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Any Carb experts out there?.

 
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Oggie
Green Cloverleaf


Joined: 06 Mar 2008
Posts: 811
Location: Whitehill

PostPosted: Mon Aug 24, 2015 8:14 pm    Post subject: Any Carb experts out there?. Reply with quote

Hi all, I recently took part in a few hillclimb runs at Shelsey Walsh at the weekend, 1st run great, second run suffering from a misfire which I believe is due to a hot engine causing a fueling issue. If I back off the accelerator for a few seconds it sorts itself out an I can gun it again.

1st Run Video. https://www.youtube.com/watch?v=GwQDJxxI_ok

2nd run with the misfire after 2nd corner. https://www.youtube.com/watch?v=3gxr-m5vGqE

anyway Im using 44Idf weber carbs and wondered if there are any experts as to the cause of the issue.
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paulhide
P4


Joined: 20 Dec 2003
Posts: 1607
Location: Oh Beautiful Billingham

PostPosted: Mon Aug 24, 2015 10:15 pm    Post subject: Reply with quote

Yeah, already watched your runs. Shame with the second one. What fuel are you using? Can you locate if there is a particular cylinder with the misfire? Does it always happen after hard cornering? Eliminate earthing problems of course.
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Oggie
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Joined: 06 Mar 2008
Posts: 811
Location: Whitehill

PostPosted: Tue Aug 25, 2015 8:52 pm    Post subject: Reply with quote

1st run was good no issues but was finding my feet so was a little cautious, I might have a sticking float needle which is not allowing the reservoir to fill enough which just might be due to the steep hill and left corner combo or could be the fuel line from the drivers side carb to the offside carb getting hot as it rests on the engine block causing vaporizing of the fuel.
I have the fuel pressure set just below 3 bar at the moment so can up it a little more to 3.5bar.
I must point out the issue has never shown up before nor has it driving 140 mile back home?.
Weber carb re-built kit on order so will try that as well and work my way back to the pump.
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paulhide
P4


Joined: 20 Dec 2003
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Location: Oh Beautiful Billingham

PostPosted: Tue Aug 25, 2015 11:07 pm    Post subject: Reply with quote

I reckon moving the fuel line back to the bulkhead might be a good idea??
I wonder how many G you were pulling? We had a misfire running round Redgate at donington years ago which was cured by filling the tank to more than the quarter-full we had!
Maybe some crap was moved about or wiring/leads?
Have certainly had sticking float needle jets in the past, though mine caused flooding of the carb rather than starvation. (though I've always had dellortos)
Increased bio-ethanol in modern standard fuels is not helpful.
Rebuild kit has to be a good move.
Good luck.
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Oggie
Green Cloverleaf


Joined: 06 Mar 2008
Posts: 811
Location: Whitehill

PostPosted: Sun Aug 30, 2015 5:05 pm    Post subject: Reply with quote

Well so far so good, found a couple of issues when re-building the carbs and also tuning them back in once I re-fitted them.
Issue number 1 was that ether sides float needles were different sizes
Issue 2 was the float levels in one set was a little low.
Issue 3 when the car was hitting the limiter the fuel pressure dwindles down to less than 0.5psi which was rectified by the regulator. it now sits at 2.8 psi no matter how much rpm is given.
Possible issue 4 with the Facet fuel pump so I still have to check the filter in it.

The car feels like its got a bit more mojo now there is fresh gaskets/accelerator pump diaphragm and has been easier to balance them up. Starts better also.
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ZeNiTh-PbArM
Alfa 33


Joined: 14 Mar 2003
Posts: 388
Location: Paris, France

PostPosted: Sun Aug 30, 2015 8:33 pm    Post subject: Reply with quote

Hi,

You may consider :
1. Fuel flow depends on rpm and engine load. If the pump doesn't provide enough flow, you can't detect it unless you actually drive the car (full throttle in 3rd gear) with a pressure gauge connected.
2. If you are running a Facet pump, ditch it. Those are unreliable at most. Use a low-pressure rotating in-tank feed pump from a VW, those will get you a good 120ltrs/min no matter what

regards,
zp
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Oggie
Green Cloverleaf


Joined: 06 Mar 2008
Posts: 811
Location: Whitehill

PostPosted: Sun Aug 30, 2015 10:00 pm    Post subject: Reply with quote

Hi ZP,
I have yet to test the car under-load but believe fuel pressure was the primary factor in the misfire when the engine was hot, it will be difficult to replicate the misfire as normal driving on the roads over here have never shown any sign of the problem.
On a plus point I have never had an issue with the Facet pump in the last 10 years and I do keep a spare , I have seen that RSfruitbat had a problem with reliability a few years back with a Facet pump, I'd like tohave a pumpthat is easy to get to and service, I have looked at other high flow pumps and the costs can be high, have you fitted a VW pump straight in to a 33 tank without modifications?. It had been mentioned that fitting a pump in the tank is a good idea but I think without an access panel it would be more hassle should it have a fault. I will stick with the set I have been using for the time being, if the fault shows up I will switch to a new bespoke alloy tank and consider a different pump.
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ZeNiTh-PbArM
Alfa 33


Joined: 14 Mar 2003
Posts: 388
Location: Paris, France

PostPosted: Mon Aug 31, 2015 7:48 am    Post subject: Reply with quote

Hi,

I have experienced fuel starvation symptoms in the past due to reaching the fuel pump limits. With carbs it can be quite tricky to understand because you need to stay at full load for more than a few seconds to drain the float chamber.
In other words, transient starvation can be undetected and yet cause running problems. Use a pressure gauge and/or a wideband O2 sensor to be sure.

I have successfully fitted rotary VDO fuel pumps to race cars and would not ever install a facet again. These pumps are inexpensive and last forever. I usually install two pumps per tank, one each side, to avoid air pickup under hard cornering when no foam is present in the tank. Never had a single complain nor failure on such a setup.

regards,
zp
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eagle3
Alfa 33


Joined: 11 Nov 2007
Posts: 402
Location: France

PostPosted: Tue Sep 01, 2015 1:03 pm    Post subject: Reply with quote

Nice work ZP Wink

I think I would have got a hernia trying that. Laughing
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