300/440/550 550sx wont start after running

[dpenta13]

Having trouble with a 88 550sx.

Has a factory 550 piston port motor. I believe it is bored to a 600cc or close.
Has milled head. Coffman Pipe, Side draft manifold with mikuni 44.

Thats all i really have for specs at the moment, until i look at it again next week.

The ski runs perfect the first time you start it for the day. After you shut the engine off weather you run it for 5 min or full tank of gas it will not start up again. Cranks fine just wont fire. Weather its in or out of water.

 

[flgliderpilot]

Try pulling the plugs (and then installing them back into the boots and grounding against the cylinder head), and then turn it over for a solid 15 seconds or so. Do this twice. Then re-install the plugs and see if it starts.

I had this problem with mine, seemed to be due to water getting back into the engine when it was rolled over on it's side, etc.

 

[dpenta13]

Ok i will have my buddy try that.

It does it when it is started out of the water and ran dry for a bit then shut off, wont start again for a while

it is def harder after a ride in the water then it really wont start and usually the battery will die before you can figure out why.

Possibly a fuel pressure issue, should we be running a restrictor on the return to keep fuel pressure and fuel in the carb longer?

 

[flgliderpilot]

Does it have a primer? yeah it could be a fuel pressure problem too. If you run it for awhile, and then open the gas cap, do you hear the hiss of air escaping?

Might want to check your one way valve at the tank also.

With mine, it would run fine until it sat for awhile... say a few hours or more and then it wouldn't start. Pulling the plugs and clearing it out would make it start right up. Haven't had the problem lately so maybe the fuel had water in it from sitting.

 

[screaming440]

it is most likely "fuel dumping". A problem the early 44mm round body carbs had that causes all the fuel in the "floatbowl" to dump into the engine after you shut it down. try starting it with the throttle wide open as it is likely flooded.

also put new plugs in it cuz fowled plugs will make it harder to start if it is flooding.

also yes there should be a return line restrictor if the revlimiter has been removed.

 

[flgliderpilot]

I figured all these carbs were diaphram carbs? Ie no float bowl?

 

[amitchell]

No float bowl, needle and seat in both BN and SBN.

Is it a BN or SBN 44?

BN 44's need return line restrictors if I remember correctly. I used a mig wire tip.

I had a hell of a time with the BN's and switched both 550's over to SBN's and it was like night and day for starting them.

 

[screaming440]

yes that is why "floatbowl" is in quotes it is actually called a fuel chamber but effectively does the something. it fills with fuel and atmospheric pressure pushes on the fuel through the diaphragm low pressure in the venturi creates a differential pressure that allows fuel to move through the jet.

this is a fuel dumping problem. it is well documented problem in old articles on this carb. replace your plugs if they are old and if it has been sitting for a short time start it with wide open throttle cuz it is flooded.