First Cafe Racer Mod! 1983 XJ750 Maxim

This is a typical situation with an engine that is out of synch.

By the way, just to clarify, when using the Carbtune synch gauge, you do NOT adjust the idle mixture screws, you will be adjusting the SYNCH screws (located on the throttle brackets, between each set of carbs). Once you get the engine stabilized and "in synch" (the sliding metal rods on the Carbtune gauge at equal levels), only then do you start fiddling with the mixture screw (using a Colortune plug, hopefully) to adjust the pilot fuel mixture screws....which, in the process of adjusting those, will also throw off the engine synch a bit, and so the Colortune and Carbtune processes are typically done "in tandem" with each other: colortune a cylinder, adjust the engine synch; now colortune the next cylinder, and re-adjust the synch, etc. Anything that affects the power output of one cylinder alone (such as, adjusting the mixture screw for that cylinder) changes the power output of that cylinder (relative to the others), and thus results in the engine being "out of synch"----the power output of each cylinder is unequal, and thus the engine must be re-synched to bring cylinder outputs back to an equal level.

The Carbtune gauge measures the power output of each cylinder using the strength of the vacuum draw from each cylinder as a "surrogate" way of determining the power output of a cylinder; this is why the synch gauge uses resistive springs over each sliding metal rod; they apply a pre-set level of resistance to each rod, so the strength of the vacuum draw in each cylinder can be "measured" by how far the vacuum can pull the rod up. There are many factors which determine why a cylinder might have a greater (or lesser) vacuum pull than others (fuel supply, piston ring seal, valve adjustment (i.e shim clearance), etc.). But whatever the reason(s), the purpose of synch is to equalize the vacuum draw (power output) of each cylinder:

https://www.xjbikes.com/forums/threads/82-xj-650-startng-problem-surpise.46358/page-2#post-414262


Also, this why an out-of-synch engine will experience this type of run-away engine speed............one (or more) cylinders are fighting for "domination" over the others, and in the process using more and more fuel (= more rpm's). One cylinder has a much greater vacuum draw than the others, and "vacuums" more fuel out its carb, and this starts the run-away (that's why it slowly builds and then goes into a runaway situation, the other cylinders can't counteract the dominance of the one great, almighty cylinder which is controlling the idle speed and so the rest of the cylinders are just "along for the ride" with no counterbalancing effect........).

 

Wow!
That’s pretty interesting! Then would it be a good idea to “lean out” all of them, turn them out maybe 2 turns and then adjust from there to at least get them somewhat close to each other?

 

So I leaned out the adjustment screws and then put them at 2 turns which made syncing easier but it’s still doing something weird...
I got #1 to flutter closely to #2, I matched #4 to #3 but when I start matching 1/2 to match 3/4, it starts revving up again when they’re getting close to each other and when I press down on the adjustment screw (middle one) with the screwdriver it brings the idle back into reasonable range but then they’re all mismatched again while pressing down to control the idle runaway.

 

use the idle knob to bring it back down.

don't do dat, that's just wrong in so many ways, use the idle knob

 

It doesn't respond to the idle knob at all...

 

your sync is in left field then. two ways to get it back, pull the carbs or
unhook the TCI, hook up you charger, turn the idle knob in till it hits the tang then two full turns more, hook up your sync tool, crank it 5 seconds and see what you got, then do the 1/2, 3/4, thing
but only 5 seconds on each set so be quick and don't be to picky close is good, feel the starter, hook the TCI up, start it it's going to race, use the idle knob
if that idle knob ever stops working, make it work