78 xs400 stalls at idle when warm

[lugnuttz]

new rider here ..my first bike 1978 xs400

when i first start it in the morning and the engine is cold it runs gread and idles fine ...once the bike has been running a while and everything has gotten up to temperature if i let the bike idle it will idle for a short time then possibly miss then stall ..now when this happens in my driveway it is not too big of a deal but when i am out riding and i come to a stop light and i let it drop to idle if it stalls i freak out because of the lineup of cars waiting behind me..so i am in the habit of at stops i keep revving it so it doesnt stall ...

fortunately enough every time this happens it starts right back up without a problem but this is getting aggravating ...

any suggestions are greatly appreciated ..i have brand new properly gapped NGK plugs in it and the carbs were cleaned

 

[chris]

hey lugnuttz. I'm no carb expert, as a matter of fact i may be carbtarded but it sounds like your bike is running lean possibly because of an air leak. Check the intake boots. Leus know when you solve the issue.

 

[lugnuttz]

thanks for the reply...the carb boots were brand new may of last year and the vaccum tube that runs from the carb boot to the fuel shutoff is also brand new .

..i read elsewhere that it could be one of the ignition coils that is being affected by the heat and not firing does this seem possible??

 

[HoughMade]

It's interesting you mention this. I just got back from a 40 mile ride and my bike is doing something like this too. I don't think mine is really "missing" it just that many times, it will idle fine (when warm), but other times, the idle will just slowly get lower until it wants to stall unless I blip the throttle. I will be interested to hear thoughts on your issue.

 

[Ace4569]

I'm guessing it's the carbs. I would take em apart and check em out or take them to a shop and get them looked at. You might also look into a tune up I had a similar problem and they told me to change the spark plugs and it worked fine after that

 

[hmusket]

Have you tested the coils. for info check the manual in the tech section
View attachment 3882
go to page 24 of the pdf

Another problem could be your gas caps not venting.

 

[HoughMade]

In my case, I know it's not fuel flow. Long story, but that ain't it- and the carbs are clean (another long story). I will check the coils.

 

[Ace4569]

if the coils are good change the spark plugs one of them could be misfiring causing your bike to stall. My bike did that and when I would shift into 1st the whole damn bike would just shut off. also if it is a spark plug you can tell if you leave your choke open when it's warmed up and it still shuts down on you while idling it could be a misfire

 

[drewpy]

might just need to mess with idle setting. mine changes with the weather all the time, they are old bikes and thus, not perfect!

 

[OliverB]

And, SU type carbs are notorious for changes in air pressure affecting them.

 

[HoughMade]

might just need to mess with idle setting. mine changes with the weather all the time, they are old bikes and thus, not perfect!

No one has to explain to me that perfection isn't in the cards. However, I used to have a nice steady idle. I had some issues (fuel starvation after a long ride- had restricted flow from the petcock and a too high float height, problem solved, also had a top speed of 65 followed by gradual slowing and then backfiring- timing was off, problem solved). Now it runs great, pulls stronger than I have a right to expect and the top speed is where i haven't found it yet.

Problem. When I set the idle speed at about 1200 after the bike has run for a few minutes, after a little riding, it droops to 1000 so I bump up the idle speed. After a little more riding, it does OK about 60% of the time, but the other 40% I have to blip the throttle to keep it from drooping below 1000 and stalling. If I try to bump up the idle speed with the knob again, it'll shoot up close to 2000 without much success adjusting in between 1000 and 2000.

I am curious to see if Mr. lugnuttz has found anything. I haven't had a chance to look at this problem since Friday.

Y'know I spent most of Saturday replacing a hub, ball joints and a stabilizer link on my car. A Saturday spent tinkering on the XS is relaxing. Spent working on the car is just- Blech!

 

[vonnieglen]

Check your compression. A cheap tester costs under $20. If you get a low reading on one side, you can position the piston on top dead center with both the intake and exhaust valve closed and run some air into the cylinder by hooking the hose from your compression tester to an air compressor. You then listen and feel where the air is going. If it is coming out the exhaust pipe you have a sticky or damaged exhaust valve. If it is coming back through your carb, your intake valve is sticky or damaged. If it is coming out of the oil fill hole in your crank case you have a problem with your rings.

I have a sticky exhaust valve in my 81 XS400, it has been causing some similar issues. They can be a little intermittent depending on temperature and hard to diagnose. Usually it is the exhaust valve which gets over-heated when your carb has been running lean.

 

[buddyhull]

Lugnutz - I have the same problem with my '78 XS400E. It warms up really nice but once the engine is hot it spits, coughs and eventually dies. I found the problem! My left side spark plug hole has stripped threads. The plug won't go in and it won't come out! When I put a spark plug wrench on it the plug just spins forever. I called my favorite bike shop. They said they can get the plug out, drill a new hole and insert a heli-coil to repair the stripped threads. Vonnieglen - you were right on the money when you said ''Check your compression". My 400 was losing compression on the side with the bad spark plug threads. Thanks for the good leads...

 

[gordonfi]

I don't know if mine falls into this category but, same thing, cold, seems to idle ok and rpms go up as it warms up. Then when choke is all the way off, will stall if I don't tweak the idle screw. Trouble is, I tweak it, it idles for a bit then if I rev it once the idle climbs and engine races up to about 4k untill I back idle back down again. BUT, when I do and get it around 1200 it just keeps dropping and stalls, unless I rev it. Carb's have been apart and cleaned about 6 times. I'm running that injector cleaner with the hopes of cleaning anything else out. This bike was stored for a long time before I bought it and there's been a lot of tinkering went in to getting it running. I'm riding it now but as was said above, I also dislike having to rev it at the light to keep it running. Gas mileage aint so hot this way either. Oh, yea, carbs are synced. What about those two jets that they say to adjust to 1 1/4 turns for XS400? I think mine are adjusted out a little farther than that (Maybe 1 1/2). Maybe? Maybe not. I think I still have to run it for a while anyway because the dipstic side of my oil stays clean while the other side gets dirty after running it for a while. Was told it needs a few hundred miles to get all the crud worked through. So that might be complicating the carb idling to?

 

[xschris]

You don't adjust jets by screwing them out. They must be screwed all the way in. The idle mix screws are the ones that get turned out from a gentle seat to 1 1/2 for a FULLY stock bike. But most often 3 turns works the best. With the 77-79 carbs you need to make sure the idle mix screw needles have not broken off in the carb bodies. This will cause the issue you have also.

 

[gordonfi]

Sorry, I meant to say Idle Mix Screws. Will this change as the engine is run and the crud works through the oil chamber, say in a hundred miles or so of driving? It seems to improve a but every time I take it out.
I will remove and inspect them. Thanks!

 

[ScottLuke]

Pilot jet/mixture screw-If you can start it cold with no choke, the pilot circuit is too rich.If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

"The best way to set a pilot circuit is with an RPM gauge. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again. You will get better adjustment by going with the correct pilot jets."

If it stalls in corners, flames out, and is really tough to start, the pilot is too lean. If it feels a little dead until you wind it out, the pilot is too rich. Another test it to rev it out a little in 2nd and then let the throttle snap shut. As it's decelerating, there should be very little backfire or popping - if it pops the whole way down, the pilot circuit is lean.

You can always adjust the throttle cable itself to help with fine tuning the idle speed, After you have followed the steps above. Always Jet according to your weather conditions, Cold air and hot air are different densities. The difference in air density from a winter day to a summer, therefore your bike that was perfectly tuned for winter feels all sluggish on a hot day and your bike that runs crisply in summer will run lean in winter. Unless you change the jetting.

Lots of jetting information can be had all over the internet, there's no magic number for any single bike. The best way to have your bike running perfectly is, riding, plug reading and plug chops.

 

[Sesty]

THIS ^^^

 

[gordonfi]

You don't adjust jets by screwing them out. They must be screwed all the way in. The idle mix screws are the ones that get turned out from a gentle seat to 1 1/2 for a FULLY stock bike. But most often 3 turns works the best. With the 77-79 carbs you need to make sure the idle mix screw needles have not broken off in the carb bodies. This will cause the issue you have also.

Thanks xchris!

I removed them(idle mix), cleaned with acetone along with q-tips in the holes. It idles now with about 3 turns out. Temperature changes seem to affect the idle, but it's now manageable. Much better!!!

 

[xschris]

You can go as many as 4 before going up a jet size. (pilot) My 82 that is all stock runs perfect at 3 1/2.