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Discussion Starter · #1 ·
Hi guys,

I'm new to the forum but have been wrenching for some time. I have ran into an issue I cannot figure out. I just bought a 67 C20 with the 250 6cyl, manual on the column. When you drive the truck 1st gear takes off and engine revs fine, when you hit 2nd and 3rd as soon as the engine gets a load it starts bucking until you let off the throttle. Some things I've done. New HEI distributor set correctly and triple checked for timing. Plugs, wires, fuel pump. The manual choke is wired all the way open because it doesnt work right. I have also sprayed the carb and intake down while running for vacum checks. Any help would be appreciated. Thanks.
 

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The older 6cyl. engine used to have a problem with the carb staying tight, not only at the base gasket but at the next gasket up above the throttle blade portion on the bottom of the float bowl. I think there are just 3 screws that fasten there.
If that gets loose you have a lean condition as the top of the carb shakes around.
 

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How many miles since new or rebuilt?

A 67 is pretty old one may assume that many internals are worn out.

67’s didn’t have HEI out of the factory, thus how is this wired ?

Bucking as the crankshaft takes load can indicate:

- Detonation after all the gas in 67 was a different blend let alone changes as to how we get to the octane rating.

- HEI is a 12 volt system check the volts going into the distributor. Assuming this is a GM style coil in cap. Keep in mind that for the old points system the Run voltage is different from the Start voltage the former being reduced 6 to 9 volts through either an external engine compartment mounted resistor or a resistive wire between the ignition switch or potentially off an “I“ terminal of the starter solenoid. This opposed in the 67 time frame during cranking only did the ignition receive full B+ voltage.

- Too much spark plug gap or a conductive coating having developed on the internal insulators .

- Secondary wiring to the plugs either leaking voltage, not the proper resistive 8mm wires.

- Too much or too little advance for the engine RPM. As you go up in gears the crankshaft load increases. The carb mixture the advances have to track this. You need to be sure the vacuum and mechanical advance systems are functional.

- If PCV is present check it for function.

- Cam drive falling out of time, hard to check on these engines it’s a gear drive probably with GM’s fiber reinforced plastic cam gear, they wear out. Normal wear is to fall retarded this gets the cam to crank out of synch and takes the ignition with it since the distributor is using the cam as jack shaft it times independently of the cam to crank relationship, which is to say you can correct the ignition timing to the crank but that doesn’t fix the cam timing to the crank.

- In this age of crap parts you cannot assume the new equals functional.

- Mixture falling lean. It could be carb with floats too low or insufficient pump delivery which can be caused by the tank pick up, lines or filters not permitting sufficient flow. It can be with jetting i’d r the enrichment system of the carb, check manifold vacuum with a gauge. my builds always include a vacuum gauge a misunderstood instrument that tells so much about engine health. Follow the link for all the data that gauge tells you, yes it’s a Ford forum but here we’re talking the basic physics of engines not who did the engineering solution. Keep in mind when transferring this info to performance motors that lower idle readings and some rhythmic wandering of the readings is normal as cams get big to bigger.

Keep in mind the old saying “most carb problems are electrical”. “Most” doesn’t mean all but more frequently than not the root cause is in the ignition system. This isn’t a directive but it is a guide to your thinking and processes of elimination.

Not spell or grammar checked, the spell checker on this form is junk and it has no grammar checker and I’m too time pressed to go back and read this.

Bogie
 

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Discussion Starter · #8 ·
Ok guys,

The issue was the spark plugs. Very odd, I put a brand new set of champions in when I first started working on the truck and never even got to drive it. I replaced those with a set of AC Delcos and boom we are good. Upon closer examination of the champion plugs the gaps closed up on them, I always gap my plugs before install. I dont know how the gap would have closed on them. Thanks to everyone for the advice. Seems like I am always learning something new!!
 

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I know no one ever sells the wrong part these days. I bring a majority of my parts in with me and inspect all my plugs as I pull them.

I would be pulling those plugs NOW and measuring the threads. At the very least compare old to new. Also when your plugs are out run your hand along the unhooked wire in loose areas where it may rub things like exhaust or firewall.

When the plugs are out I would peek a endroscope down the cylinders. Maybe you have plug material sitting in the recessed area of the pistion. If you find anything a telescoping magnet may be enough to get it out.

Endoscopes can be used to peek inside cylinders, in the oil pan, inside the radiatior, along frame rails, inside hvac ductwork(being careful you don't get it stuck). Even suspension movments if the cord is long enough.

They are also helpful around the home.

Most are under $40 and the newer ones can connect wirelessly allowing you to download a app on almost any phone.

Recommend tool in your specialty tool box.
 

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I know no one ever sells the wrong part these days. I bring a majority of my parts in with me and inspect all my plugs as I pull them.

I would be pulling those plugs NOW and measuring the threads. At the very least compare old to new. Also when your plugs are out run your hand along the unhooked wire in loose areas where it may rub things like exhaust or firewall.

When the plugs are out I would peek a endroscope down the cylinders. Maybe you have plug material sitting in the recessed area of the pistion. If you find anything a telescoping magnet may be enough to get it out.

Endoscopes can be used to peek inside cylinders, in the oil pan, inside the radiatior, along frame rails, inside hvac ductwork(being careful you don't get it stuck). Even suspension movments if the cord is long enough.

They are also helpful around the home.

Most are under $40 and the newer ones can connect wirelessly allowing you to download a app on almost any phone.

Recommend tool in your specialty tool box.
Glad you found it.
One more that people have problems detecting is a shake up can of paint can. How? Many do a touch up on their pretty engine after getting it installed and spray to close to the spark plug and the paint on the plug creates a path to ground at the base of the plug to the head.
 

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Discussion Starter · #14 ·
Congratulations Steve 92307, you’re one of the very few among the very many that posts what you found wrong. Now all you need to do is figure out why.
Hahaha, thanks. I'm pretty sure I know why. The first set of new plugs I had put in were the wrong ones. I had originally just pulled one of the plugs out of the head and took it to the auto part store and cross referenced that plug. The original one I took out wasnt the correct one for a Chevy 250. That I believe was the problem the whole time.
 
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