'78 XS650 Special - Low Speed Running Rough

[Madmartigan]

Hello Everybody!

I picked up my first XS a couple months back and have been working like crazy to get it up and running before the snow falls here in Minnesota! I've been lurking for a while now doing a metric ton of research on this forum while I work on the bike. I got it non-running but have it generally running alright with one caveat that I can't seem to shake.

The bike idles for days and revs and pulls pretty well once it hits half to full throttle or above 3k, but at 1/8th to 1/4 throttle runs like crap, it farts and pops and has no power. I attached a video to demonstrate my woes..

This is what I've done since I've gotten the bike:

-New battery
-New plugs
-Cleaned, lubed, put new Mikesxs advance springs in
-Checked Timing
-Cam chain Tensioner adjusted
-Valve Lash Set
-Put on MikesXS pods
-Carbs cleaned and gone through multiple times (BS38 Carbs)
-New float Needles and Seats
-Stock 24mm float height (no holes in floats)
-Upped pilot from stock #27.5 to #30
-Roughly 2.5-3.0 turns out on the fuel screws seems to get it to idle well
-Stock Middle Needle Height

I tried the 27.5 Pilots and tweaking the air fuel to adjust but even with the screws out to 3.5-3.75 it still didn't run nearly as well as the 30's. The bike doesn't idle well choked but it revs quick and strong when it is, so I think I'm going to raise the needle a notch tonight and see what that does. Any one have other suggestions, I'm running out of logical ideas?

Thanks everyone!

PS: I know these bike need strong power, I've test ridden it maybe 3-4 miles so far over the past 3 weeks and the battery is now flat so I either have charging problems or I just haven't spun it enough to power it up much. My slow speed problems have been around all along though, even when the battery was freshly trickled so I don't think necessarily that the two are linked. Just food for thought

 

[scaf77]

Sounds like carb problems.
Did you sync them?
Also check the brushes, I think they are supposed to be 7mm minimum. Mine were 4mm,
Cheap and easy to replace.

 

[5twins]

The MikesXS advance springs are sending up a red flag here. Many times they are too weak right out of the box, allowing the ignition to advance too quickly. Full advance should happen by about 3K and you say it runs pretty well from that point on. It wouldn't run right or well below that RPM if the ignition had advanced too early. Put a timing light on it and watch the rotor mark advance from idle ("F" mark) to the full advance mark, and watch the RPMs on the tach as it's advancing. Verify that it's reaching full advance at about 3K and not earlier.

If it is advancing too early, you'll need to stiffen the springs to slow it down. Clip one loop off of only one spring to start, that's usually enough.

A second red flag is the type of pod filters you've chosen. They may not be at the root of the problem (I think the advance springs are) but may very well be contributing to it. The pleated K&N style pod just doesn't work that well on these CV carbs. The pleats create turbulence in the air flow and that messes with the slide lift. Straight foam UNI pods are the best choice.

 

[peanut]

welcome to XS650

There are potentially several factors that could contribute to this problem and some have already been mentioned.

A common cause of misfiring like yours is a battery that is not holding a good charge or one that has a parasitic drain like a short or lights permanently on.

I noticed that your charging system is not functioning. You might try and pull all the fuses except the one needed for the ignition, coil etc and see if there is any change whatsoever .... That might eliminate the battery or a short as being the cause. There have been several threads from owners recently that had exactly the same problem as yours which was cured by connecting a fully charged battery or running the engine with a car battery hooked up temporarily with jump leads

if that doesn't cure it I would be meticulously checking the valve settings again . The gaps are tiny and its easy to get them wrong. if you had a valve that was sticking or not closing properly that could cause incorrect firing in the inlet or exhaust. Remember the cylinder you are setting has to be at TDC on the compression stoke then turn the crank to set the other cylinder valve lash when it is also at TDC

 

[Madmartigan]

Thanks for the replies guys!

I'm going to double check the brushes, I maybe will just order some new ones unless they look brand new already.

I synced the carbs, and I knew about the mikes springs being weak so I wound up clipping one loop off of each and it opens all the way at 3000 like it should.

I had heard the Uni's were the better way to go after I had already bought these unfortunately, ha, but I may order those up instead..

After charging the battery a bit and raising the needle it definitely helped out a lot, when it's warmed up it revs fairly clean now and isn't bobbling too bad at 1/4 throttle, I think I'm getting close! I'm going to go through the charging system using that one guys guide and see how everything checks out, then I'll double check my valve lash again if it seems like that stuff is alright..

Thanks for the suggestions, I'll let you know what it find out!

 

[scaf77]

Check out this video.
Xs650 valve adjustment made easy.

 

[ThatXS650Guy]

Are you showing decent voltage when running? I'm with peanut, you may have a charging issue.

 

[fredintoon]

Hi Madmartigan and welcome,
remember the XS650 mantra:- clean the carbs clean the carbs clean the carbs
Poorboy quickie check on the charging system;-
Point the headlight at a wall and rev up.
If the light get's brighter, it's charging. If it don't, it ain't.
And unless you are wallowing in nostalgia about your bike's points and mechanical advancer, replace them with the advanced Pamco system.
And be effin' careful to install the Pamco EXACTLY like the instructions tell you.
Seems pete's real nice about replacing blown-up Pamcos but it ain't worth the hassle for the lack of a proper install.