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Discussion Starter #1
Hi, I ride a 94 Superglide, fairly stock but with these mods:
Harley Slash Cut exhausts, Screaming Eagle Air Filter, breather to external filter, 148 jet, Accel 8.8mm RFI HT Lead set, Accel Platinum U Grooved Spark Plugs - and everything else is standard.

Here's my problem:

The bike starts/stops/idles normally and does exactly what it says on the box however, when I get up to approx 80-90mph it will suddenly lose power, start dropping in revs and obviously slow down. When I dump the throttle and open it slowly again it will begin to pick up but will continue to do the same for a short period after. If I stay at 65-70mph I do not notice this problem although it has occurred on one or more occasions. I do not think it is a fuel problem as it does not occur at any other time and I have checked the carb and jets for any blockage.

Any ideas out there?

Cheers Paul
 

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Have you tried it at the same engine speed, but a lower gear(lower bike speed)? See if it will do it in 2nd or 3rd gear at the same RPM's.
 

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Sounds like a fuel problem. Where it happens at a higher speeds and more specifically higher revs, I would check to make sure you are getting a good supply of fuel to the carb. It sounds like you are running the bowl dry and when you slow down it has time to catch up. Check/adjust float level. Check/replace the fuel line (they can become swollen inside and restrict flow). If there is an in-line filter, remove it and see if the problem still occurs. Check/clean your petcock screen or consider a high flow petcock like a Pingel.

Start with the simple stuff and that is all simple to check.
 

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It sounds like you are running the bowl dry and when you slow down it has time to catch up.
Had this occurring on mine for a while also. Turned out the fuel cap wasn't venting properly. It wouldn't allow the required amount of flow at interstate speeds(around 80mph).

Flow seemed good when I checked by pulling the supply line to the carb. But when I loosened the cap the stuff really gushed out. Some mechanical assembly inside the cap had broken. This created a vacuum inside the tank that acted against the natural gravity feed.

Replaced cap, problem solved.

BTW-The problem would occur at even lower speeds when the tank was full or near full. I later figured out that this was because there was less air to displace within the tank.

Good Luck
 

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Springer, and Ka-Ka both have good points.
It most certainly sounds like a fuel problem.
I have to agree with Springers line of troubleshooting, check your fuel lines, your fuel filter, etc.
after reading Ka-Ka's comment, that is a much quicker and possibly more likely the culprit.
take your bike out for a ride, and if it starts missing and cutting out, just loosen your gas cap, listen for air rushing into your tank, if the vent is blocked, you will be breaking a vacuum.
your bike should start running better, leave the cap loose, or take it off, and continue riding trying to get the problem to re-occur.
if the bike still does it, then you must continue troubleshooting following what Springer said.
Good Luck!

Bart
 

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Petcock

I'd be blaming the vacum petcocks inability, under low vacum, to keep the bowl full.
 

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Discussion Starter #7
Thanks for all your help guys.
Gas cap vent was smashed internally so replaced that - bloody expensive here in the UK.
Took it for a run, noticed an immediate difference with the pickup but did not cure the misfire at speed problem.
Took the carbs apart today, adjusted the float, checked all the lines etc and took her out for a spin.
140kph about 80-85mph for 30 miles and not a problem.
So once again thanks for all your help - I know where I will come back to the next time:thanks:
 
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