Hello all,
I posted this in the GMC section the otherday with no response, maybe someone here will.     Hopefully someone reading this will be able to give me some insight. I have an 85 GMC K2500 350HO/400 Turbo. Problem I am having is in 1st,2nd,or 3rd gear, and while under full acceleration,tach reaches around 3000 and the engine starts lossing power and popping out the exhaust(some out of the intake).First happened when I made a jump at high speed, when truck landed is when the missing problem started. Does not happen with half throttle or less and accelerating through full RPM range and shift points, even for an extended period. Also while driving around 65mph(2950RPM) if I hit a bump in the road, it starts missing and power loss.Let off throttle and push back into it, works good till another major rut/bump. Problem occured with previous engine, carb, hei distributor. All have been replaced for various other reasons(cracked block,upgrade from edelbrock to holley truck avenger, worn distributor shaft)Have verified wire for ignition is not loose or any voltage drop.tested coil and hei module. Fuel side, have verified lines, tight connections and good flow. I am starting to lean toward the tranny as the culprit. Question is am I going in the right direction or am I totally out of the park.Below is a list of other mods that might help.
350 HO Goodwrench crate
Edelbrock air gap intake
Holley Truck Avenger with 1 1/2" spacer
K&N filter w/ Tornado insert
K&N Xtreme filter top
LUND hood scoops
ACCEL bluepoint distibutor w/ accel super coil
Taylor 8mm wires
E3 Sparkplugs
Hooker headers w/dual flowmaster 40 series Race ready exhaust cutouts
200 amp powermaster alternator w/ dual Optima batteries
High flow water pump
March performance pulley set crank, alt,wtr pump
March Alternator bracket/ Turnbuckle
Be-cool aluminum radiator w/dual 15in. elec fans
Turbo 400 tranny w/ B&M shift kit Royal Purple ATF
NP 208 27 spline
Fr axle stock corp filled with Royal Purple
Rr axle corp 14 w/ Powertrax locker Filled with Royal Purple
16x12 Mickey Thompson Classic II Wheels
Super Swamper(Interco) TSL 36x12.5016R
6 in. supension Fr,spring(superlift) Rr, Spring(Procomp)
Greaseable Shackles and bushings rear spring(offroad design)
2 in. body lift
Raised steering arm
Drop pitman Arm
Skyjacker Doubler shocks FRT, single rear
Warn winch bumper
12,500ib T-Maxx winch w/recovery kit  

u may have a plug wire grounding out before the plug, check the wires for any worn areas or burns and cracks in the insulators. look carfully around the header area and should u find dammage there perhaps exhaust wrap may prevent it or even prolong the wires from burning through again.

do u have a lock up tourque converter?
perhaps it is locking at rpm's too low for the speed being traveled and is bogging the engine.  

the 400 is a non lockup, the plug wires is on the right track for sure.  

too much coffee?  

ooops, sorry, this went in while doing instant message  

I did inspect the wires when the problem showed up originally. I swapped motors and with that new wires and plugs. Everything is installed clean and away from the wires. I have had a problem with coils before causing high rpm problems. Did find my coil bad and replaced it. Problem still shows up. Thank you for taking the time to shoot some ideas out, I am lost on this and every bit helps  

tranny's generally dont cause an engine to miss or backfire.
timing issues cause this to happen, perhaps when u jumped it the sudden force on the powertrain either caused the timeing chain to skip or bent the cam wich would cause the valves not to open and or close completely as well as mess with the distributer timeing and efficiency.

check compression and see if the timming is consistant by watching it for a few minutes with a light.also did u check the ditributor shaft and bushing sice this started?  the origional post states that u had issues with the HEI bfore and since replaced it but i dont understand it to mean since the missfires occured.


sam  

If the engine wasn't properly broke in a cam lobe or lobes could be wore off already,

check for bent push rods, you don't need to pull them out, just pop off the valve covers and fire it up you'll notice that some rockers move less than the others. (quickly you'll have oil squirting all over unless you have some rocker clips.) that indicates a bent push rod or bad cam. one bent push rod or worn cam lobe will cause popping. high revs and immediate rpm drop can bend them. (from the landing)

also check the vacuum advance and be sure the cap is fitted properly.
the chain on a small block won't slip if it does it will barely run or not at all, I've had a few erson and comp cams break but never seen a bent one. that would cause immediate bearing failure. small blocks can destroy a new cam in 15 minutes if they aren't broke in.  

the problem with bogging down/power loss and missfire happened with the previous engine that is when I found the bad coil. At that time I repaced the distributor with a new unit from accel. Problem did not go away. Ended up putting a hole in the head and top of the block between the #4 and #6 cylider. This is when I installed the 350 HO took me awhile to find the problem again because it wasn't till after 3000 miles on the new engine that I stepped into it and the original problem showed its head again. what baffels me is that I can run over 3000 RPM as long as I take it up slow. Only and issue if I fully accelerate.But what really baffels me is new carb same problem new ditributor, same problem new engine same problem  

This may sound a little off, but are you using the same tach that you always had?

I ask because my uncle used to be big into dirt-track racing.  He was having the same problem, but at 4000 RPMs.  We went through everything at least twice, distributor, msd box, carb, valve lash and pushrods, wires, EVERYTHING.  We just couldn't seem to find it. Then one day after wrenching on it some more, he got ticked off and just started pulling wires looking for a short.  He checked a few things then hooked most of the wires back-up, except the tach.  He cranked it up and gave it hell, listening for the pops and stuttering.  They never came.  He hooked the tach back up and his problem cam right back.  Turns out that his tach had a short in it, causing the misfirig.  He contacted pro-comp and got a new one for free.

It may sound crazy, but its easy to unplug a tach.  It might not help you but i would definetly try it just to see.

You might check the fuel filter and fuel pump as well.  It may be starving for gas above 3000 RPMs or have varying pressure.  A fuel pressure variation can make an engine act crazy.  A fuel problem would explain why you get the problem under heavy acceleration.  The carb is using gas ALOT faster under acceleration than when not.  If the carb is not getting quite enough fuel when you slam the throttle, it will start to run the fuel bowls dry and cause your missfiring.

You might also check the power wires going to your distributor and coil.  Make sure that they are getting steady voltage.

These are just a couple of things that I would check.

Hope this helps you,

Randy  

if you have a cadillac converter it may be cloged. it would be worth a check. does it misfire only when you get on the throttle hard??