Could Gas cap Leak, cause Low Milage?

kcscooterfan · Post by **kcscooterfan** » Sun Aug 02, 2009 10:55 pm

I have had an ongoing struggle to find out why my gas milage on my Blackjack is so lower than my Wife's. We've tried switching, spark gap, etc.and today I switched gas caps. I noticed, mine was easier to turn than hers. The result, less backfiring, the gas milage score was: Mine 70, her's 77 mpg. Usually with mixed driving, hers is in the 80's and mine in the low 60's. The second tank with hard driving, mine: 63.5, hers: 71.5. A much lower difference. Normally this type of driving would have her's being the high 70's, mine high 50's.

Does anyone know if this makes sense? I assume that since you're not suppose to overfill the tank because of vapor lock, the gas cap does not vent. Therefore if my gas cap was leaking air, maybe that's why my milage went up and the backfiring went down. Perhaps more gas dribbles in without a slight vacuum, if that's what causes backfiring.
I don't know if this is the only problem. The dealer has ordered a 92 and 95 jet to see if makes a difference. Right now I have a 92 jet that must have been bored out slightly (I think slightly too much).

If anyone has any thoughts on rather this theory makes sense, I'd appreciate it.

Shane Wilson · Post by **Shane Wilson** » Sun Aug 02, 2009 11:07 pm

Are you comparing milage with you riding both bikes?

Otherwise, riding style and rider weight are huge factors that make it less than an apples to apples comparison.

charlie55 · Post by **charlie55** » Mon Aug 03, 2009 12:36 am

I can't answer to the gas cap question, but backfiring is usually a sign of running too rich. So, I'm wondering why they would even think of going to a #95.

If I'm not mistaken, I believe that you had already asked (in a previous post) about the possibility of a loose choke being the cause if the issues you're having. I responded to that by stating that a faulty choke (enricher), or one whose plunger is not fully seating once the engine is warmed up, could leave you running too rich. Did you ever have the dealer pull the enricher and bench-check it with a 12V power supply?

Short of that, does your scoot go into high idle for a minute or two when you start it (assuming that the engine is cold) and then drop down to a lower idle speed? If it starts high and stays high, then my money would be on the enricher sticking open. If it does behave normally, then I'm wrong (wouldn't be the first time).

kcscooterfan · Post by **kcscooterfan** » Mon Aug 03, 2009 1:56 am

We switched our scooters and found that mine got about 5 mpg more, hers was 5 mpg less, still not the 15 to 20 mpg difference.

kcscooterfan · Post by **kcscooterfan** » Mon Aug 03, 2009 1:58 am

About the choke, once it was screwed back on, it idles fast at first then slows down. I had to give it some gas before it would start. I assume the choke is working now. thanks

kcscooterfan · Post by **kcscooterfan** » Mon Aug 03, 2009 2:07 am

I'm going to check the idle speed at startup again. If it doesn't idle faster at startup consistantly, I'll mention it to the mechanic. I don't understand why he'd suggest a 95 jet either, I told him that I didn't think that would make it better.

Buddy_wannabe · Post by **Buddy_wannabe** » Mon Aug 03, 2009 4:47 am

kcscooterfan wrote:We switched our scooters and found that mine got about 5 mpg more, hers was 5 mpg less, still not the 15 to 20 mpg difference.

Im a little confused . Didnt you say "Mine 70, her's 77 mpg" & "mine: 63.5, hers: 71.5." .... soooo where does the 15-20 mpg come in?

jijifer · Post by **jijifer** » Mon Aug 03, 2009 6:08 pm

does anyone else here have "backfiring"? I sure as shootin' don't. I'd think "NO backfiring" is what you might aim for. I've got a prima pipe too. Never any back firing in 3k miles with it.

oryx · Post by **oryx** » Mon Aug 03, 2009 6:18 pm

mine backfires when going down an incline at a fair amount of speed and off the throttle. or if i accelerate very quickly and let off the throttle all of a sudden.

dealer said it was OK. i trust my dealer.

jijifer · Post by **jijifer** » Mon Aug 03, 2009 6:55 pm

oryx wrote:mine backfires when going down an incline at a fair amount of speed and off the throttle. or if i accelerate very quickly and let off the throttle all of a sudden.

dealer said it was OK. i trust my dealer.

I'm not saying it's not 'ok', I'm just wondering if this is common since in 7k miles, modestly modded, mine never does. What I understand about backfiring is well, limited, but I think the basics is that it's an improper fuel/air ratio. A wrong size jet could also be making the mpg's tank. Stock jet is going to be the average jet that works in MOST cases. For instance, does Genuine build bikes jetted for specific elevations and ship those to those elevations? If not, then yeah, it may not be too big a deal but ideally you'd have the right ratio of air to fuel. I believe that back firing is an indication that your air/fuel is off even if just slightly and the jet would be the first suspect.

kcscooterfan · Post by **kcscooterfan** » Mon Aug 03, 2009 8:44 pm

Buddy_wannabe wrote:
kcscooterfan wrote:We switched our scooters and found that mine got about 5 mpg more, hers was 5 mpg less, still not the 15 to 20 mpg difference.
Im a little confused . Didnt you say "Mine 70, her's 77 mpg" & "mine: 63.5, hers: 71.5." .... soooo where does the 15-20 mpg come in?

When we switched scooters there was a 5 mpg difference. When we switched gas caps, it was "Mine 70, her's 77 mpg"

Her's might be trending a bit downward as it gets broke in, but generally it's about 15 to 20 mpg difference.

As to backfiring, hers almost never backfires, mine a lot. When we switched caps, mine did very little backfiring.

So I guess my question really is: Does the Blackjack gas caps suppose to vent...or not? Once I know that, I can deduce rather its the venting that is decreasing my mileage or a vacuum that's causing the problem.

I still think the main problem is the jet...but the latest experiment got us wondering if the cap plays a part too.

As to riding styles, we both ride together. If anything, she guns it a bit faster.

jijifer · Post by **jijifer** » Mon Aug 03, 2009 9:41 pm

kcscooterfan wrote:
Buddy_wannabe wrote:
kcscooterfan wrote:We switched our scooters and found that mine got about 5 mpg more, hers was 5 mpg less, still not the 15 to 20 mpg difference.
Im a little confused . Didnt you say "Mine 70, her's 77 mpg" & "mine: 63.5, hers: 71.5." .... soooo where does the 15-20 mpg come in?

When we switched scooters there was a 5 mpg difference. When we switched gas caps, it was "Mine 70, her's 77 mpg"

Her's might be trending a bit downward as it gets broke in, but generally it's about 15 to 20 mpg difference.

As to backfiring, hers almost never backfires, mine a lot. When we switched caps, mine did very little backfiring.

So I guess my question really is: Does the Blackjack gas caps suppose to vent...or not? Once I know that, I can deduce rather its the venting that is decreasing my mileage or a vacuum that's causing the problem.

I still think the main problem is the jet...but the latest experiment got us wondering if the cap plays a part too.

As to riding styles, we both ride together. If anything, she guns it a bit faster.

I hope someone who knows more than me will speak up. The venting of the cap was on scoots prior to 2009, from what I have read. 2009 scoots have metal tanks and caps (at least caps, I think tanks) and they put a little ledge in there, 2 inches from the top, with 4 holes, so that folks don't even try to over fill it. 2009 tanks, with that ledge make it very easy to see where to stop filling and I don't think I've read that anyone has had that problem with the new tank.

None of that means your cap isn't a suspect. That's just something else I haven't ever read or experienced myself. Again, I hope someone with more info feels like speaking up.

Have you hit the mileage that is "broken" in yet? I seem to recall you all getting your scoots in June, maybe, is that right? I'd hate you have you tinker something to a bad way thinking you had a problem when really you just need to be breaking this bad-boy in. Of course people with disagree but mostly, during those first 1500 (for my scoot it was closer to 2000) WOT shouldn't be held very long. In fact, no single speed should be held very long as you get all those bits and pieces to set. When I had to break my scoot in all over again with the big bore kit, it was PAINFUL to variate my speed and keep WOT to a minimum.

Give your BJ a chance to get settled. I wonder if some of these things you're noticing are just the breaking in pains.

Post by **Lostmycage** » Mon Aug 03, 2009 10:20 pm

Check the gasket on the exhaust. If the gasket is bad (it happens) you'll get backfiring (most noticeably on deceleration) and the mpg will suffer.

Also, try a 100 and 105 main jet as well. Don't just stop when you get to a jet size that works better. Try them all and stick with the one that works the best.

Dooglas · Post by **Dooglas** » Tue Aug 04, 2009 12:33 am

kcscooterfan wrote:So I guess my question really is: Does the Blackjack gas caps suppose to vent...or not? Once I know that, I can deduce rather its the venting that is decreasing my mileage or a vacuum that's causing the problem.

All scooter gas tanks including the Buddy have to vent or they will eventually develop a vacuum and the engine will stall. The scoot is designed to vent through a tube and a charcoal cannister rather than through the cap. You've probabaly read some of the problems others have had with gasoline in the evaporation cannister interfering with venting. As I understand it, Genuine/PGO solved this by using a cap with a vent hole as a back-up. If I understand all of it right, your cap is probably vented but doen't need to be unless you have overfilled your tank and caused problems with the evaporation cannister. None of that should explain backfiring or poor gas mileage.

kcscooterfan · Post by **kcscooterfan** » Tue Aug 04, 2009 12:39 am

Lostmycage wrote:Check the gasket on the exhaust. If the gasket is bad (it happens) you'll get backfiring (most noticeably on deceleration) and the mpg will suffer.

Also, try a 100 and 105 main jet as well. Don't just stop when you get to a jet size that works better. Try them all and stick with the one that works the best.

There's another idea to ask the mechanic about. Thanks

kcscooterfan · Post by **kcscooterfan** » Tue Aug 04, 2009 12:41 am

Dooglas wrote:
kcscooterfan wrote:So I guess my question really is: Does the Blackjack gas caps suppose to vent...or not? Once I know that, I can deduce rather its the venting that is decreasing my mileage or a vacuum that's causing the problem.
All scooter gas tanks including the Buddy have to vent or they will eventually develop a vacuum and the engine will stall. The scoot is designed to vent through a tube and a charcoal cannister rather than through the cap. You've probabaly read some of the problems others have had with gasoline in the evaporation cannister interfering with venting. As I understand it, Genuine/PGO solved this by using a cap with a vent hole as a back-up. If I understand all of it right, your cap is probably vented but doen't need to be unless you have overfilled your tank and caused problems with the evaporation cannister. None of that should explain backfiring or poor gas mileage.

Thanks, that explains the cap situation better.