Faltering above 3,500 rpm

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Dave Woodward
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Faltering above 3,500 rpm

Post by Dave Woodward »

Following an IRS conversion on my Hawk, I had a new exhaust fitted late last year. I was only able to do a few miles in it before putting away for the winter.

Having now run the car some more this year I’m finding that there is a very mild misfire above 3,500 rpm which becomes more noticeable the higher you go. The car is perfectly drivable but I’m keen to understand if this may be linked to the exhaust.

The new exhaust has a wider bore so I’m wondering if the two are related and if they are do I just need to tweek the mixture or will it need re-jetting.

Cheers. Dave.
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clive
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Re: Faltering above 3,500 rpm

Post by clive »

Hi Dave,

I would suspect the fuel first. Is the misfire only when the engine is under load or cruising at 3,500+ RPM? What carb/fuel system are you using and what ignition? Do you have a Filter King Regulator fitted, and if so what pressure is it set at? Have you refuelled the car since the winter or is it old fuel? How much fuel was in the tank over winter? Do you keep your car in a warm environment or is it cold enough to cause condensation in the fuel tank? Have you looked at the filter between the tank and pump?
Cheers, Clive.

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peterc
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Re: Faltering above 3,500 rpm

Post by peterc »

Have you recently been running the engine to warm it up / test something and not driving anywhere. If so it is most likely the spark plugs are fouled. In changing my hoses I warmed the engine up to get rid of the trapped air. When I used the car a few days later it had a misfire around 4500.
Not finding the answer I contacted the garage that performed the rolling road adjustments. They advised that modern spark plugs no longer have their inner ceramic core coated. This is fine if you run a modern lean burn fuel injected car but bad news for use with carburettors. The ceramic core gets polluted with fuel.
They advised they had 12 classics all awaiting sorting and suggested everyone would merely need new plugs. Indeed I changed mine and all was fine again and revved to 5500+.
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peterc
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Re: Faltering above 3,500 rpm

Post by peterc »

Hi Dave,
If it’s the exhaust and not the manifold that’s changed I would have expected that might affect the revs at which max torque is achieved rather than upsetting the whole mixture but I could be wrong.
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StewbieC
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Re: Faltering above 3,500 rpm

Post by StewbieC »

Every time I've had a misfire it's been the coil. Change the coil problem gone. Latest Accel coil designed for electronic ignition has lasted the longest.
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peterc
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Re: Faltering above 3,500 rpm

Post by peterc »

Which particular Accel coil do you recommend as they seem rather expensive.
I have been running a relatively cheap Lucas Sports coil for years without any issue but my Lucas distributor users a basic electronic amplifier pack for the sensor. E.g. not MSD.
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Dave Woodward
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Re: Faltering above 3,500 rpm

Post by Dave Woodward »

Hi Clive,
The misfire happens when cruising and when under load above 3500. It’s gets slightly worse with increasing revs though is only ever mild. I’m running a Holley 570 cfm at 5 psi. None of that has changed though only the exhaust. I’ve also put a couple of tanks through so should be clean fuel by now.

Hi Pete,
I’ll check the plugs as you suggest but I don’t tend to leave the car idling without taking it for a run.

Hi Stu,
I’m like Pete, running a non MSD electronic setup. May be time to move to MSD :). Can you confirm the Accel coil you refer to.

Thanks all. Good to know it won’t need rejecting.

Cheers.

Dave.
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clive
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Re: Faltering above 3,500 rpm

Post by clive »

If I remember, Holley recommend 7 psi. Might be worth checking.
Cheers, Clive.

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Re: Faltering above 3,500 rpm

Post by simonjrwinter »

Probably good housekeeping just to check your fuel filter for obstructions…..
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Dave Woodward
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Re: Faltering above 3,500 rpm

Post by Dave Woodward »

clive wrote: Sat May 14, 2022 11:27 pm If I remember, Holley recommend 7 psi. Might be worth checking.
They recommend 5-7. Always ran very well in the past. It’s another thing I can test 👍
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