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Jet sizes AAAAARRRGGGHHH!!!!

  • Thread starter Thread starter Daffyd
  • Start date Start date
D

Daffyd

Guest
Hi me again......I have to ask here, dealers don't know, or don't want to tell. I have a 77 750, I got a Laser 4 into 1, so after consulting Laser direct, I don't have to modify carbs (??????). Anyhow, I got some nice pods, and want to upjet so does anyone have any idea what sizes. I got some 130's (main) but think this is too big. If I put 115's in is that more realistic, and is it essential to change the pilot jets????? cheers guys:confused:
 
If I remember correctly, Laser's are pretty benign so they are right - you won't need bigger mains. You might need a tweak on the pilot circuit.

If you're swapping to pods as well no one on here will be able to tell you what will be ideal - your bike is 'unique' and will tell you what it wants itself (do a search on plug chops). Having said that, you will almost certainly have to fit bigger mains and 130s wouldn't be a bad place to start. Again, if I remember correctly, I had 137.5s or 140s on a 750 many moons ago but I've also ran with low 120s on a similar set up to your proposed.

You'll have to dial in the pilot circuit - I bet you won't need to change the jets, just the fuel / air screw settings (as there's loads of adjustment available). Very unlikely you'll have to change the slides (for different cutaways) but you very likely you'll have to raise the needle. I suggest trying position 4 first and working from there. Don't try and adjust the mixture using float height - makes sure that's set to spec and leave it there - trust me, you'll only end up in a pickle.
 
thanks guys

thanks guys

Hey guys, thanks for the excellent tips...."intelligent tinkering"....this is a quote to remember :). I think I will try the 130's, adjust the needle to the 4th and work from there ( and keep the stock setup in case !!!)
Just a point of interest, somthing I seen on "Top Gear" (English motoring programme, do you see it in US?), US fuel isn't as volatile as the stuff in UK, so does that make a difference???
And just to confuse matters, I have had to replace my lump, I got a GS750E engine, imported from US !!!! It has never seen rain or salt....boy is that in for a shock on our roads. (it was actually cheaper to get a new engine than do a re-build, I saw piston rings for $200 on e-bay in UK.
Anyhow,thanks for tips, I'll keep you posted :)
 
One of the first things to do is to realize what STOCK settings were, then adjust from there. My information only goes back to the '78 750, and I know there were some differences from year to year. I don't know if the main jets changed from '77 to '78, but the '78 carbs call for 100s. Following a general rule of thumb for about three, maybe four sizes different for the pods, you would be looking for 107.5 or 110. I think the 130s would be WAAYY to much, but the spark plugs will tell the story. Since you already have 115s, I would start with them and be prepared to go down from there. I know you said the pipe manufacturer says you don't have to re-jet for their pipe, but since the bikes were a bit lean anyway, another size or two for the pipe wouldn't hurt. That would put it right into the range of those 115s.

Oh, you don't have "pilot jets". What you do have are two screws. The one on the bottom is your pilot fuel screw, the one on the side is a pilot air screw. In stock form, the pilot fuel screw is about 5/8 turn out from lightly seated. Most find that it works better to be about 3/4 turn, instead. On a modified engine (pods and/or pipe), maybe 7/8 to a full turn might work. As a starting point, set the air screw to double what the fuel screw is.

.
 
Steve said:
Oh, you don't have "pilot jets".
Click to expand...

Steve - I think you've been doing BS carbs for too long

Item 21 -pilot jet



VMcarb.gif
 
Big T said:
Steve - I think you've been doing BS carbs for too long

Item 21 -pilot jet
Click to expand...
Quite possible, so please educate me. :o

Is item #21 a piece of brass that has a fixed orifice, and is screwed into a fixed position
or is it a screw with a taper on the end, which can be adjusted to an infinite number of settings?
(It's really hard to tell by that picture and a few others I checked.)

I have always called a device with a fixed orifice a "jet" and the adjustable device a "screw". :-\\\

.
 
OK, just spent a few minutes doing some self-education. :-\\\

Seems that VM carbs have both devices, fixed-orifice jets and adjustable screws. :eek:

Found this picture in my 850 manual. Item #2 is the pilot fuel jet :oops:, item #3 is the pilot fuel screw.

VMpilot.jpg


.
 
Last edited:
I knew that's what I replaced!

Yeah, it's fixed and it has a number on it -I went from #15 to #20 and my bike idled much better
 
Steve said:
OK, just spent a few minutes doing some self-education. :-\\\

Seems that VM carbs have both devices, fixed-orifice jets and adjustable screws. :eek:

Found this picture in my 850 manual. Item #2 is the pilot fuel jet :oops:, item #2 is the pilot fuel screw.

VMpilot.jpg


.
Click to expand...

Steve - you must have had one too many as you've 2 number 2s. I think you must have meant 1 of your #2s to be a #3 - the pilot fuel screw (commonly referred to as just plain old 'fuel screw').
 
hampshirehog said:
Steve - you must have had one too many as you've 2 number 2s. I think you must have meant 1 of your #2s to be a #3 - the pilot fuel screw (commonly referred to as just plain old 'fuel screw').
Click to expand...
OK, I'll change it. :oops:

I usually proofread every post, somehow that one slipped through. :o

.
 
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