Throttle Body Tuning

I continue to work through jobs on my recently acquired K6 project. It is running very well, the next thing on my list is to check out the fuelling system.
So far I’ve run some Redex through the first couple if tanks of gas, fitted a new air filter and checked the TPS (which was spot on, somewhat surprising for a 13 year old, 134,000 mile bike).
I’ve read up on the fast idle check and balancing the throttle bodies in the manual, and neither seem too daunting. My question is: are there any pitfalls I should be aware of with those two tasks? I’ll probably do those jobs at the same time (as they require similar stripping for access) - in a couple of weeks (when I’ve borrowed a suitable manometer from a mate), so any advice would be appreciated in the meantime.


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[andy650]

If doing a valve clearance check is on your list, you should do that before you do the TB balance.

Originally Posted by andy650

If doing a valve clearance check is on your list, you should do that before you do the TB balance.

I’ve made a conscious decision not to check the valve clearances - because I think there is a good chance it will cause more damage than it might solve. This is a 13 year old, well used bike - it would only take one rounded off bolt head or broken stud to make it beyond economic repair, so it is best left alone. I’ve checked the compression (from memory: 150 PSI on both cylinders) which is fine, and the valve gear is not noisy, so I conclude I don’t have a valve seat problem.

Sometimes working on a project bike is a bit like a doctor fixing a person with an ailment, the first principle is ‘do no harm’. I had a bit of a mission extracting the bolts from the front cylinder exhaust header - just because they had not moved for 13 years (of being covered in road crud and having gone through about 10,000 hot and cold cycles), and so have decided to keep internal mechanical risks to a minimum.

That was a long winded way of saying I’ll just get on with making sure the fuelling is right

Many thanks.




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[Craig380]

I think that's a wise approach. On SVs, the clearances will close up with age & miles, rather than get wider, so they are unlikely to become noisy. But I agree that given the age & mileage, looking too deeply isn't really worth it. 150psi compression is 10.3:1, which is good (especially if the engine was cold at the time of the reading, 'cos the reading will be even better when at working temperature).

On the fast idle check, if it already starts from stone cold on the first press of the button, and immediately idles at around 2,000 to 2,200rpm (depending on the ambient temp - higher ambient temp = lower fast idle), then settles down to about 1750rpm after a few seconds, the fast idle is absolutely fine. Don't waste your time following the manual and reading voltages etc, just go by the rev-counter and how the engine behaves (on the K3 and K6 anyway, with the K7-onward twin-spark bikes it's a different procedure).

The TB sync IS worth doing. I used to do mine visually - lift tank, remove airbox lid & filter, and manually turn the cog on the STV motor to open the STVs. Then look closely at the main butterflies and make sure they both start moving at the same time as you gently open the throttle. If you have vac gauges or use the 'long U-tube' method, of course that's more accurate.

[Craig380]

Oh, and while you are under the tank, follow the instructions in the pic in this thread to lube all the throttle & STV linkages with GT85 or similar to keep them moving freely:

https://forums.sv650.org/showthread.php?t=220760

Originally Posted by Craig380

I think that's a wise approach. On SVs, the clearances will close up with age & miles, rather than get wider, so they are unlikely to become noisy. But I agree that given the age & mileage, looking too deeply isn't really worth it. 150psi compression is 10.3:1, which is good (especially if the engine was cold at the time of the reading, 'cos the reading will be even better when at working temperature).

On the fast idle check, if it already starts from stone cold on the first press of the button, and immediately idles at around 2,000 to 2,200rpm (depending on the ambient temp - higher ambient temp = lower fast idle), then settles down to about 1750rpm after a few seconds, the fast idle is absolutely fine. Don't waste your time following the manual and reading voltages etc, just go by the rev-counter and how the engine behaves.

The TB sync IS worth doing. I used to do mine visually - lift tank, remove airbox lid & filter, and manually turn the cog on the STV motor to open the STVs. Then look closely at the main butterflies and make sure they both start moving at the same time as you gently open the throttle. If you have vac gauges or use the 'long U-tube' method, of course that's more accurate.

That is really helpful.

I’m pretty confident the valve gear is okay, and given the good (and equal) compression on both cylinders I don’t think it is worth risking a mechanical calamity with such a high mileage engine. I checked the compression with the engine stone cold (whilst changing the radiator), so I’m quite pleased with the CR.

The cold tick over is not quite as you describe it on my K6 - it starts straight away from cold, then ticks over at the lower part of the range (1200 RPM), as the engine warms up it gradually goes up to 1400 RPM. I didn’t even realise the engine had a fast idle mechanism until I found it in the workshop manual (whilst looking for the TB balance procedure) last week, so it’s behaviour on start up didn’t cause me any concern. It looks like the fast idle is not doing quite as it should though, and it appears to be a simple voltage check and adjustment to sort it, so I think that is something worth doing.

I see your point about checking that the throttles are coordinated visually, I think that would work pretty well. As I’m going to have to strip the bike a bit anyway to have a look at the fast idle setup I might as well use the vacuum gauge I can borrow from a mate and balance the throttles at the same time. It looks like a pretty straightforward procedure.

Many thanks for the advice - you have understood exactly what I’m trying to achieve with this project.


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Originally Posted by Craig380

Oh, and while you are under the tank, follow the instructions in the pic in this thread to lube all the throttle & STV linkages with GT85 or similar to keep them moving freely:

https://forums.sv650.org/showthread.php?t=220760

... now that was a really useful article - and absolutely in keeping with my philosophy on maintaining this K6 (optimise what is there and enjoy the rest of its natural life without spending too much money).


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[Craig380]

I needed to adjust the fast idle on my K6 as it would fire easily from stone cold but the actual cold idle speed was low (around 1400rpm). All you need to do is turn the fast idle screw clockwise a TINY increment - by which I mean NO MORE than 45 degrees / one-eighth of a turn. Seriously, don't waste time with measuring voltages.

Just turn the screw a little, and it WILL raise the fast idle. If you measure the voltage, all you're doing is measuring, then turning the screw, then checking what voltage that gives you, and so on. Instead, you can just tweak the screw and see what actual fast idle speed on the revcounter that gives you.

Originally Posted by Craig380

I needed to adjust the fast idle on my K6 as it would fire easily from stone cold but the actual cold idle speed was low (around 1400rpm). All you need to do is turn the fast idle screw clockwise a TINY increment - by which I mean NO MORE than 45 degrees / one-eighth of a turn. Seriously, don't waste time with measuring voltages.

Just turn the screw a little, and it WILL raise the fast idle. If you measure the voltage, all you're doing is measuring, then turning the screw, then checking what voltage that gives you, and so on. Instead, you can just tweak the screw and see what actual fast idle speed on the revcounter that gives you.

Excellent. I follow your line of reasoning exactly and that will save me having to take so much apart. It sounds like my K6 has exactly the same issue as yours had - and I hope it will be the same simple remedy.

This forum is so useful - and the members are so helpful, that is just the sort of advice I was looking for.

Addendum: I have just tried tinkering with the fast idle screw to see if I could adjust it from the outside without taking off the air box - it was irritatingly close, but I just could not get a small right angled ratchet with a cross head bit to get enough purchase. Grrrr, I’ll take the air box off one day next week and try again.

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[Craig380]

It's a RIGHT pain in the backside to get to. I managed to avoid dismantling by turning mine using a pair of LONG tweezers, I was just able to get enough grip to turn the screw a couple of degrees at a time.

As I mentioned above, the adjustment is very sensitive, you won't need to turn the screw more than 45 degrees. Even a 30-degree turn might do it. The fast idle cam only needs to move the main throttle plates a tiny amount to give the correct idle speed.