Autocross Fuel(?) Problem

[albatcha]

Took the 2002 for the first time to an autocross, in the same spot on my fourth and fifth runs I had an issue where there was no power and it sounded like a truck engine braking when I tried to press the gas. Any ideas what it is? It's a tii with a weber 38, fuel pressure regulator in the engine bay, and MSD ignition. And float, jets, and regulator pressure were adjusted about a year ago.

 

You can hear twice around 47 to 50 seconds I try to give it some gas and it just makes that deep rattle noise.

 

 

 

[Son of Marty]

I don't believe that's a fuel problem, it cuts out to cleanly fuel problems usually sputter before cutting out.

[Tommy]

How much fuel in the tank? If less than half it can easily suck air in the system and cause that. Next time try with more fuel and if you plan more autocrossing maybe a surge tank with a transfer pump in future. There's more experienced autocrossers here who surely know what works and what doesn't...

[TobyB]

I hear you being very far down in the rev range.

 

But this is why I datalog- it tells me a lot more than a video.

 

The other thing to look at is what's happening in the carb itself.  The 

DCOE's have a 'feature' in a left- hander where the fuel spills out of the float

bowl and into the main stack.  It shows up a a really rich spike when you go back to throttle,

and the engine has to clear itself before it picks up again.

 

When I've had fuel lack- of delivery problems, it's always been towards the 

end of a longer straight when the float bowl runs low.

Yours is more likely overfuelling than underfuelling.

Since you say it happens after being off- throttle, I'd really look for flooding/

rich running.

 

t

 

[tzei]

Enter the EFI

[Son of Marty]

Toby, maybe try milling a small ledge between the jet well wall and at front of the carb and fitting a baffle like the one on the left side of the float bowl

[albatcha]

Thanks for the advice. Now I've got some idea where to start.

 

A little more info in case it helps narrow anything down. Gas tank was between 1/2 and 3/4 full and the gauge is reasonably accurate.

 

On the fourth run I had a similar problem in the same spot but not as bad, I was on the gas through that last "slalom" and it was accelerating then would stutter then accelerate then stutter.

 

On the fifth run in the video I swing out to the left away from the shed which someone crashed into earlier this year, back to the right, then I lift a little to make that left turn to go straight into the last "slalom" and after that lift and fast left I try to ease back onto the throttle and nothing happens, it's like I'm coasting in gear. Then I lift off completely wait a second and press the gas again and I get that little stutter, lift off completely again and press the gas again and get that stutter again. Then I coast to the middle of the final sweeper and try to accelerate out of it and everything is normal again.

[Mike Self]

5 hours ago, TobyB said:

The other thing to look at is what's happening in the carb itself.  The 

DCOE's have a 'feature'

Toby, he's running a 38/38 downdraft...could it be a low float setting allowing the fuel to pile up in the float chamber away from the passage that feeds the jets?

 

mike

[albatcha]

Right its a 38/38 dgas downdraft. I followed somebody's instructions and measurements for setting the level I forget whose but I remember there was something weird about it like the numbers suggested didn't quite make sense but i set it there anyways.  

 

I did it a couple years ago and don't remember the exact details. 

Edited September 26, 2022 by albatcha

[stephers]

I agree with Mike, sounds like it is running out of fuel as you corner hard and the fuel moves away from jetting        I know on the 911 IDA's for performance driving, there is a baffle that is recommended to be installed for similar issue.  And as Toby said, the DCOE's can be baffled.    Maybe first try raising the float level, larger needle and seat?   Maybe look into baffling on the carb that you are using

 

Thanks, Rick

[kbmb02]

As noted above, check float level - particularly as you’re unsure of the setting and method.

Having a bit of experience with the 38/38 in racing environment, I will suggest it’s also possible your float level is too high (whereby the fuel sloshes and dumps over the top of the air correctors). Similar issue if your fuel pressure is too high (do not trust the screw-in pressure gauges).

 

You can race the heck out of a 38/38 carb set-up, when it’s right. -KB

[TobyB]

You'll never uncover the main jet in a 38 (or a dcoe) unless you're shiny side down,

or have a fuel delivery problem.

 

Again I say, this is where logging eliminates 7/8ths of the variables.

In this case, if it's lean or rich.

Lean, you're looking for junk in the jets, crap in the carb, punks in the trunk.

After overrun, it will not be fuel delivery- you're at minimum demand, large supply.

Rich, splashover or overpressure on braking (at 1g, the fuel pressure against the carb can

easily triple).  On the dcoe, the splashover problem's solved with

(wait for it) 

some JB Weld.  You block half the mickey mouse ear cover...

I've not had trouble with the 38 splashing over on the track, but I was running

lower float levels to try to sort transition issues.

 

t

 

 

[albatcha]

Looked at the float, it was at 36mm, I remembered that what was weird about it was that I couldn't easily get it at the right level and still have enough travel at the ball. I realized someone previously had bent it in a weird way, not sure if thats normal, so I bent it back and set it to 34.3mm and 44.3mm at the lowest as per the haynes manual. Don't think I have a good way to test it until the next autocross so we'll see then. I'm also going to check how accurate my fuel pressure gauge is and maybe try a different one, not going to mess with jets yet without actually measuring A/F, since the problem has only shown up in a very specific situation and not when accelerating and lifting in a straight line I have doubts that it's directly caused by jets and mixture.