Two stroke bike lack of power?

[Paul223]

Correct points setting and timing done with a dial gauge and test lamp

 

Carb stripped and ultrasonic cleaned in carb cleaner then blown out, jets are only half the story

 

Exhaust fitted and sealed correctly ie not blowing and with the correct baffles fitted

[TriBsa]

"seems lke most stuffs covered, have you checked the fuel tap is flowing properly and the cap breather is clear ??"

 

 

Good basic point, often overlooked. Ethanol in todays fuel can often destroy tank sealers leading to blocked taps/tap filters.

 

Is the float in the carb set correctly, is the float sinking due to a hole?

 

It needs a methodical approach of going through the ignition system and fuel system step by step. You'll get there eventually and it'll probably be something blindingly obvious and simple (with hindsight).

 

You say you have returned everything to standard spec carb wise, but you have a Micron exhaust (expansion chamber). I'd be inclined to put the standard exhaust back on till you find the fault. Expansion chambers usually require jetting changes to get the engine running right and it's easier to fault find from a known baseline i.e. stock parts and settings.

Edited November 14, 2014 by TriBsa

[VULTURE]

OK will try some of your suggestions re test lamp,carb cleaner etc,

I don't think its the exhaust as I tried it with just a down pipe,made no difference apart from a racket.

I've checked the slide in the carb and it does open fully,fuel flow is good,cap is clear.

Compression is good,spark plug is clean and gapped to spec.

I don't understand why its fine up to 1/3 throttle but beyond this,doesn't misfire or pop and bang just doesn't go any quicker?

Have also checked gearing it's standard.

Cheers. Vulture.

[VULTURE]

Did a plug chop last night,it was a milk chocolate brown colour so the mixture is fairly good.

[Paul223]

Jetting and using a micron wouldn't have any short term effect, you'd be able to get the engine to pull through rev range and it'd certainly feel a bit peaky, extended use could well end up melting a piston if the fuel air ratio was incorrect but the only way to determine fuel air is to do a plug chop.

 

If the carb jets have been changed then this could also be a problem, some kits are bad for incorrect jet sizes, are the jet standard?

 

I may have a carb you could borrow to double check

[TriBsa]

Have you used a timing light and seen that the ignition is advancing when you rev it?

[Paul223]

And does the timing need to be in the advanced position when being set?

Edited November 14, 2014 by Paul223

[VULTURE]

Hi. It's not got any form of advance retard system on the bike.

Paul223 I will borrow that carb if I can't get it sorted over the weekend.

Cheers

[FatFreddysCat]

Check the exhaust port isn't narrowed with carbon deposit. Should be able to check this by removing the exhaust and shining a torch up the port. If not you'll have to remove the barrel to check it.

[Rupert]

Has it ever run right in your ownership?

 

You say new micron,put on by you? did it run right before(I mean seen by you and not he said so)

 

Is it the right exhaust for the bike?

 

Spark plug colour spot on,..... you cant do a plug chop without getting full chat.

 

Will it rev out/clear in any gear? on the stand?

 

You mention rebore rebuild etc. have the bike ever run right since (in your presence)

Edited November 14, 2014 by Rupert

[TriBsa]

Hi. It's not got any form of advance retard system on the bike.

Paul223 I will borrow that carb if I can't get it sorted over the weekend.

Cheers

 

It must have, or it wouldn't rev out......

Edited November 14, 2014 by TriBsa

[fat_jay]

Had the same issue with one of mine years ago, it was the HT lead breaking down, symptoms exactly the same as yours.

[VULTURE]

Hi. Bike has not got an advance retard fitted,the points are fixed to the stator and there are no Bob weights or any moving parts to alter timing.

I don't think small two stroke have it fitted, my fizzy and KH250 didn't have it.

Cheers. Vulture

[VULTURE]

Rupert, I bought the bike off my son who owned it for two years,it has never performed as it should but has been reliable and my son wasn't bothered about the speed as he only used in the city for work and back,I bought it for nostalgic reasons as I had a new one years ago.

I rebored the barrel and replaced the crank seals thinking this was the problem but it made no difference whatsoever.

The micron is a replica I made copying one my mate has on his bike.it performs no different with either pipe on it,the standard one is rusty but it's not blocked.

It will Rev out in neutral and in first gear it will Rev but not pull as it should.

Today I'm going check the ignition coil,leads,new spark plug etc.the higher you Rev the engine the less power it seems to produce which does point to a lack of advance as TriBsa points out!

 

I'm confused now.

Vulture

[Rupert]

then in inclined towards an ignition problem.

[Rimfireboy]

Vulture, you say the carb isn't worn. That doesn't mean the jets aren't blocked or partially blocked. Have you removed the jets and checked that they're completely clear?

[Paul223]

I'll say again jets are only half the story, it needs to bleed air too......

 

It does sound like a carb blockage or ignition breakdown, can you cobble the coil and HR from the fizzy to try it out?

[Rimfireboy]

Vulture, you say the carb isn't worn. That doesn't mean the jets aren't blocked or partially blocked. Have you removed the jets and checked that they're completely clear?

[Hubris]

I know boats not bikes, but that sounds like air starvation not fuel as your plugs would start to show symptoms of running too lean. Ignition coil breakdown can have similar sympoms but I would expect to find a wet or rich plug in the problem cylinder. I'd go simple and with the filter off check that the butterflies are fully opening at full throttle. Also check that the butterfly for the choke is fully open. Also check the linkages for both of these as (from cold, wet, everyone is overtaking me and I want to go home now experience) its too easy to manipulate the valves at the carb when checking them and never notice you've forgotten to re connect the choke linkage /set throttle cable too long / or disconnected linkage bars between carbs if you are running more than one! As mentioned above, could be sticky/broken reeds but I wouldn't expect such a loss of power.

 

Good luck,

 

H.

[neil82]

if the plug is `milk chocolate brown` then to me its running too rich, drop the needle if possible(stick circlip in higher groove) and do another chop, it sounds to me that's its not running right on the main jet, which may have been changed when the micron was fitted

[Paul223]

Milk chocolate brown to rich?

[Paddy Galore!]

Hi Vulture, I had an old yb100 about a century or so ago, I had the same problem due to coil break down under load.

[bornfree]

It might be worth checking the rubber inlet manifold for cracks.

[RedRobin]

If autolube rather than premix make sure carb balance perfectly with lube system when opening throttle. Theres normally a drilled 'dot' on the carb throttle slide that has to be aligned with the oil pump

Is carb Pilot and Main jet at standard size, Throttle slide needle set mid point?

 

Rebuilt hundreds of 70's 80s 90's air and water cooled 2strokes, easy to work on, Can be awkward to fault find

Edited November 14, 2014 by RedRobin

[hodge911]

had to put my thinking cap on here .............

 

many many moons ago I had a bultaco Sherpa trials bike that I bought with a popped piston any how head off barrel of and away to local engine rebuilding company [ Newcastle cylinder services] who new their stuff .

re-bored new piston / rings / base and head gasket new oil and plug put back together and started third kick did rev but not past 1/4 - 1/2 throttle changed fuel checked plug wasn't oiled up again second kick started ticked over spot on that's when I started to think I,d bought a duffer checked all of the above timing points etc carb slide blah blah so sfter talking to an old boy who had a bike shop on westgate rd in the toon [who turned out to be "Sammy miller"] changed the NEW plug for another one and hey ho FIXED !!!!!

not saying that's what wrong with yours but it is not unusual a duff new plug for what it costs I,d change it to rule it out