Easiest way to find compression of pistons

C

cwis

Full member
Hi all,

I've been doing a lot of reading about ignition, compression, etc. I think my Boyer fitted Bitza is set to 31* at 5000 RPM or thereabouts. If I have high compression pistons this is a little too advanced, isn't it? (4C cams, original type airbox and filter, Jota pipes wiv der big collector).

How can I easily tell which pistons I have fitted? Poke about down a spark plug hole? I don't really want to get the pipettes out....

Secondly, If I'm at 5000 odd rpm on a wideish open throttle and I back off the bike accelerates before slowing - too rich or lean? (128 mains).

It doesn't really want to pull through the midrange - higher revs are just fine. Ohh yes....

Best regards,

Chris
 
Do you mean raise the clip one notch on the needle (dropping the needle lower into the main jet)?
I would have thought raising the needle in the slide would do the opposite of leaning it out.
Or have I got it wrong myself? (entirely possible).
 
quick check for piston type.
get a compression tester, if all is well with your engine you will have;

110 - 125 psi with flat top pistons
140 - 155 psi with Jota type pistons

at engine cranking speed, that is, turning engine over with the starter motor, all spark plugs out, throttle wide open

HTH
Red
 
redax5 said:
quick check for piston type.
get a compression tester, if all is well with your engine you will have;

110 - 125 psi with flat top pistons
140 - 155 psi with Jota type pistons

at engine cranking speed, that is, turning engine over with the starter motor, all spark plugs out, throttle wide open

HTH
Red
Click to expand...

Red , I'm not sure a compression tester is a reliable way of assessing which pistone is fitted. When I rebuilt the top end of my flat -top piston equipped 3C and fitted new rings it would comfortably crank out 140psi on a compression tester.

Chris,
I don't think 31 degrees of advance will create any problems regardless of what pistons you are using.
Bob
 
Bob, it is rough and ready.
After doing numerous compression tests on Laverda's those figures are in the general range of what I expect to find.
Works pretty well.

Tchau
Red
 
Chris,

If the engine is in really good nick, you might even be able to hear the difference.... HC pistons give a definite sharp bark, the LC items sound a bit woolly. :D

Peering through the plug hole you may be able to recognise a raised plateau = HC.  Stock low comp are flat-topped.

piet
 
redax5 said:
Bob, it is rough and ready.
After doing numerous compression tests on Laverda's those figures are in the general range of what I expect to find.
Works pretty well.

Tchau
Red
Click to expand...

Hey Red , You've done a lot more triples than I have [or ever will!] and, come to think of it , I usually use Rufus' comp tester which lives in a puddle under a plank on bricks workbench so the calibration might be a tad unreliable
 
cwis said:
Hi all,
I've been doing a lot of reading about ignition, compression, etc. I think my Boyer fitted Bitza is set to 31* at 5000 RPM or thereabouts. If I have high compression pistons this is a little too advanced, isn't it? (4C cams, original type airbox and filter, Jota pipes wiv der big collector).

How can I easily tell which pistons I have fitted? Poke about down a spark plug hole? I don't really want to get the pipettes out....

Secondly, If I'm at 5000 odd rpm on a wideish open throttle and I back off the bike accelerates before slowing - too rich or lean? (128 mains).

It doesn't really want to pull through the midrange - higher revs are just fine. Ohh yes....
Best regards,
Chris
Click to expand...

Hi Chris

it is not the rpm that is relevant, it is how much the throttle is open.

throttle closed to make 1/4 max is the slide cutaway height, idle jet, idle mixture screw, and for starting, the choke plunger

from 1/4 throttle to maybe 3/4 throttle, is the float height base setting, the needle, and the needle jet. Most of your riding is probably on the needle jet and the needle. If you are somewhere between 1/4 throttle and 3/4 throttle, and the engine speeds up as you roll OFF the throttle, the needle/needle jet combination is lean. Assuming you are using a K1 needle with an AB 265 needle jet, you want to richen your setting, by raising the needle, which means moving the needle clip down on the needle to one of the lower notches.

the main jet has some effect on mid range, but mostly is wide open throttle, 3/4 throttle and up, where the needle has cleared out of the needle jet and all that is metering the fuel is the main jet. However, float height and the float jet also have some effect on available fuel. Make sure your float height is correct, then if on full throttle, when you roll off the throttle the bike accelerates when it comes back onto the needle, you are lean on the main jet, and need to go larger. I wouldn't go much larger than 130 for starters.

but, to give accurate advice, we need to knbow what is in the carbs now - float height, float jet, idle jet, slide, needle jet and needle, main jet. Also, are your choke plunger tips in good shapoe, and seating solidly?

for ignition timing, stock on a triple is full advance of 32 degrees at roughly 2,750 rpm or so, no timing changes after that on a stock ignition. For really high compression you could pull the timing back to maybe 26 to 28 degrees at full advance, but I doubt it would make much difference. The bigger issue is at what rpm the Boyer is reaching its 32 degrees? Should be by at least 3,000 rpm, if not sooner.

also, carb setup and timing timing depend on valve clearances beiing correct, no intake vacuum leaks, etc. carb setup also depends on slide heights being equal and carbs vacuum synched......

for detailed answers, post more detailed info on your base setup.

Paul LeClair
 
Hi all - here's the full detail of the carb setup:

Standard airbox with an air filter from Laverda Paradis - looks like a bit of air con filter, or a scouring pad.

32mm Dels, 128 main, K1 needles with AB265 needle jets. Floats 18mm (measured on bike to point fuel cuts off) with 250 (I think) float jets with the rubber tips. (Originally metal tipped jets were fitted).

Standard idle jets, with the air screws 1 3/4 turns out (when 1 1/2 turns out sometimes idle drops a little too low and is less steady)

I'm not sure exactly where the throttle was - I was defenitely not completely wide open, so I would estimate between 3/4 and full initially, and definately below 3/4 when the bike sped up.

The carbs are fitted to billet carb stubs, with no heat insulator between them and the head, just the little plastic inserts in the carbs.

Choke rubbers and sliders are newish, and seal. Throttle slides are standard, accelerator pumps disconnected.

From memory, the advance gets to 31 at 3000RPM and stays there - I remember not having to change the position of the triggers when I took the original ignition off and fitted the Boyer, and that was retarded a timy bit from the book.

As to symptoms, the bike pulls fantastically on part throttles up to 5K (in top, let's say) but from then on, to accelerate you have to drop it down a gear, as opening the throttle makes the noise louder but doesn't really increase forward thrust as much as you (Well, I) would expect. Because of the long throttle action I doubt that I'm using full throttle.

Dropping a gear, or revving higher in a lower gear and banging up through the box and the bike will pull, and hard.

I noticed the roll off acceleration about 5 miles from a cold start - is the bike preperly warm by then?

Chris
 
G'day Chris

Sounds like it's too rich on the main jets (standard spec is 118 on mains, not 128).  I have some 125's that I could send you, but they're not much different to the 128's you have. Try dropping down to about 120 and see what happens. The PO of my Jota had put 135's in it and it was way over-jetted. I put some 120's in it and it runs much better. Strong acceleration right through the throttle and rev range. I'd like to put it on the dyno to check the power and mixture, but my local dyno guy has been off crook for months.

Cheers,
Cam
 
Hi Cam!

I think I need to mark my throttle with half, quarter, three quarter and full, and go and replicate my issue.

I may need to raise the needle, get smaller jets, or both etc, but I think a day logging stuff first would be best. Might do in gear acceleration timing with the aid of a video camera actually....

I should just bite the bullet, buy a pile of brass and stick it on a dyno to see which ones taste nicest....

Best regards,

Chris

dellortoman said:
G'day Chris

Sounds like it's too rich on the main jets (standard spec is 118 on mains, not 128).  I have some 125's that I could send you, but they're not much different to the 128's you have. Try dropping down to about 120 and see what happens. The PO of my Jota had put 135's in it and it was way over-jetted. I put some 120's in it and it runs much better. Strong acceleration right through the throttle and rev range. I'd like to put it on the dyno to check the power and mixture, but my local dyno guy has been off crook for months.

Cheers,
Cam
Click to expand...
 
Chris, raising the needle will make it richer.
Smaller main jets will make it leaner.

Raising the clip on the needle effectively drops the needle.
 
I know Grant. I could be too rich wide wide open and too lean at part throttle (1/2 to 3/4). That would entail a smaller jet and raising the needle (lowering the clip).

I need a nice sunny weekend day and I'll go and find out.

Best regards,

Chris


Grant said:
Chris, raising the needle will make it richer.
Smaller main jets will make it leaner.

Raising the clip on the needle effectively drops the needle.
Click to expand...
 
Hi Chris

those older style solid mount carbs can result in a lot of fuel frothing in the bowls at higher rpm, makes it tough to get wide open throttle main jetting just right.

on your further description, I agree with Cam, first try going down to a 122 or 120 main jet, maybe even a 118, and test ride. I would do that before moving the needle clip.

if the Boyer ignition uses the original Bosch pickups, I don't think you will ever get the timing "just right", those old pickups are probably even cruder than a set of points would be. Consider switching to an ignition with a single trigger setup, like the later WITT system.

oh, and SPIDER , I knew that was what you were thinking, I just thought I would try to put your thoughts into words for you..... :D

Paul LeClair
 
Hi Chris,

Your symptoms sound remarkably similar to mine when I first purchased my Sprint. In that case I reduced the main jet from 135 to 106 to fix, making the following difference to the power curve.

SmallDyno.jpg


Notice that before the power died completely above 5,000 but after pulled well all the way to the red line.

I'd try that first!

--
Dick
 
Thanks for all the replies gentlemen!

OK - on advice I'll order the smaller jets now (118). Eurocarb will love me by the end of this - I can tell!

OK - in the post...

I've ordered some new K1 needles too - I can't remember whether I replaced them last time I was in the carbs.

I think after the smaller main jet test, if I have to drop the carbs to replace the needlles I'll get some new AB265's as well. I've got some new AB262's knocking about if anyone wants them? Ordered them by mistake a few years ago....

Best regards,

Chris
 
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