dead spot in carbys @ 5000 rpm

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noish
Cucciolo - the Lil Pup
Posts: 19
Joined: Mon Jan 17, 2005 5:50 pm
Location: San Diego

dead spot in carbys @ 5000 rpm

Post by noish »

I have a 77 500 GTL with Dellorto PHF carbs. After cleaning, lubing, and balancing them multiple times per Steve's instructions, they seem to run excellent -- except for a dead spot at 5000 rpm. Going up a hill with full throttle at 5000 rpm it seems like they lose vacuum and have zero power - problem is resolved by downshifting and powering through. I tried backing out the accelerator pump screws to give it more gas - this actually helped make it more responsive at lower rpm acceleration but did nothing for my problem at 5000 rpm.

I also put my needles on the nearest clip to back them out and to richen the mixture a bit - no change there either.

any suggestions?

thanks,
tim

noish
Cucciolo - the Lil Pup
Posts: 19
Joined: Mon Jan 17, 2005 5:50 pm
Location: San Diego

more info

Post by noish »

If I open the choke up during the dead spot, I get lots of power immediately. Also, if I open the choke as I accelerate, I get lots more power throughout the entire range of acceleration.

I would think that my carbs are running lean. But with the needles on the nearest clip, am I out of options to richen the main circuit mixture?

I am running PHF 30B's - can I get a narrower needle size or larger jet orifice?

wdietz186
Cagiva Alazzura
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Post by wdietz186 »

Check your float levels. If they are too low you will be effectively running out of fuel. The level also controls to some extent the transitions from idle circuit to needle jet[midrange]. With the carb on a flat surface with the inlet facing down you should measure 17.5mm from the bottom of the float to the gasket surface of the carb body. That is with the float needle seated and the tang of the float resting but not compressing the spring loaded pin on the float needle. Also check the flow from the petcocks is not restricted. You might want to take them out and clean the screens that protrude into the tank.

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BevHevSteve
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Post by BevHevSteve »

"needles on the nearest clip"

Do you mean you have the needles so that clip is on the space closest to the fat end of the needle? If yes then you have dropped the needle and leaned out things. To make the needle richer, put the clip in the middle groove or the groove farthest away from the think end.

Let us know how things progress
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