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Rough idle when hot


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The idle on my daily driver is steady at 800rpm until the engine is very hot, say after 30 minutes driving on a warmer day. It then idles ok for maybe 10-20 seconds when it will drop to 500-600 before recovering a bit and range between 500 and 700 and rough. It doesn't stall but is sometimes close to it.

TRs are not very good idlers when very hot but this was not acceptable.

The problem sounded like a rich cut and fouling plugs. I balanced the carbies, they were very close to spot on, and checked the mixtures. They were both too rich, particularly on the rear carbie.

I adjusted the mixture which did improve the idle but I ran out of lean adjustment on the rear carbie before I got it how I wanted it.

I checked the float level using a 7/16" rod under the fork as per the manual. It looked ok but I lowered the float maybe 50 thou to lean the mixture a bit. The idle is now pretty steady at 700-800 with a occasional drop for a second or two. The carbie adjustment nut is full up.

The only thing I can think of is the needle and jet could be worn. The carbies were brand new about 30,000 miles ago and I would have expected them to last a lot longer.

What are your thoughts?

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Check needle heights and that they are definitely the same needle, adjust the return springs on the shafts until they give the same pull back..

Stuart.

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Perhaps the needle in the rear carb is located a little too far into the piston?  I don't think playing with the float height does anything much to the mixture provided the fuel level is below the top of the bridge. The adjustment isn't critical. 

 

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Thank you. I'll do a check later this afternoon.

I drove it this morning and the idle was good, although they were only 15 minute legs.

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19 hours ago, John McCormack said:

The idle on my daily driver is steady at 800rpm until the engine is very hot, say after 30 minutes driving on a warmer day. It then idles ok for maybe 10-20 seconds when it will drop to 500-600 before recovering a bit and range between 500 and 700 and rough. It doesn't stall but is sometimes close to it.

TRs are not very good idlers when very hot but this was not acceptable.

The problem sounded like a rich cut and fouling plugs. I balanced the carbies, they were very close to spot on, and checked the mixtures. They were both too rich, particularly on the rear carbie.

I adjusted the mixture which did improve the idle but I ran out of lean adjustment on the rear carbie before I got it how I wanted it.

I checked the float level using a 7/16" rod under the fork as per the manual. It looked ok but I lowered the float maybe 50 thou to lean the mixture a bit. The idle is now pretty steady at 700-800 with a occasional drop for a second or two. The carbie adjustment nut is full up.

The only thing I can think of is the needle and jet could be worn. The carbies were brand new about 30,000 miles ago and I would have expected them to last a lot longer.

What are your thoughts?

Hi John,

was it really a 7/16 rod !! on the TR4A HS6 carb it uses a 3/16" rod (I think)

 

Roger

 

     

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2 hours ago, RogerH said:

Hi John,

was it really a 7/16 rod !! on the TR4A HS6 carb it uses a 3/16" rod (I think)

 

Roger

 

     

Yes Roger. The TR2 manual with TR3 supplement says 7/16".

I adjusted the needles today. They were both set to be level with the needle mount which is recessed slightly in the piston. . I set them both to be level with the bottom face of the piston. That leaned them both so I now have some adjustment on the mixture nut on both carbies. Not as much on the rear as the front but some.

It idled and drove very well, it seemed to have a bit more power and revved freely. A drive tomorrow should reveal if it has solved my idling problem.

 

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