1979 XS400 sporadic shut down

[Kev]

Newby here but have rebuilt a couple old bikes but this has me stumped. 1979 XS400. Left plug perfect, right plug shows rich. Starts great when cold and runs down the road good, although I have only put on 4 miles. I have two questions. One, when starting cold on choke if I blip the throttle hard the revs will climb hard until I reduce choke. Second, w hen idling (1200 rpm) it will occasionally "hiccup" and then quit. It starts right back up but eventually will hiccup and quit again. Carbs cleaned and sync'd, pilot screws out 1 1/4 turns. Where do I go from here?

 

[Tate]

Hey Kev. Welcome to the forum. Sounds like you have more a lot more experience than me. But I've read an awful lot about carbs and similar issues to what your having.
Apparently it's not uncommon to clean the carbs three times ensuring all those awesome micro holes are blown out with air compressor. Floats to be checked for possible holes then ensuring the heights are to spec. The idle and jet screws are turned in and out x amount of turns. All of this is found on this site.
Maybe the air filters are dirty or there are cracks or holes in the boots?
Not sure if this was useful.
Cheers

 

[NewHavenMike]

When you cleaned th carbs, did you make sure everything worked properly inside the fuel bowl? I'm talking about the float height and the needle valve. Did you inspect the carb boots to make sure there's no vacuum leaks? Stock air filters? Do you have a vacuum operated petcock and is the line connected to it off the carb boot?

How are your points and timing looking? Valves adjusted?

Cant remember off the top of my head, but I think the mixture screws are 3 turns out.

 

[Kev]

Thanks for the replies. Vacuum line connected to the carb boot. Carbs boots have some cracks on the outside but just surface cracks. Haynes manual says float height is supposed to be 32 mm!! Didn't work so I adjusted back to 27 mm. New stock filters. I will try mix screws out 3 and look into the points and timing. If that doesn't work I will go through the carbs again. I went through my Honda Super Sport carbs 4 times so I know the routine! Thanks again.

 

[NewHavenMike]

Getting the carbs spotless can take a few tries lol

First make sure that gas from the main line is clean.

There's little filters inside the carbs on the brass needle housing. Those can get jammed. If you see rings on the needles, I'd get replacements just to start ruling stuff out. The spring inside the float needle should not compress at all when the float rests on it. Then make sure you're getting fuel, open up the drain on the 2nd carb and let it flow into a cup. See if you get debris. Should be good for a start up.

 

[Kev]

Getting the carbs spotless can take a few tries lol

First make sure that gas from the main line is clean.

There's little filters inside the carbs on the brass needle housing. Those can get jammed. If you see rings on the needles, I'd get replacements just to start ruling stuff out. The spring inside the float needle should not compress at all when the float rests on it. Then make sure you're getting fuel, open up the drain on the 2nd carb and let it flow into a cup. See if you get debris. Should be good for a start up.

Thanks Mike. I plan to go through the carbs a little more thoroughly this time, including the filters and needles. Take care.

 

[xschris]

Any mods to the intake or exhaust will cause tuning issues. The idle mix screws on these earlier model carbs tend to have the tips break off in the carb bodies. Make sure this has not happened. I would try at least three turns out from lightly seated and go from there. If you need to go more than four turns to get a good idle you will need to go up one on the pilot jet. I would remove those small nets on the float valves and just use a in-line fuel filter. It will work much better and is way easier to change.