Ran great - now will not start

davidjbeatty · New Member Joined: Feb 28, 2009 Posts: 8

Hi,
First off, I don't know which category to list this under, so I apologize if I am in the wrong place.
I recently purchased a 79 xs650 that the previous owner put a lot of new parts on.
It has Pamco ignition and Mikuni carbs and was running (somewhat rough) when I purchased it.
I was tweaking the carbs the other night and had it running great, I could kick start it effortlessly even after a couple of hours of cooling down.
Unfortunately, I had killed the battery during the tweaking process, so before I went to bed, I put a 1.5 amp battery charger on the bike.
The next day when I tried to go for a ride, I could start the bike and get it to idle, but it would not accept very much throttle, it would die when I gave it the gas.
Now, it won't even idle. It will turn over a couple of times and crack off a loud backfire (scares the crap out of my horses and my neighbors horses).
I set the carbs back to where they originally were and nothing changes.
Anyone have any suggestions?
Thanks,
David

jayel

get the battery charged back up first

penn71911

Agreed.

That big backfire is a classic sign of a low battery... been there... done that. Put a multimeter on it, should be 12.5v fully charged.

Pull the spark plugs and make sure they aren't fouled also... low battery=weak spark=fouled plugs

davidjbeatty · New Member Joined: Feb 28, 2009 Posts: 8

Hey,
Charger is on it now and I'll check the plugs in the morning.
Thanks!

pamcopete · Posted: March 1, 2009, 12:29 am Post subject: Re: Ran great - now will not start

davidjbeatty

Are you out of gas after all that tweaking?

Retiredgentleman · Posted: March 1, 2009, 2:48 am Post subject: Re: Ran great - now will not start

If the engine will idle, but won't rev up when you give it gas, I'd recommend you clean the pilot jets in the carbs.

yamaman · Posted: March 1, 2009, 9:19 am Post subject: Re: Ran great - now will not start

The link to the VM tuning guide on this thread is pretty cool, that is if your's are VM's?

650rider.com/index.php...pic&t=6268

davidjbeatty · New Member Joined: Feb 28, 2009 Posts: 8

Hi all,
Battery shows 12.9v, plugs look great, have plenty gas, still won't run or idle.
I have no idea if these carbs are VM. (how can I tell?)
Should I pull the carbs and check all of the jets? There are some pretty serious filters on the fuel lines. I have pulled the fuel lines off of the carbs and fuel flows freely.
David

jayel · Posted: March 2, 2009, 12:46 am Post subject: Re: Ran great - now will not start

check timing first... fire to the plugs/on the right stroke?

Jake68 · 650Rider Supporter Joined: Mar 23, 2007 Posts: 841

You can tell a VM Carb because the throttle slide is under screw cap and the throttle cable pulls the slide up through the cap on the top of the carb.

The other type of most commonly fitted standard Mikuni's had flat caps with a screw in each corner over rubber diaphagms and the throttle control was via butterfly controlled through the side of the carb body.

Heres the interesting bit...

VM carbs are the bomb, they are simple and easily cleaned, and IF setup correctly just work. HOWEVER, they are often sold poorly or incorrectly setup with the wrong internals or have been incorrectly adapter from bikes that had these fitted as OEM. So check out the VM XS650 Links and take your carbs apart and check them, if you have VM's.

..and then there is the original Mikuni CV Carb. These carbs are slightly over complex in attempt to deliver a more regulated fuel air ratio they mostly seem to go wrong a little more often than well setup VM's. They suffer terribly from crapping up and can be expensive to repair if new diaphagms are needed......HOWEVER...if you clean them right out really well and set them up properly with the correct mains and pilots if the engine has been modified in ANY way..and then use very fine Micron (I use 30 Micron agricultural filters from Briggs and Stratton) to keep them clear of any of the crap than can get in fuel from tank contamination...if you do this, they will run and run and run and never need attention aside from external tuning like balancing and the odd low down mixture tweak..

Credit goes to RG for pointing out that tank filters (fitted to petcocks) are useless and inline filters are a MUST...and the square ones just arent up to the job...

Two of the things the Standard Mikuni carbs suffer from badly are incorrect float height adjustment, and crap in the pilot circuit. Both could cause your problems, but usually in my experience the pilot circuit get blocked and lean the bottom end out causing the opposite of what you have..ie no idle but runs on the throttle...

Pete's ignition system requires the standard advance mechanism, so it would be worth checking the timing with a strobe to make sure the ignition is being advanced correctly as the revs increase, and or ...check the advance mechanism is actually spinning out correctly ..just check visually ...a strobe is gonna be needed for a proper check...

Doesnt sound like there is too much wrong with it...usually if these engines are mechanically quiet and dont smoke or spout oil from anywhere...all is good Smile

pumps · Posted: March 2, 2009, 6:14 pm Post subject: Re: Ran great - now will not start

jake you said a mouthful but well stated.

davidjbeatty · New Member Joined: Feb 28, 2009 Posts: 8

Thanks All,
Yamaman, thanks for the link to the carb manual - I do have VM carbs.
Jake, these were bought new from Mike's XS for this bike. I'm going to pull the carbs in the next few days and clean them (now that I have that manual). I hope to get into this Wed. or Thurs.
I REALLY appreciate all of your help on this issue, I just wish I had this bike at the beginning of winter instead of the end, it's supposed to be 80 here on Sat. so I will probably be out on my Roadstar.
David

davidjbeatty · New Member Joined: Feb 28, 2009 Posts: 8

Penn, you were absolutely correct!
I pulled one of the carbs and checked the jets and everything looked very clean, so I pulled one of the spark plugs and spun the starter and I saw a weak spark. I pulled the other plug and had absolutely no spark. The weird thing is that both plugs looked a little sooty, but not bad. I replaced both plugs and everything works great now.
I know better than to go by looks on a spark plug, but the guy I bought the bike from said he had replaced the plugs "not too long ago".
I guess I'll be riding the xs on Saturday instead of the Roadstar
Thanks again all!
David

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