Honda Twins banner

1 - 20 of 20 Posts

·
Registered
Joined
·
54 Posts
Discussion Starter #1
I found this in one of my carbs tonight; it's so solid that it looks like it never even had a hole bored in it (it is open at the other end and crossways in the middle, like the jet from the other carb). It's currently soaking so I can see if I can open it up tomorrow, but there's not some weird scenario where it's supposed to be this way, is there? This is the carb with the choke on it.

_20190915_185135.JPG
 

·
Registered
Joined
·
54 Posts
Discussion Starter #3
have a look at the pilot jet (PJ) on this drawing, and see if it isn't the same:
View attachment 290652
Huh. That is exactly what it looks like, I'm glad I asked. The pilot jet in the other carb does have a hole through the end, does that mean it's wrong, or is it actually supposed to have it? There's no context on that drawing, so I don't know if they're supposed to be different.
 

·
Registered
Joined
·
2,421 Posts
The carbs should be identical in all the important ways. The pilot jet gets fuel through the main jet. If the other carb is different, it is either not the same type, or someone has put the wrong jet in. The carb number should be stamped into it, somewhere.
 

·
Registered
Joined
·
54 Posts
Discussion Starter #5
The carbs should be identical in all the important ways. The pilot jet gets fuel through the main jet. If the other carb is different, it is either not the same type, or someone has put the wrong jet in. The carb number should be stamped into it, somewhere.
That is the most logical scenario. Unfortunately I don't know the history of this bike, so I have no idea what was done to it over the years, which means I wasn't sure which way was right. Thanks for clarifying!
 

·
Registered
Joined
·
2,421 Posts
For the standard carbs, there is no hole at the bottom, but some will feed directly from the bowl; the number stamped on the carbs may tell that story.
 

·
Registered
Joined
·
54 Posts
Discussion Starter #8
I went out to compare and here's what I found. The pilot jet without the hole is a #38 with a manufacturer's mark on it, and is in a carb body stamped "NF<illegible> 751A". The pilot jet with the hole through it is also a #38 and has the same marking on it, and is in a carb body stamped "NFL 751A"; this is the carb with the choke lever (I may have gotten that part backwards earlier). Does this help clarify anything?

_20190916_135618.jpg _20190916_135709.jpg
 

·
Sensei
Joined
·
27,018 Posts
THEORETICALLY, it makes no difference WHERE or HOW the fuel enters that jet.....
It ONLY matters that the orifice size of that jet is correct to control the flow speed/volume of the fuel that passes through it.....

HOWEVER, in a 450 carb, IF the jet (threading) is NOT long enough to extend up beyond the transfer passage from the main jet, then the flow speed/volume is being controlled by the passage diameter, NOT the jet.......

I have a spare #38 450 style "plugged" jet I'll gladly donate if you'll cover the mailing expenses........Phone me at number in red below......
 

·
Registered
Joined
·
2,421 Posts
Looks like someone has used an engraving tool on the letters of those carbs; the numbers are undamaged, anyway. I've never had a 751 type to take apart; anyone know which type jet is supposed to be in a 751 carb? Or if it has the transfer passage from the main jet?
 

·
Sensei
Joined
·
27,018 Posts
The "illegible" mark is probably the Japanese initial or indicator of which factory worker produced that carb...(think paid by quantity of completed pieces)....Both carbs are 751A carbs

Another consideration would be that supply to the slow jet would be the additive capacity of the transfer passage and the "drilled" jet......
Fortunately, this would have a limited effect at idle as the fuel then has to go through the #38 slow jet as well......

IT DOES become problematic as an additive volume once the main jet is in use as you would now have the additional fuel back-feeding FROM the through-drilled pilot jet, through the transfer passage to ABOVE the main jetting's restriction....(essentially increasing the size of the main jet)....
 

·
Registered
Joined
·
2,421 Posts
^^What he said, which is why it is important to know if the transfer passage is even there, and whether the threads of the through-hole jet closes it. If it is not there, then you need two through-hole jets for the engine to idle properly.
 

·
Sensei
Joined
·
27,018 Posts
Rick....There IS a transfer passage and the 450's and 500T carbs ALL use the "plugged" type pilot jets......
The threading of the through-drilled jets is NOT long enough to seal the passage........
Which is why I offered one..........
 

·
Registered
Joined
·
6,495 Posts
I went out to compare and here's what I found. The pilot jet without the hole is a #38 with a manufacturer's mark on it, and is in a carb body stamped "NF<illegible> 751A". The pilot jet with the hole through it is also a #38 and has the same marking on it, and is in a carb body stamped "NFL 751A"; this is the carb with the choke lever (I may have gotten that part backwards earlier). Does this help clarify anything?

View attachment 290700 View attachment 290698
That hole looks bigger than 0.38 mm to me.
And when I think about it, I don't recall seeing that jet with a hole in it on any 450.
 

·
Registered
Joined
·
2,421 Posts
Rick....There IS a transfer passage and the 450's and 500T carbs ALL use the "plugged" type pilot jets......
The threading of the through-drilled jets is NOT long enough to seal the passage........
Which is why I offered one..........
So, you're saying that, in this regard, the 751A carb IS a standard 450/500 carb, and is mostly the same internally as the 723? Something for the memory bank, in case it comes up again.
 

·
Registered
Joined
·
54 Posts
Discussion Starter #16
Thanks, all. This has been a fascinating thread, and I'm glad I was able to spark some discussion and learn some things. I'm definitely going to take 66Sprint up on the offer of the jet. If things get weird after I install it I'll update here.
 

·
Sensei
Joined
·
27,018 Posts
So, you're saying that, in this regard, the 751A carb IS a standard 450/500 carb, and is mostly the same internally as the 723? Something for the memory bank, in case it comes up again.
Only if you ignore different jetting and air-cut valves....751A is only used on the 500T (that I know of offhand)
 

·
Registered
Joined
·
6,495 Posts
Only if you ignore different jetting and air-cut valves....751A is only used on the 500T (that I know of offhand)
I think you may be right about only being used on that model, Steve. Not sure what early Goldwings used, their carbs were very similar, except for the obvious linkages. In fact, the air cutoff thingies are the same parts on 500T and GW 1000's.
I haven't worked on a lot of 500T's, and can't remember if that jet was drilled or not, never noticed.
 

Attachments

·
Registered
Joined
·
2,421 Posts
Only if you ignore different jetting and air-cut valves....751A is only used on the 500T (that I know of offhand)
OK, good, for future reference. I've never worked on a 500T, and don't have a parts list for one - guess that's obvious. I knew about the valves, and jetting seems to vary with each carb version, but never knew what the stamped number is.
 

·
Registered
Joined
·
196 Posts
The jet with cross holes is plugged at the bottom and the actual jet is on the top toward the body

Sent from my SM-J320V using Tapatalk
 
1 - 20 of 20 Posts
Top