Vacuum

Typrus · Post by **Typrus** » Thu Feb 08, 2007 10:20 pm

Well, brought my baby into class today to do some compression and vacuum demos. Easy to get to everything, so why not?
Compression- First puff's- 47,48,50,50 Finals-125,126,126,127
Good enough considering the whole 210,000-ish on the engine thing.... lol.

Vacuum- 11 inches Hg... What the....? Well, I went over things. No hisses I could hear, nothing of the likes. I went to ALLData- 17.78inches Hg at sea level recommended. That means about 12-13inches here, in theory. I wasn't happy. My instructor (the Ex-Toyota Tech) came over and commentedon carburetor dinosaurs briefly. I then pointed to the guage. He looked at it for a sec, then looked at me, "She's running lean." I thought that was kind of a fast declaration... I killed it, pulled the cleaner housing, plugged the big hose to the cleaner, then called him over again. We fired it up, and he looked and listened again. We pulled the vacuum hoses off of the manifold VSV, plugged the nipples, and checked the guage. 13 inches. Sweet. "Its still running lean. See it fluctuate and smell the exhaust?" He pulled out his pocket screw-driver, backed out the mixture screw (out=rich, in=lean) a bit. 14 inches. A bit more. 15.5 inches. It settled here. The exhaust stopped smelling kind of rancid, the engine smoothed up, and the exhaust cut its "puffing" freqency to about a quarter of the starting. I thanked him profusely, and went on to examine some other things.
I eventually isolated the distributor by plugging in 2 of its lines... The one with the check valve to the manifold VSV, and the lone one that normally goes to the HACV, but I re-routed to ported vacuum. Ported vacuum being a vacuum tap ABOVE the throttle plates. Theres a source on the head-side of the carb BTW.

Words on it? Its running great. A bit better than before. I say a bit because its power band has shifted. It doesn't like the higher revs as much. Lower down, I'd say I picked up some torque. Engine-braking? Much improved. Brake pedal also stiffened up. More vacuum on the booster.
We'll see where mileage goes.

So... I had a leak of vacuum that I had no idea about. Roughly 2 inches off of 13. Thats a lot of vacuum. Not only that, but I was running so lean that I was low 2.5 Inches ON TOP of the 2 low from leakage.

I am now 2.5-3.5 Inches ABOVE the theoretical for my altitude. Hows that speak for my engine? And yet I run it so hard... lol.

This also speaks about lumpy cams. All of my instructors said that the lumpiest cam they've seen (that is street-legal) has made the engine in question, just from pre-to-post cam, go from 14 to 11 inches at this altitude. I think they each were saying they were something like 330 degree cams. Holy crap-duration. Thats a massively lumpy idle right there.
So, if I was driving around with 11 Inches of vacuum without actually noticing I was having a problem (now that I have a "proper" baseline, I'll notice in the future...) I don't think that even the 300-300/256-256 (gross int-exh/@.050 int-exh) cam will create a problem.

Also.... On WOT snap-testing, she went to +3 inches (pressure!) then back to 22 Inches (vacuum) on release. Healthy. Awesome that it makes actual pressure on off-idle to WOT snap-acceleration.

Now figure this out- at a steady 2500RPM test, she sat at 17 Inches Hg vacuum.... 2 more than idle... Kind of counter-intuitive, eh?

takza · Post by **takza** » Fri Feb 09, 2007 6:12 am

Next...try setting the ignit advance using the vac level? You want to just sneak up on the highest vac you can manage at idle, but listen for pinging when driving it.

I set mine at around 20" hg at idle.

Mac · Post by **Mac** » Fri Feb 09, 2007 6:22 pm

is it the one tucked behind the EGR valve, or is this another screw?
i ajusted the hell out of that screw and it made no differance on my vacuum gauge.

currently i only have 10hg in, and i'm way closer (100M above or so) to sea level than you!

Typrus · Post by **Typrus** » Fri Feb 09, 2007 7:45 pm

Its behind a plug typically, unless the carb has been rebuilt.

10Hg? I'd vote either huge vacuum leak, bad ignition and/or cam timing, bad rings, bad valves/valve seal.
Thats pretty dang low.

Mac · Post by **Mac** » Sat Feb 10, 2007 2:32 pm

i replaced all the vacuum lines, and i don't have much reason to suspect the engine is bad.

I think the screw has just been backed out way too much, infact i wen't to my old carb and decided i would back it out 2.5 turns like it said to do in the FSM, so i wen't to screw the thing in and it was backed out about 5-6 turns! considering both these engines run about the same (stumbling) and the caps were removed on both carbs, i'm willing to bet some clown face was screwing around (no pun intended) with something they didn't know about.

Typrus · Post by **Typrus** » Sat Feb 10, 2007 2:48 pm

Well, try a compression check to see if thats suspect.

Get a vacuum/fuel pressure tester. I believe NAPA sells them for about $25. Worth it in my opinion.

Let the vacuum and the exhaust odor be your determiner.

Also... Can you be sure that one of the vacuum accessories isn't leaking? One of the umptillion actuators could have a cracked diaphragm causing some leakage. Just a thought.

Also.... Check manifold bolt tightness. Don't overdo it though, for obvious reasons.
Check the VSV and booster port tightness. The VSV is in the manifold and has 2 vacuum ports coming off of it. One goes to the TVSV and one to the distributor. The booster port is fairly obvious, just follow the vacuum hose off the brake booster. My booster port thingy was about 2 turns loose. Noticed it because of some fuel leakage evidence around it. I might take it out and plumbers-tape it. Check to see the carb is tight on all 4 bolts.

So you were running ridiculously rich. Too rich or too lean will stumble you. And both smell bad. Lean smells really rancid... Especially when your cat has collapsed.

Mac · Post by **Mac** » Sat Feb 10, 2007 5:13 pm

i checked it out today, I swapped my old carb on, the one that came with the 4a-C liked to dump fuel rather than atomize it (stickey float needle)

i checked all the vacuum lines, and it turns out the advance/retard diaphram is leaking and i lose about 5-7 hg because of it, but i would still be a little shy of 17.78 that the factory recommends.

funny though, when i plugged the lines to the dizzy, it still stumbled and ran pretty craptacular.

takza · Post by **takza** » Sun Feb 11, 2007 3:27 am

Typrus wrote:Get a vacuum/fuel pressure tester. I believe NAPA sells them for about $25. Worth it in my opinion.

Let the vacuum and the exhaust odor be your determiner.

Think Harbor Freight for less than 1/2 that price?

Typrus · Post by **Typrus** » Sun Feb 11, 2007 11:36 am

Well, with no advance or retard, I'm sure that it wasn't happy with the timing. Without advance or retard it will only run well in a small window of RPMs.
Do you have any spare distributors lying around you could try?

Mac · Post by **Mac** » Sun Feb 11, 2007 8:43 pm

yeah, and i tried them, they are bad too.

I checked how much of a differance disconnecting the advance/retard lines made by using my timing light. to my supprise, timing retarded the same, only slower, so i'm beginning to think the diaphram only makes the ignition change the timing quicker.

i'm thinking, which one is really worse? a vacuum leak that makes the engine run super lean, or slow reaction ignition timing? if disconnecting the dizzy makes it run smoother i might just do that.

Typrus · Post by **Typrus** » Mon Feb 12, 2007 3:27 pm

There is a mechanical advance inside our dist's. Its just aided by a vacuum advance and retard.

keith · Post by **keith** » Mon Feb 12, 2007 6:36 pm

With the vacuum line removed (I disconnect mine at the filter instead of the distributor, much easier), the timing should be 5 BTDC at idle. With the vac advance connected, it should be 13 BTDC at idle.

Post by **ARCHINSTL** » Mon Feb 12, 2007 10:47 pm

Thanks for the tip for a disconnect at the filter. You are correct.
Wonder why the FSM recommends doing it at the distributor?
Tom M.