Weber 26 & 28 IMB Carbs

[Rusty's Uncle]

After last year's relatively successful project to get more power out of a 499ccs engine whilst keeping it looking pretty stadard on the outside one of the remaining jobs was to rebuild a carb as at the time I got impatient and just pulled a clean looking carb out of my garage and whacked it on the engine and it worked so just ran with it. The increased power thing worked and I could accelerate up hills & go faster but I had problems that are well documented in previous posts.
My experiment was to fit a 652 camshaft , fit enlarged (34mms) inlet valves to an unskimmed , unleaded
standard 499ccs head that has been fully ported, inlet port in head opened up from 26 to 30mms in line with the 126, 126 carb and exhaust. Idea being to get an engine that breathed better. It all works but not without its problems due to the power increase. So as part of the ongoing work I want a suitable 28 IMB carb for a "breathed upon" 499ccs engine. So I have four Weber 28 IMB carbs & two 26 IMB carbs in bits on my table. The mechanical rebuild is no problem but I am totaly out of my depth when it comes to jet sizes , etc. No problem I thought as I will pick a cross between the 500 Sport and 126 data then hope. Got more complicated when the Italian 28 IMB 1 , 3 & 5 carbs all had different jets then the FSM IMB 5 also had some variations . Some of the internals did not match the spec of any of the manuals plus some of the jets on a 500 carb are larger than the 126 

[damon500]

Hi Rusty,
Interesting post- I have done and am about to do the opposite to you... I have a 500 with 126 650 barrels and 500 Sport spec cam but still running the standard head and 26IMB10 carb.
Really the only way to set it up properly is to spend some time on a chassis dynomometer with somebody familliar with Weber Carbs- (with a collection of jets preferably) really your standard 28mm carb shouldn't be too far off- you could look at a smaller main jet and air correction jet if you run rich but there really isn't too much that you will need to (or can) play with.
I'm planning to put 650cc barrels on my Giardiniera although I will keep the 500 camshaft- i'm thinking of boring out the venturi to 28mm and using 126 jetting to start with- will be spending some time on a dyno once it is run in to help it run as sweetly as possible!

[GTMaster]

Jets were what i played around with when trying to get the carb to run leaner. It's still a bit rich on idle but that's because i reckon it has the bigger jet (50 instead of 45). If it's put onto an exhaust emission tester then finding the optimal jets should be easier. The only thing i miss from these carbs is an accelerator pump...

[Bambino]

The mechanical rebuild is no problem but I am totaly out of my depth when it comes to jet sizes , etc. No problem I thought as I will pick a cross between the 500 Sport and 126 data then hope. Got more complicated when the Italian 28 IMB 1 , 3 & 5 carbs all had different jets then the FSM IMB 5 also had some variations . Some of the internals did not match the spec of any of the manuals plus some of the jets on a 500 carb are larger than the 126 

You already know the answer to this puzzle. After years of fiddling with side draft Webers on old Alfas, the only way I can jet a carby properly is by using a dyno or 'rolling road'. It's a black art for which I do not pretend to have any great expertise, but I know guys who do

If you get it sorted, record the details - they may be of use again sometime.

Chris

[Rusty's Uncle]

Thanks for the thoughts guys , nothing like trying to push back the boundaries of that 50+ year old techology and have a car that will now go up hills without too much fuss  [-/
I have started an e-mail dialogue with the tech department at Webcon & the 1st response was to buy a bag of jets & go to a sourcerer to work their magic but with such a simple carb I am going to play with it myself for a while as there are not that many permutations with jets etc.
I will take notes if I can remember where I left my pencil 
Just going to check that tappets & timing are spot on before the next phase 

[Rusty's Uncle]

Engine now starts very well but not so good on low choke , low revs & tickover. I forgot to make a note of the jet set up I installed but I am going to remove it anyway for the next trial. So I used an Italian made Weber 28 IMB 5 body as a base . First step I fitted a new butterfly & spindle kit. I noticed that the screws that came with the kit for holding the butterfly in place had a cheese head (like the 126 carbs) but the spindle had countersunk holes (like the 500 carbs). Net result was , on full throttle , there were two big screw heads obstructing the air flow , not good.
Found a couple of original countersunk screws from a 500 carb & fitted those  [-/
Picture shows one of each screw in position.

[Rusty's Uncle]

Had a little think about what jet , etc set up I might try so I decided I would start at the beginning. By that I mean I fitted all jets and components exactly as a standard Fiat 500 carbbut in a 28 IMB. Out of interest although the main jet set up is totaly different on the 26 & 28 IMB carbs the jets themselves are interchangeable and therefore works both ways if you want to try a 126 main jet in a 500 carb. Also as I am not putting in a return fuel line at this moment I wanted a neat & secure way to block the carb fuel return outlet without permanently altering it. I found that I could tap the inside of the brass outlet with an M5 taper tap to give about six full threads then screww in a small grub screw that will lock into the taper then to finish it off pop a bleed nipple rubber cover over the top . Picture shows the tap in position in the outlet tube.
Carb is all ready for fitting , hopefully in the morning and the picture shows it along with the standard 500 inlet elbow and the 126 elbow which has a much better flow design.

[grubscrew]

Hi There.
Ummm,I was thinking.To throw a spanner in the works of the "standard" carb arrangement has any one thought about a twin choke weber ie 28/36.Certainly the initial port of the 28mm seems okay.Is there an adapter out there,or has anyone done the conversion as its a fairly stout arrangement?

[Rusty's Uncle]

Hi Gruby , not a spanner , there has been loads written on this forum on different carbs that have been fitted or considered if you search back. The carb that has been used a lot on tuned aircooled engines is the Weber 30 DIC twin choke. They are hard to get hold of but there was one on eBay last week. Next problem is that you need the manifold which will set you back the best part £100 for a little ally casting . It may have the same rectangular mounting flange as the one you mention but I have not had the chance to compare   [-/

[Rusty's Uncle]

I have now had about 50 miles of road testing over a period of 3 days & with the current carb I now have a pretty steady tick over and when fully warmed up quite smooth running through the range. I can tell that I have a bit more power than the completely standard engine but not as much as I had with different jets. This was pretty much to be expected as the 0.112 Fiat 500 main jet is the smallest I have & the Fiat 500 Sport used a 0.125 main jet. Other observations are that I do not need to use a lot of choke on cold start or a lot of throttle on hot start which kind of defies the laws of the Aircooled. The engine has major flat spots & coughs & splutters until it is well warmed up. The exhaust note also has that lovely "rounded" burble where it had a little more of a rasp with the bigger main jet. But on a flat road at about 50 mph it was so smooth it felt like you could drive all day on a cup full of petrol. I think that with the standard 126 carb it was running a little on the rich side & now I think it is a little lean. So I think that if the idle is fine & the car generaly runs OK then the 0.45 idle jet , starter jet & emulsion tube are OK . So that just leaves me with the Main Jet & Air Jet to play with. Phase 2 will start next week, the task is to retain that tickover but get back a little more of that rasp and power when you press the pedal  [-/

[Rusty's Uncle]

Went onto the next stage today. I am a bit of a slow learner but I think that I am now getting to understand & have a feel for the little IMB carbs. So I considered just upping the main jet size to 125 so it would be a 28 IMB jetted as a 500 Sport 26 IMB. Instead I removed the 235 air jet and replaced it with a 190 as fitted in the 28 IMB 5 on the 126 up to May '85 (but with a 115 main jet , I still have the 500 112 jet fitted. My theory being that the air jet sits on top of the emulsion tube and along with the main jet controls the fuel / air mixture and as the engine seemed to be running a little lean & down on power then a smaller air jet could be the way to go. Well it seems to work pretty well. First impressions are smooth running , nice tickover and more power . The noise pedal makes more noise and it goes up long and short steep hills a whole lot better. I will run with this set up for a while & hope to do a fuel consuption test before I try the next phase.
Thanks to Pete by the way for sending me a link to the idiots guide to how a carb works , he knows me too well  [-/

[dajwid]

Could you let me have the link to the idiots guide to how a carb works too. I have one in bits on my bench trying to work out what happens in these little boxes of mystery.

[Rusty's Uncle]

OK , if I have done this properly , not too good at this stuff , the link is shown below. It is very basic but it gets you thinking from first principles which is a good place to start plus there is a "spot the 126" bit in the footage 

http://www.youtube.com/watch?v=p95ZNM24A5w&feature=related

One of the carbs I messed with had been on an engine that had been out in the open for some time and was quite messy inside . It seemed to clean up OK but would not run on tickover. There are 3 very tiny holes that feed into the venturi by the butterfly flap and are fed from the Idle Jet. Did further cleaning then used an old electric guitar 1st string (about 10 thou) to prod the holes. Tickover now fine so worth checking if you have a car that has been standing for a long time or fitting a carb of unknown origin  [-/

[alapimba]

hello

can you tell me where is the idle jet in a 28 imb? I wanted to try change that jet as the idle on my car is too fast for me. Thanks

[Rusty's Uncle]

Hi I was going to ask you how your tickover was on the other thread. I can not do clever things with arrows on pictures so I have resorted to small sticky labels on a Weber 28 IMB 10 carb. The Idle jet is the one marked 1 . It is a fixed size jet , the IMB 1 & 3 carbs used a 0.45 jet and later models used a 0.50 jet. If you remove it you shpould be able to see the size marked on it. Label no 2 is where the Idle Mixture screw is. Some carbs were factory set then the hole plugged but you can remove the plug to adjust the idle mixture for smooth running. No 3 sticker is on the throttle screw that will be the main control for your tickover revs. Off the top of my head I tend to screw the idle mixture screw right in then wind it back one & a half turns. Then with the engine fully warmed up lower the engine revs on screw 3 as low as you can then adjust the idle mixture screw to get the sweetest running. If the engine revs increase as you adjust the mixture you can ease down screw 3 . Then keep playing until you are happy  [-/