Author Topic: Running on three... stumped (Read 5483 times)

twincityrider · « **on:** February 20, 2012, 04:59:53 PM »

72 350f.

Any thoughts would be GREATLY appreciated. Here's the story:

Full resto, engine rebuild and all. Bike didn't run before I dug into it. Put it all back together and I can get it to run... on three cylinders, sometimes only two. I've been working on this problem for the last month. 2 and 3 pipes typically run the hottest right away (not just a middle of the engine thing). I've timed it when it runs on three. Typically 2,3,4 fire but not 1. I'm dane sure carbs are NOT the problem (I've thoroughly cleaned and gone through all of them probably a dozen times). I've also double checked my valves and they are good to go. I've had 1 running and not 4 when I switched the spark plug on 4 and 1 but not a guarantee (4 didn't run then). I thought it was just a bad "new" spark plug. That's not it as I bought another plug, gapped it, and the same thing happens. I've tried to diagnose if it's a bad spark plug wire by switching 1 and 4 wire... 4 will fire but not 1 so I don't think that's the problem. Weirdly sometimes 2 or 3 will drop out for no reason but not as usually as 1. I've also switched the coils and had the 2-3 coil run 1-4 with similar results. I'm baffled. 1 plug is always wet after I run the bike so I know it's getting gas, overflow tube is clear so I know it's not flooding.

The frame has been powdercoated along with some of the handle bar switches and and handle bar grips. I know I don't have a good ground on my ignition switches and handlebar but I'm under the impression that a weak ground up there makes no difference if I can turn the starter from the switch and get it to run.

At this point I need to know:
1) do the stock coils need to be grounded to the actual frame (remove the powdercoat)?
2) does a weak ground at the handlebars matter at all?
3) do coils fail or get weak? I thought they just work or they don't.
4) what am I overlooking?

Thanks guys! I'll be sure to respond to any comments and update when I figure this mess out.

-Matt in Minneapolis

chewbacca5000 · « **Reply #1 on:** February 20, 2012, 05:09:23 PM »

Hi Matt,

Sounds like you have done all the right troubleshooting and if you reread your post you will find the answer. From the sounds of things the Number plug wire is broken on the inside. When you swamped 1 and 4 plug wires 1 fired, but 4 did not. You are probably going to have to get a replacement coil.

Your other questions:
At this point I need to know:
1) do the stock coils need to be grounded to the actual frame (remove the powdercoat)?
Not sure. It does not matter as it would impact the other cylinders.

2) does a weak ground at the handlebars matter at all?
Weak ground may affect starting ability. Pretty sure this is not your problem.

3) do coils fail or get weak? I thought they just work or they don't.

They do fail, but not that often. The wires can get brittle and break inside.

4) what am I overlooking?
Nothing.

Thing you need to get new plug wires or replacement coil.

twincityrider · « **Reply #2 on:** February 20, 2012, 05:17:32 PM »

Hi Chewie,

Thanks for the response. I guess I wasn't clear in my post. When I changed 1 and 4 spark plugs (but not the wires) 1 fired but not 4 (at least the first time I started it up again). So I thought it was a bad spark plug. Then I switched out what I thought was the bad spark plug and 4 fired but not 1. When I switched 1 and 4 spark plug wires the result was the same (4 fired but not 1). I don't think it's a spark plug wire. The plugs are gapped correctly. I'm running original coils with new spark plug caps. I have bought a second set of used coils with identical results. Confused yet?

twincityrider · « **Reply #3 on:** February 20, 2012, 05:24:32 PM »

Oh and I'm getting spark on all four cylinders. I'm not convinced that the spark is strong though. I don't know how these coils split the energy from the battery/points. If it's weak could higher pressure inside a cylinder suppress it?

cbr954 · « **Reply #4 on:** February 20, 2012, 05:37:50 PM »

I know you said you rebuilt the motor, but I would do a compression check just to rule it out, probly isn't the problem but if you check it you will now for sure. If you are absolutely sure it's not the carbs then it has to be ignition. Have you cleaned and set the points? The Honda coils of this era do not have a lot of output. I have had a bad coil on a 750. Do 1 and 4 miss at all rpms or just idle?

twincityrider · « **Reply #5 on:** February 20, 2012, 05:42:09 PM »

I'll do a compression test but I'm getting great pull off all the carbs and wet plugs. The points are brand new. Either 1 or 4 fires but not both (or very weakly with the second) at either idle or at higher rpm. Strangest thing. Thanks for your input. Any other ideas?

scottly · « **Reply #6 on:** February 20, 2012, 05:53:57 PM »

Pics of spark plugs, please. Also, measure the resistance of the plug caps. I suspect you have a float level problem on #1 carb; even if it doesn't over-flow.

Bill/BentON Racing · « **Reply #7 on:** February 20, 2012, 05:54:55 PM »

Get another known good coil,try it,still the same,isn't the coils,are pts and cond's new? Just had to replace Dyna coil from 6/83,would work fine til it got hot.A Dyna S and coils would improve your ignition,also check out Hondaman's stuff for points. I think you have a bad coil if pts etc are good.Good Luck,coils get weak,more compression etc need more spark!

Bill

cbr954 · « **Reply #8 on:** February 20, 2012, 05:57:15 PM »

I guess I would try a new coil if it was mine. A lot of intermittent electrical problems are the result of poor grounds, I would make sure you have a good ground between the frame and the motor and the battery.

BobbyR · « **Reply #9 on:** February 20, 2012, 06:07:11 PM »

It is a long shot but you may have a cracked plug. They can act weird. Also if you can run the bike in the dark, you can sometimes see leakage that way.

twincityrider · « **Reply #10 on:** February 20, 2012, 06:21:47 PM »

Thanks guys.

Bellcow, I was leaning towards just going with new Dyna coils period. Can I use Dyna coils w/ stock points? I may as well eliminate the coil/wire issue for once and all. Thanks.

Scottly, I can't grab a picture tonight. This weekend though. My plugs foul out pretty easy at this point. 1 is just wet and oily. The rest when they don't fire are dark and dry. I will measure the resistance of the plug caps as well.

CBR, I'll fix my powdercoat problem at the battery and groundstrap to the frame. Thanks.

Bobby, I'll look at my plugs. Appreciate it.

Updates this weekend!

tlbranth · « **Reply #11 on:** February 20, 2012, 07:47:24 PM »

Can you switch coils, change primary connections, plug wires and see if problem moves to 2 or 3? Don't run it too long with gas and no fire. You can damage the no-fire cylinder, piston & rings.

scottly · « **Reply #12 on:** February 20, 2012, 07:52:36 PM »

Quote from: twincityrider on February 20, 2012, 06:21:47 PM

Thanks guys.

Bellcow, I was leaning towards just going with new Dyna coils period. Can I use Dyna coils w/ stock points? I may as well eliminate the coil/wire issue for once and all. Thanks.

Scottly, I can't grab a picture tonight. This weekend though. My plugs foul out pretty easy at this point. 1 is just wet and oily. The rest when they don't fire are dark and dry. I will measure the resistance of the plug caps as well.

CBR, I'll fix my powdercoat problem at the battery and groundstrap to the frame. Thanks.

Bobby, I'll look at my plugs. Appreciate it.

Updates this weekend!

Black and wet indicates rich mixture; white and wet indicates lack of spark. No fire =no soot.

Bill/BentON Racing · « **Reply #13 on:** February 20, 2012, 08:30:39 PM »

Yes you can,see Hondamans stuff,can take the load off pts(will last almost forever).Bill

trueblue · « **Reply #14 on:** February 21, 2012, 02:53:00 AM »

Have you checked to make sure you are getting 12v to the coil, if there is resistance in the 12v circuit you won't get a good spark, also check grounding on your points, also are your points adjusted correctly. Higher compression from a rebuilt engine will only emphasise any weakness in the ignition system. When diagnosing a fault don't forget to check out the whole system.

TwoTired · « **Reply #15 on:** February 21, 2012, 03:46:02 AM »

Quote from: twincityrider on February 20, 2012, 04:59:53 PM

At this point I need to know:
1) do the stock coils need to be grounded to the actual frame (remove the powdercoat)?

The coils don't need a ground. But, some bikes have the green harness ground at the coil mount, and must have a clean metal to metal connection there.

Quote from: twincityrider on February 20, 2012, 04:59:53 PM

2) does a weak ground at the handlebars matter at all?

Well, yes. But, not for spark. Frame ground is a pathway to the battery NEG terminal, For the bar is must go through the neck bearings and grease. Ok for the momentary contact for the starter solenoid and the horn. But, you don't want constant current flowing through roller bears and their races. If water ever gets in there, you'd get electrolytic corrosion.

Quote from: twincityrider on February 20, 2012, 04:59:53 PM

3) do coils fail or get weak? I thought they just work or they don't.

Anything man made can fail. Coils are pretty robust if not physically abused. The wire leads are what usually fails first.

Quote from: twincityrider on February 20, 2012, 04:59:53 PM

4) what am I overlooking?

A few things.
1- the points may have a preservative coating on the contacts that hasn't worn through on the 1,4 points. Burnish the contacts to remove the coating with some card stock.

2- You can try swapping the condenser between 1,4 and 2,3. If the problem follows the condenser, then replace it.

3 - Be certain the power lead is solid to the coils B/W connections. Measure the voltage there to see if it gets all of the battery voltage. If not, you have lossy connectors or switches.

4 - Do check your battery voltage to be sure it's well charged. Turn off the headlight/lighting to increase voltage to the coils.

5- Look at the coil spark leads while running in the dark. Arcing through faulty insulation points will glow.

However, if you have spark on all four spark plugs, then it is time to revisit carb issues. Verify actual fuel level in the carb bowls. It doesn't take much zap to start the proper air/fuel mixture.

Cheers,

twincityrider · « **Reply #16 on:** February 25, 2012, 02:40:07 PM »

Thanks guys,

I will post a proper followup this week. Unfortunately I'm pretty sick this weekend and won't be dragging myself out to the garage today. Thanks to everyone who's posted their thoughts and ideas.

Matt

twincityrider · « **Reply #17 on:** March 06, 2012, 05:06:09 PM »

Wanted to do a quick follow up. Well after some tinkering and experimenting I've discovered I must have a carb issue. And I swore two weeks ago it couldn't be carbs. For as many times as I've had the carbs apart I thought I had that issue licked... guess not. I installed the dyna s ignition along with dyna 5 ohm coils (both look like really quality products). Really great spark now but only running on cylinders 3,4. 1 and 2 will not fire. I will pull the carbs again this weekend and go through 1 and 2, re-clean all passages, and adjust floats to match 3,4. Thanks guys for all your help. I'll be sure to update when I know more.

scottly · « **Reply #18 on:** March 06, 2012, 05:15:10 PM »

Be sure to check your float levels using the "clear tube" method.

Imago · « **Reply #19 on:** March 06, 2012, 07:32:08 PM »

I was running on 3 cylinders on a trip to the States many years ago... a great Honda dealer in Montana finally found the culprit... an intermittent short in the spark plug cap. Nearly drove me crazy trying to track it down. Try wiggling/bending each of them while checking impedance with your multi-meter.

twincityrider · « **Reply #20 on:** March 11, 2012, 11:09:12 AM »

Question: using the clear tube external float height check, what is the measurement between the gas mark and the float blow seal (350f)?

Yesterday I took apart the carbs again and cleaned 1,2. 3 and 4 had been running but 1 and 2 had not. I didn't find anything unusual. Reassembled the carbs after making sure my floats seemed adjusted properly (21mm). I did not adjust 3 and 4 as they had run. Put it back on the bike and bike won't start. I've since checked the external float heights and 1 and 4 are 4mm below the seal and 2 and 3 are 7mm. What's the holdup? Thanks guys.

mycb750k6 · « **Reply #21 on:** March 11, 2012, 11:21:24 AM »

I was running on three cyclinders until I found the problem. All of the removal and replacing of spark plug wires had broken one of the wires inside the insulation. The plug cap kinda flopped around like a - well you know. Pulling the plug wires off the 2-3 plugs can be hard on the wires.
So try replacing that coil and put new caps on. Worked for me. Sometimes a weak spark connection is hiding a more significant electrical problem.

twincityrider · « **Reply #22 on:** March 11, 2012, 01:28:14 PM »

I know it's not plugs or plug wires. I just put a new Dyna S ignition and coils/wires on with new boots. No problem there. Thanks for the response. Any ideas on the gas level in the bowls?

mycb750k6 · « **Reply #23 on:** March 11, 2012, 01:41:46 PM »

After fixing the problem I described, I next found that the new fuel tubing I used had a kink in it preventing any fuel flow. Doesn't take much to keep these things from starting. Make sure you have fuel coming out of your float valves!!!

twincityrider · « **Reply #24 on:** March 11, 2012, 04:37:37 PM »

mycb,

Thanks for the response. Fuel flow might be something i should look into further. It seems like I'm getting good flow to each carb but I don't like the air bubble that is in my fuel line about halfway up. I'm using a three foot test line above the bike so I can tinker with the carbs without the tank on. The bubble never really goes away even with the fuel cap open. Still stumped.

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Author Topic: Running on three... stumped (Read 5483 times)