Author Topic: Anyone rejet for elevation while touring? (Read 9263 times)

markreimer · « **on:** July 09, 2012, 11:24:34 am »

Hi there, this Saturday I'm heading out on a long tour on my cb750k4. I live in Winnipeg which is pretty close to sea level, and some of the passes I'll be taking are around 11,000 feet above sea level. Last year while riding Highway #3 in Canada through the Rockies, there was a pass or two where I felt like the bike would barely make it over. Top speed dropped to about 60km/h, which made me feel a touch vulnerable on an otherwise speedy highway.

I was wondering if any of you take along different mains to switch out in higher areas or for high passes? Or do you just suck it up till you make it over?

Here's the route:

https://maps.google.ca/maps/ms?msid=212220975008335473536.0004c2859773fcf3fddf5&msa=0

Duke McDukiedook · « **Reply #1 on:** July 09, 2012, 11:29:29 am »

And run rich and foul plugs the rest of the time? Doesn't make sense to me.
Maybe wire up a hazard flasher when you are putt-putting through the passes might be a good idea but a rejet when you spend 99.9% of your time at or near sea level is a waste of time.

markreimer · « **Reply #2 on:** July 09, 2012, 11:33:45 am »

Quote from: DukieFrankenkit on July 09, 2012, 11:29:29 am

And run rich and foul plugs the rest of the time? Doesn't make sense to me.
Maybe wire up a hazard flasher when you are putt-putting through the passes might be a good idea but a rejet when you spend 99.9% of your time at or near sea level is a waste of time.

Sorry I meant if I knew I'd be heading into much higher elevation, is it worth pulling over and swapping out the jets for the time spent higher up? I can change the jets with the carbs on the bike in about 10 minutes so it's not a huge job really, just don't know if its worth it or common practice.

Deltarider · « **Reply #3 on:** July 09, 2012, 11:37:16 am »

Quote

Anyone rejet for elevation while touring?

No.

DJ_AX · « **Reply #4 on:** July 09, 2012, 11:53:19 am »

Although I haven't taken that trip yet... wouldn't it be enough to just turn the idle screw.

camelman · « **Reply #5 on:** July 09, 2012, 12:02:00 pm »

I haven't and I've been to 7500 feet without any real power loss. The issue with altitude is that the air gets thinner but fuel flow remains mostly the same. You can pull your airbox lid or even the air filter to account for those changes. So, if you start running rich, try pulling the airbox lid, and then the filter. That'll likely be all you need to do. No jetting changes or carb adjustments required.

flybox1 · « **Reply #6 on:** July 09, 2012, 12:05:57 pm »

Quote from: DJ_AX on July 09, 2012, 11:53:19 am

Although I haven't taken that trip yet... wouldn't it be enough to just turn the idle screw.

idle screws have little to no effect above 1/4 throttle. Unless your are going over the passes in a traffic convoy or at parade speed, don't bother.

markreimer · « **Reply #7 on:** July 09, 2012, 12:49:04 pm »

Quote from: camelman on July 09, 2012, 12:02:00 pm

I haven't and I've been to 7500 feet without any real power loss. The issue with altitude is that the air gets thinner but fuel flow remains mostly the same. You can pull your airbox lid or even the air filter to account for those changes. So, if you start running rich, try pulling the airbox lid, and then the filter. That'll likely be all you need to do. No jetting changes or carb adjustments required.

Great idea to lower the lid!

Old Scrambler · « **Reply #8 on:** July 09, 2012, 06:17:12 pm »

Great Route

If you have 120s now I would install 110s for the entire trip. I had 105s on a stock K3 with good power at altitude......switched to 120s for more everyday power but notice the loss when above 12,000 feet.

Regarding the timing of your trip.....take a look at this:

http://forums.sohc4.net/index.php?topic=105559.125

Maybe you could help out

Take a look at the North Central group for a potential handoff in N. Dakota............

andy750 · « **Reply #9 on:** July 09, 2012, 06:37:54 pm »

No problem with loss of power on a CB750K4 (110 jets) in the Alps:

Passo del Stelvio, Italy

Glossglockner, Austria

or the Rockies

Coal bank Pass 10,600 ft

Red Mountain Pass, 11,000 ft

Good luck!
Andy

Mooshie · « **Reply #10 on:** July 09, 2012, 07:13:31 pm »

Andy you're the man. I want to ride with you...

andy750 · « **Reply #11 on:** July 09, 2012, 07:48:01 pm »

Quote from: Mooshie on July 09, 2012, 07:13:31 pm

Andy you're the man. I want to ride with you...

Would love to ride with you Mooshie!

BTW did you notice the 550 in some of the photos? That belongs to Ute a very cool female rider from LA.

scottly · « **Reply #12 on:** July 09, 2012, 07:50:21 pm »

I never realized the effects of altitude until I rode over Monarch Pass in CO, (11,312') when I noticed the lack of power. There is simply less oxygen in the air, and no amount of re-jetting will correct that. You may need to run it a lower gear and rev the motor more.

gregimotis · « **Reply #13 on:** July 09, 2012, 09:07:00 pm »

I live at 7000' and have ridden all around the southern rockies and Utah and 9-11,000' including a run up Mt Evans (14,200') without any put-putting around (as an aside, I got a little air on a frost heave - but no picture to prove it).
I don't know for sure that my jets are stock. I do know that atmosphere thins at an increasing rate as one gains altitude. Therefore I'm not convinced the bikes can't take sea level to 7,000' in stride w/o other factors... But I'm also not smart enough to know what those factors might be.
Hoping someone who is chimes in.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #14 on:** July 09, 2012, 09:15:47 pm »

If your jetting is correct at lower elevations don't worry about rejetting for a few quick transits over a few high elevation passes. Keep it in lower gears and keep the rpms up for those few high passes. Not worth the trouble to rejet for a few minutes. If you STAY at higher elevations then do a permanent jetting change.

I too rode Mt Evans with 2 size larger jets but didn't go above 3rd gear.

gregimotis · « **Reply #15 on:** July 09, 2012, 09:23:01 pm »

Quote from: Jerry Rxman Griffin on July 09, 2012, 09:15:47 pm

I too rode Mt Evans with 2 size larger jets but didn't go above 3rd gear.

I'm sure I didn't get out of second for the high end, and it wasn't sporty riding. My point is only that I wasn't struggling for power as the OP seems to have been.

754 · « **Reply #16 on:** July 10, 2012, 06:41:10 am »

Jet it a tad lean for touring, better mileage..plus LESS tendency to run rich when hitting high er altitude passes.

lostmykeys · « **Reply #17 on:** July 10, 2012, 02:38:51 pm »

My house sits at 5000 above sea level.
The only thing I notice is when I'm going 70 mph or above and give it the gas there is a slight hesitation before the power kicks in.Reminds me of the old cars with a 4 barrel carb.

scottly · « **Reply #18 on:** July 10, 2012, 06:54:31 pm »

Quote from: lostmykeys on July 10, 2012, 02:38:51 pm

My house sits at 5000 above sea level.
The only thing I notice is when I'm going 70 mph or above and give it the gas there is a slight hesitation before the power kicks in.Reminds me of the old cars with a 4 barrel carb.

Lost, if you ever ride your bike at sea-level, I can guaranty you will feel a difference. I had gotten used to riding my bike at 4500', and the last time I rode it at sea-level, I thought my clutch was slipping. Turned out the rear tire had been spinning..

super pasty white guy · « **Reply #19 on:** July 10, 2012, 07:53:54 pm »

Went from sea level (washington dc) through trail ridge (11k+) and beartooth ( 10.9k) with a loaded bike and no problems. Just adjust for where you are and don't worry. If you're going to worry about maintainence, concentrate on chain lube and motor oil. Enjoy the ride.

Spwg

HondaMan · « **Reply #20 on:** July 10, 2012, 08:22:47 pm »

Worst case: drop a gear, even two. At 55 MPH, the 750 isn't happy in anything above 4th, anyway. I ride the high ones frequently: one of my favorite morning rides is to get up at sunrise, run it up to the top of Mt. Evans (14,260 ft), watch the sunrise, then buzz back down the hill to go to work. I frequent many passes well over 10,000 feet, no issues after dropping a gear to keep up the speeds. At 50-60 MPH (100km/h), 3rd gear is appropriate above 9000 feet, and the engine won't mind a bit.

I still have [almost] stock jetting: I admit to dropping a 5 size when the ethanol showed up in the gas, just to stop the plug fouling problems. My K2 came with 110 which I kept until then: now it is 105, which most of the 750 were after 1973 (K4 and later). I live at 6000 feet, commute to 6800 feet every day to work.

Sniper X · « **Reply #21 on:** July 11, 2012, 07:55:27 am »

I can tell you that jetting for a median altitude once before the trip will be better. I would just go one step leaner on both pilot and main and lower the needles a notch. Higher menas leaner on fuel since it is less air. I live at 7420ft and commute to 5200 every day and that isn't enough to change but if I lived at sea level which I used to, And was going on a cross country ride I would jet in the middle and be good from sea level to 10K ft and on those REALLY high mountains, I would just deal with it as it would still be close enough to be safe. In reality, the difference between 10k ft and sea level on SOME bikes might be as much as about 3~4 jet sizes, here at 7420 it is three sizes leaner than sea level on a 4 cycle single and 2 on a multi. AND the transition has to be changed too, the needle. Now it isn't as important to swap in a leaner pilot because that is off idle only and the needle and main are more important for regular riding. I alos suspect you would get away with changing the main only out to a 2 step leaner after getting to about a mile or so high and be good for the highest highs.

scottly · « **Reply #22 on:** July 11, 2012, 06:25:20 pm »

Quote from: Sniper X on July 11, 2012, 07:55:27 am

I can tell you that jetting for a median altitude once before the trip will be better. I would just go one step leaner on both pilot and main and lower the needles a notch.

I disagree. I would rather risk a fouled plug from running rich at altitude than an over-heated/seized motor at low altitude. Ask me how I know.

In any case, the power loss from altitude is about 2.6% per 1000 feet; at 10,000 feet, the motor will only produce 73.85% of the power at sea-level. I looked at a dyno curve of a stock 750, and found that to produce the same power as at 5500 RPM, the motor would need to spin at 7000 RPM at 10,000 feet.

markreimer · « **Reply #23 on:** July 26, 2012, 06:29:05 pm »

Thanks guys for all the suggestions. I got back last night - I rode 8,128km in 12 days. Manitoba to North Dakota, Montana, Idaho, Washington, Oregon, Idaho again, Wyoming and South Dakota. Highest pass was just over 11,000 feet and the bike was a lot weaker but the road was too twisty for it to matter really. Other than one highway crash

requiring some road-side repairs to get back on my way, the bike performed flawlessly. These cb750s are indestructible it seems.

I shot the whole thing on film and am getting it developed now, I'll post a link to a ride report and photos when it's ready!

lucky · « **Reply #24 on:** July 26, 2012, 06:52:05 pm »

Quote from: markreimer on July 09, 2012, 11:24:34 am

Hi there, this Saturday I'm heading out on a long tour on my cb750k4. I live in Winnipeg which is pretty close to sea level, and some of the passes I'll be taking are around 11,000 feet above sea level. Last year while riding Highway #3 in Canada through the Rockies, there was a pass or two where I felt like the bike would barely make it over. Top speed dropped to about 60km/h, which made me feel a touch vulnerable on an otherwise speedy highway.

I was wondering if any of you take along different mains to switch out in higher areas or for high passes? Or do you just suck it up till you make it over?

Here's the route:

https://maps.google.ca/maps/ms?msid=212220975008335473536.0004c2859773fcf3fddf5&msa=0

Sea level to 11,000 feet!!! WOW!!

The real problem is going to be at 8,000 feet and up.
It will not want to idle because it will be very rich.
If you will not be there long then, just remove the air filters and it will run fine.

Seal level will be fine, just put the choke on a little if necessary.

BTW if YOU are above 10,000 feet you will need an oxygen mask. NOT kidding.

News:

Author Topic: Anyone rejet for elevation while touring? (Read 9263 times)