It seems like my L28 engine with SU Carbs wants to run away to high RPM at idle. I'm new to tuning, so I might be missing something obvious.

The timing gun starts out at high hundreds of RPMs, but grows quickly to above 1200 or 1300 RPM, and I get spooked and cut the ignition.

I've been using this as a guide for tuning: https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/

• My fast-idle is backed-off, and it looks like the butterfly-valves are mostly closed.
• Both idle adjustment screws are backed off.
• I followed the guide to lift the mixture screws underneath the carbs fully up and then backed them off two revolutions.
• Choke is off. I noticed the mixture jets underneath the carbs might not be fully returning to the top of their travel, so the springs which pull them up might need to be tightened / replaced.
• I have 3-in-1 oil in the carb damper area.
• I haven't adjusted timing yet, but as far as I know that wouldn't cause this issue.

Any ideas on what I could look at / adjust next? Is the oil that I used maybe too thin? Before I had some ATF in there, and it seemed like the pistons were way too hard to lift, but now I'm wondering if the 3-in-1 is too thin and they're lifting too easily.

 

 

  The engine has to be drawing more air to increase revs. Have you checked for vacuum leaks? The pistons should be difficult to lift, They will rise slowly with constant pressure and drop much faster with an audible tap when they hit bottom. Heavier oil delays the piston rise during acceleration which acts the same as an accelerator  pump. The nozzles must return to the top when the choke knob is in the off position. Misadjusted cables are often the cause but frequently the flat bar linkage that pulls the nozzles down needs a bit of twist to precisely align it's mating surfaces. Lube the pivot points and the exterior of the nozzles with lithium grease.

Do you have springs on the throttle plate return levers?

Those "flats" on the linkage next to the carbs have a certain way they stack together. I rebuilt my engine and on first start up it shot up close to 2,ooo rpms with the throttle rod unhooked. Really scary. After I got those right it was fine. Hope you find the problem.

 

I have it idling pretty well now. See the video linked at the bottom.

42 minutes ago, siteunseen said:

Those "flats" on the linkage next to the carbs have a certain way they stack together. I rebuilt my engine and on first start up it shot up close to 2,ooo rpms with the throttle rod unhooked. Really scary. After I got those right it was fine. Hope you find the problem.

 

I haven't taken the linkage off before, so maybe the previous owner assembled something incorrectly. Thanks, I'll take a look.

15 hours ago, 240260280 said:

Do you have springs on the throttle plate return levers?

I believe so, yes; the springs which return the throttle to closed after opening it?

19 hours ago, Mark Maras said:

  The engine has to be drawing more air to increase revs. Have you checked for vacuum leaks? The pistons should be difficult to lift, They will rise slowly with constant pressure and drop much faster with an audible tap when they hit bottom. Heavier oil delays the piston rise during acceleration which acts the same as an accelerator  pump. The nozzles must return to the top when the choke knob is in the off position. Misadjusted cables are often the cause but frequently the flat bar linkage that pulls the nozzles down needs a bit of twist to precisely align it's mating surfaces. Lube the pivot points and the exterior of the nozzles with lithium grease.

That's what I was sort of thinking. I haven't checked for leaks. If you watch the video below, you can hear a high-pitched sort of buzzing / sucking noise, so I'll have to figure out what's going on there.

One of the problems I discovered was I just didn't have enough oil in the carb dampers. I made sure to fill them almost all the way up, and that made the carb pistons way harder to push up.

I tried lubing the moving parts, which might have helped a bit, but I'll try giving the linkage a bit of a twist.

 

 

Looks like a head gasket leak, huh?

 

 

Are you using florescent blue coolant?  Maybe your camera took some Viagra.

That is a common area for a coolant leak.

 

Edited August 14, 20204 yr by Zed Head
No offense...

Looks like it took ecstasy and went to a rave. 

 

Edited August 14, 20204 yr by siteunseen

This is with UV dye in the coolant under a UV light

I had a similar problem and it was caused but a leaky intake gasket.  Spray some carb cleaner around where the intake meets the head and also where the carb meets the intake.  A change in RPM indicates a leak.  

Those "flats" on the linkage next to the carbs have a certain way they stack together. I rebuilt my engine and on first start up it shot up close to 2,ooo rpms with the throttle rod unhooked. Really scary. After I got those right it was fine. Hope you find the problem.

 

I’m so very pleased to hear a very experienced person also did this!!!! #metoo!!!!