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Author Topic: Back-fires???  (Read 3996 times)
flew
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« on: October 15, 2008, 09:32:39 pm »

I'm having a small problem Grin

A 2,2 with a weber 34 that backfires when i give it some throttle. In idle it's fine Smiley

I had the same problem when I had my solex on, but swapped because I coudn't tune the CO for MOT.

The reason why it backfires is that there is alot unburned fuel (HC) in the exhaust, but don't know what the problem is.

Hopefully some can help me Huh
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krede
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« Reply #1 on: October 15, 2008, 10:05:08 pm »

Sounds to me like you have a leaking gasket somewhere at the INLET side.. If it sucks in "false" air, it will backfire through the carburettor.
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flew
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« Reply #2 on: October 15, 2008, 10:15:29 pm »

it doesn't backfire in the karb, but in the exhaust because of all the unburned fuel.

Can that be a caused by the intake-gasket???
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Anders Dinsen
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« Reply #3 on: October 16, 2008, 07:15:00 am »

it doesn't backfire in the karb, but in the exhaust because of all the unburned fuel.

Can that be a caused by the intake-gasket???

It could, but since what you have is not "backfires", but unburnt fuel exploding in the exhaust, this is a misfire problem. A misfire can be caused by too lean mixture, but more likely a too rich one, or even more likely, by ignition problems.  Since this also happened with your old carburettor, we can rule out the weber, but the manifold could have a leak near the cylinder head. The gasket is very thin and breaks easily if the manifold is not perfectly fitted against the head, and when it does, it is sucked into the engine and a big gap is quickly opened up. This can cause all kinds of running and starting problems, but they would be more likely to show up in idle and low revs than in higher revs, where the leak would have less significance.

So, I think you should check the ignition: Coil, leads, rotor, cover and the plugs. If all that looks perfect, take the distributor out for a stripdown. The Murena has electronic ignition, which is generally very reliable, but the mechanical components in the distributor can seize up and stop working as they should.

Here's a few photos of the internals of the Bosch distributor:
http://gallery.dinsen.net/v/biler/Murena/technical/dizzy/
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'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
suffolkpete
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« Reply #4 on: October 16, 2008, 08:30:02 am »

Before you take it all to bits, check the ignition timing and also do a compression test.  The problem can also be caused by leaking exhaust valves.
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lewisman
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« Reply #5 on: October 17, 2008, 12:30:43 am »

One way to check if the manifold is leaking at the gasket is to run a bit of light oil around the edge. The oil will be sucked in forming a seal for a few seconds.  Wathch out for leaks around the headlight vacuum pipe as well.

Having said that it sounds more likely to be an ignition fault so do as Anders suggests and check dizzy cap, leads and plugs.

Good luck!

David
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flew
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« Reply #6 on: October 17, 2008, 01:24:46 pm »

Did a compression test and it's fine. Did the oil around th manifold - nothing. Did the timimg - nothing.

Need at control of the rest of the ignition-system.

What's that about the headlight vacuum pipes?? When I have ignition on I can hear the thing "sucking/humming"
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flew
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« Reply #7 on: October 17, 2008, 03:55:42 pm »

NEW INFO Roll Eyes

When I turn on the headlight the engine runs alot smoother..

There are two hoses going from the intake-manifol.. One is going stright the headlamps the other is attatched with the "reserve-tank" and going the brake. It must be one of them that is messing with me Tongue

Can't understand the problem Huh
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Anders Dinsen
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« Reply #8 on: October 17, 2008, 05:27:59 pm »

Sounds like there's a small mixup of the hoses here.

The left vacuum takeoff should go directly to the one way valve on the brake booster. The right one should go to the one way valve on the back of the engine room. This is connected to a T-piece through a short fat hose. One leg of the T is connected to the top of the vacuum reservoir on the right side of the engine room, the other end to the vacuum valve on the left above the spare wheel.

This valve is a simple valve controlled by a solenoid. There's a metal piece fitted over the valve, and this can become depressed so the valve does not seal in either "up" or "down" positions. It sounds like your valve is leaking in the "down" position and not in the "up", and this could be because of the metal piece being slightly bent. Try working with it until it stops "hissing". If that doesn't fix your problem, you will have to look vacuum leaks elsewhere.

Vacuum leaks in the headlight system are not uncommon, so I suggest you seal the takeoff's off when diagnosing running problems on the engine as they can make it really difficult to diagnose running problems. I have a leak in the brake booster on my car, which I haven't yet diagnosed, but I think it's the booster itself and not the hoses.

- Anders
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'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
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