Main jet sizes for V50 Monza with K&Ns

Sunday tinkering part 2. Been experimenting with main jet sizes and backside dyno has settled on 135. 130 felt slightly flat in the upper mid rpm range, 140 no different from the 135. For completeness should probably try a 132 or 133 but I think I’ll stick with the 135s for now. From what I have read 135 is at the top end of the recommended rule of thumb increase for K&Ns.

Anyone have any views or experience with this? Especially interested in any results from doing the job properly on a dyno.

General advice is to increase by 10 to 12% on standard.Tim Hook nose Hicks is the man to chat with, although he has not been on here for a while.

Apart from Tim the guys at Eurocarbs are real good and helpful.

If you check what is the standard out of the factory jet then add 10% gives you the optimum jet size.

Have a 750 version of the same engine running135s with cone filters. There is a fair bit of room for adjustment via adjuster screws and even float height.
Most important are how it goes/feels and what colour the plugs are I guess,
Best of luck

Factory main jet for the Monza is 118. So 130 would be factory + 10%. I’m on 135 (factory + 14%) and for my particular set up I suspect that the ideal lies somewhere between the two so call that factory + 12%. I’ll wait and see if Tim shows up and/or drop him a PM in a couple of weeks.

270T Needle Jet, 130 main.

Thanks for that Tim. I assume original needle in 3rd notch? I’ll pull the carbs apart over the winter and see what I’ve got I there.

Unsure which notch, but don’t move it up one. That will be way too rich.

hook-nose hicks2013-10-13 19:40:49

A cheap way of jetting up,Buy a small metric set of drills.
So a 90 jet will be .9 of a mm.So if you want a 130 that would be a 1.3.
Also you can do custom buying a standard jet is often cheaper.

I don’t think that would be strictly correct…the size relates to fuel flow in cc’s per minute. Not hole diameter.

I have measured the jet sizes with wires,so i dont think you will be that far out.Plus you can start small and work up.

Hmmm…I think I will refrain on that one!

Puzzled I thought that Dellorto main jet sizes were in 100s of a mm. Either way I don’t think you would get an accurate enough hole with a DIY jewellers’ drill. You could make it bigger for sure and the flow rate might be close to desired but with genuine jets available for less than £2 I’ll stick with those rather than trust my precision machining skills.

I have a lathe so find these little mods quite easy to do.

New jets are not worth the hassle of drilling out old ones unless of course you have a full workshop and enjoy that sort of thing, the outlay of drills and a lathe seems a bit OTT for the average enthusiast…


Let me see 2 main jets under £7 inc P&P vs a Lathe, drill bits and the skill to actually use it erm

That lathe over the years has saved me a fortune.
Need a spacer.5 minute job
Need to press a bearing out .Not got the right size.
With a lathe you have.
Its amazing how many mates you have when they know you have a lathe.
Ok not all have the skills to use one, I have and put it to good use.
Its also a milling machine.
Sits out in the shed,Not eating anything.Not like the son.
And just makes me money.

Yes…I would LOVE a lathe in my garage, and a milling machine. Mind you…I’d just be happy with a garage for now…

When manual states ‘3rd notch’ etc., is that counting up from the bottom, or down from the top?



Down from the top.

hook-nose hicks2013-11-11 19:22:32