Looking for Dyno evaluation/SU carbs-

[madkaw]

You will definitely have to notch the cylinder walls.And yes both valves are the bigger valves on an early e-88

So there must be something I can do with these SM needles to get more fuel. I would think I could get 200hp with these needles. It was suggested to up my fuel pressure, but i'm running a mechanical pump.

Edited August 2, 2010 by madkaw

[Bruce Palmer]

Just hypothesizing now but a couple of factors Steve and I were kicking around the other night when this first came up were.

1. Mechanical fuel pump may be weak hence not keeping up with demand at speed. Not keeping up at speed will drop the fuel level in the float bowl AND the fuel nozzle. As the fuel level drops down the nozzle tube (as the level in the float bowl drops) the carbs WILL go lean as the air stream is working harder to pull the fuel up from farther down the tube.

2. Everything I said in point number 1 could be aggravated by float levels being a touch low to start with.

If you tweak the float levels, you will more than likely need to tweak the mixture settings as well. And around the barn we go......

Cheers

[Shade133]

why a mechanical fuel pump? why not a electric one?

[Bruce Palmer]

why a mechanical fuel pump? why not a electric one?

You can do either. Biggest thing to handle is keeping fuel pressure down below what will overpower the needles and seats in the float bowls. ~3 1/2#

[Walter Moore]

Steve,

This is a little off subject, but what was your actual fuel mileage on your way to ZCON and back? To my great surprise I recorded 25mpg on the highway portion of the trip. Since we were running basically side by side on the way down, and have (I think) the same transmission and differential, ratios that might give you a hint as to whether or not you were running rich.

(I skipped the dyno at the show because I guess I just don't want to know...)

[Shade133]

You can do either. Biggest thing to handle is keeping fuel pressure down below what will overpower the needles and seats in the float bowls. ~3 1/2#

ah i see! do they sell adjustable pressure electric fuel pumps? or would you have to buy a electrical one, and then install a adjustable fuel pressure regulator?

if that rout would work, would it be plausible to install a FI 280z's fuel pump and buy a aftermarket adjustable pressure regulator for the carbs? that way you can raise it and lower it when needed?

just a thought i had.

[madkaw]

Walter,

I believe I saw close to 28 on the way down, but can't prove that since my odemeter is probably off with my speedo. Average speed on the way down was probably less then on the way back for me-thus less mpg on the way back 26+

I believe at 65 or 70 I would get 30. Diff ratio 3.7

Bruce , I might have been compensating my piston needle being off by adjusting the float level, so maybe I can readjust the floats and try that. I have it idling perfect right now and the mixture seems good, so I have a baseline. The fuel pump is new!

[Walter Moore]

ah i see! do they sell adjustable pressure electric fuel pumps? or would you have to buy a electrical one, and then install a adjustable fuel pressure regulator?

if that rout would work, would it be plausible to install a FI 280z's fuel pump and buy a aftermarket adjustable pressure regulator for the carbs? that way you can raise it and lower it when needed?

just a thought i had.

Fuel injection pumps have way too much pressure for the carbs. A quick search for electric fuel pumps should answer your question.

Steve,

Don't discount the possibility that the mechanical pump is the limiting factor in your horsepower. I had a mechanical pump (which I purchased new) on my car for about one year after I got it running. At that time the car developed a strange symptom in that it just "hit the wall" (in runners terms) at 60mph, and wouldn't go any faster. (Didn't matter what gear the car was in either.) I switched to an electric pump at that point because I didn't want to risk another bad, new pump.

That "fixed" the problem, but introduced all the electrical gremlins associated with electric fuel pumps.

Anyway, it seems like the leaning out problem that you have could easily be a symptom of the mechanical pump running out of capacity. From my experience that seems to be one (of several) failure modes of the mechanical fuel pump. When it reaches its maximum fuel delivery rate the pressure just drops to nothing and the carbs run lean. The system equalizes and the engine keeps running, but it can't produce any more power.

The rich mixture at lower speeds may have been an across the board issue if the pump could have kept up with the demand.

[madkaw]

I here you Walter about the pump. I think I will try some tweeking on the floats and see how that works out. I should be able to tell by the butt dyno whether I made any changes. It's almost a shame to mess with it because it runs so good right now, but I can't leave all that HP laying around doing nothing-can I?

The hardest part is being patient and not throwing those triples on right now. I really want a fair comparison between the two set-ups before I sell one off.

[madkaw]

I went in there and richened up the floats and WOW! Now she revs out fast and strong. not sure if I went to far with the floats, but I still have a very nice idle and mixture screws are about a turn or so out. I will put some miles on it and look at the plugs.

The engine used to get weezy at around 5k+, but now she wants to jump right to redline with a vengence. I hope this adjustment stays because i can get used to all this power:classic:

Thanks for the advice guys.

[madkaw]

Well after putting some miles on the new float set-up, I am convinced i am still weezy on the top end. The plugs all look tan with just a liitle carbon at the end of the threads. Basically they look the same as they did before. I swear I made a huge adjustment on the float and was expecting to go the other way(rich), but it didn't. I believe at first I set them at stock specs by inverting and measuring as the float tab just touches the needle-I think 13-15mm. I bet I went to 10mm on the the last adjustment.

How far can I go, or should I go until it starts running rich? What other options besides float adjustment do i have with the SM needles?

[d240zx2]

I was the car with triples dyno'd before you. I was truly running way richer than when I had run an AFR check about a year ago. On the 31st, my AFR was 9.4 or less. A year ago, it was 13.4. Still a tad rich, but not gonzo rich. Gotta get the jets right.

My instructor on track day seemed to be impressed with the car's performance, though. Or am I in denial???