Rejetting for Altitude

[micellis]

Hey guys, I bought a 97 250 last October and it ran great for a month, then when warmed and ridden after about 20 minutes it just bogs down and dies. If I let it sit for 30 minutes or so it will crank back up and go for another 10-20 minutes before bogging down and dying. There is a nasty hanging idle, If I rev it above 4-5k it comes back down to 1500 very slowly. Saw the thread below this and I'm going to check the air filter/box, as well as the slides. I was told a hanging idle could be vacuum leak?

Now that summer is on it's way I plan on pulling the carbs for my first time and cleaning them. While I'm in there I was curious about rejetting for the Denver altitude. I've been told that it isn't needed and to not worry about it, but I've found a couple topics around different forums that state a #102 main jet and #35 pilot jets. Most of my riding will be around 5-6k feet but I do plan on some mountain riding and could be around 9-10k.

Any advice here?

Thanks in advance.

[spooph]

If the pre-gen is anything like the new gen you'd actually want to go down in size... Modified exhaust/intake? I have a very open air box and slip-on and I'm still rich on the top end with a #100 in there...

[jkv45]

Not sure if compensation for altitude would be required or not. I believe that increased altitude will cause you to run richer (less dense air charge).

As far as the other issues, it does sound like a vacuum leak causing the revs to hang-up. Spray some carb cleaner around the boots from the carb to the engine and listen for a change in rpms.

The other issues sounds like fuel-starvation for one reason or the other. Either the tank is not venting (open it when it quits and listen for air rushing in) or a restriction in the fuel flow to the carbs that eventually won't let them fill to an adequate level. That would most likely be an issue with the petcock or debris in the tank clogging the filters/screens and reducing flow.

[micellis]

Originally Posted by spooph

If the pre-gen is anything like the new gen you'd actually want to go down in size... Modified exhaust/intake? I have a very open air box and slip-on and I'm still rich on the top end with a #100 in there...

I'm completely stock as far as I know. PO brought the bike up from KC from what the titlework looked like and didn't ride it much. I put some seafoam in the tank for the winter, would it be worth running a full tank of with seafoam through it before I pull the carbs?

Originally Posted by jkv45

Not sure if compensation for altitude would be required or not. I believe that increased altitude will cause you to run richer (less dense air charge).

As far as the other issues, it does sound like a vacuum leak causing the revs to hang-up. Spray some carb cleaner around the boots from the carb to the engine and listen for a change in rpms.

The other issues sounds like fuel-starvation for one reason or the other. Either the tank is not venting (open it when it quits and listen for air rushing in) or a restriction in the fuel flow to the carbs that eventually won't let them fill to an adequate level. That would most likely be an issue with the petcock or debris in the tank clogging the filters/screens and reducing flow.

I was planning on draining and cleaning the tank, changing the fuel filter when I pull everything off for the carbs. If it turns out to be a vacuum leak would it be better to just replace all of the lines?

[jkv45]

Did you store it with a full tank? If not, you may have enough condensation in the tank to cause running issues.

I always suggest completely removing and replacing the fuel - if there is any question at all - before going into more complicated troubleshooting. Use ethanol-free 87 if possible.

A vacuum leak would be caused by leaking intake boots most likely, but you should check the condition of the vacuum like going to the petcock from the carb. If that was damaged and leaking it could cause what you described.

[micellis]

Originally Posted by jkv45

Did you store it with a full tank? If not, you may have enough condensation in the tank to cause running issues.

I always suggest completely removing and replacing the fuel - if there is any question at all - before going into more complicated troubleshooting.

It was about 3/4 full, with seafoam added for stability. I've ran it every 2 weeks for about 30 mins, even just idling to cycle gas through.

[jkv45]

Originally Posted by micellis

It was about 3/4 full, with seafoam added for stability. I've ran it every 2 weeks for about 30 mins, even just idling to cycle gas through.

I would still replace the gas.

In the future, add the stabilizer and fill the tank to the top, then drain the floatbowls. Best thing to do is change the oil and filter before storing and not start it again until it's time to ride. Starting and idling just contaminate the fresh oil that is coating and protecting the engine internals from corrosion.

[spooph]

Originally Posted by micellis

I'm completely stock as far as I know. PO brought the bike up from KC from what the titlework looked like and didn't ride it much. I put some seafoam in the tank for the winter, would it be worth running a full tank of with seafoam through it before I pull the carbs?



I was planning on draining and cleaning the tank, changing the fuel filter when I pull everything off for the carbs. If it turns out to be a vacuum leak would it be better to just replace all of the lines?

If you're gonna remove the carbs, no sense in running more sea foam through it...

No need to replace the lines, fuel filter or any parts if you can clean them. If ANY lines however are cracked, brittle or hard it would be best to replace them...

It sounds to me like this might be a compound issue. Gummed up carbs can also cause your symptoms, so it would be best to start taking things apart and investigating further...

If you require any assistance, I'm in Golden... I'll be gone the first 2 weeks in June, but after that I'd be happy to assist where needed. Post up some pics as you take things apart and we can help you further on here.

[crazymadbastard]

Originally Posted by micellis

Thanks in advance.

Did you buy this bike at a lower elevation and then took it up high?

[micellis]

Originally Posted by crazymadbastard

Did you buy this bike at a lower elevation and then took it up high?

No it was bought in North Denver, but looking at the title work the previous owner had registered it in Kansas City and brought it up to Denver.

[InvisiBill]

Originally Posted by micellis

Hey guys, I bought a 97 250 last October and it ran great for a month, then when warmed and ridden after about 20 minutes it just bogs down and dies. If I let it sit for 30 minutes or so it will crank back up and go for another 10-20 minutes before bogging down and dying.

This could be the gas cap vent not working properly. It causes a vacuum lock in the tank so fuel can't easily run out the petcock. It will act like it's running out of gas, because the carbs actually do run out of gas.

For a quick check, just use a spare key and open the gas cap when the problem happens. That'll allow air into the tank so the fuel can run out properly. Obviously it's not a permanent fix, but it's a free and easy way to verify if that's the problem.

[greg737]

Most of my riding will be around 5-6k feet but I do plan on some mountain riding and could be around 9-10k.

If you're riding in a group when you go up to 9-10k you'll notice that the performance of motorcycles with high-revving, small displacement cylinders will fall off before the motorcycles with fewer, lower-revving cylinders. Even if the displacement of the two bikes is equal.

Example: The performance of a 600cc inline 4 will fall off before a 600cc v-twin. And the 600cc v-twin will fall off before a 600cc single cylinder engine.

Both bikes have to contend with the reduced amount of oxygen in the atmosphere but there's a secondary effect of high altitude: the reduced atmospheric pressure causes the Volumetric Efficiency of the engine to fall off.

While the effect of reduced oxygen percentage is equal for all engines running at that altitude, the effect of reduced atmospheric pressure is not equal.

The effect of reduced atmospheric pressure is that the individual cylinders begin having difficulty "filling" themselves (using intake suction to accelerate a gulp of atmosphere into the cylinder). A small fast-moving cylinder will have a bigger problem than a large slower-moving cylinder.

Even though it's not absolutely correct you can get a pretty accurate idea of the problem by imaging that the atmosphere gets "stretchy" at high altitude, so the "long pull" of a big, slower moving piston will be better at moving it.

This means the EX250 isn't a very good choice for high altitude riding.

[spooph]

Originally Posted by InvisiBill

This could be the gas cap vent not working properly. It causes a vacuum lock in the tank so fuel can't easily run out the petcock. It will act like it's running out of gas, because the carbs actually do run out of gas.

For a quick check, just use a spare key and open the gas cap when the problem happens. That'll allow air into the tank so the fuel can run out properly. Obviously it's not a permanent fix, but it's a free and easy way to verify if that's the problem.

Doesn't the pregen have an overflow tube like the new gen though? Shouldn't it be able to breath out of that as well?

Yes, good recommendation for the test, just wondering for my own edification.

[InvisiBill]

Originally Posted by spooph

Doesn't the pregen have an overflow tube like the new gen though? Shouldn't it be able to breath out of that as well?

Yes, good recommendation for the test, just wondering for my own edification.

That's a good question for someone who knows about these bikes. =)

It's a huge problem on the VN750 (they even gave it a name - Phantom Out Of Gas Syndrome, or POOGS) and pretty common on the EX500. I've seen posts about the vent here before, so I'm just assuming it's similar.

On the 500, there's an overflow drain tube that's connected to the area around the fill hole. But the air in the tank still has to go through the vent system to breathe out to that area.

The tank has to have some sort of vent. If air can't get in to replace the fuel that's leaving, it'll vacuum-lock and the fuel will stop flowing. They use a few rubber discs in the vent so that it's not just completely open to the atmosphere, which would allow for faster evaporation, contamination, and more spillage if it tipped over. Over time those rubber discs get hard and stop working properly, leading to that vent not actually venting.

[Motofool]

Originally Posted by micellis

............... when warmed and ridden after about 20 minutes it just bogs down and dies. If I let it sit for 30 minutes or so it will crank back up and go for another 10-20 minutes before bogging down and dying..............

http://forums.ninja250.org/posting.p...&t=79820&tro=1

http://forums.ninja250.org/posting.p...&t=81112&tro=1

Originally Posted by micellis

............ I was curious about rejetting for the Denver altitude.............
Any advice here?

Thanks in advance.

http://faq.ninja250.org/wiki/Jetting...26_temperature

http://faq.ninja250.org/wiki/Why_wou..._my_jetting%3F

http://faq.ninja250.org/wiki/How_to_...e_your_jetting

[DaBlue1]

Originally Posted by micellis

.....Now that summer is on it's way I plan on pulling the carbs for my first time and cleaning them. While I'm in there I was curious about rejetting for the Denver altitude. I've been told that it isn't needed and to not worry about it, but I've found a couple topics around different forums that state a #102 main jet and #35 pilot jets. Most of my riding will be around 5-6k feet but I do plan on some mountain riding and could be around 9-10k.

Any advice here?

Thanks in advance.

There is some good info here on jetting for Altitude, Humidity, and Air Temperature.

[spooph]

Originally Posted by InvisiBill

That's a good question for someone who knows about these bikes. =)

It's a huge problem on the VN750 (they even gave it a name - Phantom Out Of Gas Syndrome, or POOGS) and pretty common on the EX500. I've seen posts about the vent here before, so I'm just assuming it's similar.

On the 500, there's an overflow drain tube that's connected to the area around the fill hole. But the air in the tank still has to go through the vent system to breathe out to that area.

The tank has to have some sort of vent. If air can't get in to replace the fuel that's leaving, it'll vacuum-lock and the fuel will stop flowing. They use a few rubber discs in the vent so that it's not just completely open to the atmosphere, which would allow for faster evaporation, contamination, and more spillage if it tipped over. Over time those rubber discs get hard and stop working properly, leading to that vent not actually venting.

Thanks! Gotta find the holes in the knowledge base and eliminate them. I had no idea the over-flow was on the atmosphere side of the gas cap... I'll have to look into this more... But it certainly sounds like the issue! Can't wait to hear what the OP says and if this fixes the issue..

[micellis]

Originally Posted by spooph

Thanks! Gotta find the holes in the knowledge base and eliminate them. I had no idea the over-flow was on the atmosphere side of the gas cap... I'll have to look into this more... But it certainly sounds like the issue! Can't wait to hear what the OP says and if this fixes the issue..

Sorry for no update, bike is locked in my parents garage and they are out of town.

[spooph]

update us when you can, PM me if you need any hands-on assistance after mid-June...