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Makotosun

76 DT400 modified air filter

  • Bassini
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76 DT400 modified air filter was created by Bassini

I have a 76 400C. Chasing some surging I've ended up with a new VM32 Mikuni. It's working good with a 160 main and a 40 pilot jet. Real good, just normal two stroke surging when I get off the gas, almost none when the throttle is completely closed coming of the gas. Pretty happy guy, lots of chasing that problem.
Now I have to find a way to get a filter onto my new carb. The new carb has an inlet bell that is a much larger diameter than my stock carb. Also I think it may be a bit longer. Plus the weird offset to the air box. Any experience with adapting a pod? I could cut and weld onto my old air box. I really don't want to remove the air box, it's also acting as a mounting location for my updated CDI rectifier. 
Also, my reed cage only has a gasket when it mates to the cylinder. I am using an after market intake manifold. No gasket between intake manifold and the reed cage. I double checked using a parts breakout and drawing. I believe the rubber for the intake manifold acts as some type of seal, or it should. I added a gasket. I used the same gasket from the reed cage to the cylinder between my intake and the reed cage. It's possible that this is the entire reason I had success fighting surging. I could go back to my stock carb and see, but it's running so good I don't think I will.  Air filter ideas?  I have a DT400 that starts on the first or second kick, idles, doesn't surge, and has no flat spots. Was beginning to think it would never happen. I hope it works tomorrow....
The following user(s) Liked this Post: Jammyyammy
Last edit: 19 Sep 2023 18:02 by Bassini.
19 Sep 2023 18:00 #1

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Replied by yam-fan on topic 76 DT400 modified air filter

Why not modify airbox, so you can fit a larger diameter intake hose? Try a VForce reed block, and use a 10mm reed spacer, with gaskets on both faces.
20 Sep 2023 02:24 #2

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Replied by MarkT on topic 76 DT400 modified air filter

Bassani,

Sounds like you have quite a few major challenges with the carb swap which is why many of us do not advocate doing so on a 400.  The stock Mikuni is very tunable and swapping to the jet sizes and needle sizes in the new carb might have yielded the same running results for example.  

I love modifying my machines too...  but sometimes the challenges are not worth the effort.  I wish I could tell you how to make a bigger and shorter intake boot or recommend one from another bike that would fit, but I can't.  

One very important safety question.  Did you modify your throttle cable for use with the new Mikuni?  Your original carb should have had a "tall cap" for the slide.  the original tall cap will probably thread right on to your existing carb.  The problem is the new carb has a short slide.  If you use a short slide with a tall cap, the short slide can clear the guide pin and not go back down creating a very dangerous "stuck full throttle" situation.  You must use a tall slide with the tall cap. 

There are other solutions, but please don't use a short slide with a tall cap. 

I wish I could tell you what pod works or how to make larger and shorter offset boot...  but I'm not sure that info is out there.  I've tried carb swaps and some of the issues you're facing might be nearly insurmountable which is why I don't recommend it in most cases.  Please share whatever you come up with as it could be valuable to others who might want to try the same modification. 

P.S.  Yam-fan loves to recommend generic modifications and advice and make them sound easy even though they are often very expensive in time and cost or not practical at all for little to no return.  It's easy to sit and type "advice" like modify your airbox or suggest swapping reeds...  actually doing it is not always so easy or even possible/practical in many cases.  Do your research. 
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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20 Sep 2023 06:33 #3

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Replied by Bassini on topic 76 DT400 modified air filter

I did notice the difference in the slides, I did use the cap that came with the new Mikuni only because I thought the stock cap may have an air leak at the throttle connection. Thank you for that information. Black cap, black carb only with the long slide. New carb, use the new cap and the new slide. Got it, that's important info. Thanks! Still not sure about an air cleaner, never liked the look or really trusted a bike that had a pod filter installed.  Stock air box is tuned to the stock carb.
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20 Sep 2023 12:18 #4

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Replied by yam-fan on topic 76 DT400 modified air filter

In actual fact, it's pretty easy to make a custom air box from scratch.

First you need to decide on the shape, and location of the box. Make a solid plug out of polystyrene foam, slightly smaller than the air box you want to make, then lay up GRP around the plug, layup needs to be 3-4mm thick.

Then, when it's fully cured, pour acetone though the air intake slot, that you have cut into the top of the box, which will dissolve the polystyrene foam, which will leave you with a light and effective air box, which will fit your bike perfectly.

If your stock box is steel, and simply needs the intake tube modifying, the best way to do this is to see if you can modify it to fit directly to a length of silicone hose, fitted onto the back of your new carb.

Pod filters are bad news, filtration is poor, and you will no longer have any resonance effect, that is provided by a properly designed air box............
21 Sep 2023 15:26 #5

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Replied by Tinkicker on topic 76 DT400 modified air filter

As far as modifications are concerned.. I take the stance of:

Yamaha employ highly educated and experienced, world class development engineers, in a state of the art research and development department.
These engineers spend millions upon millions of Yen developing parts for motorcycles that will perform well in all riding regimes, from winter cold, to summer heat. From sea level to the top of a mountain.

What makes a bloke in a wooden shed at the bottom of his garden think he can do better?

If you are a racer altering gearing and carburation for a specific area of riding, fine. By necessity, any factory machine is a bunch of compromises to run well and meet government regulations but not run optimally in any situation.

You may think that loud pipe is cool, but bet you are not popular with the neighbours.
You may have your carburation spot on for your neighbourhood ride, but find you are riding an intractable beast after accepting that invitation to go on a riding and camping trip into the mountains.

You may find yourself to be the one stood at the side of the freeway with a holed piston on that group ride out to the beach..

If you are a sunny Sunday afternoon rider that likes to take his bike out on a ad hoc basis, leave it stock.

If that was not enough. In the vintage motorcycle market, any radical and or readily visible modification from stock devalues the bike. Period.
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Last edit: 21 Sep 2023 23:06 by Tinkicker.
21 Sep 2023 22:55 #6

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Replied by yam-fan on topic 76 DT400 modified air filter

I first made a larger resonant type air box for an 80s 2T bike, more than 20 years ago. That, in combination with a more effective flat slide carb, improved the bike tremendously. At that time, I had no real idea of the reason for the gains, and had simply made something closer to the set-up used on modern machines.

Years later, I came across an article by Kevin Cameron, which explained the reasoning behind resonant type air boxes: www.dootalk.com/threads/airbox-design-th...evin-cameron.703930/

Stock intake systems on old Yamaha's seem particularly poor, and I have found that on something like a TY175, replacing the stock air box, with a resonant type, and a V Force reed block, makes an enormous difference.

Seems a real shame that, so many seem to believe that designs dating back to the 70s, are the ultimate, and that it's impossible to easily update and improve older bikes, using simple and inexpensive alterations?
21 Sep 2023 23:53 #7

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Replied by Sneezles61 on topic 76 DT400 modified air filter

I really don’t think that “it’s the ultimate” is the point around here… Many folks here have an original bike and will keep them that way. That is the whole point as I see it. Some choose to make them look like they just rolled off the factory while others just like the worn, old patina. To keep them running with the parts they were made with is the desire.
Keep it simple stock…
Sneezles61
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22 Sep 2023 04:30 #8

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Replied by Yamfan on topic 76 DT400 modified air filter

For anyone who actually rides their bike regularly, I would have thought better performance, lower fuel consumption, and improved reliability would all be welcome?
22 Sep 2023 05:46 #9

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Replied by MarkT on topic 76 DT400 modified air filter

Yam-fan, Tinkicker is absolutely correct. Your article by Kevin Cameron is way late to the party.   "Resonant Intake Tuning" is not a "modern concept" that Yamaha was not keenly aware of in the 1970's! 

Intake resonance and tuning is discussed in detail in Gordon Jenning's seminal book "Two Stroke Tuner's Handbook" from 1973.  Jennings worked with Yamaha racing in the 1960's and much of what is in the book is the result of Yamaha R&D.  I still have my original copy that I got for my twelfth birthday...  a copy was available in the Tech Library here as well.  (and may still be). 

I used the concepts and formulas from Jennings book (along with a lot of trial and error testing) to build several very successful air cooled two stroke racing engines in the late 80's and early 90's.   

What I find ironic is that some people think everything designed in the 1970's was junk and who, without any actual experience or hands-on knowledge with Yamaha Enduros, continue to claim these bikes can be easily improved with generic methods that are often not easy at all....  in fact the suggestions can easily worsen performance, not improve 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
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22 Sep 2023 06:05 #10

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