3TJ carbs on a 4JH hybrid

[plodder]

I have a spare 4JH engine and a spare 3TJ trackbike that I robbed after I crashed my hybrid a couple of months back, so I think you can see where this is going.

Now the 4JH engine is an odd one in that it has a 3HE generator/rotor/pickup and 3HE inlet rubbers.

I have examined the ignition assembly and have tested that it will run on a 3TJ loom with about the same ignition advance as the 4JH would give it, albeit the 3TJ has a 14k redline comared to the 13K of the 4JH, so this part of it is OK.

Now to the question of carbs, which is the main subject of this thread:
The 3TJ has 32mm Mikunis and the 4JH has 34mm Kehins (there is also the option of fitting 36mm Kehins off the thundercat to 4JH inlet stubs as well but I never got that to work very well on the road due to a big mid-range hole). My existing hybrid (with 34mm Kehins) does not really do much below 7K and then takes off (even managed to keep up with an early R1 on my last trackday).

So the question is would the 32mm Mikunis improve the bottom end without robbing too much of the peak power? I was thinking I might use one of the bikes as a road bike and make the other one a dedicated track bike, so the Mikuni carbed one would be for the road.

If these will do the job then does anyone have experience of the jet sizes to use for this combination? I've got the 3HE sizes to use as a base line but only got 3TJ jets, which will be too small or FZR1000 jets, which will be too big. Obviously I can suck it and see or get a dyno setup but I'm not going to pay out for a dyno and a bit of experience may save time on the initial setup. and wasting money on the wrong jets.

If I can get some 3HE carbs cheap then that may be useful. Anyone got any?

Ta

[Evilchicken0]

I'd just look for a set of 4JH carbs. I'm sure what your suggesting is possible but it sounds like a world of pain to me.

[plodder]

I've already got a set on my other bike and a set of thundercat ones on my spares too. What I don't have is any 4JH or thundercat inlet stubs.

I don't really see it as a world of pain because it's only a bit of jetting and I did it well enough on my existing hybrid by feel.

Any technical onfo out there would be appreciated.

[Evilchicken0]

True .....
http://cgi.ebay.co.uk/YAMAHA-FZR600R-4J ... 5634d26143" onclick="window.open(this.href);return false;

[plodder]

Ta for that Evil... I just bought them to give me another option of using the t'cat carbs on my track bike and leave my road bike as it is.

Just need some cheap t'cat or foxeye headers now.

[sparkles]

I have a 4jh (foxeye engine in a 3he frame with 3he complete exhuast system even standard end can

[plodder]

Did the 3HE exhaust need any mods to fit? I seem to remember someone talking about heating & bending sime time ago

[Marb]

I theory, smaller inlet ports give you better midrange. Im my 3HE hybrid I'm running 115 jets with K&N dropin filter, open exaust and exup.

[sparkles]

I have a spare 4JH engine and a spare 3TJ trackbike that I robbed after I crashed my hybrid a couple of months back, so I think you can see where this is going.

Now the 4JH engine is an odd one in that it has a 3HE generator/rotor/pickup and 3HE inlet rubbers.


I have examined the ignition assembly and have tested that it will run on a 3TJ loom with about the same ignition advance as the 4JH would give it, albeit the 3TJ has a 14k redline comared to the 13K of the 4JH, so this part of it is OK.

Now to the question of carbs, which is the main subject of this thread:
The 3TJ has 32mm Mikunis and the 4JH has 34mm Kehins (there is also the option of fitting 36mm Kehins off the thundercat to 4JH inlet stubs as well but I never got that to work very well on the road due to a big mid-range hole). My existing hybrid (with 34mm Kehins) does not really do much below 7K and then takes off (even managed to keep up with an early R1 on my last trackday).


So the question is would the 32mm Mikunis improve the bottom end without robbing too much of the peak power? I was thinking I might use one of the bikes as a road bike and make the other one a dedicated track bike, so the Mikuni carbed one would be for the road.

If these will do the job then does anyone have experience of the jet sizes to use for this combination? I've got the 3HE sizes to use as a base line but only got 3TJ jets, which will be too small or FZR1000 jets, which will be too big. Obviously I can suck it and see or get a dyno setup but I'm not going to pay out for a dyno and a bit of experience may save time on the initial setup. and wasting money on the wrong jets.

If I can get some 3HE carbs cheap then that may be useful. Anyone got any?

Ta

I found the 3he stator (gen) cover didnt fit the 4jh engine very well and did best to make it fit but it would catch on something and stop the engine turning over

To get the 3he stator (gen) to fit the 4jh casing you have to drill extra holes but i ended up getting new casing for the 4lh engine from local yam dealer for £70 (last yr)

I have some spare 3he carbs but there not complete and sat outside for a year

I have stripped them since and cleaned them but need some work on them to free the throttle bit were cable connects also some parts replaced in the float chamber

[sparkles]

Did the 3HE exhaust need any mods to fit? I seem to remember someone talking about heating & bending sime time ago

No mods needed and everything lined up just like it did with the 3he engine

Some of the 3he engine cases fit the 4jh engine but there are some parts that are different like the clutches are different amonst other things

front sprockets are the same

I replaced the 3he rectifier with a R6 one as the 3he is known for cooking the eletrics like it did with my fuse box.

The R6 one is bigger and has cooling fins unlike the 3he one butr you will need to a little bit of re wiring to get it to work as the 3he one has 4 pins and the 4lh one has 5 pins

It was so easy to do that i reckon a 12 yr old would have no problems doing it