poor top speed

[peter Whitwell]

Hi every one, help needed if possible, bike runs lovely up to 60Mph , starts lovely, revs freely, but after 60 it seems its no go, if you wind it open it splutters and does not want to go faster ease the throttle back it settles down immediately running smoothly again but not wanting to go more around 60 -65 tried a new needle in carb, lifted the needle, no difference- just seems to want to stick to speed limits lol, Any suggestions?

[ukpco]

Just like mine - see the current bhp thread. The lack of power is so frustrating but I haven't found the cause yet.

[hendred]

What air filter are you using?  The Ramair filter recommended by Paul seems closest to the original.  I use a genuine K&N cylindrical filter with good results (wasn't aware of the Ram Air filter when I purchased it), people have had poor results with the conical filters often called K&Ns but I suspect they are not genuine ones.  Use the search function to get the details of the filters.  Check the main jet for correct size for your carburetor.  The choke/enrichment valve has caused problems - check it has a good rubber seal.  Have you checked the silencer - the internals have been known to detach and restrict flow and I found replacing the wadding at the end of the silencer made a big difference to performance.  Have you checked the fuel flow to the carburetor?  Blocked filters or passages in the body will restrict fuel flow at higher throttle openings but let the bike run ok at lower throttle openings.

Finally will the bike go faster and rev higher in the lower gears - the max speed in third is quoted at about 90mph at 6000 rpm (depending on gearbvox sprocket) in the manual.

Hope this helps.

[hendred]

Had a similar problem at the weekend, not on a Silk though.  It was caused by dirt blocking the main jet at  large throttle openings.  Ran ok up to half throttle.  Do you still have the gauze sleeve around the main jet?  Might be worth checking it isn't partially blocked with dirt or shellac from old fuel.

[hendred]

Had a similar problem at the weekend, not on a Silk though.  It was caused by dirt blocking the main jet at  large throttle openings.  Ran ok up to half throttle.  Do you still have the gauze sleeve around the main jet?  Might be worth checking it isn't partially blocked with dirt or shellac from old fuel.

[peter Whitwell]

Thanks for your help so far, any more suggestions are welcomed, but i will have systematically work through these items, unfortunately carb seems spotless,  and air filter seems to be as your recommendations, choke works well, I've lifted the needle position ----

[peter Whitwell]

As mentioned I'm using ram air filter, spark plugs seem to be very clean suggesting to me lean mixture is this correct or do i need to raise the needle yet again?

[hendred]

You may have said earlier what carb you have on your Silk but I can't remember if you did.  If you post the details here giving the jets, etc. this can be compared with the settings on a bike which is running ok. It may be helpful.  

[Smokey_Paul]

Absolutely be very careful and use the stock jetting for your carb and B8HS plugs. They should run chocolate brown with the oil burnt colour if white something is not right. Add a picture its worth a thousand words

[peter Whitwell]

Hi thanks for your help in advance,
R2036-- 302

Needle jet 108
main jet 480
pilot jet front 25
pilot jet side 50

throttle valve is 2036 35

2036 063 needle,      position 4 of 5 groves

spark plugs champion L4G

Plugs are clean

[peter Whitwell]

Hi paul tried up load images but i can't get the site to accept image it says to large but its only taken on an i phone

thanks in Advance

Champion L4G very clean burn - if it was on a four stoke i would suggest perfect but unsure as to what two stoke should look like but certainly not oily.

[hendred]

Your carb jetting is close to the settings in the Technical section.  Reading this thread again I thought I would throw in a slight curve ball.  The symptoms you described have been seem on bikes with a weak magneto.  The high combustion pressure at higher throttle openings snuffs out the spark, close the throttle and the spark recovers.  What ignition system are you using?  The original Lumenition or a replacement?  Are the coils the original Lucas or replacements? Plug leads and caps in good condition?  My Lumenition system is sensitive to low battery voltage, are all the electrical connections good and clean?  

Just some thoughts

[Smokey_Paul]

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