Bubba's chevy conversion dist.

trainguy

trainguy

Well-known member
I have a Bubba's chevy dist for my 51.I had vac advance added to the dist.What should the static advance be? I believe that the vac adv. must be from the manifold not the carb port as with the loadamatic,right?Phil
 
Did you ask this question when you bought the distributor from Bubba? Seems like he should be the one to answer the question.
 
The vac comes from ithe intake, not the carb. Disconnect the vac and plug it. Bring the rpm up to 2000 RPM set timing to 20 degs. re connect the vac read timing, should be 26-28. Let engine idle, remove vac plug it, read advance should be 2-4. Add vac should be 10-12. Now drive the car,it should start easy and idle nice and give great mileage. You can adjust the vac and turn the dist to make minor adjustments. If dist has more than 20 degs mach send it back to bubba.
 
Our latest conversions are shipped with 16 degrees (engine) of mechanical advance ( may be adjusted up or down with allen screw). The vacuum cans as supplied usually are 10-12 degrees of additional advance with full manifold vacuum applied and may also have the rate adjusted as to when that may happen.
Assuming the engine timing is set on the mark at 2-4 degrees btdc ( idle with no vacuum on vac advance) The engine when reved up ( 2500) would add another 16 degrees at 2500 rpm. This is with no vacuum.
Now lets add the vac unit to the deal with manifold vacuum applied. The engine would start with the 2-4 intial set on the pointer and when the engine starts the vac will add the 10-12 degrees giving the engine approx 16 degrees . (at idle) Now lets rev up the engine , when the engine is reved up the vacuum will drop and the mechanical advance witll increase . ( you wont have max vacuum with 2500 rpm or engine load).
This will allow the vac and mechanical advance to over lap each other to maintain a constant 16 degrees all the time the engine is running.
Both can be tuned to make this overlap perfect. Richard in Fla has mastered this adjustment with many trial and error drive sessions etc....

Clear as mud right ??? :>)
 
Ol' Ron said:
Jim, this setup would give 30-32 degs in cruise, alittle much, don't ya think?
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The numbers i used were just for explanation , the vac cans vary from 6 to 10 degrees typically and must be adjusted ( we use the adjustable accel units) .

We now use Richards adjustable set up on the cam and adjust all to 16 degrees engine .

Hopefully everyone understands the vac must be adjusted.....

Yep 30-32 degrees at cruise would be a bunch for the flathead.... good catch...
 
trainguy said:
How do you adj. vac adv.?
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You should have received a small allen wrench with your distributor. If you insert the long end down inside the vacuum canister's snout (where the vacuum hose goes) it'll engage the adjusting mechanism.

To adjust the vacuum advance without a distributor machine and a vacuum pump/gauge you will need a timing light and a degree strip on your crank pulley.

Once the engine is warmed up, remove the two little springs you'll find under the rotor. This will allow your mechanical advance to go to maximum and stay there. This makes your job easier.

Now start the engine and let it run at idle.

With the vacuum hose disconnected from the canister, make a note of the advance setting. I'll use 20 degrees BTDC. (It could be anything)

Now connect the vacuum hose and read the advance again. The idea is to adjust the canister so it adds 8 degrees of advance (that's the number we like).

If the advance reading is higher than 28 degrees (20+8) remove the hose, insert the allen wrench, and turn it counterclockwise a couple of turns. Now check the combine advance again to see where you are.

If the advance reading is lower than 28 degrees (20+8) turn the allen wrench clockwise a couple of turns and check again.

Once you arrive at the amount of vacuum advance you're after, remove the hose again.

Since your distributor is already at full mechanical advance, this is a good time to also set the maximum mechanical advance your engine will see. Loosen the holddown and rotate the distributor body until you see 20 degrees on the degree strip. Then lock it back down.

Now when you reconnect the vacuum canister you should see 28 degrees +/-.

Now put the springs back in and go for a ride.

The vacuum advance will come into effect ONLY when the manifold vacuum is high as at idle or easy steady cruise. The instant you hit the throttle for some power the vacuum drops out.

Hope this helps.
 
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trainguy said:
Where do I get a degree strip? Phil
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Is your engine otherwise stock?

If you can determine the diameter of your crank pulley, I'll e-mail you a PDF you can print out at 100% and it'll give you a pretty darn accurate degree strip. It's what I use; created it with my computer.

It looks like this.
 

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Now that we have that cleared up, we can take advantage of the vacuum advance, by leaning the main jets. A lean mixture requires a longer advance. When you get into the throttle the PV will richen the mixture, the timing will fall back and you'll have all the power you need, Richard uses an AF meter that shows fuel ratios of 15.5 to 16:1 in cruise and the mileage of 20Plus in a hat 276/ L-100 cam is no fluk , I waz there and drove it. 4.11 gears and a T-5 makes it come off the line with no trouble at all.
I was impressed. Tuning can be fun.
 
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