Honda Motorcycles - banner

Help! Bike won't Start!?

11596 Views 29 Replies 8 Participants Last post by  jaraxle1zx
I have a 1994 CBR900RR which after rebuilding ran great and rode nice. One day while starting it up after a 2 day rest, it started roughly and and while applying a little choke it stalled, never to start again! Spin, Spin, Spin no bang! Checked Spark, it's good. Checked fuel, it's getting it.

After many attempts to start it, it finally started backfiring as the only sign of life. After another 6 days of checking, testing, verifying electrical, etc. it started making the weirdest gurgling sounds in the exhaust (like it was under water and sloooowww...), neat, but not the sounds I or anyone would really want to hear from a Blade... it started backfiring along with the gurgling and at one point it started and ran! Rev'd somewhat decently, but still with loud backfiring intermitently. Turn it off, won't start.

Checking the plugs throughout the trials and tribulations, it seemed they were consistently wet with gas each time. This leads me to believe 2 things, the bike is flooding for one of 50 different reasons, or the cam timing has skipped a tooth and valve timing is off. Checking the carbs and float needle operation turned up no visible signs of malfunction.

Is it possible that the pressure switch has gone on the fuel pump? Does it have one on this model? Can the fuel pump provide too much pressure and force fuel past the needle valves? This is extremely frustrating given that the bike was running fine 2 days earlier! Several bike mechanically/electrically inclined friends with loads of experience are scratching their heads as their particular area of expertise seems to check out.

Your help and wisdom is greatly appreciated by a new member without his ride... Thank you.
See less See more
1 - 5 of 30 Posts
I can't see how a carb would function though if the float bowl were vented other than through the jets. The principle of the carb relies on it being sealed from atmosphere except through the jets.
Where did the overflows come out of?
Float bowls are vented blade. They need atmospheric pressure to operate. Sometimes bowl vents also double as overflow protection as well. It'll dump the fuel on the ground or into the engine instead on all over the engine and create a fire.
You know all this stuff blade, you're just not thinking of it right. Bike carbs typically had the hoses come out the bottom of the bike, They are teed in at the top of the float bowls to vent and supply a path to dump fuel if the float sticks and overflows. Here's a couple of pics I founf of both types of venting.
I think the vent tubes and overflow lines are one in the same. They are at the top of the bowls so they could do both jobs.
All good stuff but I still haven't seen a carb with an overflow tube.
And I've never seen fuel flowing out of an overflow tube because a float valve is jammed open, the fuel flows into the engine.
If I get a chance I'll check some of my books and see if I can find any carbs with overflow vents detailed.
I don't think you will find actual overflow tubes on a downdraft carb. If it overflows because of a stuck float, it just pours out the vent (which is just inside the air cleaner) and leak all over the engine or into the engine where it's safer if the vent is inside the throat. Marine code is into the throat for obvious reasons.

On side draft carbs like on most bikes, the vent and overflow are probably one in the same as several have said. The whole reason the pipes go down to the bottom is for that purpose.
I thought they're for venting vapour, not fuel?
The vent pipes should be above the level of the jets so any overflow should flow out of the jets first.
They are way above the jets blade. The jets can't flow that amount of fuel quickly. It has to go somewhere. I think we talked enough about vents for a while. :smilebig:
1 - 5 of 30 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.