1981xs400: engine revs too high after initial start-up

[walterfrancis69]

Bike operates as follows: Ignition on, killswitch on, headlamp on, start button depressed, engine starts and starter motor properly turns off went button is released. Next, during warm-up, engine will automatically rev too high( no tach on bike). This increase is easily 100% of initial idle. This occurs regardless of engine temperature. Fuel enrichment( choke) is full open. The increase in revs occurs approx. 20-30 seconds after each start- up. Throttle adjustment has little/ no impact to correction. New ngk plugs gapped and installed. When leftside sparkplug wire is removed engine idles at close to normal speed. Interchanging coils had no impact. What say u?

 

[levdir]

Are you leaving the choke on after it cranks over and starts? If your idle is adjusted correctly keeping the choke open will make your revs go sky high after it starts. You only really need to have it at full choke long enough to get it started. Some people leave it at half choke when they first take off from cold to let the oil warm up.

Barring that, is your idle set correctly? How are your idle screws set? The factory spec is something like 1.5 or 2 turns out but we've found that something closer to 2.5-3 turns is better. Have you synced your carbs? Once the engine is warm how quickly do the revs return to idle when you let off the throttle?

 

[walterfrancis69]

@ levdir- im sorry to report that i yet to learn how to properly adjust my carbs. I have a shop manual on order. Without illustration and exploded views, the most i can accomplish is a float adjustment and a cleaning which has been done. I dont use full choke exclusively. Even at half, the engine will rev too high. In fact, the engine will maintain normal idle speed( throttle closed) for a brief time before it idles excessively ( throttle still closed). Its at this point i turn the engine off. Your advise is helpful and i will start in that direction.

 

[levdir]

What year of bike do you have, incidentally? The carbs are slightly different depending on the year. Early bikes (77-79) have idle needles with tips that can tend to break off in the carb bodies, causing weird behaviour. I'm hoping Chris will hit this thread, he's a genius at diagnosing these carbs.

 

[xschris]

What exhaust and air filters do you have? Are they the stock ones? When you said you cleaned the carbs what did you actually do? The best way is to COMPLETELY take them apart and make sure all the tiny holes through out the carbs are clear. Compressed air ( 125+ psi works the best). On the 80-82 sohc xs400 carbs the idle mix screws where capped from the factory. If your are still capped you will need to remove the caps to get to the adjustment screws. There are many threads on how to do this as well as carb cleaning on this forum. Post some pics of your bike it help show us what you are working with.

 

[walterfrancis69]

@Levdir/Xchris-im really appreciating this forum and your responses. My bike is 1981 xs400cc. It is recently purchased and is running better than when i bought it. Its appearance is far better than when i bought it. Carbs are missing filters and exhaust is open. Air filters will be installed soon. Your comments are sound- carbs need adjusting. More than once, i disassembled carbs and adjusted floats by sight. The next time, ill be prepared with specs and ruler( learning process). All jet orifices are clear, no varnish or gum. Needle valves, diaphrams and springs look good. What i really need is shop manual. Should arrive this week. More to come.

 

[walterfrancis69]

Afterthought- when i purchased bike, throttle cable was broken and starter pushbutton did not work. I replaced cable with oem. I wired a switch across starter solenoid. With that, bike ran as follows: with left side spark plug removed from cylinder and grounded to frame ( wire connected to plug) and ignition switched to on posiion the plug would arc repeatedly- even though the bike hasnt been started. Once started ( both plugs installed) the revs during idle would increase as previously mentioned. Disconnecting the left side sprk plg (engine running) would bring the idle speed to more normal range. I have since repaired start pushbutton and temporary switch is removed from solenoid. Normal start sequence for engine has been accomplished but the over- revving during idle still exists. I still dont understand why any plug would arc prior to engine being started. I like your carb related theory. I hope the solution is as u say.

 

[xschris]

First off don't run the bike with a plug wire disconnected it can damage the TCI box. Also running the bike with out mufflers and a set of air filters will also cause high reving and overheating on the motor. Until you get air filters and mufflers on the bike getting it to run proper will be extremely tough. What air filters are you getting? I would recommend the stock setup or a good set of uni filters that are oiled.

 

[walterfrancis69]

@XSchris- thanks for info. I dont run engine with plug wire removed. I do on occasion run engine with spark plug removed and grounded to frame, plug wire still attached. I believe this to be acceptable when diagnosing ignition issues. Im in a good mood because my shop manual has arrived. Im understanding that changes in stock eqipment will affect performance. So many bikes have open exhaust( harleydavidson) that i did not realize this. Based on your info i will immediately focus on carburation.