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1981 Honda: Im consistently getting 19-22 mpg

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I'm new to motorcycles, but...
I'm new to motorcycles, but I'm learning fast. I have a 1981 Honda CB900 Custom and it is a serious gas hog. I'm consistently getting 19-22 mpg! Here is a list of the things I've done, but am still getting crappy mileage:
- checked air filter (its really clean)
- Tires are to pressure
- changed the oil added Lucas heavy duty as well
- ran plenty of seafoam through it and sprayed carb cleaner directly through the outside two.
- Adjusted the fuel enrichment screws (from 1.5 rotations to 3 and currently at 1.5 rotations measured from the seated position)
- Recently changed the spark plugs (there was a noticeable increase in horsepower)
-Looks like 2 were running rich - one had what appeared to be oil - The other carbon stained.
- The one of the carbeurators does appear to be leaking some gas (but no where near 1/2 the gas tank)
- There is some light grayish white colored smoke when rev'ed up to the 7-8rpms
- I've spoken with some people and they seem to think I have to remove the carburator and clean it and this will help me reduce the amount of gas being used? I'm new, but if a carburator jet(s) is(are) clogged how would one be using twice as much fuel? Could this have something to do with the fuel filter?

The bike sounds great and runs great, but the mileage has got to get better somehow. I really don't want to sell it because of this and I really enjoy riding it. Could you please grant me with some advice. Much appreciated.

Greg K
La Crosse, WI
Submitted: 8 years ago.Category: Motorcycle
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Answered in 19 minutes by:
6/25/2010
Motorcycle Mechanic: ...., .... replied 8 years ago
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Good day and welcome to Just Answer. My name is XXXXX XXXXX I'd be happy to help you with your problem today. You really aren't going to get stellar mpg out of that heavy bike & yes you need to do a carb service, not so much to alleviate clogged jets but more to get the carbs set up & working together & not against one another as it appears they are. You also need to find out where that oil is coming from on the one plug. If you have a cylinder with worn rings & blow by causing the oil on the plug then you are losing compression there & losing a lot of un-burnt fuel. That oil could also be coming through the crank vent & air box because the oil is over filled, either because there is too much oil in there or the carbs are over flowing & fuel is washing out the cylinders contaminating the oil.




To get to a starting point of figuring out your issues you need a bike set up properly to run. In order to do that; You need to check & adjust your valves & do a compression test. After you do that & a complete carb service which requires removal, disassembly, cleaning, replacing worn rubber parts, re-assembling setting up properly, re-installing on the bike & tuning/synchronizing the carbs with fresh NGK spark plugs. Only then when you have a bike that should run well can you start to diagnose what is off & final tuning. The bike can't run great with one plug oil fouled & another fuel fouled. Additionally, if that bike is still a points ignition you need to replace your condensers & set the points, as that also contributes to poor mileage. The smoke being white-ish gray is oil burning, that needs figured out, because if you need a top end rebuild, no amount of carb tuning will straighten that out.




Also this is a motorcycle, is fuel mileage really a concern? I mean I've ridden well over 2,000,000 miles on bikes & not once have I ever worried about that, because ultimately to accurately assess fuel mileage there are so, so many factors to consider that it is extremely difficult to reproduce them all simultaneously in order to get an accurate mileage reading. Humidity alone can alter fuel mileage from one day to the next. Let me know if you have other questions or need more info.



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