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4K Bike Carbs?


ketowagz

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After a while of thinking/working out budgets/how much time I can have the car off the road I've landed on the conclusion that 4age or anyother motor swap is out of the price range and would have the car off the road too long. So I've seen some videos of bike carbied 4k motors. Just seeing if anyone here has done it? Suggestions on bike carbs? Supporting mods? Is it worth doing? I had a quick look and bike carbs seem to be cheaper then twin side drafts.

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There are rarely cheap answers to these problems. I'd suggest if you can't afford to do a twin Weber conversion you're not actually going to do that much better with bike carbs unless you can do all the fab and tuning yourself. Remember they aren't a really common setup, so finding places to tune them may be difficult. If it's a car you rely on, don't do it.

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They have interested me for a long time. Keihins and Mikunis.. There are different models within the range too. You get one carb throat per cylinder, like twin Webers, and they have a "demand-regulated' slide like SU carbs. They have rubber diaphrams in them to do this, like Solexes but unlike the SU's mechanical vacuum slide. The rubbers wear and perish, so need replacing at some time. Hunt around for the prices of a rebuild kit, you can expect to need one.

 

I assume they don't have pump jets, also like an SU, so will have a different feel when you boot them, and seeing they run needles instead of fixed jets you can reshape the needles to get the mixtures you want. I did that for the SU's in The Girl's KE70, take a read of it from my sig below. I'd expect bike carbs to be a lot cheaper to run that DCOEs.

 

You'll have to make the inlet manifold, a plate of steel cut to suit the head and 4 exhaust tube curves welded onto it. Then four rubber hoses to clamp the carbs on at the right angle, they point upwards quite steeply. The fuel bowl seam should be horizontal. Air cleaners next, the original setup from the bike would be nice, but that would take just as much fabrication to fit as making up something completely new.

 

I think they run quite low fuel pressures, so you will need a pressure restrictor in the pump line. I use a $50 adjustable one on the SU's, they run anywhere from 1psi to 3 with no noticeable difference.

 

A vacuum line for the dizzy, another for the brake booster, two lines for the 4K PCV system, one into the manifold and one into the air cleaner box. The throttle setup witll need making from scratch I expect.

 

It would take a while, well, a month or two for me to do it, but I'd do it all myself and would sort tuning and everything. Anything custom like this costs a fortune when you take it to a mechanic, most of them can only change stock part for stock part.

 

Jono, what were the hard parts of your conversion? Have you got it written up somewhere?

 

Supporting mods? Well, to make them work you'll need extractors and a free-flow exhaust, a cam grind, porting the head and skimming for more compression. That would be the minimum to make it worthwhile, you can do more over time and end up like Filfrederick with his twincam 4K! If your bottom end is still strong then its all head work, otherwise start on the carbs using a spare head and swap it over when you are ready to rebore and rebuild everything.

 

Here's a couple of pictures from my "awaiting projects" file... dreams for the future!

 

The red car is Tommy's, the 'car of the year' on here. You can see they made a whole inlet plate that bolts onto the head, then the exhaust manifold bolts onto that again. The other motor has the carbs set too steeply, they are bolted onto a Weber DCOE manifold by the look of it..

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Edited by altezzaclub
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Tuning 4 carbs sounds like a real pain.

 

Unless you have doubled (or more) the power output of a K motor, I think twin sidedrafts are overkill too. A cheap K motor with a cam and extractors just doesn't suck that much air.

 

If I did a different carb, I would use a Nikki from a ~1600cc Isuzu/Datsun/Mazda/etc, or twin SUs.

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  • 5 years later...

There are a number of places where you can buy a full throttle body kit. Efihardware comes to mind, but it won't be cheap however you will get more reliability over carbs for example. However you will need an ecu, cam sensor, fuel pumps, filter and surge tank to make it work if you're replacing the carb. 

 

Twin Weber's are an option, but you will still need an intake manifold. Lynx auto make an intake manifold for Weber dcoe. 

 

Or check the thread on efi conversion

 

The Holley sniper might be a quick win

 

 

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