Is this fuel starvation or something else?

[richierich]

I have a 2276 type 1 motor in my street /strip Oval.
- Carbs are 48 IDA's with standard float bowls;
- Facet Gold Flo electric fuel pump mounted under the tank;
- stock fuel line through tunnel then as photo below;
- Cam is a Web 86C;
- Have a Malpassi petrol king pressure regulator which is set to 3psi (my gauge has not been calibrated so this figure could be somewhat off and the real figure could indeed be slightly lower if anything);
- Ignition is a Pertronix billet distributor.

Here's the issue - when I floor it, at around 5800rpm it falls on its face. When I first had the motor built and dyno'd it made power to 7400rpm and I ran it to that a good few times so I know it's there. I had a Pertronix billet distributor in it at first and then changed to a CB Magnaspark - which is when this problem manifested itself. I have since changed back to Pertronix (not the one I had originally) and it's still doing this thing. It almost feels like the spark is being cut like a rev limiter (i do have a limiter but it's set at 7200). It will do this in all 4 gears.

In a long winded way, my question is, could this be fuel starvation or is that unlikely? Should i try bumping the fuel pressure up 0.5 psi at a time and see what effect it may have?

If not that, any idea what else is likely?

I am interested to know how quickly the bowls can be drained at wide open throttle aswell.



If anyone can give me any advice on this, I'd really appreciate it - it's quite a frustrating issue.

[HotStreetVw]

What kind of rev limiter is it?
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4inBhore
50 Split - 2724cc NA. Haltech injected
62 Notchback - 2542cc Turbo WIP

[Howard 111]

Is it the original gas tank?
_________________
1973 Karmann Ghia
Turbocharged, Fuel Injected
http://www.thesamba.com/vw/forum/viewtopic.php?t=531270

[modok]

If it feels like the spark is being cut, it probably is.

[Kauai/5]

To me, spark cutting out feels like a bucking horse, unless it gets cut out completely and then it’s as if someone tripped over the power cord.

Since you are using electronic ignitions and it’s happening with both, double check your grounds including your main strap from transaxle to pan. Voltage is very important for electronics, as soon as you’re out of the range of operating voltage things can get whacky.

What’s weird is that the behavior seems consistent which goes against it being an electronic gremlin.

You say it happens in all four gears at the same RPM?

What happens when you get out of throttle after hitting this wall? Does it recover and then cut out again if you get back into it?

Sometimes one of the easiest and most revealing tests is to attach a strobe/timing light to each wire including the coil wire and rev the motor up and hold it steady and watch for a murmur in the flashing pattern of the light. Aim it at a surface for it to reflect on, otherwise you might fry your brain and start acting crazy. When it’s flashing that fast, a miss really stands out as it disrupts the pattern. This might help you rule out ignition, although the problem could be occurring only when under load.

Also try swapping coils if you have a spare. It might be breaking down under load.

[Dauz]

Clutch could be slipping

[maui]

To me it no sound like fuel starvation, or ignition. I had the same symptoms with my 2500cc now and what I did was went up in venturies to 42s from 37s and jet accordingly and problem went away and I have stock fuel lines and 3.5 lb pressure. For me the small vents no can and my engine wen go flat quick. I only saying maybe you get the same problem.

[Kauai/5]

One other stupid thing to check is linkage and throttle cable. I see in the photo your cable has a lot of slack at rest and is somewhat kinked.

Will the motor rev all the way to 7K by hand at the linkage?

Are you getting full travel from pedal to cable to throttle arm?

Is your throttle cable in good condition and not frayed inside the tube?

Stupid possibility, but sometimes it’s the stupid stuff that’s not obvious so I’m just throwing it out there.

[[email protected]]

Yep...!

I ran one of the little Carter rotary pumps dry, that has more flow than a Facet, around 700ft. with a 2276 that had 44 HPMX’s. This was with a 5/16 line. Went to a Holley Red pump with a 3/8’s line, and had to go down a couple jet sizes in the carbs.

A friend picked up .36 at the track going from a Holley Blue with a 3/8 line to an A1000 with a 5/8 line.

[richierich]

Hi all - thanks for your suggestions so far.

The rev limiter is an Omex Clubman and the carbs have 42 vents in them.

Its weird to me how this started when I fitted the Magnaspark however, is still there now I've switched back to Pertronix. This kind of takes the coil issue out of it as that's the Magnaspark coil and the Pertronix coil and its done it with both.

[Pruneman99]

Does it fall flat instantly when you floor it, or does it pull for a second then fall off? If it was fuel starvation I think it would pull, then fall off as the bowls ran dry. Probably get some lean pop at the same time.

If it's falling on it's face instantly, I'd look into increasing pump squirters, lack of timing, or weak spark.

Just a few ideas.

[Howard 111]

And again I ask, is it the original gas tank?
_________________
1973 Karmann Ghia
Turbocharged, Fuel Injected
http://www.thesamba.com/vw/forum/viewtopic.php?t=531270

[richierich]

[richierich]

[j-dub]

Maybe leave the gas cap loose to see if that changes anything or bump the fuel pressure up as a test.
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"POLICE STATION TOILET STOLEN ... Cops have nothing to go on."

[Howard 111]

My guess is that the fuel tank is the problem. Over the years, it has scale and rust built up, and the screen at the bottom of the tank stops it all, but get's clogged up, thus causing the fuel starvation.

Remove the tank (after draining it of gas) and take it to a machine shop, and get it boiled out. Remove the outlet which contains a screen first.

Once that's done, use a gas tank sealer from Eastwood, and seal the tank.

Get a new outlet screen from Aircooled.net, put everything back in, and it will run like you want.
_________________
1973 Karmann Ghia
Turbocharged, Fuel Injected
http://www.thesamba.com/vw/forum/viewtopic.php?t=531270

[richierich]

[Bugsy61]

This is either a faulty rev limiter, or just by coincidence, the intake valve springs have failed, or gone soft. Disable the rev limiter and see if that cures it. This is not fuel starvation, or it would not recover after you shifted. It would stay flat.
Oh, could also be an ignition coil failing. As the demand for more sparks per second with increasing RPM occurs, the coil can have trouble keeping up. That also goes for current supplying the coil. Put a jumper positive wire from the battery to the coil positive to see if that helps. That will bypass ignition switch and harness, and any resistance that may be built up.

[67rustavenger]

I'm having the same issue with a Magna Spark 2 Digital distributor on my 2180 with 45 Dellorto carbs, 86b cam and Tim's Stage 2 heads.

It's a weird issue for sure. At about 5000 RPM the engine stumbles. And will recover after a shift to a higher gear. To me it feels like spark cutout.

What makes this even more strange. Is the fact that I had this distributor on my 1679 to learn. It didn't stumble at 5K RPM. But that engine had a hard time getting up that high.

I need to figure out how to data log the ignition system. Maybe the fault will show up in the log.

I'll try to get a data log tomorrow and see it that's where the problem lies.
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[nsracing]

What size venturis are in the IDAs? I would guess they are too big for the street.

For drag engines, you can go larger on the chokes. But for street use, gotta go smaller. Otherwise, you will lose all the pressure differential across the throats when you open them to atmospheric pressure instantly.

Put another way, you will stop pulling fuel when the pressure equalizes above and below the throttle plates. Hence the stumble. Gotta have that negative pressure to pull fuel and feed the motor.

My 2 cents.