mantisgb
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| posted on 9/4/11 at 05:38 PM |
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fuel pressure for x-flow engined 750mc racer
Hi there,
I've got a weird problem with my race car. It was recently rebuilt to meet the tech regs of the 750MC locost series.
At a track day at Brands last week I'm finding the engine stuttering and losing power in right handers, and after about 20 minutes it is
stuttering all the time then finally cutting out completely with some backfiring.
Fuel tank pick up is from the left and the tank was full at start of session (I happened twice at Brands track day, second time I pitted in time not
to red flag the session  ) , so I'm pretty sure its not a pick up issue. The engine won't fire at all for at least 30 minutes, but after
that time it seems to burst back into life fine. When cool, the engine idles quite nicely.
The carb has been in bits and air cleaned then reassembled.
The fuel pump is something I'm not sure about - just after the cutout the fuel pump is racing at about 8-10 clicks per second (sounds like... at
least it sounds faster and louder than normal). Anyway, I'm going to replace the fuel pump, but I'm not sure what to use. The guy I got
the car from has said that his engine builder recommends 3-4.5psi but his dyno guys recommend 6-7psi...! He is running his car this year with a facet
silvertop competition pump this year, and I'm wondering whether to replace like with like (it has a Facet 40105 at the moment) or go for
something with a little more ooomph.
I'm going to fit a clear filter in front of whatever pump to see if there is much crap coming out of the tank.
As usual, any advice gratefully received!
thanks, Keith
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norfolkluego
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| posted on 9/4/11 at 05:50 PM |
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Are you sure it's a pump problem before you go to the expense of swopping it.
I didn't know Facets went click, the Redtop one I had on my Crossflow made a constant humming noise, the bike pump I've got now is a
clicker.
It couldn't be the extra noise you hear from your pump is it sucking on air by any chance
[Edited on 9/4/11 by norfolkluego]
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Rob Palin
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| posted on 9/4/11 at 06:28 PM |
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Hi Keith,
This is a common problem with our cars and seems to have a broad range of different solutions that have worked for different people. Some people have
changed pumps, tweaked float levels etc, but in my case the solution was a new fuel tank with a sponge/foam filling and a feeder straw dipping into a
central sump. The tank it replaced had a rear- left-hand corner feed and internal baffles but it still suffered in prolonged right-handers or uphill
(Gerard's at Mallory and the main straight at Brands were my worst nemeses).
A pressure checker for the other end of the fuel line is a good idea, to see if the pump's getting a bit weak or if there's a blocked
filter or other resistance in the system.
The Facet 40105 pump should be ample (it's what I used for 6 years), and your engine builder's pressure recommendation should be fine. I
think I used about 4psi. 6-7psi is probably too much, but other people will have better advice on that than me.
It's also worth investing in a spare pump as well (with all the fittings ready to go), since although solid state pumps are pretty reliable, but
when they do fail they don't always give you any warning, and it's not necessarily a spare that other people might be able to donate in an
emergency.
Hope you get it sorted.
Rob
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MikeRJ
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| posted on 9/4/11 at 06:40 PM |
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Might want to check there is nothing loose in the tank that could be obstructing the fuel outlet.
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Bare
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| posted on 9/4/11 at 06:53 PM |
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Time to make and Install a 'Chicken Hopper' type intermediary tank/sump that the Fuel pump can reliably draw from.
Google for it.
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samuelgill
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| posted on 9/4/11 at 07:49 PM |
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I have had a similar problem in the past. I fitted a fuel pressure regulator and reduced the pressure getting to the carb. To much pressure can
overcome the floats in the carb (when going round corners) and fill the bowl with fuel, cause the carb to choke. I just kept winding the pressure down
until it was cured.
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JacksAvon
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| posted on 9/4/11 at 07:51 PM |
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so, for chicken hopper see swirl pot
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ernie
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| posted on 10/4/11 at 06:07 PM |
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I use a facet red top, when these go wrong, its not easily identifiable. I had one pack up during race, which gave me similar symptoms as yours. it
was a loose cable inside the pump body. Also at one meeting the pump was not giving full pressure. As said it's worth getting a spare just in
case
PANDE FIRE & SECURITY SYSTEMS
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Robster
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| posted on 12/4/11 at 04:35 PM |
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Hi Keith,
I experienced a similar problem last year, but only when exiting tight right hand corners, a few people have struggled with this before -
http://www.locostbuilders.co.uk/forum/28/viewthread.php?tid=148626.
My problem was nothing to do with fuel pickup - swirl pot in the tank, left hand pickup from tank, known reliable tank design from Procomp, pump is
working fine, fuel pressure was ok & consistent - and from asking around I'd determined likely cause was carb overfueling. I previously
tried adjusting the carb float levels & tweaking fuel pressure without much success.
I've now reset float levels and following Sam's advice have further reduced the fuel pressure. Unfortunately not been able to test this
year before the first race so will see how it goes at Brands this weekend (not that my problem has previously manifested itself around Brands, so
I'm keeping my fingers crosssed it doesn't start now!!)
Cheers, Rob.
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