vacuum testing

[1977MGMidget]

I've been trying to find the reason my 1500 Midget was running a little lean. Here is what I found.

Timing 8 Advanced
Compression #1 140 #2 140 #3 140 #4 145

That all seamed fine but when I did a vacuum test it was zero?????? I sprayed everything I could find with wd40 and found nothing. So I keeped looking and I found the u shaped tube that go to the egr valve was rotten inside the white heat rap. But I still get a zero reading if I go to the vacuum nipple for the vacuum advance. If I go to the intake manifold I get a 15 reading. Are the vacuum ports in the carb plugged?

 

[CraigFL]

There are several places you can get "vacuum" from. If you hook at the intake manifold, you will have vacuum all the time while running. If you connect to a port on the carb just ahead of the throttle plate, you won't have vacuum until you start opening the throttle. Distributors are set up for either one of those vacuum sources as well as the other devices...

 

[1977MGMidget]

So I was checking from the wrong spot?

Thanks
Ron Garant

 

[jcatnite]

Right on the intake is the best place to check engine vacuum. You never really know where those ports on the carb actually come from unless you've had the carb apart. BTW, I haven't found the secret to getting more than 15 inches at the intake yet either? You can fiddle with the timing and have a dramatic effect on the vacuum though... as with fuel mixture.
JC

 

[dklawson]

I wouldn't say you were checking the wrong spot. You may have just misinterpreted your findings. As mentioned, the connection on the carb is for "ported vacuum". You won't measure much there until the car is cruising and a lot of air is flowing through the venturi. If you're trying to diagnose an engine the connection point is the manifold. You can play with the mixture and the advance to optimize your tuning. This will typically be seen as the condition that gives the highest idle vacuum at the desired idle speed. 15" is not a good idle reading. You should aim for something closer to 20"... assuming you have a stock cam. If you've got an aftermarket cam all bets are off.

 

[SilentUnicorn]

how about here? https://www.free-ed.net/sweethaven/MechTech/Automotive/Automotive02/lessonMain.asp?iNum=01030305

mark

 

[jcatnite]

Very comprehensive write-up regarding intake vacuum. Thanks for the link!
JC

 

[dklawson]

Thanks for the link above. I've added it to my bookmarks.
Here's another link on vacuum tuning.
https://www.centuryperformance.com/vacuum.asp

JC, was it you who was looking for liners for the 1500 engine? If so, I checked the VB Spitfire catalog last night and they are available from them for $40 each.

 

[jcatnite]

It was me that was looking at the liners. I was basically asking if they were replacable to bring the cylinders back to stock bore without a lot of machine work. The old cummins diesels I used to work on were easy to exchange after a couple million miles...lol. I'm still toying with the idea of an all out rebuild but would like to get this 1500 running right without it....pipe dream...lol
JC

 

[sparkydave]

The vacuum connection to the timing advance doesn't show any vacuum at idle on mine, either. If you can look inside the carburetor you will see that the throttle plate blocks that port at idle. Interesting, because the procedure in the service manual for adjusting the deceleration bypass valve implies that there is some vacuum advance at idle, so I just guesstimated at the deceleration valve setting until the car stopped backfiring when I let off the gas. /ubbthreads/images/graemlins/tongue.gif

 

[1977MGMidget]

I can't thank you guys enough for the information.

Thanks again
Ron Garant

 

[jcatnite]

Ron,
Just don't get sucked into looking at vacuum all by itself. You can tune an engine based solely on intake vacuum but must know the signs of going to far. It is easy to advance the timing too much to try and make up for some problem. Be mindful of how easy the engine starts and shutsdown and how it sounds under load. If the timing gets too high it will be difficult to start and will tend to run on when you try to shut it down. Also it will ping when underload due to pre-ignition since the plug and cylinder run hotter if the timing it too high. Best just to balance intake vacuum with what a good tach and a good timing light tell you...lol
This place is full of people who want to help. I've never been a part of a discussion board that was nearly this helpful.
JC