driveability problems with the tow rig

[gonecheenin]

79' k10 4x4 M/T truck 350ci

running a swapped in 74' 350ci engine out of my old K20 road driven trail truck
Never had a problem like this in the 74', the 79's old engine ran once and a while like it had a bad valve (that came and went from time to time, i finally took it out when i spun a rod bearing in it)

i swapped the intake manifold from the 79 onto the 74' engine because i wanted to retain the appearance of still having all the emmisions stuff hooked up for inspection reasons (the 80 and back vehicles are no longer sniff tested so all i have to do is pass a visual)

I have been having problems with this truck missing and stuttering under a hot load condition

when the truck is cold it runs great, the warmer it gets the worse it stutters and misses under load
seems almost like it's loading up at low RPM-high load
let off the gas and it smooths out
get the RPM's way up and it smooths out

when just cruising on the level it runs fine

Also idles smooth and strong

So far i've replaced the Carb with a rebuilt, checked for Vaccum leaks, plugged off the hoses running to the evaporative emmisions canister and i'm currently blocking off the EGR ports with epoxy (that way the EGR is a big but legal looking block off plate)

if I get this EGR valve back on and it still acts up i'm at kinda a loss at this point

when i replaced the carb i saw that inside the intake runners was alot of carbon buildup as if the EGR was letting too much exhaust and such into the intake

do you suppose that all that carbon being built up is causing enough turbulence to make the fuel puddle at times when hot and cause a over fueled condition?

if so then i suppose i'll buy an aluminum preformer intake manifold next

 

[Mad-Dog]

The carbon build-up in intake manifold is the result of a fair amount of exhaust reversion...............but i doubt this is causing the sputter you describe..........I'm gonna eat some lunch and think about this one.

 

[gonecheenin]

Thanks

I'm not familiar with the term exhaust reversion

would you mind explaining further?

 

[Roz]

timing? u have a vacume advance? mine was same found out it was off a little. still havent worked out all the buggs but that made it run tons better

 

[gonecheenin]

trying to remember anything else important

All wear item ignition components are new and high quality NAPA
plugs are delco R45TS
All filters are new
Base timing is set at approx 12-14 degrees
curve kit from Summit starts timing advance at about 1800 RPM brings Max timing of approx 30 degrees in at about 4200 RPM

intake was swapped on using FelPro gaskets
74' engine had HD style head castings

never set the valves, but it ran fine in the 74' last year

nothing else relevent comes to mind

 

[gonecheenin]

timing? u have a vacume advance? mine was same found out it was off a little. still havent worked out all the buggs but that made it run tons better

yes, most definitly has a vaccum advance

what was wrong with your vaccum advance, was it leaking, not advancing, sticking?

for all the more it will cost i'll pick up a new one tonight
Might even try throwing the stock advance weights and springs back in it also

what is the proper style vaccum source to hook the vaccum advance too on a old GM, i always hooked to manifold so it would advance when idling, but i think it's actually supposed to be hooked to port isn't it?




I am SO ready to swap this thing to TBI injection

 

[1-ton]

when the truck is cold it runs great, the warmer it gets the worse it stutters and misses under load

This statement is the key to your problem. The two things I can think of that could cuase an engine to have problems only when it is hot is ignition and vapor lock problems.

 

[chev4life]

I had a similar problem on my 87 with TBI...turned out it was part the coil and part the fuel filter. Once I replaced both I was in smooth sailing.

 

[86chevybanshee]

had this problem with my s10, put all new ignition parts in, and i mean everything, pickup coil, module, cap ,rotor, esc,plugs, wires, coil, all ac delco. and it still did it. so it kept getting worse and worse.

i took the plugs out to do a compression test, and i found a couple of broken plugs, like the glass part was loose in the metal part.

so id say check all the plugs

 

[Roz]

was set wrong or moved-dunno. but was off so it ran good and as it warmed up sputtered spit died all the time. could barly get it to move. so pulled vac line to dist. set timing and put it back on ran good. then had to adjust carb cause it adjusted it cold and with timing off

 

[gonecheenin]

This statement is the key to your problem. The two things I can think of that could cuase an engine to have problems only when it is hot is ignition and vapor lock problems.

A friend of mine thinks i may have caused this with the curve kit, he's thinking perhaps i'm getting too much timing too soon in the RPM curve?

make any sense?
never had that problem with my blazer 350, but that engine had flat tops, a bigger cam and an automatic

i'm also gonna try to move the fuel feed line back from the water pump a bit and check to see if the headers are to close to my incoming fuel feed.

wouldn't vapor lock cause a problem the more you tried to pull fuel though?
cause if i drop a gear and stand on it to get the RPMs up it picks up and takes off pretty good and smooth


The ignition coil!
high load, low RPM, would equal high cylinder pressures wouldn't it?

so that would be the most difficult situation to fire a plug in right?
I wonder if my Coil is overheating?

 

[1-ton]

cause if i drop a gear and stand on it to get the RPMs up it picks up and takes off pretty good and smooth[/QUOTE

It might still be a fuel starvation problem (or even exsesive amounts of fuel), if this is the case, which has nothing to do with heat build up.

 

[Mad-Dog]

I didn't read what he replaced but it sounds like the ignition is breaking down after the motor warms up, vapor lock would cause a no-start condition.

I diagnosed a hot-soak ignition problem once by wrapping a bag of ice inside a towel and placing it on top of the distributor.......ran fine, go figure.

Swapped out the coil and module......all fixed

 

[Mad-Dog]

If the advance curve was too fast it would cause low-speed driveability problems, i don't think it would cause sputtering and missing.........but then again ya never know

preignition would also be a problem if the octane level was too low for the installed CR with a fast advance curve

Vacuum advance (when incorporated into the total advance) should be routed to a ported source

 

[gonecheenin]

whelp, it isn't the brake booster or the EGR valve
(also not the coil or the module i found out later)

I started playing with the ignition timing like another buddy suggested and it started running alot better
so after i got it running the best i could that way, i climbed under the hood again with the timing light this time to see what the timing had come to rest at
while under there i heard a ticking sound (that was unlike my header ticking sounds that are always there)
upon placing my neck in many weird positions and looking at a few of the places many times over i finally caught an intermittent blue spark out of the corner of my eye

the number one spark plug wire had touched the header just enough to heat up and split wide open, i guess because they were brand new high quality wires it only threw the arc from time to time and only when the header heated up the wound

I haven't got a new wire on it yet (i only found it a few minutes ago), but i have my fingers crossed

thanks for the help guys, i'll keep ya' posted

 

[gonecheenin]

new wire on, test drive complete

she still isn't a race truck with the half header, half stock y-pipe cobbled together temporary exhaust system i got on her

but she's running purty damn good,
and thats plenty good enough for the trip to Paragon next weekend

thanks again for all your help guys