Dramatic drop in MPG. Why?

[Restrictor Plate]

       My 2005 KYMCO People 250 has had a substantial decline in gas mileage since last summer. It is down about 25% now, from an average 71 mpg last summer, to about 54 mpg now. I’m trying to figure out why? I’d appreciate any thoughts on this.

   First off, I have been logging every refueling since the beginning of 2013 on Fuelly.com. I have logged over 6,800 miles, so I have a great sampling of what this bike is capable of. You can click on the Fuelly.com banner near the end of my post to see my fuelly records.

   I’m in Florida, so the bike is ridden pretty much year-round. It is relatively low-miles for it’s 9-year age: 36,500 km (22,680 miles). (For several years it was mostly stored while I lived overseas, just ridden a few weeks while I was back in the States on visits.)

   For years, the bike had been averaging in the low 60's mpg. It was also chronically vapor-locking for years. Nobody could tell me why.  Early in 2013, a mechanic finally knew what it was and informed me of the fuel vent in the gas cap (which was plugged up with dirt.) When that was cleaned out, my gas mileage went to an average 71 mpg and stayed there throughout the summer. In October, it started dropping steadily and continuously (look at the monthly bar graphs on Fuelly.com.)

I’ve had numerous maintenance items worked on in the past year, not in pursuit of the mileage issue, but with each one I was hoping it would be the answer to the mileage issue. Unfortunately, that’s not been the case.

The bike is now running great. Top speed is in the low 70's (gps), and within two miles per hour of what it did when new. It rolls fine, and tire pressures are good; a recent compression check showed 150. This past weekend, I took a long ride and (just to confirm) I ran the entire tank with the gas cap opened, just to rule out any new issue with the cap vent being plugged again. It had no effect, and my last tank (on an easy 50mph run) averaged 54 mpg.

Recent service and changed parts (since the mileage went down) include: vacuum hose leaks in carb area corrected; carb jets were plugged, reducing top speed; air filter replaced; ignition issues (missing) resolved with replacement of CDI and coil; exhaust valve changed (bent, may have been due to lack of periodic valve adjustments); Spark plug and cap changed; drive belt and rollers changed (all OEM); none of these made any change in the lowered mpg I’m experiencing.

Prior to the mpg decline in October, the following issues were taken care of during the year last year: water pump replaced; air filter and fuel filter replaced.

Anybody have any thoughts on what I’m missing here? Something that could drop the gas mileage by 25%, yet not be noticeable in engine performance?

Thanks!

 

[zombie]

This is odd. I tend to read/think of solutions as I go. As I follow your post the solutions are all listed as done.
Optimal air fuel ratio is 14:1. A 25% drop is almost 10:1 ratio, and a performance drop would be noticeable, and exponential to the air fuel ratio. So:

Two things I would suggest... Buy a Vacuum gauge, and an appropriate size "T" fitting for the vacuum line closest to the intake/carb. Install it, and run the bike as you always do. The gauge should stay steady at idle (around 20 IM), and drop during accel. When at cruise it will again be steady at approx the same reading as idle. Post up your results...

Second is another compression test. Ride the bike till warm, and let it cool down enough to pull the plug. With the throttle WIDE open do the test, and post the results.

These will tell EVERYTHING about the engine condition.

The next step is drain, and refill the gear case. If the oil is black (mostly normal) but smells burnt there may be a bearing/bushing issue back there.

Since you say there was a "bent" valve I am curious as to how that happened... Thesae are NON interference engines, and the piston cannot hit a valve. Something had to get into the cylinder, and physically hit the valve for this to happen. We might chase that dragon if the other stuff I suggested doesn't lead anywhere.

[Stig / Major Tom]

Vivo PM'd me to ask this:
Has there been any changes in your trouser size?

[zombie]

Vivo is my God! There is NOTHING he does not know.
Another mystery issue solved. Thread closed / Topic locked, and a brand new 4XL Kymco Forum "T" shirt is on its way.

Thank Vivo for Vivo!    Next!

[Restrictor Plate]

Thanks for the prompt replies. I will try to implement and reply back next week.

First of all, no change in trouser size. 

Re Compression check: Please explain why the throttle needs to be wide opened? What does that do?

Bent valve is quite a story; here's the sequence of events:

Top speed was off by 10 mph, so I took it into a trusted independent mechanic. Asked him to run a compression check, which showed 150. He worked the problem and ultimately found the carb jets plugged, cleaned the carb, and I had my top speed back. However, I still had a suspected ignition problem because I could hear the engine missing at times when it was idling. Shortly after the carb cleaning, the bike got progressively worse at stalling out after warmed up, and being hard to start when warm.

Went on a scooter group ride, and it stalled on me both times we approached a toll booth. Took a few minutes to restart. Riding home, we had a stretch on the interstate. I was riding with bigger scooters and I was wide opened for about 20-25 miles. Near the end of that stretch, it suddenly sputtered and lost power. I slowed down on the shoulder, it didn't stall, and I babied it along. Eventually got back up to 55-65 mph and it seemed to be okay. Rode it the rest of the way home okay. Was the sputter at speed a possible ignition issue? (Read further.)

Though I had no problem with my independent mechanic, I decided to take it to the KYMCO dealer, mainly because of the belief he could get it back to spec and would have ready access to parts. (What a mistake!) I rode it into the dealer to drop it off. Described in detail what I was experiencing. Dealer asked about the last valve adjustment. Frankly, I couldn't remember. I know they are way too frequent in the maintenance list, but mine had been missed for quite a while. So I told him to go ahead and check them. When the dealer went to start the bike and take it around back, he could not. Perhaps just the difficult starting when warm I thought.

Dealer checked valve adjustment (don't recall result), and checked ignition. Told me CDI was failing. I suspected an ignition issue, was not surprised. After CDI replacement, he said he still could not get the engine started. Dealer implied the bike I rode in to drop off for service "died" when I dropped it off. Seems a bit too coincidental to me. He tore the engine down, and found a slightly bent exhaust valve. Showed me with it still in the cylinder head. It was leaving a small visible gap of light when closed. Made me wonder when this happened, since I had a recent compression test at 150? No explanation. He said everything else including the piston, head, etc. were all okay.

He replaced the valve. He rode it, found it was still hard starting when hot, and perhaps still stalling out. Swapped an ignition coil, and it resolved that issue. He replaced the coil with a new one.

I got the bike back, after 5 1/2 weeks since dropping it off (Totally unacceptable!) During that 5 1/2 weeks. they had maybe 1 week of delay while waiting for parts to arrive. Not impressed with this dealer!

Talked to my independent mechanic about the whole ordeal, he asked to see the valve. In his opinion, it had gotten too hot. Don't know if the wide-opened running for 20-25 miles could do that? Engine temps did not overheat.

Lots of confusing stuff going on, but I still have the MPG issue, which existed before all of this valve and ignition business started.

As for the valve, attached are a couple of photos. The stem is bent slightly; can't really see it in the photo, but you can if you roll it on a counter top.

[zombie]

This is all Very unusual. I'll start w/ the valve... When an engine or component over heats the carbon on the part will turn either white/grey or bright red like rust color. Bending or distorting from the heat is also a sign of overheating.
What makes ths unusual is it appears your valve didn't really over heat due to engine condition (lean mix/air/coolant flow). It looks like it had a structural failure that led to the distortion. Almost like it rusted or was full of impurities at the base of the stem. Then as it weakened it created the leak, and led to the running issues. Being in that weak a state I can see it bending all over the place, and sometimes correcting to almost true.
I have been wrong before but this may lead somewhere... I cannot imagine the valve seat being in acceptable condition after all of that, and my GUESS is your dealer does NOT own a valve seat mill. At best he may have lapped it with compound but ... I don't know.
Obviously the bike ran/runs but not well, and even a MINOR valve leak will do what you are describing. A compression test may not see this either because the carbon, and oil will in effect seal the valve until it is hot enough to be cleaned away by the exiting exhaust gas.

All just speculation. I would strongly suggest removing the head again, and filling the exhaust port with Carb Clean to see if it leaks.
You can try the same test on the bike thru the intake port, and see if you get any of the cleaner seeping into the cylinder thru the intake valve but that is not as conclusive.

Since the trouser size didn't pan out (or so you state) I'd get back into the mechanic guy, and ask him to order an exhaust gasket, a head gasket, and a valve cover gasket so he can pull the head, and test it as I I I described. No other method will verify this except pressurizing the exhaust port thru the opening, and doing a "leak down" test. I guarantee neither the dealer or the mechanic have the adapter needed to do this, and it will take as long to make one as it will to pull the head.

That's my 45 cents worth. Maybe some one else has another slant on this

[Restrictor Plate]

Thanks for all your input Zombie. I appreciate your efforts.
I'm going on a scooter club ride tomorrow, about 160 miles RT. We'll see how that goes.

I guess another thought I have on this problem is that although it is elusive now, it may progress and provide additional symptoms that will then make it easier to diagnose.

Whether the compression test is still applicable or not in my case, I'd be interested in your thinking on why to open the throttle during a compression test?

[zombie]

Oh yeah I forgot... If the throttle is left closed there is no air (or not enough air) entering the cylinder to compress. You will get false readings.

Good luck on the ride!

[mrbios]

The most I ever got with my Grandvista 250 was 61 mpg.  Normally, I get 50 to 55 mpg.  I don't but the bent valve theory I think the dealer bent it and has no idea what he is doing.  You would not have gotten 150psi with a visibly bent valve.

I don't buy the CDI / coil either.  I think the entire thing was a fuel issue with the carb and or possible vacuum leak.  Motor running lean = hot.

[Restrictor Plate]

Thanks for the reply mrbios.

I don't know how chronic the fuel-cap vent clog issue is on this era of KYMCO bikes, but when looking at the database in Fuelly of what other People 250 riders are experiencing, I saw a lot that were getting mileage in the low 60's. But clearly, I had a 5 month stretch and some significant miles that were averaging 71 mpg, so the bike is capable of that. And I'm no light-weight.

Since my last entry here, I've had a significant amount of riding (probably 6-8 tanks) and it has been between 50-56 mpg. And running great. 

As for the bent valve, I've described what I know on it. I agree it seems somewhat suspicious. Don't think that I could have ever gotten it started and ridden it to drop off for service at the dealer if it had little or no compression. It would have been interesting to run a compression check just before he tore down the engine.

As for the CDI/Coil, I know there was a problem with at least one of those, as I'd been hearing it missing at traffic lights, and once while warm and idling, it just killed suddenly and completely (as if it lost complete ignition.) Working theory is that maybe one dragged down/took out the other.

I've been busy and running all over for the last 10 days, but hope to get the bike in to my independent mechanic soon for at least another compression and a vacuum check. Think we'll do that, weigh the results, then talk about the "leak-down" tests Zombie described.

Thanks for the input everyone.

[mrbios]

Thanks for the reply mrbios.
significant miles that were averaging 71 mpg, so the bike is capable of that. And I'm no light-weight.
...

My mileage could be higher but what kills it is I have the super tall Givi windshield and I run about 80 to 100% on the highway to keep up with traffic. 

Regarding the leak-down and compression tests... if it's running fine and mpg is normal I wouldn't bother. 

Basically, the leakdown test (simple way, without using the fancy gauges) is where you put compressed air into the cylinder when it is at top dead center - all valves closed.  You then listen for air.  If air is coming back through the carb then the intake valve is not sealing, if air through the exhaust the exhaust valve is not seating, if air is escaping from the dipstick (you remove it) then the rings are worn.  If you get bubbles in the radiator fluid the head gasket is bad or the head is cracked.  Probably the best use of the test is to help diagnose a bad head gasket that causes anti freeze to disappear. 

I think the compression is closer to 140psi not 210psi as stated in the manual.  These "tests" on a good running scooter can stress out an owner when there is nothing to worry about.

My basic tests: (which neither the leakdown nor compression shine any light on)
 
- Is mpg ok (55 to 60 is normal).

- Is there "excessive" oil consumption?  (I add about 3 oz? between 1500 mile changes) Running at 80 to 100% really pressurizes the crankcase on any motor - larger motors are never run at such high revs / loads because you would be going over 100 mph.

- Is the antifreeze level and operating temp normal - I might have added just a few ounces in 13K miles.

- Performance - stuttering, hesitation indicate a vacuum leak or lean mixture - both damage a motor with excessive operating temp (which can be masked by the fact that the motors are liquid cooled).

[tyguyc]

I know that this is old but Ill put out my thoughts in case it helps future someone.

What if you were running lean (ie: better fuel mileage) and hotter (ie: bent exhaust valve)

Then you fixed all the issues and make then run perfectly, now you get the correct fuel mileage.
\\could be totally off my rocker.

[mrbios]

I know that this is old but Ill put out my thoughts in case it helps future someone.

What if you were running lean (ie: better fuel mileage) and hotter (ie: bent exhaust valve)

Then you fixed all the issues and make then run perfectly, now you get the correct fuel mileage.
\\could be totally off my rocker.

After spending countless hours on the carburetor and experimenting with different jets.... I came to the conclusion that if a person cleans the carb two or three times and still has fuel / idle issues it is  best to just replace the carb with a new one ~ $250 for the Grandvista.  A friend had a GV 250 and cleaned the carb endlessly.  Some how it got damaged so we took my carb off and installed on his scooter and it started and ran perfectly.  I could not see anything wrong with his carb and it was spotlessly clean but something hidden was wrong with it.  He then bought a new carb and installed it and that solved everything.

[CROSSBOLT]

After spending countless hours on the carburetor and experimenting with different jets.... I came to the conclusion that if a person cleans the carb two or three times and still has fuel / idle issues it is  best to just replace the carb with a new one ~ $250 for the Grandvista.  A friend had a GV 250 and cleaned the carb endlessly.  Some how it got damaged so we took my carb off and installed on his scooter and it started and ran perfectly.  I could not see anything wrong with his carb and it was spotlessly clean but something hidden was wrong with it.  He then bought a new carb and installed it and that solved everything.

The jets may be cleaned to spotless shiny but a passage may be plugged. Unplugging will take a big, nasty bucket of foul chemical to do this if that stuff is still available. EPA, tree huggers, save the planet restrictions.....