Dumb 2 stroke question
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Author:  TooBusy [ December 20, 2016, 10:55 am ]
Post subject:  Dumb 2 stroke question

Hey guys. In my A Mod project, it's running a 900cc Arctic Cat with a trio of Mikuni carbs.

I'm having cold start problems and looking for advice.
Does it make more sense to tear them down and rework the idle circuit to be happy with cold weather, then tune them again in the spring when temps stay up above 70; or add a primer and shoot a little fuel mix into the carb throats for that first start.

Help me out please, I'm a total 2 stroke noob.

Author:  horizenjob [ December 20, 2016, 12:36 pm ]
Post subject:  Re: Dumb 2 stroke question

Probably a couple of things going on here.

If you have the kits or at least the float bowl gaskets it makes sense to open them and check their condition. People leave fuel in race cars, they sit around etc. and you don't want to start off with an unknown. If you can get race gas for this thing then do so. It does much better at sitting around than pump gas. Also consider the really expensive stuff, if it's the right mix, that you can get from Home Depot or whatever. It's worth it for off season when it sits around and it won't use much just to warm it up a couple of times during the winter. You don't want the fuel to sit and evaporate in the fuel bowls.

The old timer approach is to put fuel ( premix for you ) into a pump oil can and you squirt behind the throttles a couple of seconds before starting and you can squirt some more for the first few seconds. I use my hand to choke the carb on my formula car, but it is a two barrel and much easier. Once it fires though it may keep going, so you may be able to do that too.

It used to be fun watching a mechanic drape over the bodywork of an F1 car with their pump oil can and squirt the intakes on a Cosworth DFV to get it going. We had a couple of old ones show up at local events in the 80's. It looks easy to do that in pictures or the movies but it's another ball game in real life. They were scary fricken machines and most people within about 30 feet had trouble even walking when these fired up.... :rofl: By scary, I mean your spinal cord refuses to do the things you tell it to, like even run away...

Author:  FastG [ December 20, 2016, 12:54 pm ]
Post subject:  Re: Dumb 2 stroke question

It's a snow mobile engine, it should start first pull in sub zero conditions, and run over a wide range of temps. So as horizenjob said I would start with a good cleaning, 2-stake carbs have a lot of small jets and orifices, so give them a good detailed clean and then see what you have. Also try and see if they have been modified, maybe lookup the motor and see when the jetting should be.


Author:  turbo_bird [ December 20, 2016, 1:01 pm ]
Post subject:  Re: Dumb 2 stroke question

My skidoo is the same way, mikuni carbs with no choke or enrichener circuit. The primer pump works really well, give it 2 shots before starting, then a half shot after it fires and a bit more to keep it running as it warms up for the first 30 seconds or so. It can be a bit of a dance if I'm trying to put my helmet and gloves on while warming it up. If I try to richen up the idle enough to start good, it won't idle without loading up like crazy when its warmed up, so primer pump it is for me. Some of the old 2 stroke dirt bikes had mikuni's with an enrichener circuit for starting. It's not a choke, it just let's a bit more fuel through at idle. Usually it was a little plunger you'd lift up on I think the left side of the carb body. If you don't have that on your carbs, I'd go the primer pump route.

Author:  BHRmotorsport [ December 20, 2016, 2:32 pm ]
Post subject:  Re: Dumb 2 stroke question

Arctic Cat is the only snowmobile manufacturer that doesn't put primers onto their machines and it is common to see Arctic Cat owners having a hard time with the first start from cold. That's a good reason to be a Skidoo man!

I'd make sure that the carbs are clean inside and install a primer (be careful not to prime too much and flood the engine). And I use 100LL avgas in my snowmobiles when I know that they will be sitting for a while after use. Good luck!

Author:  TooBusy [ December 20, 2016, 2:57 pm ]
Post subject:  Re: Dumb 2 stroke question

I just drained the fuel and pulled the carbs. I haven't taken them apart yet.
They are Mikuni flat slides with an enrichener circuit. Looks like I get to take them apart and start investigating more.
carbs.JPG [ 127.43 KiB | Viewed 4128 times ]

My gut tells me the enrichener isn't working like it should

When I pulled the exhaust a stream of oil and spooge ran out of all the exhaust pipes.
This is what :puke: onto the floor after an ounce or so from each pipe ran down the front of the engine.
spooge.JPG [ 83.44 KiB | Viewed 4128 times ]

Engine definitely not getting hot enough to burn the excess oil.

Author:  BBlue [ December 20, 2016, 3:15 pm ]
Post subject:  Re: Dumb 2 stroke question

Running the wrong mix leading to fouled plugs? I've found 2 strokers need a hot spark to light off. My leaf blower was difficult to start, got to where it would not start, even with a splash of fuel. Plug was somehow gapped to about 10 thou. Opened it up and it fired right off.


Author:  horizenjob [ December 20, 2016, 3:18 pm ]
Post subject:  Re: Dumb 2 stroke question

Do you have or can you get manuals for the carbs? Especially exploded diagrams showing placement of washers and clips for the needles. Since the oil is mixed into the gas if you run it lean that's like running it with low oil pressure. I've seized 2 stokes over 100 MPH, so it's a real thing... :-)

Probably makes sense to try and warm the engine before you start it next time. Hair dryer, 500W halogen light whatever. Until you get it running right the first time, you may have other problems too. You'll want some spare plugs etc. so might as well buy 2 sets now and put one in.

What does it have for air filters? Does it have an air box? Anything to quiet it on the expansion chambers?

Author:  TooBusy [ December 20, 2016, 6:32 pm ]
Post subject:  Re: Dumb 2 stroke question

Playing with Google trying to find carb manuals now.
Picking up a couple of sets of plugs tomorrow. Going with suggested plugs from the engine manual.
Spent a bit of time this afternoon gutting the rats nest of wiring. There are (were) several wires going nowhere. A couple of hot and a few ground and one I have no idea... it wasn't connected on either end.

I'm putting fresh power and ignition on wires to the MSD box. I cleaned up and bundled the grounds from each coil and the control box to an engine case ground.
Fresh power wire to the fuel pump.

Tomorrow I'll check the 3 timing signal circuits and the ignition timing dip switches. Probably wont attempt to fire it again until after Christmas as we're headed to see family early Thursday morning until day after Christmas.

Baby steps, but at least I'm moving forward.

Author:  TooBusy [ December 20, 2016, 6:35 pm ]
Post subject:  Re: Dumb 2 stroke question

What does it have for air filters? Does it have an air box? Anything to quiet it on the expansion chambers?[/quote]

Nothing. Not much extra room between the carb face and the final drive housing. Investigating how to make some extra space is on my to do list. I'm not fond of running without air cleaners.

No air box

There's a tiny little resonator on the ends of the exhausts. Still sounds like an angry chainsaw.

Author:  Bent Wrench [ December 20, 2016, 8:21 pm ]
Post subject:  Re: Dumb 2 stroke question

There is some technique involved as well.

A Mikuni enrichener will not work with the throttle open, the throttle must be closed to draw a strong vacuum in the enrichener circuit.

Opening the throttle dramatically leans out the mix and will result in a no start.

Author:  horizenjob [ December 21, 2016, 3:14 am ]
Post subject:  Re: Dumb 2 stroke question

I couldn't tell if from the pictures if the carbs even have a place to slide on an air filter. Maybe snow mobiles don't use them? Probably not much dust involved there, :) .

I think you'll need to look at two paths, one is what the engine is set up like in the snow mobile and the other is to figure out what was done to it to put it in the car. Anything you do to the intake or exhaust is likely to make you need to change the jetting. Just putting air filters on it could do that. Just changing the brand or kind of air filters and adding an air box might or might not be a big change... It all depends on the pressure waves.

On the one hand you don't want to change much until you get it running, but on the other hand who knows what the previous owner did with it. Maybe he messed with everything and then gave up on it. :( After the holidays open the carbs and look inside. Write down the jet numbers, needle position and float heights and compare to manual. You should get an idea were things stand. If your lucky it will make sense. Those exhausts don't look stock, so you likely will have jet changes.

One thing in your favor is that race engines don't need to do as much part throttle and slow running as street engines so that may help a bit.

Oh the other thing to do is make sure you really go over all the stuff, look for frame cracks and damage, bent suspension, marks from things hitting each other that shouldn't etc. There's a long list, but your eyeballs and looking at everything is a powerful tool. If it has adjustable brake bias make sure it's set up right, not too loose and not too tight.

Two strokes are fun, life begins at 130!

Author:  TooBusy [ December 21, 2016, 7:41 am ]
Post subject:  Re: Dumb 2 stroke question

Thanks for the input Marcus.
It seems like everything affects 4 or 5 other things, but I guess that's why we call them projects.

I was reading up on carbs last night and came across Mikunis for Dummies.
I like this approach, very straight forward and I hadn't even considered creating a carb data log book. Strange sense at work if I'm troubleshooting a process it's one of the first things I do. If nothing else a data log will help me fine tune by running the numbers through an ANOVA and build a DOE model. Might as well let math help me with a complicated variable set.

The exhausts are full on custom. Different bores and tapers than a sled would run, but these are the same exhausts from when this car was running very well a few years ago in Florida and Texas. Chad had rejetted for the higher elevation near Knoxville / Bristol , TN, so I'm sure that I'll have to richen it up some for a return to sea level.

You're right about the throttles, This car will pretty much see idle and WOT with not much in between. I've just got to get the initial settings to get the dang thing to start when it's cold. Saturday and Sunday when it was in the mid to upper 70's here all I had to do was hit the starter and she'd fire right up. Next week we're supposed to get a return to the 70's so I won't know if I've got the root problem solved before I go back to work. All I can do is get close and keep refining as I go.

Full inspection is going section by section. I've found a couple of points that want minor attention immediately and some rework if she stays in my life very long. The belly pan has a few gouges and the bottom sheeting on the side pods is in rough shape. Looks like it's seen asphalt multiple times at speed. Following the "If you give a moose a muffin" approach, I'll probably have the final drive out of the frame before it's all said and done which will give me a chance to look at the highest stressed points.

First frog of the day is to complete the wiring clean up.
Once wiring is good I'll open up the MSD box and check static timing and record settings for all the dip switches before I make any changes.


Author:  davew [ December 21, 2016, 11:18 am ]
Post subject:  Re: Dumb 2 stroke question

First do not put in hotter spark plugs. You have the correct heat range if you see heat marking on the first two threads. A racing two stroke needs to run on the slightly lean side of the mixer scale, for the best throttle response. [the last thing you need is a hot plug] Do not buy a spare set of plugs, you need to get a box plugs!!! It is difficult to get a good mixture read on the plugs when they are masked with oil. So the initial carb set-up is critical, which means changing to new plugs to get a good reading. File the ground electrode on the plug so it ends half way across the center diameter to improve starting. There is a reason why carb bowls have drain plugs. If the gas is more than a week or two old drain it from the carbs and tank. The oil will separate from the gas while setting. A can of two stroke mixture of gas is OK if you remix before adding it to the tank. Mix the gas by filing the can half full, so it’s easier to get oil mixed with the gas, then add the rest to the gas can and mix again. The higher the performance of the two stroke the less likely that it that it will idle. The GP two strokes did not even have idle circuits in the carbs. Installing air filters on a racing only two strokes will really kill the power because of their short intake duration. Last buy the best oil you can. Dave W

Author:  TooBusy [ December 22, 2016, 11:42 am ]
Post subject:  Re: Dumb 2 stroke question

Thanks Dave

I sure hadn't thought of filing plug electrodes. That's a nugget all by itself.
Plugs are cheap, so no reason not to have a bunch of spares.

It's going to take a bit of playing with to get dialed in. Right now what I'm having some difficulty finding is a timing map. I have an MSD box on it, so I can program nearly any curve up to 22 degrees of retard. I need to lay hands on a degree wheel or establish some timing marks on the flywheel so that I can dial in the correct initial timing.

Yesterday I spent the day gathering supplies and going through the wiring from top to bottom. Now it has a complete rewire and all of the extra miscellaneous wires are gone. one thing I noticed is it's dang near impossible to solder the wires on the MSD box. I don't know what the coating is, but it isn't happy with my flux. I ended up stripping about 2 inches of each end and doing a Westinghouse splice that I covered with a barrel crimp and electrical tape.

Baby steps...

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