Low top end power and surging

[CoyoteDan]

Hello,
I have recently rebuild a 1979 XS1100 special. It got new rings, bearings, redid the valves, set new valve lash and cleaned the carburetors, new petcock valves and I ditched the octopus valve. The compression is good and even, bike starts and idles fine, has good throttle response as well, but doesn't Preform well over 6000 rpm. Originally it was like 4000 rpm and then it would surge, and loose power and the rpm would become erratic bouncing up to 7000 and coming back down. I checked the ignition control box and saw that I'm getting resistance on its ground so I cleaned the ground terminals really well and fixed it. Now it's coming up to the 6000 rpm mark and behaving better but is doing the same surging above 6000 rpm. I pulled the spark plug boots to test spark and got zapped through the boot on a few of them, but I checked the bike at night and haven't witnessed and arcing. I took the coils off and tested them. Both primary coils are showing 1.6 resistance and the secondary is 15.4 amd 15.8.
Any suggestions or advice would be much appreciated
Thank you,
Mike

[LAB3]

Have you checked the resistance of spark plug caps?

Even a very small and seemingly trivial air leak on the intake can cause surging too.

[CoyoteDan]

I haven't checked cap resistance, but I ordered new caps, haven't got them yet.
So I thought about vaccume leaks and I believe it is fine. I replaces the boots as well, maybe the hose clamps that came with the new boots are a little weak.
My understanding and (I could be very wrong) was that a vaccume leak would be more prominent at idle when manifold vaccume is at its highest, and not as bad a wide open throttle when manifold vacuum is at its lowest.

[LAB3]

After going bikeless for about three years I picked up little project and cleaned the he!! out of the carbs. After cleaning up the petcock and swapping out the spark plug caps for non-resistor ones along going to resistor plugs my problem all along was a very slight misalignment of the intake boots on one of the carbs. That bike was doing a lot of the same as what you're describing. One thing is for sure, it was a combination of all the little stuff that needed attention.

On my current XS after doing all those same things I was surprised to learn that eliminating the ballast resistor by going to 3 ohm coils really helped things smooth out at highway speeds especially after things warmed up over a couple of hours. Keep picking away at the little stuff and only do one change at a time before you decide it wasn't what was causing the problem.

That's about the best advice I can throw out there, stick to the basics first and you'll find yourself eliminating a lot of things costing big money to prove that wasn't a problem.

[Radioguylogs]

You cleaned the carbs, so I don't know if this helps,....but I experienced the same behavior on two XS750's and it it was caused by corrosion in the main jets.

Surging happens as the mixture leans, then it becomes too lean and coughs. Then it surges and poops out again.

Boths of my carb sets had been rebuilt a few years prior, but they were stored in an a garage with temperaure fluctuations and the condensation got to them. For the record, I drained them every winter and I think that made it worse.

[CoyoteDan]

Yeah originally I thought it was a fueling issue and was some sort of blockage, so I took the whole assembly apart sonic cleaned them and put all new needles and jets in but it didn't make a huge difference. Fixing the ground made a big difference which makes me believe it may still be electrical. I'll keep chipping away at it, I believe the alignment is OK on the intake side, but the Alignment on the air box side is a little wonky and might not be 100% sealed, I don't see that as much of an issue though.

[DEEBS11]

More questions please:

1.) Has this bike ever run correctly since you owned it?

2.) Stock air box?

3.) Stock jets?

4.) Cheap Chinese jet kits and/or float parts?

5.) Stock exhaust?

[DiverRay]

One last question from me, Did you sync the carbs? You need to sync ANYTIME you remove them! The plastic bottle behind the gauges is the temp fuel tank I use to sync carbs.

[CoyoteDan]

No the bike wasn't running when i got it
everything is stock
I bought the jet kit off ebay, it looks like their kits are made In Taiwan

[CoyoteDan]

I have in the past but I have not synchronized them since the last time I cleaned them.

[DiverRay]

I would not trust the jets to be correct. We've been through this many times here, and the only ones that seem to work are Mikuni. I've had problems with carbs due to not using Mikuni jets, and it resolved with the correct size jets made by Mikuni. The other jets had the "correct size" stamped on them, but it was wrong.

[DEEBS11]

No the bike wasn't running when i got it
everything is stock
I bought the jet kit off ebay, it looks like their kits are made In Taiwan

Take all the parts you put in and trash them. Hopefully you still have all the old parts. They can be cleaned and re-installed.

[DEEBS11]

I would not trust the jets to be correct. We've been through this many times here, and the only ones that seem to work are Mikuni. I've had problems with carbs due to not using Mikuni jets, and it resolved with the correct size jets made by Mikuni. The other jets had the "correct size" stamped on them, but it was wrong.

Pure_Chinese_F*ckery.

[CoyoteDan]

Ok ill give that a go, I should have everything still, if not those new kits by mikuni should work yeah?

[DEEBS11]

There is not a kit from Mikuni available that I am aware of. Not for a bike that old. Try these guys for individual parts.

https://www.jetsrus.com/a_jets_by_ca..._selection.htm