Jump to content
Sign in to follow this  
blu260z

Low Vacuum Has Got Me Stumped!

Recommended Posts

Hey guys.

 

For a wile now I have been battling with low vacuum at aprox 5 inch of vacuum. The vacuum wasn't checked before the engine was stripped down and the top end was freshened up so I am unsure if this is a new or old issue.

The cylinder head is an E88 and was checked over and was skimmed to make sure it was true, the valves were lapped and the ports were smoothed out but nothing to serious. It had an aprox 74 degree cam installed from Camtec camshafts and have included a picture of the specs sheet. The bottom end wasn't touched in the rebuild process.

On its first startup it bent a valve due to me not setting timing correctly, the bent valve was replaced and the head and engine checked for damage. Other than a little dent in the piston that was all the damage that I could see, since then the engine has been running and the timing issue was fixed.

  • I have checked the compression and with the throttle CLOSED and received 140 / 145psi across all cylinders, I also did the oil check to check the rings and only noted a 5psi change on 1 cylinder. I know I should have done my compression check with the throttle open but completely forgot but to my understanding it will only increase the cylinder pressure.
  • I have checked the valve clearances and they all check out.
  • I have put all new gaskets on intake and carby's and checked all the bolts are tight, I have also checked over with aerostart and didn't find anything.
  • The engine has no aircon or aircon gear all vacuum tank removed and aircon controls into the cab have been unplugged.
  • I found the vacuum advance to be leaking so that was unplugged and just mechanical advance is being used.
  • I have disconnected the charcoal canister and plugged the vac lines incase the diaphragm was split.
  • I checked the brake booster check valve and its direction and that was all ok.
  • All of the vacuum hoses and booster hoses are new but I checked them anyway
  • All unused manifold ports have been plugged
Have I missed something important or just stupidly obvious? I have searched high and low but cant seem to find anything else. Could I have cracked a valve guide when the valve got bent and that is causing me troubles

Or could this be just the camshaft stealing all the vacuum? I searched and found an article on a zed that had low vacuum and a larger cam but the topic never stated if the issue was fixed. However another member said he had aprox 13inches of vacuum which is still low but not to bad for a larger cam. This makes me think 5 inches of vacuum is not right!

 

I have included pics of the cam spec sheet, timing gear, dissy/ ignition timing these pictures were all taken with the engine at TDC

 

hopefully this issue can be something reasonably simple :( help me fellas its almost got me beat.

 

thanks Chris

 

20150818_172220_zps5l1xydi_edit_14398971

 

20161014_152305_zpspckgu2qw.jpg

 

20161014_152321_zpsmcug1umd.jpg

20161014_163947_zpsxy52jiip.jpg

20160708_205248_zpsvltscqc2.jpg

Edited by blu260z

Share this post


Link to post
Share on other sites

You should compression check with the throttle open (and if you really want to be pedantic, all plugs out)
BUT, so long as you have consistent test conditions over all the cylinders you'll get valuable info, so your test is good.
ok, so, your measuring the vacuum with a gauge, obviously haha, but what is the needle doing when its running?
Does it sit steady at around 5inHg? or does it fluctuate and flicker over a range?
Does it fluctuate evenly? Or seem to have an intermittent flicker to it?
Or maybe be timed to match a particular cylinder firing?
Your not testing really high above sea level are you? its roughly 1inHg lower per 1000 feet (300 meters) but unless your right up in the hills it shouldnt affect you, and it wouldnt change your readings much anyway.

http://www.gregsengine.com/using-a-vacuum-gauge.html
Some is applicable info for you, but its all good stuff to know.

Edit: as for your cam specs affecting it, I dunno. Id have to have a real good look over the specs and then re-learn how it affects everything, its been a very long time since ive had to think about that sort of thing

 

384576d1332266723-vacuum-gauge-reading-i

Edited by nizm0zed

Share this post


Link to post
Share on other sites

Thanks nizm0zed on the picture you have attached It would be a steady extremely low indicating a manifold warp or gasket issue. I will look a taking it off and check it with a straight edge maybe. Thanks Cozza apart from difficulties starting and rough idle nothing else but I put all these down to vacuum issues.

Share this post


Link to post
Share on other sites

I would guess cam timing still out. I would presume to advanced. is low end response poor?

It stumbles on acceleration. Should I rotate my cam sprocket another number ? Its set on 1 at the moment and the triangle mark is aprox 1mm to the right of the mark. Unless I chuck the old cam back in. ?

Share this post


Link to post
Share on other sites

It stumbles on acceleration. Should I rotate my cam sprocket another number ? Its set on 1 at the moment and the triangle mark is aprox 1mm to the right of the mark. Unless I chuck the old cam back in. ?

That's Quite a bit.. Have you confirmed TDC for a starters?

 

Once you have confirmed TDC, try to avoid using the Harmonic balancer if possible as there not always accurate.

 

you want that little triangle at least making part of it lining up with the groove.

 

If it is that advanced yes you may get a stutter.

 

As chains stretch and heads are machined sometimes you have to try a few different location until it lines up, Particularly with aftermarket cam's

Ideally you should use a degree wheel but is hard to do with the L series.

 

Cheers Doug

Share this post


Link to post
Share on other sites

Thanks Doug I got a better picture of the triangle and groove. To my understanding its ok but I may be wrong ? One thing I have noticed is my distributor/ oil pump shaft may be out a tooth so I included a picture of that aswell.

 

20161017_191412_zpsqjvvwsbv.jpg

 

20161017_191430_zps2kgiwuos.jpg

 

Thanks Chris

Share this post


Link to post
Share on other sites

So after a serious look I decided to turn my dizzy/oilpump shaft back 1 tooth. The way it was setup before it was relying on the distributor adjustment to get the spark correct. However now its correct I have a total of 9 inches of vacuum, so with this still not right I setup a leak test. Chocked the throttle open got my wet and dry vacuum and set it to blow, blocked up all ports and disconected all hoses. Started the vacuum and sprayed soapy water around and found 4 leaks, both ends of the throttle shafts and front and back of the manifold.

 

The manifold will be a fairly easy fix with a bit of manifold sealant but the throttle shafts I have no idea where to start. Surely the throttle shafts cant be air tight considering they are only a shaft/ bush setup with no seal ?

 

Would these small leaks be what is causing my vacuum problems?

I have even considered putting the original cam back in or even a 12v vacuum pump to bump up the vacuum.

Edited by blu260z

Share this post


Link to post
Share on other sites

I have completed a compression test but not a leak down test.

 

I will get onto fixing the intake manifold leak and see what happens to the vacuum. The throttle shafts unfortunately will have to wait, try explaining to the lady of the house that the carbies need to go to America to get some love. Oh and I need $700+ that blank stare that follows says it all..

Share this post


Link to post
Share on other sites

Sounds like you're onto it, but a couple of other leak points could be the brake booster diaphragm, which is usually evident by air hissing around the brake pedal, and if you have an auto transmission with vacuum going to it, it could also be causing a problem.

Share this post


Link to post
Share on other sites

Thanks neRok. The Booster is a r33 gts booster and was tested before purchase and its a manual gearbox so that shouldnt be an issue. For anyone who is following this thread, I made a decision to machine O-ring groves into the shafts. I will then be putting on Viton O-rings hopefully this fixes the issue of throttle shaft leakages. This process is fairly well documented on SU carbs and when I spoke to and old mechanic he said straight away "we use to put O-rings on them".

The O-rings are a Viton 9mm × 1.5mm

The shaft size is 9.9mm and bush size is 9.9mm

O-ring gooves are 7mm × 1.6mm

 

This should allow for a 10mm >10.1mm O-ring swell so should seal it up nicely.

 

Hopefully it all goes ok

Chris

20161106_184129_zpsosmv4whk.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×