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Re: Mikuni frustration [Re: John Healy] #781447 08/14/19 11:58 am
Joined: Oct 2012
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Hillbilly bike Online Content
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Originally Posted by John Healy
Quote
Even at idle the slide and needle are lifted slightly, so the slide and needle and needle position actually do affect the idle mixture. I've found with my VM32s that changing the pilot jet had less of an effect on the idle mixture than the needle and needle position.


I read it on the internet it must be true.



While tuning the Mikuni TM flatslides on my T140 there are about 12 different needles and as many needle jets. The needles and jets also fit the round slide VM carb..Of course only a few needles and needle jets are close to correct..The taper on all the needles starts in a different location, but I noticed none of the tapers start before the slide has opened beyond the idle postion...I did notice that the thick needle diameter and or needle jet orfice does have a small effect on idle mixture..
However I can not say what works on a Mikuni flatslide works exactly the same as round slide...
My recent experient with mufflers leads me to say it has a noticable effect on engine performace much more so than an AMAL..I had been using a 19 inch straight through 1-3/4 inch core glasspack on the stock T140D two into one exhaust..The moderately modified engine ran excellent with the Mikuni TM's but i felt it had a slight dip in power as the the engine went part to full throttle at around 5000 rpm...I tried a few "lollypop" exhaust restrictors but it made no difference
I bought a well used Supertrapp mufller on eBay, massaged out the dents and fitted it to the Triumph using 12 diffuser discs...Holy crap, what an inprovement, it not only cleared the power dip but the engine seems to run smoother at all rpm's..I don't believe it's the so called "you need back pressure" , but its more got to do with complicated matters...
So...keep the exhaust system in mind when tuning carbs...

[Linked Image from i.imgur.com]


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
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Re: Mikuni frustration [Re: reverb] #781465 08/14/19 7:05 pm
Joined: Jun 2007
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HawaiianTiger Offline
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I wanted one of these bikes back when they were new. I ended up with a KZ550 Kawasaki instead which looked a lot like it, was faster, good handling(not quite Triumph league) less maintenance, and about half the price.
I wonder if I should have just bit the bullet.
I fitted a two into one Jardine exhaust on my old '67 and with the Mikuni's I put on there, the bike was a rocket. (Routt 750 kit, too, to be fair). The Jardine looks a lot like this set-up.
So, this bike is punching a lot of buttons for me.
Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: Mikuni frustration [Re: reverb] #781475 08/14/19 9:06 pm
Joined: Mar 2005
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John Healy Offline
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Quote
While tuning the Mikuni TM flatslides on my T140 there are about 12 different needles and as many needle jets. The needles and jets also fit the round slide VM carb..Of course only a few needles and needle jets are close to correct..The taper on all the needles starts in a different location, but I noticed none of the tapers start before the slide has opened beyond the idle postion...


While there are a page of needles and needle jets if you sort through them there are actually only 3 or 4 that work for a given model and throat size. AND these cover both two and four stroke models limiting choices even farther. There is not the "vast" choice of needles and needle jets which some have come to believe. Actually the false impression of those vast choices draw people away from the few combinations that will actually work.

Quote
I did notice that the thick needle diameter and or needle jet orfice does have a small effect on idle mixture..


This is not going to be the case for round slide AMAL or Mikuni models. For one thing both of these rely on a two transfer port idle system. There is one hole at the bottom of the venturi on the back side of the slide and a second just under the back of the slide. At idle the vacuum is high on the back of the slide, and near atmospheric under the slide. In this position of the slide, the transfer port under the slide draws in air because of the differential in pressure. As the slide is lifted the pressure balances and this causes fuel to be drawn out of the hole under the slide that was delivering air at idle. It acts like a little accelerator pump. Because the straight portion of the needle, both the AMAL and Mikuni, doesn't clear the needle jet orifice until approx. 1/3rd throttle AND with the needle-needle jet orifice not changing diameter at idle, it will not have any effect upon idle air mixture.

Quote
However I can not say what works on a Mikuni flatslide works exactly the same as round slide...


The flat slide is a different animal. It doesn't have the "Tea cup" area under the slide. With a Monobloc, Concentric, Mikuni VM, GP, TT, etc there is a delay in throttle response off idle. It takes time for the air flow to draw done the vacuum under the slide and begin to draw fuel out of the needle jet orifice. Obviously it is worse with GP's, TT's and Monobloc type slides with their deep "cup". But when you experience a flat slide you realize that this is common to the Mikuni round slide and AMAL Concentric and MKII with shallower "cups". There is no draw down time with the flat slide. The throttle response is instant. Now, before you run out an buy a flat slide it makes sense to understand if wheel spin is going to be a problem. If I was heading to the Salt Flats, or dirt circle racing I would think twice before using a flat slide. Road racing where you found some real HP, but have little racing experience you might pass.

Quote
I bought a well used Supertrapp mufller on eBay, massaged out the dents and fitted it to the Triumph using 12 diffussr discs...Holy crap, what an inprovement, it not only cleared the power dip but the engine seems to run smoother at all rpm's..I don't believe it's the so called "you need back pressure" , but its more got to do with complicated matters...
So...keep the exhaust system in mind when tuning carbs...


I discovered SuperTrap's when all of the racing organizations started to clamp down on noise. I never looked back! I found HP I never knew I had.
John

I read it on the internet, it must be true!


Re: Mikuni frustration [Re: reverb] #781518 08/15/19 8:52 am
Joined: Apr 2012
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jimmymckenna Offline
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Hillbilly

Did you do the rear master cylinder mod.

I've owned 2 "Ds" and it was a nuisance accessing it tucked in behind the frame

Re: Mikuni frustration [Re: jimmymckenna] #781521 08/15/19 11:06 am
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Originally Posted by jimmymckenna
Hillbilly

Did you do the rear master cylinder mod.

I've owned 2 "Ds" and it was a nuisance accessing it tucked in behind the frame



Yes, I did the rear master cylinder when I moved the foot pegs and controls to the rear...


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Mikuni frustration [Re: John Healy] #781524 08/15/19 11:41 am
Joined: Oct 2012
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Originally Posted by John Healy


I discovered SuperTrap's when all of the racing organizations started to clamp down on noise. I never looked back! I found HP I never knew I had.
John

I read it on the internet, it must be true!


Besides the tight piston to head squish, during the rebuild I retimed the stock intake cam to 100 degrees lobe center and changed to a 71 nitrited exhaust cam timed at 103..The engine made good power but had a small but noticable flat spot when going from part throttle to full throttle at around 5000 rpm..Carb tuning moved it around but it was aways there..As mentioned the Supertrapp muffler eliminated that problem and made the engine rev more freely..
John,this engine runs .020 Triumph branded 8.6 compression 750 pistons..The cylinder barrel is milled .022 and a .008 copper base gasket , .040 head gasket. gives a .032 squish around around the piston circumference rather than about .060 of the stock set up.I shortended the stock pushrod tubes and had had custone pushrods made to size.. By the numbers the engine has 9.2 compression and runs without signs of pinging on 90 octane non ethanol pump gas..I have even lugged the engine on purpose and it won't ping.Stock Lucas Rita igntion timed at 35-36 degrees total.With more turbulence and faster charge burn there is no loss in power...I believe it will probably run fine on the 89 octane E10 although I haven't tried it...I encourage anyone to come by and take this lump for a ride...


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Mikuni frustration [Re: reverb] #781788 08/17/19 10:05 pm
Joined: Apr 2012
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jimmymckenna Offline
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Loving all the flatside chat, but did fernando get a resolution to his problem

Re: Mikuni frustration [Re: reverb] #781791 08/17/19 10:51 pm
Joined: Jan 2014
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reverb Offline OP
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...hello guys;
as you know I put km on these old bikes. I am returning from a 1300 km trip; not arrived yet. I have a problem with the STUPID external filter rubbing to the center stand; two times, yes two times! in this trip, almost fucked the engine due to this crap.
Also, I see that with the rotary pump and constant speed cruising at 5000RPM all this retrofitted oil system is near the limit regarding pressure.

Anyway; plenty of road and plenty of city cruising with traffic jams et all; in Buenos Aires; that is one of the biggest cities in the World; the sparks remained sooty in low gears.

Hi Jimmy; I am waiting for other jets that I ordered again from USA.
Today I bought new plugs, chain, filter, oil and a gel battery (that for some reason the old is discharged-I used it for about 7000 km or so-not too much in my opinion)

Hi Tony; regarding the mufflers; these are the Emgo copy of the 79 T140E but I think PO removed some material from inside perhaps trying to get rid of the richness problem.

Today in the Triumph dealer the mechanic showed me a NEW NORTON; only 500 were made (this is #84) with 600 km (!) in the clock; the owner (a collector) decided that the original exhaust were too much noisy and the guys at Triumph made new ones for him.
The bike sound at idling and cranking a bit the RPMs is just marvelous but very possibly; as you imply, that would modified the performance.
By the way; the bike is just ideal; the engine looks very similar to the old one. Brute power and fine urban racer look with great components and I think, done by hand like the MV.
50000 Dollars.

-the carburetors are not in the same plane (due to no space); maybe I need to touch a bit the float level of
one, to fix one of the plugs that is always a bit more sooty than the other.

Re: Mikuni frustration [Re: reverb] #781995 12 hours ago
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reverb Offline OP
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...today arrived the jets from USA. I removed the #25 pilot jets and fitted #22.5. Removed the #200 main jets and fitted #210.
Slightly changed the float level on the cylinder that has the plug always sootier than the other.
I set the air screws at 1 1/2 (were at a bit more)
I still have sooty plugs at idle and in first and second gear. I did not solved anything. I fitted #17.5; #20; #25 and now #22.5. Too lean with smaller ones that you cannot even put first gear due to the engine shut off.

Now I have hesitation if at idle I snap the throttle grip. May be I need to change from #2.5 slide to #2.0?

With both (#25 and #22.5 sets) pilot jets the engine wants to shut off in first gear; like a clogged jet; but these jets are new and you can see through the holes...may be some passage is obstructed┬┐?
The only way is to keep accelerating and pull the clutch to gain momentum; then when you put second gear all ok.
I need to test these on the road not around the block but is late here and like a good neighbor, better to not do more noise.

Thanks

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