 1968 CST LWB with a 327, factory air, TH400, Posi, and other goodies. The body's a little rough, but nothing out of the ordinary. I'm having some ignition and/or fuel troubles, though that are driving me wild.

Here are the symptoms:

- Runs best when cold (choke on), but a little miss can be heard. The miss gets worse when warm.
- When warm, she "stumbles" a little off the line. In other words, the truck hesitates and cuts out for a second or two when throttle is applied from a dead stop.
- On occasion, the truck is hard to start (almost like it's flooded) when allowed to warm-up then shut off for 5-10 minutes. When it does start, a cloud of grayish smoke comes out the exhaust.
- Good oil pressure except at hot idle when it drops to nearly zero and a slight knock can be heard from the bottom-end.
- Starting today, the truck is throwing an all-out hissy fit. Now it's backfiring pretty regularly through the carb when under a load and I'm hearing a "ticking" noise that varies with engine speed and sounds like it's coming from the distributor. If I put it in drive, take my foot off the gas, then tap the throttle it will backfire once through the carb then rev up relatively smooth.

These problems have been getting progressively worse as I've been driving the truck. What started as a little miss has manifested itself into a truck that doesn't like to run. The engine was supposedly rebuilt a few years back by a competent mechanic, but I'm treating it like a completely "unknown condition" engine. Here's what I've done so far:

- Replaced cap, points (yes, I know... HEI is on the way), condenser, plugs, and wires. BTW, the plugs looked okay, but were carbon fouled.
- Confirmed the timing at 8-degrees.
- Replaced the fuel filter, air filter, and PCV valve.
- Visually inspected for obvious vaccum leaks.
- Verified that the power valve (or whatever that damn thing is called on a Quad) wasn't stuck.
- Verified that the secondaries were closed all the way.
- Literally drowned the carb with B-12. Once while running and again while shut off (and left to sit overnight).
- Confirmed that the distributor looks okay. The base plate can be moved, the rotor doesn't turn backwards, nothing is cracked/missing/loose, the springs are on.
- Confirmed that the vaccum choke pulloff is working as it should.

Note that the engine doesn't ping (detonation) or smoke (other than described above).

My thinking is one or more of the following:

So, does anybody have any ideas what I might be looking at? Stuck valve? Broken spring? Flat cam? Possessed carb?

[DavidM]

Sounds like the possessed carb.....
Just my 2 cents....

[shuttermutt]

That's what I'm leaning towards, too. The truck did sit for at least a few months (reports vary from 1-3 years) before I got it. Plus, I have no idea what the yahoos did to get it running (bought the truck at auction - used to be a well-respected mechanic's daily driver until he passed away). Guess it's time to start carb shopping.

[chevy_fatman]

Originally posted by shuttermutt
Guess it's time to start carb shopping

Get your current carb rebuild or just buy a new one, I also believe that might be the reason to your truck not wanting to go.

BTW, Welcome to the board from PA, got any pics of your truck?

[cdowns]

i'd change the oil and filter and add a quart of marvel-mystery-oil and runn it for awhile to clean out the engine

[cableguy0]

sounds to me like your backfiring isnt ignition related but actually a lean pop due to the carb either being fubarred or wayyyyy out of adjustment. im gonna lean towards carb needing a new home in the trash

[Blue Beard]

try some carb cleaner..........and maybe fuel system cleaner..........

if it sat with old fuel...........

[EGLIDERIDER]

'Mutt
<Guess it's time to start carb shopping>

I wouldn't jump off of that cliff just yet. Q-jets are pretty easy to rebuild, at least if memory serves (and that's a BIG if these days). I've had a number of them over the years and used to rebuild them occasionally.......25+ years ago. Have you checked into availability of a rebuild kit?

However, if your set on a replacement there are several on E-bay. Here are links to a couple of them

http://cgi.ebay.com/ebaymotors/ws/eB...tem=2439556638

http://cgi.ebay.com/ebaymotors/ws/eB...tem=2439952116

Mike

[hanssolo]

Hesitation off idle sounds like not enuff accelerator pump shot. Also on old carbs, the bodies tend to warp and you can see it by running a straight edge across the throttle body. Even with a new gasket there can be a slight vacuum leak. A flat file will smooth it out easily.

[71GMC_3/4T]

My truck backfired a while back. I didn't know it at the time, but a vacuum cap had blown off when it backfired.
It would idle okay, but there was a distinct miss on acceleration. The vac cap was on the #8 intake runner. I was checking plugs, wires, dizzy cap, rotor, carb and all kinds of stuff when I stumbled upon the missing cap which was not in an obvious place. Fixed her right up!
Use a can of carb spray to look for vacuum leaks if everything looks to be in place.
I also agree that it could be your accelerator pump.

[shuttermutt]

A quick update...

I did a visual inspection for vacuum leaks. The PCV valve was about 1/2 way in, but that was it. I also did a carb cleaner check for leaks and came up empty. I also snuck out last night to see if I had a "light show" under the hood. It was pitch black. Finally, I pulled the vacuum advance line while idling. The engine slowed down and stumbled. The line pulled good vacuum.

Normally she runs better when cold, but not this time. The truck had been sitting for over 12 hours and the miss was immediately apparent first thing. Bummer.

I'm probably gonna do a two-fer swap. I've got a line on a good carb and HEI unit. Meanwhile the truck will have to sit in the corner and think about what it's done. Bad truck! BAD TRUCK!

[O'l Buck]

yours sounds like my 72 C-10.....same symptoms. Mines definitely the accelerator pump (long story short, don't take shortcuts when rebuilding carbs.......lesson learned)

[gonebad2]

Did the truck sit for an extended period of time? How old is the gas in the tank? Just a thought..

[shuttermutt]

I've driven several tanks of gas through it. What started as a little miss has progressively turned into this problem.

[adaminvegas]

vacuum leak at the brake booster if you have power brakes can cause all sorts of problems.

[adaminvegas]

oh yeah watch your heat riser 2could cause some problems also

[mrein3]

Originally posted by shuttermutt

- Runs best when cold (choke on), but a little miss can be heard. The miss gets worse when warm.
- When warm, she "stumbles" a little off the line. In other words, the truck hesitates and cuts out for a second or two when throttle is applied from a dead stop.
- On occasion, the truck is hard to start (almost like it's flooded) when allowed to warm-up then shut off for 5-10 minutes. When it does start, a cloud of grayish smoke comes out the exhaust.
- Good oil pressure except at hot idle when it drops to nearly zero and a slight knock can be heard from the bottom-end.
- Starting today, the truck is throwing an all-out hissy fit. Now it's backfiring pretty regularly through the carb when under a load and I'm hearing a "ticking" noise that varies with engine speed and sounds like it's coming from the distributor. If I put it in drive, take my foot off the gas, then tap the throttle it will backfire once through the carb then rev up relatively smooth.

These problems have been getting progressively worse as I've been driving the truck. What started as a little miss has manifested itself into a truck that doesn't like to run. The engine was supposedly rebuilt a few years back by a competent mechanic, but I'm treating it like a completely "unknown condition" engine. Here's what I've done so far:

- Replaced cap, points (yes, I know... HEI is on the way), condenser, plugs, and wires. BTW, the plugs looked okay, but were carbon fouled.
- Confirmed the timing at 8-degrees.
- Replaced the fuel filter, air filter, and PCV valve.
- Visually inspected for obvious vaccum leaks.
- Verified that the power valve (or whatever that damn thing is called on a Quad) wasn't stuck.
- Verified that the secondaries were closed all the way.
- Literally drowned the carb with B-12. Once while running and again while shut off (and left to sit overnight).
- Confirmed that the distributor looks okay. The base plate can be moved, the rotor doesn't turn backwards, nothing is cracked/missing/loose, the springs are on.
- Confirmed that the vaccum choke pulloff is working as it should.

Note that the engine doesn't ping (detonation) or smoke (other than described above).

My thinking is one or more of the following:

So, does anybody have any ideas what I might be looking at? Stuck valve? Broken spring? Flat cam? Possessed carb?

The last time I had backfires under load and a nice loud tick it took a cam to fix it.

The first time, about 10 years ago, it took a valve job to fix it.

Do a compression test. At least test the side with the noise. If you have compression pull the valve cover with the noise under it. I started mine with the valve cover off. It was obvious what lobe on the cam was almost flat. That rocker arm hardly moved and ticked like crazy.

[sneakysnake]

Like mrein3 said"do a compression check" You may have a bad head gasket or worse a cracked head from setting(improper antifreeze)and white smoke is a sign of water.

[adaminvegas]

smoke=black ritch fule blue oil white coolant
step1 verify problem
step2 atempt to duplicate
step3 check tsb or recal(known concerns)
step4 locate diag.info
step5isolate root cause
step6 fix or repair per diagnosis
step7 verify repair
step8 back to step 2
step9 bye bye

[8.1PWR]

If the the truck has a lot of miles on it you may want to check the play in the timing chain and gears. If they have not be replaced or the engine rebuilt there could be enough wear that it could be throwing your timing off. Just my 2 cents.