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To "Leak Test" or not?

01 Dec 2020 19:31 01 Dec 2020 19:37 by LuckyLuke.
LuckyLuke
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To "Leak Test" or not? #1
My question(s) to the experts relate to whether or not there is good value in doing a leak down test (LDT) before I do some maintenance on my DT1 engine. My research tells me that an engine with air leaks will be difficult to tune and will not run optimally. I know my engine has a oil leak on the right cover side because when I filled the crank oil, about 50% of it leaked back out. Let me first qualify I am a serious novice at this stuff and have a lot to learn. Here are my questions.

I did a leak down test and the engine did not hold any air. I checked for leaks(soap and water) and believe the mag crank seal at a minimum has a significant leak. The engine at LDT time does not have any oil as I drained it first.

Questions.
1- should there be oil in the engine to do a proper leak test?
2- should I repair the clutch side gasket where the oil leak is b4 doing the leak test?
3- I am I placing too much value in a LDT?
4 - do Dt1 engine benefit from a LDT or do they run fine with some air leaks?
5 - can I replace engine seals without spitting the crankcase if repairing leaks is essential? Particularly the mag seal which is leaking noticeably?
6 - how relevant is it that I was able to run the engine, albeit very rough, before I started looking at replacing the crank side gaskets and other seals that might be replaced without splitting the crankcase?
7 - Am I just over thinking this and should I just replace the crankside gaskets (ie; right side) where the oil leak is and move on?

Sorry for all the questions, but I'd sure like to learn more about this before I journey on to next steps. I don't think I am at an experience level to split the crank as a best case scenario. I would appreciate any help from anyone with more experience and help make sure I don't go down a rabbit hole.

Thanks a bunch. Luckyluke

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02 Dec 2020 15:54
Pillsbury
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To "Leak Test" or not? #2
you can replace the seals without splitting the crankcase. See the on line manuals on this site. You will have to remove the mag (special low cost puller) for the left seal & the clutch (need to make or borrow a clutch housing tool) to replace the right. Only the crank seals, Head & base gaskets can leak during a leak down test. Doesn't matter if the gearbox has oil for the test. There are posts written about how to do both if the search feature is nice to you. Good luck, have fun!

1970 DT1-C Grand Prix race bike
1970 DT1-C Street tracker
1968 Honda CL90
1973 Honda CR250
1974 Honda MR-50
1966 Triumph Tiger Cub
1947 Mustang
1974 Suzuki TC185
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02 Dec 2020 18:03
turbodan
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To "Leak Test" or not? #3
Sounds like you may have other issues that need to be addressed before testing the crankcase for leaks. For example, if your main bearings are toasted there would be no point in replacing the crank seals and leak testing. That's why some manufacturers require case splitting to replace crank seals. More often than not they should be replaced together.

Once the engine is in generally good condition you can start working on chasing down small air leaks. Get there first.
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03 Dec 2020 08:46
nhsteve
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To "Leak Test" or not? #4
I'm in general agreement with pills and turbo here. Good description of what basically needs to be done in the second post.

So, do you have a leak down tester, or would you be borrowing one? Also, have you done a compression test, as an indicator ( I know, I know...)? Might give you better idea of overall condition at least.

Consider getting a mag rotor puller (good to have anyway), remove the flywheel and stator plate (you were going to reset the timing, riiight???), and do your leak down test. Squirt some soapy water around the seal during the test to confirm/refute your suspicions.

Does the bike oil injection line feed into the base of the cylinder, or into a spigot on the side of the carburetor? That could also be a source of an air leak. It doesn't take much to create a problem.

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03 Dec 2020 12:43
MarkT
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To "Leak Test" or not? #5
You can check for bearing play... not common at all on one of these for the bearings to "wear out"... 50 year old seals will go bad sooner...

Most "bearing" issues are either a worn crank on a few models (usually 12v starter/generator model) or bad case inserts on later models. Both are easily detected.

I've already written volumes on here about leak down testing. I do it religiously. Some never do.

I can't say one approach is better than the other. What I don't like is "guessing" and replacing parts that don't need replacing. I have a 1964 Yamaha with the original seals. I've had much newer models with less miles that have had seals that leak like a sieve. I've had new seals that leak bad. You just never know. Checking for leaks allows you to know.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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03 Dec 2020 14:49
LuckyLuke
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To "Leak Test" or not? #6
Wow!! This is amazing feedback and advice.... much appreciated as it gives me lots to think about. I wish I had a mentor that could walk me through this step by step because I'd really like to get this right, especially on this one. Some of my thoughts and I welcome any additional thoughts for sure;

- Good to know I can replace the seals without splitting the crank as I was hoping as MarkT indicated it is not common for these motors to need bearings.
- I think splitting the crank for me would be next level and hope I don't need to go there. I will try to get comfortable with knowing what to look for regarding bearings before replacing the seals as I agree better to have a purpose to what I am doing, rather than it being random. I may need to call a friend on that when I get there.
- I do have a special mag puller on its way and just need to get a clutch grabber. Will a universal vise grip type work for this?
- I have ordered a complete gasket kit as well as Yamabond. Fingers crossed I don't need to split the case.
- Once I have the mag and the clutch off is that the time to another leak down test? This time I will make sure all holes are plug such as the oil tube that feeds into the cylinder head just after the carburetor as well as the crankcase breather. (... and yes I was planning on redoing the timing even though I am not sure how to at this point. I was thinking that maybe I can I just mark the mag with the shaft before pulling and just line up on reassembly? otherwise, I will have to learn how to do this when I get there.)
- given what happens in the previous step I will at least be able to know if the seals need to be replaced. I have a complete seal kit on it way as well. I was thinking of doing these regardless and may need to think about maybe only doing the ones that need it.
- I don't think I will have enough know how to evaluate whether the bearings are good or bad. I will do more research on how to do this. Is there some room for error here in that if they are not too bad then not replacing them is not lethal? My use of the bike is going to be very gentle, regardless.
- Also, if I do a compression test will that be enough to evaluate the piston and cylinder's condition? I am also hoping to avoid replacing the rings etc. I ran the bike already after cleaning the fuel system out and it seemed to work fine other than the rough idle and muffler missing a baffle. But then what do I know...
- Lots of things to learn and do. I am a bit concerned I might be getting over head. I did get an estimate from a shop to do all this and worst case I was quoted 12hrs, plus parts, if available. Problem with that is I wouldn't learn anything and not as much fun.

I just want to be able to ride this thing around a bit and dream about being young again. I might be too old before I get there :)...

Thanks guys as you gave me lots to think about and more homework. I am working on maybe getting someone who understand this better than me although he has mainly worked on newer bikes..... Makes me think of the analogy of having to fly a plane for the first time with few lessons and having this forum coach me to land it. Hope I don't crash...:(

again....Much appreciated....Luke

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04 Dec 2020 08:54
nhsteve
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To "Leak Test" or not? #7
MarkT is definitely a 'go-to' guy for a lot of things here ! :Buds
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04 Dec 2020 17:25
turbodan
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To "Leak Test" or not? #8

MarkT wrote: You can check for bearing play... not common at all on one of these for the bearings to "wear out"... 50 year old seals will go bad sooner...

Most "bearing" issues are either a worn crank on a few models (usually 12v starter/generator model) or bad case inserts on later models. Both are easily detected.


Yeah I'm sure it's super uncommon for bearings on a 50 year old bike to have eaten dirt or been damaged by rust at any point over the past five decades. Unfortunately, the older the machine the more likely that is has been neglected or abused at some point.

Not much point in leak testing until you have verified the engine is mechanically sound. That's all I'm saying.

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04 Dec 2020 22:37
LuckyLuke
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To "Leak Test" or not? #9
Got it and good point. Thank for taking the time add you thoughts.

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