ST1300 – RPM pulsing, fuel smell, and exhaust rumble after spring service

[newowner]

HI All,

Pulled my ST1300 out of winter storage and did some basic maintenance before riding:

• Changed engine oil
• Changed transmission/final drive oil
• Replaced spark plugs (old plugs looked normal)

After the spark plug change, I started noticing a few issues:
1. RPM pulsing / blip

• On cold start, idle sits around 2000 RPM, but every 3–4 seconds there’s a noticeable blip (about +200 RPM).
• Once the bike is fully warm (3 bars on the temp gauge), base idle drops to about 1000 RPM, but the same RPM blip every 3–4 seconds is still there.
• Took the bike for a short ride thinking it might clear up—no change. (but unfortunately i filled the fuel tank)

2. Strong fuel smell when cold

• There’s a strong unburnt fuel smell during cold running.
• This goes away once the bike is fully warm.

3. Exhaust noise when cold

• When cold, I hear a rumble / abnormal exhaust sound, almost like excess fuel is burning in the exhaust.
• This also disappears once the bike warms up.

Relevant history: Last season, I removed the airbox and pulled the 5‑way tee (vacuum/air hose junction) and cleaned it. One of the branches was blocked, which I cleared at the time. The bike ran fine afterward and through the rest of last season. And I did not have that rpm blip when i put the bike in storage last season..

Where to start ? and Is it safe/ok to use the bike for an upcoming 400km trip ? thanks

 

[jfheath]

The only thing that I can think of is related to the fact that you say that you have replaced the spark plugs. It is possible to get the washer cross threaded / at and angle so that when you think it is seated before performing your half turn tighten up (or whatever it is ) , the plug is actually not properly seated. So when you perform your tightening turn, it is just trying to straigten out the washer, adn the plug remains unsealed. Depending which one - or which ones - this has happened to, the behaviour of the engine will be different.

Handbook says: New plug : tighten the plug after it seats : NGk half turn; Denso 1 turn. Then loosen the plug and tighten it as if it is a re-used plug:
Reused Plug: One eighth of a turn after it has been seated.

The first turn from new squashes the washer.

Whether or not that is the issue, you can expect all of those syptoms from a plug that isn't sealing properly. Remove the plug, check the washer is properly in place, insert the plug and this time you will have to torque it to the specifed torque for a re-used plug, No more. No less.

 

[aniwack]

Also gapping would make it misfire. I've had two this month with that problem where they bought plugs at the auto parts warehouse and just stuck them in.

 

[Larry Fine]

Air filter condition? Mouse nesting?

 

[newowner]

When i installed the new NGK plugs (CR8EH-9) - i installed them by hand first and them torqued them to 18 N.m with a torque wrench. I removed air filter (it was clean) and the air box was good (no sign of mouse) except for a minimal amount of engine oil..
I don't think i have a misfire. After the spark plug change, at idle and when i took the bike for a spin, there was no vibration and the bike performance is still the same (maybe slightly better)

the fact that the idle blip (every 3–4 seconds) is rhythmic - i think the ECU is trying to do something and it corrects itself so maybe one the vacuum lines is blocked or leaking - thoughts ?

Since i have a full tank, i cant reach underneath the air box easily - so I will use the bike this week till i get to flashing one fuel bar and will dive into these vacuum lines - since the forum database is gone, do you guys remember the specs of these lines ?

 

[newowner]

i will share the pics of the pulled spark plugs if it helps with the diagnostics - I know they are in bad shape. I drove around 30k KM with these plugs

 

[jfheath]

This doesn't cure your problem - but originally, the torque spec for plugs was 16Nm, not 18. The USA owners manual had this information:



The later service manual (not sure of the year - but later than my 2004 version) says to re-use, seat it and then torque to 16Nm.

Also the spec for NGK is the CR7EH-9 - The CR8EH-9 is for extended high speed riding.

(someone else will chip in - but 18Nm is too tight).

 

[Nightmare]

The fuel smell is very alarming, I would double check the hose that goes from the upper to lower tanks and make sure there isn't any kind of crack/leak as others have run into that problem on this site. My gut feeling is this sounds like an air leak, I would suspect between the airbox and the intakes. Make sure you reconnected the breather hoses and reinstalled the mass airflow sensor (if you removed it from the airbox). I'm assuming the F1 light goes off when the engine is running.

Alternatively, I would absolutely double check the vacuum lines to the 5-way T, or even just replace them. That 5-way connects to the manifold air pressure sensor and could be getting intermittent readings (eg, going from low pressure to atmospheric pressure and back again as air is sucked in through the leak in those little lines). Might be worth just replacing the lines if you get back in there, its just common automotive rubber lines and you could make them equally slightly longer to ease your next starter valve sync.

 

[Andrew Shadow]

since the forum database is gone, do you guys remember the specs of these lines ?

If you are referring to the five-way Tee vacuum hoses they are 4mm inside diameter.

 

[diferg]

Have you tried simply reinstalling the old plugs and go for a ride? Sometimes we get aftermarket plugs that are VERY hard to distinguish from the real deal. These aftermarket plugs are often purchased at reputable sales outlets for full market price.

 

[newowner]

Have you tried simply reinstalling the old plugs and go for a ride? Sometimes we get aftermarket plugs that are VERY hard to distinguish from the real deal. These aftermarket plugs are often purchased at reputable sales outlets for full market price.

Actually this is a good idea and good timing as the garbage will be collected tomorrow - i will extract the old plugs and keep them for future testing in case the vacuum lines turns out to be good. i just hate to remove the fairing again to install to old plugs.
I'm expecting good riding weather next week - so i will be commuting with bike which should consume the fuel that i filled in the tank (plus the old fuel from last season that was still in there).. when the tank is empty i will remove it along with the air box to get to the vacuum lines and will post some photos.

 

[jfheath]

It should be possible to remove plugs without removing the fairing. The rear one is a bit tight and it will be impossible if the HT lead has been routed behind any other cables. But I always preferred to take the fairing off rather than risk stripping the copper core out of the lead at the plug end.

I never throw my old plugs away. I keep them and throw my older old plugs away. Reason - even with genuine plugs it is possible to get a rogue one - or it was in the late 1970s when it happened with my BSA Bantam. Lesson learned.

Recheck the gap if fitting your discarded ones. They might have landed electrode down.

 

[Nightmare]

I'm curious if you were able to check those lines and if it helped with the issue.

 

[newowner]

Turns out to be a gapping issue as @aniwack mentioned - so after i emptied the tank and checked the 5 way tee and the hoses, all was good - i decided to recheck to two easily accessible spark plugs (cylinder 1 and 2) and to my surprise cylinder 2 spark plug gap was much smaller than the one in cylinder 1 .. so a new plug installed in cylinder 2 and all issues disappeared.

 

[aniwack]