Lectron or stick with the Mikuni

Hi,
I have a 1983 WR 240 which I can't seem to get running nicely through the rev range. It had a recon crank new mains and crank seals, new boysen dual reeds and a new piston ring about 30 hours ago, (ring has worn quite a lot in that time). Bore dia is standard and it has a 38mm Mikuni VM carb on it, running 32:1 fully synthetic oil and an original SEM ignition. After changing the jetting a few times I have had a few different reactions but couldn't get it right from 1/4 to full throttle, and even though the plug shows it to be more on the rich side if the bike comes to an abrupt stop as in a bog or deep rut and I pull the clutch in to prevent a stall, the engine revs it's nuts off and is hard to stop!! So as in the tittle do I stick with the Mikuni or bite the bullet and try a Lectron?
Thanks

You have an air leak somewhere.

Yes, as above. Do a leak down test to check.

And after getting your air leak sorted go with a Lectron!

the bike should be able to run tits with the stock vm...a lectron wont fix the air leak or solve problems.
first step if its unknown, do a compression test, followed by leakdown..

the mikuni vm is a solid performer and capable of being jetted perfect. what jets are you changing? have you tried the stock spec jetting? what all jetting changes have you tried? where is ignition timing set?
clean air filter? is the filter over oiled? exhaust clear?

Sounds like a air leak or maybe a worn carby?

I also am leaning towards a air leak.

To check for air leaks I have used carby cleaner,

When you spray where there is a air leak, idle just drops right down.

Doesn't it make it rev higher Dave?

the lean condition makes it do the hanging idle..the carb cleaner eliminates the lean condition for that instant when sprayed on the leak

Rito I always thought it was that the engine would rev up from idle wen it sucked in sum 'start ya barstard'(carb cleaner etc) as u sprayed it around.

No I found carby cleaner did the opposite, with the motor I last tested after spraying with carby cleaner,

spraying one crank seal, it almost stalled.

I think carby cleaner has hydrocarbons like fuel.

So it enriched the mixture.

I just looked at my tin of "Start Ya Bastard"

and that is Ether which should make your motor revs rise when sprayed on a leaking seal?

I don't have any carby cleaner at the moment but I am pretty sure it is Hydrocarbons,

same as fuel so it makes the mixture richer.

just sealing the leak sorts the issue, you can use crc etc even aeroguard...you just need to identify where its leaking and any kind of spray provides an instant gasket, letting you know what's died... my money is on a leaking reed manifold or base gasket just to buck the main seal trend....

Thanks for the reply's, I will make some gaskets to seal the barrel, then re-assemble the barrel and head and make a leak-down tester. I have the clutch cover off and will also pull the flywheel off so as to have a good inspection of the crank seals. Do you guys leave the reeds in when testing or take them out?
Thanks

I leave the reeds in, and the inlet manifold on.




You can just see the alloy block at the top of the picture. The exhaust stub is sealed with a rubber cap and jubilee clip from a fish tank shop.
Hope this helps.

What pressure do you test at? 10psig?

10 is fine, it doesn't matter what the PSI is so long as it holds pressure long enough.

I test at 6 psi. It should hold 6 psi for 6 minutes without a visible movement of the gauge needle. I don't go to 10 psi as I worry about the seals popping. They probably won't, but I feel 6 is enough.

I leak down test it overnight sometimes longer at 10psi.

To check for a running air leak when the engine is in the bike I use starting fluid.

For jetting a mikuni carb I just purchased a hand held tach. So I can see the change in rpm with the balance screw so I know when the pilot jet is the correct size. I am getting hard of hearing.

Jetting,

Check the float level.
Make sure the needle and seat isn't leaking.
Make sure the air cleaner is clean and lubed.
Good clean sparkplug.
Make sure the rubber boots on the carb connections aren't cracked.
Put the needle clip in the middle position.
Adjust the balance screw for the best idle. The screw should be between 3/4 to 1 1/2 turns. I prefer one turn so you have adjustment on the final setting. You need to go rich or lean jet wise depending on the amount of turns on the screw. Then I go to the main jet. You can go rich till it blubbery then go lean till it's crisp. Then you can move the needle. Once it's all set if the transfer from idle to the main jet is hesitant turn the pilot 1/8 turn till it's smoothed out.

Sometimes a carb can be so far off you need to do this twice till it's better. Take your time, take notes from when you started and what you changed.

Like my 570,

the mixture screw is only out 1/2 turn out with a 62 pilot.

According to George, I should be using a 60 pilot.

what brand of carb?

The mikuni carb has the balance screw adjusts the air. On a delorto carb the balance screw adussts the gas flow on the idle.

My first 390 cr had a air leak I didn't know. The engine was so flooded I tip the bike upside down to get the gas out. This bike came from the junk yard. I did' this figure that the wet gas melted the base gasket. This bike was even quicker with the air leak but never stumbled. The 390 power never scared me but I did respect it. I road a 1200 bandit for my streetbike. I was used to speed and pulling power. The small '78 390cr frame I could toss around. My point is if a bike has been flooded for a longtime at least change the base gasket. This may save you the cost of a new piston. I had three running 390cr before I was told to give up riding. My biggest mistake.

You should be a proud papa the engine is purdy.