Topic: Main jet threads?

[The Gurg]

Are these threads ok? 4th turn down has a small section that looks rough. The main jet doesn’t thread all the way, which is the way it’s supposed to be I think. But with less thread engagement, wanted to see if this was a concern. The rest of the threads further down and on the opposite side look good.

CB750K5 Keihin Carbs

[robvangulik]

The emulsion tube threads in there,the main jet should thread in that tube completely.

[The Gurg]

Oh oops, I misspoke/mistyped. Main jets seated. I meant the emulation tube has threads showing. You can see that in the second picture.

[bryanj]

That will be fine, more than enough threads to hold, just dont go he-man with a 4foot spanner

[ekpent]

 Yes its normal for threads on the emulsion tube to show. Too much force can crack the carb body or break the threads. I've seen pictures where they were cranked down so hard the small end of the emulsion tube was bent and deformed.

[HondaMan]

Looks perfectly normal!
The main thing is: the tip of the emulsifier is the venturi at the needle, under the slide. It should stick up between 2.5 and 4.5mm into the throat of the carb.

[The Gurg]

Hondaman-
Like the emulsifier threads in 2.5 to 4mm deep? I’m thinking that’s what you mean.

Ekpent-
That’s what I thought, thanks for confirming!

BryanJ-
Had to use a 1/2 to 3/8 adapter and then a 3/8 to 1/4 adapter to get a 7mm socket to fit on my breaker bar, am I doing it right?

[Bodi]

No. The emulsion tube incorporates the needle metering orifice. That part, the inner end of the tube, should stick up into the carb throat under the slide and project 2.5 to 4.5 mm above the bottom of the throat bottom. Usually no problem but sometimes some goop or corrosion blocks the tube from bottoming.

[bryanj]

You forgot the 1 inch to 3/4 and 3/4 to 1/2 adaptors Luke

[PeWe]

Easy to make mistake when removing tbe tube when carbs are mounted on bike. Tighten instead of removing.
I always use a 1/4 socket wrench I carefully check, removal only to not make mistale stripping the threads.

I have been close once back in the days. Changed procedure after that.

[The Gurg]

Bodi/Hondaman-
The throat in my keihins have a small brass fitting (the brass fitting being the Venturi I think?) that the needle goes through at the throat floor. So the emulsion tube can’t thread through into the throat because that fitting blocks it. Do you mean that the emulsion tube seats against the underside of that small brass fitting?

BryanJ-
Ah I see the problem, my breaker bar only has a 1/2” drive. Do you think a 1” drive four foot long  handle breaker bar is something I could find at Target in store? Can’t seem to find a name brand spanner on Amazon.

[69cb750]

The throat in my keihins have a small brass fitting (the brass fitting being the Venturi I think?) that the needle goes through at the throat floor. So the emulsion tube can’t thread through into the throat because that fitting blocks it. Do you mean that the emulsion tube seats against the underside of that small brass fitting?

Yes, the main jet holder seats against the needle jet.

[bryanj]

Luke i'm glad you get my humour and nah dont bother with the breaker just nip to the nearest commercial tyre shop and use there 18 wheeler air gun😂😂😂😂

[The Gurg]

Luke i'm glad you get my humour and nah dont bother with the breaker just nip to the nearest commercial tyre shop and use there 18 wheeler air gun😂😂😂😂

😂 😂 😂
Haha I’ll take the suggestion under consideration!

[The Gurg]

Just remembered, have an emulsion tube on carb number one that is shorter than the other three emulsion tubes(wrote this post when I was working on carb 3). Doesn’t show threads and seats fully into the carb body. I measured (but it was a couple days ago, and I was checking how much thread I should see, not determining seating at the needle metering orifice) and it’s close if not exact to the length to seat against the needle metering orifice. Could have a small gap though. Should I remeasure, swap out the emulsifier, or is it good?

Of course it ran that way for 8 years since the last carb rebuild. Did have low compression on the first cylinder before I pulled it apart. Don’t know if that somehow could have contributed? Could see a gap causing turbulence just under the needle metering orifice that results in a lean mix damaging the piston rings, et cetera?

[bryanj]

Tube shorter is not good, if screwed in father main jet will not be as deep in the gas and that could affect mix.
Q has the tube been shortened or is it from a different bike?