Still odd that it would only do it under constant throttle, but not WOT. But yeah, the stock fuel pump has to go. I'm just waiting until I can get a Mallory 4309, then it will go in along with a bigger return line and an MSD EFI pump.

 

How do you know that it wouldn't do it under WOT unless you held it there? And if you're revving it without the engine being under load, i.e looking at an inline gauge with your head under the hood, then I'm not sure you'll get an accuarate reading either.

Regardless, the stocker may be able to give enough fuel and maintain pressure for short spurts, but nothing more. To reiterate what Viper said, it just can't keep up. The proper fuel level in the bowls can't be maintained, as the fuel just cannot be replaced fast enough with the stock pump to meet the demands of the engine.

I'm confused by you saying that you're getting a mallory pump and also an efi pump since this is a carb.

When you're running the DCOE a return line is unnecessary, dead heading is the way to go.

 

As I mentioned in my first post, this is going to be a blow-through turbocharged setup. That's why it will get an EFI fuel pump with a boost-referenced return-style FPR. But yes, currently it is dead-headed.

It doesn't appear to have problems at WOT because I've done some freeway pulls and hard runs up a very steep, long hill by my house (Jeff knows the one I'm talking about) and the power delivery is consistent and smooth. Plus, when I was revving it under no load, the fuel pressure remained steady. But when I kept it at a constant throttle under no load, the fuel pressure went crazy. This is consistent with what I've been seeing on the road: give it some throttle and it's smooth and powerful, but at low load and constant throttle, it bucks and surges horribly.

It's all academic at this point, because I won't have the stock fuel pump for very much longer, but I would still be interested in finding out why it's acting the way it is.

 

What hyper4mance2k says makes sense. At WOT the carb is getting the full force of the fuel pump, and it seems to to keep up. But then at part throttle under light load such as cruising, the pump goes into internal-bypass mode (or whatever you want to call it) where its internal bypass thing opens and closes causing the heavy fluctuations in the guage the the herky-jerky engine. But then at idle the engine settles back down because the fuel pump enters 'dead headed' mode as the entire fuel line pressurizes and the pulsations smooth out.

It's sorta like how at low RPM a rotary has strong reversion in the intake runners but doesn't at higher RPM. At idle the throttle plates are closed and that sort of minimizes the reversion effect but at low load and part throttle the reversion is at its strongest. As RPM rises the reversion pulses smooth out as vacuum signal decreases and forward runner velocity increases.

What I think is happeneing in your fuel system is a simple harmonic oscillation having to do with the floats+needles opening and closing in and out of sync with the internal bypass of your fuel pump as it goes in and out of dead-head mode, and it can't keep up which is causing the terrible surging you've been experiencing. At idle there is little flow and the jets open and close only a little bit and less often; the fuel pump can easily handle this. At WOT the fuel pump and floats are fully flowing; not problems here. But then at part throttle the two systems fight each other.

Do you remember the bad shuddering the baja had sometimes in 1st gear? Aside from a grease damaged disc, the main cause was a bowden tube not bowed enough so as you let the clutch out, the engine torques over to one side, changing the relationship of the clutch cable to the tranny and the lever position of the clutch fork, causing a fast in and out effect on the throwout bearing, producing the chattering sensation. The whole process happens so fast that it can produce this clutch chatter effect even in a healthy clutch. Once I added some spacers to bow the tube just a bit more, the shuddering virtually stopped.

Another example, and one with a much lower frequency, was in my bro's GSL-SE. That car had some torque, but with worn motor mounts and lots of slop in the rear end, and my unfamilierty with the car, funniness ensued. I didn't change the seat position so I had to reach my foot kinda forward to step on the gas pedal. So I'd go to accellerate, which would throw me back in the seat, which would lift my foot off the pedal, which would close the throttle quickly slowing the car dpwn and causing me to slide forward in the seat, causing me to hit the pedal again, accelerating the car and throwing me back in the seat+lifting my foot again etc. It was emberassing so I shifted into 2nd and went on my way.

Another example just today. I finally got around to installing the spare solenoid on the spare automatic starter that's been sitting in my garage for the past couple of years. It's an actual Mazda reman built in '86 and apparently for '74-'78 auto rotaries. Anyway I hook up some jumper cables to give it a go and jumped the solenoid tab with a small piece of steel. The result was a fast ch-ch-ch-ch-ch. The first words out of my mouth were oscillation! I finally broke down and went looking for the convenient remote starter pistol grip switch thing with the nice alligator clips. Found it, hooked it all up and zoom zoom! It works fine and really feels strong.

It's new fuel pump time for you!

 

wat

 

That makes sense. The stock fuel pumps weren't designed to be run dead-headed, so the bypass might just be a simple "binary" open/closed valve. If that's the case, and if it can't cycle very fast, it could certainly cause oscillations in fuel pressure at certain engine load levels. (In my case it happens to be during cruise and light-throttle acceleration in low gears.)

 

Plus the stock systen didn't have a fuel pressure reg, so you've got the stock pump trying to regulate flow and pressure along with the aftermarket FPR, so in theory you've got two regs fighting eachother.

 

Okay, update time.

So I finally got around to swapping fuel pumps. I'm now using some German pump that was on the car this Weber came from. It's temporary because I still don't have a return-style pressure reg, so I can't install my MSD pump yet. It sucks being poor.

Anyway, when we got it in, the first thing we noticed was that the fuel pressure is now rock-solid at 3 PSI. The second thing we noticed was that it didn't fix the problem. So we were like, WTF? Jeff played with the mixture screws and cranked the idle way up and got it to drive better, but still not very well. Basically, it seemed like it was running way too rich at cruise, rather than way too lean like I previously assumed and the fuel pressure seemed to indicate. I was getting a little fed up with it, so I swapped in the only other set of idle jets I have: 40F9. And guess what? Now it drives beautifully with a smooth 750 RPM idle. This whole time, my idle jets were just pig rich.

It's still not perfect, and it now has a stumble at the idle-main transition. I'm guessing that's due to the huge drop in idle jet sizes. Decel also isn't as smooth as it was with the 65F9s, but at this point I don't really care much. I'm probably not going to touch it anymore until I have an AFR gauge and a turbo, aside from possibly playing with the fuel pressure.

 

It's actually smoother at part throttle cruise now? Congrats!

 

I just achieved victory with my 45dcoe on S5 13b. My setup is as follows. 41mm chokes(had the 36mm bored out on a lathe) 4.5aux, 3.00 needle n seat 205 main and 185 a/c, f-16 emulsion tube, fuel prseeure set to 4 psi with mallory 4309 and stock SE FI fuel pump and the big one is this 70 idel jet............no choppy throttle no hesitation to wot(well very little) and surging at cruise, my idle is a nice and steady 900. With a mild streetport

41mm choke
4.5 aux
3.00 needle/seat
4 psi fuel press
200main/185 a/c
f-16 tube found better acceleration in mid range than f-11
70idle

 

175 to 180 main jets I think is what we did yesterday. He said it made a noticeable difference.

 

Nope, it was the air correctors that went from 175 to 180. The main jets stayed at 180. Seemed to help somewhat. Again, though, no point going out and buying some bigger air correctors when I don't have an AFR gauge.

 

i thin you might want marginally bigger mains with smaller a/c's I've got a few sets of idles jets you could try as well.

 

bigger mains than a/c jets?

 

Brandon, always run bigger mains than a/c on a rotary. Didn't you just say you had 200 mains with 185 a/c's

I think Percent might fiar a little better with a smaller a/c and bigger main. His stumble problem is on the progression from idle circut to the main circut. There is a change in the a/f ratio between the two circuts and that is what is causing his stumble at cruise. If both circuts are equally too lean or equally too rich it won't stumble, so there are plenty of ways to get it to go away. You want the main circut and the idle circut to give the same air fuel ratio at the transition point. Then the ultimate goal is to get the idle circut stoich and a nice smooth progression to rich at the top end. It's the uneven swap from the idle circut to main circut that you're dealing with.

 

I mis typed that i am running 200 a/c and 185a/c. Ill swap some stuff around. After reading a ton of weber threads i purchased all the mentioned jets and a/c and a few emulsion tubes and idle jets too.

 

I think Adrian is right, that I need a bigger main jet. I tried some F7 emulsion tubes today because I had them and I was bored. Made the idle jets too rich again, but largely eliminated the stumble. It seemed to have a little more up top, though. Thus my conclusion that my mains are too lean.

 

f7's should be richer in the midrange and leaner on the topend than f-11's. I think... Robert or Wacky might have a better guestimate. The problem with e-tubes is that they were made when webers were fitted to new cars. So f-7 was for the 1974 fiat and f-8 was for the 1975 lancia, so they have no sequance to them. I just made up the years and cars, but that's how the f numbers for the tubes came about.

 

I read on one site that F7 is richer everywhere than F11, but particularly in the low- to mid-range. But who knows how accurate that is.

 

link! I'd like to get as much weber reading as I can.

 

I would if I could find it again. It was a while back. It only mentioned F7 and F11 in passing, as an example of how emulsion tube choice can affect the mixture curve. As I remember it wasn't a particularly useful site; I just took note of that particular piece of information because I had both F7 and F11 e-tubes.

 

Where's wacky when you need him, lol. He can straighten you out on jetting and emulsion tubes.

 

Wacky is an IDA guru, same circut I know, but DCOE's are for beginers.

 

not to jack the thread but what all did u do to turbo prep the carb? i got a side draft im wanting to do a blow thru on and so far all i have for turbo preping is fill the floats so they dont crush? delet starting circuit (choke)? sealing up any and all leaks? am i on track here?

 

I know, it's hard to find information on boost-prepping on the web. But it sounds like you're on the right track. Instead of filling my floats, I'm using the solid plastic ones from a later-series 45 DCOE.