bogging down around 50 mph

[bluesman]

Hello All,
First, I have a 79' r100 [formerly an rs] w/ 40 mm carbs. So yesterday AM I went for my usual Sun ride all was well for about the first 10 miles then starting up a long straight hill I went to pass a HD chugging along and suddenly it started losing power It felt like it was running out of gas but if I pulled in the clutch it would rev freely. So I down shifted it and I got up the hill and pulled off. It would idle fine, rev, and ran normal until I got over 50 then it would start bogging down. So I have checked the fuel cap vent, checked the screens in the fuel taps, pulled the the float bowls didn't find anything but didn't go any further. Is the next suspect the diaphragm? Or should I be looking at something else? If I do need diaphragms what sources do you recommend or to avoid? Also is there a test I can perform without removing them? I've seen mention of "drop test" on the XS 650 forum.

Thanks,
Bluesman

[melville]

Sounds like a fuel flow problem, if it's fine except when you put a big load on it.

Check float levels first. Also make sure the float needles are moving freely and passing fuel.

[bluesman]

I did check fuel flow also something else I forgot to mention is that I checked the plugs and they were almost white like it's burning very lean! It just started suddenly and did improve somewhat the more I rode it but then would start again. Should I maybe check the advance weights?

[Seth]

Assuming both plugs look the same (white) I wouldn’t think diaphragms. What are the odds both would fail at the same time. I agree it seems lean. How did you test the cap vent? Did you try just loosening the cap and see if it still does it? Doubt it would be the advance weights, especially with the white plugs. I would assume with a retarded spark, you’d get incomplete burn and some carbon buildup.

[melville]

Pull the main jets and the pilot jets and clean them out. You may have crap clogging that area if it's lean enough to bake the plugs.

This sounds like a load problem, not a rpm problem. Ignition would be misbehaving at any load at or above a certain rpm.

Check the float level--they should be releasing fuel when the floats are just below parallel with the bottom of the carburetor body. The float needle probably has a spring inside, which can make it seem like it's in the right spot if you are checking the adjustment dry. You need to know that fuel comes out as soon as the float leaves that parallel spot.

[ME 109]

Pulling the choke on when bogging down can identify fuel supply as the issue, if there is an improvement in performance.

[Rob Frankham]

Carburettor Diaphragm (or a looose carb top), blocked fuel filter, blocked tank vent, blocked main jet. Any of these will do it...

Rob

[bluesman]

Thank's for all the advice. I will do more checking today and report back.

[popete]

Thank's for all the advice. I will do more checking today and report back.

Any luck? Did you figure out what was wrong?

[bluesman]

Well, I did all the checks that everyone suggested, like float level, fuel flow, main jets etc. but found nothing that I could point a finger at. But as what usually happens I managed to fix it without knowing what was wrong! When I finished up all my checks I went for about a 50 mile ride and it ran fine. What still gets me is that it seems like it was both sides and like Seth said what are the chances that both sides would fail? I have fixed more things in my life by taking something apart finding nothing, putting it back together and having it work fine!
Thank's again for everyone's help.