Banging, cracking on start up

[pinky]

Hi, I am having a problem on start up,  the engine starts, but bangs cracks pops in the exaust pipe,

When the engine warms this stops,  I recon on to much air getting in some were, then when warm expands and stops 

Or could it be blocked jefs  in the carb 

Running on webber's, and 123 unit 

What do you think 

Thanks pink

 

 

[JohnD]

pinky,

Fuel burning in the exhaust pipe casing such effects cannot have fully burnt in the chamber.   So insufficient fuel due to blocked jets seems unlikely.   A very weak mixture might burn so slowly to cause that, but more likely IMHO is a missfire.    

John

[pinky]

Thanks John, listening to the engine it appears to be running ok, the banging is exstremly load until it warms up,

Could it be air getting in 

Cheers pink 

[richy_rich]

Does it misfire at higher revs, or just at cold idle?  As it's Weber carbs I assume you're not using any choke? 

As John says banging in the exhaust is usually unburned fuel, with Webers I'd guess that your idle settings are too rich.

[Nick Jones]

Has it always done this or is it a new thing?

Definitely banging in the exhaust or spitting back through the carb throats?

As it happens cold but not warm as Weber’s rarely have their chokes connected l’m thinking either spitting back or lean misfire in one or two cylinders with the unburned mix being lit in the exhaust by the firing cylinders. TR or DeTomasso?

[Neil T]

For it to come on just like that take your pick,

Misab plates/washer shot or not set correct

Rotor arm tracking 123 common problem

Coil

Leads

[PeterC]

Tracking in disy cap allowing a plug to fire when a valve is open ?

[pinky]

Thank you gentlemen

I will put to practice what you have suggested to tomorow, 

No choke used

When warm it stops 

This has just started, definatly in the exaust pipe and spitting back through the carbs

Been to bulie today, so havnt had a chance to have look 

Thank you for your advise 

Pink

[RedRooster]

My bet is rotor arm, is it a proper one or Chinese?

RR

[Nick Jones]

1 hour ago, RedRooster said:

My bet is rotor arm, is it a proper one or Chinese?

RR

Only happening cold....?  These are usually worse when hot.  Most ignition issues seem to be worse when hot.

I think the Misab plates / O-rings is a very good call.   Small air leak making it very lean when cold and a bit less lean when warm......

[pinky]

Thank you for all your replays,

Cleaned all the jets, 

Checked ht leads all ok 

Replaced the distributor cap, thing improved had 2 more bangs then ok, the cap is a buro 

Now maybe by this time it had wamed up, so I will try again tomorow, 

The first carburetor spits back, so maybe I need to replace the thackeray washers to 

Cheers Pink

[pinky]

My apologies, not the distributor cap I replaced the  rotor arm, it's a buro 

[pinky]

Well thank you for all your help ,

Did all the things you suggested,  nothing, so sat back and thought about it a few years ago I changed the plugs on a motorbike ,

Run for a while then stopped went over every thing, changed the plugs, bingo, sorted 

So I did this on the 6 changed all the plugs bingo running a treat now,  so what happens to the plug when it goes tits up  and is there any way of finding g wich plug it was 

Cheers pink,   thanks for your help 

[JohnD]

Which confirms several opinins above - misfire!  Unburnt fuel burning in the exhaust.    Ignited by hot exhaust, + glowing carbon from previous late burns.

John