engine and drivetrain 40Dcoes At Autocross: Stalling After Run

[KFunk]

OK, mildly annoying re-occurring problem that I've had for many years:

 

Car runs great at autocross with lots of hard acceleration and hard cornering.  Then I cross the timing lights and slam on the brakes to come to nearly a full stop, and grab my time slip if they have one.  Then car stalls, and I have to crank it for a bit to get it to re-fire again.  Embarrassing, and I need to get off the course in case the next car is coming.

I can prevent it if I remember to keep my foot on the throttle slightly to keep the revs above idle.  Just annoying, and sometimes I forget to do it.

 

I've got 40DCOEs that run great otherwise.  Its done it for many years, with 2 different engines, stock cam and 292 cam, stock pistons and 9.5:1, both with anti-vibe mounts and paper gaskets, with carter rotary pump and IE electric pump, and no regulator.  Changed needles and such a couple years ago, set fuel level perfect, etc.  I've never changed the jetting in the carbs.  I don't remember it ever doing it when I had the 32/36 DGV.  I also usually run with only a couple gallons of gas in the tank (weight savings).

 

Anybody else encounter this problem?  It seems like it has something to do with going from super high fuel demand and rotational forces, to almost no demand.  Something can not adjust itself quickly enough, and must flood it.  It hasn't been annoying enough for me to throw a lot of money and time at it, but I'd kinda like to know what could be causing it.

[TobyB]

Try a fuel pressure regulator.

 

Also check your float levels.

 

Shouldn't do that.

 

Believe it or not, G's can increase/decrease your fuel pressure a surprising amount,

which is why having the regulator under the hood's good practice.

Mine's in the trunk, and you can SEE pressure change under hard braking.

Some under acceleration, but not nearly so dramatically.

 

t

[KFunk]

Try a fuel pressure regulator.

 

Also check your float levels.

 

Shouldn't do that.

 

Believe it or not, G's can increase/decrease your fuel pressure a surprising amount,

which is why having the regulator under the hood's good practice.

Mine's in the trunk, and you can SEE pressure change under hard braking.

Some under acceleration, but not nearly so dramatically.

 

t

Yeah, I was wondering if that could help.  I don't want to get one of the $25 spectre ones from the parts store, though.  Tried one before, and it just seemed like a really poor quality mechanism that it seemed like it would impede performance more than it helps.

 

Before I go spendin big bucks on some higher performance mechanism to solve a mild occasional inconvenience, I wanted some confirmation that it's the actual problem.  If other people have had this happen and there's a more sure-fire solution, then I'd rather do whatever they do.  I don't think its the float/fuel level.  I've been over that a couple times in the past couple years for unrelated reasons (stumbling while under load in straightline acceleration),  and carefully adjusted it to make sure that the fuel itself is at the proper level within a millimeter by measuring it with a stick into the carb while the engine is running.    

I'm not quite sure how the changing dynamics of fuel flow work though, myself.  I guess both the carter rotary (supposedly self-regulating), and the IE/airtec model (which is a very different mechanism), can ratchet up fuel volume quite a bit under demand, and then can't quickly slow it down when you don't want it?  I just assumed they pumped a high volume/low pressure, and the needle valve could slow down any excess as long as it wasn't too high of pressure.  Maybe I'm the only guy thats putting an unregulated pump through this much of a challenge, though.  99.9% of the time there's no problem, but just a couple times when I'm getting my timeslip, dammit!

Edited August 27, 2013 by KFunk

[allbim]

Try a fuel pressure regulator.

 

Also check your float levels.

 

Shouldn't do that.

 

Believe it or not, G's can increase/decrease your fuel pressure a surprising amount,

which is why having the regulator under the hood's good practice.

Mine's in the trunk, and you can SEE pressure change under hard braking.

Some under acceleration, but not nearly so dramatically.

 

t

Interesting. I never thought of that before. Just running the numbers through my head, assuming an 8 ft fuel column and a specific gravity of the fuel of roughly 1, I come up up with a 3-4 PSI increase in pressure during a 1 g stop. That, and the floats binding a little, could mess things up a bit.

Edited August 27, 2013 by allbim

[LimeySteve]

Filter king by Malpassi ,Italian & really work well.look at eBay I get them from a seller in Canada. about $85.

[williamggruff]

carefully adjusted it to make sure that the fuel itself is at the proper level within a millimeter by measuring it with a stick into the carb while the engine is running. 

 

I've read about various methods for measuring the fuel level in the bowl, but am still curious as to EXACTLY how and where the measurement is being taken, and the range of acceptable values.

 

In the photo below, I've assumed I should measure from flat surface C to the upper surface of the fuel, but suppose it could be from A or B.  I've also found differing values for measuring the height of the fuel using this method, including 25.0mm, 25.0mm +4.5mm (whatever that might mean), and 29.0mm +/- 0.5mm.  The distance from flat surface C to the upper surface of the fuel in my DCOEs measures 28.75mm with plastic floats set according to the 12.5mm/25.0mm specs described elsewhere.

 

Am I doing this right?

 

[KFunk]

I've read about various methods for measuring the fuel level in the bowl, but am still curious as to EXACTLY how and where the measurement is being taken, and the range of acceptable values.

 

In the photo below, I've assumed I should measure from flat surface C to the upper surface of the fuel, but suppose it could be from A or B.  I've also found differing values for measuring the height of the fuel using this method, including 25.0mm, 25.0mm +4.5mm (whatever that might mean), and 29.0mm +/- 0.5mm.  The distance from flat surface C to the upper surface of the fuel in my DCOEs measures 28.75mm with plastic floats set according to the 12.5mm/25.0mm specs described elsewhere.

 

Am I doing this right?

 

 

I'm pretty sure you are doing it right.  From 'C' seemed like the only reasonable place for me, and it fixed my stumbling issues.  The instructions I read came from 'dapbmw', and he used an acrylic rod with a housing around it, and the housing would rest on the surface while the rod extended to touch the fuel.  The only way that would work, is if the housing is resting on 'C'. 

 

I'd have to do another search, but the 29mm (edit: nope, its 25), plus or minus 1mm, sounds like the proper spec.  Any variation near 4 to 5mm sounds like you'd be way off, so I wouldn't trust that other spec.  If I'm just 1.5mm too high or too low, I get stumbling.

Edited August 27, 2013 by KFunk

[KFunk]

OK, here's what I've always done:

 

Begin by setting the fuel level. If the fuel level is not set correctly, it will have an adverse effect on the ultimate state of tune you will be able to achieve. I’ll explain why it is so crucial later when discussing the main jet circuit. The standard procedure involves gapping the floats to the top cover. Unfortunately, this method is error prone. The floats are buoyant in a pool of fuel and should be measured as such in-situation. Construct a simple optical gauge for this wet level measurement. This tool works much like a dipstick. The split nylon sleeve clamps gently onto the acrylic rod, this allows it to slide freely along the rod but not slip. Rest the sleeve on the top surface of the well by removing one of the main, air and emlusion jet assembly and slowly push the acrylic rod down. When the bottom end of the rod just touches the fuel it will make the top end look dark (look carefully!). When it does, measure the tip to sleeve distance. Ideally, this should be done while the engine is idling and level. If the
engine is in a poor state of tune and shuddering severely it can be done within sixty seconds after shutting it off.

The desired level is predetermined by the design of
the DCOE carburetor. The passageway that connects the well to the auxiliary venturi is 23mm below the mating surface of the cover plate. The fuel level must be 2mm below that point or fuel will spill into throat when accelerating or braking. Therefore, the ideal fuel level is 25mm +/-1mm.
 

Edited August 27, 2013 by KFunk

[williamggruff]

OK, here's what I've always done:

KFunk,

Thank you for your detailed reply. According to that method, my fuel level is 3.75mm too low. I'll try adjusting the floats.

[KFunk]

KFunk,

Thank you for your detailed reply. According to that method, my fuel level is 3.75mm too low. I'll try adjusting the floats.

 

Just remember, if it's a tad too high, you'll know it.  The fuel will spill into the auxiliary venturi when you're accelerating or cornering hard, and it acts like the fuel is suddenly being cut-off or a rev-limiter is kicking in, but in reality its just intermittently flooding the engine. 

[KFunk]

Try a fuel pressure regulator.

 

Also check your float levels.

 

Shouldn't do that.

 

Believe it or not, G's can increase/decrease your fuel pressure a surprising amount,

which is why having the regulator under the hood's good practice.

Mine's in the trunk, and you can SEE pressure change under hard braking.

Some under acceleration, but not nearly so dramatically.

 

t

Oh, and thanks for the help Toby, but how many people you got locked in the trunk watching your fuel pressure?    Heh.