Topic: Drill your own jets ?

[themanxman]

Hello there everybody.
Nearly finished building a CB650 cafe racer and struggling to get it running properly. I think realy it needs bigger jets. Has anyone got experience of drilling out their existing ones ? or do I buy some at £60 ? It is a 1980 engine with Keihin 51a carbs, a not to noisy Motad 4 into1, Cheap pod filters which I have lined with additional filter foam. Float heights are correct (checked with PVC tubing fuel is at the top of the bowls), air screw is out 1 3/4 turns, carbs are in sync (checked with a cheap vacume gauge which fluctuated wildly but they were all in the same region). carb internals are clean and I`m using DR8ES plugs.
The engine runs at idle but wont return to idle when revved, some popping from carbs, It will accelerate ok but not as strongly as I would expect. Plugs are comming out brown. It`s burning some oil but thats probably from the newly honed barrels.
What say you, good people of the SOHC world?

[70CB750]

How are your advance springs?  I had to shorten mine to make them snappy again.

Just a thought;)

[lucky]

Hello there everybody.
Nearly finished building a CB650 cafe racer and struggling to get it running properly. I think realy it needs bigger jets. Has anyone got experience of drilling out their existing ones ? or do I buy some at £60 ? It is a 1980 engine with Keihin 51a carbs, a not to noisy Motad 4 into1, Cheap pod filters which I have lined with additional filter foam. Float heights are correct (checked with PVC tubing fuel is at the top of the bowls), air screw is out 1 3/4 turns, carbs are in sync (checked with a cheap vacume gauge which fluctuated wildly but they were all in the same region). carb internals are clean and I`m using DR8ES plugs.
The engine runs at idle but wont return to idle when revved, some popping from carbs, It will accelerate ok but not as strongly as I would expect. Plugs are comming out brown. It`s burning some oil but thats probably from the newly honed barrels.
What say you, good people of the SOHC world?

Fuel level should NOT be at the top of the bowls!!!!
Just measure the float level like the book shows.

[BobbyR]

The fuel level is generally just a bit below the float bowl gasket. Not at the gasket, at the lower flange. That may not be your problem, but it can become a problem.

[oldschoolcarbs]

To answer your first question, there is nothing inherently wrong with drilling jets. But make sure that this is what's called for before you jump.

Just don't do it with the carbys installed. (Rim shot) LOL.

And don't enlarge them more than two sizes or you'll upset the geometry of how petrol exits the jet upwards into the emulsion tube. It needs to fan out properly.

Aside from that, drill away. Do an internet search for metric "jobber drill bits." A #100 main jet orifice is 1mm, a #120 is 1.2 mm, and so on.

[phil71]

you put foam INSIDE the pods? Don't you worry about flaking and sucking it in?

[dave500]

if you drill jets file the original size marking away and try and scribe the new one,save someone else a #$%* load of trouble a few years from now.

[scunny]

nothing wrong with drilling them to put you in the ballpark then buy a couple of sets for fine tuning. I have a 79 650 with the same carbs, running pods and reasonably straight thru 4into 1. happy jet size turned out to be 125. dont pay that much, order them from Z1 enterprises.
your hanging idle and popping sound like a leak between carbs and head, how are the rubber manifolds and clamps ?

[Bailgang]

if you drill jets file the original size marking away and try and scribe the new one,save someone else a #$%* load of trouble a few years from now.

+1 on that.

[scottly]

Standard twist drills should not be used to drill jets, as they do not make an accurate hole. Each jet may flow a different amount of fuel. Jets should be reamed, not drilled. It is acceptable for experimentation, as Scunny said, or when applied to a single carb, as with Harleys.

[themanxman]

Thanks chaps. Plenty to go on there.
 Manifold rubbers may be suspect, they are very hard and the jubilee clips (or whatever you call them where ever you are) may not be  nipping them up tight enough.
I adjusted the floats as per the manual and that is the height i got. I will try loweriing them anyway.
I`m pretty sure the foam in the pods is staying put, If not I`l soon know about it !
The sliders seem to snap back down well.
I`l hold back on the jet enlargement for now, until I`ve had a play.

[jdubyaj]

Spray some carb cleaner on the manifold rubbers. If they leak  your idle will jump.

[themanxman]

Good tip. I was going to carefully duct-tape around them but that sounds easier.