cold start idles at 7k then revs to 9500 within thirty seconds!

branson

branson

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So I got the bike to fire up for the first time in ages. I've redone the carb and whatnot but when I fired it up it went to 7000 rpm instantly at idle. The moment I read the gauge it began to climb till it hit 9500 and I shut it down. It was still climbing! I didn't even have time to turn my choke off. I haven't hooked up an airbox yet either but I can't help but find a idle speed over 9k to be mildly disconcerting. Any thought? My screws are 3.5 turns out and all that good stuff. My jets are clean and my seals are sealed. So much for the usual remedies.perhaps my throttle is whacked? The bike was dumped by the previous owner and idk if its ever run before today and since the dumping. I know it was dumped cause he broke the brake off mauled the kill switch removed the mirror caliper cable etc. Could this bugger my throttle? Is the enrichment device aka choke able to make that much of a difference?
 
your rubber boots probably are leaking and without any kind of air filter on its sucking way to much air. Also try it with the choke in and see what it idles @
 
how much restriction is to be expected from the air filters? i was gonna use some surgical foam and metal tubing to whip some up but dont know if thats going to create enough drag
 
Wow. Well I read my handy dandy story book and it says my float height spec is 32mm+/-1mm. Is that right? I just made sure they matched when I redid the carbs. They were both presett at 24mm. That's about a quarter inch off. Bike won't even start today. Prolly fuel fouled. No air filter plus a quarteer inch extra fuel in the bowl seems like a bad combo. Any idea if that would make the bike try to start at redline?
 
its a 1980 xs400 sg sohc with the lever style choke. I suspect the carb came from a different pwerplant originally as I was given three of them when I bought it. Do you have a list of float heights by bowl diameter I could crossreference and set to?
 
it looks a lil bigger than the two with the pull style but if you open your haynes manual it mentions both styles. Is there a better (more accurate) way to tell? The insides looked rather odd to me. Almost like it needed more guts.

What makes you suspect an xs 650 or a 250?
 
Nice! I got my first xs400 for a b-day gift from my girl friend (now wife) 13 years ago.
 
Does the throttle move freely? Have you tried backing the idle air screw out? My guess is that the throttle is being held open by something.

The idle air screw is just above #5

View attachment 3926

Try loosening the throttle cable all the way and see what happens.
 
i agree with hougmade.on my old gs 450 the trottle cables would hang up every time i had the carbs off.the cables would bind a little and open the butterflies in the carbs and it would rev just the same:banghead:
 
Judge rules throttle cable. I dc'ed it and wa-lah! Instant idle just add starting. Much oblidged to all the wonderful folks at xs400
 
branson said:
Where did you find this pic houghmade? It looks like it came from a guide I should be reading
Click to expand...

That's actually a pic of my carbs that someone here used to explain all the parts of the carb. I'll see if I can find the threads....it was used numerous times.
 

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