Looking for Mikuni jets

[Sam280Z]

I'm in the process of attempting to get better gas millage out of my 280Z. It seems the best I can do is 16mpg, but I usually get more like 13.5mpg.

 

I have triple 44PHH Mikuni carbs on it. Right now I have 52.5 primary jets in it. When I lean the idle mixture screw about 1 3/8 turns out I get good idle mixture (~14:1 on my WBO2). But I get lean mixture (~16:1) and intermitent miss and backfire through the carbs at part throttle cruise.

 

I can get rid of this if I richen the mixture screws up another 1/2 - 3/4 turn. But my idle mixture is then 12:1 - 13:1.

 

I think I need to richen the idle jet, but do not have an assortment of jets to try. I'd also prefer to not have to buy a bunch of sizes, because when I get it right, I'll have a bunch of jets I'll never use.

 

Any opinions on what size idle jet I should try first? If anyone has a collection of jets, or some slightly richer than the ones I have, I would be interested in them.

 

If there are many people here who have Mikunis, what are the chances of setting up some sort of jet exchange program?

 

[Guest Magnum Rockwilder]

If they're the same as Mikuni jets for motorcycles, I can help. Got a pic of the ones you need?

[RB26powered74zcar]

Sam, I think I may have a bag leftover from my tripple mikuni days. Let me search for them, and I'll let you know what I have...

 

joel

[srgunz]

I don't know how your car even runs with those jets. You need a minimun of 57.5's. I went thru that with my L28. I have 60's in now but ran 57.5's for quite awhile. Mine runs best now at 1 full turn out. I think I have an extra set of 57.5's in the garage, I will check.

[Sam280Z]

Thanks Magnum, I doubt that they are the same.

 

Thanks Joel and srgunz. I'd really appreciate it if you guys have anything.

 

The car runs and drives with the 52.5s. Just not as well as I think it could.

 

A side note is that I bought another set of brand new Mikunis a couple of years ago fom a guy on ebay. It had a jet kit that you were supposed to use to swap out the stock jets in the carbs. The carbs had 50 pilots in them and the jet kit had 52.5s.

 

Also, somehow in all of the jets that I do have I ended up with three 57s.

 

I know a bunch of people must have tons of old jets lying around. I'd like to set up a deal where people can trade jets they don't need for jets they do. What do you guys think of that?

 

Sam

[JMortensen]

I feel your pain, I just spent about $150 on jets. I agree the 52.5's are probably too small. The best advice I've gotten is that the car should run well and transition as well as possible with the idle jets 1.5 turns out. If you have to turn them out farther they're too small, and if you have to run them at less than 1.5 then they're too small.

 

The Mikuni motorcycle jets are definitely different.

 

The biggest problem with jets is that you can never be fully sure of what you have, due to the existence of people like me who own jet reams.

 

Try wolfcreekracing.com or Nissan Comp for jets. You should be able to get them for $3 or $4 per jet.

 

The biggest help in tuning mine came by installing an O2 sensor. I'm just using a narrowband single wire hooked up to a voltmeter, but man what a difference. Can't stress enough how much easier that makes the whole process.

 

EDIT--Apparently you already have a WBO2. Don't know how I missed that.

[Sam280Z]

Well, I was doing a bit of work on the house when I looked down at those new Mikunis. Lo and behold, they still had jets in them. I thought I had taken them out to fill my little box of jets... On closer examination, the pilots were... You guessed it, 57.5s.

 

Of course I stopped working on the house and installed them in my car. It made quite a difference. Lean pop and stumble totally gone. One thing I noticed is that it feels like I do not have to push the accelerator at all for cruise. Just the weight of my foot provides enough throttle. I hope my gas milage improves.

 

Now my idle screws are 1 1/4 turn out to get 13.8:1 - 14:1 (it fluctuates a bit) on my WBO2. Cruise is just a bit richer than that (13.2:1 - 14:1). I will try another 1/8 turn in later this week and see if that helps.

 

If it doesn't, What would be the first jet you would try. I know they have to be between 52.5 and 57.5. That leave 9 potential jets. I don't have a good idea of how much change each jet size imparts.

 

I do have three 57s. And, my exhast is set up with a WBO2 for each bank of three cylinders. Would it be a good idea to try the 57s on one of those banks with the 57.5s on the other? Or, is .5 of a jets size not going to make a difference?

 

I think the reason it was jetted so lean is because of the crap job the PO of the engine did for the header collector (). What you are looking at is the series of improvements I have made since I got the engine. The one on the left was something a muffler shop did for me because the two pipes were joined by running one into the other at a bout a 90 deg angle. The second is my handywork to deal with the collectors necking down from 3 inches to tiny pipe by flaring the tiny pipe A LOT (you can see the tiny pipe in the left one. It was like that to deal with the close fit of the Clifford Research Headers)... Now you can hold the collector up and see straight through either pipe. You could not do that before.

 

Would excessive back pressure cause the car to like or accept leaner jets? BTW, I have had the thing run and drive well with 130 mains with that exhaust setup. I have 150s in there now.

 

Thanks,

Sam