High speed throttle issue ?

[Peder_y2k]

1988 VR - runs great, well tuned/maintenanced, no problems.............yet.

I don't usually ride fast, but yesterday (hot day for here -90°F) on a group ride with some church-types through the moutains, found myself at a sustained 80mph. on the interstate. When I opened the throttle for more speed to catch the prayer group, the engine would not respond to the increased throttle demand and happily stayed at 80.

I know these bikes are fully capable of two-up riding at 120mph all day long.

 

Beginning to think the fuel pump is not keeping up. Anyone know the fuel delivery rate?

Note: 1000 miles ago, I'd cleaned the carbs and fuel tank replaced the fuel filter, and replaced the pump to carbs fuel line with 5mm urethane hose. Maybe the hose is too small, but it matches the pump and carbs fitting diameters.

EDITED NOTE: On hard acceleration up to 7000RPM shifting thru the gears, there were no issues. Issue occurred only at sustained high speed

 

-Pete, in Tacoma WA USA

Edited July 13, 2014 by Peder_y2k
Added Note

[Razorback]

Pinholes in diaphragms? If you can't pull enough vacuum against diaphragm, you can't pull needle open far enough to get all it will do.

 

 

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[Peder_y2k]

I'll check the diaphragms. Last time I had the carbs apart, the diaphragms were OK.

Thanks

-Pete, in Tacoma WA USA

[MiCarl]

I think it's unlikely to be fuel pump or filter. If the fuel wasn't getting to the carbs it'd draw the bowls down and it'd fall on its face.

 

Make sure diaphragms are good and slides move freely. You could also try spark plugs - I've seen plugs that were just fine until you ran them under load, then misfire.

[Prairiehammer]

the engine would not respond to the increased throttle demand and happily stayed at 80.

 

Twigg and a couple of others had the same complaint: couldn't go faster than 80-85 mph. In all those cases, the carb diaphragms (usually just one or two) were holed or ripped at the edge. One also had a sticky slide. Cleaned up and was good.

[Neil86]

In particuliar....ensure the vacuum on your "reference" intake that the pressure sensor uses

is reacting normal (usually #2 but might be connected to another on your machine).. You'll lose most of your timing advance if it isn't.

If all good, vacuum test hose and pressure sensor.

[Peder_y2k]

In particuliar....ensure the vacuum on your "reference" intake that the pressure sensor uses

is reacting normal (usually #2 but might be connected to another on your machine).. You'll lose most of your timing advance if it isn't.

If all good, vacuum test hose and pressure sensor.

Went thru the pressure sensor system 1000 miles ago, but will check it again for leaks and go thru the test process again. - thanks

-Pete, in Tacoma WA USA

[jasonm.]

take the carb TOPS off and look for holed , torn diaphragms. Could be just one or 2. But I had the exact issue at 45mph. vacuum "boost" sensor got disconnected...and that was all it was. Could be bad vacuum(boost) sensor not advancing the ignition properly. It's above the left front cylinder