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Thinking of doing the unthinkable... *Rotor to cap gap. Normal?*

The vacuum at a hot idle on mine is i think around 15ish or so if i remember right. In this video it shows about the best vacuum i can pull out of it. Its a cold start so high idle and all that still never really hits the normal range.


On my buddys stock tbi 350 his was in the green at the worst times. Like in gear at a hot idle his is still in the green. When we put a vacuum pump on my map and brought it up to where his was, thats when mine runs perfect.

In this video you can see that its slightly higher and im really not sure why other then that my temp sensor took a poop right before this video and it was running in "error mode". Also the knock is gone. It was loose converter bolts.

 
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I had a tuning business in the '80's (in Portland), and deal with tuning issues somewhat today.

The facts I know are that low cylinder pressure means low vacuum. Now, what causes the low cylinder pressure? Bad rings, or a cam with too much overlap (cam timing).

So, I don't know for sure whats up with your engine, but i'm guessing it's a cam related issue. Advancing the cam may resolve your low cylinder / low vacuum problem.

It would be cool to see 19- 20" of vacuum on your rig.
 
Thanks for the help! I do appreciate it! I dont think its bad rings(although it could be) becasue this motor has ran like this sense the day i put it in about 10k miles ago. Also, the compression now is the same as the compression then. The motor was a fresh rebuild and done all stock with the exception of the cam. Any idea how many degrees advancing the cam one tooth on the sprocket would give you? Folkenheath and i had pretty much come to the conclusion that it was a bad idea, but maybe you have something to add here?
 
Just took this off another site after some searching on google.

Say your cam gear has 44 teeth, and your crank gear has 22 teeth. If you move the cam one tooth from straight up, you have adv./ret. the timing 4.09 degrees. Now, if you move the crank gear one tooth from straight up, you have adv./ret. the timing 8.18 degrees, a big enough change to make the motor run like "crap". Best just to install straight up with a stock engine and valve train, and make any adjustments with the distributor setting.
Click to expand...
 
Well, you could go just tooth as you mentioned. But I never wanted to pull it apart again, so I used timing gears that made it retard proof, for me. :-)
 
Retard proof! Ha! Quite the pun! Any how, It looks like it basically depends on how many teeth you have on your gears as to whether or not you can get away with moving a whole tooth. Looks like you gotta have 44 or more. And im not too sure how many i have but looking back at pics of the timing chain i just put in my blazer, it had 36 teeth which if my calculations are correct would give me 10* per tooth. Much too much im afraid. If i remember right, the crank gear on this truck did have the three keyways. Who knows. I should have it pulled apart by the weekend. I think(if you'll notice, plan A changes about every 5 minutes...)what im going to try is cranking it over now and reading the vacuum and compression, then advancing the cam timing and re-checking to see if i gained any compression or vacuum without even bolting the timing cover back on. Just cranking, not starting. And i dont seem to have gained anything, i will just continue on with the tear down and cam swap plan!
 
i had a motor 1 time couldnt get timmed to spec. had to set base time around 35-36*.

come to find out had wrong balencer/timing tabs on it. mix matched and not correct as gm used few diffrent ones over the years.

also balencers have been known to spin off mark on the rubber dampaner ring.
 
I dont THINK thats my problem here as i can advance the timing so far that it starts to run worse with no improvement in vacuum.
 
It sounds like you've got a plan, hopefully it will all work out well. In the last 20 years or so, many quality timing chain sets have came with the ability to advance or retard the cam, so yours may be that way.

My opinion only- I wouldn't worry about checking vacuum until it's running, the reading won't be very accurate without the motor actually running.

The cylinder pressure test will tell you instantly whatever the gain was. If you can get the motor to 160-170#'s, it will run smoooth and have great "in-gear" idle vacuum.

Best of luck.


*** Edit *** with these low compression motors, the best we can hope for is 160#'s of cylinder pressure.
 
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I haven't read the entire thread but what are your cam specs @ .050 ?

Low vacum on big cams can be tuned in the chip.

Does it run better at high RPM? Good pull at WOT?

I would definatly get a degree wheel and check the cam. I have seen new cams out 4 degrees. You can fix that with offset keys, or timing gears made with offset holes, lots of options.
 
Basic Operating RPM Range: Idle-5,500 RPM
Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 262
Advertised Exhaust Duration: 272
Advertised Duration: 262 int./272 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.420 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.442 in.
Valve Lift with Factory Rocker Arm Ratio: 0.420 int./0.442 exh. lift
Lobe Separation (degrees): 112
 
That cam, degreed properly can be tuned in the chip! :waytogo:

It will never idle good on a stock chip!

from my experience your idle vacuum is around 14 to 15 depending on your elevation.

HTH!
 
That must be a Comp Cams eXtreme energy 262? If so, thats why your truck won't run. EFI does NOT like that cam, I tried it with a TPI setup and never did get it running.

You ideally want an LSA of 114 or higher to get a strong vacuum signal for the EFI setup.
 
Sorry, your wrong! It can be tuned to idle and run great! I can and have done it.

I would not do it by mail but Dave W in the injection section may take it on!

EDIT: **** your in Portland and he is not far from you!
 
Well, I'm not saying that it can't be tuned for!

But bone stock, no way that an EFI setup is going to run it even reasonably well. My engine ran, but it had no power, and stumbled / behaved really poorly in the low end of things. But, again, my TPI setup was bone stock. Once I got it revved up a bit then it started to run a bit better.

If I recall correctly, you pretty much have to set the fuel and spark tables up really well at idle and slightly off idle then have the ECM ignore the oxygen sensor and MAP sensor during those periods. There is just too much fresh air skipping from the intake valves to the exhaust valves due to so much valve overlap with the tight lobe separation angles. Messes with the O2 sensor really bad by making the exhaust seem very lean when it is just the opposite. The low vacuum pressure doesn't help either.
 
Yep, i know that i could have the chip tuned to accept the cam, but even threw dave it was going to run a couple hundred bucks. THe junkyard cam swap should come in right around 40.
 
I got a hook up at the junk yard so the parts will be free! So $25 for the gaskets(should only need intake set and timing cover set) and $20 for oil and filter. I got plenty of time. I dont drive the truck so for it to be down for a couple weeks while i work on it is no biggie. If i had more money then time i would be all over Dave for a chip! This way i wouldn't have to do any real physical labor! But for whatever reason i just cant get myself to spend the extra $150 or so to save myself the trouble...
 
That is a bummer! So much work to get where you are and so close to having a sweet running engine...
 
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