10 replies to this topic

[Dom_Joly]

Well, my mini has a 1310cc 12H engine, 286MD kent Cam,, Maniflow LCB exhaust manifold, 2.8cc race head gastket, R/C 9.3 : 1 and Weber 45-45 DCOE carburator with 5" inlet intake manifold, rubber o-ring gasket and 2.5" rampipes .


I have some problems with the weber, it`s all adjusted for the engine , but the cylinder 1 and 4 doesn`t work at idle and progression throttle, i spend too many hours trying to solve that problem, in the car and looking for some solution on the net, but is almost impossible to fix, the carb works perfect at medium and full speed , really fast car .

I change the idle jet, first it had 55F8 jet and now the 55 f6, but the problem on low range stays !!
No air leaks
The head is a STD one 12g940
All the valves are opening and closing properly
New and dry spark plugs


Yesterday i change the DCOE for a SU HS4 and the engine works great !

I have 2 inlet intakes, a swan-necked and steel 5" , with both it`s the same, but with the 5" of length it works better but of all ways they fail the cylinders of the ends.

What can i do ? anyone had the same problem with a weber ?

Edited by Dom_Joly, 05 October 2007 - 09:31 AM.

[Dan]

It's nomally what you'd associate with a stubby manifold but with a 5" steel it should work fine. What make is it? What's your valve train like? Your head being completely standard won't help the flow much, this may be the problem as you have a bigger engine and an inlet setup that can flow more but are choking it up with standard valves. You say it's a standard 940 but which type? 12G940 is just a casting number, not a head build type. Have you tried running it on the 5" manifold without the stubstacks? Just to see if slowing the flow helps with distribution. You shouldn't really need one but a scatter cam would help.

Edited by Dan, 05 October 2007 - 12:40 PM.

[Dom_Joly]

It's nomally what you'd associate with a stubby manifold but with a 5" steel it should work fine. What make is it? What's your valve train like? Your head being completely standard won't help the flow much, this may be the problem as you have a bigger engine and an inlet setup that can flow more but are choking it up with standard valves. You say it's a standard 940 but which type? 12G940 is just a casting number, not a head build type. Have you tried running it on the 5" manifold without the stubstacks? Just to see if slowing the flow helps with distribution. You shouldn't really need one but a scatter cam would help.

Thank`s Dan , first the carb was without the stubstacks and it`s the same with stubstacks, the valave train is absolutely standard, complete engine belongs to MG 1300 and the inlet intake was made by me .

[Dan]

I do think that a 286 cam with a standard head and valve train is pushing it but it would be interesting to hear what others think.

Edited by Dan, 05 October 2007 - 11:43 PM.

[lunchboy]

Number 2 & 3 cylinders get more fuel at idle.Try a balance pipe between the branches of the manifold just below the flange to mount the carburettor.

[Dom_Joly]

I do think that a 286 cam with a standard head and valve train is pushing it but it would be interesting to hear what others think.

Hello Dan, you think that the problem is the head ? that is because the flow is given back ? but with a single carburetor HS4 and a standard inlet intake a problem it`s solved, the 4 cylinders work well, is strange. Of all ways I need to install it, I must work the head ?

Number 2 & 3 cylinders get more fuel at idle.Try a balance pipe between the branches of the manifold just below the flange to mount the carburettor.

Hello lunchboy, you say something similar to the twin HS2 inlet intake balance pipe ? how that it works ?, me I have always questioned it .


Thanks to both

Edited by Dom_Joly, 07 October 2007 - 09:08 AM.

[Dan]

Because SUs are variable choke, fixed depression carburettors using the manifold depression to vary the choke size both carbs in a pair must see exactly the same depression to keep the mixture strength the same in each all the time. They need a balance pipe between the manifold legs to achieve this. The Webber of course is a fixed choke carb and it's not so important to balance the depression at each choke, they are very rarely truly accurately metered anyway because they are not variable and that's why they are less fuel efficient than an SU. Webber inlets hardly ever have a balance pipe.

The SU will always work better at idle than a Weber, it's a better designed carburettor. The Weber you are using is a performance oriented race carb, the SU is designed for a road car. These Webers aren't great at dealing with light subtle differences in throttle and loading especially at the low end, they operate more like a fire hose.

You may understand this but I'll say it anyway. The problem you are having is charge stealing, the outer cylinders don't get enough charge because the inner cylinders rob it away thanks to the siamesed port design of the head and the timing of the engine. I am by no means an expert but I imagine that the restrictive standard head and the standard rockers keeping the valve durations and speeds low is going to reduce the gas speed through the manifold into the port. The 286 cam, being longer in duration is going to increase this problem and this cam is often said to be poor at idle for this reason. This contradicts what I suggested before but having a low gas speed over a long duration will not provide enough draught to get the distribution right. I think that's what I'm trying to say, it's difficult to explain this. I don't know how you've made the manifold but it could be this isn't helping either, a well shaped manifold is an important thing. It could just be that this setup simply doesn't work well and you will have to use a scatter cam, as I say I'm not an expert. Hopefully someone else who knows more will read your thread and offer an opinion. You might have to rename the thread though, not a lot of people know a great deal about Webers and simply naming the thread after the carb might put people off. I can rename it for you if you want.

[JetBLICK]

Try smaller chokes, a friend of mine had a similar problem i think on his 1380. When he first but the weber on it was an arse used to kangaroo, cough and splutter up to a point, then it would shoot off like a rocket. He took it straight to the rollers and they sorted it straight away, i'm pretty sure they used smaller chokes. Might have to check that tho

[Dom_Joly]

Because SUs are variable choke, fixed depression carburettors using the manifold depression to vary the choke size both carbs in a pair must see exactly the same depression to keep the mixture strength the same in each all the time. They need a balance pipe between the manifold legs to achieve this. The Webber of course is a fixed choke carb and it's not so important to balance the depression at each choke, they are very rarely truly accurately metered anyway because they are not variable and that's why they are less fuel efficient than an SU. Webber inlets hardly ever have a balance pipe.

The SU will always work better at idle than a Weber, it's a better designed carburettor. The Weber you are using is a performance oriented race carb, the SU is designed for a road car. These Webers aren't great at dealing with light subtle differences in throttle and loading especially at the low end, they operate more like a fire hose.

You may understand this but I'll say it anyway. The problem you are having is charge stealing, the outer cylinders don't get enough charge because the inner cylinders rob it away thanks to the siamesed port design of the head and the timing of the engine. I am by no means an expert but I imagine that the restrictive standard head and the standard rockers keeping the valve durations and speeds low is going to reduce the gas speed through the manifold into the port. The 286 cam, being longer in duration is going to increase this problem and this cam is often said to be poor at idle for this reason. This contradicts what I suggested before but having a low gas speed over a long duration will not provide enough draught to get the distribution right. I think that's what I'm trying to say, it's difficult to explain this. I don't know how you've made the manifold but it could be this isn't helping either, a well shaped manifold is an important thing. It could just be that this setup simply doesn't work well and you will have to use a scatter cam, as I say I'm not an expert. Hopefully someone else who knows more will read your thread and offer an opinion. You might have to rename the thread though, not a lot of people know a great deal about Webers and simply naming the thread after the carb might put people off. I can rename it for you if you want.

Thank you very much for that explanation Dan, you have left me much more calm.
Dan , now I feel bad because I`d not make the things better, I knew the influence of a head worked for that type of cam and carb, is very important, but I didn`t make it to finish the engine in short period of time because i needed the car in that moment.

Please rename the thread as better you consider it, thanks again Dan

Try smaller chokes, a friend of mine had a similar problem i think on his 1380. When he first but the weber on it was an arse used to kangaroo, cough and splutter up to a point, then it would shoot off like a rocket. He took it straight to the rollers and they sorted it straight away, i'm pretty sure they used smaller chokes. Might have to check that tho

Thanks Jetblick, you know, for some days i'd think the same, try with smaller chokes, now the carb has a 36 mm choke, do you think that is large for the engine setup ?
of all the ways i must to work on the head and change the config of that "headache", thanks for the advice

[JetBLICK]

I've no idea mate, just thought id mention it, as i say sounds like you may have a similar problem to what he had. I'm not too clued up on webers to be perfectly honest, my new engines going to be running on a weber, but i'm probly gonna have to get one of my mate's to come help me or show me how they work They seem to be far more complicated to set up than SU's which im used to

good luck tho

[lunchboy]

36mm chokes should be running approx. 145 mains,F2 or F16 emulation tubes,45 pumps,190 air correction jets,50 F8 or 50 F9 idles.Smaller chokes (34mm)with 130 mains will be more tractable in the lower rev range.Maybe try 200 airs with 34 chokes,all other jets should be near enough as a starting point.