1937 Special Engine Vacuum

[Jack Worstell]

For a 1937 Special  ( 248 ci) ....what should manifold vacuum be at  ,say,  1000RPM ?

 

I'm getting only 12" Hg.   I know

that "modern"   engines get more like  18" Hg...maybe a little more.

But should I expect this on a 1937   248ci engine  ??

Or is there something not right with the engine "

 

My question is triggered by last week the starter engaged four times while cruising around.

Yes...each time the accelerator was "floored".  But still this shouldn't have happened.

So I figure that either the accelerator vacuum switch has a problem or

something happened to reduce engine vacuum

 

By the way...the VR was switched out years ago...so we don't have

the back-up safety relay.

 

I'll test the vacuum switch separately this evening.

 

Jack Worstell

[Gary W]

Hi Jack!

I get a steady 20 from my car at idle.

 

[Bloo]

12 is in my opinion too low and not normal.

 

As for the vacuum switch, when the car is floored, there is theoretically no vacuum because the throttle plate is open. In practice there is a little due to the restriction of the carb and intake system, but depending on it to keep the starter disengaged is shaky at best. That's why the safety contact in the regulator was put there.

 

I understand that Buick recommended bypassing the safety when changing to a later style regulator. I have seen those instructions. However, on some later models, Buick found a way for the relay to "find" a ground through the generator and sort of have a safety again. It might be worth looking at the diagrams for some later models (1939 seems like a good choice), and compare to the diagram for the 37. There might be a way to get that safety function back. I don't have the proper manuals here to look it up.

 

 

 

[hchris]

Got yourself a big vacuum leak somewhere, is the reading steady at idle  ?

 

Just because, have you tried advancing the distributor a little to see if the vacuum rises  ?

 

 

[Jack Worstell]

Gary...thanks.     WOW   looks like I have a problem.   

 

And where did you measure vacuum ?      in my case I disconnected the vacuum tubing going to

the windshield wiper and attached the vacuum gage there

 

Jack Worstell

 

 

 

 

 

[Jack Worstell]

Bloo....thanks for your comments.....good point

 

I believe our 37 Special has a small relay mounted on the starter solenoid

similar to the set-up on our 37 Cadillac Series 60

If it does...I.ve been thinking about adding one of those small

cube relays to the system........and run

the ground for the small solenoid relay thru it

Set the cube relay to be NC....and have it

actuated by voltage from the A terminal of the VR

 

Or follow your suggestion....ground the small solenoid relay thru 

the generator armature.   I have seen this set-up before

but I'll have to find a 1939 schematic to refresh my memory on details

 

I definitely don't want to continue to  rely only

on the accelerator vacuum switch to keep the starter 

from kicking in.   The vacuum switch is more complicated than I realized

and hence more prone to something going wrong

 

Jack Worstell

[Jack Worstell]

Hchris........I only checked vacuum at about 1000RPM.

But I should also  have checked it at idle...might have

picked up a clue.  I will do this

 

Nor did I try to advance the distributor...another good thought

....I will try this too.

 

Your comments prompted something else...I will check the vacuum 

advance diaphragm in the distributor

    Something I didn't mention in my first post was

that on the same drive we didn't seem to have as much power as usual

I'm pretty sure that a faulty vacuum advance diaphragm would explain

both low manifold vacuum and loss of power.

 

Jack Worstell

[Daves1940Buick56S]

Is your fuel pump the dual type with the vac pump? If so dont look at engine vac on the line going to the vac wipers. In either case it's better to tap off the intake manifold vacuum port.

[Jack Worstell]

No....fuel pump is single action  not double action

[hchris]

Re the distributor vac advance, it picks up vacuum  above the throttle plate so unlikely to be the  cause of your problem, not having it working is more of a tuning issue and road running impact.

 

Two very important things about manifold vacuum are where your measuring it (as already mentioned) and how steady are the readings. 

 

By design your highest vacuum reading will occur when the throttle is closed, therefore initially for troubleshooting purposes, that needs to be at idle, and because you are measuring below the throttle you need  to tap your guage into the manifold, somewhere like a brake booster port. The distributor vacuum line is not the place to do it.

 

So at idle on a reasonably good engine I would expect about 20" and steady,  if it's jumping around then you need to follow a well defined sequence of checks to find the problem. If I'm reading you right though, your problem is just a permanently low reading and that would indicate a manifold / carb base or manifold fitting leak; somewhere in the induction system air is getting in where it shouldn't.

 

Having said that, I'm curious about your troubles with the starter vac switch, that's an item which would be reading below throttle vacuum, have you got a leak there perhaps  ?

Edited October 16, 2021 by hchris
Word change (see edit history)

[Jack Worstell]

Hchris.....right........ the distributor vacuum advance isn't big enough to cause a large vacuum leak

esp in view of the fact that it is connected to the carb via about 4'  of very small

1/8" dia copper tubing.    Rather....I was thinking a ruptured vacuum advance unit

would cause poor engine performance and hence low manifold vacuum

 

 

I removed the accelerator switch from the manifold  

and last night I checked it with a vacuum tester.

It would not hold vacuum !!   I think this explains the starter "kicking in"

several times during our drive last week.

But I don't think it explains the low manifold vacuum because

there is only about a 1/16"  passageway drilled thru the 1/4" NPT fitting

at the base of the unit.....the switch screws into the intake manifold 

via this 1/4" NPT fitting

 

So do I have two problems ??

 

We don't want to depart from originality too much....but if

I can't find a used switch  we may have to simply use a push button arrangement

 

Has anyone else ran a vacuum leak test on a starter accelerator switch ?

These things are more complicated than meets the eye and 

I want to be sure that...because of this....there's  not   some special way

to do the vacuum test on one of these

 

Back to the low manifold vacuum...Hchris....I will go back and repeat the vacuum test

being careful that I do  it at engine idle RPM.    Maybe I wasn't careful enough about this

and got a mis-leading result

 

Thanks for the comments

 

Jack Worstell

[MCHinson]

Dave Tacheny likely has a starter switch. If he does not, they are usually available on ebay. Here are the cheapest NOS ones on ebay at the moment: https://www.ebay.com/itm/284212460870?epid=1358310367&hash=item422c61fd46:g:16QAAOSwaRFgRsOl (if you have a Marvel Carb) or https://www.ebay.com/itm/284483023590?fits=Year%3A1937|Make%3ABuick&hash=item423c8272e6:g:sCgAAOSwnzBhYkDE (If you have a Stromberg Carb)

Edited October 17, 2021 by MCHinson (see edit history)

[Bloo]

I don't know whether it explains all of your low vacuum or not, I suspect not, but 1/16" is an enormous leak.

[hchris]

How about leaving the switch out and putting a plug in the hole temporarily, then do your vac check. 

[Jack Worstell]

Bloo/hchris

 

I'll be working on the Buick Thursday.....and first thing I will

plug the manifold opening with a 1/4" NPT pipe plug and check engine vacuum again