Rough running and hard to start. Ignition coils maybe?

Hey all,

So, I just recently bought an '83 XJ750R Seca and it's a real beauty. It is my first bike. You can hardly tell the age of it. Anyways, the problems I'm having are troublesome. When I first got the bike, it was hard to start. I had to roll the throttle numerous amounts of times, choke all the way on, and crank it over many times in order to get it to stay idling to warm up. Once i got it started and even warmed up, i couldnt take the choke off, otherwise it would die. Once it warmed up and I hopped on, it was a blast, except for acceleration issues I was having. The bike felt like it was being bogged down and I couldn't get any power. I'm only 140 pounds so its not like the bike is too small, but I'd have to put the clutch in and rev it to 5000 in order for it to smooth out and get on my way.

So, that's when I decided to take apart the carbs and clean them out and give 'em a good bench sync. Put them back in and it starts right up. Very happy. But alas, another problem arose. Every time she begins to warm up after starting, right as the revs start to increase from being warmed up, the revs drop down to below 1000 and it struggles to stay running. It sounded like it was misfiring and i only had a 2 cylinder bike (the bike is 4 cylinders, so obviously that wasnt right). Then a couple seconds later it dies... what a bummer. After a few minutes she'd start back up and then repeat the same thing.

I start to attempt my diagnosis (I'm no expert mechanic, but i mostly know my way around an engine). I put a new battery in. No fix. I left the gas cap open and left the petcock on "prime" to see if it was a fuel issue. No dice there either. I finally took the ignition coils off and checked those. Primary resistance readings were average for a bike (3.5-4.0 ohms is what i got. Spec levels are 2.5, but 3.5 didnt seem too wacky). Secondary resistance for the 2nd and 3rd cylinder coil read 11,000 ohms, give or take 20% or so. Normal. The 1st and 4th cylinder coil, however, I got nothing. Nada. Zip. No resistance readings to speak of. (i checked secondary resistance by putting my multimeter thingys in each spark plug wire with the I also happened to be feeling around the exhaust pipes too, and i noticed that the pipes for cylinders 1 and 4 were slightly warm, where the 2nd and 3rd cylinder exhaust pipes were hot! Getting close, i thought. i still get sparks from 1 and 4 though. when i checked.

S0, do i ahve a bad coil, or something else entirely?

 

swap the coils around and see if the problem follows the coils

 

welcome to the forum. fire it up in the dark and see if the plug wires and or their ends are shorting to ground.



I replaced my wire ends and it runs much better.

FU

 

You said you did a bench sync.....have you done a running sync?

Have you checked to make sure that the valve shim clearances are in spec? Did you clean the idle circuit or the enricher circuit thoroughly? What did you set the mixture screws at? What height did you set the floats at?

Dave f

 

I did a bench sync and was planning on doing a running sync with some vacuum gauges i have, but the bike will not stay running long enough for me to do it. (i've got my YICS eliminator installed and everything. Thanks Larry!)

I have not checked valve shim clearances yet, although i will after i replace the bad coil. Its just something im afraid of doing for some reason... ill get over it.

It is said in my manual that it is NOT recommended that i tamper with the mixture screw. My manual (Haynes) says the screw is preset and doesn't need to be adjusted.

Float heights were set to spec according to my manual (0.7 inches, i used a small ruler that measured in 16ths, so i did the math and used a proportion to figure 0.7 inches in 16ths and went from there). i had never done a carb job before, but i'm nearly certain they're correct.

Im new to motorcycles and it was my first ever carb job, but i think i did a hell of a job. watch hella tutorial videos. made sure each jet was cleaned and i saw light through it. soaked them all in carb cleaner (except for the float needles, since they had rubber tips, i touched them up a bit with carb cleaner but took them out before any of the other jets). i spent hours doing the job. When i get the coil replaced (because i need it anyway), ill see how it runs from there, but im really appreciating the suggestions. every little bit helps ya'll.

As for idle circuit and enricher circuit... i have no idea what those are... enlighten me, if you dont mind.

Chacal told me it may be circuits of some kind. a fuel related issue, because i mentioned i was still getting a spark on the number 1 cylinder. I took the spark plug out with it still connected to the plug wire and grounded it against the engine block. i saw spark. I will definitely get out at night and check for the spark grounding out.

Jacob B

 

REGARDLESS of what the manuals say about the mixture screws, you MUST pull them out in order to be thoroughly clean. If they have never been removed or adjusted, they will be under a round brass cap pressed into the top of the carb body just to the left of the enricher plunger boot, and be dead center of the body. You must carefully drill the cap then pull it out. There's plenty of tutorials here for that, as well. You should turn the screws down, counting the turns in til it soft-seats, record that number and you can put it back there when you're done. Now, unscrew it completely. There is also a spring, a washer, and an O-ring in there. I'd recommend, though, putting it back at 2.5 turns out for factory base setting, plus an extra 1/8 turn. From there you do your colortuning to dial it in.

If there's any dirt, old fuel, or bad o-ring here, you'll always have idle issues.

enricher circuit is the brass tube that extends from the carb body down into the well into the carb bowl. There is a non-removable jet at the bottom of the well. If that is clogged, you'll always have trouble starting.

If your fuel valve rubber-tipped needles are worn, hard, sticky, etc..... you'll have trouble with either flooding, or starving.

If you're floats are not at the correct height, you can have the same issues. Correct height is 17.5mm, then wetsetting to confirm final height.

Dave

 

Excuse me while I play BigFitz for a moment. Ahem.

First things first. Get the valves in spec. Out-of-spec valves can cause a variety of problems that make you think it's something else. There's a thread under "XY DIY How-to Instructions" that explains how to check and set valves. New shims can be purchased for member Chacal or Hogfiddles runs a shim swap. Until you get the valves in spec, you're just chasing your tail.

I feel better, now. Has anyone heard from Fitz lately? I miss that cranky old cadger.

 

Rubber tipped needles were in good shape. flexible, no scoring or wearing.

I will se about getting a shim and doing the valves. I've heard they can cause all kinds of problems and make me think it's something else.

Good to know about the mixture screw. Iwill take it out and check it out. I just always worry about breaking something, but I have the resources of this website in case that happens. I'll be careful!

As for wet setting the floats I'll find a tutorial on that and do it. I've been meaning to, but unfortunately I can't get my degree in accounting by working on my bike all the time like I want to... I wish that was the case Though...

I love how fast the responses are yall. It's much appreciated. I'll post when I have some more info and I've done some more tinkering and stuff. Thanks again

Jacob B

 

Already said that..... But keep repeating it---- shim clearances should be in spec before doing all your carb work.... Otherwise much of that work will be negated once the shims finally DO get put in spec.

 

*Cough* shims and carbs *cough*

 

Okay, getting a little pissed off now at my bike. It's not the bikes fault, but it's very frustrating.

Put the new coil on, and it fired right up. I let it idle for a while and it started to drop down in rpms again. Below 1000 and then dies after a bit. Next day, it won't start at all. I crank it and crank it to no success, and occasionally a loud pop is heard afterwards (backfiring?) From the exhaust.

I'm in the process of ordering new valve cover seals and rubber fasteners so I can do a valve job. Could the calves be so outta wacky to cause a no start issue? I wish I had someone near me who knew more and could check it out. I'd do anything to get back on the road.

Thanks again ya'll.

 

Valves** not calves

 

That's just a typo.....
But yes, valves out of spec CAN cause that, so can out of whack, dirty, clogged, etc....... Carbs

 

Don't rush anything. Keep your goal in mind, and don't allow time to be a factor in reaching your goal of having a roadworthy motorcycle. Be the water, not the rock.