Honda Rebel 300 & 500 Forum banner

1 - 20 of 21 Posts

·
Registered
2018 Honda CMX300A
Joined
·
90 Posts
Discussion Starter · #1 ·
Just over 250 kms this season on the motorcycle and things are a little off. For the last few years, I generally got 3.2 - 4 L/100Km (58 - 73 MPG), depending on the mix of city/highway riding.

This season so far, my first tank gave me just about 5 L/100Km (47 MPG) and it looks like I'm on track to get even worst with the second tank of gas. It looks like I might only be able to get 120-140 km on this tank of gas, which is absolutely terrible. I normally get at least 200kms before the gauge start flashing. Originally, I just blamed it on old gas from last year (I had about 20% of my first tank that would have been 4 months old gas), but with the second tank looking the same or worst, I don't know if I can still deem it the likely problem.

I can't say it feels like the bike handles very differently; it doesn't feel like it's lacking power or anything. Maybe it feels like I need a little bit more revs than normal to keep it at high speed, but that's hard to tell without a tach.

I'm at work right now, but when I get home, I'll get to diagnosing. I have a few ideas for things I have to investigate, but I figured I'd crowd source ideas as well.
  • Brakes: I'll check to make sure that the brakes aren't dragging. The rear brakes felt really warm even though I didn't use it much on my way here. I'll try to lay off the rear brake on my way home (if possible for safety) and see if it's warm when I get home.
  • Air Intake: I'll change the air filter. I'm pretty sure the current one is done. I've got 12000K on the bike and haven't changed it yet. I've got a K&N one in the mail. I don't know how that would cause a very sudden change though, I feel like it would be more gradual. And I can't say I smell gas from the exhaust so it doesn't seem like the engine is running rich.
  • Tires: I changed tires at the very beginning of the season for heavier ones. I was expecting to get a little bit of a MPG hit, but getting a 40+% difference seems a little bit excessive just for tires. It also seems like the 50Km I did before the tire change had more of less the same terrible MPG.

What else should I check? Exhaust header/gasket? Could an exhaust leak cause this? Wheel bearings? Fuel system? Any chance it's something I screwed up during winterization?

Any help/guidance would be appreciated.
 

·
Super Moderator
Joined
·
1,420 Posts
is there a possibility that someone is stealing gas from your bike?
 

·
Registered
2019 Rebel 300A/2020 CB500F A
Joined
·
450 Posts
With a fresh pump of gas, I would have to start pointing fingers at the tires because that is the only new variable by the sounds of it. That's a massive drop though...btw, I usually get up to 280kms before flashing starts on my 300. It did that from day 1, and 10k kms on (even with the aftermarket exhaust).

On the topic of exhaust, have you made sure there isn't a potential leak maybe?
 

·
Registered
2018 Honda CMX300A
Joined
·
90 Posts
Discussion Starter · #7 ·
Ok, so I got some time to troubleshoot tonight. Tires were good to go, good pressure and all. I checked the brakes and everything is good to go there too. There's a few things I'll have to wait a little to investigate.

Air filter is dusty. I've got a new one in the mail that should get here in a couple days. I was overdue for a change so even if it's not exactly what's causing the issue, I still got to the maintenance.

My new prime suspect is, as @SC48 mentioned, an exhaust leak. I don't feel like I've lost power, but I definitely feel I have to reach higher revs to achieve the same riding style I'm used to. That would obviously eat into my gas mileage. Maybe a combination of higher revs and lost unburnt fuel to an exhaust leak could explain the MPG discrepancy.

I remember having popping noises at the front of the engine at the end of last season (which I assumed at the time to be a leaky header gasket) and it went away when I tightened the header bolts. Maybe the cold/warm cycles of the winter did something there and I have a blown gasket. I was gonna replace the header gasket anyway, so I've got the parts on order at my local bike shop and hopefully that fixes the problem when they get here.

If neither of those things solve it, I might have to walk into the dealership with my tail between my legs and get them to figure it out.
 

·
Registered
Joined
·
233 Posts
OK, but I'm just not getting why a 2018 bike has such a problem??
If it was a 1988 I'd get it.
 

·
Registered
2019 Rebel 500
Joined
·
67 Posts
Does Canada use different fuel blends during winter vs. summer? I know that in Ohio winter blend fuel has a drastic reduction on mpg vs the summer blend.
 

·
Registered
2018 Honda CMX300A
Joined
·
90 Posts
Discussion Starter · #10 ·
I think it's time for a bit of an update: based on the second half of that second gas tank, it looks like the problem has been more or less rectified; I'm back to what I expect in terms of gas mileage. I didn't really control for all variables 1-by-1, so I can't say for sure what made the difference.

In the end, I suspect it was one of the following:
  • Even though I ran the bike pretty dry on the first tank, there might still have been bad gas in there. Or maybe the EFI got "used" to the bad gas (artificially running the engine rich to compensate) and took a while before it started dispensing good fuel at a decent rate.
  • On the subject of fuel, I generally run regular, with a tankful of Premium from time to time to degunk the fuel system. For some gas stations in Canada, Premium gas has no Ethanol and a healthier dose of additives. A little bit of Premium from time to time keeps the fuel system happy. I've only used Premium so far this season to eliminate that as a variable, so maybe the additives finally did they job and cleaned up the whole system.
  • Air filter was more black than green. I installed a K&N and almost immediately saw a difference in acceleration power. Now mind you, I am NOT saying K&N performs better than stock (I don't have nearly enough data to make that assertion), but the new K&N certainly made a huge difference compared to the clogged up stock filter.
  • The new tires (ME 888) are definitely heavier and harder to get moving. I may have unconsciously over the last 2-ish week adapted my handling & acceleration to account for that new weight, which would result in smoother acceleration, and therefore better mileage.


Other things considered that were probably not the issue:
  • I ended flushing my brake fluids, although I really can't see how that would do anything to improve my gas mileage.
  • The header gasket & hex cap are still on order at my local dealership, so I haven't been able yet to make that repair. I have to conclude from this that a popped gasket/exhaust leak was probably not the source of my woes.
  • Canada most definitely uses different blends of gas for summer and winter, but I've always ridden from March to November with just slight variation in gas mileage, nothing as significant as what was seen here. Either way, I'm seeing a return to the baseline within the same tank of gas, so that likely wasn't the issue.
 

·
Registered
2019 Rebel 300A/2020 CB500F A
Joined
·
450 Posts
Amazing. Glad you were able to go about this methodically and that the situation has improved.
 

·
Registered
Joined
·
119 Posts
My money is on the old air filter being most if not all the cause of your problem. If the engine is low on power, which it will be if it’s not getting enough air, you naturally and subconsciously try to compensate by giving it more gas. All those other variables might have contributed, but I’d bet not by much. Anyway, glad you got it straightened out.

At the risk of starting a whole other argument, I’d also suggest that running premium hasn’t helped your mileage, either. Higher octane fuel is designed for higher compression engines and actually needs a higher temperature to ignite than lower octane. Despite what all the seat-of-their-pants dynos say, premium gas simply does not burn as well as regular in an engine that’s not designed or tuned for premium. There’s a good chance you’re not getting a complete burn just from that alone, and then combined with a dirty air filter it just destroyed your mileage. Instead of running premium gas to clean out the fuel system, I just throw in a half can of Seafoam whenever I think of it, and all has always been good. Which reminds me, I need to grab another can!
 

·
Registered
2018 Honda CMX300A
Joined
·
90 Posts
Discussion Starter · #13 ·
My money is on the old air filter being most if not all the cause of your problem. If the engine is low on power, which it will be if it’s not getting enough air, you naturally and subconsciously try to compensate by giving it more gas. All those other variables might have contributed, but I’d bet not by much. Anyway, glad you got it straightened out.

At the risk of starting a whole other argument, I’d also suggest that running premium hasn’t helped your mileage, either. Higher octane fuel is designed for higher compression engines and actually needs a higher temperature to ignite than lower octane. Despite what all the seat-of-their-pants dynos say, premium gas simply does not burn as well as regular in an engine that’s not designed or tuned for premium. There’s a good chance you’re not getting a complete burn just from that alone, and then combined with a dirty air filter it just destroyed your mileage. Instead of running premium gas to clean out the fuel system, I just throw in a half can of Seafoam whenever I think of it, and all has always been good. Which reminds me, I need to grab another can!
Just to be clear, I'm not saying the Premium would have improved gas mileage because of the octane content. At the risk of carrying on this argument, I fall starkly in the camp that the extra compression means nothing in an engine that doesn't need it, hence why I use Regular the large majority of the time. However, there are advantages to premium gas that aren't just octane level, namely a higher concentration of additives (that clean your fuel system), and (at least in Canada) no Ethanol, which has worst gas mileage than regular gasoline and likes to attract water.

I'll look into Seafoam. It looks like a good option to keep the engine clean.
 

·
Registered
2018 Honda CMX300A
Joined
·
90 Posts
Discussion Starter · #14 ·
My replacement exhaust gasket and hex cap arrived at the dealership yesterday and I picked them up. I already had kinda ruled that out as my mileage is pretty much back to normal, but I had the parts, so I figured why not. I took the exhaust off and replaced the gasket. Looking at the old gasket, it looks like I really did a job on it last year when I tightened up the screws. I guess I wasn't very good at tightening both sides equally as one half of the gasket was quite a bit more compressed than the other.

Anyway, new gasket is in, header is tightened to specs with tension on both bolts balanced and everything seems to be working fine. I can officially turn the page on this (mis-)adventure.

Thank you to everyone for your suggestions.
 

·
Registered
Joined
·
1,177 Posts
I'll look into Seafoam. It looks like a good option to keep the engine clean.
not to argue with Danny or anyone else but seafoam was originally designed for marine applications, or small 2 stroke engines. it really doesn't do anything in modern cars or bikes. just a waste of money.
 

·
Registered
Joined
·
119 Posts
not to argue with Danny or anyone else but seafoam was originally designed for marine applications, or small 2 stroke engines. it really doesn't do anything in modern cars or bikes. just a waste of money.
They’re all internal combustion engines, no? They all have to get fuel from the tank into those cylinders somehow, and that all needs to be kept clean.
 

·
Registered
Joined
·
1,177 Posts
2 stroke engines run on a mixture of oil and fuel, ratios vary but many are 32:1. there are different goings on in those engines than any modern car or motorcycle.
but at the end of the day, it's your bike, do what you want. you'll find wildly varying reviews just like any product that people will be on either side of the fence about.
guess i just wanted to let @Frankster and anyone else that may come across this to know they should check things out for themselves and do research. not just grasp at the first thing they see in print here or anywhere else.
 

·
Registered
Joined
·
88 Posts
Leaky gasket = poor fuel mileage

If there was an air leak present due to a faulty exhaust gasket (or one on the intake side aft of the throttle body assembly, or from a loose or damaged MAP sensor tube) the Lambda sensor would register the combustion gasses as being too weak and add more fuel to compensate for it.
 

·
Registered
Joined
·
119 Posts
2 stroke engines run on a mixture of oil and fuel, ratios vary but many are 32:1. there are different goings on in those engines than any modern car or motorcycle.
but at the end of the day, it's your bike, do what you want. you'll find wildly varying reviews just like any product that people will be on either side of the fence about.
guess i just wanted to let @Frankster and anyone else that may come across this to know they should check things out for themselves and do research. not just grasp at the first thing they see in print here or anywhere else.
Completely agree. Just throwing my own experiences out there.
 
1 - 20 of 21 Posts
Top