83 MHR top end rebuild - low compression pressures

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PJDucati
Cucciolo - the Lil Pup
Posts: 11
Joined: Tue Nov 19, 2013 4:46 am

83 MHR top end rebuild - low compression pressures

Post by PJDucati »

Just completed a top end rebuild on an 83 MHR. (88mm pistons/42mm inlet valves/38mm exhaust valves). Ducati (Goetze) piston rings were fitted. After finishing the engine rebuild the engine would not start and found that I was getting 75psi compression pressure on both cylinders. Cylinder bores were in good condition, light polishing but no scuffing/scoring and moderate wear. The bores (not nikasil) were deglazed before fitting new rings (correct way up.). The cam timing was reset correctly with the dot markers realigning after 4 rotations of the bevel gear. My concern is that the bike may have been raced in the past (oversized pistons and valves and fettling/porting of the inlet manifolds) with evidence of lockwiring etc. and hence the cam timing may be non standard, however the engine turns over freely with no valve contact. I have measured the inlet valve lift to be 9.17mm. Is this standard for the engine? I will look to do a leakdown test tomorrow, but wondered if the good experience here could advise on other diagnostic checks to perform. There is sufficient oil in the bores not to add oil to test for rings. Has anyone got data for valve lift at TDC on the overlap, as a means of checking valve timing? The engine was a runner before the strip down, but not a brilliant starter. Valves were lightly lapped on both cylinder heads approx. 500 miles ago. Cylinder head nuts pulled down securely. Any advise appreciated!
PJDucati
Cucciolo - the Lil Pup
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Re: 83 MHR top end rebuild - low compression pressures

Post by PJDucati »

Still to carry out the leakdown test, but my concerns are around the wear levels on the cylinder bore. If I have to strip the engine down again, I will measure the compression ring gaps at various points down the bore. Has anyone got any data for ring gaps on new rings in new bores?

A free mince pie to anyone that replies.....
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81mhr900ss
750 GT
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Re: 83 MHR top end rebuild - low compression pressures

Post by 81mhr900ss »

This may help. It's from a Darmah manual but I believe the ring specs are similar for the 900SS motor. 88mm is 2mm oversize compared to an older MHR like mine:
http://ducatimeccanica.com/darmah_900sd ... es/036.htm
andybaggies
Mariana
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Re: 83 MHR top end rebuild - low compression pressures

Post by andybaggies »

Has anyone got any data for ring gaps on new rings in new bores?
Just being doing this on a Darmah and the top two rings are 0.30 to 0.45 and the oil scraper is a touch tighter at 0.25 to 0.40. These are the same as for the 900 SS.
PJDucati
Cucciolo - the Lil Pup
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Re: 83 MHR top end rebuild - low compression pressures

Post by PJDucati »

Thanks to both for this feedback. If the heads do come off again, I have a spare new compression ring to measure ring gap. Those measurements on the Darmah were at the bottom (or top) of the bore, outside piston ring travel (ie on an unworn part of the cylinder bore)?
Completed the leakdown test today and at 90psi, the cylinder pressure was fixed for about 10minutes (on the vertical cylinder). No perceived loss of pressure and no sound of leaking air from the inlet side , exhaust side or the cylinder head barrel interface.
Still struggling to understand how I am getting low compression pressures.
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81mhr900ss
750 GT
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Re: 83 MHR top end rebuild - low compression pressures

Post by 81mhr900ss »

Just a regular thickness base gasket? If compression is holding then either the piston is not tall enough at TDC or the barrel and head are too high (overly thick base gasket). Very, very weird. Has the combustion chamber been modified?
Last edited by 81mhr900ss on Wed Dec 16, 2020 5:57 pm, edited 1 time in total.
andybaggies
Mariana
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Re: 83 MHR top end rebuild - low compression pressures

Post by andybaggies »

Those measurements on the Darmah were at the bottom (or top)
These are the specs taken from Ian Fallon's restoration guide but they don't specify where in the bore they are measured. I do them at three points and ensure they are all (pretty much) in spec.

Could it be that there is a leak in the head which only manifests itself at pressures higher than 90 psi :?
andybaggies
Mariana
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Re: 83 MHR top end rebuild - low compression pressures

Post by andybaggies »

Could it be that there is a leak in the head which only manifests itself at pressures higher than 90 psi :?
Perhaps I should clarify that... I mean a leak due to a crack in the head (that only manifests at greater than 90 psi)
ducadini
860 GT / GTS
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Re: 83 MHR top end rebuild - low compression pressures

Post by ducadini »

I suppose You took good care of the seating of the valves, checking it with liquid in the chamber.
And before fitting the heads, lapping them on the cylinder, making sure there's no debris left.
Did You check compression with an electric start or kick ? And of course fully open gas ?
When You say the engine has been raced, it could be that a PO modified compression in a way. I've seen engines where the oldhamcoupling prevented the cylinderhead/cylinder from making contact.
Is the lack of measurable compression on both or on one pot only ?
Rear heads can crack (mostly on 860 and Darmah, never seen a SS go bad) between the valveseat and exhaustport, guessing the head heats up too much (all that bodywork covering the beauty of it). But this crack just leads to backfiring and popping, nothing to do with low compression.
The 9 mm lift of the cam is standard.
A different base gasket will not have such an effect on the measurements (it can on performance).
We've seen a RC with the conrod of a SC (that's 4 mm short) that started good, ran good but just couldn't satisfy the owner with topspeed (only 180 kmh). We persuaded him to settle for "less" and have an engine that will run "forever" instead of spending €€€€ and going faster once a year.
Keep us posted
ciao
ducadini
Nego i ricordi peggiori
Richiamo i migliori pensieri
PJDucati
Cucciolo - the Lil Pup
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Re: 83 MHR top end rebuild - low compression pressures

Post by PJDucati »

Many thanks for all the helpful replies. After getting a very positive leakdown test (and both cylinders giving a similar compression test) I decided to re-assemble the engine. Although not an absolute test, the kickstart would virtually take my whole weight, so I thought it worth a second investigation to start the engine. While puzzling the problem, I fitted new spark plugs (from iridium), replaced the spark plug caps and added fresh fuel, really just to eliminate some easy issues and also double checked the fuel taps and inline filters to make sure I was getting fuel. The engine started second kick, much to my surprise! After clearing the rebuild oil on the cylinders, the engine ticks over comfortably at 1000 rpm and starts first kick with no kickbacks or carb spitting and with a clear exhaust. It probably runs now better than ever before.
My only thoughts are that the gauge was reading low (would an engine run with 75psi compression pressure?) and I may have improved issues on the ignition or fuel system.
Thanks again and happy safe riding.
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