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THE WEBER THREAD


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#26 jd lj

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Posted 11 October 2015 - 06:40 PM

If you drill out any jets make sure you grind of the numbers stamped on the jets to avoid confusion down the track.

#27 jd lj

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Posted 26 January 2016 - 07:57 AM

Tip: the choke and auxiliary ventruis retaining screws should only be done up using an 8mm socket (if using the genuine screws) held in your fingertips. Don't use a spanner etc only use finger pressure and locking tabs or lockwire. The retaining screws will locate in the recessed holes and secure the choke and auxiliaries in place that way. Too much pressure also distorts the auxiliary out of shape and makes removal difficult.

I've seen quite a few where these screws have been done up to tight and the bottom of the barrel broken off around the screw.

The same idea applies to when you're setting up the idle mixture screws, they should be only LIGHTLY seated before winding out to the desired amount. Otherwise they can break through and enlarge the hole into the barrel making setting the idle mixture difficult.

Remember these are just made out of aluminium not steel and don't need you putting all your weight behind any fasteners.

#28 tsn007

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Posted 03 February 2016 - 11:16 PM

will 40mm weber sidedrafts be ok on a mild 308 ?

or 

45mm be better or worse



#29 jd lj

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Posted 04 February 2016 - 11:25 AM

That may depend on how many you're using and the rev range you want etc.

#30 tsn007

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Posted 04 February 2016 - 07:33 PM

twin webers for street use



#31 jd lj

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Posted 07 February 2016 - 08:26 AM

Most of the information on selecting the correct size carbs is in relation to an IR set up.

It may be worth considering it to be 2 x 2500cc engines running a single dcoe each (depending on the manifold design).

For a street car I'd stay with a smallish choke size for your engine capacity. A larger one may help you achieve more top end power but you'll drastically reduce the drivability in the lower rev range which is where you realistically spend most of the time on the street. A larger choke will also reduce the signal strength at the auxiliary ventruis and can give a flat spot just prior to the main circuit kicking in, this is the biggest problem dcoe webers have.

I think 45dcoe's will be a better choice than 40's and probably something like 38mm chokes, tbc.

On a street driven 6cylinder 40mm dcoe's are actually a good choice, they're cheaper than 45's and a 40dcoe with a 34mm choke actually flows more than a 45dcoe with a 34mm choke (as tested in David Vizard's book "how to build horsepower vol 2,carburetors". Once you use a bigger choke then the 45dcoe starts flowing more. But on a street driven 202 34-35mm chokes are a big improvement over 36mm chokes.

Anyway, back to your situation. You need to work out a choke size for your engine capacity and I'd give it a top rpm of say 6000rpm whilst doing the calculations (it may rev harder than that but remember that smaller will be better on the street). The best airflow and atomisation combination occurs when the choke size is close to 80% of the butterfly size and the best working range is from 72-80%. (taken from the above mentioned book, page 101).

It shouldn't be too hard to find information on carb size selection for non IR set ups and then relate it to the above information.

If you get stuck, just ask.

James

#32 LC-GTR-1969

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Posted 07 February 2016 - 03:43 PM

Quick question... I have always believed that we should use choke sizes that are the most compatible with where the engines peak HP is...

 

However, I often read conflicting info from various weber tuning places... some refer to choosing chokes for max rpm (as opposed to max power rpm), however, this can and often is very different to where peak HP is.

 

My thoughts are to choose choke size to where the ideal airspeed is around where peak hp is, as this would create the best volumetric efficiency, and most tuned response. For example, my engine produces peak hp between 5800rpm and 6000rpm, yet revs cleanly till 7500rpm... power is dropping off at 7k though. Some would say choose chokes for 7k as this is max rpm, others, would say choose chokes for 6000rpm as this is max power point.

 

What do you think?



#33 jd lj

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Posted 07 February 2016 - 04:30 PM

That's a good point worth clarifying. My personal opinion is that you should choose a choke size to suit the rev range that you actually use.

For example when I bought my car it was being prepared for circuit racing and had 40mm chokes, it'd easily rev to around 7000rpm and make good power whilst doing so. However it was a pig below 3000rpm. I don't know the exact cam specs but from what I do know it makes peak power around 7500rpm going by the information I do have. Obviously this redline is not suitable for use on the street.

Initially I dropped down to 36mm chokes and found a huge improvement in drivability and I'm currently using 35mm chokes which is again a big improvement over the 36's. Whilst the engine may not rev to the rpm where maximum power is made I now have a much better car to drive in the rev range that I will use.

The signal strength at the auxiliary ventruis when using 36mm chokes is apparently only 50% of what it is when using 34mm chokes (as tested by David vizard). Poor auxiliary ventruis signal will delay the tip in point of the main circuit and cause a flat spot at this point. This flat spot can be reduced by using an overly large idle jet but that will cause its own problems of being too rich.

For an IR set up calculate the individual cylinder cc's x the max rpm you want, then divide that answer by 2600, then the square root of that last answer is a good start for your choke size after rounding down to the nearest full number.

The same formula should work if you want to choose a choke size to reach your cams peak power at the desired rpm's for that cam.

I hope all that makes sense.

#34 jd lj

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Posted 07 February 2016 - 05:24 PM

I've recently been wondering if using a smaller cam with the peak power at lower rpm's if my engine would make more power than it currently does with small chokes that don't allow it to reach the rpm's where my existing cam makes max power.

#35 EunUCh

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Posted 06 June 2018 - 08:21 AM

What we need to know for those of us that don't know is around about what rpm should the main circuit start to pull over as the transfer starts roll off and

what happens to the fuel level in the main jet well as rpm increases with the float level set as per specs, given that sometimes the fuel level gets changed

which does affect pull over and afr's ?

 

 



#36 jd lj

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Posted 09 June 2018 - 09:19 AM

To test what rpm's you need the main circuit to tip-in on a quiet street remove the emulsion tubes and go for a drive with light acceleration you should feel the engine start to cut out as the progression circuit drops off. Don't do this around traffic as your power will be limited.

The rpm's that this occurs will vary depending on the engine specs and dcoe models. On my car this happens around 2500 rpm IIRC, but my friends torana went over 4500 rpm's. However if it goes this high though it can sometimes continue to rev further even without the emulsion tube assemblies fitted if the vacuum level in the emulsion tube well is enough to raise the fuel level high enough to make the fuel start flowing down the auxiliary venturis passageways, it'll mean however that the fuel volume will not be metered as it normally would be by the main jets, emulsion tubes and air correctors. In this situation it would be better to plug up a set of main jets and do this test as this will stop any fuel flowing at all.

As for the fuel level question, as the rpm's rise so to does the vacuum signal acting on the emulsion tube well coming from the auxiliary venturis. The bottom of the auxiliary venturis passageways are 23mm down from the top of the emulsion tube towers, so as the vacuum level rises in the emulsion tube well the fuel level will rise due to the increased vacuum. If you have the fuel level set too low then it'll take a stronger vacuum signal (and higher rpm's to produce a sufficient signal) to raise the fuel level high enough for the fuel to start spilling over the auxiliary venturis passageways and make the main circuit tip in. With a higher fuel level the amount that the fuel has to rise before it spills over into the auxiliary venturis passageways is shorter so this will happen sooner. This is why the fuel level controls the tip-in timing of the main circuit.

The diameter of the emulsion tubes has an impact on the rate that the fuel can travel past it due to the amount of surface area it provides, a larger diameter E tube will slow down the rate of fuel travelling past it compared to a narrower tube. Therefore a fat diameter tube like a F16 will tip in later and leaner than a narrower tube like a f2. Keep in mind though that both these tubes have the bubbling holes situated high and dry above the fuel level and I don't recommend either of these, I've just mentioned these as these are common tubes that others may be familiar with.

Another factor influencing the tip in timing of the main circuit is if there are holes in the emulsion tubes above the fuel level then the air coming out of these holes from the air correctors will be supplying the vacuum demands of the auxiliary venturis vacuum signal before the fuel does. Fuel is 600 times denser than air on average and therefore the air will start flowing before the fuel.

The emulsion tubes still do need one hole above the fuel level otherwise there can be a siphoning problem when the throttles are closed suddenly causing the fuel to continue to flow. A 1mm hole is sufficient. This is basically the difference between f7 and f8 e tubes. Other than that they are both the same but the f7 doesn't have the anti siphoning hole and this also makes it a richer tube. Atleast with these tubes the bubbling holes are below the fuel level which means that they can actually do their job. Due to this these two tubes will tip in earlier than a f2 but will also be richer than a f2. This richness can be reduced without impacting on the timing by plugging some of the bubbling holes. Or you can just buy some much better designed tubes as I've mentioned in the other thread about tuning triple 45dcoe's.
James D

#37 EunUCh

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Posted 10 June 2018 - 10:21 AM

Nothing is ever as easy as what it sounds is it , choke size would probably play some role in main system start as well ?, bigger choke , less signal , later

start , smaller choke , early start ?.

So as the fuel is being raised in the well around the tube and drawn in early in the start of the main system , would it be right in saying that at some point in the rpm range that the reserve of fuel around the tube would start to get lower because of high vac. pulling fuel out , and that because the main jet is the only way fuel can re-fill the well there must be some point  where the fuel is being ripped through the main jet alone and air is controlled by the high speed air bleed in

this situation ?



#38 jd lj

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Posted 10 June 2018 - 07:03 PM

Yes, the choke size does play a role in the timing of the main circuit tipping in. As you described above a smaller choke will generate a stronger vacuum signal earlier than a larger one.

I believe that the main jets need to be sized sufficiently to be able to maintain the fuel level as the rpm's rise if it can't supply enough fuel to maintain the fuel level then that is when the mixture will start leaning out. Obviously we can't actually see what is happening inside the e-tube well when this is happening so we need to make an educated assumption. Keep in mind though that all std emulsion tubes tend to lean out at higher rpm's compared to what they were doing at lower rpm's. The VF emulsion tubes don't suffer from this problem and are capable of maintaining a steady AFR through the range of the main circuit.

James D




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