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Tue Apr 17, 2012 2:49 am

1974 Californian bay with full smog equipment, with an 009 distributor with no vacuum advance.

I have replaced the distributor with an electronic one and it has a vacuum advance can on the side. However, when I checked the vacuum port on the outside of the LH carb. it seems to have been sealed with a brass insert:

Should I just drill it out ?

Many thanks

Ian

Tue Apr 17, 2012 3:53 am

My carbs look exactly the same but the previous owner still put a rubber plug over it. I assumed there must still be some vacuum through the small point in the middle????

Tue Apr 17, 2012 5:33 am

I'll check tonight, but if you look at the first picture on ratwell.com:

http://www.ratwell.com/technical/DualCarbs.html

the port looks wide open?

Cheers

Tue Apr 17, 2012 7:09 am

Those are earlier carbs in the Ratwell pics, smog carbs are different, that looks like the advance port to me but it could be the EGR control port, any other myterious nipples on it?

Tue Apr 17, 2012 8:28 am

ianthurlbeck wrote: when I checked the vacuum port on the outside of the LH carb. it seems to have been sealed with a brass insert:
Should I just drill it out ?

I do not believe that is sealed. I would not "just drill it out". I would first test it with a timing light, rev engine to 3,500 rpm with vacuum hose on, see that the timing hits 36-40* btdc at throttle tip-off. Take hose off. Now timing should not exceed 28* when you rev it to 3,500 rpm. If you do not get the 36-40, suck on the hose at idle to make sure the advance unit itself is holding vacuum and advancing the timing, you can hear the engine rev up and your mouth can tell if there is a vacuum leak in the advance unit. If it works correctly and you still don't get vacuum advance, get a properly functioning vacuum gauge with air-tight hose, and read the vacuum as you rev the engine and let it drop down. If no reading . . . then! I might think about drilling that thing out.
Colin

Tue Apr 17, 2012 11:13 am

It's a restrictor put in at the factory to steady the vaccum signal. Without it the vaccum advance would flutter wildly due to the engine firing order and close location of the carbs to the intake valves.

Wed Apr 18, 2012 1:43 pm

Finally had a look tonight. I should confess now that I have removed the smog equipment, and according to my notes this vacuum port was connected to the 2-way cut-off valve for the EGR (also removed). There is another port on the inside of the carb., but I think this is the retard vacuum port and is sealed with a rubber cover.

Now, the "advance" vacuum port doesn't seem to move the distributor advance at all. If I suck on the pipe to the distributor I can get it to advance 10 degrees or so, but the carb. vacuum port doesn't shift it at all.

So, to some questions:
Was the vacuum port reduced in size on smogged buses to limit it's control of the 2-way valve ?
If so, is it likely that it isn't strong enough to work the distributor advance?

Cheers

Ian

Wed Apr 18, 2012 1:47 pm

ianthurlbeck wrote: Finally had a look tonight. I should confess now that I have removed the smog equipment, and according to my notes this vacuum port was connected to the 2-way cut-off valve for the EGR (also removed). There is another port on the inside of the carb., but I think this is the retard vacuum port and is sealed with a rubber cover.

Now, the "advance" vacuum port doesn't seem to move the distributor advance at all. If I suck on the pipe to the distributor I can get it to advance 10 degrees or so, but the carb. vacuum port doesn't shift it at all.

So, to some questions:
Was the vacuum port reduced in size on smogged buses to limit it's control of the 2-way valve ?
If so, is it likely that it isn't strong enough to work the distributor advance?

Cheers

Ian

only the engineers know. It is common to limit how fast a vacuum signal can rise and fall with EGR ports. This lets them open and close slowly to avoid engine stumbling. Also if the EGR system has any reasonable vacuum capacity to it, it prevents that capacity from causing the engine to stumble. Pretend you had a big pool of water tee'd into your main household water line that had to be filled before your sink flowed water. You would have a huge lag when the faucet was opened and a long trickle afterwards. A limiter in that pool line would limit the effect both ways.

Sat Apr 21, 2012 11:03 am

I checked the vacuum port and it seemed to be completely blocked with the insert, so I drilled the insert out (it was about 2 or 3mm long). The port hole goes down into the throttle body and connects to a very small hole. When I blow down the port I can hear a slight hiss of air. Re-connecting everything and running the engine still doesn't operate the advance.

According to the specs. for the new electronic distributor it needs at least 8in (200mm) Hg vacuum to work the advance. I don't have a vacuum gauge, unfortunately, but there's clearly not enough coming from this vacuum port.

What to do? Is there somewhere else I can get the required vacuum from ?
There was a T-piece inline on the brake-booster vacuum line which fed back to the EGR system. I took this off with the EGR system, but I could out it back I suppose.

Cheers

Sat Apr 21, 2012 11:17 am

ianthurlbeck wrote: There was a T-piece inline on the brake-booster vacuum line which fed back to the EGR system. I took this off with the EGR system, but I could out it back I suppose.

Cheers
No, that will produce vacuum whenever the engine is running and give you full advance even at idle.
Sure it isn't working as you currently have it? you shouldn't expect to see anything by just hooking it up but if you watch the timing mark with a strobe and rapidly open the throttle you should see a short burst of advance that drops off quickly every time. The mark should move faster and slightly more than with the hose off.