advertisement
Forums

The Forum is sponsored by 
 

AAPL stock: Click Here

You are currently viewing the Tips and Deals forum
Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: microchip
Date: October 13, 2011 01:31PM
As you may recall, I've found myself in Silicon Valley. I brought a motorcycle out with me (From Colorado), with a catch - it was jetted for sea level already.

I went to fire it up this morning, and it's not happy. It'll only run well at about half-choke. If I turn the choke all the way off, it'll stop running - even after warming up quite a bit(fifteen minutes, maybe more).

My most immediate thoughts is gasoline might have something to do with it. It's got 91 octane(91 octane at 5000 feet), with ethanol(which I guess they don't have down here).


Anything else I should pay attention to? I'm a "Zen and The Art" person, so I'll fix anything that needs to, but I wanted to seek prior knowledge.

Thanks!
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: numbered
Date: October 13, 2011 01:48PM
California premium is 91 octane with ethanol as well. I don't think we are on the "winter formula" yet for the Bay Area...so I do not think new fuel will help.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: davester
Date: October 13, 2011 01:55PM
Octane only affects detonation and has nothing to do with mixture richness. I doubt ethanol has anything to do with your problems either. Based on your description it is jetted too lean, so perhaps they didn't get the jets set up right. If it's a constant depression carburettor (typical of older motorcycles) you will need to adjust the jet height to get the mixture right. You can't just throw a different jet in and expect it to run properly without adjusting the jet.



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: MacArtist
Date: October 13, 2011 01:56PM
Did the bike sit for quite a while?

Gas is crap. If it sits for very long, it will gunk up the carbs unless the person used Stabil (fuel stabilizer).



I sit on a man’s back, choking him and making
him carry me, and yet assure myself and others
that I am very sorry for him and wish to ease his
lot by all possible means — except by getting off
his back. - Leo Tolstoy, novelist and Philosopher
(1828-1910)

Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: microchip
Date: October 13, 2011 01:58PM
It sat for probably a month while it was making it's way out to California. That shouldn't be too long, should it?

Dave, nope, it's fairly new-ish. A 2000 SV650.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: davester
Date: October 13, 2011 02:19PM
Month old gas is not a problem. Your SV650 does in fact have Mikuni constant depression carbs (based on the SU design found on all old british sports cars [something I know a lot about]). These are very elegant and simple carburettors, but they require a complete understanding by the mechanic to get them set up and working correctly. It sounds to me like either the jets are wrong, the jet needle is wrong, or they are not adjusted correctly. I stumbled across this discussion on how to set up your carbs properly. [mickk.net] . You definitely need to get into some zen to get this straightened out. I am sure you will soon be a zen master of SU-type carburettors, grasshopper.



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan



Edited 2 time(s). Last edit at 10/13/2011 02:22PM by davester.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: Filliam H. Muffman
Date: October 13, 2011 02:24PM
It might just need a little cleaning but a month should not be a problem. Like mentioned, California gasoline has lots of ethanol. If the carb was optimized for Denver, it will be too lean at sea level IIRC. Good luck on getting it right. Got a manual?



In tha 360. MRF User Map
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: Dik2
Date: October 13, 2011 02:50PM
5000 ft vs sea level is not enough difference to make a bike that runs well at one, behave as you describe at the other.
First thing to check is last thing worked on.

I've done a lot of SU carb rebuilds (in the late 60's, early 70s), and it's easy to get it wrong enough that the needle binds in the jet, preventing free smooth movement of the piston. There's also the issue of jet height setting which has a big effect on mixture at all running speeds.
If the piston moves smoothly, I'd tear the carbs down and blow choke cleaner through the tiny holes in the main bore.
If you can run it without the air box and look down the carb throats while running (use a small flashlight held right near the carb), you might be able to see the difference in fuel spray between the two carbs, and tell if only one is working.
Be carefull!
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: chopper
Date: October 13, 2011 03:02PM
If you're a Zen and the Art kind of person make sure you sit in a room for days on end mulling over "what is quality, anyway?". Don't leave the room even for bathroom breaks, because that's--according to MN lore anyway--exactly what that nutjob Bob Pirsig did to that house in Mpls. He just used the floor.

The pilot jets in your bike are probably plugged, yes, from bad gas not even a month old. This is why it won't idle.

"constant depression"--actually, I think that was Pirsig before he started taking antidepressants. The SV carbs are constant velocity.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: microchip
Date: October 13, 2011 03:15PM
All, thanks a bunch. Off to see if I can find a place to actually work on it. Yet another negative point for living at an apartment.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: davester
Date: October 13, 2011 03:27PM
Quote
chopper
"constant depression"--actually, I think that was Pirsig before he started taking antidepressants. The SV carbs are constant velocity.

SU carbs were originally referred to as "constant depression". Later on people started also calling them "constant velocity" or "constant vacuum" carbs. [en.wikipedia.org]



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: mrbigstuff
Date: October 13, 2011 03:30PM
two words (but actually one brand name):

SeaFoam.

It will cure your problem almost guaranteed, if there is no other issue when the bike last ran

you can find it nearly everywhere now



Hurts like a bastid...



Edited 1 time(s). Last edit at 10/13/2011 03:31PM by mrbigstuff.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: chopper
Date: October 13, 2011 03:39PM
SU carbs were originally referred to as "constant depression". Later on people started also calling them "constant velocity" or "constant vacuum" carbs. [en.wikipedia.org]

Dude, come on. We're well past archaic terminology in this thread. We now have carbs that have pistons (slide?), jet height (needle height?) and pulling the airbox off the bike to see if one can watch the fuel spray with a flashlight to gauge how well the fuel is atomizing. Next time I see my motorcycle mechanic friend I'm going to tell him to put "bound needle" on the service order.

Do they even make choke cleaner today?

"A lot of people fail to understand that in dealing with carburetors, it's just a controlled leak." -- K Cameron



Edited 2 time(s). Last edit at 10/13/2011 03:40PM by chopper.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: davester
Date: October 13, 2011 04:07PM
Quote
chopper
Dude, come on. We're well past archaic terminology in this thread. We now have carbs that have pistons (slide?), jet height (needle height?) and pulling the airbox off the bike to see if one can watch the fuel spray with a flashlight to gauge how well the fuel is atomizing. Next time I see my motorcycle mechanic friend I'm going to tell him to put "bound needle" on the service order.

I'm not sure what you're getting at. You are the one who brought up the terminology by incorrectly correcting the term I used. It's not even archaic terminology...many folks working on brit cars and @#$%&/brit bikes use the terms interchangeably these days, and that does in fact appear to be the type of carburettor that we're talking about here. The link I gave him goes over tuning the carb in detail, and yes, it may very well be that the pilot jet is plugged (covered in the link), though that seems unlikely since it was apparently running OK just a month ago.



"In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion." (1987) -- Carl Sagan



Edited 2 time(s). Last edit at 10/13/2011 04:11PM by davester.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: microchip
Date: October 13, 2011 04:12PM
Gentlemen, no need to spur into an argument here.

Seafoam is probably worthwhile anyway. Then choke cleaner. Then I'll go from there.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: Psurfer
Date: October 13, 2011 06:45PM
I'm so glad that I sold my scooter last month -before another winter layup- that I want to post this out of pure joyful relief.

Btw, eBay has one really incredible bargain left: the "Powersports under 50cc" category charges only $10 for the final value fee -flat rate, and no insertion fee.
The scooter sold for a few thousand, the fee would have been $100 in the bigger-engined Powersports cat.
(Mine was modified; really a 70cc, but the model was originally 50cc, so what the hell.)



Edited 2 time(s). Last edit at 10/13/2011 06:50PM by Psurfer.
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: motopsyco
Date: October 14, 2011 03:15AM
[www.bayarearidersforum.com]

Register into the forum and post your query. There are plenty of SV650 owners that are members. Members in the south bay can direct you to a tuner in your area.

I'm motopsyco59 in the forum.

Cheers!
Options:  Reply • Quote
Re: Motorcycle + Carburetor + Choke + Altitude + Gas Question
Posted by: Yoyodyne ArtWorks
Date: October 14, 2011 12:10PM
Quote
chopper
The pilot jets in your bike are probably plugged, yes, from bad gas not even a month old. This is why it won't idle..

+1. Seafoam may help, but the passages in the pilot jets are very small and Seafoam or another chemical carb cleaner added to the gas tank may not work. You might need to remove the carburetors and manually clean the pilot jets. Use a strand of copper wire to clean them, copper is soft enough that it won't score the inside of the pilot jet.
Options:  Reply • Quote
Sorry, only registered users may post in this forum.

Click here to login

Online Users

Guests: 160
Record Number of Users: 186 on February 20, 2020
Record Number of Guests: 5122 on October 03, 2020