tuning weber 28/36

[johnc]

Hi all,
I'm struggling to get my Rootes 1725cc engine fitted with a Weber 28/36 carb running anything like smoothly. It starts reasonably well from cold with some choke but when warm it will not tickover.I have tried adjusting the idle misture screw in varying degrees but some choke is still need to maintain even a 'lumpy' idle.The carb has been cleaned,a service kit fitted and has a new pancake type filter.
Any input would be appreciated.
regards
johnc

[Grease Monkey]

Hi Johnc,
I should start from scratch, set up the points, timing & spark plugs first, also check the idle jets aren't blocked,these are located adjacent to the foat chamber on either side of the carb behind a couple of flat headed screws, then when engine is thoroughly warm turn your attentions to the carb,firstly screw in the mixture screw fully and then undo 2 full turns, this should give a good starting point, then adjust the throttle stop screw to get an acceptable idle speed and then 'tweak' the mixture until idle is smooth, the engine speed should vary when adjusting mixture, adjust it till it runs 'sweetly'.
I have a a Weber 28/36 on my Rapier and it's a lovely carb, runs as sweet as a nut.
I hope this helps.


John Simpson.

[megadethmaniac]

foat chamber ????

Just how do you get the Laughing smilie?

[johnc]

Thanks for that John,
I too have a Series IV Rapier,I've fitted a new electronic distributor,coil,plugs and leads.I'll recheck the timing and then have a go at your recommended way of setting the carb.
I'll let you know how I get on.
Regards
John
Saga continues...been trying to set the timing.It doesn't seem possible to set static timing as the electronic distributor doesn't enable the connection of a test lamp to the LT feed.The rev counter doesn't work (a problem which I have read can occur when an electronic distributor is fitted) so setting dynamic timing is a bit hap hazard.However reasonable running can be acheived but timing is around TDC + 25 degrees on a fast tick over.Setting the carb as suggested helps but even accelaration is cannot be obtained. I've had one thought though,for reasons I will not bore you with I have had to fit the exhaust using only the back box with a straight through pipe from the manifold,could this be having any effect on the timing/mixture issues.
Suggestions anyone?
John

[johnc]

My edit to my last post has highlighted other issues regarding timing with an electronic distirbuor and non-functioning rev counter.
Any help would be appreciated.
Regards
John

[Luxobarge]

Do you really mean TDC +25 degrees? This is 25 degrees retarded, I'd expect something more like TDC -25 degrees, that would be advanced. Set the timing up so it's advanced and runs the best you can get, then do a run under load (i.e. up a hill) and keep advancing it until the engine pinks. Then back it off until it just stops pinking and you're there. Note this setting for future reference.

If it still doesn't run right you've got problems elsewhere - I'd start with a compression test.

[Grease Monkey]

You need to check the timing with a strobe light, it should be 7 to 9 degrees before TDC at about 900rpm, with vacuum disconnected. This is the proper way to do it, not moving the distributor and trying till it 'pinks', the timing figures are given for a reason, so it's best to use them!
As to your problem with the tacho, I'm assuming you've changed to negative earth because of the fact you have electronic ignition, have you changed the rev counter? Original series IV Rapiers were positive earth.

John Simpson.

[Luxobarge]

You need to check the timing with a strobe light, it should be 7 to 9 degrees before TDC at about 900rpm, with vacuum disconnected. This is the proper way to do it, not moving the distributor and trying till it 'pinks', the timing figures are given for a reason, so it's best to use them!

Ah, but the timing figures in "the book" were originally devised when the fuel used was significantly different to that available today. Modern fuels are more volatile and burn faster than the fuels used in the 60's and 70's, and it is this burn speed that is critical in ignition timing. As the flame front in the combustion chamber is faster, it doesn't require as much timing advance as they might have done originally, and so setting it by the book can - sometimes - possibly - give too much advance, which can cause pinking and potential engine damage. I can't speak for the Rootes engines specifically, but my classics run considerably better doing it the "pinking" way than they do on the book setting, and I know that many others have the same experience. Mind you, the Midget also has some engine modifications, so that could very likely further exacerbate the issue of not running perfectly on the book settings.

I do agree though that setting it up to the book figure won't be far off and it should run perfectly satisafactorily like this as a decent starting point, so if you do that and still have a problem, I'd suggest that the problem is somehting other than timing, and needs sorting before you mess around with the "pinking" method, if you do decide to do this.

Cheers!

[johnc]

Hi guys,
I've made some progress.With stobe light I have set timing at 11 degrees,this is at fast tickover,not sure of exact revs as tacho doesn't work.
However,steady tickover and smooth acceleration can only be acheived with about 6mm of choke,no matter what adustments I try with the idle mixture screw I can't get a steady tickover or smooth accelartion. Could the exhaust mod refered to in my earlier post be of any relevance.?
I've had a look at the tacho (which is neg.earth) and the wiring has been modified by the previous owner who convereted to neg. earth.There are two wires on the same spade terminal,one from the coil and one from the ign.switch;another screw post terminal has a small plastic connector block receiving two wires,one from the ign.warning light and one from the ign.switch.The way the block fits to the post leaves it insulated from the terminal I've tried making a direct connection with the two wires no movement of needle but a few sparks so they are live (with engine running of course).Any ideas how to sort this? I wouldn't be adverse to rewiring the feeds to the tacho if someone could let me have a diagram.
P.S road testing is not an option as there is no interior,no doors etc.etc.so the local gendarmes would have a field day.

Regards
John

[Grease Monkey]

I've been using a strobe light for setting up the ignition timing on all my cars & when tuning customers cars for years, I've never ever had a problem with 'pinking', motor companies didn't spend thousands of pounds adjusting their engines to run efficiently and economically for nothing, manufacturers settings are not guess work, they've been devoloped over the years. Just because an engine is not 'pinking' by the 'adjust and try it' method doesn't mean it's running at it's best, if the engine is set to spec and it pinks try a higher octane fuel, and not the crap they sell at supermarkets, a reputable brand, if that doesn't work then use an octane booster additive.
The problem with the Rapier Weber carb must be that it is jetted incorrectly, it may need larger main jets, different idle jets and pump jets. What's the carburettor off? it may be totally unsuitable for your needs, the only way to get the full potential is to have it set up on a rolling road, but beware (speaks from experience ) find one that's sympathetic to your needs.
I wouldn't have thought the mods to your exhaust would make for such problems.
As for your tacho wiring, Google it, there's usually a diagram to help.


John Simpson.