Please visit the Club's website https://www.dloc.org.uk/ to join. Visit https://www.dloc.org.uk/adhoc to DONATE towards the cost of the forum.
Please don't post someone's email address to avoid it being harvested by spambots and it's against GDPR regulations.
Always look at "ACTIVE TOPICS" to see all posts in date & time order as they are sometimes moved; or look at "Your Posts".
Please add Reg. nrs. when posting a photo or anything about a car as this will help searches. Don't add punctuation next to nr. as this negates search.
CHANGED YOUR EMAIL ADDRESS since registering?, click your username and check your address in User Control Panel, Profile, Account Settings.
If you want help to register, use "contact us" at page bottom for help.

spitting back through Carbs

bobtills
Posts: 186
Joined: Wed Sep 25, 2019 6:05 pm
Location: North Wales

Re: spitting back through Carbs

Post by bobtills »

I don't understand all this new fangled electronic stuff so maybe you are all right.

So I think the important thing is - is it really spitting back through the carbs? As in 'spitting back'?

Because to me, if it is actually spitting back through the carbs there must be some gas movement from inside the manifold to the outside air, through the carbs, and the only place this gas can come from is inside a cylinder and the only way out into the manifold and then through the carbs is through a badly sealing inlet valve.

And this might fit with it only happening at high revs if an inlet valve is sticking and doesn't shut in time, or bouncing because it has a broken spring.

I've only had my 250 since last autumn and haven't dug deep into it yet, but it looks like quite an easy job to take the rocker covers off and have a look …….

bobtills
Posts: 186
Joined: Wed Sep 25, 2019 6:05 pm
Location: North Wales

Re: spitting back through Carbs

Post by bobtills »

Or an exhaust valve and the manifold gasket has blown.

A.N.Other
Wide Man
Wide Man
Posts: 551
Joined: Tue Feb 16, 2016 12:45 pm
Location: Leeds

Re: spitting back through Carbs

Post by A.N.Other »

Another way it would spit back is if a spark was ignited while an inlet valve was open.
Colin,
I may be slow but I’m rough as well !

Ian Slade
Wise Man
Wise Man
Posts: 791
Joined: Sat Feb 13, 2016 9:54 am
Location: Sevilla Spain

Re: spitting back through Carbs

Post by Ian Slade »

Not quite true, you can have spitting back if the ignition fires before the inlet valve closes, either highly advanced, or very, very retarded, almost to the point of running backwards
Owner since the 70's, Genghis is slightly to my left.

Brian-H
Very Wise Man
Very Wise Man
Posts: 1095
Joined: Fri Aug 09, 2019 6:04 pm
Location: UK

Re: spitting back through Carbs

Post by Brian-H »

On a fixed valve timing 4-stroke SI engine, even when the valves are all working properly, and the valve timing is set properly, it is possible to get exhaust gasses in the inlet manifold (without an external EGR). This depends on the design of the valve overlap and is basically an internal EGR, and occurs if the inlet was designed to open early. However, AFAIK, it shouldn't happen at high RPM (because the gas flows in the manifolds and cylinders are not the same throughout the rev-range), but this may also be affected by throttle position and load on the engine.

With such an internal EGR design, if the mixture and spark timing and spark quality are all working properly, then there is no backfire. Strangely, if the mixture is slightly rich, you're also unlikely to get backfire. But if the mixture is lean, then the exhaust gas will be very hot, so it is possible for lean mixture in fixed valve timing with designed early opening to cause backfire in the inlet manifold. Even if the mixture is correct, a weak spark will cause the flame front to break up, and so you can also get some burning droplets back into the inlet in fixed valve timing with designed early opening.

So, having read through this thread, fuel mixture (and/or petrol quality), as well as spark quality, can cause backfire even when the valves are all working properly. But it depends on the way the overlap works on the engine in question.

Edit - if an engine has external EGR, if there are problems there, then you can also get backfire.

Sydsmith
Extremely Wise Man
Extremely Wise Man
Posts: 1369
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: spitting back through Carbs

Post by Sydsmith »

Thanks Brian, confirms my point in the second posting on this thread,

Sad as it will sound now, when I were a lad, we used to deliberately reduce the mixture on our old bangers to gain attention and get exactly that result, played hell with the exhaust system though :D

daimlersteve
Helpful Person
Helpful Person
Posts: 496
Joined: Wed Mar 09, 2016 11:20 am

Re: spitting back through Carbs

Post by daimlersteve »

Have you checked how much actual advance you are getting ? are the advance wts. fully touching while static ?
Steve

Brian-H
Very Wise Man
Very Wise Man
Posts: 1095
Joined: Fri Aug 09, 2019 6:04 pm
Location: UK

Re: spitting back through Carbs

Post by Brian-H »

Sydsmith wrote: Sun Mar 29, 2020 2:47 pm Thanks Brian, confirms my point in the second posting on this thread,

Sad as it will sound now, when I were a lad, we used to deliberately reduce the mixture on our old bangers to gain attention and get exactly that result, played hell with the exhaust system though :D
LOL the way I used to do it, was to turn the ignition off leaving the car in gear and throttle open, then turn the ignition back on. Had to be careful not to leave the ignition off too long otherwise the silencer could split open :shock:

But I think Kevin is talking about backfire in the inlet.

I had to refresh my memory by looking up the difference between fuel burning in the exhaust, and fuel burning in the inlet. I found this useful wikipefia description (note that I didn't use wikipedia until now) https://en.wikipedia.org/wiki/Back-fire

I'll quote from there
"Additionally, improperly adjusted carburetors that create a lean condition during acceleration can cause the air–fuel mixture to burn so slowly that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a popback. In this situation it is conceivable that there is a popback occurring in the intake manifold and exhaust manifold simultaneously.

In both cases (combustion occurring before and after the combustion chamber), the result is a sharp pop, which is colloquially referred to as a "backfire". However, for troubleshooting, engine mechanics more strictly define an ignition of fuel within the engine exhaust system as an "backfire", while a "popback" is this same process taking place in the induction system."


I've also heard that "backfire" refers to fire in the inlet, but when in the exhaust, it's called "afterfire".

The reason I wrote the earlier post, was to affirm for Kevin that, as well as problems with valve sealing or with ignition timing, it can just be mixture problems or spark quality problems.

bobtills
Posts: 186
Joined: Wed Sep 25, 2019 6:05 pm
Location: North Wales

Re: spitting back through Carbs

Post by bobtills »

Big Col wrote: Sat Mar 28, 2020 6:27 pm Another way it would spit back is if a spark was ignited while an inlet valve was open.
Ah. Yes. You see, I don't understand all this electronicy stuff!

In my day, with a set of points, a condenser and a coil working properly I never saw this. It's one reason why I would be very reluctant to fit electronic ignition to mine - I don't understand it!!

I suppose if there was a large build up of carbon it might ignite the mixture at the wrong time as well, but this doesn't fit well with modern fuels and (just) a winter layup.

It might be something else of course, but when I was doing this stuff in the late 1960s and early 1970s people were bringing their cars to me for a decoke. They were always spitting through the carbs and after the decoke and grinding the valves they didn't

User avatar
Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: spitting back through Carbs

Post by Kbeal »

UPDATE:

Car starts and runs fine at low RPM

I have now double checked the timing with a new strobe timing gun (the old one gave up on me) and set it spot on. It was marginally out but only a couple of degrees. Confirmed that the advance seems to be working but maybe not fully - does anyone have the figures for the advance at higher rpm?

Carbs cleaned and dashpots, synchronisation and mixture checked - all good

Floats and needle valves checked - all ok

Compression tested all cylinders - all between 160 and 165psi so no valve problems

Fuel tank dipped using water finding paste - all clear.

During all of this I noticed that it's only the LH carb spitting back and only at high RPM, the RH carb is fine. The fault must be in the LH carb so the next step is to remove it to check. I'll have a look at the RH too. I suspect either a worn spindle bush allowing air in or a failed jet diaphragm. Or both!

Kevin

Post Reply