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Engine "stumbling"

R

ramses

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Engine \"stumbling\"

The motor in my 73 K20 has had a "stumbling" problem for a while and I just cant figure it out. The motor only has 50,000 miles on it. It does it mostly in 3rd and 4th gear when the motor is under load. It does it the worst in 4th gear on acceleration and while pulling a hill. It just seems like its missing and it jerks the whole truck. Ive rebuilt the carburetor twice and everything with it is good for sure, and it has new plugs and wires, distributor cap and rotor button, fuel filter, and air filter. There is a clicking noise coming from the engine somewhere, my dad thinks its a rocker arm or something, we are going to adjust them all tomorrow to see if thats it. I checked the vacuum advance and its working fine. A new coil and adjusting the valves are the only ideas I have left. Does anyone have anything else to add that could be causing the problem?
 
Re: Engine \"stumbling\"

Not a big help, but when my car was starving for fuel (q-jet) when I was really getting on it, the vehicle would buck violently. I used leaner secondary rods, and the problem disappeared.

This would likely only be the cause if it has always existed.

The rocker arm stud I just broke started out with a hesitation in a certain RPM range, so a loose rocker could veery well be your problem too.
 
Re: Engine \"stumbling\"

The things that come to mind that you did not list are fuel pressure and flow( too high or too low) , timing, and or a vacuum leak. Might want to try eliminating various vac hoses with caps including the one going to the trans modulator. Also try disconnectig the vac advance and see if anything changes. Or switch it from ported vac to full vac or vice versa.
 
Re: Engine \"stumbling\"

Should the fuel filter be comepletely full of fuel when its running? The filter is only about half full when its running, maybe the fuel pump is going bad?
 
Re: Engine \"stumbling\"

I would check the spark plugs for loose or cracked ceramic insulators. Sounds like a problem I had a while back, I pulled the plugs and found the ceramic part loose on one plug. There was no visable crack and the ceramic part was not that loose you had to twist it. I used a plug socket with the rubber insulator on it to install. They were AC plugs which are made by autolite, sadly. They come from the factory like that so you have to inspect them closly.
 
Re: Engine \"stumbling\"

No,the fuel filter may appear to be only half full,but it has gas vapor trapped in it--a normal condition.There was a full page explanation in my wix filter book about that very same question--I dont recall all the details witout looking it up,but they say its normal for filters to appear half full or less,especially if they are horizontally mounted. As long as the fuel pump pressure and volume is up to speck,there's nothing wrong with the pump--I replaced a few pumps unnessasarily because I thought the same thing. /forums/images/graemlins/blush.gif
Your stumble may be either fuel or ignition related.Take a long hard look at the plug wires,make sure the plugs are good and not fouled,check distributor cap and rotor for arcing or cracks,and the pick up coil could have a break in one of the wires that shuts off the ignition when the vacuum advance moves it.The other post about switching the vacuum advance hose from ported to manifold vacuum may help too--not every chevy used ported vacuum certain years,though mine seem to run better with ported vacuum. /forums/images/graemlins/crazy.gifThe coil might have a crack or be weak also.
Could be the accelerator pump not sqirting enough fuel in too.
 
Re: Engine \"stumbling\"

I re-read your post,and noticed you said its a 73--does it still have points or HEI distributor??.If its HEI it was swapped in(first year was 74 for HEI)someone may have hooked the HEI to the original coil wire,that is a resistance wire that will not flow a full 12 volts to the distributor,and though it will start and run,poor spark at high load will result(like when lugging up a hill,like you said when it is doing it worst)so thats worth checking into also.You should run a new 10 or 12 gauge wire for the HEI to a spot on the fuse block that gets power only when the key is in the "ON" position. /forums/images/graemlins/crazy.gif
 
Re: Engine \"stumbling\"

If it is HEI check the magnetic pick up coil in the dist. They have similar symptons as you are having when they go bad.
If its a points type it may have a bad condenser
 
Re: Engine \"stumbling\"

Mine started doing the same thing today...Maybe there is a K-5 virus...

I have a feeling mine might be caused by the accelerator pump...I run an Edelbrock 650...and there is a little bit of gas deposit on the top of the carb, where the pump is located, like a leakign accelerator pump.
Ill let you know if that fixes it...Im also thinking mine might be vapor locking, with the heat around here...ill move my line and let you know too.
Eddie
 
Re: Engine \"stumbling\"

Its the stock 4 barrel carb. The motor is completely stock the way it came from the factory in 73 and it has just over 50,000 miles. It has points, I put new points in it when I put new plugs and wires on it. I noticed something odd last night though. I turned off all the garage lights when it was running to see if I could see any arcs from the plugs or anything, and I can see sparks on the 2nd and 4th spark plugs back on both sides of the motor? I didnt have time to take the plugs out yet but the wires are on tight and in good shape. I dont see how 4 different plugs in corresponding cyllinders on both sides of the motor could all be arcing?
 
Re: Engine \"stumbling\"

Your plugs might be cracked near the hex part of the plug,or the wires could be bad(I have had brand new out of the box wires that arced!)but also some "corona" is visible around the plugs even though nothing is wrong--so its difficult to say if that is part of the problem or not. I'd try another coil if you have a spare,it might have an internal short or a crack in it,that will reduce the strength of the spark,seeing how you have replaced most of the other ignition stuff with no results.If you have an EGR valve that could be opening at the wrong time or sticking open slightly,that could be a possibility too. /forums/images/graemlins/crazy.gif
 
Re: Engine \"stumbling\"

That "clicking" noise could be the fuel pump,the rocker arm that operates the diaphram in it sometimes makes a clacking noise that is often mistaken for a loose timing chain( something else that can produce a stumble too,hope its not that!)so it wouldnt hurt to see if the fuel pump is making that noise,if it is its on its way out.Hope you figure it out--I know how frustrating it can be to diagnose problems like this!. /forums/images/graemlins/crazy.gif
 
Re: Engine \"stumbling\"

Yesturday I took the spark plugs out of my truck and they all looked good but some were a little wet. I thought it was just oil from when I took the valve covers off but im not sure now. I wiped them off and put them back in and it ran great for a while then it started missing again. My dad said gas could be getting on them from the valves not sealing good and causing it to miss? Are there any other causes of wet spark plugs? Could the valves just need adjusted? If this is the case, will I have to take the heads off to fix it?
 
Re: Engine \"stumbling\"

No I gave up for a while because ive been too busy to mess with it anymore. Sometimes I start it up and it runs like a dream then other times it misses so bad it almost stalls when I take off I cant figure it out. The best clue ive got so far is that after I wipe the plugs off it runs great, beyond that im still lost though.
 
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