Lack of acceleration?

[speedy king]

Hi there,
Haven't been posting for a while due to moving house and we still haven't been connected so still don't have a land line phone or the internet!

Anyway, few weeks ago at my last race i was suffering with a really weird problem which ruined my day and caused a very nasty accident. Basically it's a FR125 Rotax Max (Junior) engine and it kept just not accelerating! For example i would come out of a corner, go on the throttle, it would miss fire really badly for maybe 3 or so seconds and then accelerate! Losing loads of time. It would do this about 4 times a lap and at some non-flat out corners i would actually be flat on the throttle as it just wasn't accelerating. We think it's electrical, we changed the battery, that made little difference. The loss of power was mainly happening under acceleration however two or three times it happened half way along the biggest straight where it de-accelerated so much i felt my body almost over taking the kart and my uper body being forced forward.

We changed the battery, cleaned the jets, checked the carb, changed the battery and am totally baffled!We're going to take it up to the circuit during the week and try it again by replacing the main electrical unit black boxy thing (great technical knowledge i have!!!) attached to the engine.

Does anyone have any ideas what this is and why it's happening? Has it ever happened to you?

Thanks,
James

[Revracing]

James, I have a rotax max (senior) myself, I don't race karts any more but I still use it from time to time, anyway, in the past I have experienced hessitation on acceleration, not as servere as you describe tho, what we found was that the engine was running too lean, the carburation on these engines is very critical as i'm sure you are aware.
What I would suggest is that you inspect the fuel pump for diaphagm failure or a split pulse tube, also make sure you were running the correct jet size for the atmosperic pressure and temperature of the day.

One other thing, the extreme slow down on the straight may have been due to a partial seizure (from over lean running), I would suggest you take the head and barrel off and check. (this should'nt break the seal as it is located on the crank case, on my engine at least)

[speedy king]

Thanks for that You know alot more than i do about these engines! I'll check that out over the weekend

[1200Datto27]

I had the same sort of issue with my race car, and after chasing down and replacing all of the electricals, it turned out to be the throttle arm was slipping on the spindle, allowing the throttle butterflies to vibrate in the bodies. Have a quick check to make sure that the throttle moves with the pedal.

[BoatOwner]

Check your floats.

[Sirio]

Sell the Max and buy a real kart engine.

[AlesiUk]

Depends on whether its electrical misfire or not i guess. It could be something like fuel surge,i would check that the fuel line isnt kinked anywhere,or even check that the fuel pick up isnt dirty or damaged. Id think that if it was simply a case of running too lean top end speed would be affected first,and most likely the engine would seize totally.

If its electrical,experience tells me that is best to replace everything if possible. Otherwise you can spend a day chasing your backside trying to figure out which component or wire is causing it. Worth trying changing the plug lead and cap,there are sometimes a bit cheap and prone to becoming faulty

[speedy king]

Turned out it was the coil. Main electrics part, once it got hot it started to miss fire and not give enough power to the spark plug, normally happening from about lap five onwards. There goes another £140...... + normal stuff like tyres needed for the next round

[Alex Hodgkinson]

Originally posted by Sirio
Sell the Max and buy a real kart engine.

HAHA