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Just been having breakfast with the guy that owns the 250 I'm riding..and we were discussing hot starting problems with the ZXR.

Now that mine has flatsides it is a complete bullocks to start when it's hot.............it takes a lot of pushing.

So try this for size

Hot bike after 75 miles of racing stop it and fuel gets warm in the carbs and vapourises filling the airbox with petrol fumes creating a very rich mixture...........try to start the bike after say 2 minutes while it's refilled with fuel and it wont start cos it's way too rich ? ? ? pushing it evenutally gets some air into the airbox and it fires.


The solution.............

fuel vapour is heavy very heavy in fact open your tank and look to see the fuel vapours which cascade downwards from the cap.

So if a man was to drill a hole in the bottom of the airbox the fuel vapours will vent down and out of the airbox ? ? perhaps even drawing it some air via the inlet scoop?
and so there will not be a fuel rich mixture in the airbox and the bike will be an easy start ? ? ?

Thoughts please but please sensible ones.

would you be allowed someone to stand with a Leaf blower or fan into the snorkel for airbox??

I suppose thats possible, but arent we potentially looking at a backfiring when starting with an airbox full of fumes? And why would some bikes do it and some not??

Do owners with this problem report a strong smell of fumes at all?

Simple way to test your theory. On coming to a stand with the engine hot, hot, hot turn off the fuel tap and wait until the engine dies through starvation. Carbs are then empty so no chance that any fuel could vaporise and fill the airbox. Leave the bike to stand for a little while, turn the fuel tap on and try to hot start. If it's still a giraffe your theory's blown. If it fires easily you're on to a winner.

I run with trumpets so no air box to hold fumes.

Since fitting "kit" needles starting has become a lot easier - instant responce from cold and once up to temp or boilin hot a dab on the button then another with partial throttle and it fires up.

The kit needles are about 2/3rds thinner at the tip so fuel is getting atomozed to a finer mist - what needles are you using ? EXRs ?

Where the bike becomes a bitch to start is when I fit the F3 race CDI , but a bump gets it going no prob's - I am guessing it is needing the cam's to go along with it for this stop. As soon as things get a bit quiter here I will be dropping in a set to see if they do indeed help. I would suggest anything other than an SP running the high lifts will need a different CDI to bring the best out of them.

So another factor might be what cams are in the bikes - there are at least 4 different types used.

Interesting one it's still kicking about and not been solved.

My VFR developed a hot starting problem but this was the battery!

It started the bike perfect from cold but would not restart when hot. Put a new one in and it fixed it, never did figure out exactly why this happened... battery deteriorating from heat? Or there being slightly less compression perhaps on a cold engine?

Who knows.

Still it'd be good to check all electrical voltages, battery, fusebox, genny, igition box when the problem arises and check them against a clean starting bike, if nothing but to rule it out.

Other possible culpirts that come to mind are.

Low compression
Bad valve clearances
Bad igition timing
Excessivly unbalanced carbs
Worn valve gear.
Crap fuel pump

Making it worse is that it could be a combination, or different factors on all bikes.

Since most bikes have similar problems though i'd like to believe there is a root cause.

Kawasakis' explanation of fuel draining away should be simple to measure.. attatch some plastic tubing and measure the volume of fuel in a fully filled cold carb and then again in a hot carb after standing.

My main reason for believing it to be more electrical / engine related is that the problem seems to occur on several bikes that all have radically different carb setups... CVK's, FCR's, air-trays, ram-air boxes and standard kit.

For so many different set-up's to have such similar problems makes me think it lies else-where.

Who knows

Mine suffers from this problem too. I need to use at least half, usually full throttle and the bike sounds like it's running on three before the fourth catches and it's away sounding fine.

Tirpitz - The problem with your idea is that it would take several seconds to get fuel back into the carbs so you wouldn't necessarily know if it was hot start or no fuel causing the problem.

Cargo - I think you may be onto something here. I have noticed the carbs do throw quite a bit of fuel back up into the airbox. I have a feeling fuel (as you say, heavy vapour) is collecting, possibly all the way into the inlet tract, this would make it run very rich. And, tbh, my bike stinks of fuel when it's not running. I think this is comming from the air box / secondary fuel tank

stevezx7r wrote:Tirpitz - The problem with your idea is that it would take several seconds to get fuel back into the carbs so you wouldn't necessarily know if it was hot start or no fuel causing the problem.

Why? They're gravity fed, or if the head pressure is low the fuel pump comes on to force feed them. If the bowls are empty the floats will have dropped. The second you turn on the fuel tap petrol will run into the bowls and fill them. So you turn on the tap, give the bowls time to fill, then hit the starter. Now if you were talking about fuel injection, where you had emptied the injectors, you'd have a point. The injectors will take time to bleed the fuel lines and fill with fuel as you crank the engine. But we're not, this is a bog-standard carb fuelled bike. Fuel sits in the bowls and is drawn up into the carb mixing chamber by venturi action as soon as there is an airflow.

Fair doo's, it's just that the last time I was Effing about with mine (empty carbs) it did take a while to get the bike running - 20 plus seconds iirc. Probably my fuel pump on it's way out.

Simons race bike had hot starting problems particularly when a race was red flagged for some reason,we cured this by using a 3mm thick sheet of rubber cut to fit under the carbs (CV)this guided the rising hot air away from the float bowls whilst the bike was stationary, I have seen this done on RLR cbr 400s but they add a length of flexible hose about 20mm dia this runs from the front of the bike down the side of the engine and across the back with four holes just in front of the carbs to cool the float bowls,this was done to reduce the drop in power when the engine got hot, we never tried this so Im not sure if it works or not,I know this is not a help with the hot starting issue but it does point to problems with fuel overheating in the carbs and the problems it can cause.

rather than drill a hole in the air box, why not take off the end of the air box water drain (little pipe exiting left hand side of bike). If these vapours are heavy as said earlier they'll exit easily through that. Saves drilling and you can plug it back up after.

But then you will be sucking unfiltered air....

there is no filter in my airbox and a hole will actually be blowing air out............

I refitted the two air inlets mostly cosmetic but now apparently with a purpose to blow cool air on to the top of my airbox and I fitted a bit of hoover pipe to direct cool air to the area below the air box.

For my MGP pitstop was very hot bike restarted with ease...............however I've no idea if what I did helped and after finished 16th I dont much care anyway

cargo wrote: I refitted the two air inlets mostly cosmetic but now apparently with a purpose to blow cool air on to the top of my airbox

We told you it wasn't a ZXR without the hoovers Them Japs are not daft, everything's there for a reason

Ah but I fitted some very clever aluminium ones......I'll try and find a pic