Hot starting problem - XJ550

Only just realized that I have this problem just wondering if the collective have come across this before.

Bike starts great from cold with a little choke. It soon warms up and fuels cleanly on the road. The tickover is steady and reliable.
Valves are in spec, this has been double checked.
Carbs were balanced (with the YICS tool inplace) and are nice and even.

Once fully warmed up, if I stop for fuel and fill up and she fires first press.

If I stop for 15 mins or more, shes very reluctant to fire up. I end up opening the throttle about 1/4 and eventually it catches. It then behaves perfectly as soon as its fired up.

If I leave it for an hour after a run, she fires striaight up no problems.

Basically, after a good ride it is hard to start if I go back to her after 15-60 mins-ish. During the ride shes perfect at all revs.

The only non-stock item is the manual non vacuum fuel tap (original vac feed from carb blocked off).

I have run the fuel down in the carbs (tap off) to see if maybe the fuel level was causing issues, but it made no difference to hot start issue.

Idle screws were colour-tuned at 2 1/2 turns out. I have tried setting them at both 2 turns and 3 turns out. No difference.

Any idea chaps?

 

My 550 does the same thing. Check the float levels. I have one hi one carb 3. This will be a winter project.

 

Same with my bike. It started with 99 problems and now it only has 1, this problem. Literally no complaints at all except the warm start issue. Very manageable though, never in danger of it not starting. Just takes turning the throttle open 1/4 of the way and letting it catch and rev up.

Starting good cold but more difficult when warm would be an indicator of being too rich, but you say you have colortuned it as have I. This is not a rich pilot circuit problem. I don't think it is electric either. I have put the colortune plug in when this issue was happening and I always see a good spark, but no combustion until the throttle opens.

The "starts cold, starts hot, but not warm" concept is where my mind usually goes when I try to figure it out. That could indicate that something is heating up when parked from the hot engine that normally would be cool due to airflow, like the carburetors. I've heard the term 'heat induced flooding' thrown around, and that lowering float levels BELOW spec would help. But spec is spec, right?

Hoping others can throw out some ideas. The crazier the better.

 

I have the same problem on my Seca 550, bike runs fine when warm and always starts when cold with choke but hot starts are sometimes a problem. Carb floats and tune is good. Seems that a 30 second or 1 minute shutdown has no issues as a quick push of the start button with no throttle whatsoever works fine to start it up. However, after a 10 to 15 minute shutdown the bike often wont start without throttle (and lots of it) in order to catch. The hotter the weather the worse it gets. Sometimes it helps to put it on prime and bounce the bike a bit - so here's my theory:

After 30 second or 1 minute of shutdown, area above crankcase is hot but not enough time for any gas to evapourate.
after 15 minutes of sitting, crankcase heat has caused a little gas to evapourate, leaving bowl levels a little low.
Putting on prime for 30 seconds and bouncing on and off the seat forces a little more fuel into the bowls. Then she starts easier.
Colder weather causes less overall heat and less evaporation, therefore less or no problems restarting.

Just my 2 cents but anyone else with theories?

 

I can second the hotter vs colder weather issue.

 

it would be interesting to see what your cold and warm (at hard start) compression numbers you have. also, I would check the valve clearances at the warm hard start condition. I might also check the cranking vacuum at both cold and warm conditions.

CN

 

Have you tried putting the bike on prime for a few seconds before?

 

For me 'prime' vs 'on' has no effect.

Should be able to test float levels during hard start conditions without too much trouble though.

I always thought that compression numbers should go up when warm? The piston expanding more than the cylinder? Or would thinning oil lower compression? Or somewhere in the middle?

 

Our bikes aren't fuel injected. Imo during the first 15min or so after running at full op our bikes are in a "semi-shocked" state for lack of a more technical term. They're not completely at rest, like from a cold start, nor are they near full op temp like when they're hot. These bikes cool down fairly quickly (as they should). Imo in 15min after running the fuel left in the holes is not enough to start our bikes. I put my petcock on prime and put my ec on half and she'll start right up (i turn the pc back to on and take the ec off as soon as she fires). I let her run for a few seconds to let the carbs pick back up the fuel feed then i'm off. Like all of you, had this same issue for a while with everything in spec. She cold started great and ran great but had the darndest time starting from sitting for 15-30 min after running. I've timed how long it took from running at full op for 1hr @ 70mph before I could touch the headers and exhaust with my bare hand. Of course the headers took longer but it wasn't nearly as long as i thought it would take (about 15min).

Gary H.

 

Thank you all very much, some great comments and theories and my issue certainly reflects what you guys have experianced.
Vuxtable, your description is exactly how I would say mine behaves to the T!
My initial thoughts were that it is some kind of fuel vapourization. Many years ago I had a sporty little Renault 5 GT Turbo (a car I know, but bare with me). This would refuse point blank to restart when hot after a run if left for a similar 5-50 mins. The cure was vented louvres on the bonnet (hood) and relocating the fuel line. Eventually this became a recognised issue with this model of car, although some, for no apparent reason, were not affected in anyway. The xj feels just the same but will fire up with a quarter throttle.
I know without doubt the valves are well within spec and set towards the higher clearance end. It would be interesting to check the hot clearance, but there obviously no factory figures for this and would vary depending on how hot the engine is.
I will certainly check out all the suggestions including compression. The fuel levels will have to wait until the weather cools though as Im having to much fun riding her . It really does ride so sweetly.

 

It's not unusual to experience some cantankerous behavior trying to hot-start even a perfectly tuned 550R.

Things that may or may not help; depending on all sorts of variables they have all worked for me under one circumstance or another:

-flip it on PRI for 30 seconds or so while you put your helmet and gloves on, etc. Switch back as soon as it fires.

-Move the "choke" lever to full on, count to two, then off again once or maybe twice (squirts whatever's in the enrichment chambers into the carb throat.)

-Or go ahead and give it about half "choke" until it fires.

And remember that as soon as you so much as touch the throttle you're doomed; until it lights up.

My bike usually fires as soon as I touch the starter, usually less than half a revolution and it's running. If NOT, then I apply one of the above techniques.

If you stall the bike or shut it off before it's fully warmed up, NONE of the above applies and you're on your own. Probably will need the "choke."

 

Hello guys!

Did someone solved this problem. I have exactly the same problem with my Yamaha xj550 seca 4V8. I changed the battery, new spark plugs, check the pressure in the cylinders, adjust valves, adjust the floats, cleaned the carburetor, set up a pilot screws, changed oil, etc; but I still have the same problem. I was wondering if any of you, who have had this same problem, managed to solve this mystery.
P.S. I must say that the only thing that is not factory-provided on the engine, fuel filter between the carburetor and fuel-taps (petcok).

 

well it could be your getting vapor lock. the fuel filter must be below the petcock and full of gas I use clear blue line and when I had this problem could see an air bubble in the fuel line. I use two 90 degree elbows and route my fuel line around the back of the carbs then to tank with filter placed so it is below petcock
first try popping the gas cap to check if you have a sticking vent. then prime and the air bubble will suck up into the gas tank.
make sure your clamps are tight.
and as BIG Fitz states DO NOT touch the throttle and if needed apply some choke.

you can test this by removing your filter to see if it fixes your hot start issue

 

are your valves clearances in spec?