Engine backfire

[bmcecosse]

Looking for learned opinion on possible reasons for my engine sometimes tending to backfire through the carb - when it's working hard say slogging up a long motorway hill with a decent load in the car. It sometimes seems to go 'flat' and then there will be a loud backfire through the carb... I can only guess an inlet valve is failing to close properly.....anyone have any other ideas? The valve gaps are set at 15 thou inlets and 18 thou exhaust, standard cam but of course 940 head on the 1098 engine, and all the valve springs are good, no broken coils..... It's far from a new head - I don't think the valve stems will be particularly tight in the guides.. All suggestions very welcome !

[minor65]

Once my blue minors engine would shutter violently underload and spit/backfire though the carb, after the long time working the problem out. It turned out to be the distributor points (they where overlapping each other and not sitting square). New points instantly cured problem.

[chrisryder]

Didn't IslipMinor have a similar problem with localised overheating around one of the valves causing the valve to stick open?

He mentions something like that whenever we get onto thermostat and bypass-hose elimination discussions.

[JOWETTJAVELIN]

I expect your mixture will be weakened to within an inch of its life...

[bmcecosse]

The mix is correct...but even if it does go weak (if perhaps the fuel pump can't keep up) -why would it backfire through the carb? I can only see it is possible IF an inlet valve sticks open - but keep the ideas coming! This what I need - a brain storming session!! I have no idea how hot the engine is under these circs - I expect £$%& hot - never quite managed to get round to fitting that temp gauge I bought 10 years ago....but I expect the valves to be a rattling fit in the guides, which makes it hard to imagine them not closing...

[MarkyB]

Does it "run on" at all?
I guess a bit of glowing carbon could ignite the mixture as it arrives in the cylinder.

[bmcecosse]

No - it's not guilty of that one Marky - nor does it 'pink' - although it is set 'not quite' pinking. In fact - when it started doing this, I did set the timing back very slightly - but it still does it. Thanks for the suggestion - and keep them coming folks!

[JOWETTJAVELIN]

Didn't you have a very similar problem about a year ago, under the same circs, i.e. full load uphill at high speed? Did you ever get to the bottom of it?

[autolycus]

Let's say the mixture is a tad on the weak side, and the cylinder pressures at compression is at its highest (size 13 to the floor, so little throttling, so maximum charge). The HT voltage to initiate firing is thus as high as it ever is, and might just be a bit too much for the leads, cap, rotor arm or whatever - so that cylinder doesn't fire, and the mixture passes unburned into the exhaust.

Next piston up fires OK, and a blast of hot gas meets the unburnt mixture in the exhaust. Pop. But around tdc one cylinder has both valves open, so the pop dissipates through 9 feet of narrow bore exhaust pipe, or a few inches of inlet manifold - or a bit of both.

Hence the general hesitation, spitting, and popping, made worse by any one or more of weak mixture, HT deficiencies, fancy cam with big overlap, combined with heavy hoof.

Reasonable?

Kevin

[bmcecosse]

Yes - same problem as last year JJ - the car only gets hammered up M/way with heavy load on a few occasions - towards the end of each 'season'.. Slightly wrong Kevin - it's size 14s.... I like the general picture you paint - but the back fire is very definitely through the carb - not the exhaust - and yes I see your point about both valves open on one cylinder at overlap (but only very slightly..) , and I suppose that could be it - although not aware of any 'report' at all from the exhaust when it happens.... The cam is bog standard 1098 AEA 300 - no big overlap... It happens when doing ~ 60 mph up a long hill - the Trav has 3.7 final drive, so I guess the engine is doing ~ 4000 rpm, and yes size 14 to the floor with HIF 38 carb . Thanks for the 'picture' - good food for thought... keep it going folks!

[JOWETTJAVELIN]

I'm pretty sure the Owner's Manual says something about backfiring through the carb and what the likely causes are. I'm out of guesses now...

[MarkyB]

http://www.youtube.com/watch?v=M92CGjpiMs4

Could be number 2

[Mog-nuts]

Excellent (sorry Roy)

[moggiethouable]

Its like Alec Ferguson asking me if hes got the team balanced right in mid-field
However, where angels fear to tread is my motto.
So how about a leaky exhaust, resulting in low back pressure and therefore a backfire further up in the carb?
This just doesnt feel right at all.

[bmcecosse]

Not leaky exhaust - and we ALL know it's NEVER the coil.......geezzz But so kind of you all to come up with ideas - that youtube vid just covers his a$$ by mentioning just about every part of the engine as a possible cause....... Keep them coming lads (and lasses)...

[MarkyB]

To be fair to you tube guy it does seem like there can be many possible causes.
Weak mixture caused by leaks is another popular answer.
I wouldn't rule out the ignition side of things, end of life plugs tend to start failing at high revs.

[bmcecosse]

Could be weak mix - if the fuel pump can't keep up, but why/how does this manifest as a backfire through the carb? That's what puzzles me.......

[kennatt]

well if you are convinced that its through the carb then as you say it has to be on the inlet side via the inlet valves.could it be weak inlet valve spring letting the valve bounce when under extreme load,would also cause hesitation,had a similar problem on an mgb, head refurb cured it,but don't know if it was the new springs or the overall refit.

[MarkyB]

why/how does this manifest as a backfire through the carb?

The explanation I saw said that the weaker mixture burns slower so can still be burning when the inlet valve opens again.

[bmcecosse]

Thanks folks - both possible of course, although the inlet valve spring doesn't know it's 'under load' - it just gets pushed and relaxes no matter how 'hard' the engine is working. I'm loath to disturb the head - when I can find my compression tester I'll get some readings..