Big Boys Hobbies

I am sure there is a mechanic or two on here

flyin-lowe

flyin-lowe

Elite Member
Joined
Dec 30, 2011
Messages
783
Location
Henry County, IN
Just curious, I am have trouble with an older Chevy truck that I have. It is a 77 3/4 ton 4 wheel drive, 350/350. The engine was rebuilt several years ago and so far it has been perfect.

About a month ago I was trying to pull a buddy of mine out of the snow. I had it in low lock and was watching the revs, but I was pulling pretty hard to get him out. Once he was out and I was unhooking the chain I came back and the truck had died and there was a little bit of oil under the front end of the engine. My first thought was that I blew it up, I started it back up and it never leaked again, has full power, and good oil pressure. So I am not sure what that was all about.
Anyway every since then it starts up just fine but when it idles it idles rough and shakes and and vibrates a little. As soon as I take off it smooths out. It has a little bit of a miss in it when I am running at speed but it runs pretty smooth for the most part. My gut instinct is that it is something in the distributor. When it is running rough and I start to drive or raise the RPM's it literally hits a certain RPM and then takes off to full power. It almost feels like it is only running on 6 or 7 cylinders at idle because you can feel a huge power boost as soon as you hit about 1000 rpms and the engine smooths out. It just comes to light like the flip of a switch. It has an Edelbrock Carb with an electric choke. A friend of mine raised the idle speed on the carb a little and that helped some but as soon as you drop it into gear and sit there it runs rough. As soon as you build a little speed (rpms) it pops back to life and takes off.
I know it's hard to diagnose something like this but I was hoping there was something obvious that would case it to act like this.
 
Pull one plug wire at a time with the truck idling.
When you pull the wire from a working cylinder, the RPM's will drop. So you know that wire is good. When you find one that does not make a difference in the RPM, you will know that that cylinder is not firing so the wire may be bad.
Lastly, you can swap the wire that made no difference in rpm with the known good one and see if the problem moves to that cylinder as you move the bad wire...

sent from the secret hidden screen on my V3i via mental telepathic link using Crapatalk
 
ecoast said:
Pull one plug wire at a time with the truck idling.
When you pull the wire from a working cylinder, the RPM's will drop. So you know that wire is good. When you find one that does not make a difference in the RPM, you will know that that cylinder is not firing so the wire may be bad.

sent from the secret hidden screen on my V3i via mental telepathic link using Crapatalk
Click to expand...

^^^that will get you electrocuted do NOT do that^^^

your condition OP sounds VERY much vacuum related

find yourself a vacuum gauge & check it at idle - then go for a drive with the gauge connected (affixed to the hood perhaps) & see what the engine vacuum is doing

if there is a leak somewhere...while the engine is idleing, wave an UNLIT, but gas flowing propane torch around the intake manifold...if the idle speed goes up then there is your leak

always have a fire extinguisher handy
 
Always good to run an engine in the dark and watch for sparks. Can be a lot of things. I spent thousands building a motor and screwed up putting a 670 street avenger on it with vacuum secondaries. Messed and messed with that carb. had a bad flat spot. Finally traded the vehicle for a boat. later a friend told me Holley finally made a different squirter to cure the problem. I tried every other part available. Now if it's not a double pumper I don't want it!
 
351maverick said:
^^^that will get you electrocuted do NOT do that^^^

your condition OP sounds VERY much vacuum related

find yourself a vacuum gauge & check it at idle - then go for a drive with the gauge connected (affixed to the hood perhaps) & see what the engine vacuum is doing

if there is a leak somewhere...while the engine is idleing, wave an UNLIT, but gas flowing propane torch around the intake manifold...if the idle speed goes up then there is your leak

always have a fire extinguisher handy
Click to expand...

MSD box will make you wet yourself if you touch a plug wire:lol:
 
That was quick, it gives me a few things to try. I first also thought vacuum leak because a few years ago it acted similar and one of the plastic plug/caps came off the carb. The only reason I was thinking ignition is that I used to have a friend who had some drag cars. One of them had what he called a "stutter button" in it. He would hold it in while doing hole shots. it would cut part of the cylinders out and once the button was released the car would launch. That is what this feels like to me, just obviously not that extreme. It just hits a certain spot and instantly comes to life.
 
My first impression was a vacuum leak also. See posts above.

If not:
You can isolate the problem by looking at the porcelain on each spark plug where the electrode is. (You have to take the plug out to do this)
The color should be tan to brown. If it's all wet or black you have located which cylinder is the problem.
Distributed cap could be bad also. Take it off and look inside. The electrodes should be clean and NOT have black carbon tracks between them.
Plugs, wires and distributer cap are the 1st steps on an older engine (also points and condenser if old enough)

Good luck and let us know the outcome.
 
351maverick said:
if there is a leak somewhere...while the engine is idleing, wave an UNLIT, but gas flowing propane torch around the intake manifold...if the idle speed goes up then there is your leak

always have a fire extinguisher handy
Click to expand...
Do NOT do this! ↑↑↑
Much safer to use a spray bottle of water. If you spray a vacuum line and the motor slows or dies you've found the problem.

If it's a shorting plug wire, then it might start arcing when sprayed...but it might not. With no lights on, open the hood with the motor running in the dark. Any arcing will be obvious.

The "little bit of oil on the ground" is the part that bothers me. It could have been the result of heavy load @ low RPM beating up on the rear seal.
Have you looked under the motor since to see if you can tell where it came from? Has it leaked or dripped since?
 
Kinda tough to diagnose without being there but here`s what I would do.
The oil could have been blown out a crankshaft seal from excessive pressure.
In my driveway I start with this in any order you like.
Pull all the plugs, observe the colors, like it has been said they should all be tan. Then while they are out check compression in all cylinders, they say within 10 percent of each other and nothing under 100PSI.
Check out OK? move to a vacuum guage placed on manifold vacuum port, not on a carb port unless it is a direct port under the throttle plates.
Observe the guage movement at idle, may shake a little if you have a hot cam but shouldnt swing wildly.
also while your in there sometimes small leaks dont show up easy on a vacuum test so shoot some ether or wd40 around the manifold where it bolts to the head and under the carb , listen for sharp rises in RPM`s.
Also do like the other member said and lift the hood in the dark while running in park and look for any stray spark, you will usually hear it snapping too but I have seen some that barely show like between two wires.
Take your distributor cap off and check the cap and inside of your distributor, check the distributor shaft for play as well.
That is all mechanical and electrical but what about fuel?
Could you have water in your fuel? there are drying addatives you can try., fuel pressure OK?, fuel filter clean?
It will only take a couple/few hours to go over it top to bottom, I would suggest it all be checked at least for preventative maintenance.
 
Thanks for all the input, I think I have it figure out. My neighbor is a mechanic and he came to look at it. He pulled all the plug wires of one side and it didn't make a change. He said the carb is messed up and the fuel distribution is not going out at idle but once I start driving it is getting fuel to both sides. Now to find somebody to rebuild the carb. Good news is I know that if I need it this weekend I can get in it. We are supposed to get 6-12 inches of snow this weekend.:mad:
 
Hopefully you have it figured out and its not something worse. I was going to suggest though that a bad bearing could cause similar problems. Though it will usually ( but not always ) knock pretty hard if it was that bad. The small oil loss you mentioned is what had me thinking of a bad or spun bearing.
 
Mntnclmbr said:
Well, I was kinda close :shrug: :D
Electronic ignition started approx. 1970

Heck, my first car had a generator :D[/QUOTE

I remember when my dad put an after market electronic ignition on the old dodge van
we called it the bee box cause you could hear it buzzing:laughing:
Click to expand...
 
Man it has been so long... oh the good old days when you could work on this yourself. Not now a days. Boy I miss being able to work on my vehicle.

Not one mention to check the timing?

I sold my timing light, rpm meter, and all the good stuff on a yard (garage) sale years ago. :mad:
 
flyin-lowe said:
Thanks for all the input, I think I have it figure out. My neighbor is a mechanic and he came to look at it. He pulled all the plug wires of one side and it didn't make a change. He said the carb is messed up and the fuel distribution is not going out at idle but once I start driving it is getting fuel to both sides. Now to find somebody to rebuild the carb. Good news is I know that if I need it this weekend I can get in it. We are supposed to get 6-12 inches of snow this weekend.:mad:
Click to expand...

Really...and he didn't get 'ELECTROCUTED" ???:lol:

lol

glad it helped; it's a standard test.
 
I know you said you have the problem fixed but I wanted to add some more tips. You can buy high tension plug wire pliers at your auto parts store to perform the plug wire test. Pulling a plug wire by hand will shock you but not directly kill you. (The secondary reaction may when you jerk your arm back, fall and hit your head! ) It's best to first loosen the plug wires from the plugs before you start. This is the fastest and easiest way to narrow down any miss. It's already been said but your listening for little or no change in rpm"s when you pull the wire. This can give you loads of info. After isolating the cylinder that isn't pulling you can narrow down the issue even further. 1st put the end of the wire next to a ground like the block or manifold and listen for the snapping of the spark jumping. If none you have an issue with the wire or distributor cap. If you hear the snapping then put the wire on the plug again and slowly pull it back to make the spark jump from the wire to the spark plug. This increases the voltage it takes to jump that gap and in turn the spark plug gap itself. If by pulling the wire back slowly your cylinder starts pulling then you have a bad or dirty plug. If it doesn't change it still could be the plug, vacuum leak or low compression. Now the easy way to check for weak compression without any gauges -- Pull the power cable from the distributor. Now crank the engine over with the key like you are starting it. It should turn over evenly if the cylinders compression is within 10% of each other. But if you have a leaky valve or worn or broken ring it will not turn over evenly. You will hear the cranking rpm speed up every time that weak cylinder comes around. Then you know it's a deeper issue.
 
I remember the 'good ol' days'. My first car was a 1950 Ford (in 1960). Ok, about your problem. I doubt it being this, but I had a 1977 Chevy with a 350. I had 237k on it and it was beginning to run a little ragged if I idled it down to about 600 rpm; at 750 rpm it was great. Not bad, but enough to be noticeable. I was returning home one night and slowed down from about 35 mph to about 20 mph to change to a left turn lane. I stopped at the red light and the engine died. When I tried to start it again, I could here something grinding in the front of the engine. Got it pulled to the house, and when I pulled the timing chain housing, the chain fell out on the floor of the garage! The plastic teeth on the cam shaft gear were gone. I replaced the chain and gears, and started it. It sounded like all h*ll broke loose. When the chain let go, the pistons hit and bent some of the valves. I replaced it with a new 'Mr. Goodwrench' engine. I ran that engine for about 330k miles; but sold it when the tranny went out instead of repairing it.
 
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