Full throttle power sap

[ZacharyB]

Ever since I switched to 20W-50 and dropped a tooth off the rear sprocket I have trouble pulling past 80 mph in fifth gear. Seems normal enough given the nearly 30-year-old unconditioned engine, but when I open the throttle completely power seems to drop slightly. This is when the bike is ALL on the needles. The thing is, I don't know if it's leaning out or being swamped.

What are your experiences?

Thanks for any reply.

[duaneage]

Open it up, cut the engine with the kill switch, pull over to the side and pull a plug.

Then you'll have some idea of what is going on.

I think lean out, maybe you disturbed somethingand don't realize it so the gear change and oil are making you think they caused a problem. Try a run with the petcock on PRI instead of ON.

[ZacharyB]

I can't use PRI at this time because the diaphragm is torn in that position, making a leak. But I see your point and I'll look into it. Thanks

[Anonymous]

Generally if you have more power at 3/4 throttle than you do at WOT you're lean on the mains.

[ZacharyB]

That's exactly my case. It must be the holes in the old muffler/old engine compression. Thanks for your advice

[ccratin]

Unless you have some damage on that old engine, I wouldn't call 21K miles old at all. You likely still have great compression. Holes in the exhaust, if they are significant in size will definitely contribute to your problem.

[ZacharyB]

I was browsing around in search for main jet increase intervals and came across a post by earl that said that if one possesses the stock exhaust and airbox, one should not increase the jet size. I have the stock airbox and exhaust, except there are some corrosion cracks in the baffle where the exhaust exits. I am wondering if this would cause a lean condition at WOT.

I got the bike to 88 mph using 4th gear but I was running out of rpms at that point. It's already super-legal, and I would leave it, except I can't imagine it being an attractive selling point to have a 750 that only does 90 mph.

[LW_Icarus]

shouldnt be very hard to find- clogged mains, a big ass intake leak, something drastic. my 850, all factory, will pull easily past 100, at 3/4 throttle. Im gonna get in trouble because of that.

its supposed to be slower or the same speed as a 750. I also have a 30,000 mile engine, and plan on seeing much much more on it. Ill decide how far its gonna go after I roll the odometer over the first time

[ZacharyB]

If my air filter were over-oiled, would that cause an intake problem at high rpms?

Heh heh, it's easy to type on the computer and not try any of this stuff out for myself. But I don't want to remove my air filter for a 90 mph jaunt on the highway for the sake of the scientific process just yet

[gspaul]

If the diaphragm is damaged in the petcock you may not be supplying enough gas when you open her up, need's fixing.
If you have holes in you muffler's you are making your carb mix lean!

[ZacharyB]

One more question, how long does it take for gas to varnish? The longest I've gone without riding the bike since cleaning the carbs is two weeks excluding winter storage (supposedly properly stored)

[gspaul]

It can start as little as 3 week's,but even then should be only minor and will clean out again with use.

[ZacharyB]

Thanks again for your answers everyone.