Cutting Out in Fourth Gear

[miwenner]

I have a 1989 Festiva L. 4 speed carbureted. I recently had a spark plug blow out of the head. I got a helicoil from NAPA and tapped out my hole then put in the coil. I started it up and I could hear a slight noise like the plug hole was leaking. I used a paper towel to see if compression was causing air to blow by. I could not tell that any air was blowing through. I took it out for a drive and it ran fine through all the gears until I got to 4th gear. At about 40 MPH in 4th gear it would cut out and sound like it was going to die. It was really jerky like the plug was about to shoot out and only run on 3 cylinders. I shifted down to 3rd gear and it was fine. I thought maybe with higher RPMs it was too much pressure and was blowing by so I put it in 4th at about 35 MPH and low RPMs but it did the same thing. I then put it in 3rd and rev'd the RPMs pretty high and got to about 40 MPH in 3rd and it didn't jerk or hesitate at all. I am confused as to why it would jerk in 4th but not at all in 3rd. I had a friend tell me that the distributor or cables could be to blame but I thought it was weird it only was doing it in 4th and I didn't think the distributor would affect that. If you have any ideas that I could do or know what might be wrong I am willing to try something to get to the bottom of it. Thank you in advanced.

[bravekozak]

It's the same pressure all the time.

[tooldude]

First, I would test the vacuum advance diaphram for leaking which will throw the timing timing curve off. This can cause bucking. If it's OK, there could be just enough of a vacuum leak to cause that condition - maybe one or more of the other vacuum diaphrams and/or vacuum hose. Just enough of a vacuum leak can cause trouble at lower RPM's and steady driving, but not at higher RPM's or accelerating, only appearing to be related to 4th gear. Try driving steady at about the same RPM in each of the other gears and see if the same problem occurs. Then you could have a possible carburetor or ignition problem.

[miwenner]

How would I test the vacuum advanced diaphragm? I'm guessing it has something to do with ask the houses everywhere. Also are there any houses that can be unplugged or bypassed?

[tooldude]

A vacuum diaphragm with only one hose connection (port) is tested with a hand operated vacuum pump to see if it will hold vacuum without leaking. The distributor advance is tested at each port with the opposite port open. Your question implies that you don't have one (on sale online at tooltopia.com right now for $33). If you don't have one, I believe the following should work on the distributor advance, although I'm creating this for your situation: The distributor vacuum advance on the carb engine is unique in that it has 2 chambers separated by a diaphragm. The timing curve is adjusted by a secondary vacuum (chamber 2) from a carb port that works against the main manifold vacuum which is applied against that diaphragm (chamber 1). You need a vacuum guage. (1)Start the engine so it's supplying vacuum, (2)remove the lower vacuum hose from the distributor vacuum advance & plug it, (3)connect a vacuum guage to that lower port - make sure your hose/reducers are tight fitting - I believe 5/32" hose fits the port, (4) look for NO vacuum - a vacuum reading means the diaphragm is leaking. When that diaphragm leaks, it pulls air FROM the carb port, through the diaphragm, and into the stronger manifold vacuum, causing problems. Also, the vacuum hoses on the advance could be reversed. Pull off one at a time and check for the strong vacuum. IIRC the strong vacuum goes to the top chamber connection. Removing/bypassing parts of a system should be done with understanding or not at all, IMO, & there is not a short answer except to remove the entire feedback carb system. You can find much on this site about that. I can tell you I have a stock carb with over 280K mi on it that has always worked perfectly, getting 44MPG when everything else is in good condition. The problems are with leaking diaphragms, hoses, or vacuum solenoids.