'99 Zip RST back fire/loosing power.

Yesterday riding along the dual carriage way under full throttle at max speed (approx 55mph) my Zip started to bog down/loose power/back fire. So i pulled up in a layby and turned it off. Checked the fuel and oil levels both fine, it didnt seem over hot either. So i started it up again and let it idle, no odd noises and seemed as sweet as ever. Giving it a rev on the stand it seemed clean and free so i got back on and continued my journey.
30 seconds later and once back up to speed again same thing, so i let it slow down, and revved it back up again, all fine as the rev's climbed but once at full throttle same story, backing of the throttle a little no problem..? Plenty of power to pull up hills etc... cratch:

I fitted a PHVA 17.5mm carb from Pedparts a few weeks ago and its been great, up jetted to a 82 from the 70 that i was running in the 13mm standard. Its also running a TNT cone filter, Mallossi carbon reeds, a tournari-moto exhaust, 4.2g rollers.

It hasnt missed a beat until yesterday so any idea's would be greatly appreciated :good:

 

The 17.5mm carb came with a 65 jet in it, so i dunno where you get your 53 from?? And also its done over 200 miles since i fitted it and has been FINE it was several miles into the journey when it started...? :blink:
The air filter will be replaced at the same time as the 70 corsa kit an S6 Pro pipe go on it until then its staying put because i know that something is better than nothing... :huh:

 

Ok i get what your saying that, metaphorically, im basically trying to make it run before it can walk... And as i have stated other stuff is coming, in the not to long term either...

I do fully understand the mechanics and principles having been in my current job as a Heavy Plant engineer for more than a decade, however i have a lack of experience in the field of scooters as this is my first foray and rely on knowledgeable people like you to give me things to test and try, not diss what im trying to achieve.

However with all respect, i find it difficult to fathom that a larger jet in a bigger carb should suddenly cause the described fault, especially when these parts have all been working perfectly previously? I was thinking more like coil break down, a stator fault or similar?

 

Now that was the kind of answer i was looking for, thank you. So you think that a smaller jet in the larger carb should solve the issue, sounds arse about face to me but ill take you on your word and try it tmrw. I was always taught more air+more fuel= bigger bang or does that principal not apply to 2stroke...?

The smallest jet i have currently is the one that was supplied with the carb and is a 65, do you think that will suffice or should i purchase smaller?

Reading previous threads it suggests that its better to go bigger and run slightly rich/cooler than smaller leaner/hotter? Will altering the plug number from a BP6ES to BP9ES help as well or should i leave it with the hotter plug in...?

 

Right update, jet changed for a 65 as suggested and the problem has got worse... Ive also run it with no end can on the pipe as it was pointed out by a mate that could be blocked, still the same problem...

Any further idea's?

 

i down jetted it as thats what i thought you were getting at when you said " a much too big main jet hence problems at full throttle" And i bodged the original air box back on as i have replaced the angled inlet with a straight one and it doesnt quite fit anymore, but it seals, and still the same problem, if anything its getting worse, i have less than half throttle now before it bogs and it now also cuts out, still starts and idles ok tho??

 

i might just knock this on the head then until i can afford the corsa kit and S6 pro pipe, then will the caarb and open filter be suitable?