81MHR idle and run issues: slide wear?!

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81mhr900ss
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81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Fri May 10, 2019 1:47 pm

I have an 81 MHR that I have been trying to get to run after a rebuild for the last couple of weekends. The front cylinder is happy but I'm fighting with the rear. I just put a new set of Iridium plugs in this morning and fired up a few hours ago and if anything things were worse with the iridium plugs; almost nothing from the rear cylinder. I put the original plugs back in and the front cylinder got things going on the first kick but the rear is still not happy, intermittent firing as before.

I pulled the rear carb (completely cleaned and rebuilt just a few weeks ago to recheck the idle circuits and all are clear. I did a test of the slide using a flashlight; there is some light passing on each side when the slide is fully down. This bike only has 23k km on it. I just swapped the slides two weeks ago to get fresh contact surfaces for the throttle stop screws. I'm going to pull the front carb and compare and while at it, swap the slides again and compare light bleed around the sides. What is acceptable slide wear/air bleed? If anyone has a spare PHM40 sitting around, would you please do a similar flashlight test and report back? Obviously light means air is getting past the slide and weakening the mix but it should at least be consistent.

I very strongly suspect that I have an ignition issue but really would like to put carburation to bed before spending another US$1k or more on an updated ignition system. There are numerous 81 900SS bikes running on the original Bosch/ND ignition system. I very carefully tested my pickup wiring, wobbling the wires while connected to a multi-meter and did not lose continuity. I also swapped Bosch units as I have two spares but the rear cylinder problem remains. I've discussed this on some FaceBook groups and there's a consistent push to go with a new ignition system; I'll gladly do this if this is what is really needed but if I did and the issues I have now remain, it would be a considerable waste of time and money. I'm almost two months behind on this project already and it's 24 deg C outside!

Any words of wisdom would be most welcome

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Craig in France
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Re: 81MHR idle and run issues: slide wear?!

Post by Craig in France » Sat May 11, 2019 5:12 am

1. If it really IS intermittent firing that you’re getting, then your suspicions should trend towards ignition. Carb problems tend to exhibit themselves as ‘constant’ problems - classically, a refusal to idle.

2. I can’t comment on your diagnostic technique for slide wear - I don’t know whether it works or not! But slide wear is normally very visually apparent.

Also: if you’ve already swapped the slides over and the problem stays with the rear, I would tend to discount it.

3. You say that you’ve swapped the Bosch boxes, but the problem remains. Have you done the same with the coils?

4. Also: have you cut back the plug lead on the rear coil AND fitted a new plug cap?

5. Did you disturb the pick-ups when rebuilding the bike?

P.s
1. I’ve never had much success with Iridiums in our old lumps - and frankly, I’m not persuaded they add much. While I do run Iridiums in my Laverda Formula, that’s only because it’s such a right royal PITA to get to the plugs. On a bevel, it only takes seconds to change the plugs, thereby negating one of the big advantages of Iridiums.

2. Distance traveled is not a good guide to potential slide wear. Run a bike using clean, quality air filters, and the slides will barely wear over tens of thousands of miles. Run it with open bellmouths over the desert, and in a 100 miles they’ll be cooked.

HTH

Craig

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Craig in France
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Re: 81MHR idle and run issues: slide wear?!

Post by Craig in France » Sat May 11, 2019 5:22 am

Another thought ...

You say you checked the pick-up wiring for continuity. Have you done the same for the cables between the Bosch boxes and the coils?

And don't forget to check the ground connections!

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81mhr900ss
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Re: 81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Sun May 12, 2019 12:20 am

Thanks for your responses Craig: I did check continuity of all the wiring and components in the comfort of my living room and replaced the plug caps. I also cleaned all the original factory ground points and all the wiring connectors that would be making contact with them. I did not disturb the pickups other than carefully inspecting the wires inside and outside the casing. The bike only has 23k km on it and the inner pickup wiring is in surprisingly un-cooked condition; I'm sure you know what I mean. When I added 12V, I was getting two strong, blue sparks so it looked like all was OK.

I've had three weeks of business travel and only very little weekend time to work on the bike. Two weekends ago, it fired up for the first time on the fourth/fifth kick but was clearly lumpy on the rear cylinder. I made a few small changes and checks and just get the same. This morning I swapped the carbs just to put this to bed and the bike fired up so happy on the front cylinder on the first kick with the rear still lumpy on the rear so it's not carburation. It wants so badly to run! I think the slide wear thing is a red-herring; they both look really good with minimal scoring.

29 years ago, it came off the road because one cylinder went dead. I traced it to a melting Bosch unit; I found a small lump of black goo on my engine casing and looked up. I have a total of three good (I think!) Bosch units a (but was not able to find any bench testing method), the original regulator that bench tested OK and my original ND coils and ballast resistors that all looked and tested OK on the bench. I'm not sure how to swap the coils as the plug leads are glued in and I'm not sure the front lead will reach the rear cylinder; I'll measure tomorrow. I think I'll just pull the tank and re-test everything on the rear head side that I can. There has been considerable pressure from many on this and other forums to ditch the old Bosch/ND system and get newer, more reliable ignition components. I understand this but was hoping to get the bike to run on original parts.
Last edited by 81mhr900ss on Sun May 12, 2019 5:13 pm, edited 1 time in total.

Lumpy
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Re: 81MHR idle and run issues: slide wear?!

Post by Lumpy » Sun May 12, 2019 1:26 am

I never had any luck with iridium plugs on my 82 SS. It became harder to start and bizarrely would let a POP!! out the exhaust just by turning on the ignition. Quite odd I thought so the old style went back in.

I would look at replacing the wiring from your pick ups to the Bosch boxes. I had to replace mine and no decay could be seen outside, but the insulation on the individual wires had rotted away. It was quite strange how it all came about. I accidentally left my ignition on for several hours with the bike not running. In the morning there was a waxy type substance that had melted out of the coils. The bike started right up and seemed ok at idle. I then rode it 5 km`s to the local service station for fuel and it was quite clear something was not right. Upon stopping the bike it was also very clear the back pot was extremely hot. After lots of fruitless searching I decided to replace this wiring. It was a big call as the wiring looked fine externally. Having removed it I cut it open and the internal insulation on the wiring was all crumbly. Possible heat damage from leaving the ignition on?? Possibly. But I have since heard that these wires will always fail in bikes of our age. Basically, if it hasn't failed yet....................sooner or later it will.................

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Craig in France
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Re: 81MHR idle and run issues: slide wear?!

Post by Craig in France » Sun May 12, 2019 3:07 am

1. See the narrative below for how to replace the HT leads in the coil. Alternatively, you might try to buy a second-hand one. They were used on (for example) the contemporary Suzuki GS400 models (I think the larger capacity GS models used a similar but twin output coil). Couple on ebay at the mo' ...

Other second-hand alternatives are TEK KP03 and TEK MP06, also both widely available. And then of course there are various new, but more expensive, options - Dynatek,for example.

2. Lumpy’s advice on the pick-up wires is also good. The signal from the pick-ups is very low voltage and any corrosion or failure in the wire could be stuffing it up.

3. Your melting Bosch story intrigues me tho’. I need to think how the devil the epoxy filler could have melted. Most strange …

-----------------------------------------------
"How to replace the HT cables on Nippon Denso Coils (pinky-biege colour, stamped 028700)
Written by John Wilson

“First, carefully remove the epoxy resin around the HT cable entry point on the body of the ND coil. You can do this, for example, by using small side cutters. Take care not to damage the body plastic.

There are two parts to the HT lead:
• the inner HT lead/copper wire with insulation cover
• an outside thin sheath.

Pull hard on the HT lead until it comes out of the coil body. This will leave the outer sheath still inside the coil body. Break this bond by carefully pushing a screw driver down between the sleeve and inner bore of the tube until you feel the remaining epoxy, then continue working around the sleeve until it frees from the bore.

There is a standard spike termination at the end of the bore. Insert a good quality copper core HT lead of 7mm diameter (8mm will not fit) into the bore (tube) until is stops. Then secure the new lead to the ignition coil body using a good quality silicon sealant or epoxy.

Do not use building grade silicon as it cures by liberation of acetic acid which attacks copper components. Instead, use a neutral cure silicon such as 738RTV. You can find it at the local electrical wholesalers; similar products are becoming more available from hardware stores.

Once done, terminate the HT leads using NGK spark plug covers with 5.0K ohm suppression resistor for clean firing of the spark plugs.”

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81mhr900ss
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Re: 81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Sun May 12, 2019 9:18 am

Thanks again for your input folks:

When the clutch cover was on my table, I very carefully cleaned the pickups, checked the wiring and tested with a multi-meter while gently wobbling the wiring inside and outside the casing; there was no indication of loss of continuity. The only visually "stressed" part of the wiring was the stiff and sightly cracked outer sheath of the trigger line where it exits the casing and bends to follow the frame tube. I have also read lots of "replace it now as it is doomed to fail later" advice on this and other forums.

I got busy late last night and ordered some new components; Dynatek DC10-1 coils and HT leads and a Shindengen FH020AA regulator; I figured that upgrading the ignition system made sense and I'd rather go with new than second hand. The coils cost CAN$186.99, the best price I could find on the Internet. I so badly want to get this machine on the road and I'm now 6 weeks behind plan. I've been considering a Sachse ignition system but I'll wait to see how the new coils work first.

The Bosch exciter that failed is a mystery to me too. I have read about the failing units but not melting ones. Upon closer inspection , it's got small cracks in the epoxy around the edge where it meets the metal. The rubber boot had a matte finish to, indicating that it got hot too it but I was able to recover it. Funny thing though, the left side system failed all those years ago which on my bike is the front head not rear.

wdietz186
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Re: 81MHR idle and run issues: slide wear?!

Post by wdietz186 » Sun May 12, 2019 10:02 am

Did you check the resistance of the pickups? Is that what you meant for checking the continuity? You can test the resistance of the pickup coils pretty easily by checking across the two pickup leads for each cylinder, approx 220 ohm as I recall. What else you need to do is check for resistance between the leads of one cyl. to the other and each lead to ground, anything other than infinite resistance will indicate a wiring fault. The insulation on the Bosch pickup wire disintegrates with age/oil and more often than not it fails at the grommet where it passes through the case cover. What happens is you will get contact between the leads from one pickup to the other causing an intermittent, weak spark. If your wiring is original it's a solid bet that's what is going on. Steve sells a kit to rectify this problem and it's not rocket surgery to install.

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Re: 81MHR idle and run issues: slide wear?!

Post by Lumpy » Sun May 12, 2019 3:37 pm

The melting waxy substance I spoke of gentlemen was from the original ND coils, not the Bosch transducers. Oddly the coils still worked fine, but they were my first suspect for the issues I had and were replaced first up.

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81mhr900ss
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Re: 81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Sun May 12, 2019 5:10 pm

wdietz186: Yes, I checked the pickup resistance, both identical and consistent and you are correct, close to 200 ohms and no bleeds to ground. The readings were consistent when I moved the individual wires inside the casing and the whole cable exactly where you mention, where is leaves the casing. I'm aware of Steve's repair kit as I thought it likely that this would be the route to take. I think at this time, I'll install the Dynacoils, new HT leads and regulator in a week or so when they arrive and see what happens.

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81mhr900ss
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Re: 81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Sun May 12, 2019 5:12 pm

lumpy: understood; you had a waxy goop from an ND coil but I had a black plastic goop from a Bosch unit and a very hot connector on the front head system. This side seems happy now.

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81mhr900ss
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Re: 81MHR idle and run issues: slide wear?!

Post by 81mhr900ss » Sun May 12, 2019 5:24 pm

Just another associated comment: Lumpy's story regarding leaving the ignition on for an extended period reminded me of my activities during testing my whole bike electrics out. Almost 30 years is a long time and although all my bulbs were working just fine 30 years ago, there were a number of broken filaments when I put 12V on the bike and I was also performing many loom tests. In order to kill the ignition while doing this work, I just pulled the power supply to the ignition under the tank, the single light brown connector that splits into two. I could have also pulled the jumper from the unused engine kill switch connector. I've been considering adding a hidden ignition switch, maybe linked to a new switch for my new side stand, using the unused engine kill switch connector.

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