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36mm carb modded 78 Dt400

26 Sep 2020 18:13
FrankC
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36mm carb modded 78 Dt400 #1
Well, with all the cool modifications I have been reading about of late. I decided I would take the leap of faith and throw my hat into the ring. So I previously modded the 34 mm stock carb by oval boring it to a little under 35mm. Today I took the carb off the bike and filed and dremmeled it to 35.98 mm. Almost 36. It took about an hour and a half. Index finger is a little sore holding that file. The bike has been burbleing a little bit at about 2 or 3 thou rpm so I am hoping if it is rich there the bigger carb may solve that, dunno will try it. I have a 200 main so will see how it performs. I decided to start another thread on the bikes progress. After the carb, it's not installed yet, I want to close the squish to about 30 thou inch and perhaps try 150 psi compression.

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27 Sep 2020 13:01
RacerClam
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36mm carb modded 78 Dt400 #2
its important to know at what throttle position tuning issues are happening in order to make sugestions . So if the new carb has issues that where you begin.

Rich

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27 Sep 2020 15:16
FrankC
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36mm carb modded 78 Dt400 #3
Your so right I will start with the new carb and see whats going to happen. Yah i hope to get it back on today. At the apex of the cressant I have a wall thickness of 1.05 mm which is why I stopped at 36mm. All is good just have to do a minor modding of the intake manifold to match. I want to give a check the reed valve while in there. Another issue I have been having is the bike does not want to start cold if I go past 2 or 3 kicks it floods out and won't start the rest of the day even turning the choke off.

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27 Sep 2020 18:50 27 Sep 2020 21:10 by FrankC.
FrankC
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36mm carb modded 78 Dt400 #4
Interesting, I found the jet needle in the last bottom groove clip position so this raised the needle. Does this position inrichen the fuel supply? I wonder which position to set it for a ported motor to start with. I know stock is the middle. That may have contributed to that blubbering at mid rpm. I found the position by accident as I have been working on getting the throttle slide to go up and clear the carb bore at full throttle due to raising the roof of the bore. It may be a combination of grinding the top of the throttle slide down a little and then a cable adjustment. I have gotten it to a half mm or so just.a little bit more hopefully tomorrow. I did accidentally drop the slide on the concrete. That sukd a little and bent the needle. I'll try and straighten it tomorrow.

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28 Sep 2020 11:58
RacerClam
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36mm carb modded 78 Dt400 #5
needle position wont affect starting if your doing it right ( throttle closed ) if your pilot jet is too small you will have issues starting

Rich

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28 Sep 2020 14:30 28 Sep 2020 16:09 by FrankC.
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36mm carb modded 78 Dt400 #6
Yes it might be the pilot thank you rich. I cleaned the carb so hoping it might have been dirty. If not I will go to yamaha and get a larger one. On a side note I checked the reeds, still looking good. I am going to ditch the gasket to use some yamabond and see about any improvement performance. Also, I didn't mention I had a oil/fuel leak between the hardened up intake manifold and reed valve but since tightening it up very good haven't been leaking oil out the bottom of the valve. So a little yamabond on that too. Got the needle straightened and slide clears the bore. Turns out the slide needed grinding at the top and cable adjusted. Edit, the por-15 porpatch is still rock solid in the cast intake manifold after 200 miles so that's good.

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28 Sep 2020 16:17
FrankC
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36mm carb modded 78 Dt400 #7
Got it together just waiting on the yamabond to harden a little then go through check the idle hopefully it will start, then give the oil pump pulley an adjustment if needed due to an adjustment I made in the throttle cable adjuster at the handle bar

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30 Sep 2020 18:00
FrankC
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36mm carb modded 78 Dt400 #8
So I think I may play around with the jets on the oval bored carb. It is now running will see with some jetting changes if it performs better than the stock 34.
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01 Oct 2020 17:25 05 Oct 2020 12:59 by FrankC.
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36mm carb modded 78 Dt400 #9
I would say definitely keep your 34mm carb if you have a stock dt. Unless you can get one with same velocity or greater. I am keeping the 36 for now because I may raise my cylinder, but with the 36 I may have just a touch less on the bottom end but just a touch more on the top end. Not enough at all of a difference to warrant the loss of some low end. Keep the 34.

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01 Oct 2020 19:21
yamahoper
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36mm carb modded 78 Dt400 #10

FrankC wrote: I would say definitely keep your 34mm carb if you have a stock dt. Unless you can get one with same velocity or greater. I am keeping the 36 for now because I may raise my cylinder, but with the 36 I may have just a touch less on the bottom end but just a touch more on the top end. Not enough at all of a difference. Keep the 34.

Thank you for an honest appraisal of your modification attempt!:likey
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