Jump to content
Jeff Berk

high idle speed issue

Recommended Posts

I’m having a high idle speed issue. I’m running between 1,600 and 2,100 rpm.

Prior to this, my 74z with a modified 280z engine running a pair of SU’s was having starting problems and would sometimes stall and idle somewhat slow. I decided to try and adjust the carb’s using https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/

Because one of the fist things to do was to check the spark plug condition and gap,  I installed a new set of plugs. I’ve never adjusted the timing because I cannot find the timing mark (I have a gun and I know how to use it). I set the idle screws all the way out and then backed them in 1 ½ turns.  Then it started right up with minimal cranking, but the rpm is too high.

To get a balanced air flow from the pair of SU’s, I had to turn in the rear and back out the front SU’s idle screws. Doing so maintained the high rpm.

There was a small vacuum leak from one of the ports on the vacuum balance tube that I stopped with some Teflon tape on the M10-1.0 bolt the PO installed. I couldn’t find any other leaks between the carburetors and engine/intake manifold. The fast idle adjust screw is out all the way. The vacuum activated dashpot between the carb’s not pulling the throttle open and the choke cables are disconnected.

Could I be missing a vacuum leak or could the timing be too advanced? If so, how do I go about re-making TDC on the harmonic balance?

  • Like 1

Share this post


Link to post
Share on other sites

I must of not aligned it properly when I rebuilt it. For some reason it was working fine for several months. The local SU whisperer knew what was wrong within a few minutes. Unfortunately, he also discovered that the exhaust cam on cylinder 3 was overly worn and I'm going to need to replace the camshaft. I  didn't like that overly aggressive  camshaft so now I have an excuse to replace it. 67299620_479331499571432_295151752456437

  • Like 1
  • Sad 1

Share this post


Link to post
Share on other sites

OMG..that camshaft lobe is unbelievably messed up. Never seen anything like that. What could have caused? Zero valve lash clearance? Are the other lobes normal?

  • Like 1

Share this post


Link to post
Share on other sites

GULP!!! Something seriously wrong there to have done that to your cam lobe!!! Is there a valve sticking too!?!?!!!

Do we know what’s caused it?

Glad you got the SUs sorted though.

Share this post


Link to post
Share on other sites

It looks like just wear.  Once you break through the surface the lobe can wear quickly.  The base circle looks intact, it's on the rocker pad in the picture, which indicates proper valve lash.

Fairly common, especially during break-in of a new camshaft.  Some of the members here have burned through some cam lobes.  @madkaw I think, for one, if I remember right.

  • Like 1

Share this post


Link to post
Share on other sites

People usually catch it sooner but, yes, that's the typical pattern.  The pressure is on the lobe.  A picture of the base circle might show a nice untouched surface.

Here's an old thread - 

 

Share this post


Link to post
Share on other sites

The fragments would get splattered up to the valve cover and around up top but once they get to the pan they have to go back through the filter before reaching a bearing.  Might be interesting to split the filter open.

  • Like 1

Share this post


Link to post
Share on other sites

I run high zinc oil, have read that is important with flat tappet valve train. Modern engines use roller bearings. I wonder if the camshaft can be saved? weld on some new metal and re grind.

Share this post


Link to post
Share on other sites
31 minutes ago, Zed Head said:

The fragments would get splattered up to the valve cover and around up top but once they get to the pan they have to go back through the filter before reaching a bearing.  Might be interesting to split the filter open.

unless the filter was bypassing due to loss of filtration capacity.

28 minutes ago, Dave WM said:

I run high zinc oil, have read that is important with flat tappet valve train. Modern engines use roller bearings. I wonder if the camshaft can be saved? weld on some new metal and re grind.

You might could build it back up but it would have to be hardened again and would be cost prohibitive.

Share this post


Link to post
Share on other sites

The SU whisperer pointed to several potential contributing factors. The performance cam was milled from a CWC core. Apparently, CWC has a bad reputation. That engine should of had an internal oiler, but the cam shaft did not have provisions for that so an external oiling tube was installed. Since exhaust cams for 1, 2, and 3 cylinders were worn out, perhaps, the front half of the oiler tube became clogged.  

 

  • Like 2

Share this post


Link to post
Share on other sites

This clearly left an impression on me - I was dreaming about it for most of the night!! ;)

Glad you have some ideas about root causes.now comes the fun bit, going shopping, improving, enhancing etc

Share this post


Link to post
Share on other sites

I just got word from the shop:

"I’ve been talking to my camshaft contact. He grinds a slightly less aggressive cam for your car. The cam itself will cost around $450. We will also need new rockers and lash pads. They cost between $400 and $500 depending on what thickness pads are required. I’m looking at around 10 hours labor to remove and replace, and setup the geometry of the new cam. I will also change the oil & filter when I’m finished. So your looking at $1800 to $2000 if no problems show up after the cam is removed."

I ask that he try to avoid having to do a valve job if at all possible since I'd be looking at an additional $500 or more.

  • Like 1

Share this post


Link to post
Share on other sites

I sent Schneider Cams an original Nissan cam that has "JAPAN" stamped on it after reading about the failures of those CWC cams they will send you if you let them.

You might look at Schneider or isky's web site.  They have a few different profiles for our cars at a lot better price, send your rockers to Delta Cams for resurfacing.  Cost $50, $3 per - $36 plus $14 shipping.

DSC00958.JPG

 

  • Like 1

Share this post


Link to post
Share on other sites

Well I'm back to where I was last year. I got this message from my mechanic when I brought the car back in to have the valves adjusted:

Jeff, For some reason your cam/head has an oiling problem. The same cam lobes that were destroyed on the first cam are bad on the new cam. I have a new cam and rocker arm set coming next week. This cam uses a different oiling system than the original cam.

Followed by:

The new cam will be internally oiled. You can use both the spray bar and internal oiling, the oil pump does not have the volume to do both. I think we should also replace the oil pump with a turboZ oil pump. I have not got the complete cost of the replacement parts yet. I will keep you posted.

Any opinions on this? The mechanic specializes in Japan and British car restoration and races a 510 so he seems to know what he is doing. 

Share this post


Link to post
Share on other sites

 "For some reason" isn't good enough for me. I'd want to know what specifically went wrong. Was the oil flow satisfactory when the cam was installed? I doubt it. The external oiler system works quite well if the tubes have not come loose from the mounting blocks or the oil holes haven't become restricted due to build-up of ? Tubes can be resoldered and the holes cleaned with a torch tip files.

  • Like 1

Share this post


Link to post
Share on other sites

@Jeff G 78

I think he runs both on his race car? I have not built mine yet but I will use both on my e31 head with a Schneider cam kit. I have turbo pumps on both my Zs and they flood the valvetrain when I spun the pump with a drill before 1st start up. I read a lot of post with people using both with the higher volume pump. Actuall my 9/76 has both on the early n47 head from Nissan. That came looked great in 2011 when I bought the car. That's from '76 to 2011 without the turbo pump. At idle it barely moved the needle on the gauge. With a turbo Melling pump it sits halfway then I installed a new head with just internal oiling cam. Now it's 3/4 gauge at idle. My spray bar 240 with the turbo pump pegs the needle at idle. 

the melling 152 is the turbo pump. Cost me $60 8 years ago from Advance Auto.

 

 

  • Like 1

Share this post


Link to post
Share on other sites

Sometimes they flatten a lobe even if everything is done right.  @madkaw knows, if I remember right.

Who is the camshaft shop that he uses?  

Share this post


Link to post
Share on other sites

Would using the turbo Z pump be strong enough to pump oil through both the internal oiler and spray arms? Is there a way to check how well it's working?

Share this post


Link to post
Share on other sites
40 minutes ago, Jeff Berk said:

Would using the turbo Z pump be strong enough to pump oil through both the internal oiler and spray arms? Is there a way to check how well it's working?

I would NOT recommend running both . Just stick with a internally oiled cam . It’s hard for the system to put out enough volume for both . I think the cam spray bar is ineffective when using both . The oil barely reaches the lash pads - at least the tests I did. Maybe with my Kameari pump it might work .

Simple test is to drop out your drive spindle on the dizzy and drop in a drill with something to drive the pump . Get your mechanical oil pressure gauge hooked up where you can see it . Fire up your drill and watch your pressure and your oil flow . I made a valve cover with a cut off top to watch without oil getting everywhere .

Ive done this test using the modified valve cover while the motor was running . 

I wouldn’t , and haven’t started a motor without performing this test . I’ve have never lost a cam yet . I’ve had a Schneider , ISKY , and now a Bonk cam . 
And I use Rotella truck oil - nothing fancy

 

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

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.