Author Topic: I am about to do something unnatural with my 80 Plus!  (Read 7380 times)

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Offline KiwiJohn

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I have been working for years, not continously you understand, on the restoration of an 80 Plus.  It really should have been running the roads quite some time ago but still it frustrates me!

It is all complete and looks ready to go, it starts easily but after about 2 minutes or less of running at approx 2000 rpm the plugs foul and everything becomes silent except for muffled cursing.

I have examined the carbs, they have been resleeved, I even bought new jets,  the flooding float valves have been cured,  the leaks around the jet block have been attended to.  All airways are clear and everything is in good order as far as any experts can tell by looking at them.

I had the magneto reconditioned some time ago and it puts out a reasonable although not startling spark when kicked over.  I am using NGK B75HS plugs.

The motor has recently had new big ends and at the same time the rings were replaced. The bores and pistons were within spec for wear.


The fouling appears to be soot rather than oil and sometimes one plug will be visibly wet rather than sooty.  I get a bit of light oily smoke when it first starts.

Someone suggested blow by may be the problem so I removed the plate over the tacho drive to release the pressure and gave it a run.  It may have been imagination but it did appear to run longer than on  previous tests.

Short of melting the whole thing down into fishing sinkers I am practically at a dead end on this one.   :x    However there is another weekend coming up and I just might give it yet another try.

Offline aggettd

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I am about to do something unnatural with my 80 Plus!
« Reply #1 on: 11 Aug 2005 at 11:56 »
John:

I can't speculate on what is causing the plug fouling, but I do have a suggestion which may help you diagnose the problem. You mentioned that blow-by may be the issue. I have cobbled up a very simple device that lets you check combustion chamber and piston ring leakage very easily. I took an old sparkplug, broke off the ceramic insulator and drove out the central electrode. I then brazed a nipple onto the plug where the ceramic insulator used to be. This allows me to introduce pressurized air into the combustion chamber using a hose connected between the nipple and a suitable source of compressed air.

To use it, I install the modified sparkplug, attach the hose and compressed air supply, back off the valve rocker adjusters so that they are always closed during the diagnosis phase. Then I remove any air filter, and the dipstick. Next, with the transmission in gear, I rotate the rear wheel so that the pistons are at TDC. Lock the rear wheel in place (it helps to have a friend step on the rear brake). What you are going to do next is slowly crack the valve to your airtank so that you are pressurizing the combustion chamber. (If you haven't locked the rear wheel, the bike may roll forward off the centrestand, fall over, and dent your wife's car. Be sure to lock the rear wheel!) Now you listen. If air is escaping you will be able to hear it. If the intake valve is leaking, the hissing will come out the carburator. If the exhaust valve is leaking, you will hear it from the silencer (or exhaust port if you have the pipe off). If you hear it from the dipstick port, it is blowby. You are not finished. You have only checked the pistons at TDC. Now unlock the rear wheel, advance the pistons so that they are at mid-bore and retest again, then again at BDC.

A compression test tells you that there is a leak, this test tells you where it is. (roughly).

Did I miss anything?

I daren't speculate on the cause of your problem. With both cylinders fouling it sounds like electrical, but conventional wisdom suggests that the vast majority of electrical problems are in the carburettor. No chance that the valve rocker clearances are adjusted too tight? Carb floats set too high? Floatbowl vents blocked? Missed church last Sunday?

Offline trevorp

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I am about to do something unnatural with my 80 Plus!
« Reply #2 on: 11 Aug 2005 at 12:47 »
are the needles at their lowest point ie u cant see the needles on the slide because they are on the lowest setting and are the slide cutaways the right type eg 5/4 not 3/4 or 6 not sure what carb on 80 plus
what sort of fuel i have heard some unleaded can do this whilst premium seems better
if it has bad blow by it will pressurise the case and cause oil leaks and should blow smoke out exhaust
get someone to tell u what color smoke is under acceleration blackish and its a fuel problem and white, ring, piston, or valve stem wear
i am working on a vent copied of alwyns bike just screw in brass 3/4 fitting into oil filler and vent off with hose i am going to try putting a closed bronze fliter screen to try and catch the oil and return it to sump rather than vent it thru a tube i will still have a tube but hoping less oil will come out
one other thing to remember is that sometimes on the rare occasion a lean mixture can create fouiling of plugs and general bad running
dont give up on it john we all want to hear it purr

Offline trevorp

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oh i forgot
« Reply #3 on: 11 Aug 2005 at 12:55 »
if u have normal amals 274 276 u can lower the float level by varying the size or number of fibre washers where the float bowl touches base of carby dont go to mad because small variations make a big difference

Offline KiwiJohn

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I am about to do something unnatural with my 80 Plus!
« Reply #4 on: 14 Aug 2005 at 07:33 »
Thanks for the information and enqouragement folks.  Unfortunately another weekend has come an gone and nothing touched on the Douglas!   Last week I found the dreaded rust coming through on a body join of my 6 year old Falcon Ute which I can no way afford to replace but I really do need to carry this useless motorcycle around!  The weekend was spent ripping that apart to find the cause which left no time for anything else.

Next weekend I will be trying some of your advice.

 If only I know what to check last I could do that first and thereby save a lot of time! :lol:

Offline KiwiJohn

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Re: I am about to do something unnatural with my 80 Plus!
« Reply #5 on: 05 Sep 2005 at 00:21 »
Another few weeks closer to summer and still it is not running.... :( :(


aggettd,  you could be right on regarding the lax church attendance!  Meanwhile,  I took the plate off the tacho drive which would have released all pressure in the crankcase I think.  The results were about the same except for the oil splattered over the front wheel!  I dont really have the equipment handy to do that blow by test but I am thinking how I can get around this.

I took the mag off, mounted it on a plank with a low speed electric drill, fitted two spark plugs and let it run for several minutes,  there seems to be good sparks although I have no way of proving it fires every single time it sure appears to.

So now I am down to grasping at straws,  this is a Plus 80 that has been fitted with a woffle box exhaust.  So this is what I am thinking.. Is it likely that a partially blocked waffle box means exhaust gas gets left in the combustion chamber  causing poor combustion and the plug fouling?

P.S.  I am still on good terms with the neighbours but I am sorely tempted to give her a run on open pipes! :D

Offline alwyn

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Re: I am about to do something unnatural with my 80 Plus!
« Reply #6 on: 05 Sep 2005 at 01:25 »
Another few weeks closer to summer and still it is not running.... :( :(

P.S.  I am still on good terms with the neighbours but I am sorely tempted to give her a run on open pipes! :D

John,
No worry with the neighbours - give 'em a packet or two of ear plugs and let her go 8)  A few minutes with open pipes might be a lot more satisfying than that unnatural act you have been contemplating for so long. :twisted:

I recall that in the early '50's I drilled 1" holes through the first baffles of the woffle box (opposite the the pipe ends) on my Mark III but that was to generate more noise rather than relieve any back pressure, tho' it probably achieved that also - I was surprised however when I discovered that the woffle on my present T35 has a fairly broad but precise symmetric pattern of 10 mm holes through the bottom of it - they look clean bored and have slightly chamferred edges as if it's OEM but the previous owner may have drilled them for the same reason that I butchered mine as a youth! - just for the sheer joy of creating more noise - more noise more speed, or so one thought in those days. :roll: Just a thought - I wonder did Douglas recognise a restricted gas flow in the woffle design at some stage of development early on and produced this variation with additional gas release :question:
 Alwyn
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« Last Edit: 05 Sep 2005 at 01:34 by alwyn »
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Offline KiwiJohn

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Re: I am about to do something unnatural with my 80 Plus!
« Reply #7 on: 05 Sep 2005 at 02:27 »
Too right Alwyn if I could get the old &^%$# to run for a few minutes with or without pipes I would be really pleased! :lol:

 

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