Cylinder missing only below 4k rpm

My 83 750 Maxim which has recently overhauled carbs and has been running beautifully, started missing a little erratically on number 2 cylinder yesterday ( that plug is black, and smells of gas). It seems that is a bit sputtery til about 4k rpm (runs a little weak and exhaust smells of fuel and sounds fluttery) and then takes off strong like it should above that. I did have a piece of gunk flow to a carb a couple of months ago right after I switched to reserve. That one caused a float to not close. My idiot mechanic (Me) did not put a fuel filter in then. So, this is self-inflicted. This started yesterday right after switching to reserve, also. I will clean out the tank and take a look at the petcock as well this time. And install a filter. So, my question is this: since the problem is limited to below about 4k rpm, is there a particular circuit/jet/other where I should look for the offending contaminant? I have no time for this right now and really want to get in and get out. And install a filter. Thanks for whatever advice you may have. -Dan

 

It could be a plugged pilot fuel jet.

When was the last time you checked the valve clearances?

 

Thanks for the quick reply Fitz. I have never checked the valves. I may this winter. The bike is just a few months old to me. It looks very much like a PO put a new head on it, or maybe pulled it and bead-blasted it. It just looks brand new and so does all the hardware that secures it and the head cover. The carbs were spotless inside and the bike runs absolutely great, starts quick and purrs like a kitten. 21k miles, lots of stuff looks updated. Even the 8 boots in and out of the carbs look new. Is there a general sequence of circuits in the carbs associated with various engine speeds? Is the pilot jet what handles everything below 4k? Or is there another for even lower speeds? And last, why do you ask about the valves? Is there a reason to suspect them? I bought it from a kid who owned it only briefly between me and the original owner so a lot of info is lost and left for me to assume. -Dan

 

I hate to be a stickler to the air, fuel, spark rule, but if plug 2 is indicating rich and unburnt fuel. That seems much more like an air or spark issue.

 

Because out of spec valve clearances can and will drastically affect how the motor runs and can cause issues similar to what you're describing. Further, if you do have a carb issue, you won't be able to fully adjust your freshly serviced carbs if the valves are out of spec.

I ask because it's one thing that's often completely overlooked until it's too late. Initial check was to have been done at 3000 miles, and then every 5000 miles thereafter. Quite often they've never been touched. Here's some good reading on the subject:

http://xjbikes.com/Forums/viewtopic/t=33042.html

http://xjbikes.com/Forums/viewtopic/t=32118.html

 

it's just that getting the carbs fully dialed in and properly tuned is a waste of time if you haven't done the valves first. You can get it making noise and running down the road sure but to do it right you have to first do the valves THEN attack the carbs. Otherwise, best case, all your work on the carbs goes right out the window when you do the valves and you have to retune. Worst case is valves get neglected and burnt or cough or you just beat your head against the carbs for EVER and never get it dialed in because the valves need done first.

 

Well go figure. When I made my original post I had forgotten about the fact that since my last ride I had replaced my nasty, old, paper filter with a Uni. Marshallnoise's comment about maybe it's an "air" issue reminded me of that. A quick search on Google revealed that perhaps I was not the first to over-oil a Uni filter and create a rich-running/air-starved condition. I blotted a bunch of oil off the filter and she ran like a champ for hours last night. Case closed. I will still heed all the advice regarding valve adjustment, but the season is just too short up here to do it before winter. Thanks for the help to all who chimed in.

 

Doing a valve adjustment doesn't have to tie up the bike. I've been riding mine to work nearly every day, and just did my latest 5000-mile valve clearance adjustment; over the course of two weekends, maybe 2 hours each time. Popped the cover, checked clearances and ordered shims, and buttoned it back up and rode it for the following week while I waited for my shims. Just re-shimmed it this AM and now I'm good for another 5K.

The season'll get a lot shorter if you burn a valve; if the bike has 21K miles and you have no proof it's been done, you should at least check them. Only takes a couple of hours and could save you a giant headache.

 

Thanks Fitz. Convincing argument. I was under the impression that it was such a job that I would want to do it start to finish, including waiting for whatever shims I might need, before closing it up. So down time would include the wait, which had me thinking winter. Is it the case, then, that I could just re-use the valve cover gasket? Or should I get two before I begin (one for after checking and one for after changing)? This bike was such a find and runs so well that I definitely want to take a long view and keep it sound for the long haul. So, valves are bumped up to the short list.

 

Rubber gaskets are usually reusable. A new one is atleast, so you'll only need one if the current one needs replaced.

As long as the current gasket isn't horribly gouged up, and is pliable/bendable with no hardened spots, you can reuse it.

A little bit of sealant is a good idea. Mainly to hold it on the valve cover for reassembly. It's a bitch without it.



Basically, check the clearances, check and reinstall the gasket and look for leaks.

A spare gasket, if it doesn't need replaced, isn't a bad idea though. Especially with higher mileage. According to the PO mine went bad at around 35,000

 

Good "bump." As for the gasket; I would replace it the first time, then re-use after that.

Is your bike a YICS-motored bike, or pre-YICS? I ask because the gaskets are vastly different, and there are considerations in regard to both.

 

It is a YICS bike. '83 XJ750MK.

 

Ahh, OK.

The gasket is molded rubber and intended to be "reusable"; HOWEVER I would replace it the first time as it's likely to be crispy in spots. It's also VERY IMPORTANT to replace the metal/rubber composite "donuts" on the valve cover hold-down bolts when you replace the gasket, and here's why:

The bolts have stop collars on them that determine how far they tighten. The "donuts" are what actually press the cover down against the gasket and head. Once those donuts get all compressed and baked hard they don't work like they should, and even a new gasket will probably leak.

Once the rubber gasket and donuts have been replaced, they can be re-used many times over.