rundwark

Old post but since someone just reached out over PM to ask me for advice about this very swap, I'll post here what I wrote them for others who find this post through search: I ended up installing the Datsun flange on the Subaru diff with an impact, just ratcheted it on. Bad idea. You need to find a better way to do it so you don't end up screwing up the input bearing on your diff. Mine started whining more and more after a while, and while I can't be sure that the way I installed the input flange is the cause, it is the most likely cause. I would take the extra time to do it right next time. How I would do this next time: get a long piece of L-channel steel from the hardware store, and drill two holes in it so you can bolt it to the input flange and hold it in place. With the diff on the ground and the L-channel bar as a means to stop the flange from spinning, use your torque wrench to tighten the nut to the exact torque spec (136 lb/ft). You can probably make this easiest for yourself if you lay the diff on the ground with the front support, and putting the L-channel piece to your right when looking at the diff. Now all you need to do to stop the flange from spinning while tightening the nut is hold the diff on the ground (perhaps get a buddy to hold it down).

Did you ever sort this out Petew? It sounds to me like the bog only happens on rapid throttle changes. If you get on the throttle very slowly and gradually, does AFR stay around 13:1 (with then 57.5 pilot jets) across the range? If so, your jets are fine. Most likely, you’re not getting enough volume in your accelerator circuit. I’m not sure which Mikuni’s have this adjustment but on mine the accelerator pump has three positions for volume. I got rid of the bog on my car by changing to the highest volume setting.

Yeah, thanks for the tip! Seems like a good option too, just a bit overkill for the torque my engine's making (and I'm not so into cutting off the old tranny mounts etc.)

Hi Xnke, do you still offer this conversion? I'm very interested for my Z. (sorry all for resurrecting an ancient thread, tried a PM first)

Yeah, so I'm embarrassed to say I actually have one, just been putting off installing it. I think I learned my lesson the hard way, hahaha.

Haha thanks! That was quite the rite of passage Probably float, will report back when I know for sure. But yeah this method worked very well and results were completely unambiguous. I recommend trying it out sometime. Tried reading plugs without a lot of luck, should've grabbed my colortune from the drawer of forgotten tools. It's usually not been super helpful for me, but I bet it would've caught this issue since the carb was so far off. Oh cool, I'll look that up! Thanks for all the advice so far

Thanks Jasper, will do!

I did have them close at about 2 turns down from where the jet sits flush, so best theory is that Miles' insistence for me to check float levels was spot on.

I did discover something interesting along the way, sharing here in case it's helpful to someone. I've always had a really hard time telling whether my SU's were running lean, rich or just right using the damper piston lift method that's described everywhere. And just listening for differences in the quality of the idle and idle speed from adjusting jets doesn't really give clear results. So I've always struggled to tell whether I'm improving the mixture or making it worse. At one point, while the engine was at operating temp and running poorly, I unplugged the vent tubes for each float bowl from my air cleaner to check if either was overflowing. I then blew into each vent tube, one at a time. Doing this on the rear carb slowed the idle down. But on the front carb, it immediately idled faster. This convinced me the front carb was running lean. This was a very quick test to do, and the results were very unambiguous. I've never read about anyone doing this, but it was a lot more clear to me than any of the usual methods. I still haven't found transparent hose of the right size to measure float levels, but I'll try to still do that before I do my engine (and carburetor) swap and report back here.

So I think I figured out what's wrong, you're all gonna laugh. My front carb was way lean. Adjusted it to run richer, problem gone. This doesn't explain why the issue occurred in the first place, before I took the carbs apart, but the car runs fine now. I found a few other minor issues along the way, I think the one that explains the rough idle was a mild vacuum leak in brake booster hose, but yeah the root cause for this was likely just user error.... smh.

Well, so much for that theory. Ran the car with the fan controller on fail-safe mode (full on), problem was exactly the same.

This is the style of fuel rail I have in my Z, I wonder where the restriction for the return line (to maintain pressure at the carbs) would be.. anyone here know?

This seems relevant too and supports my heat soak idea somewhat: http://community.ratsun.net/topic/68156-rough-idle-rpm-drop-and-stalling-at-intersections/page-1

Thanks Miles, I read that thread again (once before I started mine too and once again immediately after you posted it). I will look for some translucent hose that fits just to eliminate it but am convinced this is not my problem. My car runs fine at operating temperature for a while, and then starts to bog. I don't see how a float level problem could be fine and then change with heat soak. I think I'm dealing with something else. I did recently install a more efficient radiator, and my fans are electric and on a controller. This means they are pushing less air around the engine bay which means more heat soak. I'm going to drive the car until it happens again, pull over to wire the fans to run continuously and see if the problem disappears. One issue that's become more prominent is very low idle (and sometimes stalling) after a highway drive at operating temp. Pulling the choke raises the idle. So it seems I'm running lean, possibly due to vapor lock. I'll also check that the restrictive hole in the fuel rail for the return line isn't plugged.

Here's some pics, thx for taking a gander

I hear you Miles. I didn't check with a clear tube, but when I opened up the float bowls just after running the engine they were both at a good and similar level. Also, the car runs great when it first gets to operating temperature, the issue only happens after I run it under heavy load for a while, like a good 10 miles on the highway at high speed. So the symptoms don't seem to fit with maladjusted floats to me.

Yup, both are one-piece throwaways. It doesn't but I ordered another $20 generic pump with 5/16" outlets on Amazon that gets good reviews, so I can just swap that in to see if it makes a difference.

Here's a video I shot a week or so ago, showing what happens to the front fuel filter when I turn the electric fuel pump on after it's been off for a while. https://youtu.be/q7rO57iCIn0 It doesn't look like the filter is bad, but I have another one on the shelf, would it make sense to replace that as well? I have access to air tools and a lift, so I'm wondering also if it would make sense to disconnect the return and feed lines on both sides (one line at a time, using hose pinchers on the things they connect to) and blow them out with compressed air..

A translucent plastic one in the engine bay and a metal one in the back. I'll try to get pics of those too.

Okay, the adventure continues. Replaced distributor cap, rotor and ignition module to eliminate ignition causes: no change. Opened up floats, checked and cleaned banjo bolt filters. Tiny bit of black crud inside the banjo bolts, nothing on one filter, one dark spot on the other that I was able to scrape off, bowls themselves looked clean and floats move freely, no obvious mechanical resistance on the float hinges or switches. Have yet to test drive after this last attempt but pretty sure there'll be no difference.. My latest hunch is that the return line has a clog. At temperature, the lack of circulation leads to some vaporization in the lines causing the lean condition and hesitation at load at high RPMs. After driving for a while at high speeds and then stopping at a light, the floats become overwhelmed by the high fuel pressure and sudden lack of fuel being sucked out of the bowls, causing shitty low idle and near stalling due to a rich condition. Does this make sense to anyone but me?

Ah, excellent that'll be easy, thanks!

Thanks Jasper, I'll have to just disassemble everything between the fuel rail and the jets next.

That's good info. Also makes me believe that if there's a flow problem it's not near the tank, but it has to be under the hood somewhere. 'cause when the car is hesitating, turning the pump on or off makes zero difference to how it runs but I can still see a difference in how full the fuel filter is.

Yeah, it is working. I have it on a switch in the cabin and the level in the under-hood fuel filter clearly changes based on whether it's on or off. I often drive the car with it off (just use it to prime the system) and that's always been fine too, so maybe it's obstructed..

Where? XDI direct fire ignition? Don't tempt me