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R75/6 engine / Oil temp

Andey
Posts: 117
Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

Yup the pictures were taken are right after the riding you mentioned and the engine cooled back down.

Might richen up the fuel mix screw and see if it helps out…

I’ve done similar runs and checked the plugs before, similar looking. But that was before the rings were run in.

I haven’t had time to do the leak down test yet… need to set aside a few hours to do it.

Will be swapping out the oil filter around that time too.

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srankin
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Re: R75/6 engine / Oil temp

Post by srankin »

The only mixture screw on Bing carbs adjusts idle mixture only. To richen things up, you have to adjust the needles or change jets. Don't do that unless you have the factory settings handy, they will dial things in very well. Anything outside of factory recommendations is a gamble on what you get.

IF the bike is running well, has proper power, good gas mileage, and no pinging, ride it as is. Check the plugs again in a couple of thousand miles and see what you get, if you get the same thing, don't worry. St.
Owner of a 84, R80RT and 78, R100RS

Andey
Posts: 117
Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

An update on the bike.

I managed to do a leak down test about a week ago and both cylinders failed miserably.

80% leak, through the piston rings. I could hear it through the crankcase.

Probably explains the heat in the crankcase.

Surprisingly the side with the most pitting actually held out better…

With the cylinders apart and waiting for oversized pistons and rings, will be boring out the cylinders.

Anything to look out for to make sure it holds pressure?

I’m surprised ring gaps of 0.045mm can keep pressure

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srankin
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Re: R75/6 engine / Oil temp

Post by srankin »

Good job on pinning down things with the leak down test. A compression test would have worked but that is a moot point at this time. The leak down test is better in the long run because it also covers valves and such.

As for heat in the crankcase, to be honest I would not worry about it. The only real time to worry about heat is in extreme conditions, and more so as avoiding it rather than using it as a diagnostic tool.

LOL, I am guessing the bike was not quite right which led you to checking the temperatures. In this case you found a problem to fix, but you would have found the problem to fix even if you had not taken the temperatures.

As for higher temperature in the crankcase, I would surmise the. reason for it would be the jugs cool faster as they should as they are designed to give off heat during operation. Oil, which circulates picks up heat and most of the oil is in the crankcase. When the bike is shut down the jugs cool faster where as the oil in the crankcase cools slower, thus the difference in temperatures. Only a little thought on my part.

Forgive me, I don't recall all of the issues you have written about nor the mileage or condition of your bike. At least now you have a path forward for repairs. St.
Owner of a 84, R80RT and 78, R100RS

Andey
Posts: 117
Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

Yup, that sounds about right.

Was thinking along those lines too.

The biggest give away was the lack of power when pulling away. There was a noticeable difference after a few rides.

It’s a little strange, no smoke, but when I had the jugs off, the pistons were a little oily and there was build up of carbon just next to the exhaust valve.

I’m still waiting for the parts to arrive. Hopefully it’ll pass the leak test.

I was planning to do a bench test before installing them. Hopefully it will help diagnose any problems quicker.

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srankin
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Re: R75/6 engine / Oil temp

Post by srankin »

LOL, I find it hard to tell if any of my bikes are smoking as I can't see the tail pipes so unless I am mosquito fogging the village, I wouldn't have a clue if I am burning oil other than a drop in level and black plugs.

Loss of power or high oil consumption when it is known the valves are set properly, the ignition points if there are points are gapped properly, timing is proper and carbs are working together happily means time for a compression check or leak down test.

I am looking forward to reading your report after getting the bike back on the road. St.
Owner of a 84, R80RT and 78, R100RS

Andey
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Joined: Wed Sep 02, 2020 12:45 am

Re: R75/6 engine / Oil temp

Post by Andey »

So oversized rings arrived for my rebore to the cylinder-piston limit…

The rings were too big, so like brook reams’ /5 project, I had to grind them down.

Pressure test on the bench tomorrow to see if it works. Fingers crossed…

If it doesn’t work, plan B would be a long wait for oversized pistons.

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srankin
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Re: R75/6 engine / Oil temp

Post by srankin »

Thanks for the update, good luck. St.
Owner of a 84, R80RT and 78, R100RS

Andey
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Re: R75/6 engine / Oil temp

Post by Andey »

Ok bad news.

Ring gaps are perfect…

0.03mm
0.03mm
0.25mm

spent close to an hour filing 1 set by hand and measuring with a feeler gauge…

But still failed the leak down test. I’m thinking am I doing the test wrong… but referenced online guides and seems pretty straightforward.

The only variable is the piston to cylinder clearance… I have it at 0.1mm at the moment which is within spec of 0.12mm mentioned.

So not sure now… seems like an oversize is needed?

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cbclemmens
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Re: R75/6 engine / Oil temp

Post by cbclemmens »

Andey;

The compression test should tell you if your rings are seating or not. My Haynes manual says the compression ratio on an R 100/7 is 9.0:1. 9X14.7=132.3. You should get +/- 130 psi on your compression check. If you have less than 120 psi something is leaking, either rings or valves. But if your compression is good then you have to look somewhere else for your problem.

Craig

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