1940 248 Vacuum Fluctuation and Noise

[Daves1940Buick56S]

OK, I didn't get any responses on the other question about this, but this time I have a video! And pictures are worth 1000 yadda yadda yadda. Anyhow, take a look, I am going about 40 to 60 mph in the video and up and down some hills. Also there is a noise from the engine at heavy fluctuation and low vac, almost like a rumble. I thought it might be pinging, so I backed the timing off from 10 degrees to the spec of 4 degrees BTDC but no change. Unfortunately the noise from the rear end dominates, but you can kind of hear it at 2:18 into the video. I can definitely hear it in the car, even with the rear whine.

 

So what am I looking at here? Valve springs, sticking valve, what? I am going to do another compression test on it tomorrow.

 

Thanks, Dave

 

Video at https://www.youtube.com/watch?v=qCx4xrJIp4M

[Pete Phillips]

Looking just at the fluctuating vacuum gauge is not helpful, at least not to me, because many things could cause that, including rolling terrain, varying speeds and varying pressure on the accelerator pedal. If you are doing this on totally flat terrain, and holding a steady speed, then the only thing to cause that kind of fluctuation is an intermittent vacuum leak, which is usually caused by a leaking gasket somewhere, most often in or around the carburetor. Where does the vacuum gauge tap into, to get its reading?

P.S.: Better get that rear end replaced or overhauled soon or you're going to be left stranded somewhere. Sounds like it is about ready to blow up!

Edited October 31, 2015 by Pete Phillips (see edit history)

[Mark Shaw]

Dave,

    This should help....

 

[61polara]

Vacuum gauge testing is done at idle not driving.  Not much we can tell from this video other than the timing might be retarded.  A vacuum gauge can be a great too.  Adjust the timing to the highest vacuum and then adjust the carb to the highest vacuum at idle.  Then adjust the timing again to the highest vacuum and the carb one more time.  Your goal is to get as high as possible, close to 20 or slightly above.  A reading of 19+ is good.  Then drive the car to see if there is knocking, if so back off on the timing.  You will be amazed how much better your wiper work as well.

[Daves1940Buick56S]

Pete: The readings are taken on both flat and hills. I kept my foot as steady as possible on the pedal, you can clearly see when I did move the pedal and shifted the "center", like centered on 10 inches then shifting to 5 inches. I tried to keep my pedal movements to a minimum. I hadn't considered a vac leak, if so it can only be detected under heavy load which will make if a bear to find. I will look further into this. BTW, I have been trying to get a new rear. This one is 3.41 and replaced my old 4.44 and it was overhauled, all new bearings, but the gears are just worn out. My mechanic Bill Anderson of Berryville did the work and adjusted it the best he could. I asked him if he thought it would catastrophically fail and he said he didn't think so, at least not for another few thousand miles or so. It has actually gotten a bit quieter, the sound on the video is actually worse than it is. I tried to get another one last winter but got another dud. Still looking for a 3.6 or even a 3.9.

 

Mark: Thanks, I think #5 is probably the most likely, will get some valve springs, fortunately I can replace them without pulling the head. When I got the head machined last year he said the guides were good.

 

Dave: Actually I find it quite useful to have a vac gauge in my car. When I was younger I always had one in my cars. You can tell a lot about engine condition under load, although I agree most vac diagnosis is at idle.

 

I did do a compression test today. When I did it last spring I had a solid 85 lbs on all 8 cylinders (this is after the ring job I did and I knew the rings were not broken in yet). So now I have driven it about 200 mi total this summer this is what I get:

1   85

2   81

3   81

4   80

5   88

6   93

7   88

8   90

 

Looks like the rings on the back 4 cylinders are starting to seat. I put some oil in #3 and it came up to 93 so that confirms it isn't a valve issue. I need to get the engine running good at speed and get some more miles on this to finish the breakin process.

 

On a more discouraging note, the fuel pump started leaking again. I am done with it and will put in a rebuilt from Bob's - enough is enough!

 

Cheers, Dave

[Ben Bruce aka First Born]

 Dave, I am envious of your "Motor Minder". Have not seen one in a while. I keep looking.

 

  Ben

[Daves1940Buick56S]

Thanks Ben! I got it on flea bay. I think it is from the late 50s, maybe early 60s.

 

Cheers, Dave

[jenz38]

To your engine problem, i would check all (inner and outer !)Valvesprings for abroken off end, be possible that there one of the valve begins to chatter and stops closing under revs ..

At idle range are al lookin good, because of the one good spring alone then will have enough power and time to close the valve.

Take a very precisly look to each of the 32 springs.

Another method :

If you have a good feeling in your hand ,

while stopstanding engine you can also hit onto each valves with a

plastic hammer and open it a little bit with this hit..., sometimes you can hear and feel a differend, lower sound of the jumping valve with the damaged spring.

It gave a method to change the spring without disassembling the cylinder head, but thats would be the next topic.

Good Luck

Edited November 1, 2015 by jenz38 (see edit history)

[Daves1940Buick56S]

Thanks Jenz! Yes, I think I will just replace them all. I should have done it last year when I had the head off. I have the adapters to keep the valves in place using shop air so I don't think it will be a difficult job.

 

Cheers, Dave

[Larry Schramm]

Dave,

    This should help....

Geee, Great chart!!

 

I wonder where you got that?

Edited November 1, 2015 by Larry Schramm (see edit history)