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Backfire problem...

 
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alfafan
Alfasud


Joined: 16 Mar 2003
Posts: 83
Location: Coimbra - Portugal

PostPosted: Thu May 20, 2004 10:15 pm    Post subject: Backfire problem... Reply with quote

Hi

I have a problem with my 1991 33 1.5 Carb.
The idle speed is OK, 900 rpm.
But to have 900 rpm, I have to advance the distribuctor.
And when I accelerate to 2000 rpm and keep my foot there, the engine backfires...
Its not a bang, but a puf, paf, pof, puf....
They are small continuos pufs...
And when driving, it feels rough...
I tried everything, replace the distribuctor, HT leads, plugs, No oil on the water (clear), no water on the oil...
Help...
I´m lost...
Whats going on with the engine?
What can I do?

Regards

Carlos Oliveira
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Ben_nz
Gold Cloverleaf


Joined: 30 Sep 2003
Posts: 575
Location: Auckland, New Zealand

PostPosted: Fri May 21, 2004 8:54 am    Post subject: Reply with quote

Shouldn't you adjust the idle speed from the carbs instead of by advancing the ignition timing?
Maybe you should get a mechanic to try to tune it. It's important to have the carbs balanced and tuned properly or it seems to be easy to get rough running problems.
I think the car should only backfire (louder than pof paf puf, more like tink ta tink) when your foot's off the accelerator and you're engine braking.. Seems to work best at 2500 ~ 3500rpm..
Having said that, mine backfires sometimes in the mornings when it's idling with the choke on. Rolling Eyes
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lesthegringo
Alfasud


Joined: 23 Jul 2003
Posts: 97

PostPosted: Fri May 21, 2004 2:59 pm    Post subject: Distributor to adjust idle? Reply with quote

The distributor should be set using a timing light so that the ignition timing is correct, then the clamp bolted tight! The timing procedure can be found in any manual.

The carburettors have small adjusters 1) the butterfly stop screw, which allow the buttleflies to be kept open slightly, plus 2) idle air control screws that allow you to fine tune each individual cylinder for the correct balance and 3) idle mixture adjusting screws to allow fine tuning of the individual cylinder mixture. Just in case anyone is wondering why you want individual cylinder air adjustment, it is to take into account production tolerances, wear and slight twisting of the carburettor butterfly shaft. The amount of adjustment is very small, and only affects the carburettor at very small throttle openings.

The main butterfly stop screw should be adjusted to make the engine idle at between 900 and 1000 rpm.

The carbs should then be set using the idle air control screws to give the same amout of vaccuum per cylinder using a special vaccuum guage (or if you are competent enough a piece of tube), and then the individual mixture screws adjusted to give the hightest RPM. The butterfly stop screw should then be adjusted to bring the idle back within the correct range, and then the process repeated to make sure it is all correct.

This procedure will give a smooth idle, and you will probably find that the low speed running, like when in traffic, will be a lot nicer.

This all assumes that the ignition is correctly set, that you have not got a blocked up or restricted air filter, your spark plugs, leads and distributor cap and rotor are not shagged, and that the cam timing is all correct. Any of these can lead you a song and dance, and in my experience, many carburettor problems can be traced to ignition faults.

Les
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lesthegringo
Alfasud


Joined: 23 Jul 2003
Posts: 97

PostPosted: Fri May 21, 2004 3:05 pm    Post subject: correction... Reply with quote

sorry, that last bit should have read

"many supposed carburettor problems can be traced to ignition faults"

Les
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alfafan
Alfasud


Joined: 16 Mar 2003
Posts: 83
Location: Coimbra - Portugal

PostPosted: Fri May 21, 2004 3:28 pm    Post subject: Reply with quote

I Les

Thanks for your help.
I will try that at the weekend. Monday I will tell you if it is solved.
By the way, do you still live in Portugal?

Best Regards

Carlos Oliveira
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lesthegringo
Alfasud


Joined: 23 Jul 2003
Posts: 97

PostPosted: Sat May 22, 2004 7:34 am    Post subject: POrtugal Reply with quote

Yes, I do, Carlos

I have a complete 33 16v if you are interested in parts for an engine swap, or a donor car, plus some series 3 doors, and loads of spares

Les
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alfafan
Alfasud


Joined: 16 Mar 2003
Posts: 83
Location: Coimbra - Portugal

PostPosted: Thu May 27, 2004 11:16 pm    Post subject: Reply with quote

I Les

I did as you said and the problem is almost cured (stil a very, very, very little thing there).
I´m going to make same Km and I will check again.
This is my wife´s car, and since last september was stopped, because it had an head gasket problem.
So I took the engine out and replaced the head gaskets, water pump, cam belts, etc.
Yesterday, I went to the Portuguese Annual Inspection (English MOT) and it passed.
The inspector said it is very good. I tune the car my self without any special tool (just hearing the engine), and the CO2 was between 3.2 and 3.5.
Thanks again.

Best regards

Carlos Oliveira
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lesthegringo
Alfasud


Joined: 23 Jul 2003
Posts: 97

PostPosted: Sat May 29, 2004 12:17 pm    Post subject: Problem has gone away Reply with quote

Excellent, it's always nice to be able to help!

Les
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