BJ8 jet drop measurement

[Hoghead]

I see different references on the jet drop relative to the bridge on BJ8 carbs

How does 2.25 - 2.5 turns relate to .0XX??

 

[EV2239]

Use a vernier gauge and set them 75 -80 thou down.

 

[Hoghead]

2.5 turns is .080 so will give that a go

It ran great until I adjusted it........

 

[Randy Forbes]

I wouldn't put too much weight into what the specific value is; the intent is to begin the tuning process with both carbs set the same__note, they may not end up that way, depending on the individual cylinder's demands.

You'd have to reside in a better than Perfect World to achieve the optimum mixture strength with such rudimentary settings. Volumes have been written on the subject, so a one (1) sentence catch all isn't going to get you there (it's merely a starting point).

 

[Patrick67BJ8]

Also see" Color-tune for BJ8 Carbs" last week.

 

[Hoghead]

Also see" Color-tune for BJ8 Carbs" last week.

Doesn't works so great when one is colour blind!

 

[ozhealey]

Hoghead,

This is always a hot topic with as many opinions as there are owners of classic carbs. I tend to agree with Randy in so far as being a start point from where to start.

I recently had a challenging issue with fuel levels after a professional rebuild of my carburettors. I found the company had replaced my original floats and fuel lids with the type with the non adjustable white plastic floats. I descovered this when I looked down into the jet in the carb bodies to find one had fuel at the top of the jet when set to the start level of 12 flats down and the other had no sign of fuel until I dropped the jet to as low as I could by rotating the choke cam.

I went to "set" the fuel levels in the bowls to find the new floats which need washers under the valve seat to change the fuel level. Anyway, I decided to install the new black floats which can be adjusted. I set the floats to the same level by the book and even then the fuel in the carb jet was at quite different levels. Setting and synchronising the carbs and getting the same mixture level was a bit of a challenge.

I then was advised by a guru to adjust the fuel levels in the bowls to achieve the same "height" of fuel in the jets. He suggested around 1/8 - 3/16 inch from the bridge. (In other words lower the jets to around 4mm and set the float height in the bowls to have the fuel just at the top of the jet when in this lowered level). I did this and the results have been brilliant, even though the height of the fuel is different in the actual fuel bowls.

I know your HD8 carbs dont have the same external fuel bowls, however all I am suggesting is you could adjust the fuel levels to have the same (or at least close) levels of fuel in the jets.

While perfection is not important, I found this gave me a great start point.

I have rock steady idle and timing, great mixture (confirmed by nice light brown plugs all six the same and a nice Bunsen blue with the colour tune for each carb) and beautifully synchronised throttles. The car runs great through the entire rev range, no backfiring, popping and absolutely no run-on when the car is turned off.

Happy days.
Cheers
Tony

 

[EV2239]

The plastic floats set with a 1/8" drill under them and across the lid face.

All SUs were set with main jets 35-38 thou down except HD8 with the bigger needles and jets. They were 75-80 thou and usually, given everything is to factory spec, that's where they work best and fiddling doesn't help.

i hope this helps.

 

[Hoghead]

The plastic floats set with a 1/8" drill under them and across the lid face.

All SUs were set with main jets 35-38 thou down except HD8 with the bigger needles and jets. They were 75-80 thou and usually, given everything is to factory spec, that's where they work best and fiddling doesn't help.

i hope this helps.

Fiddling with the mixture jet did not help and everything is happy at .079 which is as close to .080 as I could be bothered to fiddle.

Interestingly though, the throttle butterflies closed, or what I think is closed, she will idle at about 700 with the large idle jet screw turned in all the way.
I settled on 2.5 turns out, then used the smaller of the idle screws to idle at 1000

Runs great and off do do some plug chops