I have a 98 superhawk with 16k, Jardine slip ons, stock filter and possibly a dynojet kit from the previous owner. I have been riding the bike at least a few times a week for the last year and have replaced only the stator and the diaphragm where the fuel lines connect to the tank after it started to cut-out at highspeed last fall. (turned out to be a very torn diaphragm)

In the last 1500+ miles the bike has been running perfect except for the popping/backfire that occurs around 3k RPMs when slowing down, but this has always happened on this bike.

Now to the problem...Riding home this weekend I had been doing a long highway ride running about 80mph and then started to accelerate hard but when I got up to 7k RPMs the bike seemed to hit a wall, I downshifted and still the RPMs would not go up but actually went lower. I slowed back down to 80 and could slowly accelerate but hit a wall again this time at 5k RPMs.

I limped the bike back home and running around 30-40mph it had a lot of sputtering but would accelerate just slowly. I pulled the tank, checked the vacuum lines and plugs and everything looked ok, and confirmed I have spark from both plugs.

Now, on idle, the bike will run up to redline but it is slower to do so and you can hear hesitation. When doing a test ride, the bike struggles and pops around 3500 and 7k RPMs.

Not sure why the bike would struggle under load and what is causing this recent problem but it really seems like something just broke.

My guess is a diaphragm inside one of the carbs or maybe a vacuum line or a coil problem.

Anythoughts on where I should start looking? Thanks.

 

+1

The front seems to go first. Dunno' why.

 

bad gas iv had it a few times this year i think bouth were from a dimond shamrock i know one was put in fresh gas and ran like a raped chicken

 

I doubt it's in the ignition. Cheap test is to run premium gas from a known quality source. Down here Conoco is the absolute pits. What did the plug reading show?

 

Thanks, I will change the gas tonight but I have been filling it from the same Diamond Shamrock and only using premium, however this would be a faster fix than pulling the carbs.

 

The plug showed a normal gray color and the gap was right on, I am running a regular NGK-R.

 

go to a differnt station and if a tankers there go some where elce

 

you can pull the top of the airbox off and run the bike - watch the needles as you rev it up.

vacuum leak. spray wd-40 around the carb boots ( from airbox & to engine ). listen for an increase in rpms.

also check that you have good fuel flow into carbs.


tim

 

Made it home a bit late for enough research but I was able to confirm fuel is flowing from both sides of the petcock and did confirm some good sputters/backfires under mild throttle.
What surprised me was the bit of oil inside my airbox and on the bottom of the filter, looks like oil is coming out of the right side plastic breather. I have a stock filter and when I had the airbox off 1500 miles ago it was dry under there. Is this something that I should be concerned about in my search?

 

Pictures from the airbox incase this gives some more insight.

 

So I sprayed carb cleaner around the front carb and it caused it to bog down, I also found spraying directly in either carb that the engine speed would slow down as well. I removed the carbs and found on the back carb that the gasket between the base and engine was worn and slighty damaged at the top. I am going to go to the dealer and pick up new top and bottom gaskets for both carbs hoping this will do the trick and since it looks like a common issue I figure I should do them now.
Will post results once completed. Thanks.

 

Well after waiting three weeks on back ordered carb boots they finally came in. I replaced both carb boots and both gaskets on the top under the airbox. The bike appears to run better from idle to about 3k RPMs but then sputters hard (again) and is sluggish above 3k RPMs.

I have looked through the service manual again and the only thing I have not replaced or tried is pointing to a possible weak valve spring.

I went ahead and dropped it off at the dealership two weeks ago since I am at the end of my rope. Will provide updates once the dealer does the diagnosis.

Hopefully I can ride before the snow comes...

 

The dealer just called and they are saying the jet needles are worn out which is causing the issue. Should be fixed in about a week. I will post back once I see what the actual fix is.

 

Will,
Please keep me posted. I seem to be having the same problems. Under hard acceleration I hit what almost feels like a rev limiter at 7k rpm. Without a load on the bike I can run it to the real red line np. I have Yosh slip ons and OEM filter. It's a 2000 SH. Resently started Stalling during stops. Idel is 1-1.5k
Any help would be welcome.

 

Finally received the bike back today and it runs like a top. The dealer replaced both jet needles and then rebalanced the carbs. They also put in new plugs even though I did not think it needed it but they do it as part of the carb service...Anyway, not only is my problem fixed but the bike runs much smoother across the the entire rpm range.

It was in the upper 30's out and I did not have my cold gear when I picked it up this morning so I only did a short ride, but I will take it out for a long ride hopefully later this week.