Trouble at 5k rpms

[Hepcat]

On my 78E, When I get up around 5 thousand rpms, the timing light dims and the spark seems erratic. The vacuum advance timing wheel starts to jump at this point too according to the timing light. Would this have anything to do with the boost sensor?

I lose power and don't get the surge of power I'm accustomed to at 4k rpms on my XJ. I don't get a backfire and the bike runs great up until 5,000.

Any thoughts guys? If you need more info, I'll check back a couple more times tonight before I go to bed.

[Ken Talbot]

Which nipple is the vacuum hose attached to - the one on the body of carb #2, or one of the manifold nipples? It is supposed to be connected to the nipple on the body of carb #2 so the vacuum supplied to the advance mechanism is dampened and steady. The vacuum supplied from one of the manifolds would have quite a bit of fluctuation and would acceft the advance mechanism.

[Winterhawk]

Just a guess, check your pickup wires by pulling on them gently. When it advances maybe the wires are seperating or something. I've heard that the boxes on the 78E will last 200,000 miles. I think that Bob Jones box went out at 215,000 miles.

[scalded dog]

Don't know about the xj, but I had three bikes that did that , drove me crazy, cleaned the carbs very good, problem went away, keep in mind, I had just cleaned them, two days before. They were all three xs 11s, Later 'Dog