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Weber tuning - backfiring through carb!
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Author:  GT [ Thu May 10, 2007 7:52 pm ]
Post subject:  Weber tuning - backfiring through carb!
I have done a search, but found nothing conclusive. I need (I think) Weber advice.

My GT starts fine every time. In the morning when its cold it has a slight cough until it warms up (not very long at all).

When it's warm its a ripper! :D I reckon it goes HARD, and cruises OK too.

Sometimes, and it does seem erratic, it will misfire and backfire through the carb like a bitch. Did it on the dyno last Saturday. Those who were there would know what I am talking about. Only in low revs, under acceleration. Under heavy acceleration it goes great!

I tried to do the timing, but PO said the flywheel was lightened, and it seems the timing marks have been machined off, so I have no real idea if the timing is correct.

When I pull up after driving, it stinks. It smells like it's running rich. No black soot on the ass of the car, and no fouled plugs. Fuel consumption is a little high.

I have put in new plugs, leads, checked points. No change....

I tried making it leaner (adjusting mixture screws) and it coughed and pluttered like a bitch (no backfire through carb though).

So....the question I ask is this.

Is it carb (45 DCOE Weber) related? Is it just too rich? Will a change of jets fix it?

The jets in the Weber say this:

F16

and further down...

150

No idea what sort of jet they are. Where can I get others and how much?

I would love to take it to a pro for a propper tune up, but I started a new job this week and i am doing 10 hour days, so NO CHANCE of getting the car anywhere.

HELP!!
Author:  drmini in aust [ Thu May 10, 2007 7:55 pm ]
Post subject: 
It sounds like the pump jets may be blocked. Then the carb `sneezes' when you open the throttle.
Bring around and I'll have a look.
Or take to Greig on Sat.
Author:  mickmini [ Thu May 10, 2007 8:18 pm ]
Post subject: 
Greig is up at Morgan Park this w/e.

yeah it sounds like bad timing or possibly pump jets. mine was bitching and carrying on with the dud pulsar dizzy, and huge back fires. but once it had a good dizzy, the fuel was not getting through properly, and had to adjust the float and muck around for a bit. Once that was sorted, it started first time every time.....

oh and F16 is the emulsion tube, 150 is the main jet.

when Graham lightened my flywheel, he marked 0,5,10 and 30 degrees with a punch and scribe, so no excuses for me!

cheers
michael
Author:  GT [ Thu May 10, 2007 9:11 pm ]
Post subject: 
drmini in aust wrote:
It sounds like the pump jets may be blocked. Then the carb `sneezes' when you open the throttle.



Interesting you say that.

The car had been out of rego since 1999. It has not moved in all that time. The owner said he wanted to start it a few weeks before I went to look at it, and it would not fire up.

In short, HE SAID the tank of fuel went off, and clogged the tank and fuel lines and carb. He removed both tanks and tried to clean them out. He also blasted compressed air through the fuel line, and put a kit through the Weber.

On the weekend I removed the fuel tank guage float, and it was covered in a white powdery substance.

So.....maybe there is crap in the tank that is periodicly being sucked through the fuel line and clogging the jets?

I have fitted a filter, and it seems clear, but still.....
Author:  Blokeinamoke [ Thu May 10, 2007 9:17 pm ]
Post subject: 
it may be rich on the idle curcuit by being jetted a little rich for road use

If I remember correctly the idle screw only modifies the amount of premixed air/fuel not the actual fuel content at idle on a Weber. So turning it down doesnt lean it out it just reduces the total flow at idle without adjusting the butterfly.
Author:  1310/71 [ Thu May 10, 2007 10:15 pm ]
Post subject: 
Hardiman's in Bullecourt Ave Milperra distribute weber parts. They are very exxy to buy new though. They also do the redline manifolds.
KB
Author:  Mike_Byron [ Thu May 10, 2007 10:16 pm ]
Post subject: 
I bet there is a varnish of dried fuel all over the carby bits that is causing all sorts of fine tuning problems.

Strip, clean, new gasket and kit and then assess.

See what Doc says but the reason I am not a dcoe carby fan is that they are a fixed carby. Tuned for a particular purpose and not that flexible. Runs great with 4000 plus rpm but farts around at much less. You can probably get it running much better lower but will fart around over 4000. Or you can comprise all over with much better response down low but pay the price much higher up.

I once paid someone seven goats, three wives and a camel, to stand behind the pitwall all day at Oran Park and do nothing else but swap bits until we got something that felt right with the Anglia sports sedan. Younger brother Kevin also had to do the same thing with his P510 sports sedan.

Neither the then Dyno on Parramatta Road at Burwood (forgotten his name) or the then Howarth tuning at Winston Hills/Toongabbie got it right for Oran Park.

Remembered - his name was Frank Kleinig
Author:  pristic [ Thu May 10, 2007 10:18 pm ]
Post subject: 
Anton, the best way to tune a weber is to take it off and throw on a 1.75" SU haha... cmon, you and I both know you want to... stuff keeping the GT original ;)
Author:  drmini in aust [ Thu May 10, 2007 10:32 pm ]
Post subject: 
If there is any powder etc floating about the pump jets are the first thing that blocks. :cry:
Hey if you are going to play, come get this Weber book of mine so you have some idea!! :lol:
I don't need it, I'm a Dellorto man. 8) Runs fine up high, and down low. After 13 years, I've sussed it out.
Author:  GT [ Fri May 11, 2007 8:53 am ]
Post subject: 
Did a test this morning.

Was driving to work no probs. Car running well.

Turned on headlights. A minute or so later in traffic...cough, splutter, pop....

So I thought I had found the cause. Turned lights off, stopped for petrol.

After filling I had a bit of trouble starting, and when I drove off it was cough, splutter, pop....

After a minute of that it went back to being OK.

If the carb was flooding the motor, would it backfire through the carb?

Fuel consumption was 120 miles for the tank (25L). Thats the first tank of juice I have gone through since registering it.
Author:  miniron [ Fri May 11, 2007 1:11 pm ]
Post subject: 
:?
Low voltage at electric fuel pump? More so with lights on. Dodgy alternator?
If its been sitting for a while take off the Weber and clean it thoroughly, it only needs a few dirt particles, varnish etc to cause problems. Spitting back is usually caused by a lean mixture, poorly seating inlet valves or timing.
Warneford put out a tuning booklet that gives some starting points and guidelines. Weber also publish a parts list and basic tuning guide.
A Weber side draft is basically 3 carbs in one. Start carb, idle /progression carb and main carb.
Presumably its your 1275 GT so the F16 emulsion tube may not be the correct one. Most written material, books, Warneford info seem to indicate an F2 emulsion tube for 1275 motors and F16 for small bore motors but setting these up is one of the great black arts of the motor world.
If you don't get the emulsion tube right you are on a hiding to nothing. This is the key component for the main jet system. The emulsion tube comes in different diameters with differing location and numbers of air bleed holes. The air corrector jet in the top is for fine tuning the mixture at high RPM.
The fuel jet should be approx 4 times the choke size as a starting point.
If you dont get the progression set up correct you can end up with a "Hole" in the mixture between the idle jet system cutting out and the main jet system coming into play.
Probably the most difficult bit is getting the accelerator pump setup right to avoid the flat spot on acceleration. Remember you can get a flat spot from both too lean or too rich a mixture under acceleration. The usual way is to play around with the so called pump jets but this is NOT the correct way. The pump jet is misnamed and is actually the high speed power jet that just happens to also discharge the accelerator pump fuel. You need to play around with pump rods and springs etc. There are at least three different pump rod assemblies and several different pump springs as well as numerous pump bleed jets. These determine the quantity of fuel in the pump well and the rate at which it is discharged into the motor. The so called pump jet should only be used for fine tuning the pump discharge but should be set mainly from the high power point of view.
The other important bit is to realise that this carb was designed to feed one cylinder per barrel and in the A series its feeding 2 cylinders per barrel. The A series also apparently needs a short sharp shot of fuel from the accelerator pump.
Oops got a bit side tracked here.
Check the fuel pump and the electrics then clean the carb and go from there.

Edit: Bit more info
Make sure its not leaking fuel from the starting jets under the cover on the inlet manifold face, the bit with the choke lever. If the manifold is at more than a 5 deg angle (and most Ive seen are more than 5 deg to make them an easy fit for Minis) you can get fuel leakage from the starting jets and it drips onto the exhaust manifold. This is not a really good idea!!!!


RonR
Author:  GT [ Fri May 11, 2007 1:44 pm ]
Post subject: 
as always....a WEALTH of knowledge! I appreciate the trouble you go to when you answer questions.

I didn't realise that spitting through the carb could be a LEAN mixture. How does that work?

RE: electrics. The alternator light does seem to be faintly on while driving. It also faintly lights up with the indicators when they are on. If the headlights and indicators are on, it is worse.

Fuel pump is a new FACET thinggy. The PO put it in just before I bought the car, because he said the original SU electric pump was full of gump from the fuel going off, and the car wouldn't start.

So......could I have a combo of dirt still in teh tank / fuel line, and a suspect alternator?
Author:  Fat Boy Dave [ Tue Jun 02, 2009 2:53 pm ]
Post subject: 
Just to dredge up an old topic. How do you tune the car accurately without any timing marks?
Author:  awdmoke [ Tue Jun 02, 2009 3:07 pm ]
Post subject: 
Measure & make some timing marks

drmini in aust wrote:
I find TDC accurately with a pushrod in #1 plug hole and a dial indicator, then mark TDC on the front cover, and on the harmonic balancer.
Then I put 15* and 30* marks on the balancer pulley by measuring around the periphery with a thin 6" steel rule.

I run the motor to 4000+ rpm with the vac advance disconnected.
You WILL need a strobe timing light.

Most stock to mild Minis run well at 30* full advance.
Author:  minifreak1 [ Tue Jun 02, 2009 3:47 pm ]
Post subject: 
ive got a 45 dcoe.fited to a 1293 ..mild cam etc ...i was having problems as well with backfireing... once the filter caught on fire....ohhh what fun cant remember what the problem was but it was fixed after a good tune and dyno run....they altered all the stuff in the weber ..
im no weber exspert but here is what they wrote on the report...after they fixed it it ran great ....
will add it still coughed a little in the cold mornings..but after a warm up session it was fine.

report as follows.

chokes = 34
sec ven = 5 ? should be 3.5
mains = 155
air = 175
emul = f2
check ball = closed
squirters = 50
idel = 45f9
n/seat = 2.0...this could be 20...

i no nothing about what all this means....
would love to know ???
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