Pregen went "pop" and now doesn't accelerate past 25-30 mph

[skucera]

Phooey,

While riding to work this morning I twisted the throttle to accelerate up an overpass after taking off from a light, and just after shifting to 2nd the engine made a little "pop" like a champagne cork, and then just wouldn't accelerate. I was already doing about 45 mph, and it just cruised up and over the overpass, but traffic usually flies over it at 55-65 mph, so drivers behind me were probably thinking I'd suddenly decided to go slowly. After the next light, I couldn't accelerate past 25-30 mph.

The throttle response is great up to about about 15 mph in 1st gear, then flattens out. I think perhaps I've torn a carb piston diaphragm, or perhaps blown a ring on one cylinder, preventing vacuum. I suppose I might have a valve stuck open too, since that would also limit vacuum and perhaps mimic this symptom. I've experienced all of these problems on British sports cars with SU or Zenith-Stromberg carbs over the years, but it is a first for me on a motorcycle.

So, I get to truck my little Ninja home and do a compression test to see if the rings and valves are OK. If they are, I'll have to check the carbs. Anyone have any other ideas of what might cause this sort of problem?

Thanks,
Scott

[tc.young]

Originally Posted by skucera

Phooey,

While riding to work this morning I twisted the throttle to accelerate up an overpass after taking off from a light, and just after shifting to 2nd the engine made a little "pop" like a champagne cork, and then just wouldn't accelerate. I was already doing about 45 mph, and it just cruised up and over the overpass, but traffic usually flies over it at 55-65 mph, so drivers behind me were probably thinking I'd suddenly decided to go slowly. After the next light, I couldn't accelerate past 25-30 mph.

The throttle response is great up to about about 15 mph in 1st gear, then flattens out. I think perhaps I've torn a carb piston diaphragm, or perhaps blown a ring on one cylinder, preventing vacuum. I suppose I might have a valve stuck open too, since that would also limit vacuum and perhaps mimic this symptom. I've experienced all of these problems on British sports cars with SU or Zenith-Stromberg carbs over the years, but it is a first for me on a motorcycle.

So, I get to truck my little Ninja home and do a compression test to see if the rings and valves are OK. If they are, I'll have to check the carbs. Anyone have any other ideas of what might cause this sort of problem?

Thanks,
Scott

running on 1 cylinder maybe?

[skucera]

Maybe.... The odd thing is that it seems to still be idling on two cylinders, but with weak compression it might still be firing on both cylinders even with compression rings blown, or a lost valve shim or a stuck valve that won't close all the way.

It's killing me that I have to just finish my workday before I can even think about beginning the diagnosis....

Scott

[Motofool]

Originally Posted by skucera

......Anyone have any other ideas of what might cause this sort of problem?

It seems that one of the spark plugs has popped out.
If so, the aluminum thread of the head may be damaged.

[alex.s]

at first glance i too read a spark plug pop. good luck. if the threads are damaged (if the plug came out they probably are) then you'll need to tap it a size larger and insert a helicoil.

[choneofakind]

^^ that's what I did last summer. Bleh.

FWIW, it made a nice pop, then ran on one cylinder horribly. and it was loud.

[skucera]

Originally Posted by choneofakind

FWIW, it made a nice pop, then ran on one cylinder horribly. and it was loud.

Loud? My bike is absolutely the same volume, which is quite quiet at idle, but racier sounding as it revs. Unfortunately, it only revs to 5000-6000 RPM in this condition. I hadn't thought of a blown spark plug. That is easy and relatively cheap to fix compared to some other scenarios.

I thought again on riding the bike home, and realized that if I had failed rings or a torn carb diaphragm, I didn't want to chance doing more damage. A jagged ring or lean mixture in one cylinder could make for a lot more repair work in the end. So... I called my wife for a ride home, and we bought ramps and straps so I can bring the bike home in my pickup tomorrow.

Scott

[Motofool]

Originally Posted by skucera

.............So... I called my wife for a ride home, and we bought ramps and straps so I can bring the bike home in my pickup tomorrow.

Just be extra careful:
http://www.ninjette.org/forums/showthread.php?t=121630

[skucera]

Those clips were hilarious, especially the ones where guys figured they could just ride a road bike over a sharp break like a ramp on a tailgate. Luckily, I had a couple of coworkers to help me and it went up without a hitch.

Scott

[choneofakind]

Yeah, mine was loud with one plug blown out. I was basically blowing all the air/fuel mix out the top of the head, then igniting it in open air whenever the plug was grounded on the head.

Doesn't sound like your problem is the same as mine.

[n4mwd]

I suggest pulling the valve cover off and inspecting the insides for broken stuff. And also, its a bad idea to run it like that, even idling, until you know for a fact that nothing is loose in the the head. Loose stuff up there will destroy the engine.

But aside from that, if it wasn't for the pop sound you heard, the symptoms would indicate the carb is bad/plugged up.

[FvnnyL3tt3r1ng]

I've had my bike pop very loudly out of the exhaust when it became fuel starved. An overly lean condition will cause that. It may or may not be your plugs but I'd check your fuel line and rate of flow.

[skucera]

I ran a compression test, and both cylinders are within 10 psi of one another. The plugs looked good. That got me looking at fueling and the carbs.

The bike will rev eagerly on full choke. I think I have partially blocked jets, and perhaps a torn diaphragm. I didn't have any split vacuum hoses. I've ordered rebuild kits, and I hoped they would be here this weekend, but they didn't arrive just yet. I hope to rebuild the carbs next weekend. If I need a new diaphragm I'll buy it then.

Scott

[FvnnyL3tt3r1ng]

Originally Posted by skucera

I ran a compression test, and both cylinders are within 10 psi of one another. The plugs looked good. That got me looking at fueling and the carbs.

The bike will rev eagerly on full choke. I think I have partially blocked jets, and perhaps a torn diaphragm. I didn't have any split vacuum hoses. I've ordered rebuild kits, and I hoped they would be here this weekend, but they didn't arrive just yet. I hope to rebuild the carbs next weekend. If I need a new diaphragm I'll buy it then.

Scott

I'd probably buy a used set of carbs off ebay before I bought new diaphrams considering the costs for what you get. Sounds fuel starved at this point. Happened to me when one of my float valves went out.