Starving?

[zeiss150]

Hey guys, I did something I've never done before today and I noticed something weird. I decided to drive my Z bare foot and I noticed that when I put the peddle to the floor my Z sort of sputtered and lost power at about 5k but if I pushed the peddle slower and not quite "to the floor" it would give me power up to 6.5k. I think with my bare feet I could get a better feel for where the peddle was at. I know that the nozzles are turned down enough to feed the engine (I tried to richen up the nozzles and that didn't help the problem). Since the problem seems to come on at "instant WOT" and not with a gradual WOT I guessing that it is a problem of not enough fuel pressure to keep the float bowls filled. I don't have an electric fuel pump (I know ... a non FI pump 3-5 psi) I've just been using the mechanical pump. So what do you guys thing? Dry float bowles? I await your wisdom!

[zeiss150]

A bump for my thread!

[Carl Beck]

If you have enough fuel pressure to fill the float bowls and keep them full while feeding the engine at 6.5K RPM - then I would think you have enough fuel pressure to get them there to begin with. Sudden acceleration/deceleration is one reason they are there to begin with.

If you floored it - and it took off at once.. ..but later started bogging down.. then maybe you would have a fuel supply problem.

The fact that you can get to 6.5K slowly and smoothly - might indicate that you have a problem with the damper fluid your using. If you floor it - and the vacuum slides respond too quickly - you'll be running too rich at that instant based on the vacuum demand from the engine... and we all know that a mixture that is too rich, not only costs you power, but money.

I'd change the oil in the dampers first - observe how thick/thin it is now -then go just a bit thicker. If your not sure - try using 20wt. first as a base line.

The second place to look, is the engine timing. Check the vacuum advance and check for vacuum leaks.

FWIW,

Carl B.

Carl Beck

Clearwater, FL USA

http://ZHome.com

[zeiss150]

Thanks for the reply carl, The damper oil is a good tip but I have fresh oil in there, 20wt (bought it from MSA just for SU's) so I don't that thats it. I tried the hard acceleration with the vacume advance pluged in and "unpluged" and there was no differance. As far as my static timing go's I have it at 17 BTDC I think that translates to 35 degrees total timing. I'm also using a 82 ZX dizzy (freshly rebuilt).

Let me tell you why I think It might be the float bowl ...

My engine is an L28 20thou over with an N42 head with a schnider cam and valve train (460 270, a mild cam but I wanted to bleed off some of the CR) My SU's are rebuilt and are running SM needles with stock 72 nozzels. So my engine with WOT can suck some gas.

I was thinking that when I punch the gass that hard and that fast, it just drains the bowls out and I looses all power, it actually "sputters" like its out of gas.

With the slower acceleration up to 6k (much higher and I can feel the same loss of power but no sputtering) it alowes enough time for the bowl to fill up just enough to power the engine.

I sort of remember reading a thread about this problem happening ... the answer was to put a non FI fuel pump in the system to keep the bowles full 3-5 PSI I believe.

So did I change your mind Carl or am I missing something.

Matt-

[Marty Rogan]

Matt,

I had a similar problem with my track car last year that drove me crazy tracking it done. Under heavy load it would bog and lose power. Long story short, I discovered that the carb insulator gasket had blown way underneather where you could not see it easily. With the car running, spray some car cleaner all around the area of the seals. If the engine RPM's change, that is your problem.

The other thought is that your floats may not be adjusted properly.

Good luck tracking it down.

Marty

[zeiss150]

Ok so I check everything and here's what it turned out to be... I was running to lean:stupid:

My floats were just fine I had plenty of fuel in the bowls I think the real problem is the needles im using. I had it running perfect for idle and normal driving conditions. But once I punched it the carbs would just run out of fuel under heavy load. I think the only way to really get the SU's "running Perfectly" is to have custom needle profiles made for specific applications.

I'm using SM needles with stock 72, 3 screw carb, nozzles.

I believe that the SM needles run rich at idel but under heavy load work great.

I tried the stock 54 needles but it ran worse with those.

With the SM needles I get about 21 mpg so I'm not running that rich but the exaust smell is real annoying at idlel.

Right now I'm about 2.75 turns down and it idles great but is a little "smelly"

Has anyone ever done some custome needle profiling and if so what did you do?

Now when I punch it I get up to 45 mph in 1st gear at 6500 rmp and it does that in about 3 seconds with no loss of power.

Thanks for your help guys!

[Bruce Palmer]

One thing to check while you're running the traps looking for culprits on that starvation issue. Check and make sure the final filters in your float bowl lids are not partially blocked restricting fuel flow.

And on the SM needles if they are fat at idle run the nozzles up half a turn and see what that does. Should still "idle fine" just wont be as fat and the half turn at idle wont affect much of anything up top. Keep playing 'til the fat at idle goes away.

[zeiss150]

Hey Bruce Thanks for the reply and advice. The banjo filters were taken out when I rebuilt the carbs 3 years ago. I'll try to turn up the needels .5 turns and see if that gets rid of the rich idle condition.

I think the problem with most of us weekend mechanics is that with the 35 year old, non computer technoligy, carborators we have a hard time getting the perfect AFR (which is what we are all hopping for). With three hundred plus needles to chose from it becomes sort of an overwhelming task to pick the right one for the custom engine that a persone has just built.

I have read hundreds of SU posts in the 3 years I've had my Z and most of the time I just end of with more questions. For such a silmple carborator it sure has a lot of tunning potential.

I think I remember reading a post from you Bruce, about a college student that was doing a bunch of tests on the SU's for different needle profile combinations with different engine applications (i.e. cams, displacment,heads) and his school was getting in the way of our SU tests, how's that comming?

It would be a wonderful list to have ... sort of like the Lengine calculator ... put in your displacement, head, cam, valves, exaust system, timing, and anything elese that might have an effect and the program would tell you which needle, nozzle, and how many turns. Dare to dream!

Matt-