erratic vacuum???

[jbh7052]

hey guys....just curious if anyone can point me in a direction here. I have some erratic vacuum readings at idle. vacuum gauge flurries from 15in to 17in and steadies up at 20in at around 2500rpm. this is a fresh engine with 3,000 miles. below are some engine specs and what Cliff had me do to the carb. I am curious as to if maybe a carb setting or ignition time can play a role into this. sparkplugs are nice and tan and the jeep runs great. Carb #7045213. rebuilt with SR kit plus 76 jets, p-rods 43, sec rods DA, apt screw idle tubes(.036 or .037 cant remember) and power piston. Cliff had me tap the idle air bleeds to .059. This is a 355 with 8.5 compression. factory ram horn exhaust manifolds and duel 2.1/4 exhaust. Low performance aftermarket aluminum dual plane intake. Cam is .444 intake and 466 exhaust, intake opens 0 btdc and closes 34 deg atdc. exhaust opens 49 deg bbdc and closes -5 deg atdc. this motor is in a 82 jeep scrambler with 33 in tires and 4:10 gears with 350 turbo tranny. I have 15 deg of initial timing but have not measured the mechanical 

[Jeffs68]

Try adjusting the idle mixture screws, I usually shut OFF the engine and turn the screws in all the way, then back them out 2-3 turns as a baseline. Start it up and turn them in one at a time until the motor starts running a little rough, then back out 1/2 a turn or so.

[jbh7052]

thanks Jeff! I ran home on my lunch and was able to increase my vacuum at idle by 1in. still bouncing between 16 and 18in of vacuum. But that is with my timing advanced beyond any reasonable amount. I don't have a balancer tape on it but I am guessing about 20%. not sure whether to try a road test or not. I have my vacuum advance on ported source but I unhooked and plugged it anyway.

[Jeffs68]

You definitely need to get the timing accurately set.

[jbh7052]

will do tonight when the sun goes down! it's about 100% in Arkansas right now! It is only that high in an attempt to up the vacuum. that didn't work so I will get back tonight. I may move the source I am currently using from the carb base plate to the port just in front of the distributor. This is all in an effort to improve my brakes. I bought an aftermarket dual diaphragm booster and chevy master cylinder. The brakes start like they are gonna be great and then they get really firm.Thanks again Jeff!

[Jeffs68]

I prefer to use manifold vacuum for dist. advance.

Sorting out the idle vacuum issue should also help keep the brake booster working properly.

[jbh7052]

but doesn't the manifold vacuum pull the advance in at idle? Anyway I set the timing and I still have the vacuum gauge flurrying between 15in and 17in. this is the first small block chevy that I have built that likes above 12 deg of initial advance. I am sitting at 17 deg right now. this is where it runs its best. Also I noticed that with the initial timing at 12 deg I had to turn the idle up and this pulled vacuum through the ported slot for the advance on the distributor. starting to think that maybe the timing marks are not correct. I had to buy an aftermarket bolt on marker with timing marks. lost the original cover that was on the engine over the years