4 replies [Last post]
bpearson's picture
Atlanta, GA
Joined: 11/13/2017
Posts: 1

Appreciate any help from the forum in troubleshooting a curious issue with my R75/5 that simply refuses to rev out.

I've owned the bike 10 years now, and over time have restored and refurbished most systems including the engine. Although I've not run compression or leak-down tests recently, the engine was made mostly "new" in the last 5,000 miles including new pistons & rings, full cylinder head rebuild on both cylinders with new seats and valves, timing chain/gears, etc. It has electronic ignition that's always worked perfectly.

Running perfect when I put it away....due to circumstances beyond my control, unfortunately the bike set idle for the past year. When I cranked it again, it'd idle but die if I cracked the throttle. So I drained the tank, cleaned it, cleaned screens/petcocks, removed disassembled and cleaned both carbs with all new gaskets, floats, needles and reset the float levels at 24mm fuel depth in bowl. Diaphrams looked fine, slides are very smooth.

Engine cranks and idles perfectly, as it always has. But it won't rev up past 4,000 in gear while riding (and it's sluggish getting there). It won't rev beyond maybe 4500-5000 in neutral and isn't smooth at higher rpm's. Both cylinders are firing, that I've confirmed.

Interestingly, the engine revs higher when not at wide-open-throttle position....it bogs at wide open, then if I back off to perhaps 3/4 throttle it revs a little higher....made me think running out of fuel...

All I've looked at is carbs (dirty from sitting with fuel in them). Any suggestions where else to start? Never experienced carb problems with the bike in 10 years, just disassembled and cleaned them every 3-4 yrs. Always worked perfectly and so simple what am I missing. THey are 64/32/09&10 series



schrader7032's picture
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6737
You've done a lot to check

You've done a lot to check things. Stuff that crossed my mind:

- you wondered about running out of fuel. Could you be creating a vacuum in the tank? Next time it bogs, crack open the tank to allow air to get back in.

- have you considered the rodent factor? Check the air filter for nests...as well as the mufflers for unexpected guests.


Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

MikeL46's picture
VBMWMO #9300
Joined: 01/01/2016
Posts: 65
Could the advance mechanism

Could the advance mechanism be stuck? Easy enough to check before looking deeper into the carbs.

Other thought is small tears in carb diaphragms.



67 R50/2 w/R100 engine/trans and Ural Sidecar
69 R60/2 76 R90S 78 R100RS
70 Triumph w/Spirit Eagle Sidecar

mark_weiss's picture
Joined: 11/17/2013
Posts: 173
Another vote for a torn

Another vote for a torn carburetor diaphragm.



RickR90s's picture
Farmington Hills, MI
Joined: 11/28/2016
Posts: 63
check these too

all of the above but I will add, check the tank petcock screens. Low flow can be an issue at higher rpm's. Don't ask me how I know. Sad




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