Low Vacuum? With good compression?

[MagmaJct]

I've had my truck for 7 years now, got it from my Uncle after he passed away. (350 SBC) As long as I've known, the idle vacuum has been low. ~10 in/mg. This has never really caused a problem, the engine idles fine with the Carter AFB. I get 10 MPG, empty, loaded, uphill, downhill, and it passes emissions.

I've decided to replace the carb with a Quadrajet 4MV. But, as you could imagine, it does not like low vacuum. The power piston is staying down, however, to maintain idle speed I have to open the throttle enough to cause main nozzle drip. I've verified the float at 1/4". (It is brass, however.)

Instead of trying to make the 4MV work, I'd rather address the low vacuum. I just completed a compression test: Range, 160 - 175 PSI. All pressures came up within 5 cycles.

I've replaced carburetor and intake gaskets in an attempt to correct potential intake leaks. None were found, no improvement. I'm not surprised, any vacuum leak that would drop the vacuum that low would cause other problems, like high idle, lean idle, and misfires.

What's left?
1) Late valve timing.
2) Uncle put a performance cam on it.
3) Intake leak from exhaust crossover.
4) Anything I missed?

How do I verify? I'm a family man, and would prefer avoiding pulling the harmonic dampener and timing cover if I can, due to limitation of my free time.

I'm considering investing in a leak down tester. It may prove useful.

[Wrenchbender Ret]

It could be a worn timing chain. You can get an idea by moving the balancer back & forth to see how much movement till it tries to move the cam. About 5 degrees is not bad. 10 or more is too much. How many miles are on it. If it is over 100K it could very well be bad. Those had plastic cam sprockets that wear out easily & the chain can jump a tooth.

[enaberif]

Originally Posted by Wrenchbender Ret

It could be a worn timing chain. You can get an idea by moving the balancer back & forth to see how much movement till it tries to move the cam. About 5 degrees is not bad. 10 or more is too much. How many miles are on it. If it is over 100K it could very well be bad. Those had plastic cam sprockets that wear out easily & the chain can jump a tooth.

From my recent rebuild. No clue on mileage of truck

http://i.imgur.com/qF8Guro.jpg

[geezer#99]

If your quadrajet with one power piston doesn't like low vacuum, then how did your afb like it. The afb has 2 power pistons.
Your idle problems and low vac are due to lack of timing.
What's your timing curve like?
What initial?
How much mechanical?
How much vac advance?

[MagmaJct]

Originally Posted by enaberif

From my recent rebuild. No clue on mileage of truck

http://i.imgur.com/qF8Guro.jpg

Holy crap Batman! Wow! I believe the engine is original, but I do have some small doubts. The truck's been in the family since 1973 the earliest I could tell. My grandmother never drove, and my grandfather was legally blind. The truck was only driven twice a year when mom and us kids would visit. I remember as of the 1990s it only had around 50,000 original miles. Around that time, both my grandparents had passed, and my uncle took possession. That's when its history becomes a little uncertain.

As it sits now, it has 131,663 miles on the clock. It's extremely likely the sprocket is as bad as yours! Wow! Thanks a ton! Looks like there's more impetus to perform exploratory surgery.

[enaberif]

Originally Posted by MagmaJct

Holy crap Batman! Wow! I believe the engine is original, but I do have some small doubts. The truck's been in the family since 1973 the earliest I could tell. My grandmother never drove, and my grandfather was legally blind. The truck was only driven twice a year when mom and us kids would visit. I remember as of the 1990s it only had around 50,000 original miles. Around that time, both my grandparents had passed, and my uncle took possession. That's when its history becomes a little uncertain.

As it sits now, it has 131,663 miles on the clock. It's extremely likely the sprocket is as bad as yours! Wow! Thanks a ton! Looks like there's more impetus to perform exploratory surgery.

LOL yeah I don't really know how the truck was running. The timing chain was so stretched it almost touched in the center.

[MagmaJct]

Originally Posted by geezer#99

If your quadrajet with one power piston doesn't like low vacuum, then how did your afb like it. The afb has 2 power pistons.
Your idle problems and low vac are due to lack of timing.
What's your timing curve like?
What initial?
How much mechanical?
How much vac advance?

It's been a couple years since I checked mechanical and vacuum advance. I don't remember the specifics, but I remember nothing unusual. Initial is 8* BTDC. I've experimented with advancing it, in an attempt to rule out dampener slip, but when I do advance the timing, the engine seems to run better, to a point. When I go on a test drive, after a couple miles, it runs terrible, the idle quality plummets.

I also tried manifold vacuum to try and boost vacuum, essentially, as a way to compensate. But again, once I take a test drive, the idle become more erratic, and performance suffers.

As for the AFB, I did attempt to lean it out a bit a couple years ago to improve economy. Again, I don't have the specifics. I installed leaner rods. Drive-ability suffered. Ultimately, I don't recall how I corrected it, if I used the leaner, or original rods.

The AFB seems to not mind the low vacuum as far as I can tell. It passes AZ emissions. (Has for 7 years!) (It did fail a recent test, because I'd just fixed an exhaust leak on the driver's side manifold doughnut, which affected idle quality once fixed. I adjusted the mixture screw to correct that, the results of the test indicate I went too rich.)

I'll test the timing advance and get back to you.

Thank You for your help.

Regards,
Mike

[MagmaJct]

Originally Posted by geezer#99

If your quadrajet with one power piston doesn't like low vacuum, then how did your afb like it. The afb has 2 power pistons.
Your idle problems and low vac are due to lack of timing.
What's your timing curve like?
What initial?
How much mechanical?
How much vac advance?

Okay, I've got the information requested.
Baseline timing 8* BTDC @ idle, ported vacuum.

The way I performed the following tests was to set my timing light such that the timing scale on the engine reads 0* BTDC for baseline. I then increased the vacuum until the next value was reached by the timing mark on the damper. I did the same for mechanical, with vacuum disabled.

10 "mg = 2*
12 "mg = 4*
13 "mg = 6*
14 "mg = 8*
15 "mg = 10*
>15 "mg = 12* max

~1,110 rpm = 2*
~1,250 rpm = 4*
~1,870 rpm = 6*
>1,870 rpm = 7-8* max

What struck me as odd is the mechanical only provides up to 8* advance. I would have expected more. I checked the weights and springs, and everything moves fluidly and returns. The timing is rock steady, unlike my VW. :-P

Now that I think about it, another problem with low vacuum is the engine will never see full vacuum advance. Hmmm. That could affect economy as well. I have tried higher initial timing before. The results are initially positive, then, after a couple mile test drive, idle quality drops considerably.

I'm going to schedule exploratory surgery, probably within the next couple weeks to inspect the timing gears/chain. Since enaberif's experience was shared, I've been searching past posts. Looks like there's an HD option where it's a double sprocket/chain. Does anyone know if this will still fit in the stock cover?

Regards,
Mike

[geezer#99]

Curious!!
What device did you use to increase the vacuum?

[VetteVet]

Do you have power brakes or th 350? Both require engine vacuum as you know.

[MagmaJct]

geezer#99 and VetteVet,

I used a handheld vacuum pump to apply vacuum to the distributor to measure the advance.

No vacuum/power brakes, all manual. When I first investigated this problem a few years ago, I did disconnect the line to the TH350 and plugged the manifold port. I recall no change.

Regards,
Mike

[RichardJ]

>> I've experimented with advancing it, in an attempt to rule out dampener slip<<
Put a chalk or paint line on the ring and also on the hub or any pulleys bolted to the hub. Start and stop the engine once or twice and your marks will be misaligned if the damper is bad.

If the problem is,
>>the idle vacuum has been low. ~10 in/mg. This has never really caused a problem<<
you needn't concern yourself with the vacuum advance. Disconnect the hose and plug the vacuum port at the carb. Vacuum advance is used for fuel economy. No need to have it working until you deal with the low manifold vacuum.

Does it have a points distributor? Does the dwell stay steady or at least within 1 or 2 degrees as you rev the engine.

My question for you, is 10" vacuum the peak vacuum as you adjust the idle mixture screws or what are you seeing as you adjust the idle mixture screws.

[motorcritter]

That worn timing component concept is pretty valid. The time spent chasing a vacuum leak/carb defect is probably better spent opening the timing case and actually verifying the conditions. A bad harmonic balancer (slipped outer ring) can really cause problems, and is a possibility, considering mileage and condition of the engine.

[Wrenchbender Ret]

Are you hooking the gauge to manifold vacuum? Ported vacuum will not give you correct readings.

[davepl]

Remember, no vacuum advance at idle. If you've got any vacuum advance at idle, you're either connected to the wrong port or your throttle blades are too far open, uncovering the transfer slots.

Problem with the timing approach is you can usually increase vacuum a lot by over-advancing base timing. More of a crutch than a fix though. It has been my experience, however, that cars need/like a lot more base timing than the old tuneup stickers say.

My 402 prefers 12 degrees, a lot more than the (4 or 6 maybe?) that the sticker calls for. People say it's because gas is different, I'm not really sure.

[Vero67]

My bet would be on the performance cam. On my 56 210, I need to run a vacuum pump and canister for my brakes. If you can figure out the cam, the specs should tell you the expected vacuum.

[MagmaJct]

Wow, a lot of good info here. I'll just respond to all in one post.

The distributor has a points replacement module, similar to the Pertronix. (It might very well be a Pertronix.) It's been awhile since I verified the dwell, I don't recall it being out of spec. I can't seem to find how to adjust it anyway. (With the lack of points, that is.) The timing stays steady, I can check it weeks later, and it'll be where I had it last. I will mark the damper/pulley to verify it's not slipping. Good idea!

Regarding using a vacuum gauge to set the mixture, that hardly works. There is very little change in vacuum regardless of whether I use the Qjet or the AFB. I just reinstalled the AFB in the interim, because it idles better with low vacuum. I set mixture by best idle RPM, then back it off ~1/2 turn.

Best vacuum at idle in neutral is 13 "mg, 10 "mg in drive.

As far as I know, the truck does not have a performance cam in it. But hey, I could be wrong!

I agree that advancing base timing isn't the way to go. Those were initial tests.

I intend to pull the timing cover. But that kind of exploratory surgery will have to be scheduled. I'll keep this thread and post my results when the time comes.

Regards,
Mike

[68 TT]

Originally Posted by MagmaJct

I'm going to schedule exploratory surgery, probably within the next couple weeks to inspect the timing gears/chain. Since enaberif's experience was shared, I've been searching past posts. Looks like there's an HD option where it's a double sprocket/chain. Does anyone know if this will still fit in the stock cover?

A double roller timing chain fits under the stock cover and clears most blocks. Some older ones do need a little clearance work behind the upper gear for proper chain clearance.

I use them on everything I rebuild. Well worth the small amount of extra cost.

[MagmaJct]

Okay! The planets have aligned just briefly enough for me to get the timing cover off, and the chain exposed! It helps the radiator was out being repaired. More on that later in its respective thread.


Above: The marks line up when rotated. (Not photographed.)


Above: Slack.

I've never done timing chain work before! I've worked with air cooled VWs and their timing gears, I've worked on more modern engines with timing belts. They NEVER have slack. This amount of slack seems excessive in comparison.

Questions:
How much will this affect engine performance?
How will it affect engine vacuum?
Can anyone tell me about this timing set? Doesn't seem stock.

Regards,
Mike

[Mike_The_Grad]

Maybe too little too late, but what the heck...
Yes, that is not the original timing chain. That is a "Roller" type timing chain.

Yes, that amount of slack is excessive. You should have no more than 1/4" of travel.
When I pulled the cover off my '72 SB 350, the chain moved almost an inch! But it started,idled perfectly,and smoked the you-know-what out of the tires...lol.

Also, yes it will affect vacuum and performance. I replaced my stock setup with a "True Double-Roller" Timing set at a tune of $35. The stock set was $17.
The toughest part of replacement is the crank sprocket,and only if you attempt it without the correct puller/installer like I did. Local parts store should be able to lend it out, other than that pretty straight forward, proper cam to crank alignment needs to be at #1 TDC. Also check crank sprocket for markings indicating "advance/retard/zero" installation timing. Anything driven on the street should be installed "zero" crank timing.
Just my 2¢.

[volksworld]

if i had it that far apart i'd just change cam and lifters too just so you know where you stand...small blocks have been known to flatten cam lobes...thats why i loved bug engines...not having to consider a bad chain or hydraulic lifters made them a heck of a lot easier to diagnose

[cleszkie]

If the timing chain has been replaced in the past (as it appears to be), that increases the odds that a cam swap also happened, which could mean a cam profile that generates less vacuum. Judging by how clean the post on the cam that engages the cam gear is, I'd say that the cam was replaced. Remove the cam gear and see if the cam grind number is identified on the end of the cam. This should give you an idea of what vacuum the cam should be making.

[MikeB]

Here's my take, and I'm open to comments:

13" manifold vacuum at idle in Park would usually indicate a vacuum leak or a cam with lots of overlap. But if your idle is smooth, the cam is probably not bigger than 10-12 degrees above stock. And if turning your idle mixture screws CCW from fully seated doesn't increase vacuum or RPM, then you probably don't have a vacuum leak. But that would typically indicate a rich mixture, caused, for example, by the idle transfer slots being fully exposed.

BTW, 8 degrees max vacuum advance is in the range of what many GM canisters had before emissions controls. There's certainly nothing wrong with it, but once you solve the vacuum problem, it would be nice to get a canister with adjustable spring tension and stop limiter. We eventually started seeing ~20 degrees vacuum (ported) with very little initial + mechanical to reduce idle emissions.

I never would have thought about too much timing chain slack, but it certainly makes sense. You might pick up a couple inches with a new timing set. However, for a stock or mildly modified engine, I suggest you skip the double roller (and potential clearance issues) and get a basic Cloyes timing set. My last few 350s used GM single roller timing sets with the same Renold chain as the GM Performance Parts big blocks. Line up the marks at #1 TDC COMPRESSION STROKE.

[Mike_The_Grad]

Thanks MikeB for adding that important note that I forgot to include about #1 TDC being on compression stroke,versus any other stroke in the cycle.

This is exactly why I love this site,people actually care to see others succeed at what ever it is they are facing with these trucks.

Not to hijack this thread, but the reason I was reading it was because I am facing issues with low vacuum at idle, about 15" Hg. On the gauge, some regular flucations of the needle always within about 2" less than the 15" it "floats" around, and always to the lesser side never more than the 15". I recently rebuilt motor starting with .40 over bore(piston's were already .30 over.), new comp cam XE250H,lifters,pushrods,roller-tip rockers,882 reman. heads,new hi-pressure oil pump, H.E.I. had about 4k miles,new module,rotor,autolite AR132 race plugs,fresh rebuilt by me original quadrajet 4mv,new base&steel gaskets,new vacuum lines (PCV line and valve,Vacuum advance line,th350 modulator rubber ends, and brake booster line with check valve and grommet,new vacuum plugs on unused carb ports, manifold gaskets were just replaced last weekend because front and rear leaks because I didn't put enough silicone the first time.) Manifold is original cast iron with exhaust crossover which is worn away and pitted around the crossover groove.
Trying to pin down low vacuum signal. How would I go about finding out how much vacuum the cam I'm using would generate? I was told this was similar to an "R.V." cam, and relatively mild.

[D.PASSMORE]

GMC Paul website:
Vacuum Gauge Use
Connect vacuum gauge to base of carburetor to full vacuum source at idle. Then monitor your readings and compare readings to chart below.
1-Steady gauge reading from 17-22 indicates normal engine in good condition.

2-A intermittent fluctuation at idle indicates a ignition miss or sticking valves.

3-Needle fluctuates as you increase engine speed indicates ignition miss, blown head gasket, leaking valve or weak valve springs.

4- Steady needle that drops at regular intervals indicates burnt valves or improper valve clearance, (the needle falls when bad valve is in operation).

5-A drifting needle goes high then drops goes high then drops in a steady movement indicates minor intake leak or improper carb adjustment.

6- A low reading but steady from 10-16 indicates late ignition or valve timing, low compression, stuck throttle valves, leaking carburetor or intake gaskets or a big cam.

7- Needle vibrates heavy at idle but becomes steadier as engine speed increases indicates worn valve guides.

8-A gradual drop in reading at idle indicates blocked exhaust system or excessive back pressure in exhaust system.

That's troubleshooting with one. Now for fuel mixture adjustment with one.


Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.
Ok now for monitoring fuel consumption with a vacuum gauge.
Connect the vacuum gauge to full unported vacuum and run vacuum line long enough for gauge to be in cab. Go for a drive monitoring the vacuum gauge. When you have high readings you are getting the most fuel economy possible, when you have low readings fuel consumption to efficient combustion ratio is lowered. By easing into your gas pedal rather than moving it rapidly you will notice vacuum readings stay higher which increases MPG.