Only firing on 2 cylinders and only idles with choke on?

I have a 1986 xj750 maxim-x that's only firing cylinders 2 and 4 all the time and cylinder 3 intermittently. I checked to see if cylinder 1 has a spark and it does. I'm new to motorcycles and working with carburetors so i don't really know where to look next to get all of the cylinders at least firing. the bike had pod air filters and the exhaust was straight piped when i bought it and i haven't replaced anything yet. also backfires i think on deceleration. thank you in advance for any help

 

Sounds like clogged pilot fuel circuits..........it happens if the carbs sit unused for an extended period of time, varnish fills up the small internal passages. Try using the choke just a bit and see if the problem is reduced or goes away...........if so, that's a sure sign of a pilot circuit issue, since the choke system uses an entirely different source of fuel supply.

P.S. how did you get a 750-X down there in NM? They were only sold in Canada originally.........and are quite rare (and fast!).

 

when i turn the choke on i get a little bit more of a exhaust puff from 1 and 3 but not usually complete combustion like 2 and 4.

my mom bought it for me for my 18th birthday from a guy locally who was selling it for like $700 it looks like its been tipped over once or twice. But i was doing some research on it and its rather hard to find any good reliable info.

 

I'm with @chacal on this one carbs are probably a mess inside.

 

^^^^+2 carbs are in need of cleaning . Since your new to this , first PODS suck makes it hard to tune and requires experimenting with jets . Go up in size might run ok at speed but not want to idle go down runs fine at idle and horrible at speed . The carbs are CV ( constant velosity) if these carbs were flat slides pods would be fine . Okay I gave my rant ...back to your problem you can try a HOT SHOT on the carbs lift tank pull the fuel line off the petcock , drain the float bowls and using a meat baster syringe fill fuel line with Berrymans B12 carb cleaner and acetone 50/50 mix will take several shots to fill . Let sit for about 20 minutes , drain all 4 carbs and reattach fuel line , put petcock to PRI and start bike up see if this helped . Eventually you will have to take carbs to church . Cheers

 

Ok, I'm getting the hump (do I have any right to as a newby on here?) with people blaming pods for everything including the end of the world. If one of your cylinders is firing, it stands to reason they all should - pods, airbox or a bloomin long snorkel.
The other issue is so called nay saying experts telling that these carbs are constant velocity - if you want to talk about constant velocity google it, and rzeppa, who invented the joint. These carbs are constant vacuum....
So, fix the issues, then blame the carb jetting and the pods if you can't sort the jetting, not arse backwards.
Purely my opinionated opinion of course....
Edit to add, otherwise good advice

 

while im cleaning the carburetors i noticed the smaller hose coming from the fuel tank petcock is hooked into carb 3 at the boot where you can check vacuum. is that supposed to be like that? thanks guys

 

Typically it is hooked to #2 but makes no difference which one, as long as the other 3 are capped off. This small hose sucks a diaphragm open in the petcock and allows fuel to flow when in the on position.

When you have the carbs apart list the jets you have to see if they are OEM. Inspect the diaphragm springs to see if they've been altered. Inspect the diaphragm hole to the vacuum chamber to see if it's been enlarged.
You need to decide if you're going with pods or returning to airbox. I'm running a 750-X motor bored to 911 with pods and it runs terrific. There are no commercial stage-3 kits available for the 750-X so it is a challenge. Without help from a professional tuner with dyno I wouldn't have achieved these results.
Running 750cc I was getting 82 rwhp on the dyno with a flat spot at 5,000rpm. The big bore took the flat spot out at 5,000 rpm. I've never put it on the dyno as a 911. I do occasionally run it to about 9,000, I'm scared to take it to the 11,000 which is what the 750-X will go to. the 750-X BTW revs a full 1,000 rpm's higher than the 700-X. Even with the longer stroke the pistons are lighter, I suspect that is the reason.

 

how do i get the part that is pictured in a previous message out of the carb? does it thread in or just sit in there? and thank you Simmy I just have never dealt with caburetors and to work on 4 at the same time has got to be a little more challenging

EDIT: Nevermind i figured out how to get it out

 

the only numbers i can find on the long tube are 318 and then y-2

and on the threaded nipple that goes on the end it has 32 stamped on it

 

32 ??. y2 is main needle jet size

http://xjbikes.wikidot.com/carb-specs
Years & Models: 1985-6 XJ750-X XN/XS water-cooled Canadian models
Carb Manufacturer: Mikuni
Carb Series: BS33
1985 Carb Model ID: 1FL00 (XN models)
1986 Carb Model ID: 1MY00 (XS models)
Main FUEL Jet Size: #105
Pilot FUEL Jet Size: #37.5
Main AIR Jet Size: #120 (non-removable, in carb throat)
Pilot AIR Jet Size: #140
AIR COMPENSATOR Jet Size: not used
Starter FUEL Jet Size: #35 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #5FZ83
Main needle JET Size: #Y-2 (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 3.0mm +/- 1mm (.118" +/- .039")
Idle RPM's: 1,050 rpms

 

well i got it put back together and its idling pretty good now. But it starts to snap like a backfire under load between 3k and about 4.5k haven't tried to take it much higher because people like to complain in my area about noise

EDIT: i pulled the spark plugs now that its cooled down after the test ride. the two cylinders that weren't running are black and the two that are running look very lean

 

I think I got it confused somehow i just looked at the vin and everything its actually the 1985 700 maxim x but the ending digits on the vin are 000210

 

It's the choke air speed (velocity across the piston at the top of the needle jet) that's kept constant, hence the refrence to constant velocity. Same type carb, different terms to refer to it. Same=Same, just like muffler and silencer, or truck and lorry.

BTW, absolutely none of us refer to ourselves as experts. Insults are unwlecome here, so please refrain from dishing them out.

 

I will never claim to be a expert ...but working on various bikes over 30 years I have seen things first hand . A new and first time bike owner can get very frustrated quickly when trying pods especially if PO hacked it up which is usually why he is the PO ...he gave up !!!! In fact just watched a you tube video on Pods vs stock couple of Australian dudes put a couple of cars on a dyno the stock set up made more HP than 2 different pods .... the guy that started this thread needs help to get his bike running better . Whether he will take advise and work thru his problem his sounds like it is carb related I like all here just want to help and in no way am I trying to high jack your thread . Good luck

 

My recommendation regarding pod installation is - don't do it unless you consider the following;

- you have a local tuner with the experience required for this job. In my case I have a local guy with a dyno and exhaust gas analyzer who knows what he's doing. The line up of racer's bikes waiting for his services is testament to his skills. When I took my hot-rodded Seca to him without an airbox he made no guarantees as to the results we might see. He had an assortment of diaphragm springs at his disposal and the experience to know which ones to try first. Most pod installers understand the stock airbox restricts/limits the airflow going in. What they fail to understand is this is by design. The CV carb as set up by Yamaha need this negative pressure to control the rise of the throttle slides. When you remove the airbox the stock diaphragm springs are then too stiff as they now have less negative pressure to overcome them. Consider the airbox as providing some power-assist to your throttle slides. Some commercially prepared stage-3 kits provide softer springs. Some kits recommend clipping off a 1/2 a coil at a time until you arrive at something acceptable. The problem with this approach is that it will never have the optimum compression rate needed. You might get the slides to open all the way when your wrist demands wide open but you will never achieve the mid-throttle control of the OEM/airbox set up. Another trick to fine tune the throttle slide action is enlarging the hole leading to the vaccum chamber. This is a black art and best left to those with the experience needed. Once this part is sorted then with help from exhaust gas analysis you can start enriching the jetting. The dyno tuner strives for a ratio of 13.5:1 of air to fuel for optimum performance. BTW those bolts at the bottom of each head pipe equipped on your XJ are ports for exhaust gas analysis.

- there are commercially available stage-3 kits available for certain XJ's. I think you would need to ride a bike equipped this way and ride it back to back with a stocker to understand if it's something you really want. In all likelihood it will never match the driveability of the OEM set up. Also riding in the rain presents issues as many have discussed at length already.

-I'm not aware of any commercially available stage-3 kit for the X so you are back to finding a local tuner you can trust. Some awesome and innovative project bikes I've seen here require airbox removal for monoshock conversion or just plain aesthetics. My opinion is that with proper help you can get your XJ to work without the airbox. You might even achieve some hp gains at red-line but you will never match the driveability of the stock bike.

- with shop labour approaching $100/hr any dyno time will not be cheap. If they will allow you to pull the carbs and change the parts between dyno runs this will save big. Do you want to spend $1000+ setting up the carbs on a bike with $2,000 value?

I don't consider myself an expert, just a backyard hotrodder who's been through it.
I will say that if you focus only on jetting your bike will never run right. You might even have the jetting correct but if your slides are not opening properly you will chase your tail till you're blue in the face.

Back to the new member GShultz - you have a nice little XJ700-X. (not quite a 750-X LOL) I suspect someone hacked the airbox off and installed pods thinking he was going to gain big hp. I agree with everyone else here, verify all internal OEM carb internals, find the airbox and rubbers and return it to stock.

 

Here we go then, my apologies for any offence caused re so-called experts, I do agree k-moe we're all learning.
Her's my bit on the pods myth (please don't take this as me saying they don't cause issues, I think we all agree they do).
Springs and needles are important, but rarely get a mention - sure the idle jet should be easy to tune just by going by the idle screw setting. The main jet will limit top en (hard to tune without a lambda sensor, or better still a wideband sensor and display). The inbetween is all spring and needle setting, and youve guessed it, lambda sensor and idealy dyno, otherwise you could crash due to looking the wrong way.

 

ok, second appology, this time for thread hijacking:

 

thanks guys. like i said im very "green" when it comes to these things as this is my first bike and first time dealing with carbs. but back to my earlier message in this thread i still have two cylinders that are running to either extreme on fuel to air mix, is there a mix adjustment screw somewhere or is it hidden behind one of those plugs that i keep hearing about that you have to drill out? thanks for any info

 

Bingo!