high idle speed issue

[Jeff Berk]

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?

[Jeff Berk]

Problem resolved. I had a stuck butterfly valve in the front carb.

[Mark Maras]

What was the cause of the sticking butterfly?

[Jeff Berk]

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. 

[jonathanrussell]

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?

[AK260]

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. [emoji106]

[Zed Head]

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.

[jonathanrussell]

ZH...so you have seen this kind of wear where the lobe has walls carved in it? Never seen that but....

[Zed Head]

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 - 

 

[Patcon]

What would concern me most is where did all the metal end up.

[Zed Head]

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.

[Dave WM]

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.