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'74 DT360 carb jet ???

01 Aug 2020 09:23
gevanske74
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'74 DT360 carb jet ??? #1
Got a question about carb jets / air box. Seems like engine starts to choke out or won't rev up much past 4800 rpm. Using stock airbox with foam Uni-filter, 180 main jet, stock reed valves, new ignition system. 4500 ft elevation, 4th overbore, so around 360cc. Runs well overall, starts easily 1 or 2 kicks, good throttle response. Does it sound like needs more jet, or is this all the power these older bikes have? Thanks!

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01 Aug 2020 09:44
yamahoper
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'74 DT360 carb jet ??? #2

gevanske74 wrote: Got a question about carb jets / air box. Seems like engine starts to choke out or won't rev up much past 4800 rpm. Using stock airbox with foam Uni-filter, 180 main jet, stock reed valves, new ignition system. 4500 ft elevation, 4th overbore, so around 360cc. Runs well overall, starts easily 1 or 2 kicks, good throttle response. Does it sound like needs more jet, or is this all the power these older bikes have? Thanks!

I posted a couple weeks ago about the filter oil and uni filters.
Try no filter in a non dusty area , see what happens

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01 Aug 2020 09:49
yamahoper
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'74 DT360 carb jet ??? #3
Could be a few things like the o-ring on the jet setter in the bowl, plugged exhaust etc.. I would start with the air filter it is the easiest.

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01 Aug 2020 09:51
gevanske74
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'74 DT360 carb jet ??? #4
Will do, was wondering about that filter. My other DT360 has a K&N style filter that was on the motor when it blew up years ago. Thanks!

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01 Aug 2020 13:07
Gr8uncleal
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'74 DT360 carb jet ??? #5
I've also read here that the DT360A runs better without the airbox cover (lid) fitted.

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01 Aug 2020 14:04
MarkT
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'74 DT360 carb jet ??? #6
I don't think you can leave the lid off on a 1974-on? That's when they started trying to cut back on intake noise. Lid off is like no filter? Could be wrong though... memory might be playing tricks on me.

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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02 Aug 2020 01:03
Gr8uncleal
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'74 DT360 carb jet ??? #7

MarkT wrote: I don't think you can leave the lid off on a 1974-on? That's when they started trying to cut back on intake noise. Lid off is like no filter? Could be wrong though... memory might be playing tricks on me.


That could apply to either of us Mark! :ROFLOL

It just stuck in my mind as something to note when (if?!) I ever get my own DT360A finished.

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02 Aug 2020 04:58
Swoop56
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'74 DT360 carb jet ??? #8
Something that might be worth checking . I seem to recall someone having a problem
with an airbox lid being crushed by the seat , and causing an intake restriction to the air box .
If I remember correctly , it was a DT 360

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02 Aug 2020 08:00
turbodan
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'74 DT360 carb jet ??? #9
How does the plug look?

I've been going through this with that same bike. I think I'm getting all I'm going to get but it's not what I expected for a 351cc two stroke. You need to check the pipe and spark arrestor for obstructions. Mine had a baffle broken loose inside the main pipe and chunks of debris partially blocking the spark arrestor. I had to cut open the pipe to remove the loose baffle and weld it back up. The spark arrestor can be cleared usually with a piece of stiff wire.

The intake system is convoluted and poorly designed. The elbow between the carb and airbox likes to kink, the upper snorkel is shaped like a tall, narrow triangle and the long sides are inclined to collapse inward and close off most of the intake area. The upper cover and toolbox tray does have two prongs that sit inside the snorkel and should prevent that but if you're missing that upper cover or it is improperly installed you won't have any support.

The carb can also cause problems with it's externally replaceable main jet design. The oring at the bottom of the emulsion tube that seals into the float bowl is usually in poor condition unless it's been replaced which will cause an excessively rich condition.

When running properly they have enough power to get around. Mine is running 16/44 gearing and it will exceed 80 MPH without much strain. The motor pulls up to 6500 RPM but without any real hit. Not a hell of a lot of low end, some midrange, a little bit of top end and that's it. With a proper pipe, a bigger carb and a less restrictive intake and air filter it would surely run better. Some porting would help too but I believe there is plenty more on tap without having to open up the engine.

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02 Aug 2020 11:31
gevanske74
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'74 DT360 carb jet ??? #10
Thanks for all the areas to check! The carb o-ring is new from Yamaha, the pipe does have broken baffle pieces inside. Will check air inlet clearances. Try without the uni-filter foam cleaner. I can remember thinking back in 2005 (last I rode it) that bike had pretty mild power for 2 stroke also. But feels like an air delivery issue. Thanks!

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