Bike won't rev above 5000rpm

drunkaroos

drunkaroos

XS650 Member
Messages
22
Reaction score
14
Points
3
Location
Alberta
Bike splutters really hard at 5000rpm, and when I hold it at around 25% throttle. Carbs are a fresh rebuild, already checked the diaphragm for leaks, properly synched, and mixture screws are tuned according to the carb guide. Coil is a new MikesXS high output. Haven't looked at the timing yet but I checked it last month with a timing light and it was in spec. Been chasing this issue for a few days, any help on the cause would be appreciated! Thanks
 
Did you mess with any Jets or move needle position? It could have something to do with the new coil. I assume that it was running good last month so I would go back to what you just changed about it.
 
I never really ran it up past 5k before yesterday. It's been an ongoing project for about a year now. At this point I'm throwing parts at it hoping something works, might just upgrade the whole ignition system other than the coil at this point and see if that helps.
 
Interesting. Does it react differently if you go farther than 25% throttle? And are you using stock CV carbs?
And just to get more info, what year is the bike? I’m curious if it is electronic ignition or points.
 
Odds are it is a fuel issue, but....Do you have a spare TCI module you could try? The dwell tends to get shorter at the higher rpms on these modules. At 50 years old, some of the components on the board will have drifted a bit. A dwell that is too short will cause some problems.
 
I don't have an extra but I might just order one. I spent 6 hours today troubleshooting (completely dissambled and reset timing system, resynched carbs) with no change.
 
So you said you reset timing, what did you do exactly? And could you see that the timing was advancing when you revved the engine? I’m not sure at what rpm the engine should have full advance, but if you see full advance when you are revving then i would think that fuel delivery is an issue. Maybe not enough fuel or too much at that higher rpm??
But back to ignition, my 81 ran great for awhile after i put it together and then randomly started to miss heavily on one cylinder, and it turned out to be the TCI module. I’m not sure why it would only be happening at higher RPM for you though.
Anyone have any thoughts?
 
Sorry, I didn't word it correctly. We took apart and cleaned the alternator, and then adjusted the timing with a timing light. Could it be an issue that it has a relatively free-flowing exhaust with stock jetting? We used the mixture screws to compensate for it running a little lean.
 
I had a weird issue that I fixed that ended up being the fuel cap. I chased my tail for a long time thinking it was the ignition system. Try opening it up while it is running and see if anything changes.
 
Yes, I believe that a free flowing exhaust would affect how the carbs need to be jetted. And like bushyeyed1 said about the fuel cap, if it is not properly vented then it can cause problems.
And when you adjusted the timing, were you able to see the timing advance when you revved up the engine?
 
drunkaroos said:
Sorry, I didn't word it correctly. We took apart and cleaned the alternator, and then adjusted the timing with a timing light. Could it be an issue that it has a relatively free-flowing exhaust with stock jetting? We used the mixture screws to compensate for it running a little lean.
Click to expand...
Sure could, open pipes and free-flow air in7ake need more fuel ! READ carb guide info in 7ech a7 7op of page.........up 1 size main je7 and 2 on low speed je7s will help, ma need more. Apologize for sevens bu7 a couple of kees are fried on compu7er...............
 
We did another few hours of work on the bike today. Cleaned the carbs again with an ultrasonic cleaner, adjusted the valve lash, and put on the stock pipes. We also re-synched the carbs. The problem has changed a little bit, the bike will now rev to redline, but it bogs down really hard when accelerating from low rpm's.
 
bakerman24 said:
So you said you reset timing, what did you do exactly? And could you see that the timing was advancing when you revved the engine? I’m not sure at what rpm the engine should have full advance, but if you see full advance when you are revving then i would think that fuel delivery is an issue. Maybe not enough fuel or too much at that higher rpm??
But back to ignition, my 81 ran great for awhile after i put it together and then randomly started to miss heavily on one cylinder, and it turned out to be the TCI module. I’m not sure why it would only be happening at higher RPM for you though.
Anyone have any thoughts?
Click to expand...
The TCI module advances the spark between 1900 rpm (15 degrees advance) to about 3100 rpm (40 degrees). The spread is fixed at 25 deg just by the fixed physical distance between the two pickup coils. You can vary the start and end points by adjusting the position of the pickup coil module itself ... say 17 to 42. Or 13 to 38 but the start and end is always 25 degrees apart. The rpms that the advance works over can change over time due to degradation of components, but typically it will advance somewhere in that range. At the high rpms that they are having issues, the advance will be fixed at 40 deg +/-2. The dwell does tend to narrow over the active range of the advance curve, but should steady out once it hits full advance, provided it is working properly. I have only looked at them on an oscilloscope. I don’t know if you could check it with a dwell meter?
 
drunkaroos said:
I don't have an extra but I might just order one. I spent 6 hours today troubleshooting (completely dissambled and reset timing system, resynched carbs) with no change.
Click to expand...
Well it’s to bad you don’t live closer. I could check the module on the bench. I have a spare board, but am in the process of modifying it. Otherwise I might lend it out.
 
Took a look at the gas tank cap vent today. It was a little crusty, but had good airflow after we cleaned it. Took it out for a ride as well, and we think the issue is the tci module for sure. Does anyone know where to find one for a decent price? We seem to be only able to find them in complete ignition system upgrade systems or for 500$ from independant suppliers. Thanks for the help so far everyone
 

Similar threads

B
XS650 Won't Rev
2
Replies
24
Views
840
BaronJess
B
S
Won't Rev past 4k
Replies
2
Views
397
SPCflattrack
S
Glowyeast
Can't rev above 5k rpm and go faster than 65
2 3
Replies
49
Views
3K
joebgd
joebgd
2ManyHobbies
77 xs650 won't start after mods
Replies
19
Views
536
sleddog83
sleddog83
G
New build will not start.
Replies
19
Views
853
BowlingSS
BowlingSS
Back
Top