Why would a great running bike SUDDENLY run poorly (carbs right)

[JayR]

Hoping for a sanity check from the forum if you all don't mind. My next move is to remove and clean carbs, but has anyone experienced this:

Background: Bought 82 heritage special barely running. Had carbs cleaned and rebuilt (GentlemanJim), to get engine running. was not ridable so did not put road miles on bike.

Then rebuilt top end, valves, rings, cam chain, cam chain guide. Rewired electrical, new reg/rec and charging system components all check out (thanks again all).

Bike was running perfectly - i mean absolutely great for 250 miles as i babied her to break in engine.

Current Situation: While on a fun ride a month ago, and with the bike running well, I grabbed a handful of throttle to go WOT. The engine responded great, with fine high RPM power. After deceleration, about 5 minutes later the bike then started to sputter and stall. Needed to run it on choke to get home. When I gave it throttle, there was a dead flat spot, then the carbs cleared and engine accelerated strongly - took off.

I messed around with a loose exhaust a little, and the problem went away. Bike running great for another 125 miles. Running great. BUT - I now think this was just a coincidence, because I just experienced the same WOT then stall/backfire issue again.

What I did this time:
1) Drained the carb bowls, seemed to be a few specks in there not much. Started it but still running very rough and backfiring. I also heard a sort of metallic "Ting" noise as the engine was idling roughly with choke. Not sure if that is related.

2) Next I lifted the slides and sprayed carb cleaner into the needle jet hole to see if I would get lucky and clean the main jet of any debris that may have been knocked loose during WOT. Drained carb cleaner from bowls.

Now the bike will not start at all. The plugs are grayish, with 400 miles on them.

I am assuming my next step is to re-read the carb manuals, remove the carbs, take them apart and clean them, inspect diaphram.

I am only 400 miles into being a hands on bike owner - have any of you more seasoned riders and mechanics ever diagnosed a similar problem, and if so what was the cure?

Thank you as always. Jay R

 

[TwoManyXS1Bs]

Hoping for a sanity check...
...next step is to re-read the carb manuals, remove the carbs, take them apart and clean them, inspect diaphram.

Yep, Jay, from your description, sounds like you're on the right track.

A cheap experiment would be to back-flush the pilot circuit, kinda like this:

http://www.xs650.com/forum/showthread.php?t=42002

Another cheap experiment (if you have TCI ignition) would be to unplug the regulator.
That's the test for a weak trigger magnet.

 

[5twins]

The last time I pulled and cleaned my carbs because of bad running, it was a loose negative battery cable. We all tend to think carbs first thing when bad running appears. Sometimes that is it but it's not always so. You might also want to check wire connections and the fuse connections relating to the ignition and main power. When you hit it hard like that, it also means mucho more vibration. That could be enough to push a weak connection over the edge.

 

[gggGary]

divide your problems in 2 find out if it's ignition or carbs.
Well, the triad is fuel, ignition, compression....

 

[jetmechmarty]

Yep. I'd make damn sure the problem isn't electrical. Are you sure it's charging? If that alternator rotor is original OEM, it's past due for failure. I recommend you at least take a resistance reading between the copper rings. If it's much less than 5 ohms, send it to Custom Rewind in Alabama. It will be fixed from now on. Don't buy a new one, as they've been reported as junk. While you're at it take all the electrical connectors apart and clean them. Be sure to clean the kill switch, but watch for escaping little springs.

 

[pamcopete]

Always after a WOT? Valves. Stuck, bent, or broken springs.

 

[JayR]

Yep, Jay, from your description, sounds like you're on the right track....

Thanks for advice all, I removed the carbs and they are on my workbench in the foil pan my wife bought for the Thanksgiving turkey. I don't think she'll mind lol. I'll have to wait until later this week to open them up. Jim did replace the float needles and seats as well as new main jets so I hope that it is as simple as clogged mix screw and by pass holes, or whatever else the guide recommends.

Always after a WOT? Valves. Stuck, bent, or broken springs.

Pete...I am a bit worried it may be more as you called out....
I mentioned in my write up that I heard a metallic "ting" or two that, at the time, caught my attention as an odd and unfamiliar sound when idling. And on my last attempt to kick start, on the down stroke I hit a "snag" - the kicker sort of got stuck midway through and then got past the hang up and I completed the down stroke.

2 1/2 months ago had replaced new guides seals, cleaned the valves, seats cut (mechanic said they were too wide), replaced the cam chain and slider, and new piston ring sets. Also honed the cylinders.

Is there a way for me to investigate / check the valves? My local motorcycle mechanic did the top end work, I know the shop and they are good people, and he knows the XS 650 and older bikes very well (he quoted the correct valve clearances for the 82 off the top of his head). I am wondering if something didn't go so well with the rebuild, or maybe just another new issue with an older bike?

The last time I pulled and cleaned my carbs because of bad running, it was a loose negative battery cable....

5Twins - good point, all of my battery connections are new, and now under my seat in a custom electronics pan. This will be top on the list of checks this week as well.

Yep. I'd make damn sure the problem isn't electrical. Are you sure it's charging...take a resistance reading between the copper rings. If it's much less than 5 ohms

Thanks Jetmech! I am pulling 5 ohms between the rings. I followed gggGary's Electrical trouble shooting guide step-by-step (with a boat-load of direct help from gggGary and DaddyG, thanks again fellas) and the charging system is working very well. I have a volt meter mounted on my bike and she charges to 13.9 - 14 volts when out on the road. But yeah, the charging system was a problem, and after following gggGary's process of elimination it turned out that my reg/rec was fried and needed to be replaced. My head hurt, but we figured it out!

 

[JayR]

As an update cleaned the pilot and main circuits following the carb guide section 5 and 6. Kicked right over, and the backfiring and stallling went away, and the bike is actually running better then ever!

It pulls noticeably stronger throughout, and actually seems like a different bike. I thought it was running well before, but even though the carbs were cleaned before as part of the build, there must have been some residual crud that cAme loose after I got it back on the street like you said TwoMany.

Got lucky with the weather here in CT today, 65 degrees and best ride yet! Once it gets cold I'll tackle the leaky seals but hopefully another couple of rides left this year.

Thanks all. Jay

 

[TwoManyXS1Bs]

Glad you got it, Jay. At least for now.
That was actually a lucky guess, and the 'real' problem may reoccur.

Universal failure mode would more likely be ignition/electrical related, like the others were suggesting. Carb problems usually manifest on one cylinder, first. But you have to be able to detect that.

Other suggestions found in this forum would include checking the gas cap vent...