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$$!??#%%! Fuel starvation when it's raining


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I replaced all hoses front to back, went through two pumps and wound up with an electric, did new float inlet valves, reset the float levels, shifted the fuel pickup in the tank, advanced the cam gear. I think I've fixed the fuel issue, now there's just the high load high RPM miss.

 

Last night I installed a NOS exhaust I got off Ebay...I forgot what it's like to not have the stank of a million tiny leaks wafting everywhere, and not to have to listen to the wheezing sound through all the gaps! It's stock past the built in muffler which comes as part of the system from collector on back, then 1 3/4" the rest of the way back. This was the easiest size to get around here. Sounds pretty cool too. Not too loud, but throaty.

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Just so everyone knows only a plug fuel filter will be full, the cleaner or newer the filter the more it appears to be empty

 

Assuming you start with a full fuel filter, how does it get air into it and become more empty as the filter gets plugged?

 

If you have the vertical mount filter, as on the 620, the inlet is at the bottom and the outlet tube at the top reaches down inside about half way. Any air above the bottom of the outlet tube can't be pulled out by the pump. It's like air trapped in an overturned boat. Any air trapped in the fuel filter means very little. I've had in-line filters replace the vertical mount ones and run with and without an air pocket and there is no difference in running. It's just trapped air. If yours changes in volume, then the air is coming from somewhere don't you think?

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  • 2 weeks later...

Rig is still running pretty good. I replaced some wires around the coil, notably the ballast resistor wire, and the miss went away. It pays to look closely and simply redo the easy stuff!

 

I've been fiddling with the mixture on the carbs in order to get things back to how it ran before my woes began. It's weird, it seems as if the needles have changed...it's changed where it's rich or lean RPM wise. Now part of this is doubtless due to all my fiddling, and part due to resetting the float levels...but will different float levels change how the needle profile plays into the mix? After all, the fuel level in the nozzle is level with that in the bowl...if I've raised or lowered it relative to where it was before, will that make the mixture behave differently other than a different setting for the mixture nuts?

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Yup, SU's.

 

I know the float level sets the fuel level in the nozzle, but I can't get my head around the subtle interaction between the fuel level in the nozzle and the gap between the needle and the orifice of the nozzle. The nozzle/needle gap at the orifice must control how much can escape between the two, but would the actual level of the fuel being below that point also influence the mixture? I guess it must. I'll have to review the diagram I saw for the system and see where the ideal level is. Obviously it matters or the float level wouldn't matter much other than enough fuel being in the bowl to not run dry.

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I have the hitachi square tops

 

thats the proplem from the begining!!!!

 

Only proplem with  Pertronix I have is the ring tape wearing out causing the magnets to stay on shaft.got new magent ring

another time my 521 would cut out on high revs. Find out my single point bushing was worn out causing the shaft to wiggel at high rpms. replace with another dizzy with same pertronix then was fine.

 

maybe since you have SUs maybe a matchbox with a EI coil would explode that overrich mixture better?

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  • 3 months later...

Update...putting an electric pump on this thing seems to have fixed all my issues. I then redid the float levels and got them to match as closely as I could, the sight glass trick worked great for that, and especially with the electric pump since you don't need to turn the engine over to get them full.

 

Since then I've been tweaking and retweaking the custom needles I did ever leaner, I'm a full turn leaner than where I initially set them and the intentionally rich reprofiling seems to have worked. I now get better numbers around town and around 14s on the interstate at cruise.

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