750 sport compression

[dden]

Are there any known compression figures for 750 sports?

[Craig in France]

Ian Fallon quotes an official factory figure of 9.3:1 for the 1972-3 Sport, adding the rider "Specifications for many 1973 and 1974 versions of the 750 GT and Sport were never oficially quoted".

Source: The Ducati 750 Bible

Ciao

Craig

[dden]

Thanks Craig, I was looking for kicking compression. Wondering what i should be getting on compression gunge.

[Craig in France]

Thanks Craig, I was looking for kicking compression. Wondering what i should be getting on compression gunge.

Easy. Once you know the compression ration, all you need to do is to multipy the compression ration by 14.7 * (which is the mass at sea level in lbs of a column of air one square inch in cross-section).

So about 136lbs.

But, for various reasons not worth going into here, you're unlikely to get a figure as high as this. But no matter. What you want to see is a figure somewhere near. At 100-110 lbs, you're probably ok. At 60 lbs, you've got a problem.

ALSO: A compression test is really most useful in telling you is which cylinder, if any, is weak. So look at the difference between the two pots.

P.s I assume you know:

1. Do the test with throttle held fully open.

2. If you do get a low reading on one or other cylinder, a quick and dirty way of checking whether it's valves or rings is to put a teaspoon of oil down the spark plug hole and repeat the test. If the reading goes up, it's most likely rings (the oil temporarily seals the rings against the bore). More sophisticated diagnosis requires a leak-down test.

* Or, if you're metric, 1.03kg.

Ciao

Craig

[dden]

I thought many other things went into figuring out what compression numbers should be. For instance valve timing, or is everything already figured into the compression ratio figure?

I'm seeing higher than 120psi-135psi... I'll have to do the test again.

[Craig in France]

I thought many other things went into figuring out what compression numbers should be. For instance valve timing ..

Yes, you're right, of course (as I kinda suggested in my previous reply * .. . But it really doesn't matter too much. The critical thing is what is the difference between the cylinders, and does the reading increase if you put a little oil down the bore.

I'm seeing higher than 120psi-135psi ...

Well, it would be nice to have more, but it's not a disaster, either! Btw, I get 140-145 on my SS Darmah (altho' there's confusion as to its compression ratio: the factory handbook says 9.3. But the piston is the same as the standard SD, which has a quoted compression ration of ... 9!)

What's the problem that you're trying to resolve?
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* Other variables include engine temperature, ambient air temperature, humidity, height above sea level - and how accurate your valve clearances are. And not to say anything about the accuracy of the gauge! .

And when you do see compression values quoted, they can be across a very wide range. For example, here's one that I just found on Tinternet for a Smart : "Ideally, the figures need to be as close to 12 bar/180psi as possible. (However) the Smart engine tolerance is +/-25%, so can be anywhere between 9 bar/135psi and 15 bar/225psi".

For more go here, for example:
http://www.pumaracing.co.uk/comp.htm

.. altho' I find their statement that, "You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio", to be a bit open to challenge.

[dden]

I thought i have one cylinder weaker than the other. I noticed when tuning the carbs and holding my hand near the exhaust (one strong, one weak). At this point i am chalking it up to possible valve clearance out of spec + weak battery (not so good spark) and points not set perfectly. I didn't have the parts needed to correct the other problems and wanted to make sure i didn't have a bad cylinder so i did a quick compression test. Since i had figures i wanted to see if they were within spec. They actually seemed kind of high to me. I was expecting 120-140. I'm seeing 165-175. Gauge is good.

Could be valve shims out of spec again, but they aren't that far off. spec is .1mm clearance, i am seeing .2mm

[Craig in France]

Well - assuming there isn't a marked difference between the pots? - with those readings, I'd be surprised if you've got much to worry about . Personally, I find a difference in exhaust pulse is normally down to carb settings (and I have to say it's not something that bothers me over much. I tend to do what the vacuum gauges tell me to do). But yeh, I can see that improper sparking might produce the same result - but I don't know.

P.s Again speaking personally: I like to get valve clearances as near to 'right' as my limited abilities permit. I grind shims down on an oilstone if necessary. Also, I recently re-set one of mine using one of Nigel Lacey's 'half size' shims (he does them in 0.05 steps) and was really surprised by the difference in how much smoother and easier the engine ran.

Ciao

Craig

[dden]

Craig, what engine were you shimming? What specs do you use on clearances. I have another thread for valve clearances but no replies yet. Too much varying info out there to make a determination.

-Dan

[Craig in France]

Craig, what engine were you shimming? What specs do you use on clearances. I have another thread for valve clearances but no replies yet. Too much varying info out there to make a determination.

Hi Dan

An SS Darmah *. Unfortunately, I can't advise you on a 750 engine - altho' I see your confusion. Most sources I have say 0.1 on both ... EXCEPT for the factory workshop manual which says 0.1 inlet and 0.15 exhaust.

Best see what someone else says - or perhaps give Steve A. a call? Or Malcolm at Syd's Cycles? (Tel 727.522.3333). Or Nigel Lacey in the UK (except I know he's away until 26 March), http://www.laceyducati.co.uk. Or pm Kev, aka machten ...


Ciao

Craig

* FWIW:
Inlet 0.08 opener, 0.00 closer
Exhaust 0.12 opener, 0,00 closer

[machten]

Dan,

Or pm Kev, aka machten ...

I'd actually prepared an earlier response to your query, but thought I'd wait and see what others had to say and maybe get the chance to check compression from kicking on my Sport before I responded. Craig's post has encouraged me to at least give my experiences.

I take the approach of never setting less than the published opener rocker clearances. I take these as 0.1mm for both exhaust and inlet. Bear in mind that opener rocker clearances will decrease over time. (If they don't you have an issue - which is why you should always record your historic settings). That being said, I ride my bikes in a hot environment, so I set my inlet at not less than 0.1mm and exhaust at not less than 0.12mm. I'm happy to go a bit above those sizes, but no less. I'm fortunate enough to have a complete set of opener shims from 2.0 to 6.95mm in 0.05mm increments (which I never stone down), plus I have access to another 40-50 shims in between (which I'm happy to stone). That's kind of the nirvana of shimming - I never have to start a shim job without having the shims to complete it at the time.

You asked what is "acceptable"...

My experience is that your bike would run fine by setting clearances between 0.1 and 0.18mm for both exhaust and inlet. The downside of above the higher end is increased top end noise, but it won't cause any harm at all up to 0.2mm. In many cases this might be influenced by your available shim sizes.

The most important thing is to make sure you don't allow clearances to decline to close to zero (or less). If you do, you are holding the valves open and allowing hot gases to pass the fine edges of the valve, resulting in burnt valves.

To be honest, based on your symptoms of a weaker pulse from your exhaust, I would consider the following:

I understand you are using the points ignition. It was always extremely difficult to set both cylinders correct points opening using a one piece points plate. It was a common mod to cut the plate in half so that the points timing and advance could be set independantly. Even better - install Dyna ignition - BevelHeaven has the them at a very good price. I would look at your timing first.

Balanced carb setup in a V-Twin is important to performance. If you're already diagnosing from the exhaust pulse rather than vacuum gauges, it sounds like you know what you're looking for. Deal with that after sorting the electrics.

Compression - You will almost always see loss of compression due to wear in the horizontal cylinder first. That's the effects of gravity! The compression figures you state are very high - unusually high and certainly nothing to be worried about if that is what they are. As Craig said, variation is more of an issue above (I'd say) 115psi. I confess I'm suprised you are gettting the figures you are, and it may be an indication of a higher compression piston, which would be worth confirming.

THe other thing to bear in mind is the imprecision of the round case cam timing. (And this is why you shouldn't worry so much about 0.05mm shimming tolerance). I have adjusted 750 cams that were out of sync between front and rear cylinders by nearly 20 degrees across cylinders. That makes 0.05mm shimming kinda irrelevent for anything other than top end noise. You can deal with the cam timing seperately, and dealing with it totally transformed my Sport's performance.

Hope that helps.

Kev

[wdietz186]

If you are only going on a percieved difference in exhaust pulses,please note that due to the firing interval one pipe will will have a stronger puff than the other.On the bevels as I recall the left pipe will be the stronger pulse as the flow is already started from the vert cyl. firing, closely followed by the horiz. cyl. So if your compression readings are fairly close and the carb and valve adjustments are in the ballpark you are OK. You are using vacuum gauges to balance the carbs aren't you?

[Craig in France]

I take the approach of never setting less than the published opener rocker clearances. I take these as 0.1mm for both exhaust and inlet.

Ta Kev.

So, any idea what the story here is? (i.e. the bit that says, "The clearance for the inlet valve should be 0.10 mm and 0.15 mm for the exhaust valve"). Simple error?



Post-script: Ah, I see this has come up before. On the version of the manual that is loaded on René and Sue's site*, someone (perhaps René?) has added this warning:
" I would also caution you to take the dimensions for valve clearance printed on page 77 of the original manual with a grain of salt if you are reading the English section; the values listed in the Italian and French sections are the correct values.".

Unfortunately, the author of the warning doesn't then say WHAT the correct values are; but I guess the implication is 0.1 for both ... ?

* http://www.ducatimeccanica.com/roundcas ... index.html

The other thing to bear in mind is the imprecision of the round case cam timing.

Ah, yes! THAT little problem ...

Ciao

Craig

[dden]

Wow thanks for the excellent information.

The left exhaust is in fact the stronger one and the horizontal cylinder is in fact slightly lower on compression. Yes i am using gauges to sync the carbs.

I ordered a bunch of shims that should put me in the .1 - .13 clearance range on the intake and .1 - .16 on the exhaust.

I didn't have time to really start adjusting the points. I will do that this weekend. I've also been looking at the dyna ignition, just haven't made a decision yet. I hate to replace things before I've given the stock setup a try.

[wdietz186]

The Dyna is a set and forget setup,I've had one on my 750 for 20+ years and haven't touched it.The only downside is it draws a bit more current than points and you have to have a good battery and be mindful of using the lights only when nessessary. Or update the charging system. Looking at the posted pics from the manual reminds me of the time I spent getting the clearance just so,using the factory tool instead of the rocker pin and then realizing the tool diameter is .002" less than the pin. Only made that mistake once though. These bikes are as much fun to work on as they are to ride!