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Tracking down a misfire ...


Jbc562l
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Hi,

I have a Herald 1200 Engine, fed with twin SU's & a Spitfire cam.

On Friday I had a new S/S exhaust fitted. I was warned by the installer I may need to re-tune !

On driving away from the fitters, all seemed well but then I did have quite a few misfires, which went after the engine got to full temperature.

Same thing happened yesterday on a run from cold. Misfire happens after 2 minutes & goes after 10 ? 

I would normally expect the back pressure has changed on the engine.

UNFORTUNATELY to complicate issues, on my way to the installers, I filled up with fuel (half a tank), the 95 RON standard unleaded pump was out of order, so in my hurry I decided to go for premium. Whilst I'm 75% sure the misfire is exhaust based, I also have a small thought I may have increased the octane above what the engine is happy with. I always add a dose of Castrol Lead substitute with Octane Booster as well.

Common sense dictates not to do anything too drastic on the engine tuning front until I have had a chance to get some normal 95 ron back in the tank.

Any thoughts on the cause of my misfire would be gratefully received, I'd like to understand both what to do & the science behind what's happened.

I haven't pulled the plugs yet. I'll do that tonight.

 

 

 

 

 

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I wouldn't expect exhaust back-pressure to cause a misfire only when part-warm. It's possible the improved gas flow (I assume that's what you're expecting the new exhaust to give) has changed the thermal behaviour and you simply need to leave the choke out a bit longer. It may be that this has revealed a pre-existing problem (weak spark or the like) that just wasn't showing up before.

Filling with the higher octane fuel should not cause any issue. However, if the garage you filled up at doesn't normally sell much of it, the actual fuel you put in could have been stale. That might well explain the symptoms. Go for a good long blat to use up the suspect fuel, re-fill with known good stuff and see if that helps.

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did they use anew manifold gasket  ...see the posts about manifold leaks , seized studs and more   this could be a intake air leak

you should be using 97+ fuel they were designed to run on 100  95 will make for retarding the ignition and lost performance

you dont need a lead addative or octane boost  there is years of lead memory in the head casting  , you can fit exhaust seats if and when the head needs any work  till then save your money, buy high ron fuel and smile

you may need to change the carb needles  and maybe the damper springs to richen up if you have  horrible pancake filters and a 421 manifold and free flow exhaust 

Pete

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Thanks for the replies. They made me a system that joined the original cast manifold, not 4-2-1, and a new gasket was supplied by me..

I feel you are correct that it's a weak mixture, the new system will be flowing much better. 

I will try to make the mixture a bit richer by a couple of flats & see if that improves matters. Failing that it's get some new needles. Can't remember the current code but they are standard profile, they will be a richer equivalent.

And I plead the 5th on the subject of pancake air filters ? I will start to investigate some proper ones ASAP !

 

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so its just a 'standard exhaust'  but made of stainless   sorry my mis interpretation.

this should not affect the running just not as clagged up as the old     .

try the lifting pin  just a gentle lift to touch the air piston then raise 2mm what happens   looking for a hint of change not a radiacal change in running 

if its lean expect a 50 rpm drop in idle   if its rich a 50rpm rise,  if nothing happens its correct.

pancakes do make lean running and take in hot under bonnet air ...not good for fueling 

pete

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Yes, the top ones look a bit richer than the bottom ones but none of them are out of range. Only a tiny tweak if any.

Have you used up the possibly suspect fuel yet?

It's not impossible that we're actually looking at a coincidental emergence of an unrelated issue. Can you describe the feel of the misfires? Sharp loss of one cylinder at a time? General sluggishness?

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I always had it drummed into me that 90% of misfires were electrical. Could your changes have just highlighted a separate  problem?

I spent weeks trying to sort out a misfire on a V8 changing/adjusting loads of stuff only to find that after its winter hibernation, the electric rev limiter had decided to start cutting in at 1500rpm!!

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6 hours ago, muddypaws said:

I always had it drummed into me that 90% of misfires were electrical. Could your changes have just highlighted a separate  problem?

I spent weeks trying to sort out a misfire on a V8 changing/adjusting loads of stuff only to find that after its winter hibernation, the electric rev limiter had decided to start cutting in at 1500rpm!!

Agree, in this instance I believe (statistically) it to be one of the two things that changed immediately prior to the fault. i.e: filled up with (possibly dodgy) V-Power. AND changing exhaust system for one that had better flow. If it's something else it's of course possible but a bit spooky ;)

 

If I'd changed nothing I'd be changing plugs, leads & swapping dizzy cap/rotor arm for sure.

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6 hours ago, NonMember said:

Yes, the top ones look a bit richer than the bottom ones but none of them are out of range. Only a tiny tweak if any.

Have you used up the possibly suspect fuel yet?

It's not impossible that we're actually looking at a coincidental emergence of an unrelated issue. Can you describe the feel of the misfires? Sharp loss of one cylinder at a time? General sluggishness?

Not enough room in the tank yet to refuel, so It'll have to be this weekend :)....

Misfires were a very brief stutter which disappeared when engine got warmer. But could not be solved by pulling out the choke a bit more. Car has always needed minimal choke.

 

Thanks all for your replies and interest !

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30 minutes ago, Jbc562l said:

Agree, in this instance I believe (statistically) it to be one of the two things that changed immediately prior to the fault. i.e: filled up with (possibly dodgy) V-Power. AND changing exhaust system for one that had better flow. If it's something else it's of course possible but a bit spooky ;)

 

If I'd changed nothing I'd be changing plugs, leads & swapping dizzy cap/rotor arm for sure.

Agreed - electrics can be spooky! 

My point was that I had done nothing to my engine apart from lay it up for the winter & tried to start it with old fuel - a part co-incidentally failed

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Thanks for all your replies, you're a clever lot ! Before any tuning I thought I'd change HT leads for a new set if had lurking in the garage unused .. took her out for a run this afternoon and ran great .. still not sure if new exhaust started it all off, but think it's more likely to be coincidence.. old cars don't you just love them !

pic attached of shiny new exhaust in car ...

 

 

IMG_3765.JPG

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