Weber Carb Problem?

Post by **moggyman64** » Fri Oct 30, 2009 6:17 pm

Hi All,

I have a standard weber adf carb on my 2ltr Fiat twin cam, my problem is that I dont get the expected power out of the engine. The car feels flat and underpowered and uses more fuel than expected. Flat out I can only get 85mph, not much more that I used to get out of the 1098 moggy engne.

I have checked and adjusted the timing and all is ok.

I cant see any adjustment on the carb other than the "idle mixture screw" which only affects tick over.

I've had an idea and hopefully someone could shed some light on it.

There is a K&N filter on the carb and the original jets are still fitted, therefore, is there too much air being allowed/drawn into the carb which is out numbering the amount of fuel being allowed in through the original jets. The carb was on my 131 prior to the transplant into my traveller and ran fine, the only change was the addition of the K&N due to its smaller size and looks.

Can anyone tell me if I need to re-jet and if so, what do I need to fit and where can I source them.

All feedback and othe ideas will be appreciated.

PS>>All runing gear is fom the same donor car.

Regards

Keith

Post by **Mogwai** » Fri Oct 30, 2009 6:50 pm

If everything else checks out ok It could be that the carb jets need a good cleanout / float height check etc. I had a similar problem with a car i had which had a weber DGAS fitted & one of the emulsion tubes had a partial blockage which really messed up the running. after cleaning it was a different beast

Post by **MarkyB** » Fri Oct 30, 2009 8:35 pm

How much smaller is the K&N and were the instructions followed when it was last reoiled?
If too much oil has been used it wont flow well and if the end plate is too close to the throat of the carb it will be restrictive too.

Post by **moggyman64** » Fri Oct 30, 2009 9:38 pm

Hi Mogwai,
The jets were removed, cleaned and re-fited but no difference in performance. Do you know what size the original spec for the jets is?

How do I check the float height?

Cheers

Keith

Post by **moggyman64** » Fri Oct 30, 2009 9:42 pm

Hi MarkB

When I said that the filter was smaller, I mean it is smaller than the original fiat air filter housing which I assume was designed and calculated for air flow into the carb. The problem is not one of air restriction but totally the opposite.

Regards

Keith

Post by **bmcecosse** » Fri Oct 30, 2009 9:48 pm

Try it without the air filter! You can't have 'too much' air - the engine is an air pump and it needs all it can get - but the fuel supply has to be matched to that air flow.
This is what the SU carb is so good at ............

Post by **moggyman64** » Fri Oct 30, 2009 10:14 pm

hi bmcecosse

Thanks for the reply, as you say I cant have too much air as long as it is matched to fuel suppy, but that is my problem cos I dont know what size to increase the jet size to, any ideas.

Cheers

Keith

Post by **Mogwai** » Fri Oct 30, 2009 11:04 pm

If you pm me your email I could send you a scan of a few relevant pages from a databook I have

Post by **fsk658** » Sat Oct 31, 2009 9:27 pm

Hi Keith. I had the same problem with my Pick up fitted with a 1600 Twin cam. I tried new jets float level Timing and got so fed up with it after two years of bad running, I bought a new Weber 32/36 DGV carburetta although it cost me a lot of money it now seems well worth it. It had to go on a rolling road to set it up which cost more than the carb (90BHP at the wheel). The top speed on a private road was the needle down to the middle of the fuel guage with still more to go. It still has a very slight flat spot but sure does pull well. I would first recheck your timing try advancig a bit and then road test it. I thought I had got mine spot on but it was way out according to the rolling road readout. I now get about 35 mpg.

Post by **Wal** » Sun Nov 01, 2009 9:36 am

Keith,
you say the carb was on your 131 before transplant. If it worked fine on that I'd very much doubt that it's the carb.

Are you sure about the engine, they are 30 years old. I'd do a compression test and check out the ignition system to make sure it's firing on all 4, as you say you timed it up properly.

If you have any doubts about the K&N filter put the original one back on and see if it makes any difference.

If there is too much air and not enough fuel the car would be running lean (hesitation under throttle) and a simple check of the spark plugs will tell if this is true.

Regards,
Steven

Post by **moggyman64** » Sun Nov 01, 2009 3:56 pm

Wal,

As I said earlier the carb ran well before, the only change made was to the air filter. I dont have the original Fiat air filter housing to refit so cant try that. Th engine was rebuilt so poor compresion etc. is not the problem.
I have ordered new bigger jets to try and match air flow to the fuel flow into the carb. I dont think that I have a major problem its just bloody annoying when I cant just put my finger on whats wrong. I will try the jets for starters.

Cheers

Keith

Post by **moggyman64** » Sun Nov 01, 2009 3:59 pm

Hi FSK 658,

Hopefully I dont have to go down that road of having to change the carb but if I do, then thanks for the advice.

Cheers

Keith

Post by **MarkyB** » Sun Nov 01, 2009 4:21 pm

Why are you getting bigger jets?
It sounds more like over rich mixture, that bigger jets will just exacerbate.
Did you look at the plugs yet?

You could waste a lot of money buying jets that would be better spent on a rolling road session.

Post by **Mogwai** » Sun Nov 01, 2009 4:40 pm

If it was running perfectly in the 131 another point to look at is what has changed on the engine to fit into the Minor for example fuel delivery pressure & flow if using a different pump, or the exhaust maybe restrictive ?

Post by **moggyman64** » Sun Nov 01, 2009 5:11 pm

How much is a rolling road session? New jets are only £3 each, so no big loss there. The plugs are pale so that would point towards being lean and not rich. I have a 2" stainless exhaust fitted so I dont think that would restrict because the original was a smaller diameter, not sure but was probably 1 1/2 diameter. Fuel starvation I have thought about this but have fitted the original mechanical pump with new fuel lines and filters . The length of the fuel lines are slightly shorter than in the 131. When I said in my earlier post that the engine is using more fuel than expected I think this is because it is being mixed with too much air and is not burning effeciently, am I correct to think this, which is why I'm plannng on changing to bigger jets. Hmmm starting to get confused by some of the replies. Please keep the opinions coming cos your keeping me from cracking at the mo.

Cheers

Keith

Post by **bmcecosse** » Sun Nov 01, 2009 6:34 pm

If you had the jets out - did you note the sizes ? It's very unlikely any 'rolling road' place will have suitable alternative jets for your car - so unless you go along with a selection in your pocket - it will be wasted money having the RR session.

Post by **jonathon** » Sun Nov 01, 2009 7:18 pm

How have you resolved the flow and return pipework on the carb. Usually folk do away with the return pipe to the tank. Not sure if doing this has caused your issue. Like BM say's try running without the filter. You may need to trim the carb settings with a free flow filter rather than the original Fiat unit.

Post by **moggyman64** » Mon Nov 16, 2009 11:03 pm

I have gone up one jet size and problem solved, I have looked up the K&N website and it tells you to do this to overcome the poblem, thanks for all the comments,

Cheers

Keith

Post by **Kevin** » Tue Nov 17, 2009 4:12 pm

Glad you sorted it Keith but its a common problem with K&N being so efficient at flowing air.

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