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Don't bring a Mikuni BST36 to a gunfight

8K views 14 replies 5 participants last post by  Andybass 
#1 ·
Hello all, I am the registered carer of my 1994 Daytona 900, one of only three in the UK apparantly, according to:
https://www.howmanyleft.co.uk/vehicle/triumph_daytona_900#!manufacture
Infact, if this data is true, then there was a total of only 15 daytona 900s in the UK as of Q4 last year. I wonder how many of these are members on this forum!

Anyway, I digress. When I got my bike a few months back, and I was doing the usual replacing and fixing of bits that you do (Oil cooler, plugs, coils, oil, filters, gearchange rubber, voltmeter, fairing mount points, derusting of bits, exhaust straightening, fuel hoses, carb to head inlet rubbers - and noticed that my carbs have a broken lip. I didn't take a picture but in this following one I have arrowed the lip that is broken off one of the carb bodies. It is missing the top arc that you can see, back to a depth of about 6.35mm (alright, or 1/4"!)



The bike seems to run ~ alright ish. Hesitates, misfires now and then but this might be plug leads and the carbs needing an overhaul (this winter's job), but getting hold of a carb body is not easy these days really. It ran fine when I got it apart from being pretty rough and overheating and stinking of petrol.

My question is: what does this chamber / pathway actually do?, and how would it be affected by being broken off like mine is. It is the carb on the RHS when looking forwards, and I believe it feeds the vacuum to the petcock - but this seems to work OK as well.

Cheers!

Andy
 
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#2 ·
My question is: what does this chamber / pathway actually do?, and how would it be affected by being broken off like mine is.
It's a vacuum passageway for the vacuum to the slides. Having that broken would have an adverse effect on the sync, idle, and tune of the carbs. Get a new body.
 
#3 ·
Thanks Ian, I suspected it was a vacuum passageway. It would go some way towards explaining the the slow throttle response, slow return to idle speed I get, and general lumpiness too.
(Also a misfire under load now and then which new coils, plugs and filters didn't solve)

Now my problem is where to find a replacement carb body. I have the feelers out on ebay - I see the bst36 was used on a number of other bikes too - RF900, bandit 12, gsxr1100 etc so they are out there, They are all crazy prices though for a bank of carbs, and triumph do not sell the individual casting. (Although they do sell a new complete bank of 3 joined carbs including everything, for £460 / $597. While this is pricey, it may match the cost in spares for an overhaul - I will have to price things up once the carbs are open and I can assess the state of the parts -
If they need slides and guides and seal sets, needles, emulsion tubes and pilot jets, plus a secondhand bank of carbs for the casting body, it will be cheaper to get the new bank of three from Triumph...
 
#4 ·
Before you spend any money on those POS carbs new find a used single or set cheap. Your better off using FCR flat slides which are damn near FI performance. I saw a set for sale on. Bike site was 1300 Euro and located in uk I thought but the currency not lbs sterling threw my a curve ball...
 
#5 ·
Flat slides are intended for race applications, and will not allow him a good everyday ride to the grocery store.

My everyday ride is a '78 XS1100 bored-over with a Wiseco big bore kit, and every other engine mod you can think of. And, the one thing that works best is the carbs engineered for the bike. That goes the same across my stable of 28 other bikes.
 
#7 ·
Ducati riders love the throttle response and increased power from the flat slides, but we start with a Mikuni very similar to those. I would not be concerned at all about the bit of missing metal as long as you don't block the flow and can keep from sucking dirt there. These carbs are notorious for wear of the emulsion tubes and needles. Factory Pro may make a tuning kit for yours as they do for Ducatis. Order the ones that contain titanium needles and nickel plated emulsion tubes, this is done to reduce wear. Also the gaskets under the jet holder and needle/seat holder can leak unmetered fuel, replace them. They are sensitive to float level. People with more dollars than sense swap them out to avoid rebuilding them, but once setup correctly they are a good carb. Be careful with the diaphragms, they are expensive. Don't use carb cleaner on them, they'll swell. Check them for pin holes or rips. Replace the tiny o-ring in the cap. My 900 SS still has the Mikunis, setup as I've outlined here, and runs great.
 
#9 ·
iandmacdonald

Yup. Was 29, but gave a GS1000 to my younger brother.

And, if I was you, I would NOT be using any of those jets in the rebuild kits. The orifices are notorious for being wrongly sized (K&L, etc.) Buy genuine Mikuni jets.
 
#11 ·
I thought I had better get some more carbs in my diet:



I got a carb body from Sprint along with three seal kits (and a set of plug leads - ouch), so this will be a carb strip, inspect, body replace and rebuild job for me, at some point this winter... Should be fun.

AC
 
#12 ·
Just an update for anyone who may be interested:

It was going so well....

I got a replacement body, took the carbs off and got them in my spacious workshop:



The new carb body ready to be transplanted:



I also changed all the internal o-rings on each carb and set the float heights correctly...



I tidied up the bank and reassembled them - no problems at all..



Until I remember to do the last mixture screw and discover it it siezed solid, and I then proceed to destroy the 22 year old head on it:



Damn it.
New mixture screw ordered from Trumpet UK (£6.90), and I hope to drill it out and extract it with a little bit of magic. (Or screw extractors [or a torx head])

All good fun...
 
#14 ·
Just a question - how "loose" should the mixture screws be? With it assembled - screw, spring, washer and o-ring, and bottomed out in the hole, after unscrewing it 2 turns, it feels quite loose - as if it would easily vibrate loose, or am I missing something?

I don't see anything on the trumpet parts diagram that covers the screw. I know there is a plug on some US models that you have to drill out to be able to adjust them, but without this plug do we just rely on spring pressure to prevent it turning?
 
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