1978 BS38 breaks up at 2500. Video included.

[Prisondawg]

Hello, I bought a dusty crusty 650 last spring and I've been slowly making progress on it. Now I'm trying to get it all tuned up but I've reached a sticking point.

The bike starts up and idles great ! But it seems to struggle getting up to around 3000 rpm. Once I get there it runs fine !

I added UNI pods so I went up to a size 30 pilot jet. Everything else is stock on the bike. I have a new battery. I've tried running the bike with the clip on the second, third , and 4 position. And the same symptom persists.

I've read the carb guide and saw that it says if it hits a flat spot off idle, you're pilot may be too large so I tried to run it with the stock 27.5 pilots as well and didn't have any luck.

I was originally having an issue with timing, and I had to swap out the centrifugal advancer for a new reproduction set in order to get it to time right. I am wondering if this after market unit is the cause of the issue ? Or maybe I need to go up a size on my pilot jet ?

I truly feel like the carb is totally clean. Ive disassembled the carbs fully and placed them in an ultrasonic cleaner, and blew carb cleaner through all the ports, as well poked guitar strings through the tiny holes in the throat of the carb.

I've tried various fuel screw settings. From 1 turn out to 2.5 turns out.

What could be the issue ?
See below link for a video of the bike running and struggling in low RPM.XS650 Break Up

 

[Jan_P]

I would check the points status and gap and timing Maybe late on one or Both cylinders
And check so full power coming to the points

 

[Prisondawg]

I would check the points status and gap and timing Maybe late on one or Both cylinders
And check so full power coming to the points

You raise a good point, whereas I felt confident on my static timing, I never checked my dynamic timing. So I broke out this old laser gun I got a swap meet, and checked the dwell and timing and fined tuned it a bit more, and the issue persists. I truly think it's the centrifugal advancer mechanism. I'm going to order a used but stock set up and see if that helps, unless someone else has any other suggestions.

 

[bosco659]

Sounds like it’s running on one cylinder?

 

[Prisondawg]

Sounds like it’s running on one cylinder?

It's definitely running on both at idle and once it kicks in at 3k rpm. I personally don't think the advancer is working smoothly between those rev ranges. But also I have not done the valves or synchronized the carbs.

 

[Jan_P]

Maybe I am telling stupid things here
But removing the cover one can see the Advance Weights moving In And Out if revving standstill
With that beautiful Strobe light at the same time one can see if the ignition advancing is OK
Maybe a 2 or three man work But a mirror at the far side can do it

Furthermore on the video the bike stalls that happens if the advance weight is not moving freely
It can be serviced some info there
https://www.xs650.com/threads/new-owner.67021/page-4#post-856043

I also vaguely remember that the weights can be moved carefully with the fingers and let go see if it retards back
Carefully not breaking.

You may well be right
If the advance unit is sticking in the more advanced position it runs good at the higher rpm not so at lower
And stalling It would not idle at all .But If the advance rod not been serviced .Maybe that is a first

Lastly the Advance Springs can be replaced

 

[Bruno_c]

I would try to disconnect alternatively one spark plug to see if the trouble affects one cylindre or both.

 

[dave_a]

A slow/sticking advance isn't likely to cause the engine to run on one cylinder. You have something very basic wrong.

When running, are your points visibly sparking? Do they have peaks on the contacts? What did you gap the points at?
What's the dwell reading on each point set ?

 

[Prisondawg]

A slow/sticking advance isn't likely to cause the engine to run on one cylinder. You have something very basic wrong.

When running, are your points visibly sparking? Do they have peaks on the contacts? What did you gap the points at?
What's the dwell reading on each point set ?

I don't think it's running on one cylinder. I never said it was. I'll take some thermal readings of the pipes if you'd like. Both sides are sparking. I don't think the advance weights are snapping back all the way.

The issue here, is that the bike revs fine at idle. And even at low RPM then around 2500 to 3k the power drops out. If I rev the bike super fast I can break through that flat spot and it revs fine. I just have to drive like an a-hole to do it.

I ordered new springs from mikes, and I've read on the forums about a way to adjust the springs. I will possibly try that out tomorrow with the stock springs.

 

[JOHNDADDY]

What do your plugs look like?

 

[member43148]

Mid range stumble ?
Transition from pilot jet circuit to main / needle jet height setting perhaps.
You have UNI Pod filters hidden behind the side covers.

 

[Prisondawg]

I moved my pilots from their stock 27.5 to a brand new size 30 to adjust for the pods. I've tried my needle clip at position 2, 3 , and 4 with similar results.

I will attempt to upload a video of my after market advancer later today and see if I can get some folks on board with my theory.

Mid range stumble ?
Transition from pilot jet circuit to main / needle jet height setting perhaps.
You have UNI Pod filters hidden behind the side covers.

 

[5twins]

Have you serviced (cleaned and greased) the advance rod and the bushings it rides in? If not then you need to do that. Most haven't been serviced since they left the factory which makes them loooong overdue. There's a bushing in each end of the cam with grooves in it's I.D. Pack these with grease. The shop manual recommends a grease with moly in it. I use VW CV joint grease because it has a very high moly content .....

Even though none of the manuals mention it, this is a required maintenance item. You don't have to do it very often, like maybe every 5 or 6 years, but it does occasionally need doing. And you needn't remove the points/advance housing like the 1st photo above shows, it just happened to be off when I took the pic. Just remove the little disc from the advance rod on the advance side and pull the rod out of the head/cam from the points side, points cam still attached/installed.

 

[dave_a]

You had your timing light on the bike.
Was the advance mark on the adv. mark at 3000? At idle, on the fire mark?

 

[Prisondawg]

Hello ! I am back ! I've been messing around a bit and have a question!

My carbs were leaking, I assumed it was a new gasket that I needed but that wasn't the case ! My carbs are actually leaking from the connecting tube. And upon further inspection, the brass bit that sticks out the side , is spinning freely. Could this possibly prevent my bike from reaching the correct fuel level in the bowls causing it to have a tremendous flat spot in the middle of the rev range? What would be the best way to seal up around the connecting tube (the area that bridges the two carbs together in the middle of the carb bank)

Thank you advance!

 

[5twins]

I think I'd remove it completely and try to expand the inner end a little to get a tight fit again by driving a small, gently tapered punch into it. Then I'd install it with Yamabond gasket sealer on it and in the hole.

 

[Prisondawg]

Ok. I fixed the leak I also found out my carb boots were very cracked. So I ordered the tour max boots and have those installed now.

The bike actually runs a little bit better now , but the main issue remains.

I still believe my issue lies within the auto advancer. So I once again removed my advance rod cleaned everything and re greased it.

I noticed that no matter which springs I use, I am unable to twist the nut to open the advancer by hand , it works easily with pliers but I've read that I should be able to twist the advancer open by hand.

I've tried different springs to see what effect it has on my advance. I've even tried different combinations of springs. With my tightest springs, I can't get the advancer to advance at all, with my loosest springs the advancer doesn't return to its closed position. When I tried the combination I managed to get it to advance however it advanced past the the mark on the rotor. Please let me know if anything seems out of place that would hinder the operation of my advancer. And if anyone here lives in the Seattle area, and you think you could solve this problem, feel free to dm me, i'd love to talk about it. (It looks like I forgot to take a photo of the advancer itself. I will report back later with a photo of that as well)

 

[Its been a long time]

Yes, there's an assembly problem for sure. The bob weights should move freely and if you advance them by touching them, they should snap back. There's certainly an issue there.

Difficult to say from here. If you assemble it bit by bit, at what point does it go tight?

 

[Jan_P]

Yes, there's an assembly problem for sure. The bob weights should move freely and if you advance them by touching them, they should snap back. There's certainly an issue there.

Difficult to say from here. If you assemble it bit by bit, at what point does it go tight?

Guessing labyrinth not greased up or damaged

 

[Its been a long time]

Guessing labyrinth not greased up or damaged

Quite possible. Then it would probably not advance/retard even when the assembly isn't secured with the end nut. That's what the poster needs to check out. Then you can work out what the problem is during assembly.