combustion, plugs and mpg's. Looking for some explaination.

Question on combustion...

I had a stuck open thermostat which was causing a rich fuel mixture in my bike due to engine running cool. I've since replaced the thermostat, and am going to replace the plugs as soon as they get in (ordered them today). How does the rich fuel mixture effect the spark plugs negatively and how does that effect my fuel efficency?

When the thermostat was stuck open I was getting about 23mpg, after the T-stat replacement I went up to about 29, and I'm hoping after the plugs are replaced It'll come up even more, at least that's what I'm told will happen according to some that claim to know.

I'm going to go with it with wishful thinking adn it's not a huge deal to do, I'd just like to also know the "why's" in addition to the result.

Thanks,
Chip

 

An engine running rich for an extended period of time will fuel foul the plugs a bit as they are not meant to burn that much fuel for that long of a period. Under normal operation once the engine is up to a proper temp the mix will lean out allowing the cylinders and the spark plugs to catch up, burn off any excess fuel and continue to run normally. Once the plugs have been fouled enough deposits form on the plug electrodes and cause a less efficient spark. This not only throws off timing just a bit but contributes to a less than even burn sometimes if the spark is not as hot as it should be. Fouled plugs also put extra strain on ignition components such as coils causing them to work slightly harder and therefore slightly reducing their lifespan. Spark plugs gain minute ammounts of deposits over time no matter how well your engine runs. The electrodes also wear out over time which is why there is a service interval.

So, now that your bike is running at proper temps and not causing an overly rich condition all the time you are getting better fuel mileage. New plugs, new air filter, etc, blah blah blah are all good for 1 or 2 MPG but not usually very drastic. Regular maintenance also tends to boost HP just a touch but it's more of a restoration of power than a gain as you haven't modified anything, just made it work more correctly. Hope this answers the questions and dosen't confuse more.

 

That's a great explaination. Thanks for takign the time to type it out. I'm hoping they make a difference as I'd love to get into the 35mpg range at some point. The current plugs have 24000miles on them and if they're reasonable fouled I'm hoping to get into the 30's range regularly. Even babying my bike on a mixed interstate/city I only get 29mpg (see fuelly in signature), and the absolute best MPG I've got was a full tank on the interstate, crouched positon and only pulled out a 31mpg I believe it was.

Not sure where to go next if the plug change doesn't do anything.

The facts I know as of now are the thermostat is good, and the plugs are about to be good. The bike has no mods aside from a removed snorkel in the airbox adn a K&N air filter.

(Edit that interstate tank was 32.8mpg according to my fuelly history)

 

To be honest, the best fuel mileage I ever got was on a day I went out and absolutely flogged her until the fuel gauge was blinking at me, and then rode some more. Got a little over 200 miles out of that tank. Multiple starts/warm up cycles, stop and go traffic, traffic lights, idiots, etc all kill my fuel mileage. As much as I'd like to get a ton of miles out of a tank of fuel, I need to stop before she is runnin low usually. In my most recent tank of jumpin around town, back and forth to work, etc (read, not REAL ridin) I got about 120 miles out of the tank before the light was blinking constantly. That's with K&N, PAIR disconnected, and PC3 on zero map. I didn't do the actuall calculation on what fuel mileage I got, I'm too busy to deal with all that nonsense!! Let us know how it turns out once you get fresh plugs in the girl though.

 

Man, your mileage is low. What speed were you cruising at on the interstate? Last summer I did a 225 mile trip with all but 70 or 80 miles on the highway. I did my best to keep the bike at 70 to 75. After the twisty section at the end I pulled into a gas station with the light blinking at 217 miles. The bike got 47 mpg with a bad thermostat.(Though it was close to 80deg. that day and the bike was running around normal temp even with the bad t-stat) The only thing I have on the bike is a slip on exhaust. Put up some pictures of the plugs on here when you get them out, I'd like to take a look. If the bike is running way rich, the plugs will show it. I also think honda actually says to change the plugs at something absurd like every 8k miles. I'll probably do mine again in another 10k or so when the air filter needs changed again.

 

I'll definately post some results, and pics of the old plugs. I'm hoping they are ragged...

As for the fuel calculation stuff, I use the SMS feature on fuelly. They issue an address to send it to, when I get gas I SMS my milage, price, gallons in that order (181 3.35 4.3) and hit sent... It updates the website adn replys with your mpg to your phone. Pretty slick.

 

We were running about 75mph and was in 6th gear. The service interval for the iridium plugs is a 16k inspection with a 32k change. Mine are currently at 24k, I bought the bike used and I'm assuming they're the original plugs.

 

BOOM! got the bike back together today and I'm runnin 39.9mpg mainly on the highway for this quick test. That's up from 28mpg average. So, the open T-stat causing a cold engine fouled my plugs. T-stat change brought me from 23mpg to 28mpg; then changing the plugs took me from 28mpg to 39.

I.COULDN'T.BE.ANY.HAPPIER!

Thanks guys!

 

Sweet mother of results my friend! If I recall off of the top of my head the Clymer manual I have for my 5th gen says to change the plugs every 16k I think. I'm gonna make it easier on myself and change them every 12-15k when I do the airfilter, that way I know the plugs are good. Too bad they are expensive little buggers...

 

Not so spensive

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