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PaulAA

Mikuni HSR

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I have an itch.

Vintage Performance offers a twin Mikuni kit for the TR6 and I just wondered whether anybody has experience of/opinion on this approach.  It looks mighty tempting...

Paul

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Paul,

There'll be no tuning the mixture wih a touch of emory on the needle. They need micron precision, unlike SU's thou. Because at cruise the depression over the jet is about  7psi- the air flwo is sonic or 'choked'. By contrast the SU jet is at about 0.25psi.  So no fidding with the Mikuni needle. You might find the mpg is bad and the factory needes not avaiable to cure it, or horriby expensive.

Peter

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No direct experience but my 2 cents worth.....

I'm perfectly happy to believe that the Mikunis are a very decent carb, which have had the benefit of development decades beyond the point when most other carb makers have stopped even making carbs, never mind developing them.

However.....  reading the website does not fill me with confidence due to the extravagance of claims made.

20% power gain on a "stock" MGB.  Really?  How sick was it to begin with?  MGB had a claimed 97 bhp when new.  Probably 65 ish at the wheels (which would agree with Pete Burgess's typical reckonings).  They show about 55 bhp, so it's 15% down to begin with.  Whereas the 70 bhp they show afterwards is a bit under 8% up.  Respectable and more believable, but not what they are telling you.  Then there is the thorny issue of rolling road calibration - lets not go there.

Also, not such an issue on the MGB with very simple, straight-shot, siamiesed inlets, but the Triumph 6 has a reasonably complex inlet manifold, which isn't particularly noted for it's equal flows and mixture distribution to all cylinders.  The carbs can't influence that.  To be fair, the long branch manifolds as used on the US TRs and later saloons are better, but still not perfect.

To put this in perspective, when I injected the Vitesse I threw away the 150 CD strombergs and the clockwork distributor and fitted a multipoint system injection system with equal runners and distributorless ignition.  The engine had a bone stock bottom end, some mild DIY head porting to the head and a 6-3-1 extractor manifold.  That engine made 120 bhp on the rolling road (flywheel corrected) vs 104 claimed by the factory.  The factory probably lied and who knows where the RR calibration was, but that is about 20% and clearly alot more involved than bolting on a set of carbs.  The figures don't tell the whole story - it drove much better and was much better on fuel too.  I get asked if it's a 2.5 alot.  It isn't.

My point?  How big your win might be is going to depend mostly on what you have fitted now, how good it's condition and whether it's properly calibrated to whatever state of tune your engine is in.  I suggest that if all is well on that front then you will drop alot of cash for relatively minor gains.  If, on the other hand, your existing carbs are in need of a refurb and perhaps haven't been tweaked to accommodate any other mods made, the gains might be more significant meaning that the Mikunis would be a reasonable alternative to fettling what is there.

Nick

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Peter, Nick

Thank you for your thoughts.  I am, of course, pipe dreaming.  'Nice idea if...' sort of thing.  Uncle Google hasn't come up with a lot of examples of the Mikuni installation on a TR6, hence the question.  I corresponded with a true adventurer a while back, who had culled six Keihins from various scrappies and strapped them onto an array of PI manifolds on a '6.  I wonder whether the outrageous appearance was matched by outrageous performance.

I believe that my engine is in decent form - solid bottom end, even cylinder pressure, US head skimmed down to 9:1, WBC518v2 cam and a late carb manifold with rebuilt HS6s.  The dissy is due for replacement to eliminate a stutter at 2k and more pronounced missing above 4.5k, but it pulls like a train and more power really is superfluous to requirements....................... sort of.

Peter recommended a blower.  I sort of regret not following his advice.

Paul

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Paul,

The M62 is still here...I'll never use it now. All you need is an engineering machinist with a lathe and Tig welder...... and a few underbonnet minor changes...

Peter

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I did, briefly, look at a blower with intercooler and keeping the Lucas PI.But sanity prevailed......a partly blocked injector and goodbye piston.

So I went with Allard and an SU and wet blower, and it turned out more heat from the Wade blower than is sensible.

Peter

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Hi Peter

Thank you - it is tempting!  I suspect I have rather messed up the situation with the high compression head, though.  Wouldn't the pistons be at pretty serious risk?

... but the whine from a blower on song is such a seductive sound!

Paul

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37 minutes ago, PaulAA said:

Hi Peter

Thank you - it is tempting!  I suspect I have rather messed up the situation with the high compression head, though.  Wouldn't the pistons be at pretty serious risk?

... but the whine from a blower on song is such a seductive sound!

Paul

Hi Paul, TimD is at 9.1:1 CR and I think it will work well on 97 RON up to about 5or 6 psi boost, except for perhaps track work with prolonged max boost due to heat build up. For roadwork eg overtaking it should be safe, there's not enough road to hold max boost fro long !! Above ca. 6 psi there's boost retard or water inejction.

Not a lot of blower scream with Eatons, they've been gas-flowed in computers to eliminate it - its a sign of turbulent heating. And the Wade is  quiet too, thanks to its angled ports. So trying to mimic an ERA is sadly not possible.

.Peter

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I’ve used mikunis on mobikes for years and highly rate them, but, you will need to know how to set them up.   Main jets at max throttle opening first, then needle setting for mid range then pilots.   I have a mikuni manual pdf if you need it

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