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Discussion Starter #1
Greetings,

I know many use Mikuni carbs on their vintage British bikes. I have this 68 Trophy 250, that I'm setting up as a ride around town bike. My friend had a set of Mikunis he used on his trident, but he went back to a new set of amals to make the bike more original. We were playing around with a Mikuni on this 250 yesterday. It starts great, runs out great, but was slow to drop back to idle speed. The first thing we thought was the Mikuni spring wasn't strong enough to force the slide to drop, so we added an Amal spring inside the Mikuni spring, it worked better, but it still took a few seconds to slow to idle.

Does mikuni have a stronger spring?

When reading some tuning tips, it says to allow 10 seconds between adjustments, to allow the engine to catch up, thats about how long it takes for the idle to drop. Is that normal?
 

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Usually if you have a handing idle, it is due to an air leak somewhere. Could also be a lean mixture. I personally don't like Mikuni's on British bikes and I always run Amals on mine, but Mikuni's can be made to run just fine on a British bike as well. How did you figure out what jets and such to use? I doubt you could use the carb the way it was jetted for the Trident.
 

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Discussion Starter #3
mach1970,

I just used the carbs he bought for his trident, I figure 250cc cylinder, is a 250cc cylinder, either times 1, or times 3.

I already had this bike running on the stock Amal. I run Amals on every old triumph I have. My friend had these, so we thought we'd try em. One thing about the Japanese carbs, is they start extra easy, in any condition, not so much with an Amal.
 

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like mach70 says, might be jetted lean on the idle circuit. turn the idle air screw in to make it rich and see whether it still takes a long time to drop to idle.

when you close the throttle the idle should drop almost immediately if the carb is jetted correctly. the slide should snap closed with a click no matter what the idle does.

i'm going to guess that you should have something around a #25 mikuni pilot jet in there to start with.

theres some information here:

 

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Your thought about a 250cc cylinder is a 250cc cylinder is incorrect for a lot of reasons. So, you need to eliminate the possibility that the carb is running to lean, and you need to eliminate any possible source of air leaks. Those are the 2 things that will cause a hanging idle.

Rob
 

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Hi Phil,

Could also be a lean mixture.
used the carbs he bought for his trident, I figure 250cc cylinder, is a 250cc cylinder, either times 1, or times 3.
a 250cc cylinder is a 250cc cylinder is incorrect for a lot of reasons.
+1. All triples had 27 mm. Amal 626 with 150 main jet. However, all 250 singles (except the Fleetstar?) had 28 mm. 928 with much bigger main jet - '68 Triumph parts book says 170, '68 Beesa book says 220! :eek:

Hth.

Regards,
 

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Discussion Starter #7
Thanks Stu, My triumph workshop manual for the 250 says the main jet is a 160, and that's what is in the Amal. Main jet is somewhat of a mute point with how I ride, the needle, needle jet, and needle position has more effect. I did raise the Mikuni needle to full rich and it ran out better, in fact it runs great. Rain stopped us from doing more dialing in. I don't think it's sucking air, but I do think I may need to use a different pilot jet.
 

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Discussion Starter #8
I still need to find a stronger spring. I think the spring that it has is for a gantry installation, just enough pressure to hold the needle in place. The Amal spring with the Mikuni spring seems to work ok, but still it needs a stronger single spring.
 

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the single mikuni spring is all that is needed for a mikuni, unless something else is wrong. you may have a distorted throttle bore, although that is typically an amal problem, rather than a mikuni. take the cap off and unhook the throttle cable from the slide, and see whether the slide moves freely up and down the carb body. there will be some friction, but it should not be tight at any point. if it does jam or if there are noticeable tight spots, try one of the other mikunis. you can mix and match slides with different bodies to get one that moves freely. there may be a kink in the throttle cable. see whether it slides freely in the sheath.

to test the pilot jet, set the idle air screw at 1.5 full 360-degree turns out from lightly bottoming. that is the correct neutral position. start the bike and let it warm up. then turn the screw in an 1/8 of a turn, then a quarter, then a half. return to the 1.5 position, and then turn it out 1/8, then a quarter, then a half.

if the idle increases when you turn the screw in to richen the mixture, then the pilot jet is too lean. if the idle increases when you turn the screw out to lean the mixture, the pilot jet is too rich. if the idle decreases when you turn the screw in either direction from 1.5 turns out, then the pilot jet is correct.

what you want is a pilot jet that gives the highest idle when the screw is in the 1.5-turn position.


you may need a different needle and needle jet, but if it runs as well as it does you're likely in the correct ballpark. if youve raised the needle all the way to make it run better, it's possible that you might need to be richer than even that. to check that, put in the next larger needle jet and drop the needle back down. if either needle jet works okay, then either one is close to right. there's overlap in these things, so more than a single combination can give the same mixture.
 

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Hi Phil,

'68 Triumph parts book says 170, '68 Beesa book says 220
My triumph workshop manual for the 250 says the main jet is a 160, and that's what is in the Amal.
Fwiw, I'd confirm with Amal what was actually supplied at the time.

Main jet is somewhat of a mute point with how I ride, the needle, needle jet, and needle position has more effect.
That'd depend how the Mikuni is tuned. E.g. Amal tuning is always (and always has been) main jet first.

I did raise the Mikuni needle to full rich and it ran out better, in fact it runs great.
... And both @mach1970 Rob and @speedrattle Kevin have raised the possibility of weakness being the cause of the slow return to idle?

Hth.

Regards,
 

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Discussion Starter #11
Thanks for the method(s) speedrattle, if I can get it to idle right, I'll be happy with how it runs out, and starts. As for the spring, the slide works fine without the cable connected, but it's too soft to feel right at the throttle grip, and yes the cable, and throttle grip work fine.
 

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You are incorrect that the main jet is not relevant to how you ride. Especially with Mikuni. Main jet is not just for WOT. You always tune the carb starting by figuring out the proper main jet size. Otherwise you will never get the carb tuned correctly. Figuring out the main jet size is easy. Get the bike good and warm. Ride it at wide open throttle in about 2 or 3 gear and just when you hit redline, hit the kill switch so the bike shuts off. Pull the plug and read it. The color of the porcelin will tell you if it is to rich or to lean. Of course, start with a brand new spark plug. After you have the proper main jet, then you move on to the needle and needle jet. Then and only then, you move on to the pilot jet. Doing it any other way will result in you chasing your tail or you not having a properly running carb. Trust me, I've been down this road numerous times.

Rob
 

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In fact, by raising the needle all the way up like you are, you may be trying to compensate for a to small main jet. All of the jets overlap each other and they must be tuned and selected in the proper order or the carb won't be correct.
 

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Discussion Starter #14
"Ride it at wide open throttle in about 2 or 3 gear and just when you hit redline"??

Everyone has told me, a trophy 250 is a POS, so I'd be afraid to run this bike to red line. I know the mixture, above idle, comes through the main jet, so it should be right, but the way I'm gonna ride this bike, it may never see 3500 rpm.
 

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Hi Phil,

I'd be afraid to run this bike to red line.
it may never see 3500 rpm.
With respect, you're quoting Rob out of context:-

Main jet is not just for WOT. You always tune the carb starting by figuring out the proper main jet size. Otherwise you will never get the carb tuned correctly. Figuring out the main jet size is easy. Get the bike good and warm. Ride it at wide open throttle in about 2 or 3 gear and just when you hit redline, hit the kill switch so the bike shuts off. Pull the plug and read it. The color of the porcelin
... i.e. Rob is suggesting using the redline just for determining correct main jet size.

Whether or not running a late 250 single around the redline more than that is a good idea depends on how it's built and how much time and money you're prepared to spend on it. I can't speak for US experience but I do know they suffered three main problems in GB - build quality and owners:-

. Irrespective of badge, certainly all the engines were built by BSA, who couldn't build the more-expensive triple engines to a consistent high standard ... :(

. The British owners that buggered 'em fell into two categories:-

.. 250 cc was the maximum engine size a learner could ride 'til he or she had passed his/her motorcycle driving test. Not only did very few have good mechanical expertise and sympathy, the competition were better-built and faster Japanese 250's - if you're going to chase Jap 250's (and many smaller) with one of these 250 singles, you better be prepared to put in the workshop time.

.. The old farts that thought they were buying replacements for their old B31's and B33's and plodded 'em around similarly ... if your bike "never see(s) 3500 rpm", you'll bugger it just as quickly as they did ... :(

Everyone has told me, a trophy 250 is a POS,
Assuming you don't have 'em already, you'll want the Rupert Ratio books - definitely Volumes 1 and 2 - to prove 'em wrong. (y)

when you hit redline, hit the kill switch so the bike shuts off. Pull the plug and read it. The color of the porcelin will tell you if it is to rich or to lean.
These two bits of advice might not work ...

. "when you hit redline, hit the kill switch so the bike shuts off" - At best, a kill button - that you have to hold pushed 'til the engine stops - was only an option on a '68. Easy to fit if your bike hasn't got one.

. "The color of the porcelin will tell you if it is to rich or to lean" - Ye-ea-ah ... might depend on the fuel you're using - iirc, both @Truckedup and @speedrattle have posted previously that mixture determination is much harder with modern pump 'fuel'?

Hth.

Regards,
 

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Actually, I do this regularly even with todays pump gas and it works just fine. If the bike is lean, the plug will still turn out to be to white, and it if it's to rich, it will still turn out to black and possible a little wet. If it's a nice grayish color then it is just right.

Rob
 

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If you seriously think you will never ever rev the bike up, pick an arbitrary rpm to call redline , say 5000 or 6000 rpm and rev to that point. The redline the factory chose was a safe rpm for the engine. You may choose your own safe maximum. It’s your bike.
 

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Hi Phil,

The redline the factory chose was a safe rpm for the engine.
Fwiw:- all '68-'71 brochures say the 250's max. bhp was developed @ 8,250 rpm.;

. same-bore-'n'-stroke triple is 7,250 rpm or 7,400 rpm depending who you believe;

. the shorter-stroke T100R max. bhp was @ 7,200 rpm;

. redline is normally a few hundred rpm higher - e.g. a triple's was 7,500.

Hth.

Regards,
 

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perhaps the difference between the 250 singles red line and the 750 triples is the valve gear. The 250s have eccentric rockers to minimise reciprocating valve gear mass, no adjusters on the end of the rockers , unlike the 750 triples.
 
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