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Altitude changing bike performance

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Jacob2

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Hello all,

I recently brought my all stock 1979 CB 750k from Illinois witch is at 700 foot elevation to out Colorado, 7200feet where I reside now.

Obviously the bike is going to run rich but how rich? What I’m wondering is am I going to be forced to rejet it? (we all know how much of a pain in the ass it is to take the carbs off)

Has anybody else had this experience and can share some insight?

Thanks
 
When I bought my bike in Albuquerque I got all the service records back to 2015. At some point the carbs were rebuilt and "high altitude" jets were installed. I have no info on the sizes of those jets.
I do know that the bike ran fine all winter at around 800-1500 feet of elevation. We'll see here in another couple of weeks how it does at 7200' where I'll be for the whole summer and fall. I'm not so sure the CV carbs are as sensitive to elevation as the round top carbs.
 
Yeah please share when you do take it for a ride.

I purchased some iridium spark plugs to throw in because I know they don’t foul as easy(worked on my old two stroke dirtbike ). Its my back up if I do run into a lot of fouling issues with the coppers.
 
Jacob2 I was wonder the very same thing for my 81’ cb750c. I just had the carbs rebuilt in Indy and moving back west again to Boulder, CO area where the roads are great riding, but will be living at 6k feet. How did that affect things? Hate to leave a good bike sitting for a year or two when I can ride it
 
Ha, I was in the same boat.

So as of today I have about about 120miles on her. No fouled plugs yet, she does run rich as hell. Exhaust stinks, backfires a little bit at high speed throttle let off, and of corse, less power. But its not to bad really, I have the jets for it, but thats a rainy day project.

At about 200miles Ill pull the plugs and check it out.

I would say send her for a ride, just be like me and bring ALOT of tools in a backpack and spare plugs.

I live up the hill in Evergreen, shoot me a message on here (or however this site works) if ya ever need a guy to ride with.

Goodluck
 
Jacob2 said:
I live up the hill in Evergreen, shoot me a message on here (or however this site works) if ya ever need a guy to ride with.

Goodluck
Click to expand...
Totally OT here, but when I was a young kid back in '57-'58 we lived outside Evergreen in a subdivision called Hidden Valley. I remember my dad towing us on our sleds behind his Jeep in the winter.

To get back on topic, I took the bike out for a ride and it runs great at altitudes up to about 8500'. When I got close to 9700' it didn't want to idle, but got it back down to 7200' and it runs fine.
 
Sounds like we need to have a CB 750 ride with all you guys living so close to Denver, haha.

But that’s awesome man, I’m on the opposite end of the spectrum. 23 years old but really just appreciate the old shit.

It’s amazing how resilient these bikes are, I think I’m gonna run my bike up to 10,000ft here later in the summer just to see what happens. Obviously I’ll have the lady on speed dial with the truck to pick me up if I break down lol.
 
got the same problem, when moved from IN to CO from 700' to 6700' in elevation, and plugs running very rich and foul after only a few miles. Assume it's the jetting but what size do you need? Any insight is greatly appreciated.
 
Jacob2 said:
Sounds like we need to have a CB 750 ride with all you guys living so close to Denver, haha.

But that’s awesome man, I’m on the opposite end of the spectrum. 23 years old but really just appreciate the old shit.

It’s amazing how resilient these bikes are, I think I’m gonna run my bike up to 10,000ft here later in the summer just to see what happens. Obviously I’ll have the lady on speed dial with the truck to pick me up if I break down lol.
Click to expand...
I'm for it, I'll bring the beer
 
Last summer I logged about 500 miles on the stock jets. I experienced no issues or fouled plugs besides what I mentioned above (loss of power, stinky exhaust crackling a little bit when you let off throttle)

I am by no means an expert in this field.

Assuming you have the same bike as me, you could probably do some research and figure out what stock jetting is on these bikes. Then throw that size in and theoretically it should run just like mine, not great, but rideable.

On the flipside, you could try and run iridium spark plugs? Like I said before I used to throw those in my two stroke dirtbike, and it was really hard to foul those plugs.

I am by all means an amateur in this, so hopefully someone else can chime in.

Goodluck
 
Jacob2 said:
Last summer I logged about 500 miles on the stock jets. I experienced no issues or fouled plugs besides what I mentioned above (loss of power, stinky exhaust crackling a little bit when you let off throttle)

I am by no means an expert in this field.

Assuming you have the same bike as me, you could probably do some research and figure out what stock jetting is on these bikes. Then throw that size in and theoretically it should run just like mine, not great, but rideable.

On the flipside, you could try and run iridium spark plugs? Like I said before I used to throw those in my two stroke dirtbike, and it was really hard to foul those plugs.

I am by all means an amateur in this, so hopefully someone else can chime in.

Goodluck
Click to expand...
Also FYI Im not sure off the top of my head what stock jetting is, thats a wormhole I never really went down…
 
Jacob2 said:
Also FYI Im not sure off the top of my head what stock jetting is, thats a wormhole I never really went down…
Click to expand...
Also, another thing I’ve heard is if you take that cover off the side of your airbox where you can access your air filter and just leave it off. That way your air filter is still doing its job, but it is allowing more air in. Thus making it run less rich.

Again, I’m no expert in this these are just hacks that I’ve heard
 
HoosierHonda750 said:
Jacob2 I was wonder the very same thing for my 81’ cb750c. I just had the carbs rebuilt in Indy and moving back west again to Boulder, CO area where the roads are great riding, but will be living at 6k feet. How did that affect things? Hate to leave a good bike sitting for a year or two when I can ride it
Click to expand...
HoosierHonda750 said:
Jacob2 I was wonder the very same thing for my 81’ cb750c. I just had the carbs rebuilt in Indy and moving back west again to Boulder, CO area where the roads are great riding, but will be living at 6k feet. How did that affect things? Hate to leave a good bike sitting for a year or two when I can ride it
Click to expand...
Another Hoosier !!! , from Indy also, and have an 81K. carbs are running very rich out here and trying to figure out the jetting equation ......HPH stands for "High Plains Hoosier"
 
Jacob2 leaving that cover off is exactly what i had to do when i put 102 main jets in the carbs because every book i had said that was the size, the only problem being my 750FZ was a Japanese market bike fitted with 75's from the factory and matched the air intake size at the back of the air box. i couldn't get it to rev above 5000rpm untill i did this. i have now put the 75's back in and everything is good.
 
Hey, in a similar boat here as I've moved from Cali to Northern CO, I was curious about what to do with higher elevation jetting! '81 CB750C - still got a lot of work to do to get the bike on the road but like the idea of removing the airbox filter cover as a simple method of airflow compensation!
 
Well that's the big question; what to downsize from the 68/102! But no one has workable values listed.
 
You might be right, I've never seen a chart, either. You're going to have to go down incrementally, one at a time, and see what happens.
 

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