Topic: Terrible mileage

[christianr]

Hi All,

I am a newbie here...I have a Honda 76 550k four and I am getting terrible gas mileage. Something like 20mpg...It seems like this bike should be getting at least 40 or so.

Overall, my bike runs ok, but I suspect that this is a carb problem. A previous owner converted this into a 4 into 1 exhaust and I am wondering if this sounds like they may have rejetted the carbs to something not very efficient.

I have never taken apart the carbs for a thorough cleaning though. Do you think that cleaning everything up may solve this problem? Could this be caused by the carbs being out of sync? Timing? All of the above? Do I need to get some rebuild kits?

Would cleaning the carbs be a good place to start? Is there a way to tell what size jets are in there once I get it apart?

Thanks for the help

-Christian

[Short Round]

I would go ahead and get some carb kits, do a proper cleaning of all of them, put them back together.  Have them synched and take it for a ride to see what the results are.  They might be running really rich, not getting a good spark, inproper plug gap.  Go ahead and get new stuff that way you  know what is in there for the future.  Trust me its worth the $.......................Chris

[Bob Wessner]

Christian, while you are in the carbs anyway for the suggested cleaning, might be worth checking what jet sizes are in there. If the PO altered the exhaust, as you mentioned, he may well have changed the jetting and may not have needed to, or went overboard.

[333]

There are numbers on the end of the jets that denote size.  The slow jet is supposed to be a 38, the main a 100.  The main seems a bit large to me, but that's what Honda says it't supposed to be.  Good luck.

[super pasty white guy]

Have you done the other tune up items (air filter, timing, tappets, etc) and checked the color of your plugs?  You might find that the problem won't require a carb tear-down in the middle of riding season.  When I bought my 750F, I rode it straight from the sellers and got mid 20s mpg.  Turns out that the timing was Way, Way off and just correcting that problem (went with electronic ignition) along with the rest of the normal tune-up adjustments put me into the 30s mpg.  

With this spring's tune-up I replaced the filter with a Uni Foam and put in iridium plugs.  I'm bumping into 40mpg.

I know that eventually I really should tear into the carbs, but the need to do that has dropped way down.

Dave

[TwoTired]

Would cleaning the carbs be a good place to start?

No. First thing, check that all your head pipes are getting hot.  If so...

Then, do a complete and thorough tuneup that it's supposed to get every 3000 miles or 3 months anyway.
A compression test will give you knowledge about the cylinder mechanicals. You can rebuild the carbs several times and it won't help a dodgy cylinder.
Do learn to read spark plugs. Important clues can be found on them.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Count the number of teeth on your front and rear sprocket  S/B 17/37.  Do you have standard size diameter tires?  19 F, 18 R

If you just got the bike, check the tank for rust.  Drain and catch each carb's contents from the bottom drain screw.  Try to measure the volume and note anything found in there besides gas.  If you found crud in there, find out where, will clean carbs won't remain clean with a dirty supply.

By this point, carbs might need to be rebuilt if your issues aren't addressed.

Cheers,

[christianr]

Thanks for the tips everyone!

Would cleaning the carbs be a good place to start?

If you just got the bike, check the tank for rust.  Drain and catch each carb's contents from the bottom drain screw.  Try to measure the volume and note anything found in there besides gas.  If you found crud in there, find out where, will clean carbs won't remain clean with a dirty supply.

By this point, carbs might need to be rebuilt if your issues aren't addressed.

Cheers,

The tank used to have rust in it, but I re-sealed it last summer. The carbs have not been cleaned since I re-treated the tank though...

[christianr]

Have you done the other tune up items (air filter, timing, tappets, etc) and checked the color of your plugs?  You might find that the problem won't require a carb tear-down in the middle of riding season.  When I bought my 750F, I rode it straight from the sellers and got mid 20s mpg.  Turns out that the timing was Way, Way off and just correcting that problem (went with electronic ignition) along with the rest of the normal tune-up adjustments put me into the 30s mpg. 

With this spring's tune-up I replaced the filter with a Uni Foam and put in iridium plugs.  I'm bumping into 40mpg.

I know that eventually I really should tear into the carbs, but the need to do that has dropped way down.

Dave

I do have a foam filter in there, I'm not sure of the brand but it's definitely not the paper kind. I don't know about the timing. The bike was timed ~2 years ago and it went from running super rough to where it's at now.

I've never looked into iridium plugs, do you need new caps to switch to those?

[super pasty white guy]

Nay, just slap them in.  At least I didn't bother to change the caps.

Could be just snake oil, but it does seem to run smoother as well.

dave

[OldSchool_IsCool]

Check also that your brakes aren't dragging. 

[jabbadeznuts]

After putting in a new UNI filter, my mileage jumped from the mid 20s to the mid 40s. Both doing sport riding.