I have a 1950 mercury V8. It idles well and has a steady 20-21 inches of vacuum at idle. I recently put rings in it and the compression is 110-115 psi. When I rev it up to 1500 rpm or so when the vacuum settles down it fluctuates rapidly between 16 and 18 inches and I can feel the misfire. It has new plugs and points but the wires have been in it for a long time. Could the fluctuating vacuum indicate a sticking valve? Any ideas on what to check? Thanks for your help.
I think a sticking valve would cause a wider fluctuation. Might be misfiring for any number of reasons....
Does the fluctuations settle back down, or is the steady 20-21” only there on initially start up? Or does the low fluctuating reading stay there at a constant 1500 rpm?
Are you running the original distributor? It’s designed for Venturi vacuum off the carb. The advance diaphragms are known to fail over very many years.
Did someone ask for troubleshooting 101 posts this week and I missed it? That's pretty darn high for an engine, but I don't have a ton of experience on flathead tuning. How about 500 RPM higher or lower (1000, 2000)? This (1100-1400) is normally where transfer ports are at the top of operation and the mains are just kicking in. Doe the miss continue under load and at higher operation? Yes, but a sticking valve is going to be constant and more choppy at idle. This is the area where it is 'above the pistons' in an overhead valve engine. Things to check include, weak valve springs, intake leaks, carb issues, ignition problems. In other words, a grab bag of issues. Repair/replace things one at a time to confirm.
Vacuum gauge diagnosis charts are very handy to ferret out issues. They used to come with vacuum gauges, I know I have lost mine over time.
It is the original distributor, and I checked the diaphragm and it is good. It has the original single Holley carb. It will miss under load but not a dead miss like a dead sparkplug would feel. I'll take a video of the vacuum gauge in operation and post it so you can see what I see.
Might check dist. shaft for slop. Shaft slop will cause the points gap to fluctuate. This will feel as you describe and can cause the fluctuating vacuum reading.
>>>Using vacuum to diagnose a miss>>>It idles well and It idles well and has a steady 20-21 inches of vacuum at idle>>> Even though it seems to idle well, pull one plug wire at a time anyway to see if you can find any where the idle doesn't drop.
While not impossible, 20" to 21" of vacuum at idle seems unusually high. And increasing ignition timing will also increase vacuum, at least up to a point. So far I've not seen any mention of the current tune-up's initial or total ignition advance. Is it possible that it could have too much initial timing (explaining the high idle vacuum) resulting in too much total timing at 1500 RPM to cause a miss? Maybe stuck advance weights and/or vacuum advance in the distributor?
USING VACUUM TO DIAGNOSE A MISS Sure, I guess, if you made it look like an accident/unintentional. Let the vacuum latch onto a part of her skirt and lift high, you should be able to get enough of a peek to tell male or female. Keep in mind, this method won't tell you if they were born a female though
Yep, lol. I saved the file from here on the HAMB, funny I could read it clearly, so saved it. Can’t read it for nothing. Now I just saved yours and can read it just fine. Thanks.
Steady vacuum at slow speed and fluctuates at higher speed makes me think ignition or weak valve spring.
Here are two videos, one taken a couple of minutes after a cold start and the second one was taken after the engine was up to temperature.
I wouldn't say it's too impossible as I've got 20 inches at idle on my basically stock Merc Flathead.
You do drop a point for every 1000' feet of elevation. Correct? I know at my elevation I'd never see 20.
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