Jack Worstell Posted October 14, 2021 Share Posted October 14, 2021 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 Link to comment Share on other sites More sharing options...
Gary W Posted October 14, 2021 Share Posted October 14, 2021 Hi Jack! I get a steady 20 from my car at idle. Link to comment Share on other sites More sharing options...
Bloo Posted October 15, 2021 Share Posted October 15, 2021 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. 2 Link to comment Share on other sites More sharing options...
hchris Posted October 15, 2021 Share Posted October 15, 2021 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 ? Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 15, 2021 Author Share Posted October 15, 2021 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 1 Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 15, 2021 Author Share Posted October 15, 2021 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 1 Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 15, 2021 Author Share Posted October 15, 2021 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 Link to comment Share on other sites More sharing options...
Daves1940Buick56S Posted October 15, 2021 Share Posted October 15, 2021 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. 2 Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 15, 2021 Author Share Posted October 15, 2021 No....fuel pump is single action not double action Link to comment Share on other sites More sharing options...
hchris Posted October 16, 2021 Share Posted October 16, 2021 (edited) 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) Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 16, 2021 Author Share Posted October 16, 2021 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 Link to comment Share on other sites More sharing options...
MCHinson Posted October 16, 2021 Share Posted October 16, 2021 (edited) 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) Link to comment Share on other sites More sharing options...
Bloo Posted October 17, 2021 Share Posted October 17, 2021 I don't know whether it explains all of your low vacuum or not, I suspect not, but 1/16" is an enormous leak. Link to comment Share on other sites More sharing options...
hchris Posted October 17, 2021 Share Posted October 17, 2021 How about leaving the switch out and putting a plug in the hole temporarily, then do your vac check. Link to comment Share on other sites More sharing options...
Jack Worstell Posted October 17, 2021 Author Share Posted October 17, 2021 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 2 Link to comment Share on other sites More sharing options...
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