Stutter at 3/4 revs??

[njhardy]

Well, after doing the plug chop on a new plug at high revs- which is fine, checking float tang height, checking the powervalve movement by removing the pipe and checking the limits to the valve opening, checking there are no air leaks in the system and no engine oil is being used, adjusting the oil pump, I still have a clear 'stutter' and lack of pull at 3/4 revs...

Initially good, clean pull from low down, the power builds, she becomes smooth as she gets into the power band with good pull (for an old girl with 100,000Km on the clock), then on the far side of the power band a distinct lag of power and splutter/cough to emerge into to tail end of the revs pulling quite cleanly to rev out...Each time I wind up through the revs, this seems to happen.

Have I missed something here?? Is there anything else I need to check??

[fallenmikethebike]

Where are you at with the needle jet position now?
2 strokes, so many things that can effect performance.
Reeds, exhaust, generator circuit, electrical gremlins, filter, fuel supply, et al.
Mike

[knackeredMk1]

What's your main jet and where is your needle clipped . What pipe have you got and got a nice oiled filter .

Like FMtB says it could be a lot of things but more likely to be jetting.

[Jeram]

100000kms of spooge in the expansion chamber?

but try the needle first

[knackeredMk1]

If the jetting is OK then Jeram could be right or the barrel might need a rebore.

[njhardy]

Thanks for your reply guys,
I recently totally cleaned out the front and tailpipe with new wadding- a graphite Procircuit front end and RSV offroad tailpipe. The plug was burning really dirty when I got her 6 weeks ago , so I took apart the carb and cleaned it- resetting the tang and cleaned the jets- the main is a 150. It still ran rich, so I have moved the needle down (twice eventually)- the clip is on its highest setting to keep it lean and its burning spot-on light brown now. The air filter is oiled with a bit of clean gearbox oil. The oil pump was also well out, so I reset that and the powervalve too. I know that other aftermarket pipes affect the jetting, so the Procircuit must do too. I am tempted to put a 145 main jet in. She revs out happily when not in gear, but when pulling my fat body around, she continues to stutter at 75%. Surely a knackered bore/piston would give seriously reduced power across the range??

[knackeredMk1]

Put a 145 in . After market pipes sometimes require leaning the jetting from standard so buy a 142 while you're at it. When you put the smaller jets in return the needle to the standard setting.

A worn bore might show up an imperfect jetting more than a good one.

[Jeram]

for checking your ring sealing/bore wear you will need one of these...

http://www.ebay.co.uk/itm/Automotive-Pe ... 539wt_1185

even better than a visual inspection :)


I believe this is identical to the one I used on my CRM... and i found it to be reasonably accurate...


But Im not sure if it could be compression, because that wouldnt explain it being flat at 3/4 throttle and fine everywhere else...
even with low compression, you should still be able to get her to run half decent with jetting changes.

how new is the spark plug?
have you noticed the powervalve malfuntioning at 3/4 throttle? does it flutter or do anything unusual at 3/4throttle?

this one has me baffled...
but try the jetting first before we go reinventing the wheel for you ;)

[back off road]

sounds very much a carb issue

[fallenmikethebike]

Gearbox oil on the filter , put some proper filter oil on a clean filter, and don't over do the oil, work it in sparingly.
Then run her again.
Mike

[njhardy]

Thanks again guys,

Yes, I will try and get some normal filter oil for the carb, but as I am Vietnam this is not just difficult- its impossible until I get back the the UK. Same with the compression tester- great idea and may well show me that I don't need a new piston/rings/cylinder. What kind of compression should I be getting?-About 140lbs?
I will try to source a 145 jet here (I assume they have a standard screw diameter, or is it dependent on the make of carb) and see if that makes any difference.

It does seem that if I back-off the throttle at 3/4 revs and let her clear herself, she seems to get back 'on the boil' again, so Im thinking jetting too....

[fallenmikethebike]

You could clean the filter, firstly with some paraffin, or a propriety oil remover.
Wrap the filter in clean newspaper for a day or so, if you have the time, the paper will absorb the residue cleaner.
If time is an issue, wash the filter in WARM soapy water, [Daz, fairy , persil whatever],this is what actually removes the dirt from the filter, rinse with copious clean water and allow to dry, or again if time is an issue, a gentle warming with a hair dryer will do the job.
Once fully dry,gently tap the filter a few times to shake out any stubborn grit particles,then work in some light viscosity maintenance type oil.
This is not ideal, nor really a replacement for the proper filter oil, but it's light enough to be invisible to the incoming air.
It will hang on to any crud that gets into the air box.
I would recommend a much shortened service interval if you do go this route.
You may also need to go one notch richer on the needle jet with the lighter oil on the filter.
I am at the same conundrum with my MK3, but have more or less settled on a bit less HP, in favour of longevity of service, as I only potter around Trail riding these days.
I hope this helps.
Mike

[back off road]

for the price i find 2 filters are the best bet ,use 1 while the other is been cleaned ,dried and oiled properly rather than scrimping and not cleaning it properly. once the spare is done pop it in a sealed poly bag until you need it

[fallenmikethebike]

for the price i find 2 filters are the best bet ,use 1 while the other is been cleaned ,dried and oiled properly rather than scrimping and not cleaning it properly. once the spare is done pop it in a sealed poly bag until you need it

Ditto.