Carb Tuning after Pod Filters

I am somewhat hesitant to post this here as it seems the general forum consensus is that pod filters were invented by satan to ruin yamahas. Yet, here we go. When I heard that pod filters provide increased airflow and that I would almost certainly never get them tuned right to run well...I just couldn't help myself but give it a shot. My Bike is a 1986 XJ700 airhead.

Well, new pods are on the bike and I am pretty close starting out, just not quite there. So, thought I would get some opinions.

First of all, I decided to tackle this because I think that the issue most people have with pod filters is not due to a lack of vacuum from the increased flow at all. Yes, it is true that these CV carbs use a vacuum to suck up the diaphragm in the hat and lift the needle out of the main jet. This vacuum does not come from the engine vacuum though, it comes from the venturi effect. (https://en.wikipedia.org/wiki/Venturi_effect) When air flows past an opening, it creates a negative pressure across that opening.

The problem with pod filters that creates strange performance is that the venturi effect requires laminar flow and pod filters attached directly to the opening of the carburetor create turbulent flow. To fix this, I used the boots from my old airbox and put them between the pod filters and the opening of my carbs. This gives the air time and space to become more laminar. If this doesn't work, I have an idea to test out using the same technique that wind tunnels use to create laminar airflow from choppy wind produced by their large fans. Hopefully, everything goes smoothly with this simpler solution though. Of course, the airflow will still be increased, requiring carb re-jetting. Helping to create laminar flow will just help to solve the odd mid-range dead spots which are probably just points where the flow becomes too turbulent to raise the needle sufficiently.

So here is where I am at. I rebuilt my carburetors with all new hardware. I increased the idle jet from 36.5 to 40 and the main jet from 107 to 112. When replacing the mixture screws, I turned them out 4 turns as this was about where they were when I started. The bike starts up fantastically with the fuel enrichment fully open, although a little throttle is necessary to get it to wake up. I just shimmy the throttle a little until it starts which happens in 2-3 seconds of turning over. It then idles fine but once it warms up with the enrichment on, the RPM jumps up to 2 or 3 thousand, even with the idle adjustment screw all the way closed. If I turn the enrichment down to about 1/3 it will idle right at 1k very nicely. Sadly, if I close the enrichment completely, even when well warmed up, it will die immediately. The bike revs like a mad-man if I open the throttle a little. I have not ridden it yet, but revving it in neutral seems very happy.

My question is, should I change the jetting or just open up the mixture screws some more? It seems odd that the bike will rev so high with full enrichment on, even with the idle adjustment full closed. This makes me nervous that it is getting too much air with the throttle fully closed and I am just limiting the rpm by running it lean. Maybe I need to swap in smaller air jets? I have a colortune plug on the way but it won't be here for a few weeks and I want to get the bike running decently well enough to take it out this weekend, as we are expecting beautiful weather. Thanks for any advice!

(Valves have been adjusted less than 3k miles ago, no apparent vacuum leaks)

 

1. Adding the intake boots between the pods and the carbs has been reccomened here for years.

2. She's racing because she's lean. The mixture adjustment screws only afffect the idle circuit. You will need to rejet. +2 sizes is a starting point.

3. Don't ride her until you get the jetting right and have confirmed that there are no vacuum leaks. You are lean enough to hole a piston at speed.

 

Thanks for the warning. No piston holes for me please...

Darn, I must have missed the recommendation on 1. Could have saved myself some brainpower on that one.

So you would recommend two sizes more than what I have already done? So from 36.5 to 40 now up to 42 and from 107 to 112 now up to 114?
I can always buy more jets but the next size up I have on hand for the main is 120. Think that's too big?

Also, I'm not sure lean is the reason for the high idle. It goes up when I open up the enrichment circuit full blast.

 

I misread that part in your first post.
Once idle increases to above 2,000 RPM the engine is warm anough to not need enrichment.

Pull your plugs and check the groundstraps.
Better yet give us photos of each plug, numbered 1-4, so we can read them and see how close the jetting is.

 

After starting with full enrichment you need to adjust the lever to keep the rpm's down. When it starts racing you adjust again, then again until you need no enrichment and the idle is close to what it should be. Ride the bike to make sure engine is completely warmed up and then adjust your idle screw to get the correct rpm's and then leave it there. Doing the enrichment lever dance is normal when starting cold.

 

Yes, when it jumps up to 2k rpm I turn off the enrichment and it dies. Restart the bike and turn down the enrichment to 1/3 it runs perfectly.

I'll take some pictures of my plugs, but ill have to wait on the new ones to come in the mail in a week. The ones on there are pretty old. I don't think I have had any ignition issues so that probably isn't a concern, but those old plugs won't give a true reading. Also, I will probably need to get it to run without enrichment to be able to judge the jetting, right?

 

Thanks for the tips! I always used to do the enrichment dance before but I thought I was compensating for something being off. Didn't know it was normal.

If it's normal, I won't overthink it...but it seems weird that the rpm would go up with it on full blast. I always thought the amount of air controlled the rpm and the fuel was just supposed to match it. I figured once it warmed up it would run crappy, not happy with enrichment on because now it would be rich.

Also, my idle screw is all the way out with it now.

 

The enrichment is a seperate circuit that is not directly connected to the rest of the fuel circuits. It is essentially a seperate throttle control that manages fuel directly instead of managing air.

 

Well nearly, the enrichment also ads air.
@Dalpha22 , you can't set these up with the idle speed screw right out. You need to turn it in until it just lifts the throttles as a start - did you bench synch these? Are the float heights spot on? Plus 2 sizes on the idle jets could be enough, but no guarantees. Changing main jet won't do anything until you're on the move, so leave as is for now.

And lastly, a comment on your theory on what changes when you go to pods - the biggest change is to the pressure on the outside of the carb - this is now near atmospheric, not just below it as with the air box and paper filter, so the balance either side of the piston has changed - this will cause you some headaches on the road.

 

Correct. I was attempting to make a clear distinction between the function of an enrichment circuit and that of a choke plate (which most U.S. residents are more familiar with).

 

I do understand that the enrichment functions differently than a standard choke. I am actually really glad that our bikes use this instead of a choke, it just makes more sense to me.

So the enrichment does add more air? I might just have the idle screw too far out then and it needs the air the enrichment circuit adds in order to run... I haven't synced the carbs yet but intend to do so very soon. The floats looked pretty darn close, maybe 5 degrees above parallel but very minimal and it is consistent across the carbs.

I will be sure to let you all know how she does on the road and follow up from there. Is there any way I can be confident that I'm not going to damage anything taking it out for a (potentially spirited) test ride?

 

I think you need to stop what you're doing, step back, and go back to the basics of Hitachi carb tuning as it applies to our Yamaha's. What I mean is pull the carbs off the bike, perform a bench sync, then check and correct the wet set float heights, and set the pilot mix screws to 2.5-3 turns out from a soft bottom. Then get them back on the bike, get it running and idling as you should be close enough to do so. Then perform a running/vacuum sync hopefully followed by a color tune. From what I'm reading it seems to me like you're chasing your tail a bit so to speak. You can't have the idle screw backed all the way out and expect it to run once you take away the enrichment. You seemed determined to make it work and I think that's awesome. Keep at it and you'll get there but like I said I think you need a fresh start baseline to go from

 

sounds like a major vacuum leak
can the air jets on these get switched?

 

you want to go basically up hill

 

You're not wrong. I am ashamed to admit it, but I realized I was overthinking a simple thing and skipping the basics. I opened up the idle screw incrementally as it warmed up and I let off the enrichment and....it runs just fine as it is once it can get some air... imagine that...

I got it in my head that since opening the idle screw with enrichment on all the way at cold start pegged the rpm crazy high that it must not be right. In reality, I just don't need all that much enrichment to get it started now with the idle set right. It runs great now as far as idle goes. The mixture seems good as far as I can tell. Once it is fully warmed up it bogs down if I add enrichment instead of revving up, which I imagine is exactly what it should do.

I think I'll take it out for a test ride tomorrow and synch it and colortune it once I get the tools in the mail. Anything I should watch out for when riding in case it isn't quite right? I would hate to damage anything giving it a test run.

 

Yup! They're under the vacuum covers/hats (whatever they're called). I won't pretend to know how those two air jets work exactly or why you would change them though. Everything I read said just leave them alone.

 

Aww come on now, this beast can do more than that! Okay maybe a beginner beast, but it's been fitting me well.
Besides... there aren't any hills in the midwest so I'm out of luck.

 

keep riding west you will find some hills

 

Well, I took it out for a test drive today and it is definitely not right. It has low power throughout the rpm range.

Although not a proper plug chop, I will pull the plugs and look at them once it cools down. I suspect it is rich.
I'm not sure how much is from the new intake noise, it sounds very gurgly. Also, after cutting off it didn't want to start without a little throttle so I am thinking it was getting flooded. Also noticed some white vapors from the crankcase vent. The smallest main jet in the kit I bought was 112 and the original was 107. Going up 5 sizes was probably too much.

 

I don't get this last bit - Going up 5 sizes was probably too much.

You don't need any tools other than a screwdriver to determine this. Simply, get the machine to idle with all the mixture screws set the same (you will have done a full carb synch before this, for which you do need some kit). With the engine idling on the idle screw stop, just turn no1 mixture screw in slowly. If the idle speed goes down, maybe to the point of misfire, come back out. Set to the fastest idle speed, one cylinder at a time. If the screws are in from the factory setting a long way then your idle jets are too big, and vice-versa. It's not hard. No guessing needed.
The idle jets do idle and progression, so if they're wrong you'll get poor running in the lower speed/load area.

 

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

 

and CR for the air-cooled 700 is 9.5:1, I thought I'd correct my previous post.

 

This is kind of a reboot to this thread. I need a source for a good jet kit for the Mikuni CV carbs on my 1981 650 Maxim. I bought a couple kits from Amazon that didn't work out to well so I'm going back to the original jets. Pilot jet replacement was a 60 from the original 40 and the main jet was 120 from the 110 it started with. The carbs were a bit out of sync on the first run and I had very little power until I hit 5K RPM. After jetting to the larger sizes my gas mileage went to crap but the sync was better. also would like to get my hands on a good tuning manual for this series of carbs if anyone knows the right one to get. I also have the pod filters that seem to do ok but im not ready to make judgment on them until I get the jets right.

 

Len will have the parts you need info@xj4ever you can e mail him on.

 

You have mikuni carbs on your 650 Maxim?? thats the problem it should be Hatachi carbs.

 

I have 2 sets of 650 carbs and a set of carbs for the 550 .... all very similar... could I be mistaken about the make of the carbs?

 

Certainly that's not correct - they will be Hitachi, but equally, if they were they would not be an issue. The fj600 mikunis on my 650 maxim worked well with pods, with no changes. That's not so say they were perfect, but on the few miles I did with them I didn't detect a hole in the power band anywhere. And then I fitted the 600 airbox, without changing anything jetting wise, and have done quite a bit more miles of very mixed riding. The only issue I suspect I'm feeling is a tendency to "switch off" when backing off the throttle at lower road speeds. My thoughts are they need to go up a size on the idle jet, but that's a seat of the pants feeling thing. I might turn out the idle screws a turn to check before buying them.
So mikunis - no issues for me, I prefer them over Hitachis, maybe because of the above, maybe because I rebuilt two sets. (The first were a very rough set just to check the things would fit and run the bike - they did).

 

But you knew that Mikuni and Hitachi use different methods to designate jet sizes. If the OP dosen't know that, then he's further off in his jetting than he thinks. Especially if he put Mikuni jets in Hitachi carbs.

 

Correct - that would be a disaster. I have seen a cross referance table somewhere on the net - but why not just hunt down the correct makers jets to match the carbs, less hassle all 'round. The only non- original item needed would likely be the needle - height adjustable and different tapers - good luck.

 

post a photo of them

 

the open bowl is when I first took them apart to clean..... they had been sitting for at least 10 years if not more