[Fot] Fuel issue

Fri Aug 9 17:14:50 MDT 2024

Andrew,

Is it possible that a carburetor piston is sticking in the up position
off and on?  This problem is exacerbated by a hot engine, and it is hard to
find after you stop driving because unless it gets stuck tight it will drop
back down with vibration.

Bob Kramer

On Fri, Aug 9, 2024 at 4:47 PM Andrew Devenish via Fot <fot at autox.team.net>
wrote:

> Hi Mike,
>           The first question is a good one. The car was built for
> autocross by a guy in Indiana who knew his stuff. However, I bought it from
> a guy who had bought it to run PIB, but knew nothing about race engines. He
> had run it lean and holed #4. He told me he had not changed anything, but
> that is turning out not to be the case. (I suspect it holed the piston
> because I found he had fitted gros jets with the float level way too low).
>         I rebuilt it to same mechanical spec. It ran great but a little
> lean on the AAU needles for road course, so I’m now on AAA. It still runs
> great, unless it doesn’t 😁.
>         The car has had this issue since I built it. At first I thought
> the obvious - float heights, needle/seat, regulator pressure etc etc but
> while it always runs great I have this issues after a few “laps”.
>       2)   The car had a Summit racing 3136 pump which has around 90GPH
> and an internal blow off at 7PSI, my regulator is at 4 PSI. Today I had
> taken the pump apart and checked it but all looked OK. I fitted a cheap
> solenoid pump in its place. Issue still there.
> I had checked both the Summit and now the solenoid pump for flow, there is
> plenty.
>       As I noted the float bowls are full when it quits running.
>     3) Great point - could be what I have. I know the fuel level in the
> jets is fine when not running, but a poor flow from bowl to jet would
> change that when running. My O2 is in the collector, so I can’t be sure if
> it’s both carbs or just one. All 4 plugs look very similar, but that is
> possible as this “lean” condition is only for a few cycles before it stops
> - not enough to color the plug. Will check when I can ( unfortunately a
> busy weekend coming up as I race sailboats too ). I’ve been thinking it is
> heat related, but it could just as easily be time. When I tried to run
> autocross it started to play up after 10 or so seconds of full throttle
> (which I can’t do in my yard). On autocross it would pull like crazy, then
> go very flat, and then come back on, repeat. Your idea would do just that !
>     4) always worth a check, but it would have to be something that shows
> up only when hot.
>     5) yes, compression good. Between 220 and 225 for all, measured on
> crank with throttles open.
>
>
>        Thanks also to the other responses that I received, I didn’t cc all
> on my replies in order to reduce the clutter…
>
> Thank you,
>       Andrew Devenish
>
> Sent from my iPad
>
> On Aug 9, 2024, at 3:51 PM, Mike Harmuth <ofracer at gmail.com> wrote:
>
> 
> Hi Andrew, a couple of things to check.
> 1st, is this a new problem? if so, what did you touch last?
>
> 2d Verify fuel flow, not just pressure i.e Gallons per hour? Disconnect
> the fuel line at the carbs, and stick it into a 1 quart or one gallon
> clean/clear bottle/jug. Turn the fuel pump on and time how long it takes to
> fill (1 Qt/ 1Gal whatever size container you're using).  If you're getting
> more than 20 gallons per hour, the flow is OK. Set the bottle aside and let
> it settle out, look for particles, rubber, fuel cell foam, dirt. If so, see
> where that's coming from. For example, old fuel cell foam will break down
> and clog the intake. When the pump turns off, the fuel in the lines drops
> back flushing the intake clear until next time. Fuel filter clean? Fuel
> pressure reg OK? Not just holding pressure (I run 3.5 on my HS4s) but
> actually sending fuel down the line? The gallons per hour test will
> indicate if you need to look into any of these areas, if you're gelling
> enough fuel, more down the list for now.
> link to flow and fuel requirements chart
> https://historicracinggroup.com/resources/Reference_Material/Canton/Fuel%20Consumption%20and%20Flow%20Quick%20Ref%20by%20CM%20Filters.pdf
>
> 3. A problem I had with an identical set up, was rubber hose breaking down
> and settling into the little box shaped well in the carb bowl where the
> line to the jet connects. It's happened to me twice now, last time was in
> June with fairly new hose feeding the carbs. It restricted the flow to the
> jet, even though the bowl was full. At high load, not enough fuel could get
> past the blockage.
> <IMG_20240630_112925914.jpg>
>
> 4. Vacuum leak- Is there a crack or loose bolt on the intake manifold?
> With the engine running, spray carb cleaner around the manifold to block,
> carbs to manifold. A change in engine RPM means something was drawn in
> through a gap.
>
> 5. Compression check- Are all the cylinders within 5-10% of each other?
>
> Happy hunting
>
> mike h
>
> On Fri, Aug 9, 2024 at 3:03 PM Andrew Devenish via Fot <fot at autox.team.net>
> wrote:
>
>> Guys,
>>           My car ( 1300, 12.5:1 comp on Sunoco 110 race gas, dual HS4 )
>> is still giving me grief. I’ve chased all the things I can think of, but
>> still same problem. It starts and runs great, but after a few laps of the
>> yard ( 5-6, only 1000ft  or so per up/down lap, so quick accels then
>> braking to turn ) it goes very lean ( I have O2 sensor ) and stops. I’m
>> running it without the hood for these test runs. It always stops at the
>> entry to a turn.
>>          I’ve been working on it a lot, so know it has fuel pressure, and
>> the float bowls are full of fuel. All plugs look very similar. The needles
>> (AAA) are the spring type, and the slides move freely. I use ATF as fluid,
>> but have tried 50 weight too.
>>         After a minute or so it will fire back up, but only run enough to
>> get back to the shop before going lean again. If I pull out the choke it
>> will run, but is unstable.
>>        I have wrapped headers and a heat shield around the carbs but they
>> do still get hot.
>>
>>        Any ideas ?  Thinking to try some less volatile fuel as it seems
>> heat related and the Sunoco evaporates so fast !
>>
>>
>> Thank you,
>>       Andrew Devenish
>>
>> Sent from my iPad_______________________________________________
>> fot at autox.team.net
>>
>> http://www.fot-racing.com
>>
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