Cold start problems

[cosworth]

I thought I knew carbs. I've tried it all and I'm stumped.

Stock 1980 xs400. I have sprayed 3 cans of carb cleaner in every orifice. All the jets out, emulsion tubes out, choke out, into the ultrasonic cleaner. Carb cleaner comes out all 4 holes by the butterflies when I spray from the air jets. I got some welding wire in there even and poked the 3 holes. Took the idle screws out and cleaned them too. I have had the carbs totally apart and really cleaned them. Twice now. I'm missing something. I must. These carbs are the ones with the idle jet rubber plug and the passage from the main jet.

Bike runs fine warm. Idles ok, starts up again. One quick press of the starter button and choom, she fires up. Plugs look fine.

But cold, she won't start. The choke circuit does nothing. Once I do get it going with some carb cleaner sprayed into the airbox, the choke does nothing. So if I help it along and spray carb cleaner into the airbox she will catch and run. Idling at 900-1000 rpm. Reluctant to rev for a bit then running ok. Once warmed up she idles at 1500 and rips around town like a raped ape.

But she doesn't want to get out of bed. Once she is warmed up, I spray carb cleaner around the rubber bits and there are no leaks. No changes in revs. So no air leaks. Rubber tube from I take to petcock is intact.

I've tried the idle screws at 1.5 and 3 turns out. No change. Float levels are dead on. What the hell am I missing?

 

[16VGTIDave]

A couple thoughts for you. 1500 rpm idle is too high, should be around 1000 to 1200. If the bike doesn't want to run well at that rpm, I'd be looking for vacuum leaks between the engine and the carbs. The throttle shaft seals are often overlooked. Also, 3 times seems to be the magic number for carb cleaning. Strip, throughly clean, and replete 3 times. When you swear they are as clean as possible, do them one more time! Yes, I know how that sounds, but history shows... Mine took multiple baths in the ultrasonic cleaner, the last was for 12 hours with a mild acid cleaner, to get the corrosion out of passages. If you don't have vacuum leaks, I'd guess there is still blockages in the Pilot (idle) circuits.

The diaphragms are critical for cold start enrichment. Make sure they are holding a vacuum - raise the slide up, block the large oval port in the carb inlet and let the slide go. It should drop a small amount and then hang. If the slide drops quickly either the port isn't sealed, the cold start plungers (choke) are leaking, or the diaphragms are leaking. A slow descent is ok. Also, make sure the float bowl is cleaned throughly. There is a passage for cold start (called choke, but not a choke) fuel supply that draws from the bottom of the float bowl and is easily plugged with debris or corrosion.

That's all I've got for now, I need more coffee.
Please report back with your findings!

 

[cosworth]

I have cleaned those yes. That's why I'm stumped. I use ethanol free fuel, the plugs are gapped right, lots of spark. Diaphragms are perfect and seal nicely.

She is just a really hard start. I'm wondering if the previous owner changed a pilot jet on them.

 

[ReelRacin57]

I am having the same problem. I know I have a air leak on the left intake boot. But it will not start for the life of it when it's cold the choke doesn't make any difference, and normally you can hear it. So I have to start it with starting fluid. But like you when it warms up it fires no problem

 

[cosworth]

Ok so I stuck the charger on it for the 75th time. Drained the tank. Drained the float bowls. I put in 5 litres of chevron 91. No ethanol. I dumped a whole can of seafoam in there. Let it sit with the petcock on prime for two days.

She fires up no problem now. Thank you seafoam.

I need an ultrasonic tank that a whole carb can fit in. Aliexpress here I come.

 

[norrie]

I'm having the exact same set of issues but I've had half a can of seafoam in for a few days now. Hill starts only til I troubleshoot it out.

 

[Travis]

Ok so I stuck the charger on it for the 75th time. Drained the tank. Drained the float bowls. I put in 5 litres of chevron 91. No ethanol. I dumped a whole can of seafoam in there. Let it sit with the petcock on prime for two days.

She fires up no problem now. Thank you seafoam.

I need an ultrasonic tank that a whole carb can fit in. Aliexpress here I come.

I'm impressed that worked! I wonder what was clogged that the seafoam helped. Does it run well with that much seafoam in the fuel?