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'70 DT1 to Road Race

13 Feb 2020 16:14 13 Feb 2020 17:00 by Pillsbury. Reason: still the same dumbass :)
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'70 DT1 to Road Race #171
Laguna Seca was unbelievable! Walt Fulton (ex HD racer) & Dave Roper (1st American to win Isle of Man) Invited me to share a garage with them & Robert Aegerter all 3 riding Aermacchi 350's. Friday practice was 60-70d after 1st practice we all geared up except Dave, who was already geared high. Saturday the fog rolled in so there was no pre-race practice. It was about 46-48 during the day with cold wind. In Gp350 Dave 1st, Walt, 2nd & Robert 4th. In GP250 I got a great jump & beat everyone off the line, I short shift up thru 3rd because it keeps the wheelies down & she'll pull it. When we started over the hill going into 4th & 5th she would not pull over 8k so I quickly lost position. About 3/4 thru the 2nd lap she came alive & went back to 9500. So then I got to play the passing game but it wasn't just my division, it was 3 other divisions that were in the mix so a bit to go thru, I only took 7 of 12th place but WOW what a race!! Wining time was 2:02 and I was at 2:10. To share these racers great camaraderie, witness their skills, listen to their shared racing knowledge & watch Karl keep their lovely Aermacchi's race ready was truly a Gift. I never imagined 2 years ago after I got lapped in my 1st race that I would be here. Thank you Lord

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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13 Feb 2020 18:51
MarkT
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'70 DT1 to Road Race #172
Congratulations and love the story and pictures!

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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24 Feb 2020 13:00 24 Feb 2020 13:01 by Pillsbury.
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'70 DT1 to Road Race #173
Here are some pictures from the track at Laguna Seca.

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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14 Jul 2020 09:44
Pillsbury
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'70 DT1 to Road Race #174
Willow Springs last Saturday. I had done test runs in the 500' elv. desert @ 95d at a slight incline and jetting was spot on.
Willow was 100d and 2000' higher. I went two medium laps to scrub the new tires, the WFO from turn 9 to 1, down shift to 4th for turn 1, hold about 8800 rpm thru turn 2 and then a big slow down, grab the clutch as I hear "pop" and here is the end result. I was going to make wfo pass during lap 3 at turn 8 then do a plug chop & coast in but never made it to check the plug.I'm so hard of hearing I could not here the pinging

I'm thinking bad gas but I don't know how to measure the octane rating? I bought 110, this looks like 100 or maybe water.
Any ideas?

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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14 Jul 2020 14:05
Swoop56
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'70 DT1 to Road Race #175
Could be bad gas . The guys running TZ's here in New Zealand used to periodically test 100
octane . The sellers that didn't move it regularly tended to have lower readings ,
due to it going stale .
I'd suggest your ignition timing was a bit advanced , or spark plug a bit too hot .
Possibly a combination of all three .
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14 Jul 2020 14:34 14 Jul 2020 14:35 by LongStride.
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'70 DT1 to Road Race #176
Lean siezure indicated from pic, probably aluminum melted into exhaust side of cylinder, could be fuel related, what fuel are you running? most likely to small main jet , use your richer jets to start out with and jet down, sucks to happen, it does to all of us, double check you timing to be sure it has not advanced itself
Thanks for sharing your race pics.

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14 Jul 2020 14:49
RT325
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'70 DT1 to Road Race #177
#$^&U* & damn, that's very disappointing. No help i know but i used to use a plug out of my boys RS250 roadracer which was just an NGK 11--only had 11 on it. Was short on the top section but think that was so the front wheel didn't contact it under braking with the front V twin cylinder. In my 360 i'm running a surface discharge plug called a BUE--NGK. Not sure if that makes them a super cold plug but looks cold as no porcelain up the middle pretty much. Anyone know about these plugs. Anyway--feel for ya as love what ya doing, & it makes such a mess when that happens.
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14 Jul 2020 17:05
MarkT
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'70 DT1 to Road Race #178
Unfortunately I've destroyed a few pistons over the years... :Ugh

Lean rarely puts a cavity/hole in the center of the piston on a two stroke. Usually the worst melting and the hole (if any) is over by the exhaust port when lean.

Looks like preignition more than detonation to me.

What does your plug look like? Can you post pictures?

A friend I raced with had almost the exact same damage years ago... he would run a B7ES for warm up and switch to a B10EV for the race... except one race he forgot to swap the plug after warm up. Hot plug turned into a "glow plug" and put a nice hole like that in the center of the piston after about half a lap.

Are you running an aftermarket ignition without a key? If so, timing may have slipped? If stock ignition, key may have sheared? Either way, timing is definitely something I would check before tearing it down.

(I would have pressure tested before tear down too... just to rule out air leaks...)

Those are my best guesses based on the info that the jetting was good at lower altitude and lower temp... I use a RAD gauge these days for reference because weather fronts can screw with jetting and I'm too lazy to measure the baro, humidity, and temp and figure out the RAD (relative air density).

P.S. I see you have two plug holes... just FYI, it does not matter if the plug is not firing, it still needs to be the correct heat range. A "spare" hot heat range plug can overheat even though the "firing" plug is a cold heat range plug.

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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14 Jul 2020 19:38
Pillsbury
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'70 DT1 to Road Race #179
I don't really know what happened. Timing was set at 2.2 for the last two years and reduced to 2.1 during the jet checks in the desert. It has a 5d drop at 9500 rpm.
I went from a B8 to the B9 last year and never had a reason to use the B-10's. I would run the same plug all weekend, no previous problems. I checked the timing b4 I tore it apart because it had slipped once b4 but this time it was right at 2.1 so no movement.

If anything, she should have been slightly rich. After checking everything I'm thinking bad gas or water? Or my elect. module went batshit crazy. :EEEK
the piston from buttonwillow when the rotor advanced 20d had no where near the recent damage!

I don't know, I've never had this happen without something mechanical causing it.

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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15 Jul 2020 04:55
KennyV
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'70 DT1 to Road Race #180
A piece of trash partially clogging or blocking the main jet? Just a thought.

71 Z50 Minitrail
72 RT2
74 250A
78 XT500E
20 CB500X
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