The Stovebolt Site Forums The Shop Area Engine Shop Help...engine still misses...

I posted something a few weeks back about my engine missing. I thought it was the carburetor. After adjusting that carburetor and putting on a different carburetor, I'm not sure that is the problem.

The engine is a 250 in a 1966 Chevy pickup. I did a compression test on it a couple weeks ago and all the cylinders registered around 175. However, a couple of them leaked off as it sat. So, this weekend I'm going to repeat the test.

I replaced the plugs and plug wires a couple of weeks ago to see if that was the problem. It did not change anything.

A year or so ago I replaced the points with a Petronix electronic ignitor and the coil with a Petronix coil. At this same time I replaced the vaccum advance and it still works as it should.

I have adjusted the timing so that the engine runs smoother. It is several degrees more advanced than the manual suggests.

Last night while it was running I pulled the plug wires to see if there was any cylinder that did not cause it to run worse. It seemed that when I pulled #1 and #2 wires the engine did not run any worse. It definitly ran worse on the remaining 4 cylinders.

Another thing that I noticed was that when I reved the engine it backfired through the carburetor.

So, any ideas on what the problem might be or what I should check this weekend? Did my timing jump? Do I have a bad head gasket? Do I have some burnt valves? What are your thought?

Sorry for this long post, but I have been working on this thing most of the summer and have about come to my wits end.

Thanks in advance for any help or suggestions you can give.

Mike K.

Mike,

I have always found that checking the engine out with a vacuum gauge can eliminate some possibilities and, if you are lucky, it might identify the problem.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

It's too bad the above site is not actively maintained because it has a lot of useful information.

Tim

Are the #1 & #2 cylinders the ones that leaked down?

If not, it sounds like you you have an electrical problem. First, inspect your distributor cap. Look for burnt or corroded contacts, and look for cracks. If you have another cap on the shelf, I would try it. Another old trick is to do a "dark" test. At night and in the dark, run the engine and look for arcing. If you see arcs from the wire boot to the head or plug, you have a plug with a broken electrode. Arcing between wires is usually crossfiring due to improper wire routing.

OK, let's go through this: We know it's not wires and plugs, the coil would effect all cylinders, not just 1 & 2. Could be the distributor cap, but not likely. Backfiring is sometimes a symtom of either valves not opening or lean mixtures. So check the valve lift on #1 & 2 to make sure a cam lobe isn't down. That's not likely either because of the good compression. If the backfireing and missing is due to a lean mixture it can't be the carb since not all cylinders are involved. I would look for a vacuum leak somewhere near the #1 and 2 intake runner. Maybe a loose fitting, cracked line or the gasket itself. Squirt a little oil around the manifold to head joint while the engine is running and let us know what you find.

I connected the vacuum guage to the line that feeds the vaccuum advance on the distribitor and it read around 5.

According to what I read it should be 17-22, right?

Did I connect it to the right place?

Thanks for the replies.
Mike K.

You need to connect it to the manifold vacuum. The distributor vacuum is ported and changes with throttle position.

Do you have the #1 and #2 wires swapped?

On a Rochester B carb, where do I hook up? Is there another port on the carb or manifold?

Thanks,
Mike K.

Any connection below the throttle plate will work.

Kenc maybe on to something with that vacuum leak. I have pretty good luck using a small hose about 3 feet long and listen for leaks. With one end in your ear use the other and probe around the gasket area. First stick it in the carb so you know what listen for, then start hunting. Most likely spot will be under the intake or bottom side of the front runner. The bolts are spaced a little funny right there and don't hold to well. Keep us posted, Joe.

LIKE KEN SAID!!!!

Hey Mike,
Let us know what you found.
curious minds want to know.

I didn't get a chance over the weekend to look into it. As soon as I find something out, I'll let you know.

Update...I did a compression check this past weekend and felt like the leaking off was not in the cylinder but in not having the guage sealed good. My dad suggested a new distributor as the old one had some slop in it. I bought a new one and put it in, but the miss is still there.

So, I gave up and took it to a local repair shop to have them tell me what the problem was. In driving it there I noticed that it was pinging real bad, I must not have the timing set just right. Also, the miss was not very noticable when you were driving. Wonder what that means.

Anyway hopefully in the next day or so I'll hear from them on what they think the problem is.

Check your spark advance curve, both mech and vacuum. Six cylinders will develop a stutter that seems to be missing, but its actually incorrect timing.

Well, I got a call today from the mechanic. He said they adjusted the air/fuel mixture and set the timing. He stated that it "pulls out real good". Hopefully, I'll be able to pick it up today and see for myself.

Mike K.

Well, I picked the truck up last night and it runs like a top. The mechanic would not share his method of fixing it. Said that he set the carb and timing by ear. He told me that he grew up on this motors. He would not tell me what he did because I would go buy Craftsman tools and do it myself. Whatever....

I'm just glad it runs good. I drove it around last night just to make sure it would be up to a trip to the ATHS truck show tomorrow.

Well dangit! I sure was hoping to hear the fix for this one. Too bad my truck sounds similar and I can't get it to MO. I'll be checkin' on those vacuum leaks...

That means it was so simple he doesn't want to tell you because then he can't charge what he did.

Well, if the mechanic did the work for free he wouldn`t be in business now would he :rolleyes:

Read through all the replies, all good advice, and here's my funky addition that may or may not be of use... What was the AIR TEMP OUTSIDE? I have found that with all else working, monojets seem to like to ice up if the hot air stove from the manifold is missing or rotted out. Really. If it starts and runs well above about 5 degrees above freezing and below about 5 below, but stumbles, stalls, and runs like crap between, and if you shut it off for 5 min then and it restarts and runs smooth for another 5 before it starts acting up again.... There is quite a temperature drop as the air speeds up through the venturi, and it will freeze the water vapour in the air, stick to the bore and mess up the actual venturi effect, and block the idle passage too. you can keep it running with lots of throttle of course. I think below about -5C (27F) that the relative humidity is too low to sustain the effect because mine would clear up about then. Took me a year to figure that out, and since have run into it on 4 seperate trucks - each one ran perfectly again with the hot air inlet repaired.