Stuttering while accelerating?

[Graman]

I have a nice 1979 XS1100 Special and it has always ran very well. Recently it has developed a strange problem. When I'm accelerating up past about 4500 rpm the bike starts to loose power and starts to stutter. It's like the whole bike starts to shudder. It is most obvious in 4th and 5th gear but is noticeable in 2nd and 3rd too. The carbs have been rebuild and synced. The coils are fairly new dyna and have been working fine. I'm not sure whether it could be a spark problem or possibly a problem with the vacuum advance. Has anyone else experienced this sort of problem? Any advise would be appreciated.

[XS 1100 Rick]

I would start first with checking the wires behind the advance plate sometimes they break. The pick up wires that feed the coils. Most likely the culprit or gas starvation at higher rpm's.

Rick

[TopCatGr58]

Like Rick said,

Check the TECH TIPS for how to test and repair the PICKUP COIL WIRES, they are the most common culprit in this type of performance loss.

T.C.

[MarkD]

+1 on pickup coil wire issue. Baffled me and my mechanic on my 1st XS, but was cured in short order by an experienced vintage bike mechanic. Now I could do it, now that I am '' an experienced vintage bike mechanic'', thanks to the helpful guys on this forum.

[ViperRon]

I agree that pick ups is most likely but if you do not have success there the next place is fuel starvation make sure you have good fuel flow before removing the carbs. The fuel and vacuum lines should be checked no cracks or kinks in hoses. if you have filters make sure of good flow threw them some filters can get air locked and not flow well. Its easy to get kinks in vacuum and fuel lines putting tank back on. Some gas line the black auto type mainly can dissolve and little black rubber pieces can clog carb jets. There are plenty of threads on carb cleaning if you need to go that direction. Remember do the simple things first.

[Graman]

Thanks for the advise guys.

I am going to check the pick up coil wires. I should of added that my idle has started to bounce. It has always idled very smooth but lately it's like it's missing. Comes close to stalling sometimes. Hot or cold. I've already gone over the ignition coils and plug wires. I recently pulled and cleaned the carbs and I have my own synchronizer so that's done too but I had this problem before that. Just waiting for my days off so I can take a look at those wires. Hopefully it will stop raining.

[Prisoner6]

Hi, Graham -

One other simple thing to check for is a plugged/clogged gas cap vent. That could cause fuel starvation as well.

Try going for a short ride with your gas cap cracked open (not with a full tank, of course), and see if that helps your stutter. If it does help, that vent is most likely clogged.

[Graman]

Nothing found.

I finally had a chance today to look at the pick up coil wires. Couldn't find a problem. The wires are in good shape. I pulled on them looking for any sign of breaks and even pulled on them in both directions with the bike was running. No change to the idle at all. The advance seems to be moving smoothly up when I rev the bike. Still it seems to miss while idling and the shuddering is still there as you approach 4.5 to 5 K rpm. As an experiment I tried pushing it past the shudder in 4th gear and it did start to smooth out some as I approached 6 K rpm. About a month ago I rebuilt the carbs and increased the size of the main jet from 137.5 to 142.5 because of the K&N air filter and custom exhaust. Maybe I needed to make some adjustment to the float bowl heights. I was trying to eliminate a flat spot I have always had at around 5 K rpm. I am tempted to put the old jets back in.
Sorry to go on so much but I'm just trying to give as much info as possible. Not sure what I should check next.

[donebysunday]

Put the stock jets back and see ! In the process you will clean the bowls and check the pilot jets.

[Bonz]

I have a nice 80SG that bogs and goes flat, wouldn't accelerate from a steady light throttle cruise at random times unless I clutch it, give it a rev and then let clutch out.

My wires are good like yours, however I thought my plate was turning freely by hand until I shot some silicon lube around the base while turning it by hand, and loosened it up just a bit more. Then my issue went away.

A good disassembly and cleaning isn't a bad thing as these bikes are 37 years old now in some cases and the factory lube job inside the mechanism ain't what it used to be... and if it's been cared for, it will look clean but don't confuse clean with "still well lubed after all these years".

Try some lube.

[skids]

My wires are good like yours, however I thought my plate was turning freely by hand until I shot some silicon lube around the base while turning it by hand, and loosened it up just a bit more. Then my issue went away.

It seems that the throttle plates use spring action to close on deceleration rather than acceleration. I am not sure how sticky action would cause a bog when opening the throttle quickly...unless the plugs were wetted of fouled from the previous deceleration.

[DiverRay]

I think he is talking the mechanical advance unit. I know of a few that "quit working" due to rust. I take them off and clean/lube them when I do any major engine work. They CAN cause some strange problems, and the bike DOES run a LOT smoother with them lubed and moving correctly.

[Graman]

Thanks guys. I have lubed the advance mechanism recently with silicone lube. I usually do it once a year or so. I did find something interesting yesterday. I decided to pull the plugs and check whether fowling could be the cause of the missing at idle. I found that both #1 and #2 plugs were heavily carboned up. #3 and #4 looked ok. I have been running Iridium NGK for a while now with out any problem but I have read on this site that they are very prone to fouling problems if your mixture is off. I put the old NGK BP6ES back in and dialed the idle mixture down on #1 and #2 about half a turn. No more miss. Idles smoothly at 1100 rpm. The stuttering at higher rpm is still there but is much less intense. On my next day off I'm going to double check the carb balance again. I have one of those Motion Pro balancers and it usually works great. I wish I had a Colour Tune to adjust the idle mixture but I can't afford one right now. Will let you know how it goes.

[DiverRay]

Don't waste your money on a color tune! There are a few members here who would probably sell you the one they paid for, at a discount. The BEST way to adjust the mixture is when the engine is warmed up, with a fan to keep it cool while adjusting, start from one side and work across.
1. Valves adjusted
2. timing set
3. carbs synced
4. Start from #1 or #4, adjust mixture for highest idle speed. Go down the line, doing this for every carb. If the idle gets above about 1200RPM, turn it down before going on to the next carb, I usually start at 800RPM, and turn it down if it gets to 1100RPM.
5. RE-SYNC the carbs! DO NOT FORGET THIS STEP. things can get off just a bit. The bike will run well if you don't sync again, but it will NOT run it's best.
If Surrey is close to the coast, I'll be up that way late June/early July. SWMBO and I will be on the BMW, from Tacoma to Vancouver BC, and on to the Island for a night before going back South. We could do a quick "how to" for the price of two beers!

[Graman]

Thanks DiverRay. I'm going to attack it again on Monday. What is your starting point on the mixture screws? 1.5, 2 turns out? Are you starting lean and turning out to increase the idle or visa versa? You also mention the timing. I have checked it in the past with a timing light and it's at factory specs but my service manual says that it is factory set and not adjustable. Is this true?
Thanks for the offer to give me some help but I hope to have this resolved soon. We are just coming into some of the best riding weather of the year. Not too cold but not too hot and humid. And that's when it's not continuously friggin raining. You are used to a little better riding season than we have here. I have been to Santa Cruz a couple of times. Beautiful place. You should enjoy your ride over on the island. There are some great roads. If you have time on the mainland the Sea to Sky Highway from West Vancouver to Whistler is a great ride.