2 jets: same number, but different size??

So I've been agonizing over a couple lean cylinders and a racing idle. I tore into my carbs for the umpteenth time, and realized something interesting. When I first went into the carbs last year I ordered a couple new pilot jets from chacal to replace 2 that were chewed up and pitted. What I noticed today was that although both the original and the new jets are marked 40, the hole is clearly smaller in the new jets. I'm glad to have finally found the(probable) source of my problems, but does anyone know how this could have happened?

 

my guess is someone drilled the originals out, instead of getting larger jets, to accommodate some modification/issue

 

looks like someone drilled those out and did not file off the size, is your bike stock?

 

I guess there is a possibility someone drilled them out... however with a stock airbox and exhaust I'm not sure why they would.

 

reasons why they might... for fuel economy and similar reasons the factory jetted bikes on the lean edge of acceptable. they can stand to be a bit richer even without intake and exhaust mods. not to mention elevation...

 

C-Muck your assuming that the bike has always had stock airbox and exhaust could be at one time it did not or could be someone just wanted to see if they could improve performance or they may have had parially plugged passages its all a guess, but darn good eye catching that and thumbs up for not giving up. It just goes to show that nothing can be taken for granted when it comes to these bikes

 

The carbs that had the new jets were making those cylinders run quite lean, however one of the cylinders with the old jets was running rich even with the mixture screw turned all the way in. BUHHH I was hoping this was going to be the solution to my problems but now it just seems to be complicating things.

 

Just to make sure about a thought I had... is there any difference between sizes of mikuni jets vs. hitachi jets? The old jets have a hitachi logo stamped on them where the new ones dont, is it possible I bought mikuni jets by accident?

 

Do your jet needles have multiple slots in them and an E-clip? If so, someone put a jet kit in there. Dynajet has different meanings for the same numbers on their jets.

 

Looks like a big hole there on the old one, and it doesn't look "round" anymore. Wouldn't a bigger hole lead to it being more "rich" on that cylinder, regardless of the mixture screw position? This is interesting, and like previously mentioned, good eye for catching it!

 

There is a difference in how Mikuni vs. Hitachi Jets are measured.

In both cases though a 40 is pretty small. Hard to tell from pictures but I suspect your "original" jet was drilled out.

Your replacement jet is an aftermarket, hence no Hitachi symbol. Should be functionally identical to the Hitachi jet though. Hole also appears about right for a Hitachi 40.

I think you need to put the jets Chacal sent you back in and look elsewhere for your problem.

 

Alright, its settled then, the PO drilled out the old jets. Here's the best part though- HE ONLY DID THREE OF THEM. The fourth jet was still the size it was supposed to be. So since I had only bought two new jets from chacal, I ended up with three drilled jets and three that are the correct size. HOW CONVENIENT. So until i can get a couple more jets sent from chacal I will be running one cylinder with a larger jet and therefore a richer mixture.


On another note:

I'm still having problems with a racing idle. It seems fine when the engine is still cold, but once it warms up a little it can't help but idle at around 3500 rpm. On my hour-long ride yesterday I ended up having to kill the engine at every stoplight I hit. It seems to me that the problem is somewhere in the throttle linkage, if I push down on the middle linkage I can get the idle to come down. It almost looks to me like the springs on the linkage aren't strong enough to pull the throttle all the way closed or something. Anybody else have an experience like this? Also, here is a list of the things I've done so far to rule out any of these causes:

-Valves checked, spec'd
-carbs cleaned, all passages clear, pistons sanded with finishing paper, new idle mixture o-rings, new throttle shaft seals
-new intake manifold gaskets with RTV, new vacuum plugs and hose
-float levels checked with clear tube
-throttle cable lubed-routing seems fine
-carb sync & colortune

 

On further research through old threads dealing with this issue, I'm guessing it's probably my one bigger jetted carb that is pulling everything else up. Can anyone confirm this?

If not, it must be that when I put my rack back together after doing the throttle seals something isn't quite straight and the linkage in binding somewhere.

 

Dynojet does that sometimes too. We have to check everything in our shop when people bring in Dynojet kits as sometimes they are different sizes and sometimes Dynojet Jets don't even have any hole drilled... They seem to not have great quality control..

Dave