Sandrail build w/ mikuni carbs

johnysnyd · Last edited by johnysnyd on Wed Mar 28, 2012 12:21 pm; edited 1 time in total

I recently bought a sandrail frame, and I have been building it pretty much from the ground up. The frame is set up with a balljoint front end and an IRS rear.

Things that i have done so far:

-new poly-highback seats
-engine and trans (1600cc)
-dual exhaust (what it came with)
-newly fabricated beer keg tank
-switch to electric fuel pump

I am slowly finding all the poorly done welds, and I have plans to set up the engine w/ a sand seal and possibly a power pulley, but I'm not sure on that yet. I am also planning on putting on an external oil cooler and filter.

The main question that I had: Carburation... i know a lot of guys like to run webers, but I'm hoping to do something a little different. I want to set myself up with some mikuni carbs with a custom manifold. I was wondering if anyone has done this, if so, how did it work?

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Front view

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The crappy seats have been replaced, as well as the gastank (you can't see it in the picture, but that one had a big dent and a small leak)

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this is the engine, it runs decent, but the carb it has on it is only rated for about 70 cfm, I figure that it needs about 140, depending on volumetric efficiency, that is why I'm looking to install dual mikuni's

I will post more pics as soon as I can, but in the meantime, I'm open to suggestions Very Happy

Bashr52 · Posted: Thu Mar 22, 2012 6:45 am Post subject:

I believe there is a thread on Mikuni carbs in the performance section, try searching there. Someone has a setup on their bus IIRC. Why do you figure you need 140 CFM?

johnysnyd · Posted: Thu Mar 22, 2012 8:00 am Post subject:

to calculate cfm:

(engine displacement in CI)*(Max RPM/2)*(volumetric efficiency)*(conversion factor CI to CF)

A Simplified way to calculate is:

(Displacement in CI)*(Max RPM)*(Volumetric efficiency)/3456

So I was figuring my cfm using my 1600cc engine turning between 5000 and 6000 at about 85% efficency

at 100% efficiency and 5000 RPM a 1600cc engine needs 139 cfm and at 6000 RPM it needs about 170

I calculated for 5800 RPM redline and 85% VE and came up with about 140 CFM... This is, of course making the assumptions that I can safely turn my engine at 5800 RPM and also, I can't be certain on my VE, but I figured a good ballpark would be 85%

with all the caculations, I know that I am still going to be WAY over carburated, because the carbs I plan on using are some that I have sitting around from a snowmobile. either way, I need to have better flow than the stock system... the carb I have on now is rated for about 70 cfm

johnysnyd · Posted: Wed Mar 28, 2012 10:54 am Post subject: the carbs work

so, I got the manifold all welded up and put on. I haven't run my throttle cable yet, but I did start it up just to make sure it was going to work... If this setup is going to run as good as it sounds, I am excited!!!

I still have some tuning to do, Right now I am running rich and throwing fireballs out my exhaust, but that is just a matter of rejetting. I will probably get the cable setup on by tomorrow so that I can take it out for a run and see

I will post some pics later

johnysnyd · Posted: Wed Mar 28, 2012 12:20 pm Post subject:

thesatelliteguy · Posted: Wed Mar 28, 2012 1:38 pm Post subject:

Ohh im deeply interested in this!!
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Failproof · Posted: Wed Mar 28, 2012 2:54 pm Post subject:

Any heat on the intake runners? Might be fine in summer of utah, but major issues in winter time without it. Looks good though!

johnysnyd · Posted: Wed Mar 28, 2012 5:12 pm Post subject:

No heat on it as of yet, I am only planning on running it mostly during the summer months. I have considered putting a preheater on it that would be tied to the exhaust pretty similar to the stock setup. I was figuring that I shouldn't run into condensation issues when I'm running in the 70 plus degree temp range.

I really just wanted to put them on as see how they work. I am hoping to have the whole thing put together by easter weekend, so right now I'm in crunch time Smile

Tonight I need to figure out the throttle cable situation and start building my seatbelt mounts.

johnysnyd · Posted: Wed Mar 28, 2012 5:16 pm Post subject:

Also, I am considering running my front end w/o shocks... I have seen rails with them and without, I think that since I will be running in the sand, it should be fine to run without... any thoughts?

thesatelliteguy · Posted: Wed Mar 28, 2012 9:36 pm Post subject:

With the shocks, Id say run it around a lil bit with out them and see how springy it feels.
But, i really want to see more pics of the carb set up and how you get the throttle cable to each. And what are they off of? I have a friend that has a few mikunies sitting around and he was always trying to convince me to make a set up using them on my baja.
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johnysnyd · Posted: Wed Mar 28, 2012 9:59 pm Post subject:

I actually bought them from a friend at work, he said that they are off of a yamaha snowmobille. these mikuni's are a little different than most I have worked with. they are connected almost like if they were on a street bike, and they have one center mounted throttle cable. I am still working out the details on the throttle linkage, i was going to get it done today, but ended up working later than I thought.

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This is a closeup of the carbs I have.... I think it would be pretty easy though if you wanted to set up an average set of mikuni's w/ a throttle linkage, as long as you have a set of mikuni throttle cables to go with them. I will let you know how my linkage works out, and of course post pics

I obviously haven't run them yet, but it seems like they will work pretty good... I mean, mikuni's are pretty much made for offroad.

tdonaldson · Posted: Thu Mar 29, 2012 9:29 am Post subject:

I had a pair of Mikunis on a yamaha a couple years back. The intake runner was only a few inches, but I never had an issue with freeze. The carbs had electric heaters built into the side.
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johnysnyd · Posted: Mon Apr 02, 2012 9:48 am Post subject:

So, I got part of the linkage built on saturday. basically took two small (2 in) pieces of steel and drilled a hole on each end... one end for the size of the vw throttle cable, and the other for the size of the mikuni cable. I then put one hole in the center, sandwiched the cables between the plates, and screwed it together w/ the center hole. It seems to work ok so far, but I need to fab up a holder for the mikuni cable and re-adjust it. I just wanted to see how it ran Smile

Pics to come

johnysnyd · Posted: Thu Apr 19, 2012 9:13 am Post subject:

Sorry it has taken me so long to get back here with pictures. I am in school right now, and things are busy...

anyway, here are some pictures of my throttle cable linkage and my custom air filter box

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johnysnyd · Posted: Sat Apr 28, 2012 8:13 pm Post subject:

So, I got working on my oil cooler this weekend. I removed the cooling tin and put the adapter on. I haven't mounted the cooler yet, but I'm planning on mounting it right next to the gas tank behind the drivers seat.

A couple of questions that i had-
-What fans should I use? my cooler is 4"x12"

-There are a lot of aftermarket fan shrouds... should I buy one or just fab up some covers for the one I have now? also, if I buy one, I have heard that there are two different thicknesses of cooling fans... how do I know if the shroud i order will fit my fan?
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wbrown45 · Posted: Sat Apr 28, 2012 8:50 pm Post subject:

You will not be able to beat the stock doghouse shroud and cooler you already with aftermarket shrouds. You should have the thicker fan that comes with the doghouse shroud. I would reconsider that split exhaust. You will not get the scavenging effect and you will lose power at higher RPM's. They do sound kind of cool though! Wink

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johnysnyd · Posted: Sat Apr 28, 2012 9:06 pm Post subject:

So are you saying that i should just stay with the stock cooler and shroud? In that case, do you have any ideas on how to run a temp. gauge? I was planning on putting on inline with the external cooler, but if I stay with stock, I don't know where to put a temp sensor without drilling into the oil pan, and I want to avoid that.
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wbrown45 · Posted: Sat Apr 28, 2012 9:18 pm Post subject:

A 1600 should not produce a lot of excessive heat in a sandrail. If you want to run a gauge the best place to pick up a true oil temperature is in the sump. VDO makes one that replaces the oil drain plug, but it is very vulnerable while running off-road. You can drill, and tap the engine case for a sending unit, but the magnesium shavings can get into the oil unless you take the engine apart. I used to run my Baja with a 1776 in Phoenix. It was my primary driver for over 10 years and I never had a temperature gauge. I had the stock cooler with a full flow system and oil filter. I did use VDO's dual pressure sender that powered a gauge, and the idiot light. Gene Berg makes a neat unit that fits in the dipstick tube and turns the oil light on when it reaches a critical temperature. I never had any failures without a temp. gauge, and it always ran fantastic.
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Jamies74sb · Posted: Sat Apr 28, 2012 11:25 pm Post subject:

I thalt about putting one of my standup jet ski carbs on my bug it's like a 44 or so . Now I think I will . It should work . I put a newer Harley Davidson c v carb on my suzuki samurai & it was the best thing that I ever did .

johnysnyd · Posted: Sun Apr 29, 2012 12:28 am Post subject:

Honestly, the carb setup is working out really well... I am grossly over carborated, and running pretty rich, but it still runs really well. In a week our two when I finally get the time to tune them right, it should really scream. I would say to go fit it with your 44mm, especially if you are running stock carb right now. Plus the manifold is pretty simple to build yourself if you know how to weld. I think that I spent a total of twenty bucks on my manifold, including the rubber. The custom air filter set up cost me another ten. If you already have the carb sitting around, it is a cheap way to boost your power.
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