Fitting Mikuni TM28 flat slide carb on 250 V-Twin

[JimL]

In the process of building my Street Tracker, I really wanted the look of a classic V-Twin and a little more power. These pics show the simple manifold that worked out well.

The manifold is a simple 2-bolt exhaust flange made for 1 3/4" pipe (available at muffler shops or industrial hardware stores). The plenum is a piece of 1 3/4" pipe welded to the flange. Leave about 1-2" extra length to allow the end to be sliced, folded, and welded as shown in the first pic.

The inlet adaptors are short pieces of 1" tube, welded into holes cut in the plenum. The trick is to simply cut off the original manifold tubes, bolt them to the heads with duct tape covering the intake ports, and then mark the plenum for drilling 1" holes to weld the short pieces into. I actually positioned the short tubes into the rubber runners, and then spot welded them to the plenum while the manifolds were bolted to the engine.

After the tubes were welded to the plenum, I used a Dremel tool to dress and smooth the openings inside the "back end" of the plenum. After removing the manifold from the engine, remove the rubber intake runners and set them aside.

The end of the plenum, left extended beyond the short tubes, is sliced to allow folding over and welding. Do not try to make all the welds perfect (without pinholes, for example). After the plenum is closed, reassemble and double check fit when bolted to the engine.

To finish the manifold, use JB Weld to glue the steel tubes into the original rubber manifold runners. Coat all the welds with JB Weld to eliminate air leaks. Put wax paper, rags, etc, under the intake port area and bolt the assembled manifold onto the engine for a 24 hour cure of the JB Weld.

The TM28 carb, carb mount, and throttle cable/drum assembly were purchased on Amazon. The carb return spring will need about 3/4"-1" of coil cut off (top end of spring inside the slide) for nicer feel. While you have the slide out, remove the needle and move the clip DOWN one notch on the needle. This will richen the part throttle response.

For jetting, use a 250-260 main jet (I simply drilled an old Short Hex Mikuni jet with a 1/16" drill bit). This runs very well at Sea Level.

You will need to set the idle using "lean drop" method. To accomplish this procedure, adjust idle speed down to comfortable level, adjust for smooth idle, and then lower the idle slightly by adjusting the mixture screw leaner (about 1/8 to 1/4 turn in from best idle). Idle speed can then adjusted up slightly with the knurled knob. This process is necessary to make the engine idle down quickly when the throttle is chopped.

The air filter (small K&N) is mounted to a piece of curved exhaust tubing, to make a little more knee room.

I STRONGLY RECOMMEND angling the plenum/carb mount flange slightly forward. Mine is just a little snug on my right knee for a 6' rider.

The exhaust is constructed of 1" steel tubing, heat painted, baked, wrapped, and clear ceramic painted. The front header is easy, and joins a 1 3/4" collector below the right footpeg. A pair of the 1 3/4" flanges allows the use of an old Toyota Corolla exhaust gasket between the front and rear exhaust systems. If you don't make it 2-piece, you can't mount it on the bike. Front header length is 29" (actually extends a little past the gasket in the collector.

The rear header is a series of curves, entering the side of the collector behind the gasket joint (through a long oval slot). It is also 29" tuned length and actually extends into the chamber of the Super Trapp muffler (also from Amazon).

Results:
Great throttle response, nice slow "loping" idle, quiet exhaust, and cruises "sitting up" at 75 MPH. I was really surprised to find 10 more MPH with just a carb and proper exhaust tuning. Additionally, the fuel pump system is gone and the carb can be disassembled and reassembled without removal from the bike!

Here are the pics.....I will make a "reply" for one more pic.

[JimL]

Rear exhaust arrangement.

[Weldangrind]

Interesting. I've thought about fabricating an intake tube, but haven't tried yet. The JB Weld is a good idea.

Thanks for the lean drop tuning tutorial.

[JimL]

This was so easy, I was amazed. Who would have thought there is aluminum inside the rubber of the manifold runners?

.....and that 1" tubing would fit into the holes (I hand filed a little taper on the end of the 1" so that it wouldnt gouge the rubber.)

....and that the Amazon carb mount would bolt right on to a $3.00 flange from Amazon!

JimL

[zingshoen]

that looks good! you find the TM28 goes smooth? it took me a differently tapered needle get rid of a flat spot.
do you have a pic of the finished bike?

[katoranger]

I like it. Would you be able to make a running video?

[JimL]

The finished bike is under Diamo 250 V-Twin Street Tracker, below this thread. When it stops raining I will roll it outside for better pics. This is Oregon.....that could be a while, this time of year.

I will try to get a video and learn how to post it.

[JimL]

I was able to post a very short video on my home website. It is at the bottom of the 4th page. I am out of space allowance on that site.

The 4th page is mostly about my Bonneville stuff, but it is the only place I could put the video.
The link to my 4th page is:

http://home.earthlink.net/~leinfam/id2.html

[zingshoen]

beefy sound alright!

[JimL]

Part of the idle down problem is possibly due to component quality of these low-cost aftermarket Mikunis. The flat slide can be draggy at high engine vacuum on decel. That could be the reason the stock spring was so stiff. Unfortunately, there was so much spring in the carb that it could not reach full throttle opening!

I plan to take the carb back apart and hand lap the slide to the body with my finest valve grinding compound. I will let you know how it works....too cold in the shop to do anything at this time.

Also, the end of the idle speed adjustment screw may be too small, which tends to "stick" the slide ramp relief. Another job for warmer weather.

JimL

[katoranger]

I made an unlisted youtube video for you.

[JimL]

[Weldangrind]

Nice lopey idle. I like it.