89 K75C Idle and Popping

[DEANCOX]

I did something I have never done before. Circumstances were such that I had had to let my bike sit for about a year (horrible, I know!) I had prepped it for the layoff so there were no typical storage issues when I was ready to go again. I was in a bit of a time crunch, so I opted to take it to a local independant bike garage to have it looked over. The owner has the appropriate experience to handle my bike.

When I picked it up he said it was in good shape and he had "balanced the injectors (?)" a process he described akin to balancing a bike with multiple carbs.

It gets slightly better mileage, but now the ilde is around 11-1200 and even when I consciously return the throttle position to a completely closed position when rolling up to a stop and downshifting, I get alot of popping-style backfires....not explosive, but a afair amount of it. This bike has always done this a little bit, but I found that I when made sure to crank the throttle closed, it virtually eliminated it.

I am a DIY guy with all the tools, patience, and willingness to learn and follow instructions, I know that I hadn't really should not have taken it to the shop. The owner's attitude towards questions is to blow off the customer with sarcasm (but that's another story.) I consider it a lesson learned. I have the Clymer Manual for the bike.

I found that a good way to short circuit the research period is to "crowd source" right here. I would be grateful for direction and instruction. Thank you in advance

[deilenberger]

The comment on "balance the injectors" has me a bit scared. Hopefully he did not adjust the inter-throttle-body linkages.. those are factory set and should never be touched. Take a look at them and see if the green/blue paint is still intact (some had plastic caps over them also..)

It's possible he has simply unscrewed the big brass screws too far. This seems to be something inept mechanics like to do.

First step - look at the idle adjustment screw (refer to your manuals, Haynes or Clymer) - make certain the linkage is hitting the stop screw. If it isn't - we'll have to figure out why. If it is - continue..

Remove all the big brass screws at the base of the TB's. Make sure the O rings on them are intact and in place. It's not a bad idea to replace these. Make sure the screw points are clean and not deformed (no ridge in them.) Replace each screw and turn them about 1 turn out. Try starting the bike and observe the idle.

If the idle is still too high (correct idle on a K75 is around 900-1,000 RPM) - then adjust the idle adjusting screw to see if that lowers it. If it does, you're about done. If not - then something else is wrong (like a too tight throttle cable.. or fast idle cable..)

You do want to balance the TBs at idle. To do this you'll need some sort of vacuum gauge setup.. either a TwinMax (which you must move around since it only does two cylinders at a time) or CarbStix (commonly used on 4 banger Japanese bikes - not very available now since they used mercury for the liquid) or one of the solid-sliding-slug sort of devices that are now offered as an alternative to CarbStix.

Your goal is to get all the TB's "pulling" the same vacuum at idle. The brass screws are where you adjust it. Do not turn any one of the brass screws more then 2 turns out from gently seated. The correct position of them is 1-1.5 turns out.

One important thing - to get the TB balance correct - your valves MUST be correctly adjusted. I'd check them before even starting.

That's the quick version. DO NOT touch the screws that set the linkages between the throttle-bodies - these are never to be adjusted.

I've written only basic instructions/hints since you have a service manual. Read though the section on the throttle-bodies and if there are some specific questions, get back to us.

[35634]

The backfiring sounds like a throttle position switch issue. If they raised the idle up it may have messed the adjustment. You should hear it click when just starting to open the throttle (listen with the engine off)

[beemerguru]

Another common cause for backfire is a cracked crankcase vent hose...that "S" shaped hose behind the throttle switch..they get hard with the heat and split. Or if the bike has been sitting for a long time.

[beemerguru]

Hi Don,

I tried this technique over the weekend..replaced.. the O rings and oen turn out. I'm trying to solve a vibration and high heat problem..

Well vibration was gone but the heat got worse.The bike went into the red zone after about 5 miles in 60 degree weather. Red light came on and fan kept cycling on and off for the 15 mile ride.

One thing I did notice was the O rings seemed a tad bit thinner than what came off..ordered the right part number.

I'm open for ideas...yes, a good flush on on the list of to dos.

Greg

"88 K100RSSE

The comment on "balance the injectors" has me a bit scared. Hopefully he did not adjust the inter-throttle-body linkages.. those are factory set and should never be touched. Take a look at them and see if the green/blue paint is still intact (some had plastic caps over them also..)

It's possible he has simply unscrewed the big brass screws too far. This seems to be something inept mechanics like to do.

First step - look at the idle adjustment screw (refer to your manuals, Haynes or Clymer) - make certain the linkage is hitting the stop screw. If it isn't - we'll have to figure out why. If it is - continue..

Remove all the big brass screws at the base of the TB's. Make sure the O rings on them are intact and in place. It's not a bad idea to replace these. Make sure the screw points are clean and not deformed (no ridge in them.) Replace each screw and turn them about 1 turn out. Try starting the bike and observe the idle.

If the idle is still too high (correct idle on a K75 is around 900-1,000 RPM) - then adjust the idle adjusting screw to see if that lowers it. If it does, you're about done. If not - then something else is wrong (like a too tight throttle cable.. or fast idle cable..)

You do want to balance the TBs at idle. To do this you'll need some sort of vacuum gauge setup.. either a TwinMax (which you must move around since it only does two cylinders at a time) or CarbStix (commonly used on 4 banger Japanese bikes - not very available now since they used mercury for the liquid) or one of the solid-sliding-slug sort of devices that are now offered as an alternative to CarbStix.

Your goal is to get all the TB's "pulling" the same vacuum at idle. The brass screws are where you adjust it. Do not turn any one of the brass screws more then 2 turns out from gently seated. The correct position of them is 1-1.5 turns out.

One important thing - to get the TB balance correct - your valves MUST be correctly adjusted. I'd check them before even starting.

That's the quick version. DO NOT touch the screws that set the linkages between the throttle-bodies - these are never to be adjusted.

I've written only basic instructions/hints since you have a service manual. Read though the section on the throttle-bodies and if there are some specific questions, get back to us.