Ka100 Stuttering at low rpm

Hey guys, need some help here. So I switched from Lo206 to Ka100 this season and its been going pretty good for the most part, Bought a brand new engine, only about 3 hours on it and am having some issues, at low RPMs and getting out of slow speed corners my engine stutters if I try to put the power down, you can hear it rev up a couple of times before actually putting down power and it makes it very hard to overtake, Once the rpms finally pick up it gets going but getting it to pick up is difficult, it’s almost as if your waiting for Vtech to kick in lol. Anyone have any idea what the problem might be? I had my Mechanic look at it and he tried changing out a bunch of stuff in the carb, improved a little but still definitely off where it should be. Thanks.

To an extent what you’re experiencing may not be super unusual as the engine has to be in a certain RPM range to come on song. Outside of that range it will run rough and not pull very hard.

That said, do you have the correct gearing on for the track you run?

This sounds like a clutch problem. If it’s revving up before grabbing and going off the corner. Sounds like the clutch is installed incorrectly or has an issue.

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I would be curious about what RPM you are having the issue. These motors don’t do much below 7k. You seem to be describing several issues too. Can you clarify what you mean by:
Rev up before putting the power down

Does your gearing compare to your competitors?

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Can say what it isn’t:

Clutch - happens on the stand too
Brakes - hit and release properly
Gearing - relatively same as race winner
Driving - Marshall is driving really well

Definitely on the engine side. Thought it was electrical so swapped coil & plug. Boot was not twisted on to the wire, fixed that. Have not touched the stator (yet). All connectors are installed properly and wires are not in tension.

Fresh gasket kit on the carb, did not replace the internals (yet) or check pop off.

Reeds were floppy open on 1-2 of the four sections. Replaced that and things improved significantly.

Needles are set at 1 +5 minutes on the high and 1 on the low. Did not go back to the needles after replacing the reeds.

The stutter was very pronounced on the bottom on the stand, particularly pulling off a brake load. Less of an issue when not coming off of load but still there particularly if you don’t roll in to the throttle gently. Hard throttle dump and it dies.

Had a similar situation in another stock package and sent the carb to BBS and Brian got that package sorted. Just seems like an issue on stock engines that they’ll run fine out of the gate for a bit but then fall on their face after several hours. This has been the third instance now and other than “sending it out” it’s been stumping us. A stock engine should run 10-20 hours easily before needing service.

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Anyone else have any thoughts on Marshall’s issue? Just put another together yesterday and had a similar issue come up on a brand new engine after break in. Seemed to get it dialed in and rolling but just can’t seem to find a happy consistent place in the carb settings on the stock engines.

Hi Dan, I find KA’s are nigh undrivable out of the box…tolerances are too tight, timing isn’t right, but most of all the carb has a long flat, lean spot at low rpms. All carbs have this flat spot, esp pumper carbs, but it is especially pronounced in the KA. You can’t just dump more fuel at it with the low needle, as it will end up too rich across the whole curve. Try a softer fulcrum spring with lower pop off.

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This is an interesting idea. can you give us an idea of what number you would consider lower? I believe stock is between 9 and 10psi. Do you use the same needle settings with the lower pop off?

pop at 8, hold at 6, needle settings the same. Try a 31 gram spring, stock is 37

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Thanks! Going to get some things on order so we can get the stock packages running better out of the box. We really don’t want to be in the engine prep business as we encourage new KA racers to work with established builders. But if we can help them get off the ground easier and have a better experience it’ll only encourage more to follow suit and jump in.

There’s no doubt a KA that has been given the once over will perform better than box stock but in my experience most of these motors run competitively at the club level out of the box. Maybe things have changed as both of the motors I bought were pre covid, but other drivers at our track seem to be mostly pleased with a box stock motors too.