Triumph Rat Motorcycle Forums banner

Proper jetting for 1973 T140 V, 750

5K views 8 replies 3 participants last post by  rambo 
#1 ·
A little background, I have this nicely built 1973 750 Bonneville motorcycle. I added K & N filters and Norton peashooter style mufflers. They are essentially wide open.

Occasionally when I come across a nice hill I do a third gear wide open throttle roll on. Most recently I did this after installing the parts and the motor fell on his face at approximately 5000 rpm.

I let off the throttle a little and sure enough, as I backed off the bike shot forward. That tells me it's taking in too much air for the available fuel from the main jets.

It is my understanding the needle does not have any impact when at WOT as they are totally removed from the jets. In other words there is no overlap.

In any event I was considering an increase in jet sizing and here is the prob: the Triumph shop manual calls for 190 main jet. The parts manual calls for 210 main jet I pulled the jets on the carburetors and they are 190s.

I'm wondering if 190 is proper or if it is undersized for the machine? I am intending to start with a 210 main jet and hoping it will allow for proper acceleration. Any comments? Steve
 
See less See more
#2 ·
I let off the throttle a little and sure enough, as I backed off the bike shot forward. That tells me it's taking in too much air for the available fuel from the main jets.
Exactly, you are too lean

It is my understanding the needle does not have any impact when at WOT as they are totally removed from the jets. In other words there is no overlap.
I wouldn't use overlap. What you say is correct for an Amal, but not for a Mikuni VM.

I'm wondering if 190 is proper or if it is undersized for the machine? I am intending to start with a 210 main jet and hoping it will allow for proper acceleration
If you are truly at WOT when you roll the throttle back that is where I would start...
 
#8 ·
210 Jets Installed

I installed them and buttoned up the air cleaners and such.

I took it for a ride and found my typical WOT hill. Third gear and gradual acceleration from about 3k rpm until WOT. Zing, it went to 6,200 rpm without any hitch. It runs nicely at other speeds too!!

BTW, Rambo, I ran both taps and thanks for the reminder.

I don't know if 200 jets would be better or it this nails it. I'll monitor the plugs and adjust idle air fuel. Unless something becomes a real game changer, this looks like a real fix! :grin2:

Steve
 
#9 ·
200 mains do seem to be the correct mixture for mine,however,running a 210 might give a little protection from a weak mixture.If it runs good with a 210,i would leave it running.
Not suggesting this is the answer to the fuel flow problems at higher speeds.I made up this junction from copper tube and it sits between the two carbs.I made this so that fuel does not have to go through one carb to get to the other carb.There should be equal fuel if either tap,or both taps are on.The fuel bowl connections have to be changed to single entry ones.
Made from small bore copper heating pipe and been fitted for over 1 year so far and not fractured.Could be a risk if your soldering is not too good.Could also be made using fittings to avoid solder.
 

Attachments

This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top