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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: gard on April 16, 2025, 01:28:40 PM
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I recently changed the lower rubber manifolds on 85 K100RT with 142,000 miles, replaced radiator fan and verified temperature sensors, fuel pressure 36 psi. The idle still sucks. Measured vacuum at the throttle bodies of only 2" to 3" Hg, I assume that is low but what should it be? Plugs and Air filter changed recently, compression test OK.
The test with propane had little or no effect, hard to tell idle is so rough. Adjusting the idle speed up helps some but then I ran out of adjustment on the "choke" cable. I plan on changing the upper grommets, hose clamps, vacuum caps and TB air bypass screw O- rings. Is there any other parts I should put in the basket to save on future shipping? I guess I should check the valve clearance but I don't have the special tools if they are out. Plus its snowing out so will the cold temperature effect the gap? Open to any other suggestion of what to look at.
On a related note, anyone have a recommendation on a gauge set for synchronizing the TBs? Price ranges all over the place on the internet, reviews suggest some are pretty crappy.
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I checked the valves at 40F (5C) temperature, the exhaust valves are between 0.009" and 0.007" (spec is 0.010 to 0.012"). The Intake valves are between 0.004" and 0.005" (spec 0.006" to 0.008"). Any thought if this is enough out of spec to affect the manifold vacuum? I am guessing not. I think the clearance will measure slightly more at 70F, (20C). The Dealer adjusted the valves in 1993 about 100k miles ago, not sure if they were checked since then.
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There is a Z type hose between the crankcase and the plenum chamber, if it's leaking, that might be the problem.
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Thanks, I looked that hose over when I had the air chamber out and it looked OK, I might replace it anyway, I have a spare.
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Does the idle change if you remove the oil filler cap? It should. It should die.
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I checked the valves at 40F (5C) temperature, the exhaust valves are between 0.009" and 0.007" (spec is 0.010 to 0.012"). The Intake valves are between 0.004" and 0.005" (spec 0.006" to 0.008"). Any thought if this is enough out of spec to affect the manifold vacuum? I am guessing not.
If they are out of spec you put them into spec; it's that simple. Do what needs to be done. Don't be a guesser. Use a Carbtune to balance the throttle bodies or make an effective manometer out of juice bottles. Read this. (https://www.motobrick.com/index.php?topic=7262.msg50254#msg50254) Don't concern yourself with Hg; just balance what's in the columns, or bottles, to equal heights or as close as you can get them. Clean the balance screws and lube their o-rings before the procedure.
You don't need to pay BMW premium price for shims. Yamaha shims will fit. (https://newmotorcycleparts.net/motor_parts/valve_shims.html)
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Very interesting info on the juice bottle manometers, I had not seen that concept before. If it is using water or oil instead of mercury it should be something like a 13X more sensitive measurement, I may give it a try. The Carbtune literature suggests balancing within 2 cm Hg and has a measurement resolution of 0.5 cm Hg which would be around 6" of water difference.
Nice to know the yamaha shims will fit, looking around some more, KpartsHolland has used valve shims for $4, but shipping is up there.
I brought the bike to the local shop, he said adjusting the valves is a pretty quick and relatively inexpensive job with the right tools. He will do a leak down test of the valves and synchronize the throttle bodies. He said it is unlikely to ever run as nice as it did new. I have mixed feelings about spending too much on a bike this old but think it will be good to have a second set of very experienced eyes look at it. My wife thinks we should just buy a new bike.
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My wife thinks we should just buy a new bike.
Yep, just mention it'd be exhilarating to have a fresh, perky new bike 😉
Once back from the shop, and back in spec, bricks tend to be pretty carefree with light maintenance. The factory set throttle body screw can bugger stuff up too, the shop should be able to balance everything. Fingers crossed, Cheers
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I brought the bike to the local shop, . . .. He said it is unlikely to ever run as nice as it did new.
That sounds like a setup for a sales pitch. You cannot miss what you never had. Is this shop in Essex or down south?
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You don't need special tools to swap valve shims. You push down on the outer rim of the valve cap of with a screwdriver Easy peasy.
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I did not know you could just depress the outer rim with a screwdriver. Perhaps I am too cautious about causing more damage, it has happened to me more than I would like to admit. I like this bike, have put 142,000 miles on it over the last 40 years. Have been doing most of my own maintenance for the last 20 years or so. I bought it new at Just imports in Montpelier Vt. But Franks in Essex Jct did a lot of service on it in the 1990s, they are only about 5 miles away.
I am a little afraid to start test driving newer bikes, pretty sure they have improved some LOL. We do a fair bit of 2 up riding and the wife says she would like something easier to get on/off (old knee injury) and better wind protection. Some friends have suggested a Harley not sure I could go there.
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Some friends have suggested a Harley not sure I could go there.
I have had several Harley's, always carburetor Sportster's. My wife finds it easy to ride pillion on, especially after I cut out some foam and added silicone sheet. Of all the bike's that have come and gone, the Guzzi California is hands down the best distance cruiser/vibe/unique .. to the point of looking at the new models.
Cheers
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I did not know you could just depress the outer rim with a screwdriver. Perhaps I am too cautious about causing more damage, it has happened to me more than I would like to admit. I like this bike, have put 142,000 miles on it over the last 40 years.\
I think you're justified in being more cautious than a guy who has owned and worked on gawdnose how many Bricks relatively successfully. Using well-designed, relatively inexpensive tools to work on a machine is its own pleasure and also shows some respect for those who engineered the machine and brought it to market.
Once the principles of each component are clearly understood, you can start impressing onlookers with maintenance tricks to show the mighty depth and breadth of your knowledge; however most onlookers would likely be more impressed by your juggling Wedgwood pottery and freeweight plates while working on your Brick. :laughing4-giggles:
If you're with someone who loves you and wants to ride on your moto with you, you've got a lot going for you right there, unless, of course, that person is a drama queen hitchhiker with a death wish who falls in love easily.
Take a look at used Harleys and the Suzuki C50 and sit on them.
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I used to joke about the oil spots under the Harleys, now I have a bigger oil spots under the K100. Thanks for the suggestions on other bikes to look at. I have not ridden any other road bikes since trading the Honda CX-500 for the K100RT. I am 5-9, wife is 5-0 so next bike will be lower. As I get older I notice I am getting shorter and weaker. Back when I was working, I worked with several people who traded in their rice rockets for Harleys and were very happy. I remember one guy saying its a little different style/philosophy, a little slower, more relaxed more comfortable and less likely to get you killed, so when you are ready to grow up...
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As the topic of other rides has been discussed I'll add my experience.
I also recently added a Harley Ultra Limited to my garage. I like it from the fact that it is a lower profile bike and waaay easier for me to flat foot it at stops. With the lower center of gravity it is easier to keep upright. The Mrs likes it as it is easier for her to get on/off, and is more comfortable for her as well.
Drawback, yep it is bigger than my K or R, 200+ pounds heavier even though that extra weight is closer to the ground, still makes it harder to push around the garage. If you are looking to please the Mrs, don't be afraid of looking at the newer Goldwings. I've personally not ridden one, but I know of several couples that have gone from HD baggers to Wings. More comfortable, reverse gear, not quite as expensive, all play a part in the experience. Happy wife, happy life....
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Good news I think,
Got the bike back, all valves adjusted. Cylinder leakdown test results are good for this age. They found engine air leaks from valve cover bushings, the upper TB bushings and the large tube that goes between the air box and air cleaner, O-rings on air screws and Vacuum caps. He said he could tell from sound the alternator rubber bumpers are bad. All parts on order from kparts holland at about 1/2 of local prices pluss an additional 10% off easter sale. He said no point trying to synchronize the throttle bodies until the vac leaks are fixed (I will install all those parts).
The timing chain is worn to the limit of the adjuster so this throws off the valve timing he said no rush to install new chain and guides. Ignition timing is good. He said the Castrol oil I previously used is known to smoke more than some others, Valvoline is in it now. Shop total was $100 in parts and 3 hrs in labor, money well spent I think.
The bike seems to start and run way better now so I guess having the correct valve clearance is indeed important (who knew?). I was only charged for one valve shim so I guess he is just swapping ones in and out of his box.
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Thought I would provide an update. I have replaced all the rubber parts that were on order as well as the crankshaft cover gaskets and radiator overflow tube (see other post on collapsed radiator hose). The total cost for all the rubber bits as well as the dealer diagnostic work was $1100. I adjusted the idle, fast idle and balanced the throttle bodies. Also adjusted clutch cable.
I have had it out for a couple of rides and it is starting and running much better than it has for the last several years. I am happy I put the time and money into it. I appreciate all the help and encouragement from this forum. At the start I was wondering if it was time to put the old bike out to pasture (literally) but it will live on to ride another day.
I put together a gauge for the throttle body adjustment. There is a digital manometer that reads pressure differential up to 6 in Hg (about $40) There are 5 flow control valves ($18 for the package) some tubing and T connectors from the junk drawer. In use, each throttle body is adjusted compared to #4 by opening one valve and closing the other 2. It only took a couple of minutes. The actual vacuum level has increased from about 2 to 10 in Hg so I guess I did seal some leaks. I found the #4 valve needed to be closed all the way to get the other 3 to the same pressure. All are now within about 0.5 in Hg which I think is better resolution than some of the carburetor balancing gauge sets on the market.
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The digital manometer provides way more accuracy than needed for this job but I wanted one anyway for other projects.
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I put together a gauge for the throttle body adjustment. All are now within about 0.5 in Hg which I think is better resolution than some of the carburetor balancing gauge sets on the market.
Of course it is! Make up a few sets and sell them at the Montpelier Farmers Market. Unfortunately, the Brick is still going to be hell to lift upright from the garage floor. Turn your attention to creating a lightweight, compact gin pole assembly.