Re jetted carbs, now have stumble off idle

engine22 · Full Member Joined: Apr 10, 2008 Posts: 104

So I just rejetted my carbs after installing pod filters on my 78. So far I have only gone up one size on the mains and I am having a slow response in the low rpms right off idle. The bike accelerates really good in the higher rpms and I have noticed a good power increase from the pods. Should I try going up one size on the pilots or raise the needles or both?

What causes the stumble off idle?

tweedy64

Try adjusting mixture screws first. They maybe just out. cheers tweedy

xsjohn

Have to watch the 38's..... the needle thickness feeds the cutaway with fuel at just off idol throttle......requires the mix screws to be dead on.....run it to clean off the plugs good and let it idol for like 5 minutes......then check the plugs.....if they are getting too dark or lighten at idol adjust the mix screws accordingly....each side independently....watch the plug strap and soot...about 1/3 the way up the side of the strap being black is ok with just a "tiny tiny" bit of soot on your finger tip from the ring.....do it again until adjustments keep the plugs reasonably clean at idol and both sides the same........engine should always be at operating temperature for this procedure....heavy on the mix screws is an easy mistake.....and it kills mileage..........and at least you will have the mix screws set reasonable so other problems can be addressed if that be the case.......there is not much load on the engine at low throttle so heavy mix screws will always work against you and mileage .......then the other stages can be adjusted if necessary.........hope this helps......

xsjohn

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

Did you lower (lean) the needle to compensate for the larger main? If so, that will also lean the upper part of the idle circuit and a slight stumble or flat spot just off idle can occur. Going up one size on the pilots is the usual cure.

If you haven't changed the needle settings from stock (#3 slot or middle position), the sluggish response through the midrange could indicate your larger main is making it too rich. You might try lowering (leaning) the needle one step to see if it improves midrange response.

Stock spec for your mix screw is 2 1/4 turns out. Start there and fine tune for best running. If your best setting ends up far from that, say 3/4 to 1 turn or more, that could indicate a different pilot jet size is needed.

xsjohn

Another way I have found to fine tune the midrange “for me” if it has become slightly rich after raising the main jet is to re-adjust the floats “slightly” leaner…...finding a float position that is in between the stock and larger main jet mixtures…...lowering them 1mm at a time until the correct lower mid mixture is obtained…for mine it was about 1 ½ mm….float levels can have a major impact on mid range mixtures....but no matter how you do it the idol mixture has to be correct and getting the beginning of the needle taper off even a little causes all sorts of smooth cruise and mileage and tuning problems……for example setting the mix screws too rich to compensate for a too lean needle position can mean the difference of mid 40’s for mileage and 50's to 60 for mileage at cruise….then just slight needle position changes will make since when the midrange is not over fueled.....and light cruise does not need heavy mixtures anyway......well at least this is another way to manipulate the mixes for desired results ......and altitude has everything to do with tuning.........

xsjohn

What Happened to 650rider.com?

Yamaha xs650