Sketchy power delivery?

Insan3guy · August 24th, 2013, 04:05 AM

(gpz900r-A1)I was out riding yesterday, and I decided to have a little fun. It's one of those backroad highways with a long straightaway and no cops. Anyway, I got it up to around 130 @8-9k rpm, and I went to slow down. It might be that I let off the throttle too fast, or that I was out of gas, or something (this thing is a gas HOG), but when I got back down to cruising speed (about 60mph @ 5.5k) it started tugging. It would sort of jerk forwards, then let off, then jerk forwards again, etc. My hand wasn't moving on the throttle.
About a minute after that, it cut out. I pulled over, switched to reserve, cranked it for a minute, and went along.

Now. About a mile after that, it started doing the tugging thing again. It COULDN'T be out of gas, and the gas in the tank isn't more than 2 days old. I made it to a gas station, filled up, and then crossed the street to my house. Didn't misbehave at lower rpms, it only seemed to do it at cruising speed.

Any ideas on what could be behind this? I haven't gotten a chance to check the valves or carbs fully yet, I was saving that for a project next week.

choneofakind · August 24th, 2013, 06:11 AM

That's called surging. That's a symptom of lean fueling.

That can mean 2 things:
1) dirty carbs
2) need to rejet

Since its an unfamiliar carb'd bike, I'm voting for number 1. Checking valves would be a good idea as well.

Insan3guy · August 24th, 2013, 06:40 AM

One of the carb's airbox boots isn't seated properly on the airbox side (missing the spring wrapped around it). It looks like it's sealed but would that have the same effect? Previous owner lived in alaska. maybe elevation difference?

choneofakind · August 24th, 2013, 07:19 AM

Yes. That will give the same effect by adding extra air to one cylinder. I've experienced that on a ninjette as well. Get that seated properly on the carb and replace the spring if you can. Try a hose clamp or something similar if you can't get your hands on the spring.

DaBlue1 · August 24th, 2013, 07:45 AM

It is also possible for the the bike to slow down while riding on a flat surface if the fuel is too low. The carbs tend not to fill up as fast. I had it happen before. Any amount of throttle more than 1/4, resulted in a non response.

Low gas, go slow.

Insan3guy · August 24th, 2013, 08:01 AM

Quote:

Originally Posted by DaBlue1

It is also possible for the the bike to slow down while riding on a flat surface if the fuel is too low. The carbs tend not to fill up as fast. I had it happen before. Any amount of throttle more than 1/4, resulted in a non response.

Low gas, go slow.

How does that work? Shouldn't the carbs draw the same amount until the gas tank runs dry?

DaBlue1 · August 24th, 2013, 09:05 AM

Quote:

Originally Posted by Insan3guy

How does that work? Shouldn't the carbs draw the same amount until the gas tank runs dry?

The fuel system is a gravity feed system. It requires the weight if the fuel itself to feed the carbs. As the fuel becomes less, the flow can slow. Also if you have a tank venting problem or clogged fuel filter, the same symptoms can result.

Motofool · August 24th, 2013, 11:29 AM

Quote:

Originally Posted by Insan3guy

.........
Any ideas on what could be behind this? I haven't gotten a chance to check the valves or carbs fully yet, I was saving that for a project next week.

The poor machine is almost 30 years old !!!

At this point, if the engine is original, some oil should be leaking into the combustion chamber (via rings and valves' guides).
If so, some carbon has been accumulating in there (especially if non-synthetic oil is used).
During some fast ride, the temperature and pressure in there reaches values higher than normal.

That can cause two things:
1) The carbon gets hot-red at crests, sharp edges, etc., becoming little glowing points that induce erratic pre-ignition.
2) Some particles of the burned carbon fly and linger inside the chamber while trying to find the exhaust path. Then, some are attracted by the magnetic field of the sparks, landing between the electrodes and creating a temporary bridge that induces spark failure.

Read more about this phenomena:
http://en.wikipedia.org/wiki/Engine_...f_pre-ignition

Insan3guy · August 24th, 2013, 02:53 PM

totally forgot about the fuel filter... it's a little metal one that's about as thin as the fuel line. I'll have to grab a new one

August 24th, 2013, 04:05 AM	#1
Insan3guy ninjette.org member Name: Patrick Location: Coronado, CA Join Date: Jun 2013 Motorcycle(s): 2015 zx6r Posts: 176	Sketchy power delivery? (gpz900r-A1)I was out riding yesterday, and I decided to have a little fun. It's one of those backroad highways with a long straightaway and no cops. Anyway, I got it up to around 130 @8-9k rpm, and I went to slow down. It might be that I let off the throttle too fast, or that I was out of gas, or something (this thing is a gas HOG), but when I got back down to cruising speed (about 60mph @ 5.5k) it started tugging. It would sort of jerk forwards, then let off, then jerk forwards again, etc. My hand wasn't moving on the throttle. About a minute after that, it cut out. I pulled over, switched to reserve, cranked it for a minute, and went along. Now. About a mile after that, it started doing the tugging thing again. It COULDN'T be out of gas, and the gas in the tank isn't more than 2 days old. I made it to a gas station, filled up, and then crossed the street to my house. Didn't misbehave at lower rpms, it only seemed to do it at cruising speed. Any ideas on what could be behind this? I haven't gotten a chance to check the valves or carbs fully yet, I was saving that for a project next week.

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August 24th, 2013, 06:11 AM	#2
choneofakind ninjette.org certified postwhore Name: . Location: . Join Date: Feb 2011 Motorcycle(s): . Posts: Too much. MOTM - Feb '13, Feb '14	That's called surging. That's a symptom of lean fueling. That can mean 2 things: 1) dirty carbs 2) need to rejet Since its an unfamiliar carb'd bike, I'm voting for number 1. Checking valves would be a good idea as well.

August 24th, 2013, 06:40 AM	#3
Insan3guy ninjette.org member Name: Patrick Location: Coronado, CA Join Date: Jun 2013 Motorcycle(s): 2015 zx6r Posts: 176	One of the carb's airbox boots isn't seated properly on the airbox side (missing the spring wrapped around it). It looks like it's sealed but would that have the same effect? Previous owner lived in alaska. maybe elevation difference?

August 24th, 2013, 07:19 AM	#4
choneofakind ninjette.org certified postwhore Name: . Location: . Join Date: Feb 2011 Motorcycle(s): . Posts: Too much. MOTM - Feb '13, Feb '14	Yes. That will give the same effect by adding extra air to one cylinder. I've experienced that on a ninjette as well. Get that seated properly on the carb and replace the spring if you can. Try a hose clamp or something similar if you can't get your hands on the spring.

August 24th, 2013, 07:45 AM	#5
DaBlue1 Long Time Rider Name: Blue Location: Charlotte, NC Join Date: Sep 2010 Motorcycle(s): 2009 Kawasaki Ninja 250R Posts: A lot.	It is also possible for the the bike to slow down while riding on a flat surface if the fuel is too low. The carbs tend not to fill up as fast. I had it happen before. Any amount of throttle more than 1/4, resulted in a non response. Low gas, go slow.

August 24th, 2013, 02:53 PM	#9
Insan3guy ninjette.org member Name: Patrick Location: Coronado, CA Join Date: Jun 2013 Motorcycle(s): 2015 zx6r Posts: 176	totally forgot about the fuel filter... it's a little metal one that's about as thin as the fuel line. I'll have to grab a new one