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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.

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.

       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!