Tripple Mikuni's not running right - where to start

[heavy85]

Well I just got my 240Z this week and am in the process of working out the bugs. One of the first issues is what I think is the carbs. It's got tripple Mikuni's but not sure exactly what type - they seem to be about 40 mm dia. Anyway it backfires a lot through the carbs and really smells of raw fuel. I checked for fuel leaks and didn't find any but I noticed that you can see fuel laying in the bottom of the carb throats when you turn the car off (not sure what it looks like when it's running). I tried a search for 'Mikuni' both on this site and the internet and haven't come across anything. Everything is for motorcycles or snowmobiles even the Mikuni.com site doesn't list the two barrels that I could find.

 

Anyway my question is where to start? What do I look for to adjust the carbs, any good books out there, recommended parts suppliers, etc. Should I just rebuild them and start from scratch? Basically I know nothing about Mikuni's but need to learn real quick.

 

Any advice would be appreciated. One other piece of info it's got a MSD 6AL (+ mild cam and header) but I dont think it's an ignition problem. Last thing it's got little power but I think most of that is because the yahoo who set up the linkage made it so WOT on the pedal only opens the carbs about 1/3 the way.

 

Thanks

Cameron

[JMortensen]

If you can scrounge up a Mikuni manual, do it. If not, there is a lot of info in the How to Modify Your Nissan & Datsun book. Two things I would check immediately. Floats - are they stuck (tap on cover with back of a screwdriver. If that doesn't fix, check the float level (get the book for specs). Fuel pressure - what kind of fuel pump do you have and do you have a fuel pressure regulator. If you run too much pressure, fuel will just continually pump into the bowls until it runs into the jet blocks and flood the motor potentially causing the spitting you describe. Sounds like the PO didn't know what the heck he was doing.

 

Good luck. I also recommend an O2 sensor in the exhaust to give you an idea of what is going on. Especially with MSD it is hard to read the plugs.

 

Best source for parts I've found so far is Malvern Racing. They know their Mikunis.

 

Jon

[240Z_Master]

Tripple carbs are a very tricky thing if not tuned correectly. I havn't done it myself but am in the process of running triple 40mm Solex Carbs with a Mikuni Manifold. However, if you havn't had them rebuilt in a while, it is highly suggested that you do so, I know a guy on Ebay that sells rebuild kits for the carbs at a good price.

 

Nissan Motorsports (The American One) should have the Mikuni Service Manuals. There is a guy named Gerald at Glendale Nissan in California and his phone number is (818) 547-5550 at the dealership. Ask for him andw then ask for a Mikuni Service manual if they have it.

 

Nissan Part Numbers that may be of some help to you...

Mikuni Carb Rebuild Kit 99996-S1044

 

Okay, I don't remember the one for the rebuild manual, but go ahead and ask.

 

I can't seem to find anyone who can tune Tripple Carbs at an affordable price around here.

[ZR8ED]

Ok I'm no Mikuni expert, but I know webers and they are very similar, so I'll take a stab at whats going on here...

 

before I even start, I should mention that with triple carbs of any kind, accuracy counts. I've said this a million times. 1mm difference, or 1/4 turn differences in linkages, or settings is a BIG deal for normal driving conditions.

 

 

1st: fuel seen in the venturis (infront of the throttle plates) is VERY BAD... FIRE HAZARD. Ok I stated the obvious, and I'll give you a solution. check your fuel pressure after the fuel pressure regulator (if installed...if not, it NEEDS ONE) you should see MAX 3.5 psi... be accurate.

 

2 nd find the model # for your carbs and find a setting for your floats.. they should have a "closed" and an "open" setting measurement usually in millimeters.

 

If you can't find settings, have a look at all your floats and ensure they all have the same amount of travel... you may want to lower the floats closed setting...ie so the float level sits lower in the fuel bowls. I know i'm a bit vague, but I haven't seen mikunis up close for a long time and don't recall how to set the floats...Webers I can tell ya settings from my head.

 

3rd. make sure your mechanical linkages are operating all three bellcranks at the same time and the same amount. 1mm difference makes a big difference at idle and off idle.. make sure WOT opens the butterfly valves all the way, and that your pedal has no extra play in it, and/or that it opens the butterflys all the way.

Mechanical linkage accuracy is very important and takes time...spend the time to ensure they all are exactly the same.

 

Try that for a start. then post back on your progress and we'll take it from there..

[heavy85]

Thanks for the info. Fuel pressure is about 4.5 PSI so I'll lower it to about 3 if the regulator will let me. It's set-up pump-regulator-carbs (no return) and I've seen a lot of debate on the order but this is what I currently have so I hope it works.

 

I reved it up after dark and saw that it was only #3&4 cyl that backfire and #3 has the raw fuel ahead of in the velocity stack. Not knowing what was behing door #1, I started by taking off the cover in the top center of the 3-4 carb. It turns out this covers the jets and the long jet-like thing (emulsion tube maybe? not sure the name but a real long brass tube with holes around it and a screw-in jet at either end). Well the long jet-like thing wasn't even screwed in. The first thread of it was crossed so it would only go in about 1/2 a turn so that's where it was left by PO (I'm really started to not like POs - you ought to see the wiring, sigh). I'll chase the treads on the jet and see if that gets rid of the backfire and raw fuel smell.

 

I also checked the spark plugs and they were all BLACK with heavy soot so I cleaned them up with a wire brush. This tells me it's running real rich on all cylinders. Also notice the gap was only 0.028 but with the MSD I would think I could go up quite a bit - even our minivan has 0.050 gap (that's a fun job with transverse V6). Anyone know where the mixture screw is or which jet to change - there's something that looks like a mixture screw on top next to the mount flange but there's also a screw sticking out the bottom at an angle?

 

Once I get it running right then I have to figure out how to make the pedal actually open the carbs more than 1/3 the way....hmm wonder why it has little power! Any advice on adapting the carb linkage to the pedal and actually have the pedal open the carbs up all the way would be much appreciated.

 

Thanks

Cameron

[Drax240z]

My best bit of advice is to buy a book specific to tuning your carbs, and a uni-syn and go from there.

 

Tuning these carbs seems like black magic a lot of the time, but if you are patient and methodical it doesn't seem so bad. You should certainly be at 0.040" or slightly more with a high output ignition. I wouldn't worry about leaning out all your carbs yet, increase the gap first.

 

Mikuni's are great carbs, I personally think they are a better design than the webers as you can adjust the accel pump stroke externally.

 

ZR8ED has good info, heed it! Many problems with triples we've seen here has been that the mechanical linkage flexed enough that carbs were opening at different times. Changing jets isn't something you are likely going to have to do until the car is running reasonably well and you are looking to fix a flat spot somewhere in the RPM range. Most likely the jets in there are close. If you want you can pull the jets and post the numbers stamped on the side so we can verify the sizes for you. (assuming they haven't been soldered and re-drilled anyway)

[heavy85]

Thanks for the info. There was some play in the linkage which I have now fixed, it now goes to WOT with the pedel after playing around with it, plug gap now set to 0.040, and the jets are actually screwed in place. It'll be a few days before I can pull it out of the gargage to start it but I'll post the progress.

 

My regulator only lowers it to about 4.5 PSI - I found one just like it in the Summit catalog and it's minimum setting is 4.5 PSI so I guess I'll have to find a lower pressure regulator.

 

Jets are as follows:

 

Top end of the long jet - 200

Bottom end of the long jet - 150

Jet next to the long one - 545 (not sure as they were hard to read)

Jet out by itself under a small plug - 40

What I assume is the idle mixture screw is two full turns out

 

Engine is a 2.8 liter w/ mild cam, 6-2-1 header, tripple 44 mikuni's (I could not find any model # on the carbs but they measured 44 mm), MSD 6AL, not sure what else but manifolds are port matched to carbs - manifold has F.E.T marked on it if that means anything. Do these jets sound reasonable for this set-up?

 

Thanks again.

Cameron

[JMortensen]

Those jets do sound reasonable. I'm betting the pilot jets small ones inside the cover are 57.5. Should run reasonably well with those jets, and you can fine tune from there.

 

Jon

[heavy85]

Thanks for all the info - this is a great site. I've now done everything short of rebuild them and it runs a lot better but still backfires occassionally at idle and it stumbles real bad at midrange (not sure the exact RPM as autometer isn't hooked up yet - yet another PO thing). Still not the real inspiring performance that I would expect with this set-up.

 

I'm going to get a carb rebuild kit, Mikuni manual, sync gauge, and timing light and go from there (any suggestion on ignition timing setting would be much appreciated - it's got a MSD 6AL if that makes a difference).

 

Thanks again

Cameron

[JMortensen]

I would try to get 18-20 degrees advance at idle and 34-36 degrees full advance, should be all in at 2500 rpm or so. You can cross reference your distributor and how many degrees advance it will have on the following site:

 

http://home.att.net/~jason510/dizzy_specs.htm

 

You can also modify the amount of advance and when it all comes in by changing the springs in the distributor to lighter or heavier springs, and by narrowing the slots so the mech advance doesn't travel so far. This mainly relates to the 240 dizzy. The ZX has a pretty good range already and lighter springs according to the chart.

 

Here's another dizzy reference:

 

http://www.jrdemers.com/280ZX/distributor/distributor.html

 

Personally I disconnected the vacuum advance and JBWelded the mechanism on my ZX dizzy. They never work anyway (diaphragms always tear), and the last thing you want is the vacuum coming back up at the end of a straightaway and advancing the crap outta your ignition. If you are driving on the street, you'll get slightly better gas mileage/driveability with the vacuum advance, but IMO it is not worth it.

 

If the jet blocks (long tubes with the jets in both ends) are OA, you'll probably never get rid of the midrange stumble. I just ran across something recently that said the 8 jet blocks don't have the big gap from the pilots to the mains, so they are better for street use. I think this was due to a change in the size of the aeration (sp?) holes in the blocks. Probably better for autox too, I'm going to try and track some down for my carbs.

 

Jon

[srgunz]

I got the syncrometer for my mikunis and really like it. Motorsport had them on sale for $29.95. Check them. I paid a lot more for mine.

 

I also have the same MSD as you with the 72 distributer.

 

Take another look at those pilot jets. They don't make a 54.5. If they happen to be 52.5 you will get that spit thru the carb. They are too lean I found out. Get 57.5 or 60 pilots instead. It will make a big difference.

[heavy85]

Once agian thanks for all the info. I checked real close again and I do have the 52.5 jets so maybe that's part of the problem so I think I'll get some 57.5's and go from there. On second thoughts maybe I'll just drill them out - anyone know what dia 57.5 is?

 

I've tried to find Malvern Racing and can find a site but can't get it to load either at home or work (tried it just to make sure it wasn't and AOL problem). Anyone have contact info for them? The only other place I found on the net was toysport.com but they specialize in toyota's and I would rather find someone who's knowledgeable about Datsuns. I also looked on e-bay for parts but only found motorcycle/4-wheeler/snowmobile carbs. Motorsport doesn't list Mikuni's so I assume they dont service them. Any other good placed to get Mikuni parts?

 

Thanks

Cameron

[srgunz]

Cameron

For parts try: http://www.wolfcreekracing.com

[A. G. Olphart]

I ran across a 1985 Mikuni catalog & PHH service manual from 1979. Here's an old address: Mikuni American Corp., 8910 Mikuni Ave, Northridge Ca 91324. Phone was (213)893-2101. Odds are they've moved, but you never know.

 

One of our carbs has a jet block that shakes loose on a semi-regular basis; at that point the engine will run on 2 fewer cylinders except at close to WOT. Obvious solution (once discovered) is to pull the cover & screw it back in place. Jet assembly from the top is 1) Main air jet 2) bleed pipe (aka emulsion tube) below the main jet and inside 3) Jet block with 4) main fuel jet screwed into the bottom of the block.

 

Luck...

[Tim240z]

Current area code for Northridge, CA is (818). (213) is Los Angeles metro area only.

 

BTW, I work 2 blocks away from Mikuni Ave, in Northridge, so I can swing by there in the morning if you like....

 

Tim

[heavy85]

I went with http://www.wolfcreekracing.com for parts. They were very responsive, seemed knowledgeable, and have reasonable prices. One carb's back together now two more to go. Found float was 2 mm high and accel pump a few mm short on stroke. Also found one carb has considerably different jets than the others so order #2 will go out soon for matching jets. I'll update once everything's together and running.

 

Thanks

Cameron

[JMortensen]

One carb having different jets sounds really fishy to me. I can't think of ANY reason to do that. That's the type of thing I've seen (and done myself) when people ream their jets, though, because the numbers don't mean anything once they are reamed. It is entirely possible that none of those jets are what they say they are.

 

I think Drax mentioned soldering jets and redrilling, but that's not always an accurate way to tell. If you just want to go bigger, then you just ream the jet or redrill larger and there would be no solder to give it away.

 

I would suggest you take the jets you just bought and measure them against what are in the other jet blocks. Only way I can see to do that is to get a set of pin drills, find the bit that fits the holes in the new jets, and then make sure that the existing jets in the other carbs are the same size.

 

Whenever I ream jets, I always scratch out the number on the jet just to prevent this kind of confusion.

 

Reaming jets is not the way to go, BTW. I only did it in the beginning stages to get a rough idea of what to buy. The reams are cheaper than buying a whole tackle box full of jets...

 

Jon

[heavy85]

I think the jets are mixed because the PO bought a pair and a single from different applications. Two of the carbs are the same and one was an oddball and was definitely more well used than the other two. Anyway I got them all rebuild, accel pump and float set, re-jetted, linkage all squared away, back together and running. It runs pretty strong but still a little weak below about 3500 RPM but really wakes up near the top end (need speedo fixed so I can judge just how fast it builds speed though to really tell). One problem I still have though is the raw gas smell. There is no longer puddles of gas laying in the throats when I shut if off but there is still a sort of thin coating of gas. Main problem with that is our garage is attached so when I park it inside it stinks up the house. Is this normal? I dont have any air filters for it but would like to get some for durability reasons. I was thinking an airbox would eliminate the smell because it would contain the vapors within inself as long as it was sealed good, but TWM is the only one I've seen and they are $400 - that would be a good chunck towards a used turbo.....let alone the $180 for rebuild kits and jets not to mention I haven't even started playing with jets yet other than making them all the same - these carbs are going to get expensive quick, sigh.

 

Is this fuel smell when shut-off normal and has anyone solved it? (and by the way there are no external fuel leaks) Oh yeah and I've yet to order a sync gauge to really finish setting them up but that's next on my ever-growing list.

 

Thanks

Cameron

[ZR8ED]

The fuel smell after shut down is not normal. Air filters are a very good Idea and will assist in cutting down the odour... (I think.. sounds good anyways)

 

If you are still seeing some residue in the venturis, then you are likely still a bit high with your float settings, or your fuel pressue may be too high... (check that first)

 

To cut down on the expense of filters, you can make a nice air box out of sheet aluminum. There are probably lots of pics of members rides. I made one for my car, and ran a 3" hose infront of the rad and attached a K&N filter out front.

 

Come to think of it though.. when ever I opened the air box, (I had a removable lid for easy access) it usually had a bit of residue in there. I ran my carbs on the rich side, with no chokes, I suppose that residue was from when ever the carbs would backfire...when the engine was cold. Not a big deal though.