mcg
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| posted on 12/12/11 at 11:57 AM |
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Going wrong after 4-5 hard laps - carbs getting too hot perhaps?
Hi. I Did Donnington a few weeks back in my Formula 27, running 8v Vauxhall engine with fireblade carbs and standard points ignition.
Reliability is still an issue, and we are finding that after 4-5 hard laps, it conks out. Wait for a few minutes and it starts up ok implying that it
could be something to do with heat. Part way through the day, I met with Matt from Procomp (cheers for your advice BTW Matt!) prior to my last session
out on track who suggested refitting the heat sink on the module to make sure the heat is dissipating well. I didn’t have the resources to do that at
the time, but did move the module and coil so that they were in a high airflow area to aid cooling. Took the car out again, and after 4-5 hard laps it
conked out. However, we did notice that prior to it conking out, it suddenly starts going very lean as per the AFR reading. Wait 2 minutes, and it
starts fine so that I can drive it back to the pits.
As it is going very lean prior to conking out, someone has suggested to me that it could be due to the fuel in the carbs getting too hot and boiling,
and therefore not being able to serve the engine. I currently have an aluminium inlet manifold, with a normal gasget so wondered if it was conducting
too much heat to the carbs. He has suggested to me that I need to get a gasget that insulates the heat from the carbs.
Does this all sound plausible? If not has anyone any other suggestions?
Thanks in advance.
Matt
PS. Donnington is an awesome track!
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britishtrident
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| posted on 12/12/11 at 12:06 PM |
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Is the fuel tank vented properly?
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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minitici
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| posted on 12/12/11 at 12:07 PM |
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Could be your fuel tank is not properly vented and you are creating a vacuum which lowers the amount of fuel reaching the carbs which starts to weaken
the mixture and eventually stops the engine.
After sitting for a while the vacuum recedes and the car starts and runs as normal.
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minitici
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| posted on 12/12/11 at 12:07 PM |
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Doh! - beat me to it again....
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whitestu
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| posted on 12/12/11 at 12:23 PM |
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It is unlikely to be heat that is causing a problem with the carbs.
Do you have a spark after it stops?
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adithorp
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| posted on 12/12/11 at 12:25 PM |
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Sounds like poor tank venting to me as well.
It'd have to be very hot around the carbs to cause it to boil the float chambers dry under load. Usually it's more noticable at idle and
when the exhaust is the same side as the inlet. The Vaux 8v has the exhaust on the opposite side so unlikely.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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adithorp
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| posted on 12/12/11 at 12:27 PM |
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quote:
Originally posted by whitestu
It is unlikely to be heat that is causing a problem with the carbs.
Do you have a spark after it stops?
Deffinatly worth checking that as the Vaux ignition module was prome to failing and then starting again when given a few mins to cool. If it is,
eventually it'll fail completely.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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mcg
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| posted on 12/12/11 at 01:51 PM |
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I checked the vent on the fuel tank and it is definitly set to allow air to be drawn into the tank. It is a cheap ebay vent, so perhaps it isnt
performing well enough.
I didnt check the spark after it stopped but will do so next time. I will also check the fuel tank vacume as well if that happens.
Regarding the module, Matt (procomp) suggested that I re-mount it with new grease / adhesive that fully transfers the heat to the alluminium.
I am wondering if it would be sensible to get a new module and coil to be sure. Is it possible to get uprated versions of these or am I just looking
at new replacements of what I have got?
Thanks everyone for the suggestions.
Matt
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adithorp
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| posted on 12/12/11 at 02:21 PM |
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I'd get a new ignition module (not expensive) and try that.Have you got it mounted on the ally heatsink? It needs that. Make sure you use the
heat transfer gel that comes with the new module. Modules used to fail for fun and often came to use after the owner had replaced it themselves but
not used the gel and it'd failed again ("It's not the module 'cos I've done that" . Coil is less likely to be the
problem.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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mark chandler
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| posted on 12/12/11 at 05:34 PM |
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Bike carbs live in a pretty hostile location so I doubt if its heat, it is more likely that they could be freezing up !
Sounds odd but in the old days which a few of us remember you had to point the air intake at the exhaust manifold in winter to stop this, my GF had a
fiat panda this was awful on a run for this, fine around town but the open road in the winter....
Consider the carb as a a/c unit, the petrol is turned into a vapour by the carb so freezes 
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dlatch
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| posted on 12/12/11 at 05:49 PM |
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if its the ignition module i am thinking of i had the exact issue on my old mk3 astra sri
turned out to be the module was over heating and the car would just conk out like you had turned it off, new module i picked up had the alloy heat
sink fitted unlike the old unit.
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BaileyPerformance
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| posted on 16/12/11 at 09:32 PM |
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quote:
Originally posted by mcg
Hi. I Did Donnington a few weeks back in my Formula 27, running 8v Vauxhall engine with fireblade carbs and standard points ignition.
Reliability is still an issue, and we are finding that after 4-5 hard laps, it conks out. Wait for a few minutes and it starts up ok implying that it
could be something to do with heat. Part way through the day, I met with Matt from Procomp (cheers for your advice BTW Matt!) prior to my last session
out on track who suggested refitting the heat sink on the module to make sure the heat is dissipating well. I didn’t have the resources to do that at
the time, but did move the module and coil so that they were in a high airflow area to aid cooling. Took the car out again, and after 4-5 hard laps it
conked out. However, we did notice that prior to it conking out, it suddenly starts going very lean as per the AFR reading. Wait 2 minutes, and it
starts fine so that I can drive it back to the pits.
As it is going very lean prior to conking out, someone has suggested to me that it could be due to the fuel in the carbs getting too hot and boiling,
and therefore not being able to serve the engine. I currently have an aluminium inlet manifold, with a normal gasget so wondered if it was conducting
too much heat to the carbs. He has suggested to me that I need to get a gasget that insulates the heat from the carbs.
Does this all sound plausible? If not has anyone any other suggestions?
Thanks in advance.
Matt
PS. Donnington is an awesome track!
Hi, how can you have points ignition and an ignition amp? normally its one or the other - points or magnetic pickup with amp unit.
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MikeR
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| posted on 16/12/11 at 10:34 PM |
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If you get the problem again - jump out and open the fuel tank. If it goes whoosh then its the breather.
Could you also try driving without a bonnet? would mean lots more airflow and cooling so should rule that out.
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