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Makotosun

DT175 1979 Airbox

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DT175 1979 Airbox was created by Tinkicker

Well, have to say the devil bike is running sweet now.  Had a 40 mile ride today without incident.  Power delivery surges at 3500rpm and is fairly linear above that.

SinceI am more comfortable making it sing and going further afield, I have discovered two things.

1. The speedo is distinctly under reading.  I activated two radar controlled speed warning signs today.  One was a 30 mph one and the other a 40 mph one.  In both cases, the speedo was reading the correct speed. So maybe she is performimg better than was first thought.

2. I have noticed that in top gear pulling 5000rpm/50mph into wind ,if I roll off thethrottle about 10%, she will gradually pick up a little more speed than keeping the throttle pinned.  I am suspecting the air box restricting top end.  It is certainly very small and has a very convulated inlet arrangement that the parts book calls a "silencer".

Is airbox restriction a thing on this model and will removing the silencer improve matters?


 
23 Jun 2024 10:29 #1

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Replied by MarkT on topic DT175 1979 Airbox

Are you talking about #23 in the diagram at this link? www.partzilla.com/catalog/yamaha/motorcycle/1978/dt175e/intake

In the USA, that part only came on the 1978 model...  the 1979 model here had the same airbox but no baffle. 

It was also common to drill holes in various places in the 78-79 airbox to allow more air in...  The most common place I've seen holes drilled on 78-79 bikes I've bought is a few small holes around the outside of the filter lid... NOT in the center where the filter is located.   Not sure that the holes help, but it must have been in a magazine or something because I've seen quite a few modified that way. 
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
23 Jun 2024 17:23 #2

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Replied by MarkT on topic DT175 1979 Airbox

It could also be a little lean on the main?  The classic sign of a lean main is speeding up when throttle is rolled back slightly.  But that's often a temporary "surge" more than a gradual increase.  Still thought I'd better mention it. 
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
23 Jun 2024 18:17 #3

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Replied by Tinkicker on topic DT175 1979 Airbox

Thanks for that Mark. I have ordered a 135 main. At present I have a 130 in, which is as supplied from the factory for 2X2 UK model. Available sizes in the parts book are 125, 130 and 135.
Its nice to have someone with a lot of knowledge about carb tuning for racing. Not something I have ever had much to do with.  I never heard of roll back tuning before.

Since I have the std jet fitted, all I could think of was not enough airflow through the box to keep the engine fed at WOT.
Never thought about a lean main. I thought it was going rich as the main came fully into its own and not getting enough oxygen to burn it, esp after the holy piston affair.

Still cannot understand why the Europe 79 175 has a 130 main as std and the US one has a 140 with ostensibly the same carb.
Last edit: 25 Jun 2024 09:25 by Tinkicker.
25 Jun 2024 09:03 #4

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