Low Vaccuum on 350sbc???

Cmal7 · May 8, 2017

Hey, I have a 86k1500 with a 350 with emissions removed. I can't get it to run properly not matter what I try. I timed it to 7* btdc with the vac. Adv. plugged it idles rough at7-800rpm and pulls about 7" of Vaccuum And makes a terrible ticking noise. If I advance the timing while running it will idle up to 1200 rpm and seems to be a little smoother ticking goes away but this is about 30* advanced still only pulling about 7" of Vaccuum. I checked slack in timing chain by rotating crank clockwise by hand and then reversing from tdc and noting where the rotor starts to move its about 7-8* of slack is this too much? I checked for vac leaks and have all ports plugged that aren't in use. Checked compression and have about 145 on all 8. Is there a way to check cam timing withoutremoving timing chain cover?

ifixedit · May 9, 2017

http://www.freeasestudyguides.com/engine-vacuum-test.html

I would lean towards it's a late valve timing issue, the timing chain has likely jumped a tooth / and or it's a combination of that and a badly stretched timing chain. The ticking is probably being caused by a bent pushrod which can happen when the timing chain jumps. 145 psi for compression is pretty low and could also be a result of the timing chain jumping a tooth as the cam / valvetrain is now out of sync with the crank / pistons as the intake valves are now closing too late on the compression stroke which doesn't allow the compression to fully build up, your cranking compression should be around 160 + for a normal / healthy engine. There should also not be 7+ degrees of slack before the rotor on the distributor starts to move.

Get the timing cover off to inspect, also pull the valve covers off and check for a bent pushrod or two.

Cmal7 · May 10, 2017

Thanks for the reply.. I went ahead and took off the timing cover and found something I did not expect. I found a double roller chain and gears with a insignificant amount of slack. With #1 @ tdc the crank gear mark was @12 o'clock with the key way at 2 o'clock but the cam gear was also at 12 o'clock. From what I understand the cam gear mark should be at 6 o'clock. If my thought is right how the hell did the engine ever run the way it was???

ifixedit · May 10, 2017

I like to have the #6 cylinder at top dead center to verify things correctly. With #6 @ TDC the crank sprocket dot will be (or should be) at the 12 o'clock position and the cam sprocket dot should be at the 6 o'clock position, thus aligning with each other. Any deviation from this and you've got trouble.

Here's another post from here where I posted some info on this, scroll down to my reply and check out the links I provided >> http://www.gm-trucks.com/forums/topic/190936-i-am-confused/

Cmal7 · May 10, 2017

Thanks for the link. If I put #1 at tdc on the compression stroke #6 is also on tdc correct? When 1 is on compression stroke at tdc cam mark is 12 o'clock and so is the crank when I rotate the crank gear 360 it comes back to 12 o'clock with the cam mark at 6 o'clock and the rotor follows as it should. As far as the slack in the timing chain if I rotate the crank forward and backwards I can get the mark about half a tooth out at the most.could it be valves? Could worn cam lobes cause the lack of power and virtually no Vaccuum?

Cmal7 · May 11, 2017

I should also mention while running I gave it throttle it would rev up and not hesitate while in park or neutral with no load on the motor. And vacuum would bounce up to 20 at wot. Then back down to almost nothing as it came to idle. With load on the motor while in gear there was a huge hesitation bog down then slowly accelerate once it got to about 3000 rpm it seemed to make power.

ifixedit · May 11, 2017

Vacuum will go up to a certain RPM from idle if you raise the idle slowly, but at WOT the vacuum should be zero. Maybe a couple of questions here... How long have you had this truck? Have you done any of the engine work to it or is it as received? And, how long have you been having this problem? As far as the #6 cylinder thing goes, as long as the crank is rotated so the crank gear dot is at 12 o'clock and you turn the cam (without the chain on) and position the cam gear to 6 o'clock you can't goof things up, cam orientation will then be correct. Now, on to more possible issues.... Yes, you could have a cam that is completely worn out or has some badly worn lobes which would also explain the excessive ticking you hear. If this engine has been worked on in the past, rebuilt, freshened up, etc., etc., whoever installed the (new?) cam may have goofed badly and neglected to properly break the cam in and / or did not use oil with the proper amount of zinc in order to properly break the cam in and protect the it from excessive / premature wear. If your truck is an 86 and the engine is the original then that would indicate it has a flat tappet hydraulic (non roller) cam. In this day and age most oil companies have removed a significant amount of zinc from their oils and thus if this low zinc oil is used flat tappet cams get destroyed in short order. One oil I am aware of (and I use in my 1977 Camaro) is Valvoline ZR1 racing oil, it is available in both conventional and synthetic blends and contains the high amount of zinc needed for both proper break in and protection of the cam from excessive wear and should be used not only on break in but at each oil change as well. For break in I would not use the synthetic version, start with the conventional, then once the engine is broke in you can switch to the synthetic version if you like. You can of course also purchase a separate zinc additive that can be added to any regular low zinc oil as well, the stuff is available at most any auto parts store and folks like Lucas as an example make it.

Unfortunately, the only way at this point you are going to know for sure if the cam is 'out to lunch' would be to remove the intake, pull the lifters and inspect them for wear, and if you've gone that far you might as well yank the cam for a look at it as well. Until you do that it's going to be a guessing game.

Cmal7 · May 11, 2017

Thanks solid info..I bought the truck a couple years ago it has been acting this way since I got it. The motor is original I looked up the numbers. I haven't done any engine work at all. Judging by the gaskets on the timing chain cover, intake and valve covers also the blue paint on the block I doubt this motor has been touched in a long time if ever. Unless of course someone put the double roller timing chain in it not sure if it coulda came from factory that way because it's a " heavy half" or not. Like I said before there is some slack in the timing chain not sure it's enough to throw cam timing out. Would it tell me anything If I was to advance the cam gear by one tooth and turn it over check compression to see if it goes up at All? I really don't want to sink too much money into it thanks.

ifixedit · May 12, 2017

I wouldn't mess around advancing the cam gear, the main thing is to check to be sure when the dots are supposed to be lined up at the 6 and 12 positions as outlined above. If the dots don't line up as they should in the current state of things, then yes, go ahead and make the change so they do. I too would doubt it came from the factory with a roller timing chain, so it appears someone has been in there at some point in the past. Another thing is Chevy was known to use the cheap plastic tooth gears for the cam sprocket. Does it by chance have one of those? If so that could be the cause of the timing issue. A couple things that keep coming up for me is you report excessive ticking which could be collapsed lifters, worn cam lobes and the lifters can not sufficiently pump up to take up the additional slack, you've got lower than normal cranking compression which could indicate late valve timing or simply just a worn out motor with poor ring seal. Another possibility is you could have burned valves causing the poor compression, but if the compression is the same on all cylinders then I'd probably rule out burned valves. One thing you could do is try squirting some oil into a spark plug hole or two, then do a cranking compression test. If the compression goes up you've got bad ring seal and a worn out engine, if the cranking compression doesn't change it's then something to do with valves or valve timing.

I'm thinking there's a good chance the previous owner changed the timing set and perhaps decided to install a new cam at the same time and just didn't break it in properly, then realized that after it was not running well decided to put it up for sale?

In any case I'm still thinking you're going to have to tear into it further, I'd start with removing the intake so you can get at the lifters to fully determine what's going on.

Cmal7 · May 12, 2017

Ok. The dots definitely lign up as they are supposed to. The cam gear is at 6 when the crank is at 12. When firing on number 6 .The cam gear is not plastic. The motor is a d3p suffix number which is the same motor they used in 3/4 and 1 ton trucks which also is identified in the vin number. So I assume the motor original and thought it may have the double chain. The ticking only happens when I retard ignition timing to like0 degrees. It almost sounds like an exhaust leak or high pressure air leak of some kind. I am going to try putting a shot of oil in the cylinder and see if compression improves. thanks again for the info.

Cmal7 · May 13, 2017

So I tried a shot of oil in the cylinder ran a compression test and plugged a vac gauge to the intake 0 vac little to no difference on compression. For the hell of it I advanced my cam gear one tooth which is 88 teeth = 4 degrees not sure if that's a big deal or not I ran another compression test and got another 15+ psi out of each cylinder and registered some Vaccuum while cranking the engine. I said before there is some play in the timing chain but I didn't think it was enough to make a difference maybe I'm wrong. Worth changing it to see? Thanks for the patients!!

ifixedit · May 15, 2017

Actually, in a lot of cases the camshaft maker will advise when installing the cam that you advance it 4 degrees which is more commonly done with a special crankshaft sprocket that is marked. But in your case in doing things the way you did this perhaps has now given you the correct cam placement. Your compression is now up to where it should be as well. Just rotate the engine by hand to make sure you don't have any contact between pistons and the valves (should not be an issue), then fire it up and see how it runs!

Cmal7 · May 15, 2017

well I think that's where we're at here... Is there anyway to check and see if the cam has been changed without taking the intake off? Should I try one more tooth advanced and see what happens before putting it all back together? Is there a chance a valve would kiss the piston at that position? Thanks again

ifixedit · June 6, 2017

Sorry, haven't visited this forum in awhile. No, there's really no way to tell if the cam is aftermarket unless there is a name or something possibly stamped on the end of the cam under the sprocket which is possible. I think companies like Comp and others will put their script on the end of the cam. I would not mess around advancing any further than one tooth, and really, advancing the cam that way is not a good way to do it in the first place. If you can rotate the engine with it advanced one tooth you should be fine. More than one tooth? Well, then you might be asking for trouble. The only way to verify 100% if the cam is installed correctly is to use a cam degree wheel. You can buy a cam degree wheel kit from places like Summit and it may be worth the investment at this point vs. pistons slamming into valves and / or if you ever want the truck to run correctly. If degreeing the cam over your head, a good mechanic or engine builder could verify cam installation for you.