Hot Rods Vacuum advance problem

I have a 350 sbc in my streetrod. As far as i know its pretty much stock. Just had a points distributor set up on a sun machine , advance curve kit installed, new points,condensor,rotor. It’s at 36* at 2900 rpm. Installed it set timing to 36* initial fell in at 12*. Problem i have is when i hook vac advance to manifold vacuum it idles really bad. Adjusted idle mix screws with a vacuum gauge still cant get good idle. If i connect to port vacuum and tune mix screws vacuum gauge gets to about 20 inches and idles perfect. Edelbrock 1406. Am i losing any benefits being on port vacuum ? It does run slightly hotter in traffic but nothing to worry about. I am also getting only 10 mpg mostly puttin around town to local cruises. Spark plugs look great not black and sooty but fairly light colored. Any thoughts ?

 

Have you checked the vac can to see if it holds vacuum? I don't believe there is vacuum at idle on the port vacuum plug.

 

How many degrees of advance is the can adding? Usually 10 to 12 is what the pre emission engines had from the factory.

 

Vacuum can doesnt leak down. Correct there is no vacuum at idle on the port connection . If i connect to manifold vacuum thats when the rough idle comes in. It will run good once i get past 1000 rpm. It almost wants to stall at idle. Im just not sure if its ok to leave it on port vacuum. It seems to run just as well on port only a little hotter

 

you said V If i connect to port vacuum and tune mix screws vacuum gauge gets to about 20 inches and idles perfect.

Looks like ya answered your own question. If it runs good on ported then thats the place to run it. There is no written rule where the oems put the vac can ......some are manifold and some are ported ... depends on the emission package for the vehicle....

 

Looks like vac can is about 12*. Hard to tell, old eyes , small timing marks !

 

What is the timing change when you hook the dist into manifold vacuum? Maybe your not in the range you need to be.

 

I agree bubba. I just wasnt sure if i was losing any power or mpg on ported. It will idle well on manifold vac if i back timing to about 8*. Then it cant get out of its own way.

 

When on manifold it picks up about 12* from can

 

You can limit the travel of the can. If backing off initial back 4 degrees, then take 4 out of the can. Easy enough to do. Can make like comes with the Crane kit. https://www.summitracing.com/parts/...1ORHzlIbsppbMl_rrT9N3o5ycDAKzEWxoC75EQAvD_BwE

 

Nice call scrappbunch ! I never saw an adjustable vacuum can. That may be the answer.

 

Here are some stock can specs.

 

How much timing advance is there at idle with vac advance connected?

See where it tops out at too, you want way more advance under no load/low load cruise on flat ground than some people might think, but, some of those cans also pull in way too much. There are a gazillion different vacuum cans.

When revved up probably see close to 50°, remember the cans are adjustable. But it should run smooth when connected to manifold, 25 or 30 degrees at idle should be no problem. It will run cooler, but you'll never pass smog.

"No vac at idle" with a ported connection is a little misleading. It's true, when idle RPM is at a slow factory spec, but bump up the idle a bit to say 650 or something like that and it will start pulling in vac advance, and smooth out. Your timing light should show this. That's probably why people bump up the idle RPM a bit to begin with, it smooths out with the timing advance.

One thing with a manifold connection, some have suggested you want a vacuum can that's pulled in or pegged all the way at whatever manifold vacuum your particular engine can achieve at idle. If it isn't, this can set up a slightly uneven idle. On my compression Ford 10° to 12° initial and around 36° total works good and 52° indicated on the balancer with vac advance. Without a distributor machine it's tough to know where it really ends up. There's no load on the engine at all in neutral, there's some on the road even on flat ground.

 

Which spring combo was in the advance curve kit? Possibly coming in too quick?

 

I wouldn't do it that way, once the mechanical or total timing is dialed in, don't change it to compensate for a screwy vacuum advance. Most vanilla OHV V8 seem to like maybe 10 or 12 degrees initial, maybe 34 or 36 deg. total and close to 50 cruising on flat ground. That's where to start with, those numbers. Some take a little more, some take a little less but those are ballpark factory numbers for a gazillion motors, I expect.

 

Im seeing around 24* at idle with advance on manifold vac. 36* Timing is all in at 2900 rpm. Yes i do see it get to about 50* when running throttle up. Distributor was set up on a distributor machine. And yes vac can is pegged at idle on manifold. I just cant get a good idle almost sounds like a lean miss so i tried richer metering rods, no difference. I even tried different points hoping they were from a different part of china but that didnt help either.

 

Light,medium,heavy springs. We used one heavy and one light. Looked good on the machine but im not sure idle rpm would would bring it in quick. Beyond 1000 rpm it runs strong

 

Your misunderstood my comment. Set the initial to 12, and since it idles better with 4 degrees less timing, take the 4 out of the canister. So the can is only adding 8 degrees.

 

The more i think about it i agree with you. Im at 36 total and 12ish initial. Im only lookin for a street cruiser a couple days a week. Mpg is what i need , i could only fit an 11 gallon tank in it. The sloppy idle is buggin me. It even idles good with vac advance disconnected and the port suckin air. Whats up with that ?

 

Sorry i thot u suggested to pull 4 out of initial timing. My mistake

 

I went out and pinched vac hose a little and the idle did smooth out. Not sure how much i choked off, too many beers to mess around moving parts

 

Hm. Are you certain there aren't any ignition defects? Because it will seem just like a carburetor problem. Like a chinesium condenser maybe? Not saying for sure this is your problem but it sure wouldn't be the first time. Very common these days. Leaner fuel mixtures need more spark energy. I think that's what happens when lots of timing gets pulled in. Ignition is starting to puke? Try a known good condenser.

 

Im not certain of ignition defects !! Parts are garbage any more. I’ve been away from tinkering with cars for years. I had 2 sets of AC plugs with at least 2 in each that wouldnt spark. The car had AC R44t when i got it. For all i know they may not be correct. Its a streetrod and im trying to figure what the heads are off of. Block is a 70 chevelle. I do have other condensers never thot with frustrations to change it. Do u know how to test a coil ? Its new from napa but that dont mean its good

 

Would a slightly smaller plug gap help? Im at .035

 

I would fab up a little piece of metal that would be held by the screw that holds the canister. Easy enough to see how much it limits the degrees. Once its set to where it runs good and your happy, just tac it in place. Done this several times. Cost you nothing but time. See the black thingy in the Crane pic.

 

All mechanical advance by 2900rpm..?!
That's way too high. You are leaving both power and mileage on the proverbial table
Your mechanical advance should be all in by about 1600 to 1800rpm.

Mike

 

Should be able to run .035 OK. Ignition is weird stuff, it all has to work together as a team. And testing individual ignition components, most people don't have the equipment to really test them. Testing the capacitance value of the ignition condenser doesn't tell the whole story. Simply measuring the ohms resistance on the windings of a coil tells you it "should" or might work, but that's not the whole story there either. And both need to be tested at normal operating temperature. HOT. My WAG is the ignition coil is OK. If it is one of those el-cheapo condensers, grab an old school grubby used one out of your junk box and try that. Know anybody with a scope?

As a practical test, is the spark at a grounded plug fat and blue and snaps in the air? Or, pull the coil wire from the distributor and see if it will jump 1/2" held next to ground. If it's thin or weak and yellow, no good. Thing with carbs and ignition timing curves, if the spark is weak it won't ever tune right. Need a good hot spark at all times under all conditions. Maybe I'm leading you off into the weeds here... But it's something to check.

 

Your right. Lot of things to check. Ya know how ya get tunnel vision on one thing and cant see the obvious. I did buy a set of new old stock points and condenser but that dont make it right either

 

OK. When you mentioned chinesium points, I figured maybe it was chinesium condenser too. And a bum condenser does all kinds of wacky stuff. Go see if you've got a good fat spark at the coil wire and/or plugs. If it's weak in the open air it won't be any good under compression.

 

Really ? I didnt think you could bring it all in that soon. Easy enough to try