Faltering above 3,500 rpm

Technical Area for all the problems you have in the garage
Post Reply
User avatar
Dave Woodward
Posts: 821
Joined: Sun May 06, 2007 10:00 pm
Location: Cheshire
Contact:

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.
__________________________________
I say, ding dong!
User avatar
clive
T289R Member
T289R Member
Posts: 3207
Joined: Thu Mar 01, 2007 1:18 am
Location: Perth

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.

(If I'm not here I'm in my workshop or on the golf course!)
User avatar
peterc
Posts: 1667
Joined: Wed Mar 05, 2014 9:05 am
Location: Surrey

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+.
Peter C
User avatar
peterc
Posts: 1667
Joined: Wed Mar 05, 2014 9:05 am
Location: Surrey

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.
Peter C
User avatar
StewbieC
T289R Committee
T289R Committee
Posts: 1301
Joined: Mon Oct 13, 2008 12:50 pm
Location: out in the sticks, Shropshire

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.
________________________________________________
Stu
Hawk 289, 66 Mustang Fastback with a 289 maximum smiles per mile..
User avatar
peterc
Posts: 1667
Joined: Wed Mar 05, 2014 9:05 am
Location: Surrey

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.
Peter C
User avatar
Dave Woodward
Posts: 821
Joined: Sun May 06, 2007 10:00 pm
Location: Cheshire
Contact:

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.
__________________________________
I say, ding dong!
User avatar
clive
T289R Member
T289R Member
Posts: 3207
Joined: Thu Mar 01, 2007 1:18 am
Location: Perth

Re: Faltering above 3,500 rpm

Post by clive »

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

(If I'm not here I'm in my workshop or on the golf course!)
User avatar
simonjrwinter
Posts: 1026
Joined: Sun Jan 03, 2010 8:25 pm
Location: Upminster, Essex.

Re: Faltering above 3,500 rpm

Post by simonjrwinter »

Probably good housekeeping just to check your fuel filter for obstructions…..
TR6 (V8) Hawk 289 FIA (V8) Doing my bit for global warming.
User avatar
Dave Woodward
Posts: 821
Joined: Sun May 06, 2007 10:00 pm
Location: Cheshire
Contact:

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 👍
__________________________________
I say, ding dong!
Post Reply