750sf low compression readings (was ignition question!)

It a TR5T "Adventurer" August 1973 made for the 74 model year that never happened (factory sit-in)
Triumph's last gasp at competing with Japanese trail bikes. Triumph 500 motor in a slightly modified BSA B50MX frame.
Oil in frame, off road pretensions. Very low 1st and 2nd gear, accelerates like the clappers.
Probably one of the last 500s made. One of a few released in 1974 during the sit-in. I think they only made 2,500 ish.

Not a big seller but two years later in 76 Yamaha released the XT500, - oil in frame, off road pretensions ... sold tens of thousands!
 
k
Mr Vegetable Trousers said:
Not a big seller but two years later in 76 Yamaha released the XT500, - oil in frame, off road pretensions ... sold tens of thousands!
Click to expand...

XT 500 didn’t just have off road pretensions - it won the Paris Dakar- Triumph on the other hand would have melted and snapped the frame on day one, that’s the reality of it not being popular. Reg Allen’s in Ealing we’re going to build me an Adventurer from NOS when I was 19…still in the showroom with cat on the seat when I was forty!
 
Tippie said:
Were they W2C Bosch plugs? As you say very cold and definitely not for a road triple, it might have been ok with W3 or W4. When Bosch changed from the old (eg W240T2) type designation to the single number, they also went opposite so that the higher the number was hotter, as with Champion. https://www.boschsparkplugs.net/boschs-numbering-system
Click to expand...
Being so long ago the Bosch W22 may have been an error in conversion, came on the customers bike as i fitted the new iis. As you say Tippie, W240 T2 is specified in the Laverda 1000/1200 factory original manual here. j.
 
Head and barrels off.
No wear that I can feel in the barrels just what looks like staining from when it stood a long time?
Piston to bore clearance is spot on measured in many spots.
Ring end gap 0.4mm
New rings and a hone?IMG_4584.jpegIMG_4585.jpegIMG_4587.jpeg
 
No harm in seeing if a hone cleans it up but as Rob says it looks like deeper damage if zoomed in. How did you measure the piston clearance? And what is "spot on"?
 
Just make sure you use a decent engineering workshop. Tales of non parallel bores and inaccurate measurements abound, plus poor surface finish. Get your pistons and rings first so they can be measured accurately by the shop.
 
I have recently discovered an excellent rebore etc workshop in Crowborough (havent used them yet) and another just outside Rye, a little treck from London (not sure if North, South, East or West, for you) but well, worth the trip especialy if South of the snake, I have found that I need to emphasise that the datum for securing to the boring table is the cylinder base, this gives a better chance of the bore being square to the crankshaft, On triples I have found that when the top face is used it is sometimes not square, only a very small amount but accentuated by the length of the rods and the stroke, very often results in dissapointment, the shop in Rye are very well aware of this and go the extra mile to use the base as datum for fixing, having stopped using a local re-bore shop for the very reasons stated above, I was still wary, so my son (David) took the newly bored barrel to work and checked it using a lazer device that is used on guns of all sizes, even naval ones, (he is an engineer with the MOD) he found that it was as good as any ballsitic device he had ever tested ,ad piston to bore clearance was bang on 4 thou as well, great care also needs to be taken to get "the right" pistons, as many dealers will sell you the 1200 items, if you want the compression ratio correct and the combustion chamber also, be sure to get the right ones, they are thin on the ground, my brother in law found this very recently and he bought from Andy Wag, very expensive, like all his things, but actually the correct pistons.
CLEM
 
I doubt that you can re-use your bore without re-boring, shame as stock pistons are easiest to obtain in 80.00 bog standard size,. The Omega pistons are the ones with the very thin top ring, that I mentioned earlier, I am not sure if there is a correct crown profile avaialable for the 750 with this brand or if the 1200 profile is substituted as "second best" and " wiill do" kind of scenario, would be interested to hear from sellers of these pistons in that respect.
CLEM
 
Seen that type of horizontal witness marks before, had many stumped including University auto engineers i had access to. Was a lady at Deves whom called it out, insufficient back clearance of the control rings on the piston, ie the expanding piston caused circular increased interference fit front to back, of the rings. Ring end gap ostensibly has no part in this serious engineering error. HTH j.
 
You must log in or register to reply here.
Back
Top