Carb troubles

Sorry everyone to be full of questions, however I’m having some strange carb issues on my 85 XJ700 Maxim X. So for the past month I’ve been having the issue where when I get to 5,000 RPM’s all power on the bike stops and it bogs down. However last week I replaced the coils with new dyna coils and that got rid of the bogging part, now it just has no power. What is not making sense to me is that the PO put pods on which made me think the bike is running lean, however I ran the tank 100% empty yesterday and whenever I was running on fumes it had all the power I needed and I got up to 7,000 RPM’s??? So is there some sort of issue where it could be getting to much fuel?? Any help would be appreciated!

 

It's very possible that you are running rich, there may be another carb issue or the PO may have rejetted carbs with jets that are too big. Have you pulled the carbs off yet to inspect and see what jets are in there? What do the plugs look like?

 

Your carbs were designed to work in conjunction with an airbox.
The elimination of the airbox has altered the pressures at the carb inlet, at the venturi, inside the diaphragm chamber, everywhere.
You are now tasked with engineering new jetting sizes, main jet needle shape and size, throttle slide spring rate, and diaphragm chamber hole size.
or
find an airbox.

 

Would it make sense to stiffen the spring? Theoretically if too much air is coming in then it would be lifting the slide too much letting too much fuel in which would be causing it to run rich? Stiffening the spring would compensate for that right?

 

You need a weaker spring.
Suppose you put your hand over the carb inlet with the motor running.
This will create suction and lift your throttle slides.
What is happening is the opposite of this scenario.
Your slides are opening until 5,000 rpm then stuttering and becoming chaotic.
Think of the airbox as your hand over the inlet, it's like a power-assist for your throttle slides.
What you've described is text book symptom of what happens when the airbox is removed.

 

Oh ok that makes sense to me. I could just experiment with cutting off half a coil of the spring to shorten it some. Now that I’m hearing it outloud that could very well be the only issue besides needing different jets that I have. If the slide isn’t able to open as much because of the pods that could be causing it to not let fuel into the emulsion tube.

 

Cutting it will make it stiffer. Hard concept to explain but a similar material spring with less coils has a higher spring rate than one of the same length with more coils.

 

Think of it as putting a straighter piece of metal there

 

If you cut a section off it will be easier to mount the hat, therefore less force holding slide closed. Cutting a section out is what is recommended by the commercial stage 3 jet kits.

 

Dave that is a perfect picture.........

 

Most backyard hot rodders correctly understand the airbox restricts air flow on their XJ.
What they don't understand is how they upset the balance of the carburation by eliminating it.
Let's ignore the fuel and just consider the airflow.
The little hole at the top of the venturi (or the hole in the throttle slide) is what controls the rise and fall of the throttle slide.
The spring is spec'd to hold the slide against the idle stop screw at idle, 1000 rpms.
It is also spec'd to require redline, say 10,000 rpms to open all the way.

What the backyard mechanic really needs is, I'll call it an airflow machine.
In the case of a 700-X he would hook his BS33 Mikuni to the machine and set it to flow 87,500 cc's of air per minute.
700 cc's /4 cylinders x 1,000 rpm's /2 (4 strokes or 2 rpm's are required per cylinder fill).
This would mimic idle conditions and he'd want to visibly verify the slide is against the idle stop screw.
Gradually increase airflow and immediately watch the slide rise, at 875,000 cc's per minute of airflow he'd want to verify the throttle slide is all the way open.
At 10,000 rpm's redline it will be flowing 10x idle.

Below 5,000 rpm's the airbox is not offering any restriction. After that it is and that is where the problems start when trying to run without it.

Once the throttle slides are opening correctly he can move on to ensuring fuel flow to match the airflow.
The ideal ratio is between 13 and 13.5 parts air to fuel. This is where the dyno comes in by measuring the free oxygen in the exhaust.

This might be simplifying things, just trying to explain just doing jetting changes without slide mods can leave you blue in the face.

 

The slides don't contact the idle screw

 

Thanks Hogfiddles you’re correct. The slides bottom out in the carb body and the butterfly sets idle.

 

Too complicated for me. Just install the correct airbox and jets and enjoy the ride.

 

Amen!

 

I sure would love to do that. If only they were laying around at part stores lol

 

I have a few sitting around here.
(Your just north of the border parts store).
Ps including air filter and carb boots.

 

Do yourself a favor and go back to stock airbox and stock jets ( air and fuel jets ) for your carbs......
Pods = Problems. Period

The performance gain factor or cool factor for having pods is so over-rated.

You will be much happier will when your bike is dialed in when it has all of its stock parts back on.

 

if you go back to stock, your going to need those springs not cut up. so rather than cut the spring, put a split-shot or dime or nickle in the slide so it's harder to lift

 

It’s starting to look like finding an air box in one piece is almost impossible. What all do those dyna jet kits have in them? Would that do everything I need? Springs and jets?

 

What goes into reinstalling one of those? I heard you almost have to take the engine out to get them in, in one piece.

 

I say, once you get an airbox, start with the easier stuff: take off the starter, the carb and maybe the battery box. Then find a heatgun and see if you can heat that box up just enough to squish into place.

Well... That's my vote for Plan A, in any case.

 

the Maxim-X is probably the tightest fit of them all due to the water pump.
Modern epoxy glue is pretty effective.
If it was me I would cut it in half where the seams are going to be accessible for glueing it together later.

 

Also, be aware that the X and the non-X box are the same EXCEPT the X boot holes are slightly larger

 

Come to think of it... Maybe after the box is split and inserted, you can smooth it back together with a decent soldering gun that comes with one of those flat plastic-smoothing tips?

No idea idea if that'd work - I haven't tried it myself. Maybe, maybe?

 

Loosen the engine mounts and the exhaust, tip the engine forward

 

So this might have been a bit excessive, but in order to see if the springs were too stiff I just took them out of all 4 carbs, I’ll take it for a run and if it fixes it then I will know they are too stiff right?

 

Andddddd that fixed it! Got it to 8,000 RPMS no prob! Spring was too stiff! Going to rejet and adjust springs and just live with the pods

 

Only 8k? You're not even close to redline yet

 

I could have kept going but I stay farrrr away from that redline lol.

 

But the XJ's looooove the redline. They'll even go above redline, just don't stay up there long. You keep the oil changed, the carbs clean, and the shim clearances in check, and it'll take that and beg for more. Your roads are so flat and straight down there, you'd STILL think you're barely moving