Testing the coils by switching connectors, Not Good results

I jumped in on a post last week and I can't find it now. I have a 83 XJ750 with some confusing symptoms. Since I can't find the post I will explain from the beginning.
When I bought this bike I was told that the carbs need cleaning. No problem. Before I started on the carbs I was having a problem with #1 cylinder not firing, the pipe was cold. I decided to troubleshoot the coils and pickups. The pickups were within specs. I proceeded to the coils. The primary resistance checked good on both coils. I was having trouble getting a check on the secondaries (HT wires) on both coils. I switched plugs from a couple of cylinders for the heck of it. All of a sudden, I got #1 back and #2 & 3 quit hitting. I checked a few things and suddenly #1 & 4 quite firing and 2 & 3 started firing.
I decided to eliminate some areas and rebuilt the carbs. I spent a great deal of time making sure that the carbs were clean, especially the pilot tubes and orifices in the fuel bowl. I also spent a lot of time getting the fuel level set according to the 3mm setting according to the Haynes manual. I am not new to rebuilding carbs and even tough these were a pain I reinstalled them and started the bike.
After I installed the carbs I replaced both coils with used ones. I checked them with an Ohm meter before installing them. Both checked good. I started the bike and still no #1 cylinder. I tweeked the pilot adjustment for #1 carb and the pipe seemed to get quite a bit warmer, but still not blazing hot like the other three.
On the post I was on last week I was told I could switch the coil leads around and see what happens. I switched 1 & 4 around, still the same, no #1. I was told that I could switch the primary (input) wires between coils and see if that made a difference. It did. It won't start like that. It did backfire very hard a few times but no start. Switched them back and it fires up but still has #1 dead.
I actually saw the #1 & 4 cylinders firing briefly before doing the carbs. is it possible that the CDI box is bad causing the #1 cylinder to not fire? I don't have another CDI and there is no test to see if it is good. I would have thought that if the CDI was bad that both #1 & 4 would misfire.
Any ideas?

Thanks, Ed

 

When you switch the coil primary input leads you have to switch the secondary leads also. The left coil fires #1&4 and the right one #2&3. Switch these and then switch the primary leads. I would also pull the plug caps off the end of the plug wires and check them for correct resistance. They can short out or have so much resistance the plug won't fire under compression.

 

Yeah, if only #1 isn't firing, it isn't a TCI or Coil issue. Both #1 and #4 fire at the same time from the same TCI signal so one cannot work without the other, unless there is a wire issue. What you should do is switch the signal wires (primary if you will) and also switch the plug wires (1&4 to 2&3 and visa versa). The dead cylinder should move to #2 then if it is a wire issue. I would also try replacing that spark plug also, it isn't unheard of for a brank spankin' new plug to be bad. If the dead hole does switch to #2, unscrew the cap, trim back the last 1/8" of plug wire or so and screw in the cap onto new wire. Make sure that the copper wire is at the end of the insulation and not playing hide-and-seek. While you have the cap off, check the resistance through the cap, I can't remember the spec though, but they are resistive caps.

If it stays on #1, then it might be compression issues, stuck/bent valve, blown head gasket, or dirty carb (I know, I know, but it happens to the best of us).

 

Re: Testing the coils by switching connectors, Not Good resu

Hi Bluemaxim,

That is what I was thinking to begin with. I asked if the primaries were the only leads to be changed and was told yes because the system had a "wasted spark" and would fire every time it came to TDC.
I am trying to determine if I need to get another CDI. I have my #1 cylinder that won't fire. Something that happened or I did caused it to start firing on that cylinder and I lost #2 & 3, then lost 1 & 4, then #4 started firing and #1 is dead again..
I changed the coils out and it still does it. I even reversed the spark plug leads for # 1 & 4. It still runs the same with #1 missing. If it wasn't for the fact that I saw with my own eyes that #1 & 4 started firing and the pipes got very hot immediately and the other two got cool I would suspect something else. I am highly suspect of the ignition system.

Ed

 

I used a spark gap tester like this...



...to help determine if I was loosing spark or if the problem was fuel related. I unplugged the spark plug wire, clipped this to the end of the spark plug, then ran an alligator test lead from this back to the spark plug wire to complete the circuit. By do it this way, you can see if you're actually getting spark on each revolution. It was a big help in diagnosing my ignition troubles.

The tool shown is made by Thexton, but I know that Lisle and KD Tools also offer versions, too.

I'll add that you should put in another set of plugs just in case you've got one that is fuel-fouled. They will cause symptoms like what you're describing.

 

Re: Testing the coils by switching connectors, Not Good resu

PvtSchultz,

I have done everything you said except for switching the HT leads as I switched the primary leads. The compression is low on that cylinder but like I said, I saw the engine change and start firing on that cylinder and two others started misfiring.
I am ready to get irritated and pull the top end apart for a re-ring because of the low compression. I thought about trying to put something in the cylinder to soak for a bit to see if the rings could possibly be stuck. The compression does come up with a few squirts of oil. I haven't checked the compression on the other cylinders.
I can't get over the fact that I saw the cylinders start firing and two others started misfiring. I WILL find out whet the problem is sooner or later.

Actually, I bought this old bike at a swap meet and the owner said that the carbs needed cleaning. I asked him if it would run and he said yes. He reached over and hit the key and the start button and it fired up. I happened to be looking him in the eye at the time it started. The look of surprise in his face was amazing. he turned t someone that was with him and said "Do you believe that? It actually started". He then recomposed himself and started talking to me again.
I bought the bike to sell and make a few bucks on. It actually looks pretty good and has good tires. I gave $600 for it and loaded it up and brought it home.
After unloading it, it actually started again and I was riding it to the front of my house. I was going to put a "For Sale" sign on it as is. My neighbor saw me riding it to the front and came over. He immediately wanted it.
Well, that shot making any money on it. I rebuilt the carbs after having the prior mentioned problems with it. One more thing to eliminate. Had I known about the problems I am having with it now I would have bypassed it. Yeah, RIGHT! It is really a decent looking bike and looking back I really would have had problems walking away from it even knowing of the problems I am having now.
This IS going to be a great bike. Now that he has the title in his name I can't leave him hanging with it. It is a lot easier to sell something to a total stranger than to sell it to a close friend and neighbor. It certainly would be a lot easier to have not had to work on it. When it all comes down to it, I am probably going to loose money on it because I feel I have to make it dependable for him and I can't charge him for anything I do to it, I would not feel right about that. I will be lucky to get the money back for the parts. When I get through with it the bike still won't be worth anything to speak of because of it's age. It will just be a good ole bike.
On top of that, he wants me to paint it black. :cry: It is now red and he doesn't like red. I build Street Rods and have rebuilt a lot of the older muscle cars like the Chevelles and Camaros, Z28s and others. The last car I painted literally has a room full of trophies for "Best Paint" and "1st Place" from some major car shows. He wants that old 25 year old bike to look like that :cry: . He doesn't realize the work and expense that it is going to take to make it look like that.
Problem is that I can't just put a decent paint job on it, I would never be satisfied with it. I know just as sure as I am sitting here that it would look like that Chevelle I just mentioned. This bike is very straight with no dents in the sheet metal but it just is not worth the kind of work I am going to have to do on it.
Words of Wisdom: Don't ever sell something to a friend and neighbor. I guess I got a little off topic here.
Anyway, thanks for the replies.

Ed

 

Re: Testing the coils by switching connectors, Not Good resu

Gamuru,

Interesting looking device. It looks somewhat similar to testers I used back when I was younger on conventional ignition systems. I haven't had a need for something like that for over 30 years. Most of the test equipment I have is worthless on these bikes. I still have all of my old equipment, I never throw anything away like that.
As far as the spark plugs go, I switched the plugs from different cylinders that I knew were firing. The plug I removed from the #1 cylinder then started firing in the other hole and the one I installed in #1 started missing. Definitely related to that one cylinder.
I was really hoping at someone could tell me if the CDI box could cause a problem like this. I don't really think so but figured I would ask.
BTW, I like the avatar. Kind of right down my alley.

Ed

 

Have you checked the leads coming from the coils to the pig tail. When I first got my bike, I had the same problem. If you look at your coils, you will see two wires coming from each coil. These are the + primary wires. Mine has broke at the bottom of the plug that connects to the other plug going to the TCI(cdi). With out good connection, the coil can nnot be charged. From reading your posts, It makes sence. When switching the primaries, the problem would move. These wires are at the right age to be brittle. When I was doing my tests, as you did, I ended up pulling one of the wires out of the connector for the coil that fires 2 and 3. Then I had two dead cylinders. Its worth a try.

 

If you can't get the miss to move by switching the plug wires, it might be a dead hole and not anything spark related.

The two wires (primary) that lead into the coil are for 12VDC and for the TCI (CDI) trigger which causes the collapse of the magnetic field resulting in the spark. Switching the two wires will do nothing to move the spark since the one signal triggers the primary field collapse with both plug wires connected to the secondary field. (Thank Faraday for that discovery).

With a bad plug wire/boot, you should be able to move the miss from 1 to 4 by switching the plug wires. An easier way to check for spark is to pull the plug out and lay it on the cylinder head. You'll see if there is a spark or not. Also, you could use a multimeter to test for continuity between the two plug boots. If there is an open circuit, you have a bad wire/boot/connection along the way. If the one hole fired/ran once, I am inclined to say that your plug wires are hashed.

Otherwise, it's a carb or compression issue. Could always check the valve clearances to see if they're too tight or none at all resulting in some bleed down. But you did see an increase in compression doing a wet test. So, I'd first lay the plug on the cylinder head and see if you can see a spark in the gap. BTW, what is the plug gap set to? Just had to ask.

 

Re: Testing the coils by switching connectors, Not Good resu

Hurst, any update on that dead cylinder?
I have the same problem with the #3, I'm kind of lost at this point....

 

Re: Testing the coils by switching connectors, Not Good resu

At one point you said you had switched plug wires between #1 and #4 and it was still #1 that was cold. If that's the case, the only thing ignition related that could be the cause of your problem is the spark plug in #1 itself.

As others mentioned, the plugs fire in pairs, so if you're getting spark out of three out of four plugs then TCI is good, coils are good, and wires and caps are probably good.

So, likely causes are a bad plug, a problem with the #1 carb, or mechanical problems (bad valve, etc) with the cylinder itself.

 

Re: Testing the coils by switching connectors, Not Good resu

I have not got into it as of yet because my wife is running me ragged on home projects. After checking it out a bit better, I believe it is a tight valve causing the miss. That particular cylinder is low on compression.
I checked out one of the other XJ sites and found that they have a shim library available. You have to request the size shims you want, leave a deposit and when you return your old shims they send the deposit back. I am going to take advantage of that.
The thing that screws my mind up is that when I was fooling around with the coils and wires, that particular cylinder started firing and I lost two others that had been firing good. I never could duplicate it again. When you run the crap out of it the pipe for that cylinder starts getting fairly hot but cools down quickly at an idle.
At first glance it seems like a carb issue. I pulled the carbs three times to reset the float levels, adjusted the pilots with a home made YICS tool and still no change. After I get the compression issue taken care of I will have to re-adjust everything again. I have a new set of rings and gaskets for the bike but want to try and change the shims first.
I came close to pulling the shims out last evening but quickly snapped out of it when I was reminded that I still had not finished "other" things.

 

I just skimmed this so this may have been asked - Have you tried replacing the plugs?

The symptom of the pipe heating when you run it hard then cooling at idle sounds more like heating from the other running cylinders then it running at higher RPM - Pipe would stay hot for a long time.

My bike sat a few weeks and when fired only 2+3 ran. After chasing spark etc and cleaning the 2 plugs I finally swapped 1-2 and 3-4 plugs and 2-3 now would not fire. New plugs and the problem disappeared.

I never had 2 plugs go bad - especially just sitting and no apparent damage etc but that was the cause.

Just a thought....

 

Re: Testing the coils by switching connectors, Not Good resu

If this means that you (or someone else) might be getting USED shims, this is a bad idea.
Shims wear, they rotate under the cam lobe and the outer hardened surface gets worn away; so they are NOT the "marked" size any more nor are they to be trusted.

From the XJ4ever catalog:
NOTE: there has always been some discussion regarding the re-use of old valve shims. The factory advice and our advice is this: "DON'T". Valve shims and pads are designed to "wear items", much like brake shoes or pads are.

The heat-treating on the valve shims extends only a small distance below the surface face of the shim, and normal wear will "eat through" this surface hardness and then result in rapid wear of the shim.

Re-using old shims is false economy, honest. chacal has aftermarket shims, new, for $7.50 ea or $7 if you buy 5 or more.

Last but not least, remember NOT to use any magnetic tools in the valve shim removal and replacement process. All that being said, by all means get the valves adjusted correctly HOWEVER: It sounds to me like you are fighting an ignition or wiring-related problem.

 

Re: Testing the coils by switching connectors, Not Good resu

Hi fitz,

I have been through the ignition system on this bike until I see it in my sleep. I replaced all the plugs, checked the resistance on the pulse generators, coils and leads (primary and secondary) and even went so far as to replace the coils. The bike has right about 60 PSI compression on the #1 cylinder dry and about 65-70 PSI wet. At first I thought it was rings but with only that much difference between wet and dry I did some more checking.
Unlike a lot of gasoline engines, the XJ valves get tighter with wear instead of getting looser. It is my belief that because of the lower compression that running the bike at higher RPM and more throttle that it gets enough compression to fire somewhat, but still not efficiently. It occasionally gets hot when running hard but not nearly as hot as the other pipes. This also would explain why it cools quickly when running slower.
On the issue of using used shims. I am a firm believer in most cases of not using used lifters in an engine. Lifters in an automotive engine are not hardened nearly to the extent that the XJ shims are hardened. In my experience of being a machinist and heat treating tool steel, as thin as the shims are, I doubt that they are case hardened. Typically, when a piece is case hardened and ground to size you will remove most of the hardened material. Therefore something of the nature of the valve shims would be hardened throughout rather than case hardened.
When I heat treated material that had to be ground to size, I would machine it close to size, heat treat it and then finish grind it to size. Otherwise it would be almost impossible to maintain size of the shim. Typically when a piece is heat treated it will actually shrink somewhat making it extremely difficult to maintain size. This is another reason why it is roughed out and then reground after heat treatment.
This is not to say that the shims will not wear, because they can and will. I say all of this because the hardness is more than a few thousandths deep. There is a lot of reuse of the shims with no problems, but they should be checked with an accurate micrometer to make sure of the size before being used. A lot of the techs at Yamaha dealers will reuse the shims. The reuse of the shims has to be on a case by case basis.
You are correct in saying that the shims really should not be reused, however, if you should replace all the shims it would be a bit of strain on on the old pocket book. I am sure if a lot of guy's on here had butt loads of money they would have new bikes instead of 25 year old ones. Then again, I think I prefer the older bikes over the newer ones, depending on what it is.

 

You could be right about tight valves on that cylinder. I would definitely check it. (Check 'em all while you're in there.) Out of curiousity, what were the compression readings on the other three pots?

Shims at $7 or $7.50 are cheap compared to the cost to repair a destroyed camshaft or the damage caused by a spit shim. Used shims are false economy.

Lastly, I'm willing to bet that a large number of our members COULD afford to ride a new bike but CHOOSE to ride the older bikes, simply because there are no new bikes that strike our fancy. I could name quite a few besides myself...

 

I would highly agree with you. I not too long ago bought a 85 Honda Shadow VT1100 because I liked the way it sounded. I sold a new 07 Honda Shadow 1100 with only 290 miles on it. It was a beautiful bike but sounded like it had a tin can for a muffler and just didn't have the power of the 85 Shadow. So, what you are saying is correct.
Has anyone ever posted a poll on whether the membership could afford a new bike or if they preferred the XJ? It would be interesting to know.

 

I don't think there is much cheap about an older bike. The maintenance costs can be high but you don't have the initial high layout.

I much prefer the older bikes, like you guys I could buy a new one but what's thefun in that?

 

Enough said. That is all that counts. Regardless of the age, if you like it and it makes you happy. Especially if it is paid for. Sometimes the worst part about it is getting parts.

 

Not at all, at least for us. We have chacal (would be COMPLETELY different without him) and for the Norton I have British Cycle Supply who have as complete an "in-stock" inventory for Norts as Len has for XJs. And then there's eBay. Parts NOT an issue here.

There's also the "keepers of the flame" factor, which most of us are part of but only some actually aware. Like my fellow British bike riders, or the vintage and classic car guys, or the fanatics with their restored steam engines and 19th century farm tractors, we are the people keeping this "bloodline" alive. But like the warbird aircraft owners or vintage racers, we can't just make museum pieces. We are driven to resurrect and ride them; use them as they were intended. Some are inspired to chop, stretch and customize them like a hot rod car enthusiast with a '69 Camaro or 23 "T". I think we're all just a bit loopy...