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In a message dated 7/31/2003 2:48:21 AM Eastern Daylight Time,
rotary.coot@verizon.net writes:
> Trying to help a friend get his recently purchased 2nd
> >gen running. He has a street ported 13b with a Weber
> >copy side draft carb. The engine starts and runs fine
> >for about 3 minutes and then it starts
> >backfiring/popping (a little) and loses power and
> >quits. Let it sit for a while and it will start back
> >up and run OK again for another 3 minutes.
> >
> >I don't think it's the fuel pump cutting out since he
> >has it hardwired on and it seems to be putting out
> >plenty of flow.
> >
> >What else to look for?
> >
> >Thanks guys!
> >
> >Joa
> >
>
> It is fuel starvation, first of all you need 4 PSI and at least 14
> GPH fuel flow and it should run on a car.
> Also the passages in the Weber between the fuel inlet and the needle
> and seat are way to small to flow the fuel we need so you need to
> pull the top of the carb off and drill them out larger and leave out
> the strainer screen under the brass cap as it is to restrictive.
> Also run the biggest needle and seat they make and warning do not
> run the after market ones that are called a gross valve as they tend
> to stick in the closed position.
> With that it should run in a car but for the long high power climb
> outs that aircraft demand you will need to bump the pressure to 6 PSI.
> What I do is I have my main pump set at 4 PSI and my auxiliary pump
> set at 8 PSI and on takeoff and climb out I run both pumps for the
> safety aspect and to boost the pressure.
> I have found that for the best power I need to flow 18 GPH, 16 will
> do but there is more power at 18 GPH, if the fuel pressure is 4 PSI
> the flow drops below 14 GPH at full throttle it will starve the float
> bole.
> The best jetting that I have found for the 48 Weber is 190 main gas,
> 155 air and f-7 emulsion tubes.
> Ken Welter
>
>
What do you mean by won't run after 3 minutes? Are you talking about just
sitting still at idle? Are you talking about going down the road at 35 MPH?
I would want to know the choke size before changing things around. For just
driving down the road at 55 MPH you need maybe 20 HP or less. The above jetting
will support full throttle operation at the 240 HP at 8,500 RPM level. For
slow speed partial throttle, float height, fuel pressure and idle jet size are
in use. But if it runs OK for a while, then none of that should be the problem.
If it just will not go around the block, I would agree that it is probably
the last chance filter just above the needle seat. That is under the big brass
plug just inboard of the fuel inlet. Throw the filter away. Cut into the fuel
line near the carb and add a cheap aftermarket in line can filter. Maybe $4.00
at any parts store. You could watch inside of the bores with a light and a
mirror as it warms up, and see that the booster venturies don't start dripping.
This would indicate the float level being too high. An easy fix.
The secondary ignition would not be my first guess. It will run on either
one, and at low power settings (or idle) it is difficult to detect when one or
the other quits.
When we had full length boosters, we ran 160 mains (Fuel jets) 160 airs (air
correction jets, or as some say, air bleeds) on F7 emulsion tubes. And Kens 6
PSI on the fuel pressure.
Ken, most racers run the 3.5 or 4.0 Gross jet needle seats as they out flow
the same size Weber seats and I have never heard of the sticking problem. Maybe
because we shake them so hard that they just can't do it.
If it will do this at highway speeds right after start up, before it gets to
the three minute mark, then I would go after the last chance filter first.
The exhaust system is also suspect. A partially plugged reactor or crimped
pipe will act like that. This would be the winning malady if all of the engine
power slowly goes away just before it quits. Substitute a test pipe for the
reactor to test for that.
Fill us in with some more Data.
Welter has me thinking now, about the passage sizes. Next time the carb is
exposed I will take a look at that.
>
Lynn E. Hanover
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