Mk3 carb decision

Hello Chaps,
I’m a relatively recent owner of a Mk3 Roadster that I’m knocking into shape after a number of years during which the bike did very little with an increasingly elderly owner. As a consequence of its long periods of inactivity the carbs are in poor shape, and I can only occasionally get it to run, and then only on one cylinder. The carbs need to come off for a fettling, and at 36000 miles, possible replacement. I’m also not enamoured with the Mk3 air box, so it is scheduled for backdating. Put the two together, and I’m thinking about a switch to a single carb. I’ve spent a few days reading the many and varied opinions on the subject and I’ve pretty much come to the conclusion that I’m going to try a Mikuni. I’m chilled about the fact that the top end will take a hit, I’m more aiming for a stonking midrange and I’ll probably raise the gearing a little to give a comfortable 80-85mph cruise. The question is, would this aim be best served by fitting a 34mm or 36mm Mikuni? I want to minimise the hit on performance, but I understand that the Mk3 is a bit of a soft old thing. Is the 36mm in danger of over-carbing it?

Help a poor Triumph man out, will you!

Cheers, Jon.

Jon, good choice going with a single Mikuni sir.

Bolts right on, pre jetted, starts and idles very well and flows the mixture very well, and for your purposes of getting things fettled all over your bike the less complication with twin carbs the better.

I honestly don't think you would be able to feel any difference between a 34 and 36 so get the tried and tested 34 and you will not regret it.

Keep the questions coming, everyone here is knowledgable and willing to help out.

1up3down said:

Jon, good choice going with a single Mikuni sir.
Bolts right on, pre jetted, starts and idles very well and flows the mixture very well, and for your purposes of getting things fettled all over your bike the less complication with twin carbs the better.
I honestly don't think you would be able to feel any difference between a 34 and 36 so get the tried and tested 34 and you will not regret it.

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+1 Make sure to use the original Mikuni air filter and not one of those aftermarket flat pancake filters as they will smother power.

Thanks chaps. I'm in the UK, where Miks are less available than in the US. There's a guy selling kits on eBay, the price looks good but the filter looks somewhat skinny...
http://pages.ebay.com/link/?nav=item.vi ... 1512823089
Allen's in Nottingham recommend the 36mm for the 850, it's 50 quid more but the filter looks more like it -
http://www.allensperformance.co.uk/vm36-0032-k.html
I've read about a different throttle cable for the Mk3 - how do they differ? Allen's don't list one for the Mk3 but if I send my throttle up they can spec one to suit.

Before you buy from Allens, get a price from Motocarb in Liverpool

Ah yeah, that's the guy in the eBay ad. The only downside I can see is the slightly mean-looking pancake filter, but for the £50-odd difference I could probably buy a decent filter...

1up3down said:

I honestly don't think you would be able to feel any difference between a 34 and 36 so get the tried and tested 34 and you will not regret it.

Click to expand...

+1 I went to a single 34mm on my '74 after becoming tired of the inconsistent idle afforded by the worn slides on the stock twin Amals. Low to mid-range response is a bit crisper than the twins, and top-end doesn't appear to be affected. Granted, I rarely let it run clear out, but, suffice it to say, it'll comfortably break 100 mph with no obvious lag as compared to the twins. Keep in mind that I also went from a 20 to 22 tooth countershaft sprocket at the same time, so stock gearing might have made the Mikuni accelerate even better than it does now with the taller gearing. That's also while routinely getting 55-56 mpg in mixed driving.

Nathan

I am not personally a big fan of the single Micky, tried one once and wondered where the power went. Be that as it may, if you don't address the ignition system, (does it still have points ?) you may find that after installing the "fix all single carb" it still may only run reluctantly on one cyl. After mechanical condition, ignition should ALWAYS be #1 on the list of things to get right before delving into carburation issues.

Good thought, Biscuit. It is already on electronic ignition of some flavour, with low-mileage coils & leads.

Cheers, Jon

IMO, reliability and basic "up to date" mechanics from the Mikuni KIt (34mm) is a perfect choice with a electronic ignition...
Allensperformance's Kit with that particular bigger, fatter air cleaner (K&N) is what i use on both my 750 and 850 with 34mm miki's.
I dont race nor try to beat other's or care about fuel consumption.. But i really love that fact that both bikes will start 2-3 kicks after standing between their regular monthly startups.. (that is there not used inbetween).

Installing the Mikuni kit with an electronic ignition should give you the potential to light the fuse before completely depressing the starter button, assuming good compression. The Mikuni will give you enhanced low end and mid range power, however, unless your ISOs are fresh, properly adjusted and you are confident that your swingarm/engine cradle pivot is within spec, I'd caution you against the ton. If you are aiming at 75 to 80 I think you'll be pleased with a Mikuni conversion.

If you settle on a 34mm kit your money will go further in the US, even with the additional shipping. I apologize for being a bit self serving (which I hope doesn't put anybody off), but Rocky Point Cycle has been in this business for over 25 years, offers the 2.25" thick air filter and ever a throttle cable that mates perfectly with the Mk3 OE switch gear and with the 90 degree petcock fittings the installation is quite discrete and very easy to implement. Mention your membership on this forum and receive a 5% discount. RPC also offers you a choice of the Tri-Spark, Boyer Micro-Digital and the Pazon ignition systems.

Bill.

I'm curious about why people say the top end takes a hit with a single mikuni. If you are running an 850, it's all about the mid range anyway. I installed a Rocky point Cycle 34mm kit back in the early 90s, and didn't notice any loss in performance, but did notice improved starting and a reliable idle.

My stone stock, except for the Mikuni and a Boyer, '73 850 often out runs bikes with dual amals, pulls out of slow corners well. I rarely rev it past 6.5K, mostly running it between 4-6K.

Now, 750 would probably take a hit on the top end, I used to have to rev the crap out of my Combat when I had it.

I'd highly recommend buying from Rocky Point, I've had great service from them when Stan had it, and it has continued under Bill's ownership.

I'm just disappointed I only just learned today about a discount if you're a member of this forum

Well thanks guys, that's a great cross-section of views and first-hand experiences. Where there was fog, now there is clarity!
Bill, expect an email from me.

Cheers, Jon.

PS If I remove the air box, it looks like I'll remove the top mount for the LH side-panel. Old Brits do a bracket kit for 100 bucks - is that the way to go, or are there better / cheaper alternatives?

Have bought two LH sidecover mount brackets (have two Mk IIIs) from Old Britts. Nice unit and fits.
Bit of a hassle installing the grommets but hand cleaner lube makes it better.

Boxerfan said:

Ah yeah, that's the guy in the eBay ad. The only downside I can see is the slightly mean-looking pancake filter, but for the £50-odd difference I could probably buy a decent filter...

Click to expand...

I would have a look at JS Motorsports single 34mm flatslide set up before jumping on the Mikuni bandwagon. It comes complete with Manifold , filter and twistgrip for a very reasonable price, plus you get Jim's outstanding customer support. I like mine , just my .02 cents worth Cj

cjandme said:

I would have a look at JS Motorsports single 34mm flatslide set up before jumping on the Mikuni bandwagon. It comes complete with Manifold , filter and twistgrip for a very reasonable price, plus you get Jim's outstanding customer support. I like mine , just my .02 cents worth Cj

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I agree 100% The 34mm JSM flatslides are a better carb than the 34mm round slide Mikuni. And they look better in my opinion. The problem with the single Mic is that the fuel bowl runs dry if you run it wide open for any length of time when feeding two cylinders and running lean can damage your pistons. The JSM flatslide is modified to avoid that problem, it just keeps going faster.

I put Rocky Point Cycle's Mikuni VM 34 kit on my Mark 3 several years ago and have never looked back. It's as trouble free as the fuel injection system on my Toyota, as it should be. Comes with everything you need.

Go for it.

Out of respect I will not say the PWK is a bad carb, but the Mikuni is in another class.

Well, the deed is done. A 34mm Mikuni kit is winging its way to me. Thanks for all the help! I'm looking forward to it.

Cheers, Jon.