Weber carb jet sizes

[Beej]

can anyone please tell me what the correct jet sizes are for a bog standard 1700 X-flow super 135 (supposedly) bhp. Its the idle jets that I am especially interested in. I can't get it to pull clean on a part throttle and I think a previous owner may have been mucking around with the jet sizes. Any clues?

[Graham Sewell]

The official sizes are:

choke 32

main 120

air 165

emulsion f16

idle 45F9

 

according to Dave Andrew's program (and proven on my car)

choke 32

main 130

air 180

emulsion f16

idle 45F9

 

Other issues include the number of progression holes (should be 3)

 

Light throttle resonse is always a little awkward as the ignition timing is not brilliant - but can only be improved with 3-d mapped systems which I am not prepared to go for (at this stage).

 

Low tech luddite - xflow and proud!

[Beej]

Thanks very much - thats really helpful. I can cross check this against the current settings. I strongly suspect they will be different - it just runs too lean all the time and spits like camel having a bad day at the office.

 

Cheers

 

[Beej]

Graham - it turns out that the idle jets are correct, BUT the chokes are 30s, the mains are 110 and the air is a max at 180. So I am not really surprised at it running lean but it doesn't explain the off idle problem. Any further thoughts?

[Graham Sewell]

Working the other way, from Dave's program:

 

For flexibility (rather than outright power) which should give good manners lower in the rev range:

 

choke 30

main 120

emulsion f9 or f16

airs 170

idle 45f9

 

for a 1700 engine with a 6000rpm red line. In which case, try getting onto Burton for some new air jets. It looks as though the previous owner has probably grown the engine from 1600 and not completely recalibtaed the carbs. Depending on what you are after, the cost could be very reasonable (<£10) or serious! New chokes are not cheap.

 

Low tech luddite - xflow and proud!

[Beej]

Cheers, I bought the car specifically for track days so, although tractability is important, I don't want to lose out on outright power. My everyday car does tractability quite well (420bhp) but its too darned expensive to run on the track. I had planned on changing the mains to 130s, and if that improves the balance I would change the chokes to 32s. I am not sure about the origins of the engine, it looks like a totally genuine, straight out of the factory 1700 with all the right bits but I wouldn't rule out the 1600 theory. Either way I had planned on transplanting a twink head this winter so I'll probably have to rejet it again!!

[Graham Sewell]

The 130 mains with 30 chokes will be over-rich. Remember it is the choke size that is the driving factor in setting up carbs. See Dave Andrew's Weber Site for more info.

 

Also his jetting program can be found here

 

Low tech luddite - xflow and proud!

 

Edited by - Graham Sewell on 21 Aug 2002 08:20:23

[Beej]

Graham, smee again! Need some more help.

 

OK - I ran it with the 130 mains and it ran ok - certainly not too rich but definitely still not right. It was still popping and f@rting at both ends.. which has totally confused me. I replaced the O rings with sandwich plates and reset the rubber bobbins at the same time. Everything else checks out ok.. BUT no matter how hard I tried it would not pull more than 5,000 rpm in 4th and dropping to 5th actually lost me speed. I was at least 20mph slower than most other cars down the back straight (still stuffed them thru the corners though!!!).

 

Since then I have changed the chokes to 32s and I can now get 5500 rpm in 4th (indicated 110mph but probabaly less) - with possibly more to come but it was really struggling - and getting very hot.

 

It is still spitting back at and off idle and clearly is not happy on the main circuit.

 

I assume you can spin your 1700 up to 7,000 in all gears? Any ideas?

 

Cheers

[Graham Sewell]

I doubt that I have got my car above 5000rpm in 4th due to the paranoia of losing my licence - I only drive the car on the road and not track so these speeds are not available to me. The only time hat I have taken my car upto 6200 in 4th was on a rolling road where drag does not count

 

Low tech luddite - xflow and proud!

[eric]

Hi, me too I had hudge problem with Webers

 

My car is a 1600 with A2 cam and I bought the Caterham Kit

Main 120 Air 180 F16 iddle 50F9 (richer than 45F9 but now as std)

pump jet 35, choke 30 mm

 

The problem is that my head is non std Unleaded stage 2 Head

and the car was poping from webers and exhaust (with flames ...)

 

On the point to revert to the single Weber, I found the solution :

the iddle jet was too small, now I have 60F8

and everything is perfect, the car is like a dream

 

even with a 55F8, it doesn't work

 

my jets are now :

main 115 air 180 F16 iddle 60F8 chokes 30mm pump 40 (Ifound better response)

spark plugs : NGK BP7ES (excellent), Aldon distributor 103FXYS

 

It is a road car with some track days from time to time to meet friends

 

From your side you could try to remove iddle jets for 55F8 or 60F8

You must adjust richness screw and see

 

I hope it can help

 

[Beej]

thanks eric, sounds like maybe my original idea (that the idle jets were too weak) was correct. I was a bit surprised to discover that the "correct" jets were fitted but because I assumed that Caterham did their homework when setting the carb specification I figured the problem was with something else. It may be that my engine is not exactly to Caterham 1700 supersprint spec and it does seem worth checking out a richer idle jet configuration.

 

That being said, I am not too concerned about how the car behaves at and off idle because I tend to drive it flat out 😬. So my primary concern was to understand why the engine would not pull more than 5000 - 5500 rpm in 4th gear (and less in 5th). I posted a comment on this on another thread and various people have commented that this seems normal for a 1700 SS - equates to an indicated 110mph max speed. If so, I may need more horsepower because my nearest tracks are Goodwood and Combe and I am used to hitting 135 - 140mph on the straights in my everyday car (although its obviously slower thru the corners).

 

Before I catch upgraditis big time I just wanted some reassurance that I was getting the best out of the current spec.

[Anthony]

Wow, 130-140 at Goodwood is going well.

 

SLR pulls c. 7 and a bit with 3.64 and 6 speed 130 ? down back straight

 

To pull 140 would need a good Vx or K-series ( or of course BD series / zetec )

 

Whats the road car ?

[Beej]

Sorry, I didn't mean to imply it was a 7 that would do 130 on the back straight - although there was a burgundy R500 that was definitely in that ball park last week. My road car is a Cerbera 4.5 - which, with 420bhp, is just plain cheating of course. But I hit 130mph in a bog standard 993 Carrera 4 last week so I just assumed that this was within the realms of possibility for a 7. I just have to reset my expectations and find some twistier circuits!! Problem with that approach is it presupposes I can actually drive well, and not just rely on power to weight ratio. O well, I guess I am just going to have to learn to driver well.

[Anthony]

Hi Beej, well that makes sense 420 lovely bhp.

 

I went round in a 02 996T ( with engine upgrade ) and that was pulling well over 145/150 at the end of back straight. No doubt the carbon phone which makes all the difference ( Sorry Pete )

 

Regards

[Beej]

I followed a 996 TT into work this morning.. awesome piece of kit. So where can I get a carbon fibre phone.. its obviously worth an extra 5mph? he he.

 

I'm back down there at the end of the month in both cars. It will be interesting to see what the cerby will do now the cats are off (apart from spit foot long blue flames out the back). I am such a child.

 

Of course, the truth is it really depends how good the driver is getting the power down through Lavant - in my case that probably explains why I am so off the pace - that AND the carbon phone.

[eric]

Hi Beej

 

Difficult to find the solution from Marseille without see the car but :

 

1) do you have tested your rev counter switching a strobo lamp

and in same time you can see if you have 21° advance at 2000 rev

and 34°/36° full advance at 5000

if not you have a worn distributor, a bad rev counter or bad settings

 

2) see also from the points of view of leads and sparks plugs

BP7ES are warm, cheap and you can try

1 and 2 are electrical causes for not reviing freely

 

3) As my grandma said if it is not electrical : it is fuel

What do you think about trottle linkage not correctly fit and so the carburators not open full when the throttle pedal is completely down

Could be also internal parts of the webers not allow them to open full

Also change the quality of fuel for super 98 unleaded (once to see)

 

4) Engine : see the valves clearances

 

See if you have friends who can give you another distributor or another

couple of weber to try

[Beej]

Thanks Eric, I don't mind bringing the car down to Marseille if it helps!!!!! (I wish)

 

Your grandma is a very wise woman.. I will go back and check the ignition advance, check the throttle linkages, valve clearances etc. Bear in mind I am running 4 star LRP - not unleaded. Also although I will happily try warmer plugs - I can't quite figure how this will help with the engine running lean and I am running the coolest plugs known to man - it was quite hard to get them actually.

 

I think ultimately the idle problem will be solved by richer idle jets and the full throttle performance will be solved by me just accepting the car won't go faster than 110mph!!

 

I guess I just want someone somewhere to tell me that their 1700 supersprint does 130mph so I can justify spending more money.

[Graham Sewell]

According to the car test program a Caterham Seven weighing 1525lb (with driver) with a 0.82 5th gear (typical I think) with 180bhp at 7600rpm and 135lbft torque at 6000rpm and a compression ratio of 10.2 would max out at 124mph.

 

So go figure how much you want to spend!!!!

 

Low tech luddite - xflow and proud!

[Beej]

Ah. Ahem, when you put it like that, I should really stop whingeing and be thankful for 110mph.

 

I'll get my coat.

 

Cheers Graham.