Interesting manifold length experience

[JimL]

Nothing to do with LSR anymore, but some interesting stuff I just ran into.  I built a "pattern" copy of a 1938 Triumph Speedtwin, using a Chinese V-twin that came by way of a good friend.  I needed to move the carb from a "vertical downdraft" position between the cylinders, to a side mount position.  While trying to get an interesting look, I made up a manifold that tucked in tight and kept the carb out of my right knee.  There should be a pic attached of the bike.

Trying to get it jetted correctly turned into a nightmare.  After days of struggle, I reworked the manifold to accept an old Honda SL350K1 carb (very simple round slide, that always ran well).  It was giving me similar problems to the trouble with the flat slide Mikuni I started with.

To shorten the story...here is what I finally realized.  There were only two RPM ranges (above idle) where the engine would seem to run cleanly.  No matter which carb I tried, or what I did with the jetting, those narrow RPM ranges did not change.

Today I cut the manifold as short as possible and the bike runs well.  The key lesson here is the fixed RPM points that were useable, when everything else was ugly.  I hope this might help someone, someday.

JimL

P.S.  I built this (in this style and size) because I came across a 1938 Triump Speedtwin gas tank, and the 1935 BSA M2 forks.  It is kind of like building a car around that cool gas cap you find at the garage sale!

[Jack Gifford]

Nice work- I like the bike!
So- is your knee now hitting the carb (or air cleaner)?

[Peter Jack]

Jim, you definitely got the look right! 

Pete

[JimL]

Thanks Pete.  Regarding the new carb fit....No, but it is very close.  I have a different carb ordered that might fit a little tighter.  Truth is, with the hardtail rear and the friction damped girders in front, this thing is not comfortable to ride for very long.  I just built it to see what it would be like and for some hobby time in the shop.  It has moveable pegs/pedals and drop bars to set up as a vintage style café racer....now that it runs good I will switch it over to see how it feels.

You can build fun cheap stuff when you have a Chinese bike serial number, license plate, and title.  I recently bought a 2007 non-running Chinese scooter that will become a pattern copy of a three-wheel 1949 Mustang Delivercycle (flathead with a separate trans driving an Amazon sourced Peerless chain-driven rear diff).

My pattern copy of the 1915 Smith Motorwheel (on an Electra 7-speed Cruiser bicycle) is still what I ride the most.  It is easy, quiet, and pleasant in the neighborhood cruising at about 8-10 mph.  I may build a pattern copy of the 1910 Wall AutoWheel to see what it was like.  Oregon is very easy about these type bikes with no registration or insurance required (35cc or smaller and 15 mph max speed).

All those years building stuff for Bonneville leaves you with lots of tools, a good JD-squared tube bender, and enough experience to finish things.  Not a bad deal at my age.

[kiwi belly tank]

I think you'll find it's a pulse wave problem where the lead cylinder is killing the pulse to the trailing cylinder so there is no correct jetting using one carb with a mechanical jet control (cable slide). Give them a carb each or you might want to try an SU carb, they don't have a mechanical control on the jet & will respond to pulse fluctuations. Harley's respond well to a single SU for the same reason & they are a variable cfm carb so you really can't over carb them into a flat spot condition. CD Stromberg's work the same but nobody seems to use them.
  Sid.

[fordboy628]

Nothing to do with LSR anymore, but some interesting stuff I just ran into.  I built a "pattern" copy of a 1938 Triumph Speedtwin, using a Chinese V-twin that came by way of a good friend.  I needed to move the carb from a "vertical downdraft" position between the cylinders, to a side mount position.  While trying to get an interesting look, I made up a manifold that tucked in tight and kept the carb out of my right knee.  There should be a pic attached of the bike.

Trying to get it jetted correctly turned into a nightmare.  After days of struggle, I reworked the manifold to accept an old Honda SL350K1 carb (very simple round slide, that always ran well).  It was giving me similar problems to the trouble with the flat slide Mikuni I started with.

To shorten the story...here is what I finally realized.  There were only two RPM ranges (above idle) where the engine would seem to run cleanly.  No matter which carb I tried, or what I did with the jetting, those narrow RPM ranges did not change.

Today I cut the manifold as short as possible and the bike runs well.  The key lesson here is the fixed RPM points that were useable, when everything else was ugly.  I hope this might help someone, someday.

JimL

P.S.  I built this (in this style and size) because I came across a 1938 Triump Speedtwin gas tank, and the 1935 BSA M2 forks.  It is kind of like building a car around that cool gas cap you find at the garage sale!

Jim,

Just to make sure I am getting this right, you made a forward facing inlet manifold "extension" to move the carb away from the "troublesome right knee location" that is standard on carbed V-twins?

Without some dimensions to confirm, it appears that you might have doubled the intake length, AND, added a 90 degree turn into the mix as well?


My own experience with projects that "extended" inlet manifold length is some what mixed.    I would distill it and comment that in situations of normally aspirated carb installations, the throttle response was very poor until high enough rpm was achieved to promote "reasonably high" gas flow speeds.   Often, these experiments proved "difficult" to drive, because of the throttle response issue.

On the other hand, supercharged applications fared better, but mostly they were full race, wide open throttle situations where low speed driveability was ignored.    For a street driven "pleasure vehicle" however, low speed throttle response can only be ignored at the displeasure of the end user . . . . . . .

Older, and presumably wiser, I now spend a bunch of time with simulation programs such as PipeMax (tm) and others before cutting parts.

JMHO & 2¢, hope you and others find some value in it.

 
Mathhelpsalotboy

[fordboy628]

I think you'll find it's a pulse wave problem where the lead cylinder is killing the pulse to the trailing cylinder so there is no correct jetting using one carb with a mechanical jet control (cable slide). Give them a carb each or you might want to try an SU carb, they don't have a mechanical control on the jet & will respond to pulse fluctuations. Harley's respond well to a single SU for the same reason & they are a variable cfm carb so you really can't over carb them into a flat spot condition. CD Stromberg's work the same but nobody seems to use them.
  Sid.

Stromberg CDSE's can be found very cheap and they are easily and cheaply converted into std variable venturi carbs such as SU's.    175 CDSE's have a 1.75" throttle plate and are plentiful.    Not certain if there are 200CDSE's   (maybe Jag V12 ? ? ?)  but 2" SU's are out there on ebay and the like.

I've seen lots of older Harleys using single SU's.    Seems to work well.

 
F/b

[kiwi belly tank]

Only 150 & 175 CD's, no 2 inch model. Early CD's adjust the needle from the bottom, later CD-2's adjust through the damper tube in the piston & require a special tool.
  Sid.

[JimL]

Thanks for the thoughts, fellows.  This little 250cc engine only uses about a 1" carb bore.  These are an undersquare engine with more stroke than bore.  They aren't built to rev much, but are pleasant to cruise around the back roads.

It is running very well, now, with about 7" of intake manifold length.  Now all I have to do is figure out why the turn signal circuit keeps flaming out....Chinese wire harnesses are pretty poor.  I have always said, if China were to send their Navy to attack the U.S., we will need a much larger Coast Guard to rescue the poor fellows when their ships all break down on the way over!

[Koncretekid]

That's a fine looking build, Jim, and I'll bet no one can identify what it is.  Tell us more about the parts - - frame, wheels, etc., as I need something to build besides BSA singles.

One thing I did not understand is your statement:

"Today I cut the manifold as short as possible and the bike runs well.  The key lesson here is the fixed RPM points that were useable, when everything else was ugly.  I hope this might help someone, someday."

What I see in your photo shows a very long intake manifold??  Also, I wondered if you tried a CB350 (instead of the SL350) stock carb, as they were about 27mm CV type with the vacuum controlling the slide.  They certainly worked well on the stock motors.

Incidentally, I'm planning an excursion to "World of Speed" this year, and you know which class I'll have to run in!  It will be my first run at an SCTA event.  Last year I attended as a MC tech inspector and loved it.

Tom

[JimL]

Tom, what I am trying to explain is that the engine would only run cleanly at about 3000 rpm and at about 6000 rpm (no tach....just guessing).  At ALL other rpm's it either acted very lean or it would get so rich that it fouled plugs.  At certain rpm, in neutral, it would grow a long white cloud in front of the carb mouth.

The strangest event was the "self-opening" carb slide.  If I chopped the throttle from high rpm, the slide would close down and then begin to creep open again.  I even doubled the slide spring and it still did it.  This was the weirdest thing I have ever seen a non-CV carb do.   The only way to get it stopped was turning off the ignition key.

The carb now sits much closer to the Y manifold....not like you see in the picture.

My CX based engine (in the last LSR bike) would wet my left knee with gasoline if I tried to run too far past 11,000 rpm.  Knowing that, I should have figured out this manifold length problem a lot quicker.  We all know how a fellow gets caught in a thought process and "can't see the forest because of all the trees".

Since I am out of LSR and don't know many folks around here, I make dumb mistakes with these hobby projects.  I only posted this because it was so strange and difficult to believe what was happening.

[JimL]

Tom...sorry I didnt answer your question.  Most of that bike is homemade, including the frame.  The battery cover, for example, is made from a piece of a "saucer sled" because you can cut them and hand shape into tighter compound curves without hammering....just a weird characteristic of die formed thin metal.

The front wheel is 1969 CB 350 Honda, the rear wheel is the Chinese "virago clone" hub that I laced with an 18" rim.  The engine is a Lifan clone of the Yamaha 250 Virago.  I have had to put some Yamaha parts in it, but I still don't trust it very far.  The Chinese engines are very prone to spring failure (everything from clutch springs to valve springs to kick stand springs....they all go soft).

The fenders came from the same fellow that was running the vintage twin engine Triumph a few years ago, at Speedweek.  Very nice and strong fenders.

Those 1935 BSA M2 forks are the made-in-India clones and require a lot of work to get them accurate.  I had to form 1/8" wall sections of reshaped tube material to braze onto the cross castings.  They are known for cracking or breaking.  I also set them up with full pass-through heavier studs and made my own top links to get the trail correct.

The gas tank required making "bent" studs so they could be positioned with all 4 pointed straight down.  The studs are threaded in with JB weld to the correct position.  None of the original mount holes were pointed straight down!

Enough....this turned into a long and probably boring story for this forum.  Good luck with your runs at my records.  I kept notes of my experience and notice that not one of those records was a result of two good runs.  That APS-PF 650 record is the combination of a 164 pass mated with a pass screwed up by cross-wind the afternoon two cars rolled and a bike went down.  That is Bonneville and we always took what we got.

 I still dream of bringing my 1969 Speedweek LSR bike (cb450) back for a run at the AMA classic record (t is stored in So-Cal).  Next year would be the 50th anniversary of my first records and the bike is still about 10mph faster than the current record.  I actually brought it to Speedweek 2007 and ran time-only passes just for fun.....which is how I know how fast it still runs.....but thats just an old guy idea and there really isnt any reason to do it.

[Koncretekid]

".....but thats just an old guy idea and there really isnt any reason to do it."

I resemble that remark!

Thanks for the tips on the build - - it sure turned out nice.

Tom

[RidgeRunner]

Jim,

     Your posts are never boring, I learn from every one, great perspectives on problem solving.

      Insert Quote
".....but thats just an old guy idea and there really isnt any reason to do it."

Jim, Tom,

     A major part of what keeps it all fun eh?

                   Ed

[WOODY@DDLLC]

Bring it on Jim! 
Still miss my '66 humpback!