81 spitfire issues

[81 spitfire]

I have an 81 spitfire with the kawasaki 340 and mikuni butterfly carb. Over christmas I completely replaced the fuel lines(both inside and outside of tank) installed new fuel pump and cleaned the carb. I had running great then, but was unable to ride due to no snow. I have been starting it up every few weeks waiting for the snow. Finally got some and was able to ride for about a half hour, but now it wont run off of choke and when you try to give it gas it dies. I have a tech manual and it is jetted for the right altitudes and temps. I've heard this is a common issue with these sleds, but I'm not sure where to go from here.
Thanks

[JD600]

Have you replaced the crank seals? Blown crank seals will cause all sorts of wierd issues.

DE

P.S., I beat you to this one Todd....

[81 spitfire]

First off thank you for the quick reply. I have not replaced the crank seals. As I'm looking for parts I see that I can get the seals for around $3 w/o shipping or I could get a whole gasket set for around $30. The compression is good in both cylinders and as far as I know the rest of the gaskets are good, but if I'm going to have the engine out and apart to replace the crank seals do you think it would be worth it to replace the rest of the gaskets while I'm at it?
Thanks Again

[JDT]

Touche' Dustin.

Go for the full monte and do a complete gasket/seal set.

Then you know you are good to go.

[81 spitfire]

Last night I removed the motor and started to disassemble it. I should have the gaskets and seals in the next few days. However, I noticed that the crankshaft bearing on the clutch slide is not very good. Could someone help me out with a part number for that and a possibly a place to start looking for one?
Thanks

[JD600]

If you get it removed from the crank, any bearing distributor like Motion Industries can match it up for you. They are a pretty common size, but make sure you specify C-4 bearings as the standard C-3 bearings do not have enough internal clearance under the heat these are exposed to and will most likely fail a little faster than the correct C-4 bearings.

DE

[JDT]

I bet New Breed Parts can supply the bearings.

Note the use of the plural. If you change one change them both.

[81 spitfire]

Haven't been able to get back to this project for a few days, so no news to report. I should have the parts for the engine rebuild by tomorrow. One other issue that I have had with this sled is that it will run great for about a half hour and then it will die. Sometimes I can get it restarted, but usually only on full choke and with generous starting fluid.. Could this be caused by a faulty CDI unit? Is there any way I can do a resistance test on it with an ohm meter ( this is something I have done with the power packs on my boat)?

Thank you,
ER

[JDT]

You can not test the CDI with an ohm meter.
The best test is to swap in a known good box and see what happens.
You can check the coils, both internal and external, with your ohm meter.

Keep in mind that the free air engine runs better the colder it gets.

Once you have completed the engine overhaul see where you are at.

Don't forget to install new plugs when you are ready to fire it back up.

[81 spitfire]

thanks for the info Todd. Do you know what readings I should be getting from the coils with the meter?

[JDT]

I don't have a Spit but I bet someone else will post the values....

[81 spitfire]

Well after about a year I finally got the new seals and gaskets put back in this motor, I can get it to run just fine on choke, but it doesn't want to stay running without it. It seems that no matter which way I turn in the mixing screw there is no effect. I have checked compression in both cylinders and they are both at 125 psi, and I am getting spark on both sides. Does anyone have any ideas on what could be causing this?

Also, I installed new pistons and rings and changed out a couple bearings on the crank, what is good practice for break in time? I am running 40:1 right now and not revving up the motor for more than a few seconds.

Thank you

[JDT]

Plugged jet or passage way on the mixture screw?

[81 spitfire]

Todd,
I haven't had a chance to go through this carb yet, but I did have an extra one laying around that was cleaned before I put it on the shelf. I am still having the same issues with this. The motor will start just fine, but dies when you turn off the choke. Could this be because I am running 40:1 instead of 50:1 for break in?
Thanks again

[JDT]

I would doubt if the gas/oil mixture is the issue at 40:1.

Depending on what type of carb you have, the choke does one of two things when it is on.
It either adds fuel or restricts air. The end result is the same... a richer mixture of fuel to air.
This aids in starting a cold engine. A side effect is also an increase in RPM.

If the engine runs fine with the choke on but stubbles with it off then the carb mixture is too lean or the rpm is too low.

So the question is do you have an air leak? This would lean out the mixture making it run bad but with the choke on you are bring the mixture back to a point where the engine sounds good.

Or is your RPM too low? If you turn the choke off and slightly increase rpm with the throttle does it run smooth than? If so than this is an easy fix.

But if it really a mixture problem the fun begains.

The carb boot is often an overlooked part that can be the issue. If it is cracked or does not mate flush with the intake it can leak air and lean out maybe even to the point of meltdown.
Sometimes you find a small barb on the intake or carb that used to have a primer hooked up to it but was removed. If the barb is not sealed off it will suck air and lean out.
A bad head gasket can mimick the same symptoms.
This is where looking at your plugs will tell the storry.
Chances are if you see one nice plug and one bad is the problem is with an individual cylinder.
If both are bad than look at the intake and carb.
Also look at the gaskets on the intake. It only takes a tiny tear or folded corner to make a leak.

This gets real fun when you have multiple carbs.

The idle curcuit on a carb has extremly small passages and jets. Even an almost invisable piece of dirt or blockage will mess you up.
There is no such thing as taking too much time cleaning a carb!

Crankcase seals are always suspect.
Case halves can leak due to no sealant or too much sealent.
Cracks can happen from a previous piston failure.
The mating surfaces can be damaged from attemps to seperate them.
A bad hose on the impluse line that goes to the fuel pump can have a small cark,rub or cut and suck in air which will lean you out.
The fuel pump itself could be the sourse of unwanted air.

The list gets long but you have to check everything.
But start with the simple things first.

And don't forget the tank vent, pickup line, inline filter(s).