Took it for a spin last night and drove it into work this morning....no more sputtering/break up...bizarre. Seems to be running great again!  

3 hours ago, blodi said:

Took it for a spin last night and drove it into work this morning....no more sputtering/break up...bizarre. Seems to be running great again!  

 Possibly a little water in the gas?

How is the fuel filter?

 

 

Just now, Zedyone_kenobi said:

How is the fuel filter?

 

 

Just replaced it last fall. Drove it over lunch, ran great. But, had a hard time starting when we came back out to it after eating. Vapor lock? Had to floor it while cranking for a while for it to finally fire. Usually fires right up. 

22 minutes ago, blodi said:

Just replaced it last fall. Drove it over lunch, ran great. But, had a hard time starting when we came back out to it after eating. Vapor lock? Had to floor it while cranking for a while for it to finally fire. Usually fires right up. 

 That would be my guess. Are you missing any heat shields? What changed during the build to cause vapor lock now and not before? 

15 minutes ago, Mark Maras said:

 That would be my guess. Are you missing any heat shields? What changed during the build to cause vapor lock now and not before? 

Changed to a 6-1 header from the stock exhaust manifold. Heat shield it still in place. 

#1 here could give you some ideas.  I assume that carbs are sensitive to the more modern fuel blends also.  Even back in 1974 Nissan was going as far as cutting holes in the hood (template at the end of the document).

 

Start with the basics . Make sure nothing is loose like grounds , linkage , loose bolts that can cause a vacuum leak, or anything that can cause a vacuum leak.

a loose lead on the dizzy will make you swear it's the carbs. 

Just today I was driving around -pretty hard on her- and it ran great . Then all the sudden it acted lean and was popping a bit at the carbs. Vacuum plug had blew off of my vacuum log. Car still ran okay, but wanted to die at intersections, and didn't feel quite right. Assumed it was fuel pump, stuck float , ect....

Drove it home last night, and this morning and still runs great. Starts right up everytime since that odd time yesterday at lunch. 

As for that 4K breakup that I experienced that one day...my latest theory is that I possibly didn't take the choke fully off. I noticed this morning when I took the choke off there was a it of a hangup in the cable or something and I had to push it a bit to get the lever fully forward. So maybe on that particular day I thought I had it fully off....but didn't. 

The breakup/sputtering at 4K+ came back the other night.  Since this is a high compression build I put in 7 heat range plugs when I installed it. On a total guess I pulled the old 6 heat range plugs from the old engine and installed them last night. Breakup/sputtering gone! Game on! 

So...update and questions...up to 850 miles on the engine and overall it's been running great! 

Question is about carb tuning...and maybe I should start a thread in carb central...but I'll start here...

Running stock round top SU's. Z therapy rebuild kits, ATF in the dashpots,  SM needles. 260Z intake manifold. Electronic fuel pump.  Again...L28, high compression, E31 head, 500/500 cam. Ported, 6-1 header...etc. 

I just got an Innovate LM-2 the other day to see how things were looking in the tune. Overall...idle...for some reason...it's reading in the 30-40's...which makes no sense...but regardless...

At cruise...things look pretty good...between 15-16. Rolling into throttle at 3.5K at about 30-40% throttle...things go lean...like 17's and there is a slight stutter. But if I push throttle beyond that (50-60%) it richens back up nicely and pulls well.  

WOT...things progressively get richer down to 12.5-13.0  and hold steady...until I get to maybe 5K+ and then it just starts progressively leaning back out to 14's by 6K...and that's all the revs I'm putting into the engine so far.  

What are my options for these two issues? Is this engine asking too much of "stock" SU's? to deliver at high RPM? And suggestions for the partial throttle lean out?

I think I may eventually want to go to Tripple Webers (40's or 45's?) and fortunately when I bought this engine it came a label for what jets to use! So that will take a lot of guess work out of them down the road hopefully. 

But I want to get the SU's working well for the time being. 

 

 

I was wondering. Since you have a high compression set up, did you have to set the timing way back

to prevent detonation? I would think that would affect LM-2 readings.

You also need a working distributer vac advance. Not just hooked up, I mean working.