The Stovebolt Site Forums The Shop Area Engine Shop 350 with a miss

The 350 in my '70 C10 has developed a bad miss and sounds like it isn't firing on all cylinders. Also, it will not keep running at idle unless the manual choke is partially closed even after the engine has completely warmed up.
I installed new plugs not long ago and the truck hasn't been driven but maybe a couple hundred miles since then. I pulled the plugs anyway just to have a look and to recheck the gap setting and everything looked good. I changed the plug wires, points, condensor, rotor and dist. cap and still no luck. Is it possible that the coil could be the problem?

I have noticed during my fumbling around under the hood that if I touch or move the vacuum hose that connects to the vacuum advance the engine stumbles and dies unless I either let go of the hose or pump the acc. linkage. Is this normal or is it a sign of a problem? I removed the hose and checked for damage but found nothing. I trimmed the ends back about 3/4" and reinstalled and still get the same response whenever I move the hose with the engine running.

Any help is greatly appreciated.

Ralph,

It doesn't seem to me that removing that small vacuum line to the advance can should affect your engine as much as you say it does. If the engine were experiencing a partial vacuum leak somewhere, and you jiggled that hose and added another leak, then what you describe makes sense.

I would take a can of Brake-Kleen, or a propane torch (not lit) and go hunting for vacuum leaks. Small blocks are notorious for developing leaks where the intake mates to the heads. Good luck.

Stuart

Hy bigbassmann, It sounds to me like you have a vacuum leak, that is causing a too lean mixture situation. Is the tube in the carb that the vacuum hose goes onto loose? Does the hose have a hole somewhere in the middle of it? Try replacing the hose entirely, or disconnect it from the distributor and plug it, that will tell you if the advance is leaking, hope that helps

Sounds like you have a bad vacuum leak. Check the carb. base gasket. (What carb. do you have?)
Check the hose going to the power brake unit & the unit itself. Check the hose going to the auto. trans.

Could some trash have made it's way into the carb?

Had some water get into one of mine and gave me all kinds of fits.

Did a little more searching and came up empty, can't find a vacuum leak. I am running an Edelbrock performer series 4 barrel carb and I did a check around the base gasket and checked the mounting studs just to be sure they were still tight. No power brakes so that is out and no auto. trans. There are only 2 vacuum lines on the truck, one for pcv and one for advance and both are good. Pcv is working. The tube on the carb for the vacuum adv. hose is solid.

I did notice something though. It isn't the vacuum advance hose that makes the engine stumble but rather my hand. Let me explain. I have been doing the work on the truck with the breather off the carb so I would have better access to certain areas such as the dist. area and the vacuum hoses on the front of the carb. When ever I reach into the area in front of the carb I get the same engine stumble as before but I am not touching anything. I tried it several times and it only happens when I place my hand in front of the carb area which just happens to be where the advance hose connects. Is this because my hand is blocking or disturbing the air flow into the carb? Is it normal?
I tried touching the vacuum advance hose near the dist. area and nothing happened. I also disconnected the hose from the vacuum advance and plugged it off and it made no difference to the way the engine was running.

I tried adjusting the timing by turning the dist. just to see if maybe the timing had magically changed but that did no good either.

I am grasping at straws here but does it sound like a carb problem? I don't think it has trash in it because I have always used an inline fuel filter and I purchase my gas from only one place for both my trucks and I am not having a problem with my daily driver.

Hy again bigbassmann, It sounds like you have eliminated a lot of things, so now I think it's time to look into carb problems, it sure sounds like it is too lean, having to choke it when hot is a definate indication of not enough fuel. Have you tried adjusting your idle screws to see if opening them has any effect, hope that helps.

3B, I did try some adjustments but didn't seem to make a difference. The carb has always been set a little on the rich side.

One thing that I forgot to mention at the beginning was that the #5 plug wire was bad, that was why I replaced them. The contact end that clips over the spark plug was loose and had caused the inner wire core to burn away from the contact end. Perhaps I have at least one (perhaps more) fouled plugs. Even though I checked the plugs and they didn't look bad, maybe I should change them anyway. Any thoughts on this?

HY again bigbassmann, It sounds like a carb problem at first blush, but you may have something there thinking about bad plugs, they cause lots of problems. Do you have a vacuum gauge, you could use it while the engine is running, and see what readings you get. I have found a vacuum gauge to be a very useful diagnostic tool over the years, hope that helps. Please keep us posted on your progress.

It pretty much points to the carb. Have you adjusted the idle jets. What carb do you have? A Q-jet will act like that if the secondaries are not closeing tight. Sounds like a carb o-haul is in order. Your float level may be off.

Well, I just changed the plugs and no luck. I think you guys may be onto something with the carb. I am running an Edelbrock performer series 600 cfm 4 barrel and I have messed around with the idle jet settings and get no better results. It still seems strange that whenever I place my hand in front of the carb the engine wants to stall out. I have to keep one hand on accelerator linkage to open the throttle a little while I make adjustments with the other hand. Also, when the breather is off, the engine idles up a significant amount. With the breather on, I have to pull the choke knob out more to keep the engine running. This is really getting on my nevers.

I should have said any 4 barrel if the secondaries don,t close tight it,s like a vacuum leak.. The Edelbrock carb is pretty easy to o-haul. Did you see mine in the Gallery. Pretty similiar trucks.

Very nice truck you got there, George.

I think I will pull the carb and give it an overhaul and good cleaning. Man, I hope that works because I don't know what else to do after that. I will update as I get more done. Thanks for all the replies, you guys are always willing to lend a hand.

I finally got around to pulling the carb today. Before that though, I checked the fuel pump to see if it was working properly. It is but there is a lot of water and crud in the gas. After removing the top of the carb to have a look inside, I am amazed that the engine would start at all. The bottom 1/4" of the float bowls were nothing but crud. I cleaned everything real good and the carb should be fine now. My next thing will be to drain the tank to get rid of all the water and refill with fresh gas and a bottle of Sea Foam. I am going to install new inline fuel filters as well. I may have to look into getting a new fuel tank unless someone knows of something that can be used to stabilize all the gunk inside.