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DT1B piston blow by.

  • pmb
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DT1B piston blow by. was created by pmb

Hi All. I have been trying to get my dads old 69 DT1B back on the road. I have been reading through other posts here and made a couple posts about my struggles. The bike never would run crisp. It always bogs and seems fouled.  I have recently decided that I needed to replace the main crank seals. I first replaced the seal behind the magneto. It helped for a short time but then the bike went from better to way worse. I figured with only one new seal the crank case pressure was even higher on the other side. So I pulled the clutch case cover and replaced the other side. There was some smell of gas in the old gear oil and it did appear that there had been fluid exchanging through that seal. Here is where my stupid could show up...I thought I was chasing a lean condition due to the bad crank seals. As I said, it did run a bit better after replacement of the seal behind the mag. I didn't process that the seal on the other was not sealing to the air, but sealing between the gear oil and the crank. Now I believe I went from a way too lean condition to a way too rich condition due to getting gear oil in the combustion chamber. Then I found this (see pictures) when I pulled the barrel off and looked at the piston! It appears as though I need a top end rebuild. I would love any input that you have. Thanks. 
28 Jul 2021 18:52 #1

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Replied by pmb on topic DT1B piston blow by.

Here what a new plug looks like after a few funs up and down my street. Seriously rich looking to me...
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28 Jul 2021 18:59 #2

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Replied by RT325 on topic DT1B piston blow by.

I'm permanently stuck in exhaust mode. If in doubt, burn it out. Works for me every time. Its the lifeline of twostroke horsepower. Also, if it's been building up & restricting plus getting worse it'll cause everything to carbon up over time & plug to go black & all that if its been like it for years of running. Just my ideas.
29 Jul 2021 01:22 #3

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Replied by MarkT on topic DT1B piston blow by.

As long as the clearance is good, that piston looks fine to me.  Might want to clean it up a little but looks like rings are sealing decent.
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
29 Jul 2021 04:58 #4

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Replied by RT325 on topic DT1B piston blow by.

If ya getting rings for it i remember the wrong rings arriving by mistake of the seller who sent the proper one straight away after that.. Ones needed were keystone tapered style but parallel arrived. If keystone--apart from eyeballing the rings before ordering--the piston will have a K stamped on top for keystone. This applies to very early pistons as far as i remember, maybe "68.
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29 Jul 2021 16:17 #5

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Replied by pmb on topic DT1B piston blow by.

Thanks for the input. As far as the exhaust goes, I have done a serious camp fire burn. It smoked like crazy. I have a giant wok style fire pit so I could not fit the entire pipe in the fire all at the same time. I spent about 4 hours moving it through the fire n stages. I have a laser thermometer device and at about 500 degrees the pipe would start smoking. I waited until about 20 minutes after the smoking stopped and then moved the pipe down to burn a new section. At certain times during this process the smoke exiting the pipe would turn to flame. It was a pretty cool process. After the pipe cooled down, I banged it with a dead blow hammer. That knocked a bunch of crusty burnt surf loose and I was able to shake it out. 

As for what is happening now. I replaced the crank seal on the clutch side. I cleaned up the piston, put everything back together. It was when I was re-filling the gear oil that I noticed that the new gear oil (yamalube wet-clutch 80wt) was the color of honey. The gear oil I drained out was blue/green like the color of the 2-stoke oil. So clearly the seal I replaced was toast. I felt pretty good about starting it back up with a new plug, new coil, new points, new seals, new carburetor, a burnt out pipe, fresh gas and fuel filter...  It started up and immediately went back to the same most recent issue. After it starts and warms up, it will idle but as soon as I open the throttle it falls on it's face. It used to only do that when I tried to take off in first gear from a dead stop. Now it does it while in neutral just sitting there. This thing has been kicking my ass for a long time. I'm going to try and keep running it and messing with basic card settings at long enough so I can drain the gear oil and see if it is turning blue again. If so I figure I will have to split the case and replace the main case gasket. 

I really want to give up and trade it for a newer dual sport bike, but I am not going to let it beat me. Maybe someday I will be able to post about success and share what the problem was and tell the entire tale of this old bike. 

Cheers! And thanks to all of you guys for this wonderful resource. 
30 Jul 2021 19:42 #6

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Replied by asco on topic DT1B piston blow by.

Talk about jetting changes to lean. From what to what?

asco
YAMA-LAND RESTORATIONS, 818-521-2109 This email address is being protected from spambots. You need JavaScript enabled to view it. 1971 CT1-C (BRANDY) 1970 DT1-C (MONICA) 1973 100cc Hodaka Combat Rat (ABBY NORMAL) 1972 AT2M (ARNOLD ZIFFLE)
30 Jul 2021 21:05 #7

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Replied by Mothersbaugh on topic DT1B piston blow by.

Cleaned your carb about six times yet? I had an XT-350 which did the same thing. Idled great, but touch the throttle, and it died instantly. I rebuilt the carb with special attention to cleaning all jets and jet passageways. Ran fine after that.

Same for these smaller, less complex carbs. There is a main jet, a pilot jet, and then the starter jet, which is at the bottom of a long standpipe cast into one of the long sides of the float bowl body. You should be able to see light at the bottom of that pipe, and you should be able to pass compressed air (with a compressor, not a computer can) through the jets and the starter jet passageway. If you cannot blow air through all three of those, it probably will not start.
30 Jul 2021 21:37 #8

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Replied by RT325 on topic DT1B piston blow by.

Just flick the choke while its idling or wherever the range is that fails & see if it overcomes the problem briefly. If it does then go back into the carb. If it doesn't help it then choke fixed jet in the bowl cavity could be blocked. Otherwise raise the needle--by dropping the clip to the richest to prove a point. My DT1 carb was a mess in places not easily seen, like all the way up through the emulsion tube to where its stepped & metered near the top. Caught me out for a while as was unexpected. Stale fuel deposit from our terrible fuel if left parked for a year. Grows a crusty blue powder in my case but wasn't obvious until i put a bit of a bend in a splitpin & wound it up through there & all the blue powder came out--so carried on until clean & the metering step near the top became visible. Been good since. Poke a wire brush strand through the pilot jet too-- & the choke jet if wire is long enough.
31 Jul 2021 04:57 #9

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Replied by craddojc on topic DT1B piston blow by.

Hi PMB.  I think your problem is ignition and in particular the condenser. This happened to me on my 1975 Suzuki 500 2-stroke twin road bike during a long ride where one cylinder started to cut out above 3500 rpm, and it got progressively worse during the ride to where it would cut out above 2000 rpm. That cylinder would foul its plug and it would not blow out and run clean no matter how many times I cracked it wide open under load.  The condensers in this bike were 25 years old at the time, an one had failed.  With this bike the points were out in the open and you could see the points with the bad condenser were arcing like crazy.

People dont realize it but condensers go bad with age.  I had a half dozen NOS spare condensers for this bike, all were over 20 years old, and I checked their capacitance with a meter.  The spec for the suzuki condensers is .028 mfd., and most were below .015 mfd.

If you are having this problem you will see a lot of damage from arcing the points.

Good luck,

Jeff
31 Jul 2021 08:27 #10

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