Running lean, getting awful mileage; not sure how to fix. 1994 K75

PGlaves said:

But three quarts will get hotter than 3.5 qt so it could contribute a little to thinning I suppose.

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If this were an airhead, I'd be with you, Paul, but this is a liquid-cooled motor. Because the majority of the head cooling is via the coolant/radiator (resulting in the head temps remaining in a much more narrow operating range than an airhead (or even an oilhead)), I would expect the oil temps to be quite a bit more stable, regardless of the oil quantity (within a reasonable range -- a 16%-17% lower quantity (3 vs. 3.5 qt) shouldn't be signficant here).

That would at least be consistent with my auto experience -- oil temps on my Corvettes rising/falling in pretty close sync with coolant temps, whether 6 or 7 qts. are in the crankcase.

So, I'm still mildly concerned that there is a real low oil pressure issue here (clogged filter? worn-out oil pump? leaking seal in the flow path? nothing at all?? -- can't tell without more data). Fingers crossed that the oil/filter change cures the issue.

Thanks for the feedback. The ride home from work was an easy one and the light did not come on at all. It was much much colder than earlier today but I have no proof that that had anything to do with it.

Yes, while low, the oil level is still visible through the sight window.

Bike was dropped back off at Bob's last night. I am waiting for a call from the service department with their conclusions.

The service manager was nice and more than willing to accommodate me and get it looked at. I say "looked at" because I understand that me telling him that my valves were adjusted incorrectly is, at the time, hearsay and that he needs to confirm it firsthand.

On the service slip I asked specifically that the intake valves be adjusted as close to .15mm as possible and the exhaust valves be adjusted as close to .30mm as possible.

Over all the bike seems to be running much much better as expected. I still need to check the compression once I get it back.

Question: What kind of issues with the bike would I expect to see with having the intake valve clearances set to loose?

Having the intakes set too loose does two things: First it alters the valve timing, delaying by a few degrees the initiation of opening and advancing by a few degrees the reclosing of the valve. This shortens the open time on both ends.

It also decreases total lift by the amount of increased gap, and decreases the total open area by some factor, with a geometry that eludes me at the moment. I can't do a 3 dimensional model of that conical open area in my head.

The end result is a restriction in the air delivery to the cylinders. A modest restriction actually may improve bottom end and mid range slightly, at the expense of top end. A bit more restriction and it would suffer throughout the rpm range, top to bottom.

In doing some experimentation on Oilheads, Rob Lentini used narrower GS intake tubes on his RS, and increased intake valve gap to improve mid-range: at the expense of top end but he didn't care.

Like Paul said: Less power. It's like putting a smaller cam in the motor, one that is not optimum for the motor.

This is not AS critical on the exhaust from a power stand point as the exhaust is being "pushed out".

The reason the exhausts get set loose is because, once the exhausts get up to operating temperatures, the clearances greatly decrease.

By setting the exhausts on the loose side of spec., you allow them more time on the seat which allows them to cool better which slows valve recession, helps protect against valve burning, and extends time between adjustments. What little "theoretical power" you may loose, you MORE than make it up in reliability and longevity.

Since the intakes are cooled by the air/fuel mixture, recession is not a concern. You can "tune for power", making sure that it still has enough clearance that once it is all warmed up and spinning at 8 grand, that the valve still fully closes. So going for the tight side of spec. on the intakes works best.




The increase at low-mid range, that Paul was referring to on Rob's motor, has more to do with the increase in VELOCITY in the ENTIRE intake tract leading to better atomization and filling at low speed with the expected lose at top end. In the late 60's the thought was bigger is better when it came to ports and valves. Ford found the limits with the tunnel-port 289, Boss 302, and 351 Cleveland. All great top end motors (if the bottom end didn't blow up before you got into the real power zone). Now the NASCAR motors based on the 351 Cleveland run MUCH smaller (but better thought out) ports and make way more power throughout the RPM range.

As always thanks Paul and Lee!

I got a call from the service dept saying that my bike was done. They also told me that the oil change I requested will be on them so that was definitely a nice gesture.

They also told me that the #3 exhaust valve has only 1 or 2 more adjustments left before the valve will need to be replaced. I definitely did not like hearing that. Especially since the labor, if I give them the head already removed, was going to cost ~$700.

I questioned why the valve would need to be replaced so soon and was told that they usually don't need to be replaced til much higher in miles and that the odometers on these bikes were replaced very frequently

My assumption is that the valve receded so much because it was improperly adjusted during the valve adjustment I had done there 12,000 miles ago (as many of you have suggested) but I have no way to prove it

I would pay attention to that valve, but also cross that bridge when I came to it. I have seen a number of K75s go 100 or 200K miles between shims on any individual valve. If this holds true on yours you have a bit of a ways to go before you run out of shim sizes.

toddlamp said:

My assumption is that the valve receded so much because it was improperly adjusted during the valve adjustment I had done there 12,000 miles ago (as many of you have suggested) but I have no way to prove it

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Do you have the paper work from that first adjustment showing what shims were in it before the adjustment? If so, that could tell the whole story.

PGlaves said:

I would pay attention to that valve, but also cross that bridge when I came to it. I have seen a number of K75s go 100 or 200K miles between shims on any individual valve. If this holds true on yours you have a bit of a ways to go before you run out of shim sizes.

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There are 3 thinner shims available. If kept properly adjusted, it could be quite awhile before you run out of adjustment (unless it's burned).

Good to hear that there may still be lots of life left. I intend to keep a close eye on the valves from now on. They're so easy to check I am embarrassed I let it get to this point. I need to get back in there to double check what the dealer did today. Even though a couple of the valves were technically correct I wrote on the service sheet that I wanted the intake valves adjusted to .15mm and exhaust to .30mm. I am anxious to see if they re adjusted valves that were technically in spec to my specific specs.

toddlamp said:

Good to hear that there may still be lots of life left. I intend to keep a close eye on the valves from now on. They're so easy to check I am embarrassed I let it get to this point. I need to get back in there to double check what the dealer did today. Even though a couple of the valves were technically correct I wrote on the service sheet that I wanted the intake valves adjusted to .15mm and exhaust to .30mm. I am anxious to see if they re adjusted valves that were technically in spec to my specific specs.

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I forget exactly what the step size in shim thickness is (I know Suzuki makes some that fit in smaller steps) - but the reality is - it isn't always possible to get exact clearances on K bike valves (actually - it's more the rule that you don't rather then the exception), which is why BMW gives a range of acceptable clearances (unlike a hexhead or airhead where the screw adjuster gives a variable range..)

The best you can hope for is they try to get them as uniform as possible within the range allowed for by BMW. You don't want a case of intakes or exhausts going to both extremes of the allowable range.

Just a FWIW..

deilenberger said:

I forget exactly what the step size in shim thickness is

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.05mm

PGlaves said:

I would pay attention to that valve, but also cross that bridge when I came to it. I have seen a number of K75s go 100 or 200K miles between shims on any individual valve. If this holds true on yours you have a bit of a ways to go before you run out of shim sizes.

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I have seen two K75s suffer from rapidly decreasing exhaust valve clearance. Both of them had been over 100,000 miles, so no one was really upset by this. We were, however, quite surprised to see this when so many other K75s and K100s go so far without valve problems. As with most engines with receding valves, it wasn't long before these bikes ran out of adjustment before their normal service interval.

If I were Toddlamp, I'd be looking for a low-mileage used head or complete engine, from which I'd use the head, and scrap the rest of the motor.