Any Advice on this Vacuum Gauge Reading?

[Hot Rod Reverend]

If you have been following my build, you know that I recently went completely through an Edelbrock Carburetor that was installed on my 55 - serious ethanol corrosion problems, filter element junk in the carb, etc.

All plugs were removed, some were fuel-fouled, but 4 were looking good. I cleaned, re-gapped, and installed all spark plugs.

Take a look at this video and let me know what you think.

https://vimeo.com/331574496

fun times

[fordrodsteven]

I don't have much experience with "Y" blocks. I am wondering if you might have a vacuum leak in a line or diaphragm somewhere in the system. Do you have a way to do a smoke test to see if there's any leaks? Have you put a dwell meter on the distributor to see if there is any correlation with the moving needle on the vacuum gauge? I had a 351C that ran pretty good at idle but every once in a while would just turn to shit. My problem was a bad breaker plate in the distributor. The dwell and the vacuum would not settle down and stay steady until after I changed the distributor.

[Hot Rod Reverend]

Will check on some of those things. The distributor holds vacuum as it should, and is not that old - only 300 or 400 miles on it. But who knows? I did unhook the PCV and plugged the carburetor port with a cap - no change.

Here is another video I made about exhaust temps. Not sure what to make of it.
https://vimeo.com/331576377

[fordrodsteven]

Man! Last time I worked on a "Y" block was in the late 1970's. LOL. Anyway. My take on your exhaust temps is that one cylinder is firing intermittently or maybe not at all. The temperature you're seeing could be coming form other exhaust gases in the exhaust manifold. Have you tried moving a plug or a wire to see if the problem moves with the plug or wire? My only other thought would be mechanical in nature. Possibly carbon on the valve or a slightly bent valve? An improperly adjusted valve? If you've got a valve that is not closing all the way it could cause the vacuum gauge fluctuation you were seeing.

[wellcraft17]

I have a 56 2bbl bone stock 292, compression 155-160 all cylinders, timing 11btdc.it has 23 inches of vacuum and is rock steady. Just checked my exhaust manifold temp on all cylinders with engine hot with my handy dandy laser gun. Starting with #1 and going up the readings are 377,415,446,446,386,467,482,414. This engine idles and runs perfect. What I notice about my readings is that #1 and#5 are very close and are at the front of the engine where the water from the radiator hits first and I’m guessing it’s cooler due to coming through the radiator. I’m assuming that is the flow direction of the water. From the vacuum charts I’ve seen your reading is right in the middle of normal. What are your compression readings?

[wellcraft17]

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I have a 56 2bbl bone stock 292, compression 155-160 all cylinders, timing 11btdc.it has 23 inches of vacuum and is rock steady. Just checked my exhaust manifold temp on all cylinders with engine hot with my handy dandy laser gun. Starting with #1 and going up the readings are 377,415,446,446,386,467,482,414. This engine idles and runs perfect. What I notice about my readings is that #1 and#5 are very close and are at the front of the engine where the water from the radiator hits first and I’m guessing it’s cooler due to coming through the radiator. I’m assuming that is the flow direction of the water. From the vacuum charts I’ve seen your reading is right in the middle of normal. What are your compression readings?

[Hot Rod Reverend]

you guys are leading me in the right direction... I checked secondchance garage and they had a nice adobe flash display of different vacuum gauge scenarios. The scenario they showed that came very close to matching my reading was indeed indicating a valve not closing all the way. Almost a valve overlap it said. Which would make sense. When I removed the spark plug from #5, it was the worst, almost 3 times worse, than all of the others. It is the one in the video where the tip had just about "welded" to the electrode. Cylinder 5 was also the one that had the least amount of heat when compared to all other cylinders exhaust temps.

Next time I am in the garage I will take the valve cover off and check #5 intake and see where we are at. I did adjust the valves before I put the engine back in the car, so who knows? I may have adjusted #5 intake too tight... If it is not #5 then I will do a compression check on all cylinders. Last time I did this I was at 155 for all 8, with very little variance from that number. That was some time ago however.

I will let you know how it turns out. Thanks for the advice guys. :P

[JimNNN]

Happy Easter Hot Rod Reverend!


I'm curious as to how your car runs generally speaking. The compression readings look really good.

[39deluxecp]

worn valve guides

[Hot Rod Reverend]

Originally Posted by JimNNN

Happy Easter Hot Rod Reverend!


I'm curious as to how your car runs generally speaking. The compression readings look really good.

generally speaking - very well. Plenty of power, torque, I burned rubber today, literally. Most of the issue I have had has been idle quality lately, but with the carb issues there is no wonder.

[Hot Rod Reverend]

And of course it was cylinder #5 - watch the video, for the vacuum gauge reading tells the result of success. The exhaust was just a hair OPEN at TDC and the intake had only .010 lash. Ridiculous. My mistake late one night when adjusting these valves a while back trying to get the engine prepped to go back into the car.

Video link:
https://vimeo.com/331877284

thanks again fellas! Love the FordBarn here!

[JimNNN]

Congrats on figuring out the problem. Your car is very clean and nice.