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Discussion Starter · #1 ·
I have a 305 H.O. SBC.

I believe my initial and total timing is set correctly. With the dialer at 8° and the vacuum hose disconnected from the distributor, the timing light shows initial timing is 8° BTDC @ 600 rpm, and total timing shows 34° @ 3500 rpm with the distributor vacuum hose disconnected from the carburetor...

But the vacuum gauge fluctuates between 14-12hg...

I would assume late timing, but the timing is showing correct...

What does that indicate ?
 

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Discussion Starter · #2 ·
I've been trying to find a decent reading chart but I can't find one specific to my reading...

But the 7th image from the top looks like mine and says leaky valves... Second opinions are helpful...
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Depends on cam, the bigger the cam the lower the vacuum.

The markings on these meters are for average production style cams with not a lot of duration or lift combined with a wide LSA. The LSA also tells a story on vacuum readings where you can take the concept that wider LSA’s say from 110 degrees up there is less intake and exhaust overlap which contributes to higher vacuum readings, where narrower LSA’s under 110 degrees indicates more overlap which lowers vacuum. So at the outset you need to know the cam’s specs.

The vacuum gauge is also affected by cam timing to the crank as well as ignition timing. Here there is a little trickery going on where a miss timed cam to crank has its effects as well as ignition timing. But the trick here is the realization that as far as the distributor is concerned the cam is nothing more than a jack shaft transferring power from the crank. So setting and resetting the distributor timing is an independent function from timing the cam to the crank..

A big cam will on the type of vacuum gauge with general markings about the vacuum you’re reading always read low no matter where you put the ignition timing.

A low steady reading is also sn indicator of a vacuum leak. Especially where mixing and matching parts like aftermarket head’s or blocks or blocks and or head’s that have been milled often open up manifold to head leaks on the valley side which cannot be tested for by the usual means for top side leaks. Other common sources are failed vacuum advance diaphragms and TH350 vacuum modulators not to mention the myriad of hoses and connections.

Bogie
 

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Discussion Starter · #6 ·
Depends on cam, the bigger the cam the lower the vacuum.

The markings on these meters are for average production style cams with not a lot of duration or lift combined with a wide LSA. The LSA also tells a story on vacuum readings where you can take the concept that wider LSA’s say from 110 degrees up there is less intake and exhaust overlap which contributes to higher vacuum readings, where narrower LSA’s under 110 degrees indicates more overlap which lowers vacuum. So at the outset you need to know the cam’s specs.

The vacuum gauge is also affected by cam timing to the crank as well as ignition timing. Here there is a little trickery going on where a miss timed cam to crank has its effects as well as ignition timing. But the trick here is the realization that as far as the distributor is concerned the cam is nothing more than a jack shaft transferring power from the crank. So setting and resetting the distributor timing is an independent function from timing the cam to the crank..

A big cam will on the type of vacuum gauge with general markings about the vacuum you’re reading always read low no matter where you put the ignition timing.

A low steady reading is also sn indicator of a vacuum leak. Especially where mixing and matching parts like aftermarket head’s or blocks or blocks and or head’s that have been milled often open up manifold to head leaks on the valley side which cannot be tested for by the usual means for top side leaks. Other common sources are failed vacuum advance diaphragms and TH350 vacuum modulators not to mention the myriad of hoses and connections.

Bogie
Not sure of the cam specs to know Lobe Separation Angle (LSA). I'm assuming it's stock.

Never had this issue until I replaced the distributor with a new one. Other than a bad alternator and distributor swap, the car ran fine before.

Didn't get the chance to do a compression test before the distributor swap, but a recent test shows low compression in some cylinders...
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"setting and resetting the distributor timing is an independent function from timing the cam to the crank.." You mean the initial timing and total timing ? Or is crank timing different from initial and total timing ?

Can't be a big cam if big cams always show low readings, because I've had 19-21 hg on the vacuum gauge, but only at higher idle rpm (1,000-1200), it doesn't stall after flicking the throttle.

When I spray around the engine intake and carb, the rpms don't raise. I could try to smoke leak test it and see if anything comes out.

Vacuum advance diaphragm is good because it's new, new distributor. Not too sure about the TH350 vacuum modulator...how do I test these things ? First gear had a hard/jump shift, but only shifting out of first gear.

Here is my timing check video with a timing light...
 

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Bumping your initial timing will increase your idle rpm. If it runs at 600 rpm and isn’t a problem in gear then run it there.

I see an edelbrock carb. What intake you got? If it’s the stock one did you use an adaptor to bolt the carb on with? The wrong carb gasket can cause a small vacuum leak.
 

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Discussion Starter · #8 ·
Bumping your initial timing will increase your idle rpm. If it runs at 600 rpm and isn’t a problem in gear then run it there.

I see an edelbrock carb. What intake you got? If it’s the stock one did you use an adaptor to bolt the carb on with? The wrong carb gasket can cause a small vacuum leak.
Yea I tried in gear @ 600 rpm and it backfires and stalls, but if I raise the idle rpm to 1,000 then in gear its 800 rpm, the vacuum gauge shows normal motor/18-20hg fluctuating...never had a steady needle, and drives fine last time I drove it a short while...

I have an Edelbrock 1405 600cfm carb and an Edelbrock 2975 Victor Jr. single plane intake.
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Just rebuilt the carb, and used this gasket to mount the carb to intake...
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I also have this gasket that came with the rebuild kit I could use...
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I didn't know which one to use...
 

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Discussion Starter · #9 · (Edited)
Funny you mentioned carb gasket causing a vacuum leak... Did a smoke test and found smoke coming from under the carb on the drivers side... Looks to be coming from the crab/intake gasket...
 

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Race it, Don't rice it!
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I go 12 base and 36ish total, all in around 3500.
THEN.....
Set idle around 800 in drive,
THEN....
Turn the carb idle mix screws all the in, then out 1 full turn from gently seated
THEN....continue unscrewing them 1/8 at a time on each, giving it a minute to stabilize, untill the idle is smooth as it will get.

Your big intake is going to compromise things a bit because it's a mismatched a bot. Either change it or be okay with it.
I don't see a major issue with compression tests OR the vacuum reading. It's with in reason.

Once all that is done, Whack the throttle to about 2400 real quick and if the idle comes back down quickly to exactly 800, without dipping below and coming back up. It's about as good as your gonna get it.

Keep in mind not all spacers are flat and fit properly. Leaks are common.
 

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Why are you using a Victor Jr intake on what sounds like a pretty much bone stock motor? Anyways with your one cylinder being on the very low side compared to others have you by chance done a wet leak down test to check the rings and check that one cylinder to see if it makes a difference? You will have to look up on the method of doing it or maybe someone can post the way to do so but that is how to tell if you have a ring problem of sealing. Also are you running a flat tappet camshaft by chance? If so have you checked for any signs for lobes going flat on the camshaft? Also on those gaskets from my experience they are only good for a few times before they can start to leak of you take the carb off enough times and can start to leak.

Also how old is your timing chain? If your timing chain is old and has a lot of slack in it, then it can cause a lot of issues on readings and timing as well.
 

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Discussion Starter · #12 ·
I go 12 base and 36ish total, all in around 3500.
THEN.....
Set idle around 800 in drive,
THEN....
Turn the carb idle mix screws all the in, then out 1 full turn from gently seated
THEN....continue unscrewing them 1/8 at a time on each, giving it a minute to stabilize, untill the idle is smooth as it will get.

Your big intake is going to compromise things a bit because it's a mismatched a bot. Either change it or be okay with it.
I don't see a major issue with compression tests OR the vacuum reading. It's with in reason.

Once all that is done, Whack the throttle to about 2400 real quick and if the idle comes back down quickly to exactly 800, without dipping below and coming back up. It's about as good as your gonna get it.

Keep in mind not all spacers are flat and fit properly. Leaks are common.
Gotcha, I'll try this procedure today, thank you.
 

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Discussion Starter · #13 ·
Why are you using a Victor Jr intake on what sounds like a pretty much bone stock motor? Anyways with your one cylinder being on the very low side compared to others have you by chance done a wet leak down test to check the rings and check that one cylinder to see if it makes a difference? You will have to look up on the method of doing it or maybe someone can post the way to do so but that is how to tell if you have a ring problem of sealing. Also are you running a flat tappet camshaft by chance? If so have you checked for any signs for lobes going flat on the camshaft? Also on those gaskets from my experience they are only good for a few times before they can start to leak of you take the carb off enough times and can start to leak.

Also how old is your timing chain? If your timing chain is old and has a lot of slack in it, then it can cause a lot of issues on readings and timing as well.
Its the intake that came with the car when I bought it.

Haven't done the wet test yet... And after watching compression test how to videos, apparently I did the test wrong by keeping in the other spark plugs while I had one out at a time...in all of these videos they take out all of the spark plugs...think I need to re-test ?

I know nothing about the camshaft, only had this car a few months...

Yea I'm thinking I need to buy another gasket, and make sure its the correct one... Autozone has Mr. Gasket and Fel-Pro to choose from...
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The one I have, I had to cut the insides a bit because it was quad barrel shaped, instead of a straight square like the ones pictured... It looked like this...
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Race it, Don't rice it!
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For about the billionth time, a compression test doesn't tell you a lot. Almost nothing actually beyond a major mechanical problem like holes in pistons, broken valves, blown out walls, gaskets etc.....A leak down test will tell you all of this, and a ton more. Especially if its done early, and used as a reference for engine wear.
The compression test is generally just comparing one cylinder to another as the reference so as long as all holes were pumped the same way, it really doesn't matter much.
Throttle open/closed/ plugs in or out, etc.....Just do it the same way with each hole.
On number 8, I'd check the lash first. then retest it. If the same, It wasn't a lash problem. Actually, while your at it, might as well check them all. You never know what other mindless fools have jacked around with and messed up so going back to the basics of something new to you will always payoff because you get to do it your version of the right way.

I'm partial to Felpro because I use them a lot. ....Any of those will work though. Whatever is cheapest and they have on on hand.
What Spacer is on it? Open? 4 hole?
Honestly, On that intake and carb, I'd take it off and not use it.
 

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I use the flepro or the Mr gasket open spacer square gaskets on all my builds and I have been able to take my carb off and on a few times if the carb has not been bolted on for to long of a time. After its been on and left alone for several months and ran through many heat cycles then about 50 percent of the time when I take my carb back off if for something unexpected, then it usually will stick to either my spacer or the carb base plate and I will have to replace it. I always keep a extra supply of gaskets on hand just in case. I get a bulk buy off of ebay for like five gaskets for like $20 bucks or so which are good ones just un packaged like in the auto stores but the same quality and cost less per gasket.
 

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Discussion Starter · #16 ·
go 12 base and 36ish total, all in around 3500
When I've successfully set base timing at 12° BTDC, with the timing light dialer on 12°, and turning the distributor, I should be seeing the balancer timing mark hitting 0° or 12° on the engine timing tab ?
 

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Your compression test shows more than 10% pressure difference between cylinders. This is characteristic of a high wear engine ready for rebuild. This also speaks to a high mileage to where one of the early failures or at least high wear items is the timing set. This is a mixture of chain stretch and gear tooth wear. Wear allows the cam to fall retarded which takes the distributor timing with it. However, as I said earlier the in the eyes of the distributor the cam is only a device providing rotary motion so the distributor can be corrected to the proper ignition timing while at the same time the cam to crank timing is not affected by fixing the distributor that element remains incorrect.

So if the cam is getting out of sorts with the crank, correcting the ignition timing to the crank does nothing to correct the cam timing to the crank. Low vacuum on a stock cam indicates that the cam or the ignition or both are eventing too late. You corrected the ignition with no effect upon the idle vacuum so there is something else going on.

Yes speeding the idle will increase the vacuum but that’s cheating, running more idle than the cam needs to keep the motor ticking over presents a false high vacuum reading.

If your engine has a stock cam, 8 degrees base advance is plenty. The whole need for lots of advance on big cams is the idle compression pressure is low because late closing intakes and long overlap periods bleed out mixture either through reversion out the intake and as scavenge out the exhaust. So absolute pressure fall off slowing the burn speed in the process, thus more idle advance with larger cams to buy more burn time at low RPM’s so the damn engine will idle.

Bogie
 

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When I've successfully set base timing at 12° BTDC, with the timing light dialer on 12°, and turning the distributor, I should be seeing the balancer timing mark hitting 0° or 12° on the engine timing tab ?
 
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