rxs51
I am still trying to sort out running problems on my old 851 and thought I would seek out second opinions on the forum before investigating any further - may need to invest in a diagnostic setup!
I have done all the usual - new plugs, fuel filter, balance throttle bodies, TPS reset etc but still have an irritating misfire and when I check the plugs there is a distinct difference in colour between the 2 cylinders. Thinking it could be the injectors I swapped them them over yesterday but it made no difference to the plug colour with the rear plug still much darker than the front one. I have tried a different chip in the ECU and even tried swapping over a known good ECU but without success and am now running out of ideas.
Because of the difference in plug colour between the 2 cylinders I assumed it is a fuel problem but am now starting to wonder if this is misleading me and whether it could be due to a problem with the ignition (I have checked all the obvious connections).
All suggestions welcomed as I have run out of ideas - other than giving in and taking it to a specialist!
duck2
What's your valve clearances like? Could it be a leaking /burnt valve? Try and do a leak down test on both cylinders and see if the values are the same for both cylinders,
rxs51
e3
duck2 wroteWhat's your valve clearances like? Could it be a leaking /burnt valve? Try and do a leak down test on both cylinders and see if the values are the same for both cylinders,
Clearances were OK at the beginning of the year but it probably would be a good idea to check clearances and compression again. Thanks for the suggestion - easy to make assumptions that they should still be OK.
rxs51
rxs51 wroteClearances were OK at the beginning of the year but it probably would be a good idea to check clearances and compression again. Thanks for the suggestion - easy to make assumptions that they should still be OK.
I am not able to do a leakdown test at the minute but have checked the compression and am getting pretty close to 140psi on both cylinders. Have not been able to find any specifications for this so am not sure whether it is OK?
hindsight
Can you swap the coils over to see if the fault follows them, and while you're at it, have a close inspection of the HT leads and their resistance.
griff851
At 140psi in each cylinder, first thing i would normally say is, "Well thats rooted, time to pull it apart". However, having fallen for this myself yonks ago, i'm pretty sure you were not holding the throttle wide open. If you were, your gauge is suspect, or your bike has 12 zillion miles on the clock and the ring gap is bigger than the distance between the moon and us.
For a quick check but not the most effective do compression test dry. Then poor oil into the cylinder, do test again. If compression comes up, rings. If it stays low, valves, do leak down to see how bad.
Static compression test. Air is at sea level 1 bar, 14.7 psi etc. If i compress it at a ratio of 10:1 i will get 147 psi. Compression ratio for these engines in good nick is 11.5:1. So 11.5 x 14.7 = 169.05
Pull off the rocker and have a look. When a valve isnt closing, it can sound like ignition failing. A broken closing spring i've heard on a pantah, gave an occasional misfire. The guys spent a couple of hours checking ignition and pick ups, only eventually to have a look at rockers and find said broken spring.
If everything is reasonable in the rocker dept, get a leak down check done before you pull out all your hair. If thats good, then you know it has to be ignition.
rxs51
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Thanks for the suggestions, that all makes sense. I do not have the correct adaptor so had to use the rubber bung on my gauge which is not ideal for a stable reading and I also had to do it with a cold engine as I had the tank off to check injectors - not sure just how much difference engine temperature has on the readings and also not sure just how accurate my Gunsen gauge is. I was really aiming for a quick check to see if there was an obvious difference between the 2 cylinders. I will check the springs and if they look OK will try swapping the coils over.
It is a bit frustrating as the problem is only evident when tested on the road so making changes is a slow process!
rxs51
Thanks for the suggestions, that all makes sense. I do not have the correct adaptor so had to use the rubber bung on my gauge which is not ideal for a stable reading and I also had to do it with a cold engine as I had the tank off to check injectors - not sure just how much difference engine temperature has on the readings and also not sure just how accurate my Gunsen gauge is. I was really aiming for a quick check to see if there was an obvious difference between the 2 cylinders. I will check the springs and if they look OK will try swapping the coils over.
It is a bit frustrating as the problem is only evident when tested on the road so making changes is a slow process!
griff851
Ah more info. The guston needs to sit for 2min, then trimed to 2. At idle near 5, 4.5 better. Rev the bike an notice the change levels at rev to 6-7k and compare. That gives an idea of fueling and possible oil through a valve guide seal maybe.
If you cant get the fault on the bench, then mechanical like a spring starts to become unlikely. If it only happens on the road and randomish then loose or corroded plugs come into play as now you have bouncing and shaking as a possible cause.
Have you put new spark plugs in? Alway easier to read a new clean plug than a shitty old one.
brad black
140 psi is fine.
has it had a balance port screw fall out, or the little vac hose from the manifold to the pressure regulator come off one side?
rxs51
Thanks for the replies - now have lots to think about!
A few answers: plugs were new and vac hose and screws are all in place.
The problem is most obvious at a steady throttle opening at low speed in urban limits (30mph) when is best described as jerky - at speed on the open road when I am on and off the throttle at higher revs it is not so bad.
I thought it might be the TPS but a check with my multimeter on the ECU terminals showed a good smooth response to the throttle movement - is this a good enough test or could it still be faulty?
I have been keen to get this sorted so I can enjoy the bike during the fine weather and have been rushing things a bit but think I really need to go back to the basics and tackle this properly and systematically so will aim to investigate properly over the next week or so and go back to using the CBR in the meantime. Will post updates on my progress but any other suggestions are welcome!
griff851
Just for referance, what gear and what rpm at 30mph around town are you talking about here.
I find the engines really dont like prolonged time below 4000rpm. Driving power and where they really prefer to be is from 6-7000 to 9-9500 in standard form, this is part of its sport/race nature. It does of coarse make them not really a good commuter.
The misbehaving described, sounds a bit like a tad more wide eyed and white knukle aproach may be needed more than an a techinical issue. They were never smooth like a CBR bell curve applieance, that was not ducati's market niche.
Yes the rear plug being darker than the front certainly is not helping and needs to be addressed with a balance an tune (and a leak down check)
rxs51
Griff, thanks for the comments - I understand exactly what you mean but this is more than just the usual low rpm lumpy Ducati feeling ( I have owned and ridden lots of them).
Although I have owned the bike for around 20 years it has been off the road until recently while I played with newer and faster toys so I did wonder whether my memory was playing tricks on me as I ride a fair bit slower these days so thought I may not have noticed this so much in the past but it is really jerky/snatchy rather than just lumpy even at 4000 rpm on a steady throttle.
I agree with your comment that it is best at around 7-9000 rpm and that is how I ride it on the open road but it would be doing well over the speed limit in urban areas even in bottom gear at those revs!
A leakdown test sounds like a good idea but I need to find time to sort out suitable connectors - summers seem pretty short up in the northern part of the UK so when you have other bikes available it is tempting to get out and ride rather than spend time in the shed!
griff851
All good, just removing one more possibility. Wire and plug issues are falling of the list too, due to when its happening.
rxs51
Still struggling with trying to sort out this problem - I have tried swapping over injectors, coils and plugs but still getting a big difference in the plug colours with the front cylinder much leaner than the rear no matter what I do - there are no signs of air leaks or any popping and banging. I am pretty sure that that the old 851 does not have individual fuel maps for each cylinder so not sure what is going on and feel I am missing something which should be obvious!
I have attached a photo of the plugs in the hope that someone might have an idea?
Mr.R
There's a big difference between those plugs so something's not right but...
The front cylinders always run weaker than the rears on these bikes, which's why the P8's a good idea to use the seperate mapping per cylinder.
I've just been setting up/playing with my '79 900SS special and the pots run completetly different to each other, with the front running weaker.
Steve R
rxs51
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Mr.R wroteThere's a big difference between those plugs so something's not right but...
The front cylinders always run weaker than the rears on these bikes, which's why the P8's a good idea to use the seperate mapping per cylinder.
I've just been setting up/playing with my '79 900SS special and the pots run completetly different to each other, with the front running weaker.
Steve R
Thanks for the reply - I expected a slight difference as my early model 748 always ran slightly weaker on the front cylinder but this seems much more obvious and I am inclined to think it may be connected to the slightly uneven running that I have been trying to sort. I wondered whether it may be due to a mechanical problem but compression is OK and there does not seem to be any shortage of power so not sure what to try next.
Was the difference between cylinders so obvious on your bike?
Weed
Have you tried swapping the injectors from the vertical cylinder to the horizontal & vice versa to see if the plug colour change is reversed?
rxs51
Yes, I tried this (mentioned in earlier post) - bit of a pain to do as access is difficult but it made no difference. I can understand the differences between front and rear cylinders on air-cooled bikes but would expect only a small difference with water cooling?
Bit stumped at the moment and may have to give in and take it to a professional.
griff851
brad black wrote140 psi is fine.
has it had a balance port screw fall out, or the little vac hose from the manifold to the pressure regulator come off one side?
If 140 is fine, what are the new bike figures? Mine has been doctored and bevels come in at 165 unless they have an issue.
