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Loss of power above 3000 rpm

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

Loss of power above 3000 rpm

Post by Kbeal »

An annoying problem which I hope won't be too hard to diagnose and fix, but there could be many causes.

Engine (SP250) starts fine, pulls like a train up to 3000 rpm when it stutters and loses power. Drop back to 3000rpm and all fine. No backfire.

Advance seems to be working ok (checked with strobe)
Carburettor pistons cleaned in case they were sticking
Float chamber needle valves & floats look ok
fuel glass is full
Pertronix electronic ignition fitted so no points to worry about
Coil (pertronix flame thrower) was new last year

Before I start pulling things off an replacing them (dizzy cap, rotor arm, HT leads, plugs, fuel pump) has anyone had a similar fault and if so what was the cure. (I suspect fuel rather than spark as there's no backfiring - maybe a weak fuel pump?).

cheers
Kevin

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

Re: Loss of power above 3000 rpm

Post by A.N.Other »

The fuel bowl may be full but that is only when you are stopped and the pump has had chance to fill it.
You have a fuel starvation problem.
Colin,
I may be slow but I’m rough as well !

Vortex O'Plinth
Extra-Wise Man
Extra-Wise Man
Posts: 1065
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Loss of power above 3000 rpm

Post by Vortex O'Plinth »

I had a similar problem. It turned out to be a tiny piece of black crud lodged in a carb jet. With the suction chamber and piston removed and the needle clear of the jet it could just be seen. I winkled it out with a wooden cocktail stick and problem cured. It may have been a minute piece of the hose from filter to carb that was not adequately ethanol proof; I replaced the hoses as a precaution.
Nick

"Don't bother with the Air & Space Museum - there's nothing to see.......".

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captain bobo
Posts: 268
Joined: Tue Feb 09, 2016 7:32 pm
Location: Kimbolton, Cambs

Re: Loss of power above 3000 rpm

Post by captain bobo »

I also had similar symptoms with my V8 250 saloon 5 years ago - the problem disappeared when I changed the coil. Quite why or how the coil caused the issue I can't imagine.
Best wishes, Dave B

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

Re: Loss of power above 3000 rpm

Post by Kbeal »

Sorted,

I changed the fuel pump, but that didn't cure the fault. However the the old one was definitely on its way out so worth doing.

It was the spark plug extensions not making proper contact with the plugs, so not getting a proper burn. OK at lower rpm, but not at higher rpm. I had an original set (probably over 10 years old) of the 'performance leads' with the moulded integral plug extension. Apparently some of the older sets weren't manufactured correctly, the metal cores in the extensions were not quite long enough to reach the plug so the sparks were having to jump twice. With new leads the problem disappeared. Took the opportunity to replace the dizzy cap, rotor arm and plugs too. What a difference that made. I guess with the limited use our cars get we forget that these items degrade, but we don't notice the performance drop until there's a real problem.

Kevin

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