Multiple Bent Pushrods

[Woollybandit]

Hi all,
Just got ‘73 TR7v out of storage whilst moving/building workshop and on first ride after about 15 miles cruising at 3500rpm suddenly lost power then died. Got it home and not firing on left side when trying to kick it over but right side spluttering for a second or so. Blue flames out of bare carb.

Initially found badly bent left inlet pushrod and replaced it but still not wanting to run. Took off exhaust rocker box and both sides’ pushrods also badly bent.

Bike built by PO garage owner with 9.5:1 pistons and has been running for 50 miles before storage and fuel oil changed prior to 15 mile run.
Rocker arms are free, oiled and I’ve squirted oil through shafts to prove flow. Valves are seated and bounce if stems hit with rubber hammer. Springs are seated in caps and no sign of piston impact.

Engine wouldn’t rev above 5k which I put down to ignition advance timing not being checked yet.

Anything I should check before new rods arrive? Are high strength pushrods available but would they transfer a problem to somewhere else? Inner springs are painted red if that is significant of anything.

Does anybody have any input? I will put in new rods but is strange why three would fail.

Thanks in advance.

Woolly

 

[grandpaul]

It might just be me, but those valve stem tips seem to be longer above the keepers than I recall.

I've never seen all 4 pushrods bent before (on a classic Triumph). Maybe replacement too-long pushrods?

 

[Woollybandit]

Yes I thought that but here's a photo with an old valve from a previous 650 or 750 (can't recall which).

 

[Woollybandit]

Number on installed stem reads 70-2904.
Correct part # but yes it does look long!

 

[Woollybandit]

Just checked rocker shafts and all oilway passages are as clean as a whistle.

 

[speedrattle]

i have never seen pushrods bent that badly except with a thrown rod. its hard for me to understand how it would run at all if there was that much interference. its quite possible that the valves are bent if they were smacking the pistons.

if your PO changed the valve timing enough to cause that much piston to valve interference i find it hard to see how it would have run fifty miles. advancing the intake cam decreases intake valve clearance, while retarding the exhaust decreases exhaust valve clearance. but both at once? what kind of camshaft is in there?

more likely, if youre getting flames out of the carb then obviously the cylinder is firing with the intake valve open. check the ignition timing to see if it changed while you were riding. if the valves are correctly seated as you say then the ignition timing or valve timing is way off, and it could be catastrophic detonation that caused this issue. but you would have heard it, i would think.

take off the timing cover and compare the marks on the cam pinions to the illustration on page B33 of the manual either the 63-70 or the 73-78 manual. its the same page.

when you put it back together rock the camshafts over their noses and make sure you dont have coil binding on the valve springs. maybe they were shimmed up way too high.

if thats okay i would clay the pistons. i run 11.75 to 1 in one motor and i have to check valve to piston clearance every time i change anything. normally 9.5 to 1 is not a problem, but i have encountered valve pocket interference with oversize valves before at 9.5.

and measure the pushrods. they look stock but who knows.

something is very definitely wrong in there. you need to know what it is before you run it again.

 

[Woollybandit]

Thanks Speedrattle, the timing cover is my tomorrow job. I’ve rebuilt a couple of 650/750 twins recently but this is a mystery to me. At least I have the special tools for the job.
I don’t know what cam it is but it ran fine for 50 miles, during those miles the power didn’t seem exceptional. I can’t ask the builder because he passed away just before fitting battery and ignition timing it. I almost wondered if he’d straightened out some old rods but why save £30 after all this work? Rods when straightened measure 150.7mm ….. I have a rod from a previous Bonnie (650cc?) that is 10mm longer, btw.
When I checked the tappets last week (.008/.006) no sign of binding or piston clash whilst rotating crank. I was checking head torque at the time so I also checked tappet clearance. Nothing to concern me then.

It’s a mystery…… which I don’t like. I can only guess at valves sticking shut. Whilst looking at tappets I could see edge of the inlet valve opening up through the plug holes and there didn’t seem any close proximity to the piston then.

If worse comes to worse I’ll have to remove head and see if valve stems seem tight in the guides.

 

[speedrattle]

you cannot easily check piston to valve clearance without taking off the head. i measure it with clay on the piston tops, but you can also replace the valve springs with some carburetter slide springs and rotate the motor through several rotations while screwing down on the valve adjusters to see how far you can push before they hit the piston. safe is 0.040 intake and 0.060 exhaust. the closest approach is around 10 degrees or so near TDC.

the valve adjusters are 26 tpi, so one full 360-degree turn on the adjuster is 0.040, practically speaking.

doing the spring thing will not tell you how close the valve margin is to the edge of the valve pocket, where you need to have enough clearance not to hit when the piston rocks over TDC. you need 1/32-inch or so side clearance.

but i would remove the head anyway. youve got the rocker boxes off already so all youre needing to mess with is the pushrod tube seals.

 

[speedrattle]

the 750 T140 has cylinders that are 1/2-inch shorter than the 650 T120. one less fin on the cylinders, shorter connecting rods.

so your shorter pushrods for a T140 are correct. T120 pushrods would be too long for a T140, by 1/2-inch.

check that ignition timing. what are using? points? boyer? pazon?

If worse comes to worse I’ll have to remove head and see if valve stems seem tight in the guides.

do ^^^that anyway. you need to look at everything on this puppy.

 

[grandpaul]

Yes I thought that but here's a photo with an old valve from a previous 650 or 750 (can't recall which).

My point, illustrated! The installed one looks "tall", the free valve top section looks much more "square" height vs width.

 

[Woollybandit]

My point, illustrated! The installed one looks "tall", the free valve top section looks much more "square" height vs width.

I double checked, it’s identical and has the part number stamped on that free end above the collets. Even if they were taller you can still only push as far as the cam will allow and tappet gap will still be 6 or 8 thou. Rocker angle might be different but the free top section is as per spec for 70-2904 or E2904

 

[Woollybandit]

the 750 T140 has cylinders that are 1/2-inch shorter than the 650 T120. one less fin on the cylinders, shorter connecting rods.

so your shorter pushrods for a T140 are correct. T120 pushrods would be too long for a T140, by 1/2-inch.

check that ignition timing. what are using? points? boyer? pazon?



do ^^^that anyway. you need to look at everything on this puppy.

Ignition is Boyer Mk3, fully charged battery and big fat sparks.

 

[Woollybandit]

you cannot easily check piston to valve clearance without taking off the head. i measure it with clay on the piston tops, but you can also replace the valve springs with some carburetter slide springs and rotate the motor through several rotations while screwing down on the valve adjusters to see how far you can push before they hit the piston. safe is 0.040 intake and 0.060 exhaust. the closest approach is around 10 degrees or so near TDC.

the valve adjusters are 26 tpi, so one full 360-degree turn on the adjuster is 0.040, practically speaking.

doing the spring thing will not tell you how close the valve margin is to the edge of the valve pocket, where you need to have enough clearance not to hit when the piston rocks over TDC. you need 1/32-inch or so side clearance.

but i would remove the head anyway. youve got the rocker boxes off already so all youre needing to mess with is the pushrod tube seals.

God how I hate those PRT seals!
Is there a way to see what cam is in there without splitting the cases? Say, marked on the timing side end? I only guess that something extreme is available for these vintage engines. It certainly didn’t feel different from my last 5 Bonnies….. less grunt than the 650cc I sold last year.

 

[Woollybandit]

while screwing down on the valve adjusters to see how far you can push before they hit the piston. safe is 0.040 intake and 0.060 exhaust. the closest approach is around 10 degrees or so near TDC.

Speedrattle, are you saying set tappets to the 6 & 8 thou gap and the 40 & 60 thou is the clay minimum thickness? I’ve reread the passage a few times and this is what I’m taking away. This would mean placing clay and rebuilding the whole top end?
In any case tomorrow’s job is off with head to check valves for impact/guide clearance and open timing side case.

 

[speedrattle]

yes

take the head off, place a thin layer of clay on the piston tops in the valve pockets, oil the valve faces, replace the head, pushrods, and rocker boxes, set valve clearances, and rotate twice. take tbe head off, cut the clay on the piston valve pockets down the middle with a razor blade and measure to 0.01 with a machinist's rule.

i cannot see well enough to measure so i take photographs of the clay and the ruler and then magnify the image. it takes very little time compared to a rebuild.

a mk3 boyer is generally foolproof. the only way it would matter here is if the stator plate or the rotor in the points cavity shifted and advanced the spark enough to cause detonation. i dont see how that would bend pushrods but see whether the rotor is tight in the exhaust cam anyway.

you cant easily tell anything about the cam without taking it out, but you can look at the marks on the pinions to see what the timing is. you can measure lift with a dial indicator. but if it ran fifty miles that should not be a problem. something changed, i think.

but check everything anyway. something is wrong somewhere.

 

[speedrattle]

i will measure some 650 pushrods tomorrow

 

[Woollybandit]

yes

take the head off, place a thin layer of clay on the piston tops in the valve pockets, oil the valve faces, replace the head, pushrods, and rocker boxes, set valve clearances, and rotate twice. take tbe head off, cut the clay on the piston valve pockets down the middle with a razor blade and measure to 0.01 with a machinist's rule.

i cannot see well enough to measure so i take photographs of the clay and the ruler and then magnify the image. it takes very little time compared to a rebuild.

a mk3 boyer is generally foolproof. the only way it would matter here is if the stator plate or the rotor in the points cavity shifted and advanced the spark enough to cause detonation. i dont see how that would bend pushrods but see whether the rotor is tight in the exhaust cam anyway.

you cant easily tell anything about the cam without taking it out, but you can look at the marks on the pinions to see what the timing is. you can measure lift with a dial indicator. but if it ran fifty miles that should not be a problem. something changed, i think.

but check everything anyway. something is wrong somewhere.

Hi, I woke up with a jolt last night thinking what if the PO had timed the cams/valves on the wrong cylinder TDC ? My head is still reeling trying to work out what the effect might be. Unless there is valve/piston overlap the engine might still run on effectively the wasted spark from the Boyer. If there is overlap then that would explain 3 bent rods assuming the last one had popped off the rocker!
‘Off with his head’ later this morning! (Remembering to push with my thumbs on the PRTs so I don’t unplug them at the bottom!)

I’m curious about the 750’s shorter barrels, why do that? Anything to do with lowering the frame height nobody liked? The stroke is the same for 650 & 750 @ 82mm ……. but 750 engine has 6” con rods instead of 6.5” of the 650. I wonder if 750 barrels have longer skirts and consequently less flywheel clearance, or skirt cutouts on the sides to clear the flywheel?

 

[Rudie]

curious about the 750’s shorter barrels, why do that? Anything to do with lowering the frame height nobody liked?

Made a complete 750 engine easier to fit in the frame.

 

[Woollybandit]

Made a complete 750 engine easier to fit in the frame.

Makes sense as there was an outcry about the tall OIF bikes. That might have been one of several modifications to lower the seat height.
Seems like a futile effort when you look at bikes now. I have owned two 1200gs, modern Tiger and found those to be one-foot bikes, can’t imagine trying an Africa Twin or god forbid a HD Pan American!!! I’d never pick that up ……. & I’m used to my Dyna’s weight.

 

[Rudie]

Makes sense as there was an outcry about the tall OIF bikes.

Triumph sorted the seat height mid-72, by lowering where the seat rails are welded on the vertical of the large diameter backbone tube. Sidepanels and airboxes were also modified

Triumph modified the 650 rocker boxes and mountings to the cylinder head at the beginning of the 71 model year so a complete 650 engine could be fitted in the then new oil in frame. The modifications to the height of the 750 twin engine just made that a little easier to fit in the existing frame.

HD Pan American!!! I’d never pick that up

It used to be the big Harleys were supposedly easy to pick up because the weight was low down and the shape of the crashbars was deliberate so they could be rolled on to their wheels?