Carb Tuning after Pod Filters

Hi Mate, getting pods to work properly takes time as many will tell you not to do it.. As already mentioned get back to basics first. Eg bench sync, check float levels etc.
I’ve got pods on my XJ650, to make things more challenging I also changed to a bigger crank, oversized pistons, porting and different exhaust so I really was in uncharted waters with the carb tuning. I used a 6 Sigma jet kit, my mains are 145 and at the moment I’ve got it running well but still not perfect, just needs some fine tuning now to get the mid range right.

I suggest using colortune too as it helps check what your mixture is running like. I have a few videos on this on my channel if you’re interested, just search for Mezzmo Engineering on YouTube. Good luck with it..

 

How big did you make your engine? 145 main is huge.

 

I'll definitely go ahead and do that for my idle jets! Unfortunately, I think the issue more so lies with my main jet being too big. Any good way to figure that out other than trial and error test rides?

 

he went from 650 to 820 cc
we have a thread on it here
650 -> 820cc conversion

this thread is long but contains formula for jets
The Information Overload Hour

 

you mention floats at 5 degrees you need to read and do this to start
Setting the fuel levels

what other mods do you have after pods?
do you oil your pods?
one of the things that the sigma kits comes with is a drill bit to open the hole in the slide. once you do this there is no going back

 

MAIN FUEL JET SIZE CALCULATIONS:

Changes made:

Exhaust:
4 into 1 with Supertrapp = +4 Sizes Main Fuel Jet

Intake:
K&N Pod Filters = +4 sizes Main Fuel Jet
----------------------------
Equals: +8 main fuel jet sizes above baseline
Subtract: -2 main fuel jet size per formula above
----------------------------
Equals: +6 main fuel jet sizes due to modifications, thus:


---------------------------
Subtract: -2 main fuel jet sizes for Altitude of 2500' Average


PILOT FUEL JET SIZE CALCULATIONS:

The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.


Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000' a-s-l.

 

Once the idle jets are close you make sure your plugs are clean, take it for a spin and switch off. Have a picnic, cigarette, whatever until it cools, then pull the plugs.
Other than a wideband lambda or a rolling road tune I see no other way. But - all kinds of other things come into play, not just main jet. Here's my understanding of what goes on, by no means the bible, but it makes sense to me.
So Here goes-
Idle jet air jet - this corrects the top end of the idle jet mixture - progression if you like.
Main jet - obvious, but is it? This controls full power, max demand fuelling.
Tapered needle - works as soon as the piston lifts, metering fuel, along with the main air jet.
Piston spring - sets the vacuum at which the main jet and needle works - too weak a spring allows the piston to rise "early" in the load/speed range causing weak mixture, getting to "fully up" too soon, destroying the fuel metering capability of the carb. increasing the pressure on the atmospheric side of this (by fitting hi flow filters) causes the piston to ride higher...
Drillings in the bottom of the piston - controls the rate the piston rises when you snap open the throttles.

 

Yeah - I see no reason to drill these holes bigger - don't do it.

 

Thank you all for the info.

That looks like an awesome build, I might keep it in mind one day.

So far pods are the only mod I have. I bought the cheapo ones and I actually like them better than K&N because the boots are removable and the stock velocity stacks can fit right in tightly where the cheapo boots were. Think they will filter okay or will cheap pods damage my engine?

I haven't oiled them yet, but intend to soon. By that formula, I should be doing just +2 on the jet... that makes sense with why it seems rich. I saw that formula somewhere else but I just kind of wung it with the smallest jet my carb rebuild kit came with. I actually think that 112 was the smallest in my kit because it was supposed to be a 650 kit. With the 650 being a 110 stock, 112 as the smallest sizes makes sense. I couldn't find any 109 jets so I ordered 110. Hopefully, +3 will be close enough.

I won't be making any permanent changes unless I have no other choice. I don't like the idea of not being able to go back when I am still learning what I am doing.

 

if you ever consider the 650 to 820 start with a 750 motor and go 820 less work as the 650 used a lot of 750 parts to get to 820.

Chacal our supporting vendor sells jets in every step up the scale

that formula is the starting point so 1 extra step up may be ok

 

Could I start with my 700? What is the difference between the 700 and 750 engines? Bore, stroke?

 

Stroke and compression ratio. The 700 makes a tad more HP than the 750.

 

Wow, I didn't realize that. I assumed the 750 would make more power. The compression ratio on the 700 is a lot bigger, I'm honestly surprised that it still takes 87 octane with an 11.2/1 ratio.

It's funny, the 700 has a larger bore than the 750 but a much shorter stroke. 700 has a 69 mm bore and mezzmo overbored his 650 to 68. So, my stock 700 bore is already bigger. I wonder if the 700 cylinder length is the same as the 650, which I imagine it should be. Maybe I could use 750 rods and crank and aftermarket pistons in my 700 to get an 843 cc engine... Maybe even the same 700 pistons and just carve out some meat in the combustion chamber to get the compression ratio right. Now you guys are giving me ideas I'm not ready for...

I wonder how an engine like that would perform though with a longer stroke and still a high compression ratio. The redline would probably have to be lower because now you are increasing the piston speeds. Probably just the same redline as a 750 though, unless valvetrain is the limit and not piston speeds.

 

The 700 bore and stroke is 65 x 52.4
750 bore 650 stroke
The CR you quoted is for the 5-valve motor.
700 air cooled is 9.6 from memory.

 

Thanks! I google air cooled but often it seems online sources get their wires crossed between the air and liquid-cooled 700.

 

The online sources here do not (even though I sometimes do).
The Information Overload Hour

If compression ratios aren't listed there, then they are probably here.
XJ PARTS CATALOG, SECTION E: ENGINE

 

No sure mezemo may know.

 

HP = Torque x rpm. If the intake and exhaust (including valves and cams) are up to it, the torque will hold up, and hp goes up. It's why racing engines generally are screamers.

 

the Xj700 has a stroke of 52.4mm which is the same as the 650. The 750 has a stroke of 56.4mm.
I made it work by using the 750 crank and rods together with oversized pistons that were made for the 750. The crank from a 750 fits right into the 650.

you can’t use 700 pistons in the 750 or 650 as the piston to pin height is not right. Not sure about swapping cranks into a 700 as I haven’t worked on a 700.

big bores, different cranks etc also depend on what you’re trying to achieve. It’s a lot of work for a bit more power. Also then you need to mod the carbs to match. I went to the hassle of my mods as I just wanted something different and I like pushing the boundaries Am very happy with it but not sure it would have been worth it just for the power increase alone. I’m guessing power is up by 10 to 15 perecent over stock, will have to sync it some day.

 

I forgot to post earlier that the CR for the air cooled 750 is 9.2:1