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350 doesnt want to get the truck past 50mph

Pizza_Man

Pizza_Man

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Okay so here's the truck
71 Jimmy 350/465/205 14 rear with 3.73 gears and 36" tires
Motor has MSD ignition, hooker headers, edelbrock air gap intake, edelbrock 750 carb 1411, mild cam, K&N air filter, it is the 71 motor and hasn't been rebuilt and has about 150k on it.
For some reason it doesn't want to accelerate past like 50 mph. No matter what gear. In 3rd it starts cutting out around 3k and sounds like not all 8 are firing. And in fourth its the same story but at 2k rpm. In second, first, and low revs in third it runs fine and accelerates as normal. But then it hits 3k like a wall and just won't do any more. In neutral it normally could hit 6k and climb but not it wont get past 5k. And i havent really been too hard on it lately. Spark plugs are 3 months old and it was running just fine before this. I changed the in line fuel filter and checked the spark timing.
Is there a filter inside the carb?

Thanks everyone so much!
 
Check all plug wire routing/locations? Sounds like possibly a switched wire.

Also, what are you gauging speed off of?

RPM's should be accurate, but doubt your speedo is.
 
Some GM carbs have a small filter right where the line hooks on. You can usually tell because the input fitting has a big hex head where it unscrews from the carb body.
Its right behind that nut, inside the body. If its not that, then it sure sounds like a fuel or fuel pump issue. The higher gears would suck more fuel at lower RPMs.
Fastest way to check, would be to put a "T" in the line in front of the carb, and hook up a fuel pressure gauge.
If it goes to 0 as you get faster, you have found it.

Or just slap on a new pump, I think those were fairly cheap. Also check your oil level in the engine.
A bad fuel pump can pump gas into the block, thinning the oil and ruining the bearings.
If the oil is over full and smells like gas, stop driving it until you replace the pump and oil.
 
Could be 100 things,or a few..hard to say..

Are you sure its timed right??..maybe the distributor moved and retarted the timing--or the vacuum advance if it has one,failed..a bad module can limit the top end power if its dwell advance circuit goes bad (some cheap modules dont even have that!)..it might not be fully opining the throttle with the pedal floored,but since it is misfiring too,I'd say its a spark or fuel issue..

There is a factory fuel filter in the Q-jet carbs,maybe one was left in there when the external in line one was added..look at the cap and rotor good,or try swapping a known good one on it,I have seen perfectly good looking rotors and caps make an engine run like crap,the spark gets shunted to ground under a load rather than firing the plugs..

Worst case sceaniro,is the cam lobes may have gone flat on a few valves--easiest way to check that is to watch the valves with it running with the valve covers off,but be prepared for an oil bath,if you dont use those little deflector clips to block the oil holes off on the rocker arms..you'll see any valves that are not opening as far as their neighbors right away if its a bad cam lobe..
 
The oil looks fine, it burns some and that hasn't changed
I'm kinda gauging speed off of feel because the speedo is off
All the wires seem good, it was driving fine the day before
And its an edelbrock carb, I'll check if it has the threaded inlet
Also its an MSD distributor and I pulled the cap and it looks clean as a whistle
 
I had a similar problem with mine. Turned out to be the timing was off and the vacuum advance was dialing in too much advance. I reset everything, ran manifold vacuum, and got the Crane advance limiter. Problem is gone now, although now that I've got a tach the transmission seems to always shift at about 3500 on WOT. Going to check and make sure I'm getting full pedal travel...
 
I doubt an Edelbrock would have a gas filter in the fuel inlet,so I wouldn't bother looking for one there,there is no place to put one in them...it could have a screen on the inlet fitting ,but I doubt it..

The ignition coil might be crapping out when it gets hot too..best way to test one is to try another one that you know runs another engine OK,you can do ohms checks,but they sometimes show "good" and only act up at certain temparatures..

I had a older truck that had points--someone put an Accell "Super Coil" on,and it wouldn't pull a hill or go over 65 without bucking and misfiring..took me quite a while to discover the truck had a original resitance wire powering the GM coil,and the Accell one had a ballast resistor on it too--so I was getting about 6 volts to the coil!..

I hooked up another 12V hot wire from the fuse box to it that worked with the key in the ON position,took off the original GM power wire,and the thing would roast the tires after that..I was amazed at the difference.a few volts made a huge difference..

A lot of GM's I've owned had HEI distributors swapped in,that had the original points coil wire powering them--thats no good either,HEI needs a full 12V with NO resistor or resistance to power it properly..
 
I have had trouble with my coil in the past but it wouldn't run. It does have the MSD box on it, but I wouldn't think a bad coil would only act up under load?
 
Well it's possible. Under a load it's going to be a little rich, making it harder for the plug to fire. If the coil is putting out less voltage than it should it could struggle in a rich condition.
 
Fuel supply and or pump, check rubber lines from tank and to fuel pump from frame for collapsing. There's probably some short sections on the frame rail too.

I had this causing similar problems but only when my foot was in it. Get a fuel pressure gage like Fordum says and see what going on when your breaking up. Thats how I found my problem.

If you jumped time you should have hard starting. But a quick timing check is easy to do.

Also check the tank, make sure fuel flows easily from tank.

I'm betting fuel.
 
I'll try to get my hands on a good coil to test it
My dad keeps insisting that my secondaries aren't opening up. Could this cause it to not want to get up past a certain speed? Because it starts almost on the first crank every time and runs like a champ until its under load. I double checked all my vacuumes and my advance and its all good
 
It is mechanical on edelbrocks. Pull the air cleaner and move the throttle and watch the butterfly's opening. Should open secondaries at around 1/2 to3/4 throttle. If not linkage is messed up. Also check that choke is opening correctly. More sounds like the coil or ignition problem though. Which msd and does it have a rev limiter?
 
chulisohombre said:
It is mechanical on edelbrocks. Pull the air cleaner and move the throttle and watch the butterfly's opening. Should open secondaries at around 1/2 to3/4 throttle. If not linkage is messed up. Also check that choke is opening correctly. More sounds like the coil or ignition problem though. Which msd and does it have a rev limiter?
Click to expand...

Is it an edelbrock carb? I know on a qjet you can run with out secondaries.
Also does your dist have a vacuum advance? Mine was a real dog until I found the advanced was frozen and replaced it.
 
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