I've fitted a 4S cam in my otherwise standard MkII 850 engine. The engine works brilliantly except when dropped quickly to idling, it tends to stop running. It's also a bit hard to get started when hot. Does anyone have experience about that? Is it a feature or could it be setted somehow?
Norvil 4S cam and idling problem
- Thread starter Mr Finn
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Are you running standard compression. I would think that this cam should have a higher ratio. Check the plugs.
Please get more opinions but I suspect richness and therefore you may have to lean it up a bit. I assume you are running Amals. Drop the needle, leaner needle jet, leaner main jet and if worse come to worse, punch out the pilot and use a screw in pilot jet. One item at a time.
All that being said, I don't suspect you will ever get a perfect smooth idle with that cam, You will get one heck of a cammy idle if you like that sort of thing of which I most certainly do.
Anyway, I'm thinking compression needs a bump.
Please get more opinions but I suspect richness and therefore you may have to lean it up a bit. I assume you are running Amals. Drop the needle, leaner needle jet, leaner main jet and if worse come to worse, punch out the pilot and use a screw in pilot jet. One item at a time.
All that being said, I don't suspect you will ever get a perfect smooth idle with that cam, You will get one heck of a cammy idle if you like that sort of thing of which I most certainly do.
Anyway, I'm thinking compression needs a bump.
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STANDARD ??
Hymph ?
As the sucker appears to be a 2S with 10 thou off the base , some things are overstated .
2S in my 750 , 8.7:1 C.R. , would sit idleing at 500 rpm all day .BUT it had Mk 2 Amals ,
AND a BSA Auto Advance , giveing 30 deg retard at set at 31 fully advanced .
And W&S springs etc , set right . At .016 clearances
Knactually I think there was an advantage running that clearance .
SO , Id fit the 30 deg Adv Unit , if your ' STANDARD ' :shock:
Though Jetting , from the top down ( main ) on 100 Octane could possibly almost get a set of mk 1s working almost right , if theyre still new . :lol:
essentially , thrashing it , they wernt to worried about the coughs & hiccups , as there was less time for them to intrude / develop .
Id also check the ' overlap ' period .As In , where the open / close overlap as per T.D.C. , degrees differant ( forward , about 3 - 5 degrees ?? )
Hymph ?
As the sucker appears to be a 2S with 10 thou off the base , some things are overstated .
2S in my 750 , 8.7:1 C.R. , would sit idleing at 500 rpm all day .BUT it had Mk 2 Amals ,
AND a BSA Auto Advance , giveing 30 deg retard at set at 31 fully advanced .
And W&S springs etc , set right . At .016 clearances
Knactually I think there was an advantage running that clearance .
SO , Id fit the 30 deg Adv Unit , if your ' STANDARD ' :shock:
Though Jetting , from the top down ( main ) on 100 Octane could possibly almost get a set of mk 1s working almost right , if theyre still new . :lol:
essentially , thrashing it , they wernt to worried about the coughs & hiccups , as there was less time for them to intrude / develop .
Id also check the ' overlap ' period .As In , where the open / close overlap as per T.D.C. , degrees differant ( forward , about 3 - 5 degrees ?? )
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- Jan 20, 2011
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I have the 4s, and yes it does idle a bit lumpy.
Was much lumpier with the old Amals than the new PWKs.
I had the problem of the engine wanting to stop at the lights, after hard braking.
I tried alsorts of things from stayup floats to float level to idle jet modification etc.
Then rebiult, sleeved carbs.
I always had the suspiction it might be a little too much cuttaway, for the 'ordinary' idle circuit.
Not sure if it was a problem back in THE DAY, and if it was, what they did about it,
but new carbs fixed my problem.
I like my engine to idle at 1200, just for peace of mind, but will idle (lumpy) at 600.
Mine is easy to start, hot or cold.
AC.
Was much lumpier with the old Amals than the new PWKs.
I had the problem of the engine wanting to stop at the lights, after hard braking.
I tried alsorts of things from stayup floats to float level to idle jet modification etc.
Then rebiult, sleeved carbs.
I always had the suspiction it might be a little too much cuttaway, for the 'ordinary' idle circuit.
Not sure if it was a problem back in THE DAY, and if it was, what they did about it,
but new carbs fixed my problem.
I like my engine to idle at 1200, just for peace of mind, but will idle (lumpy) at 600.
Mine is easy to start, hot or cold.
AC.
SteveA
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- Dec 20, 2011
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Mr Finn said:
Mr Finn, please don't think I am intending to be rude, but I am inclined to ask what is your objective with the use of this cam in a standard engine in a road bike.
Most might also say that the standard cam is good in the application and the 4S is not suitable and is a race cam only, so it is interesting that you are pleased apart from the idle/cutting out.
Are you intending further mods later? meaning the compression and porting this cam needs to work properly, if so you might best do them before spending too much time sorting carbs and ignition for this setup only to change it again.
Just my thoughts.
Steve
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