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Discussion Starter #1 (Edited)
Weber jetting on hot 2L

Hi,

I am looking for some advie on where to get the necessary components idle,main and air correction jets for 48 side draft webbers.

My motor has just been rebuilt by PACE and I am now at the point of needing to book some dyno time and get it fine tuned. I will call Vin tommorow and suss out complete specs of my carbs as they stand. What I can say at this point is that the idle jets seem wrong, the car is a pig at idle and seems not to want to run till I get it into main jet territory (for AF measurements yet). I blows black smoke at idle, so figure the idle jets need to be pulled back a fair bit...maybe.

Has anyone on here got a similar spec engine to me that could give me a head start? Cams are 308/272 inlet, 308/262 exhaust, 11.6mm lift, 48mm intake valves, 40mm exhuast, 48 webbers, 11.8:1 compression.

In the mean time if anyone can point me in the direction of where to buy some jetting to play with, that would be great. I have been told to go to benincas, but have acess to a freinds dyno and am comfotable with tuning, so long as I have the parts.



Oh also, the bottom of the trumpet seems to weep fuel...normal??

Cheers
Mike
 

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Sorry, can't help with the jetting but if the carb weeps fuel it's ether a seal gone or the float valve isn't shutting the fuel off - possibly why its so rich at idle? Hope this helps, Toby
 

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.

I realise carbs are different but I know someone who made gains going from 48 to 50mm ITB's on a 1300cc engine.

Buzzed, has your distributor advance curve been altered yet? I ran a pretty reasonable 1300cc engine with quad 40mm Yamaha R1 carbs, hottish cams, cam gears, etc and the curve on it was fairly steep for a road car...BUT it made all the difference in how the car ran. As a rough idea on its' curve, base timing was 14 degrees, with 34 degrees max being reached by 5000rpm. The engine was by no means serious but made roughly 100hp atw and had a rock-solid idle at 1000rpm, due in most part to the ignition timing. It ran on pump fuel. What fuel will you be running? Whereabouts are you, I know one or two places local to me that can get the parts you need.
 

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Discussion Starter #7
Thanks for tips guys, I'll head out to shed and try and see if there is dripping at idle, the carbs do seem too wet and I am suspect the needle and seat not closing properly.

48 vales, yep they fit...with indexed spark plugs and an egine builder / machinest that knows what they are doing ;-)..Thanks to PACE.

If you read Jim's book, you'll note that the most HP comes from a PACE motor....with 48's. His curve also shows it makes good low down hp to boot. Proof is in the puding though and I'll let you know how I go on.

I'm armed with a few tuning bits so I'd better get cracking.

Thanks heaps for the webber link....very handy
 

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Discussion Starter #8
OK found my problem, got a leaky needle and seat it would seem. When I turn on the fuel pump I get one carb dry when I look down it and one dripping in both chambers.

Question is, can it be fixed on the spot...or do I need a new needle and seat (its bloody new any way!).
 

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Discussion Starter #9
So I pulled the seat out, put a rubber oring in the seat and a washer on top of that, and guess what...problem solved!

Well partly, now it runs without seriously fouling plugs...but it still wont idle, and more anoyingly, when you stick it in a gear, give it a bit and then go to grab another gear, it hangs up in the revs when you are in neutral...what causes this? Anyone?
 

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Discussion Starter #10
ok did a little old post reading and will check the following things:

idle screw (mine currently are screwed out 3/4 turn, but sounds like this should probably be more like 2 full turns.

tripple check the syncronisation (but was poretty carefull with this up front)

check for manifold vaccum leaks by using carby cleaner to see if idle is affected.

check again that linkages not binding (but suspect ok as went to a lot of effort to machine new linkages).

I checked fuel pressure and set it to 3.5psi with a malpassy fuel pressure reg. Fuel pump is a carter 8spi unit.


Will see how I go tommorow after work...too late at night for my poor nieghbours!

PS Appologies for those I may have offended by spelling weber incorrectly in my heading, it has now been fixed and I am lashing myself as we speak ;-)
 

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maybe reduce fuel pressure to 1.5-2 lbs. too much pressure will cause issues as well. from the sound of this engine, idle is limited, due to all of the mods... that is some of the give and take. Good luck
 

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Discussion Starter #12
I will try turning down the fuel pressure, but I figure that if the venturi is not dripping then the fuel pressure is not to blame?....

Anyway, I only had the idle screws turned out 180 degrees, so wound it out to a more respectable 2.5 turns, which seems to have made it easier to start and was no longer catching in the revs.

That said, its a bit hard to fine tune it when it stalls before i get a chance to get out of the bloody thing and hit it with a screw driver!...oh well, will fiddle some more then off to the dyno to see what can be done.

Appreciate that idle wont be perfect, but it does not want to idle full stop...at any revs...so I guess something is not right
 

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Discussion Starter #14
Yep, ignition curve is 10deg BTDC static and 33 deg total.

Well I am gettgin closer. Truns out it was the most basic of things, idle jets were wound out too much. That said, the idle jets may be a tad big as I only have to wind them out 1/8th of a turn to get a rather righ mixture, so will be looking at dropping a size and seeing what affect that has.


Some additional tuning needs to happen now on the dyno and I will keep all posted on the results.

cheers
Mike
 

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Discussion Starter #16
I popped the car on the dyno tonight and heres what I found:

running full exhaust, no air socks on trumpets:

180 mains
160 airs

10 deg advance, 33 locked 105rwkw's (141rwhp)
13:1 at 7500rpm at full load
12:1 just off idle, with dip in hp around 3500rpm due to very rich mixture

changed main to 175, 13.5:1 at 7500rpm at full load.
109.5rwkw's (147rwhp)

The curve it very rich down low and just about right up top, throttle response is good, but full load around 3500 will bog it, the pedal needs to be eased on.

The idle scrwes are turned out 1/8th of a turn and this makes me think that perhaps the idle jets could come down a size, but I am not sure if this is the correct logic.

Our feeling is to drop the air size down to 150 and drop the main jet down to 165 and see whether we can pull more fuel out of the 3500 rpm range. Our logic here is that smaller mains will drop the whole af curve tidying up the lower revs, whilst the smaller air will richen 4500rpm plus. But I newish to webers and am not sure if we are tackling this the right way. That said I have free access to the dyno and so its easy to see what is working for us and what is not.

I'd be very interested to hear any advice you have, my main aim at this point is to to try and lean it out down low whilst maintaining up top. I figure an air fuel ratio of about 13.5:1 is about right.

I have access to 100 octane fuel (it has 5% ethanol in it...I worked for Shell and helped put it into the market so I am familiar with its benefits and would like to exploit that). perhaps this will allow more static/total advance?

Let me know what people think
 

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Does your advance curve ramp up quite aggressively to start with? Or is it a more gradual curve? What are the AFR's at the point where it bogs down?

BTW...the engine sounds like it will be fun to drive! How much does your car weigh?
 

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Discussion Starter #18
Honesatly not sure what the curve looks like, I'll request it from Vin Sharpe as he built it.
The engine bogs a little where AF ratio is about 11:1...I know its too rich and at the moment am working on understnading how these rather complex little carburettors administer the fuel and air to try and correct this area of the curve.

So far, yeah, the engine is a lot of fun to drive, it appears to be very eager above 3500 rpm and certainly is starting to take shape, early days yet. The car has all interior striped. I plan to replace windos with lexan and install fibreglass bonnet and hatch, which should drop it a bit more. I am guessing weight must be around 1000kg at the moment, but will wiegh it soon. It is caged so that adds some back in..not to mention the driver (me) is 6foot six and 100kg.....bugger
 

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Honesatly not sure what the curve looks like, I'll request it from Vin Sharpe as he built it.
The engine bogs a little where AF ratio is about 11:1...I know its too rich and at the moment am working on understnading how these rather complex little carburettors administer the fuel and air to try and correct this area of the curve.

So far, yeah, the engine is a lot of fun to drive, it appears to be very eager above 3500 rpm and certainly is starting to take shape, early days yet. The car has all interior striped. I plan to replace windos with lexan and install fibreglass bonnet and hatch, which should drop it a bit more. I am guessing weight must be around 1000kg at the moment, but will wiegh it soon. It is caged so that adds some back in..not to mention the driver (me) is 6foot six and 100kg.....bugger
Are you going to stick a decent airbox on it? You will notice gains there. Sock filters on multi-bodies carbs tend to cost a little power. My car will weigh in at 910 with me in it....I can legally strip another 110kg out of it in my class...I know how to but it would cost a lot of money! It would be cheaper to fit a bigger engine and go up in classes and hence minimum weight! I think it will be fun as is, and I only wanted to get my bum back into a seat, really!

I have moulds here for the fibreglass bonnet and hatch - they came with the car.
 
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