Hi RPM Power Loss

[Lynn Kirkpatrick]

I have a '68 B GT with a '74 engine, twin SU carbs, pretty much all stock. I have finally finished some body and brake work, so I haven't driven it much this summer until today.

I'm getting some power loss in 3rd (a little) and in 4th gear above 3500 RPM or so. I also notice on deceleration some uneven backfire.

One thing I could do is change fuel filters, in case it is starving at hi RPM, but that doesn't explain backfire.

I could connect the timing light and see what the advance looks like through the rev band, but I have only seen timing settings up to about 3000 RPM or so. Does the curve keep advancing?

I have the Lucas points/condenser dizzy.

Whattaya think? Fuel or ignition?

Thanks everyone

 

[JPSmit]

I think mixture - what color are your plugs? (and if the backfire is in the exhaust I predict too rich)

 

[Jacmo]

Stuck or erratic mechanical/vacuum advance mechanism

 

[Lynn Kirkpatrick]

Just to close this, the dashpots were empty and the points badly burnt. I topped up the pots changed points and condenser and life is good. (except for some rust that I just found)

 

[Steve_S]

Sounds like a mixture issue to me as well. Also, never trust a new condenser. Over the years I've fallen into the habit of never changing a known good condenser unless it's actually failed.

 

[Lynn Kirkpatrick]

I forgot to mention that before changing points and condenser i looked at the plugs. All were the same cocoa-brown, at least to my bespectacled eyes. It's been a few years since I have really taken a close look at plugs, with the modern systems in our drivers. The MG plugs looked "Good" based on some websites that looked at, for comparison.

Way back in the dark ages, my dad always changed the condenser with the points. Condensers were relatively cheap,and while you are laying on top of the motor with the distributor apart it seemed like good preventative maintenance. (Does anyone else remember working on the GM's with the distributor back next to the firewall, behind the aircleaner?)