302 Running Rough, need help

[jcwalsh1]

Hello All,

My Bronco just started running rough and I can't figure out why. I updated the distributor a year ago but it had been running great.

Cylinder 3 and 8 have very low Exhaust Manifold Temperatures($20 HF laser temp gun). I have verified that they are getting spark and have done a compression test on each cylinder. Timing is set at 10 degrees @ 900 RPM.

Cylinder EMT Compression
1 706F 125
2 624F 125
3 228F 125
4 760F 119
5 775F 120
6 814F 120
7 522F 130
8 239F 125

 

[B RON CO]

Hi, the quick test would be to pull the plug wire off the spark plug while the engine is running. If the engine starts to shake, the cylinder is working, of there is no change, the cylinder is missing, you might have a bad spark plug, wire, or distributor cap. Usually the exhaust temperature is used to find lean cylinders, which run hotter. Good luck

 

[Timmy390]

How do the plugs look in #3 and #8 ?
Pop the valve cover and look for loose rockers/other damage.

I went to the junk yard years ago and got a SBF valve cover to which I cut the top out. Yu can inspect valve train while the engine is running without getting oil all over.

Tim

 

[jcwalsh1]

How do the plugs look in #3 and #8 ?
Pop the valve cover and look for loose rockers/other damage.

I went to the junk yard years ago and got a SBF valve cover to which I cut the top out. Yu can inspect valve train while the engine is running without getting oil all over.

T

 

[jcwalsh1]

The plugs all looked the same. I will pull the valve covers now to see if there are any issues with the rockers.

 

[Broncobowsher]

3 and 8 share the same port on the carburator. That is the way a dual plane intake distributes.
The 120s for compression shows me it is a stock low compression engine, probably a stretched timing chain. But no single cylinder being an issue.
What did the plugs look like?

 

[DirtDonk]

And have you checked for vacuum leaks in general? Including inspecting the PCV valve, which is probably at the other end of the carb, but can still affect things.
Definitely pull the valve covers, but those are other things that you can quickly check before digging too deep.

Another thing I like to do, involves clapping a timing light to each and every spark plug wire. See just how consistent the sparks are on the wires, especially those two.
The two cylinders don’t fire adjacent to each other in any cam configuration, so it’s probably not distributor related. However, it never hurts to check easy and quick things to rule them out.

 

[DirtDonk]

And it certainly wouldn’t hurt to pull the wires and look inside them to see if there’s corrosion on those two.
You can even swap with other cylinder wires to see if the trouble follows the wires. That also rules out something simple.

 

[73azbronco]

all of a sudden running rough...

how old is gas? Fuel filter? Another how does each plug look, what is idle vacuum?

 

[cldonley]

Spark plugs can fail and still produce a spark. On airplanes we test them under pressure and look for a hot blue spark, if it's orange then the plug is weak. Might just need a couple of new plugs. I'd try swapping plugs with other cylinders and see if the problem follows. Check inside the dizzy cap and look for signs of arcing. One last easy check-start it up, pull the hood down low and look for blue arcing from dizzy, plug wires and spark plugs. I've seen some pretty good light shows under there before. Just don't get too close-you'll get the whey shocked out of you. Ask me how I know...