Weber question:

[southwind25]

This is my first set of webers...

we always ran S.U.'s forever.

(The ginetta isnt really running yet..)

 

Weber 40 dcoe carbs on a 1600 x-flow. mains 125, airs 180 idles 50f9's. nice headers and free muffler. engine is rather plain besides.

 

ok the exhaust pipe is way too black and you can smell the fumes on your jacket after a run. ((yes i know the exhaust outlet on the birkin needs to be moved from the rear to the side)). starts and warms up ok, with a bit of pedal till it's warm no big deal.

so we seem to be running rich on the top end a bit..but might wait till warmer weather to make sure.

 

the idle ...start of idle progression is whats confusing. mid throttle thru full seems to be fine. But at idle she farts and backfires more than i would expect. and it has a little hesitation when we first start to open up the butterflies...which would indicate to lean idle jets right?....but...

when we shut the car off...about 1/4 second then...BANG..we get a massive backfire out the exhaust, which is like i used to get in the old mazda rx7 rotary!//like it's dumping raw fuel into the exhaust? uhhh...

 

steep weber learning curve here... cant possibly afford ITB system for the x-flow.

 

any suggestions? or is this more normal than i am thinking?

[JohnCh]

My understanding is that a lean condition can also cause a backfire. The mixture is too lean to fire, hence raw fuel is pushed out on the exhaust stroke and ignited by the hot exhaust. Another thing to consider is you timing, which can result in incomplete combustion. A good online Weber tuning guide is located here.

 

-John

[Kitcat]

I have zero to suggest since I am a mechanical Neanderthal. But my car did all of those things and they were remedied by a tune-up on a chassis dyno at a local shop. In other words, it can be fixed, I just don't know how. But-hey I am giving you hope!

 

As I have mentioned in earlier posts, an earlier tune-up by a knowledgeable mechanic without a dyno got it about 1/2 right. The subsequent dyno tune-up showed I was running dangerously lean tho. After some rejetting and more dyno tuning, I made the same power (93 prancin horses) but it no longer backfired, spit, coughed, etc., etc., and was reasonably smooth at all rpms. Not exactly ECU smooth but vastly improved.

 

Unfortunately I don't know what jets were used or anything that might be helpful. Others commented back then who did have knowledge so a search mite uncover something helpful.

[lowflyer]

The idle air jets carry the engine from idle to the point where the main jet circuit kicks in

 

The 50 idles are probably too lean, and on the other hand the 125 mains may be too rich.

 

Lots of variables, not only the engine itself but also the altitude at which the car is running, water vapor content (humidity) and others.

 

Try to find someone that has some extra Weber jets and see if a set of #60 idle air jets fix the problem. I suggest the use of "loaner" jets because it gets very expensive to buy 4 or each jet just to try it out. As posted earlier, a dyno with a air/fuel ratio meter would be a big help but still wouldn't give you more than a "direction" in which to go.

 

Also might think about 115 mains. As I recall 115's were stock on the Lotus Cortina I used to have. A lot depends on the set-up of your engine (cam timing, vavle size, port size, ignition timing etc).

 

There's a very good pair of books out on the Webers. One is the theory, the other is the practical. As I recall the practical (how to rebuild) had a listing in the back that showed the stock jets used on various cars that came stock with Webers, including things like the Lotus twin cam Elan/Cortina. Might be very useful.

 

I have the books around here, somewhere and if I can find them I will post some of the stock "fitments". :jonautox:

[lowflyer]

O.K., found the books.

 

First, if you're interested the books are "Weber Carburettors by John Passini" Book #1 is "Theory" and #2 is Tuning and Maintenance". They were published by Speedsport Publishing in England, in 1973. See... it pays to never throw anything away. Probably out of print, but available somewhere.

 

I had forgotten a basic test that can be easily done to determine if the idle circuit is lean or rich.

 

First warm the engine up thoroughly. Next set the engine idle, using the idle speed screw on the linkage to around 1300-1400 rpm. Now turn all of your idle mixture screws out 1/2 turn and listen to the engine - rougher or smoother?

 

If it is rougher, try another 1/2 turn, rougher or smoother? If still rough, turn them back in 1 full turn, to your original starting spot, then 1/2 turn further in. Rougher or smoother?

 

At this point, you should see a pattern becoming apparent. If it runs smoother with the idle mixture screws backed out then you're too lean. If it runs better with the screws turned in, then you're too rich.

 

This won't tell you how much of a correction you need, however. There, sad to say, it's mostly trial and error.

 

Here are the specs on a couple of "stock" engines that use 40 DCOEs.

 

Lotus Cortina twin cam 1600 cc

30 mm choke, F11 emulsion tube, 115 main, 200 air, 45F6 idle

 

Lotus 7 with a 1500 non-xflow, and A2 cam

33 mm choke, F16 emulsion tube, 115 main, 160 air, 45F9 idle

 

Volvo 122S, which I think is a 2 liter.

32 mm choke, F9 emulsion tube, 125 main, 175 air and 50F9 idle.

 

BTW, a couple of cautions... first the number on a main or air correction jet are logical, in that a 115 is smaller than a 120 or 125. When it comes to the emulsion tubes the numbers have more to do with the order in which they were created. An F9 is richer than a F18. Idle jets are numbered by size, but don't necessarily have the hole in the same place. An 45F6 is the same size but has different characteristics than a 45F9. I suspect that the idle speed test will give you a direction, then ask your jet supplier for one or two sizes richer/leaner than what you have now.

 

Hope this helps a little, and good luck.

 

 

[southwind25]

all input is greatly appreciated..and yes i will try this. I have the BIG weber book and the jets i have are very close to what a 1600 for x-flow is listed for. What i need a copy of is the theory book. Thanks though we are a sponge soaking up all the info we can.

[bball7754]

Other books you might consider are "Weber Carburetors" by Pat Braden and "How to Build & Power Tune Weber and Dellorto Carburetors" by Des Hammill.

 

My 1700 Crossflow has the Kent 244 cam and head porting. The Weber 40's have 120 Main, F16 Emulsion, 165 Air Correction, and 50F9 Idle. Haven't checked to see what the Choke size is. To my somewhat uneducated foot, it doesn't have any real flat spots (although things don't really get interesting until about 4000 rpm) and starts easily.

 

I've been told that changing things on Webers can be a slippery slope. Good luck!

 

Steve

[xflow7]

All the mentions of 1600 and 1700 xflows in Caterhams I've ever seen have run either 45F9 or 50F9. If you feel like idle is a little lean, you could try going to a 50F9. Another approach I've seen mentioned (but not seen any results reported for) is to go to a richer F# (air drilling) in the same family. Note F#'s are not logically numbered.

 

F9 is sort of 3/4 the way from lean to rich and I think F12 and F16 are both richer. So maybe 45F12 or 45F16.

 

For reference, I have summaries of a couple of jettings used in the 1600 Classic spec from Caterham (100bhp):

 

Setup 1

---------

Choke 30mm

Aux 4.5mm

Main 110

Air 180

Emul. F16

Idle 50F9

Pump 35

 

Setup 2

---------

Choke 30mm

Aux 4.5mm

Main 120

Air 215

Emul F16

Idle 45F9

Pump 35

[southwind25]

The 190 airs seemed to help this morning...have to give her a good run tonight to feel (and check plugs) if anything really changed.

 

Wish we had access to those rolling road dyno's the state of ohio e-check (emmissions) centers have. they have like 8 of them per station and only use 3 on any day...

[stevet]

I have a set of jets out of Webber 40's at home - can't remember what size they are, but I'm probably not going to need them so am happy to send if you want them to play with.

 

I used to do my set up based on the notes posted by lowflyer. I used a length or rubber tubing (one end in the trumpet, the other in your ear) to balance them on sound - which proved to be quite accurate, even if it did get some funny looks from the neighbors!

 

PM me with an address if you want the jets.

 

Steve.

[Ian7]

I'm dealing with similar richness and low rpm acceleration 'pops' and hesitation in a 1600 x-flow w/ twin 40DCOE.

 

Curiously, I have 32mm chokes as delivered from Caterham, not the 30's everyone else seems to have or recommend.

Anyone think I'm wasting my time trying to tune these 32's? better to get 30's and start over?

 

thx

[lowflyer]

The 32's should be fine, but will need the rest of the carb set up for them.

 

If it is running correctly in some areas, why not just adjust the parts of the carb that need adjusting.

 

I had a twin cam Lotus motor and put 32's into it, even though it started out with 30's. Just needed some enrichment in the mid to high range.

 

:cheers:

[horizenjob]

 

Sanity checking the basics would be a good idea. Timing was mentioned. Make sure they are sync'ed at idle. They make cute little airflow meters for this, or toxic mercury column type things or if nothing else try the hose - listening method mentioned.

 

Has the car been sitting over the winter or some other time period of months? That might mean some blocked idle or progression holes - most common thing I find.

 

Have you always had this problem?

 

Looks, from your picture, like your aircleaner sits out in the airstream. That can cause a little bit of fuel haze to be blown back onto your clothes. Since the exhaust exits rear, that shouldn't get on your clothes so much.

 

The dangerous thing is you can't assume from the color of your exhaust pipe that the motor is not dangerously lean at high speed for example. You can easily have both or multiple problems. That's why dyno shops are so helpful.

 

If it's a Birkin, does that mean it's jetted for foreign fuel? Just wondering if you think it was jetted by people who knew what they

were doing doing, like the factory with many motors or is this a custom?

 

The UK sites might be helpful, should be many people willing to share setups. I hope you get this to work well, I really like those old xflow motors...

 

 

[Ian7]

thx horizonjob:

 

 

Timing was mentioned. DONE

Make sure they are sync'ed at idle. DONE

 

Has the car been sitting over the winter or some other time period of months? YES

That might mean some blocked idle or progression holes - most common thing I find. WILL CHECK

 

Have you always had this problem? YES, FROM NEW, TWO ENGINE BUILDS BY TWO DIFFERENT PEOPLE - SAME RESULT

 

If it's a Birkin, does that mean it's jetted for foreign fuel? Just wondering if you think it was jetted by people who knew what they

were doing doing, like the factory with many motors or is this a custom? IT'S A 2002 CATERHAM, WITH JETTING AS RECEIVED FROM CATERHAM (ENGLAND, NOT USA)

 

 

[fnmag58]

So my set-up is 110 mains, 180 air corrector, F16 emulsion, and 45F9 Idle. This is the way it came from Caterham for my 1600 Xflow. Seems to run fine up to about 2500-2700 RPM and then it starts backfiring thru the carbs. When i pull out the choke cable, it clears up completely. So my question is: Do i go up in size on the mains--115? 120? or do i go up in size on both the mains and the AC? I'm thinking i need to go to 115 mains and 200 AC? Thoughts?