Hi everyone.
This is kind of a long story but here goes. My wife has a 64½ with a 260v8 that she had before we were together. It usually misses a little bit when it's cold in the morning but other than that runs reasonably well. I drove it about an hour away to a doctor's apt. and when I got off the highway I needed to drive about another ten minutes. When I pulled in the parking lot it seemed to be missing. When I came back out to leave it was still missing at idle but was fine when in motion all the way home. The next morning I got it to the end of the block and it was missing so bad it was shutting off. It will idle but acts like its being choked if you give it more than an inch of throttle. I did a compression test and from what I looked up they are supposed to be anywhere from 130-170 on the 260. Cylinders 1,4,5 and 8 came up short, the worst being number 5 at 115psi. I also replaced the spark plugs and the old ones looked a little whiteish (hot?). So the problem is it is missing on cylinders 1,4,6 and 7. It is getting spark in all cylinders and I can see that the fuel filter is full and I can hear fuel spraying with the accelerator pump. My question is, how low can the compression get before it will begin to misfire? Two of the low compression cylinders are working. My gut tells me that it's a fuel problem but wouldn't that cause all cylinders to misfire intermittently? I can't seem to find any vacuum leaks either though I could do some more looking around on that. My wife said she had a shop replace the 2bbl carb with another supposedly rebuilt 2bbl carb but it looks pretty weathered to me. Thanks for reading this novel of a story and any advice helps and is apreciated!!

 

Ok i will start with your spark plugs. I dont have a 2v manifold here for me to look at but if memory serves me right 1,4,6&7 are all feed by the left venturi. So for the lean i would take a look at the idle mixture setting, if it isnt the idle mixture screw then its something in the power circuit to the left venturi. However white plugs doesnt happen suddenly or with just a few miles on them. It is something that takes a few thousand miles do do. So i dont think this is what is causing the missing right now but something to take a look at after its running smooth.

An old engine with old rings and valves will have lower compression. What you are looking for is even compression between the cylinders. Though 115 is definatly low and a sign the motor is getting a bit tired i wouldnt immediatly call this the problem. All of them should be within 15 PSI of each other the motor will run well on a even 115-125 PSI. What was the PSI of all of the cylinders? You will want to do a leak down test of the lowest cylinder (#5) if it is not matching the others, this will let you know if its rings or valves that are leaking.

You say it seems like its choking out. Is the choke open? If you have a timing light i would throw it on to see if you have some timing issues(streached chain,etc..) Just because you have spark at the boot doesnt mean you dont have a bad plug or wire or worn points. I would change the plugs, wires and points(if you have them). What you are describing sounds like worn points. If you look down the venturis do you see a flood of gas down the venturis? This would be a sign of a float sticking high.

After you have made sure the ignition system is solid and deside that the compression is good. i would move on to the carb. As long as the choke is fully open and there's no vacuum leaks i would throw a kit into the carb and see what it does.

 

One other thing a lean to the point of miss fire will commonly backfire. Not always but it is common. Have you had any backfiring?

 

Agree with checking the timing

 

With these symptoms I would ordinarily say check the points, but with the plugs indicating a lean condition, not so much.

But still, if you are still running points, the gap/dwell would be a great place to start anyway.

 

Thanks everyone for the replies. I should have mentioned earlier that the distributor doesn't have ignition points but has one of those electronic ignition modules. From my experience and from what I've heard from others when they go bad they quit working completely. Is this correct or should I try replacing that as well? I did replace the plugs already. What I did to test the presence of spark was use one of the old plugs and try it on every boot just because I forgot to test each new one as I was putting them in and didn't want to take them all back out again. The compression test came up as follows, 1)125 2)140 3)130 4)127 5)115 6)130 7)140 8)120. I will take a look down the carberator tommorow and see of there's fuel flooding in there. I'm fairly certain that the choke is staying open although I did notice when I was poking around the first day that the heat riser tube was not plugged in to the exhaust manifold, but it didn't clear up after that anyway. Also if the choke was stuck closed I would figure it would run okay cold but not warm but cold or warm it's still missing. I'll put up an update as soon as possible and thanks again!

 

The 25 PSI difference is very curious. I would recommend checking all of the bolt on's like your planning (carb, timing, etc...) If those dont have anything wrong then it will be time to figure out the PSI difference. Do another pressure check to make sure the pressure test is correct and then do a leak down on the #5. The leak down will let you know if its valves or rings. Im saying another PSI test cause thats alot of money spent if the PSI test isnt done right.

 

I'll have to pick up a leak down tester because I don't have one. Today I looked down the carburator and didn't see any flooding. Although I don't have a timing light I played around with the timing a little bit and it didn't seem to make any difference. I did however start checking the manifold bolts and the two up front were considerably loose. After I tightened them up a bit it seemed to help a bit. I also noticed that if I get the revs up and hold them there it seems to smooth out once it gets past a certain point. I'm starting to lean towards a vacuum leak at the manifold somewhere. Either way it looks about due for some new gaskets and cleaning anyway so I think my next step is to put a 4v manifold and carb that I have sitting around on there. The exhaust also has a horrible leak so I will do that at the same time. I will update when I get through with that.

 

Ok good on you finding the vaccum leak. Sounds like changing the gaskets will take care of most of your problems.

I would recommend going to harbor freight (if there is one near you) and picking up a timing light and vacuum gauge. HF has them cheap and they can do alot for you for troubleshooting and tuning. The last time in was at HF 30 bucks would get you a light and gauge. After you get the new carb and manifold on you will be able to get her running like a champ.

 

One other thing, take a look at your compression tester. Alot of them are made to also be used for a leak down test. So you might already have what you need to do the test. Thou i would recommend waiting to see if the manifold gets her running smooth before worrying about the leak down test.