• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Vacuum advance question

Share this page

DanLewis

DanLewis

Jedi Trainee
Offline
I think I may need to reconfigure my vacuum advance, but first I wanted to see if anyone here could offer some sage advice.

I have a Weber 40DCOE on my BE. The Weber didn't have a vacuum port but the manifold did, so I connected the vacuum advance unit on the distributor to the vacuum port on the manifold. However, I have a suspicion that the vacuum advance unit was designed to work with a carburetor vacuum port and not a manifold port. I say this because of the way the car behaves. When I step on the gas, the power drops unless I back off and slowly let the rpms come up. I.e., before opening the throttle, there's a lot of manifold vacuum, and I get lots of vacuum advance. But when I open the throttle, the manifold vacuum drops significantly and the timing is no longer advanced until the rpms come up.

IIRC, the vacuum port on a carb is usually above the throttle plate. If so, then I think the vacuum would behave almost opposite to what I just described: Before stepping on the gas, there would be very little vacuum above the throttle plate. As the throttle is opened, however, more air is drawn through the carburetor causing the vacuum presented to the distributor (and thus the amount of advance) to increase.

I know someone will tell me to just disconnect the vacuum advance, but my understanding is that the additional advance from the vacuum advance unit actually is supposed to improve low-end throttle response.

Does anyone here know enough about these things to tell me if my reasoning is on track? If I need to connect the vacuum advance unit to a vacuum port on the carb, I think I saw in an exploded parts diagram that the 40DCOE actually does have a provision for adding a vacuum line. I think it's normally blocked off by a screw, but maybe it could be replaced by a fitting that could be connected to the vacuum advance unit. (See part #48 here.)

So - what do you guys think?

Dan
 
I'm surely not an expert but I've run cars on SU's both ways. Carburetor ported advance just eliminates vacuum advance at idle which gives you a smoother idle and seem's to make the engine less prone to overheating at a stop light. Above that I don't believe it make's much difference since manifold vacuum drops as soon as the throttle is opened. Perhaps there is still a differential on either side of the throttle plate that the distributor is designed for that could make a difference. Wonder if Vizard has anything to say about that??
IMHO I would think you need to dump more fuel in from the accelerator pump or equivalent on a DCOE. There is supposedly a Guru on DCOE's and I'm not him!:highly_amused:

Kurt.
 
Hi Kurt,

nomad said:
Carburetor ported advance just eliminates vacuum advance at idle which gives you a smoother idle and seem's to make the engine less prone to overheating at a stop light.
Click to expand...

Well, maybe that explains why my car overheats so much at idle - even with a special aluminum radiator! :grumpy:

I just found a post on another website that said it's a common mistake when installing a Weber to connect the vacuum line to the manifold, and that it causes a big stumble when you give it gas! Well, I guess I'm guilty!

I have a mechanical-advance electronic distributor (no vacuum advance) that I removed in favor of the one with vacuum advance, thinking that it would give better low-throttle response, but I guess my big mistake was to connect it to manifold vacuum. Maybe I'll just give up and put the mechanical advance distributor back on. :peaceful:

Dan
 
Vac on DCOE's is mech. only.. no vac of any kind on manifold... Just never works out as much as I know...The hesitation is common on them I might add...
 
You might want to contact Jeff at advanceddistributors.com and have him reprogram the advance curve on your distributor. His website has a work sheet for the specs of your engine. I have a 45DOCE with a Kent 276 cam and headers. He reset my curve and now my static timing is 16 degrees BTDC. This has pretty much solved the hesitation problem. Webers don't like to transition very much. I have a Crane electronic ignition system. Webers use mechnical advance.

Healey on.
Ray
 
Last edited:
Hi Ray,

bugi said:
You might want to contact Jeff at advanceddistributors.com and have him reprogram the advance curve on your distributor. His website has a work sheet for the specs of your engine. I have a 45DOCE with a Kent 276 cam and headers. He reset my curve and now my static timing is 16 degrees BTDC. This has pretty much solved the hesitation problem. Webers don't like to transition very much. I have a Crane electronic ignition system. Webers use mechanical advance.
Click to expand...

Good advice, and I have heard great things about Jeff's work, but I'm not ready to take the plunge just yet. The non-vacuum-advance distributor I have is new and is supposed to be designed for the 1275. I put it on the car last night and it seemed to make things a lot better, but I was only able to do a rough set on the timing. I'd like to see what setting the timing correctly does before trying something else. I'm waiting on a new throttle cable to arrive before I go any further because the one on the car is binding, preventing the throttle from returning all the way to idle.

My distributor is electronic, made by Accuspark in England (see it here). The mechanical advance kicks in with around 4 degrees of advance at 750 rpm and flattens out with 16 degrees at 4000 rpm. Of course those numbers are added to any initial (static) advance setting. However, you can't set static timing - you have to set timing with the engine running. I'd really like to find out from someone how much total advance I should have around 4000 rpm - then I could set the timing correctly.

My other option that I'm curious about is going back to the vacuum advance distributor, and finding a way to get ported vacuum from the carburetor. The 40DCOE 151 doesn't have a vacuum port but it appears to have a place for one that is currently blocked by a screw-in plug. There's one on each side of the carb, and I believe it was intended for balancing the vacuum between the two sides (or four if there were two carbs). Anyway, I saw in a post on another website that someone was considering the same thing (see here).

So many things to try, so little time. :fat:

Dan
 
Back
Top