HELP! My buddy might murder my bike! Advice needed!!

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Hodakakid

Hodakakid
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Hey Guys

I'm needing some advice/ideas on some troubleshooting. A buddy of mine (motorcycle mechanic) is working on my XS. Not long after I got the bike it stopped firing on the right cylinder. He deduced it wasn't carburetor or the coil and had little to no compression...so we tore into it looking at pistons and rings. We wound up replacing the rings (some of the rings were lined up in the gaps). Anyway the bad cylinder also needed one valve seat machined, which we had done at a local machine shop. We put it back together and it now tries to fire very sporadically but not enough to even heat up the exhaust pipe. Both cylinders have 120 psi. We have gas, fire, compression. What else do we need to look into?

Any advice would be appreciated.
 
We can get it to idle (rough) but it will try sit there and shoot little bursts of backfires. While giving it throttle/revving the backfiring increases and still runs very rough.
 
Hodakakid said:
Hey Guys

I'm needing some advice/ideas on some troubleshooting. A buddy of mine (motorcycle mechanic) is working on my XS. Not long after I got the bike it stopped firing on the right cylinder. He deduced it wasn't carburetor or the coil and had little to no compression...so we tore into it looking at pistons and rings. We wound up replacing the rings (some of the rings were lined up in the gaps). Anyway the bad cylinder also needed one valve seat machined, which we had done at a local machine shop. We put it back together and it now tries to fire very sporadically but not enough to even heat up the exhaust pipe. Both cylinders have 120 psi. We have gas, fire, compression. What else do we need to look into?

Any advice would be appreciated.
Click to expand...

If you want help, you need to supply more information on the bike.
Year of bike?................year of engine?.................what carbs are on the bike?

What ignition are you using? .....................what exhaust is on the bike?

You say you have compression, but 120 psi is really poor compression.

You say you have gas, but that is rather vague, as you really need the correct mixture of fuel/air in order for the engine to run properly.

You say you have fire, but is the spark weak or is it strong? The fire must come at the right time in the cycle....................have you used a timing light to view the timing??

Have the carbs been taken apart and checked for clean jets and correct size of jets??
Float levels correct for the carbs you are using??

Are you using a fully charged battery? What is the battery voltage??
 
If you have points ignition, could look a little more closely at the coil and condensers. Inspect the coil body closely for any signs of cracks. Also, a bad condenser can cause that type of misfire. Telltale sign is arcing/sparking of the affected point while running...
 
FWIW, the only XS650 in the Kid's profile is an '81 bobber.
So long as a dreaded PO ain't messed with it it's got an electronic iggy which has to spark both plugs if it works at all so I'd say, put in new sparkplugs just for luck and clean the carbs mebbe 3 times over.
 
Hey, guys

Thanks for the help so far - first up, I'm limited in knowledge/experience on this bike on most of this info is passed along second hand ( yeah, I know that may be a deal killer for you all being able to help- but I'm willing to giving it a shot) so anyway... Here goes...

Yes, correct it's an 1981 hardtailed bobber, the stock exhaust has been hacked off to what looks to be way short (stopping close to mid mount foot pegs)

What ignition? I'm assuming it's stock (not upgraded) Some back story/info: I bought the bike as is (with the problem) and it's spent almost all of it's time at my buddies shop so I've had almost zero time with this bike and it's my first xs - so limited knowledge about bike... Anywho...

120psi poor? What's typical/ good?

The rest of the questions from retiredgentleman... I'll need to quiz my buddy for a complete answer as again, unfortunatly most info is second hand info...
 
fredintoon said:
FWIW, the only XS650 in the Kid's profile is an '81 bobber.
So long as a dreaded PO ain't messed with it it's got an electronic iggy which has to spark both plugs if it works at all so I'd say, put in new sparkplugs just for luck and clean the carbs mebbe 3 times over.
Click to expand...
What do you mean by "dreaded PO"? And to everyone who's answered so far... Thanks in advance for any help...
 
Oh, and a side question- regarding 125psi being poor compression - Whenever we replaced the rings we checked the pistons and they were marked 74. something and the cylinders were marked 007. That would be a standard bore, correct? We installed std bore rings. I remember being a little concerned that there was a #1 stamp over on the side of the piston and at first we were wondering about if that was an oversize marking but read (somewhere) that is not the case. So the 74. something added to the 007 equals to 75 mm standard bore. Correct? If so, what's another reason for the "poor" compression?
 
Dreaded 'PO' means Previous Owner. Ie. the person who sold you the bike messed around with it and it's no longer as per spec and they did something on the cheap or just plain stupid.

125 psi isn't great but it should run with that to be honest. Im not convinced your problem is because of that. About 140-150 is mint but it will run pretty much right down to 100 ok.

If it were me i'd just work logically and by elimination. Rule out electrics/timing first. Then carbs and fuel. Then consider motor issues.

Got good spark on both cylinders?
 
Last edited:
Your first posts said 120 psi compression and now you say its 125 psi ?

Your buddy can't be much of a motorcycle mechanic, if he can't rebuild a top end and
get at least 140 psi. Where did he train as a mechanic?

You've bought an old bike that some PO has messed with, which means it's probably in need of a complete going over from end to end...........electrical and mechanical. Your knowledge of bikes in general is weak, and you're trying to pass on second hand information to the forum.

I suggest you get a book(s) on motorcycle engine repair and on ignition systems, and start reading so you can learn enough to ask intelligent questions. A Haynes manual for the XS650 might be a good book to read.

I'll just repeat the same questions as in my first post.

You say you have fire, but is the spark weak or is it strong? The fire must come at the right time in the cycle....................have you used a timing light to view the timing??

Have the carbs been taken apart and checked for clean jets and correct size of jets??
Float levels correct for the carbs you are using??

Are you using a fully charged battery? What is the battery voltage?? Does the charging system charge the battery or not??
 
This seems to be a case of the blind (your "mechanic") leading the blind (you), lol. Neither one of you seems to have a clue about what you're doing.

When you installed your new rings, did you check the end gaps first to insure they were within spec? There's no sense installing new rings that are way too loose. You won't be much better off than with the old ones. This could explain your lower than ideal compression.

RingEndGap.jpg


The 007 stamped on your cylinder sleeve was your cylinder bore size when new, as in 75.xxx mm. The 007 is the fractional part of that number so your cylinders were 75.007mm when new. The piston should also have a 3 digit number stamped on top. That was its original size as in 74.xxx mm. You may need to get the piston top pretty clean to read it. Here's one I'm working on now .....

CleanPiston3.jpg


That number subtracted from the bore size should give you a clearance within the factory spec of .050-.055mm. There is a limit slightly above that that is still considered acceptable. All these specs are listed in the factory shop manual, which any competent mechanic would have referred to before even starting the job.

I think you're going to need to go back to square one and re-check all your topend work. I have a feeling it's not right. Pistons and bores should be measured to insure they are still within acceptable limits. If they are, cylinders should be honed before fitting new rings. If worn too much, a re-bore will be needed. End gaps on the new rings should be checked to insure they're not too large or too small. Did your "mechanic" buddy do all this? It doesn't sound like he did any of it, hence the poor rebuild results.
 
Do a leak-down test..:laugh::laugh::laugh:


None the less, It should at least "Run" if it was assembled properly and the valves were at least half ass good. One would think?

1. Valve timing correct?
2. Cam chain adjusted?
3. Valve Lash set correctly?
4. Static ignition timing (If needed for your ignition)
5. Clean Carbs. (Really clean, read carb guide)
6. Strong Battery (12+ Volts)
7. Hotwire Ignition (Simplify)

It Should run... After that you can redo the top end if needed..
 
Hodakakid, By kid I assume your , what 12. that's the way your acting. The people you are talking so poorly about have not said anything wrong. They both referred to your inexperience and the poor quality of your mechanic. Both of which you talk about. They gave you good advice and you attack them for it.
If you have been on this site much you would have read up on a lot of the things you are asking about. In doing so you would have seen just how good these two people are.
On your compression the book calls for a minimum of 145 PSI.
Less will run but not be as it could.
With good even compression numbers and poor running I would check the ignition. After trouble shooting the ignition then I would clean the carbs. www.amckayltd.com/carbguide.pdf is what you need for the carbs.
As suggested I also recommend a repair manual. Free downloads at biker.net.
Leo
 

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