Help! Bike won't Start!?

11596 Views 29 Replies 8 Participants Last post by jaraxle1zx, Jan 13, 2010

1994cbr900rr

Discussion Starter · Jan 9, 2010

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.

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bladeracer

· #5 · Jan 9, 2010

1994cbr900rr said:
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.

What was the reason for the rebuild?
Was anything improved or was it built totally stock?
Have you done compression and leakdown tests?
Is the vacuum line to the fuel tap dry?

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bladeracer

· #8 · Jan 11, 2010

1994cbr900rr said:
Fresh fuel used every time we changed something and tried again, and like I said it did run for a minute or so, just popping alot and really loud!

I'm not sure how to check if the timing may have skipped, other than to follow my haynes and align the timing marks under the case cover. I do have a timing light come to think of it...

Popping or backfiring? Popping indicates a leak in the exhaust system. Backfiring is an ignition or cam timing problem.
A timing light doesn't do valve timing - it's for ignition timing.
If the camchain tensioner wasn't removed at all then it's unlikely the timing has altered - unless it's very high mileage and about due to skip anyway?
Does the crankcase smell of fuel?
Are the plugs new?
How long was the bike sitting before you've tried starting it now?
If it's been a while my guess is it needs a thorough carburetor strip and clean and probably new needles and seats.

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bladeracer

· #13 · Jan 12, 2010 (Edited)

cbr lonner said:
if the float bowls were over filling id expect it to pour on the floor via the overflows on the carbs which it isnt.

Overflows on the carbs?
The only overflow I know of is the main jet where the fuel then runs down into the cylinder.

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bladeracer

· #15 · Jan 12, 2010

cbr lonner said:
im thinking of the float needle valve sticking so if the carbs just dump all the fuel into the cylinders that woud figgure.

they used to have an over flow on all carbs once so you could tell it was leaking

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?

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bladeracer

· #17 · Jan 12, 2010

denzee said:
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.

I know it is kept at atmosphereic pressure via "venting" through the jets but those vent into the carburetor still.
I've never seen "overflows" that allow fuel out of the carb though.
There are drains on float bowls that are opened manually to drain the carbs though.

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bladeracer

· #19 · Jan 12, 2010

denzee said:
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.

You may be right Den

The first pic shows the air vents (not overflows) and I'm fairly sure I've never seen fuel come of them.
The second shows the vent back into the throat.
In both cases, if the float valve stays open fuel will flow into the engine, not to the outside of the carbs.
The only time I can ever recall seeing fuel running from an overflow is from the fuel tank overflow.

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bladeracer

· #21 · Jan 12, 2010

cbr lonner said:
that was my thought's denzee.

when i fitted dynojets to my carbs the other day fuel pissed out when i tipped them up as i didnt bother to drain them fully .
i dont smoke any more and i do miss the danger:smilebig:

carbs used to just have a pipe that ran down to the bottom of the engine and you got a pool of fuel on the floor if you dropped the bike or the floats got stuck.

suppose with the strict emission laws its changed a bit now

If you drop the bike or invert the carbs the fuel will drain out through the air vents.
Do you know any bikes with overflows so I can have a look at them?

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bladeracer

· #23 · Jan 12, 2010

cbr lonner said:
hi

looking at the bikes you have had it would be on them i believe(had some of them bikes myself as im 44)

think lots of the older bikes were like this ?

i may be confusing terms but whatever allows the fuel out is what id call an overflow ie when it falls over the fuel pours onto the floor as the float valve cant function correctly as it relies on the fuel lifting the float and shutting the valve.

think were on the same lines here just terminology

were really hijacking this thread too:O

Probably the oldest bike I've owned myself would be 1980 (actually, I owned a '74 GT550 but that was before I got really mechanically inclined) but I've worked on some seventies stuff.
And I don't recall ever seeing any overflows (even on cars).
The air vents that would allow fuel to drain when the bike falls over should be designed to prevent fuel from running out of them if the float valve sticks. Any vent should be above the level of the jets so that's where any overflow should come out - thus into the cylinders.

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bladeracer

· #26 · Jan 12, 2010

cbr lonner said:
If this level is way too high, gas can leak out the carb overflow tube or into the engine.

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.

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bladeracer

· #28 · Jan 12, 2010

denzee said:
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.

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