Fuel return line help.

[Just Joel]

Alright ratsun, I need your help... again :)

 

I’ve been tuning and working on a set of sss 38mm su's on my almost stock l20b for quite a while now and fixing small issues as I go along. A while ago, I had an over pressurized fuel system (factory mechanical pump but brand new and working WELL) that affected my float level and would subsequently cause the float bowls too over flow occasionally. After reading a lot and talking to the guys over a ztherapy, I fixed my problem by adding a fuel return line to my system. To do this I repurposed my vapor return line from the tank and vented the gas cap to compensate. I'm currently using the factory banjo fitting return system for the su's (can’t find a pick at the moment) but after experiencing some rather sudden starvation issues last weekend (one night it was running fine the next day it was lean as shit anywhere off idle. this is confirmed not only by hesitation at cruse and light load but also a wide band o2 reader) ... I'm suspecting my return line bypass fitting may be malfunctioning and allowing too much fuel by, in turn starving the system. So long story short, I want to replace this system with something simpler. I’ve notice that typical return lines in most datsuns are restricted at the end of the tube line by a small hole drilled through a closed end. My thought was since my vapor line isn’t set up this way, I would solder shut one leg of a "T" fitting and drill a similar small restriction hole to accomplish the same affect. This "T" would then go in line with the fuel supply and the restricted leg would feed my hard line running back to the tank. My questions are, am I thinking and going about this right and what size should the hole be to restrict fuel flow but still be affective enough for my application. Also, I’ve searched the internet to find a fitting or system that I can purchase but couldn't find anything that didn’t also include a pressure regulator. If you know of something or have a better idea, please feel free to fill me in ;)

 

Thanks

Joel

[ggzilla]

Datsun Twin Hitachis don't normally use a fuel return line.

 

Just replace the pump with one that's in spec, or use a fuel regulator. But regulator often go bad.

[Just Joel]

Hmm, well the new pump is a factory nos unit... sure hope it isn't bad because I gave a bit of money for it to make sure I wouldn’t have a problem :) I can try tossing on the old one and see if that works but I’ve talked to a couple reputable people who said that su's prefer to have a return line... not to mention I have a factory part designed by hitachi for the return feature... That being said, it seems that they did make provisions for a return and I wouldn't mind retaining it. I just don’t think my forty year old part is functioning correctly so I'm hoping for a modern or custom alternative that satisfies the same task.

 

Thanks

Joel

[ggzilla]

SUs may prefer a return line, but most factory Datsun hitachi sidedrafts were not fitted with a return line and ran just fine. Only later cars had a return line.

 

the new pump is a factory nos unit... sure hope it isn't bad

Don't hope -- test it. If it is under 3.8 psi then you'll know that it is good. Or you can just keep on wishful thinking ...

[haubstr]

I have had SU's on my mostly stock L18 with no return line for about 6 years. I run a Holley Red electric fuel pump with a FPR at about 5-6 Lbs of pressure. No overflowing at all.

[wayno]

I have this on my rear carb, it's the fuel pressure regulator I beleave, I also used my vapor line as a return line, but I re-piped it, deleting all the vapor stuff in the box, fact is that my carbs ran fine without the return line, but since it had the regulator, one day I put the return in.

I have found that not having a serviceable set of SUs can cause lots of issues, if you can't get them ajusted correctly, you can have issues.

You have put PS fluid or trans. fluid in the top of the SU housing (that top fitting that unscrews).

Do you have a set of SUs from Z-therapy?

If you don't, where did you get these SUs?

Are you sure they are a serviceable set?

[Sealik]

I used the hard line that feeds the carb on a 720 Z24/Z22.....just cut it shorter

The bottom of the "T" is for the return line

No starvation or excessive fuel issues

 

[ggzilla]

Joel, are you talking about SUs or Hitachis? There are many types of SUs that are unlike the Datsun sidedrafts.

 

Datsuns that come with a factory return line have an orifice, and small calibrated opening, between the fuel line and the return line. Have you fitted something like that?

[Just Joel]

These are Hitachis and have a unit just like the one wayno is using and has pictured for fuel return duties. After giving it some thought, I figure that fuel delivery/pressure isn't my problem because at wot the mixture drops plenty into the rich region (high 11's to mid 12's) and during steady half acceleration she is also more than fine. The only time I have an issue is when I'm at cruise and very light throttle/acceleration or climbing a small hill.

 

Thanks

Joel

[ggzilla]

11:1 is way too rich -- makes LESS power than 12:1. The first thing is to get the float level correct. That affects the performance across the board.

 

Sound like there also might be a vacuum leak, which can be adjusted out of the idle, but then everything will be wrong. Meaning too lean at cruise and too rich at throttle. Or maybe the fuel pressure is too high.

[Just Joel]

Float levels are perfect confirmed by clear hose trick. Cant detect any vacuum leaks but it is possible... I'll find out after I install the new Ztherapy carbs when they show ;) I don’t have a fuel pressure gage but I'm going to look into testing this shortly. Most wot runs I have done have been in the 12's with perfect throttle response and great power… nice and smooth acceleration. As a matter of fact, most of the time I can't tell there is any issue at all outside the wideband reading.

I will add that while retuning things today, I noticed that the front carb was sticking when the piston is at its lowest point. I started to investigate and found the nozzle seat was loose. I realigned the nozzle, fixing my sticking issue and the "sudden" extreme lean out issues I was experiencing (what provoked this thread) seems to be fixed but I’m still stuck with 15's at cruise (not horrible) and occasional low 16's at light throttle/acceleration (more concerned about this than anything). Like I said, I have a set of Ztherapy carbs on the way to eliminate the potential carb condition issue once and for all and I can focus on tuning spec rather than wondering what I'm compensating for.

 

Thanks

Joel

[Z chopper]

lets see pics of the carbs you are having problems with

[72240z]

Datsun Twin Hitachis don't normally use a fuel return line.

 

Just replace the pump with one that's in spec, or use a fuel regulator. But regulator often go bad.

 

S30's used a return line the entire time they used Hitachi SU's.

 

I would create a loop with a restriction just like the s30, its a good system. With my triples I halved the return line vs the feed. Triples dont need a return but those were the fittings that came so thats what I made work. Glad I did, no return can still lead to problems even if not "required".

 

Wow you ordered Z therapy SU's over this? Thats throwing a ton of money at the issue for sure lol. Hope your building a show car or something that warrants spending that much when their full rebuild with nozzles, needles and bearings can be had for like 1/8 the price. I was very happy after using their stuff to rebuild mine but would never even come close to considering paying their asking in house referbs, pretty as they may be. I got triples for like half that price lol.

 

Pics when they come for sure! :-)

[Just Joel]

Here’s the current setup...

 

 

So yeah, I am concerned that they look good but I've also decide that these are the carbs I want based on my experiences dealing with the used ones. My current carbs are in fantastic shape with little to no wear. That being said, even from the factory the throttle shafts are flawed and allow leaks. After driving the car for a while yesterday, I can say that finding that loose nozzle seat was my main issue with the rather sudden and random mixture set I was experiencing so with that problem solved, I'm right back to an acceptable range. I'm asking a lot of fifty year old technology to get these bad boys running "perfect" where anyone one else would be happy with the current performance. The carbs I have now are to clean to turn in as cores so I plan to keep them for future projects and find another set to send to the junkers. By going with Ztherapy carbs, I can eliminate any potential condition issues and focus solely on tuning and eventually will profile a set of needles to really hone them in.

 

So you mentioned a "closed loop" return system with a restriction. Can you elaborate a bit on how this works and add a couple pics? Perhaps the orifice size in the restrictor if its available?

 

Thanks

Joel

[ggzilla]

You don’t have a fuel pressure gage? Almost all mechanic's vacuum gauges can check the fuel pressure.

 

$16 to $25

[ggzilla]

S30's used a return line the entire time they used Hitachi SU's

Even the JDM S30s? JDM 510 SSS didn't use a return line.

 

Don't get me wrong, a fuel return line is good, but it doesn't fix a pump putting out 7 psi. It CAN mitigate a fuel pressure problem if it is borderline, but the correct way to fix it is to fix the pump. Either a kit (which comes with new springs) or a new pump.

[datzenmike]

Without a restriction the fuel would just pump back into the tank. With a tiny hole, pressure can build up on the carb side. The return line and the restriction is not a pressure regulator, that is built into the pump... instead it constantly circulates cold fuel from the tank past the carb. It gets very hot under the hood and fuel can boil in the hard line between pump and carb and even in the carb. Vapor bubbles are pushed back to the tank. Circulating cool fuel reduces this and makes hot starting and running much better. An added benefit is that the fuel is constantly being filtered and crap build up in the tank is virtually eliminated.

 

The lower black hose connects the L fuel pump to the hardline to the carb. The hardline loops back and is blocked at this point with a small pin hole. A hose connects it to the return hardline to the gas tank.

[Just Joel]

What size is that hole? My plan is to make a "t" in the feed line to the rear carb with a restricted side running to the return line (mine doesn't have that end on it) my current carbs have the nifty fitting but the new ones might not.

 

Thanks

Joel

[72240z]

Yep even the JDM and UK ones. If I had to guess I would say to help with vapor lock. Which was a problem anyway.

Here is a JDM l20b pic, same rail as a USDM l24/l26 one.

 

I would make a loop the same way. Here is a shot of how the stock rail breaks down from like 1/4 to 1/8. My s30 didnt have a restriction like Mike posted but it doesnt seem like a bad idea.

Here is another example that uses a loop instead of blocks like most I have seen. They look like the carbs your getting too lol.

 

Nice clean set up there. Your carbs are nice I would have just rebuilt them and installed the roller shafts. Would be the same thing your getting. SU's are very simple strait forward carbs, so long as a few key things are in spec they just work, well.

 

There were NO throttle shaft seal issues from the factory. The issue doesnt even come into play until after 30+ years. Not to shabby if you ask me. ZT roller shafts are def a solution but they require machining. The issue can actually be solved way easier with a cap over the outside shaft and an O ring installed on the inner side. If you got the money and you will make you happy though power to ya. They certainly are pretty.

[datzenmike]

Have the return rail higher so vapor bubbles will rise up it and be pushed to the tank.

[72240z]

Ya thats one of the reasons I say I think they used a return to help with vapor lock. I think they used blocks with tight radius' to create back pressure as well since there is no restriction besides the downsizing of the ID.

[Just Joel]

Ah, so the line loops up and back to the fuel pump... From there it runs around the head to the fire wall and back to the tank? Or is there a second hard line where the supply line comes up through the fender?

 

Thanks

Joel

[Just Joel]

Oh and over the past couple days, the carbs have been performing perfectly. Cruise is in the hi 14's low 15's with only occasional low 16's with light throttle/acceleration. Idle drops right back to where I set it off of acceleration and doesn't "hunt" around like it used to. Its amazing how much that loose nozzle can affect performance. I guess it took buying a new set of carbs for me to fix all the bugs in these but at least I have a nice back up set if I need them ;) maybe for the next project?

 

Thanks

Joel

[wayno]

Oh and over the past couple days, the carbs have been performing perfectly. Cruise is in the hi 14's low 15's with only occasional low 16's with light throttle/acceleration. Idle drops right back to where I set it off of acceleration and doesn't "hunt" around like it used to. Its amazing how much that loose nozzle can affect performance. I guess it took buying a new set of carbs for me to fix all the bugs in these but at least I have a nice back up set if I need them ;) maybe for the next project?

 

Thanks

Joel

 

 

Loose nozzle???

[72240z]

I think he means, and I could be wrong, jet retaining nut&bearing. It being loose besides causing a vac and fuel leak (hunting idle I'm sure) would cause binding of the needle and piston during operation and of the nozzle in and out of choke.

 

SU's are simple but thats a major thing to have wrong, it basically handicaps the entire operation of the carb.