View Full Version : carb bypass line

02-26-2002, 07:30 PM
have a yaw carb, can i run the bypass line to a tee in the line between the tank and fuel pump?

02-27-2002, 01:02 AM
Daryl, do you mean between the fuel pump and the line TO the carb? If so, no. The tee would turn both lines in to positive pressure towards the carb. It's going to have to be returned to the tank or a point between the tank and fuel pump.

Good racing

Okay, so the dumbass re-read your question, the answer is YES!

[This message has been edited by Quickshoe (edited February 27, 2002).]

02-27-2002, 12:46 PM
I thought so, my concern was air bubbles created by the carb somehow getting to the fuel pump,is the return line a steady clean flow of gas?

02-27-2002, 10:57 PM
Good question. (this is sad I'm already back...looking on the mazda site) I wouldn't think it would be a steady stream. Especially since under some conditions the fuel pumps have trouble keeping up with the demand. Wouldn't the fuel flow in the return line be interupted everytime the float is letting fuel into the bowl?

Bottom line is I've changed my answer from no to yes to maybe!!! Call Mr. YAW I'm sure he would know. If not put a temporary CLEAR fuel line on the return and see what happens to the flow.

Know anyone with some 13" panasports for sale?

[email protected]

02-28-2002, 01:15 PM
yaw hasnt returned my calls or emails in the last two weeks, but in that time he did cash my check allthough i havnt recieved any parts.
my school at t-hill is 15 days away, im getting worried.
todd lauchbaugh has used panasports for sale
[email protected]

Daryl Brightwell
ITA Mazda #90

[This message has been edited by 7'sRracing (edited February 28, 2002).]

02-28-2002, 04:48 PM
Welcome back Quickshoe...... http://Forums.ImprovedTouring.com/it/biggrin.gif That was shorter than last time you were going to go away.

02-28-2002, 05:05 PM
Daryl, here is some guesswork that may allow you to complete this return line deal.

I have not taken anything apart at the inlet & return lines at the carb, but it would seem that if you run the carb to tank return line into the line between the tank & the fule pump the fuel pump will be trying to suck fuel away from the carb through the return line & the same fuel will be pumped around in circles........

Whatcha think Daryl & or Quickshoe ???

Come on folks I am always willing to learn.


02-28-2002, 05:15 PM
you know where im going with this David, im trying figure out how not to have put another hole in my fuel cell, dosnt look like i have a choice.

02-28-2002, 07:30 PM
Since I accidently drilled a hole through my return line a few years ago, I just crimped off the return outlet on the carb...so there is no return line to the tank. The line is still there but its not hooked to the carb.

Never had a problem with it.

02-28-2002, 08:03 PM
you must not be running a yaw carb with a fuel pressure regulator.

03-01-2002, 02:14 AM
Why do you say that? I run a Yaw with a reg. and the return deadheaded. No problems.

03-01-2002, 12:20 PM
rlekun & GEO46, is the reason that you both (& maybe others) have deadheaded your return line sucesfully because of the fact that you have an aftermarket fuel pump & regulator that can constantly provide enough fule flow at the required PSI ? The stock fuel pump with no regulator had overflow capability at normal driving speeds & higher PSI than desireable therefor a return line. (?) The stock fuel pump could not provide the same overflow at higher motor speeds therefor the motor received enough fuel to run very well. (?)

Anyone, come on with your responses. This is all in a learning mode.

On my Spec-7 (stock carb)I have an aftermarket fuel pump & regulator so that I always have the correct controled fuel flow & PSI.(?) Under these conditions do I need the return line ? Without taking the rules into account. http://Forums.ImprovedTouring.com/it/confused.gif Who, me ???????

Have Fun

David Dewhurst
CenDiv Milwaukee Region Spec-7 #14

03-01-2002, 12:59 PM
well put David, even with a regulator, without a return the pressure will build when you take your foot out of it so when you hammer the throttle the motor will be too rich, less power.
remember, the reg is there to keep mixture at optimum, letting pressure build is defeating that purpose.

Dave Damouth
03-01-2002, 01:32 PM
Regulator creep is a problem with deadheaded systems. The normal mode for the regulator is to be slightly cracked open at all times, letting just enough through to maintain pressure. When you lift, the regulator may go to low enough flow that it needs to close completly. Its not consistent at doing this. Its made worse by the fact that the differential pressure is high with Yaws recommended stuff. The pump is trying to stuff 7 psi into the back of the regulator, and the carb wants 2 psi (or less). Do some datalogging, and you'll find at times that you're building 5 psi at the carb if the regulator creeps. Then you start gently opening the throttle and find that the intake is full of raw fuel, takes a second to clear out, fouls plugs, people pass you exiting corners. The stock pumps and clicker pumps only put out a few psi, the differential is a lot less. Most of the clicker pumps can't supply enough fuel at high load though.

03-02-2002, 02:19 PM
Thanks Dave DaMouth for your points, the black RX-7 I'm driving now has a "non-perforated by owner" return line, so I'll hook it back up. No sense taking a chance!

So....back to David Dewhurst's question, if plumbing for a fuel cell, and you want the return line to return fuel, where do you best return it to? The line before the pump or back to the tank or cell?
I'm about to order a new fuel cell, so should I make sure the top plate has a second inlet for the return line? Do they come with that automatically?

03-02-2002, 02:44 PM
rlekun, 7'sRacing (Daryl) is doing the plumbing project while installing a cell & yaw carb.

On my Spec-7 I must leave the return line alone. But if I was installing a cell I would continue using a return line to the cell. Don't know if a cell comes with enough holes but ya can always have em put it in.

Dave Damouth thanks for your experienced knowledge. I will refine the refinements & test for a continious psi.

Have Fun


Dave Damouth
03-04-2002, 12:01 PM
I can't remember if Spec7 lets you swap jets. If not, you can always play with fuel pressure as a method of mixture adjustment.

Now for the strange idea... Put your regulator on the firewall as close to the carb as you can get it. If you happen to put it on the driver side firewall where its EASY to install, this weird problem crops up. The Fuel pressure goes up turning left, goes down turning right. Centripetal force = height to gasoline. Not a huge problem, unless you're trying to run 2 psi....

03-04-2002, 04:36 PM
Dave Damouth, Spec-7 shall be stock carb with no mods. I do low pressure & bowl depth. I cut out a piece of the stock rubber line between the steel input line to the steel carb line & inserted the regulator there. I did not secure the regulator to anything solid. Your thoughts ?????

Have Fun


Dave Damouth
03-04-2002, 06:08 PM
You must be using one of the dial type purolators. No way you've got the holley boat anchor model hanging on that rubber hose...

I'm one of the few people that won't trash talk those. They got a bad rep somewhere, but I always hear generalizations from the naysayers. I like them. They're not perfectly accurate, but they're repeatable. Holleys have big problems with quality, and the creep can be really bad.

Of course, if you wanna make the specrx7 guys cry foul ( I have a lot of experience with that. It can be quite fun.... http://Forums.ImprovedTouring.com/it/smile.gif ) read your rules on fuel pressure regulators real carefully. Aeromotive builds a fuel pressure "regulator" that drops voltage to the pump to control pressure. It uses RPM, as the input, but you could use any number of things. I know of one person that built one that varies voltage to the fuel pump based on the 02 sensor value. (Don't try this at home) You could probably get another 500 rpm on top if you can get it to lean out a bit up there. Imagine the questions at impound....

03-04-2002, 08:54 PM
Dave, I also heard all the stuff about the Purolator so.....

Row Row Row the Holly so gently down the stream. I beleive in the KISS factor. About 2 inches of hose each side of the regulator.
All those mounting brackets & bolts add weight.

While testing for constant psi, if the constant psi is not there then it's serious time about another regulator method.

Let's not get into the class wacking mode cause then them pals of yours will come out of the woodwork. http://Forums.ImprovedTouring.com/it/eek.gif

Have Fun


Dave Damouth
03-05-2002, 12:35 PM
MY friends! Haha, you're killing me. You spec guys are the one's with a sense of community. All 4 of you.... Oops. Did it again. http://Forums.ImprovedTouring.com/it/biggrin.gif

03-05-2002, 04:03 PM
Dave, below is a message posted two months ago & all 4 from the RX-7 IT community responded on the site. http://Forums.ImprovedTouring.com/it/tongue.gif I even signed up to change my SRX7 decals to ITA to help fill up the race group. http://Forums.ImprovedTouring.com/it/smile.gif On the serious side, how many ITA RX-7's are race ready in the CenDiv district 4 & how many in district 5.

:The Detroit Region SCCA and the Waterford Hills Road Racing Club are currently investigating a combined race weekend this upcoming season. Since the Waterford Hills club runs an ITA rotary class for 1st gen RX7's, RX2's and RX3's (called ITR), this weekend could feature an "IT7/R" race.
Of all the CENDIV ITA RX7 racers out there, how many would be interested in competing in an IT7 race? Tell all you're RX7 buddies to log in to provide their input.:

The fun part of each race I attend is that there is usually someone to race with. Even if it's someone from a faster class that's having a slow day that driver gives it the 100% that he/she has. http://Forums.ImprovedTouring.com/it/biggrin.gif

Have Fun