ET: Engine (Twin) Confirming Valve Lifter/Decompression Operating Normall

[rcheli32]

I did a search but didn't get a result, so hoping the combined experience on here can advise how I can confirm the valve lifter mechanism is working properly.

I am suspicious that the front cylinder isn't firing properly, because blocking the rear carby intake kills the engine pretty quickly but blocking the front cylinder carby doesn't make a noticable difference. The valve lifter mechanism/cable had broken recently and a workshop repaired that while doing roadworthy items/paperwork, but I'm wondering if the lifter isn't working properly leading to reduced firing on the front? Is there a simple way to tell? The front cylinder is firing because removing the spark plug lead from the rear still allows the engine to run, and the front cylinder exhaust heats up very quickly when the engine is started, I'm just not sure it is operating 100% of the time especially at higher revs.

Thanks,
Ross.

 

[chankly bore]

Firstly, what cams are you using?High lift cams can cause clearance problems in the timing chest as well as on the lower valve guides. After that, try removing the inspection caps over the tappets. Go carefully here, as at full lift the tappet may jam the inside of the cap, so best to remove plugs and feel for T.D.C. compression one cylinder at a time. You should then be able to see roughly how much each cylinder exhaust valve holds off when the valve lifter is working. They should be equal, especially if the series "D" set- up is fitted. Sometimes the little 3 x 5mm. roller drops out of the lifter arm. If you're lucky it drops harmlessly to the bottom of the timing chest. While you're at it check the tappet clearances. Other things can also hold the valve off its seat. Maybe do a compression check as well. It is normal for the front cylinder to run a little hotter on a Twin, but I'd get this sorted quickly or you mightn't enjoy the bike. What sort of workshop did the fettling, maybe no names, no pack drill, but there are some graceless chancers out there.

 

[greg brillus]

You should be able to turn the engine over with the kickstart lever until compression on either cylinder is felt, then with moderate not excessive force squeeze the valve lifter lever in and whilst pushing down again with the k/s lever the engine should rotate until the next compresion is felt........Just squeeze the lifter lever enough to ease it past TDC on that compression then release the lever ready for the next cylinder, and repeat again.........This should be easy enough........To cycle the engine using the valve lifter to turn the engine without too much effort..........If it tends to pass compression on either cylinder it is possible it might have a tight tappet, easy enough to back off to let the suspect valve seal against its seat again.......the tappets run nil clearance, although i prefer to have the slightest up and down feel on the rocker. If you find the valve lifter works fine on one cylinder but very stiff on the other, there are several reasons this can happen.........Commonly the link rod that connects the 2 together is not adjusted correctly for length.......the earlier ones of these were non adjustable........I had one suffer this symptom recently........The culprit was because someone had installed 2 cams with different diameter base circles........Like out by 2 mm or so........Thus the valve lifting mechanism was lifting nicely on one but the other was not lifting at all.........Let us know how you get on.........Cheers.

 

[rcheli32]

You should be able to turn the engine over with the kickstart lever until compression on either cylinder is felt, then with moderate not excessive force squeeze the valve lifter lever in and whilst pushing down again with the k/s lever the engine should rotate until the next compresion is felt........Just squeeze the lifter lever enough to ease it past TDC on that compression then release the lever ready for the next cylinder, and repeat again.........This should be easy enough........To cycle the engine using the valve lifter to turn the engine without too much effort..........If it tends to pass compression on either cylinder it is possible it might have a tight tappet, easy enough to back off to let the suspect valve seal against its seat again.......the tappets run nil clearance, although i prefer to have the slightest up and down feel on the rocker. If you find the valve lifter works fine on one cylinder but very stiff on the other, there are several reasons this can happen.........Commonly the link rod that connects the 2 together is not adjusted correctly for length.......the earlier ones of these were non adjustable........I had one suffer this symptom recently........The culprit was because someone had installed 2 cams with different diameter base circles........Like out by 2 mm or so........Thus the valve lifting mechanism was lifting nicely on one but the other was not lifting at all.........Let us know how you get on.........Cheers.

I don't use the valve lifter at all to start the bike, and yet I feel like one compression is higher than the other. How many downward cycles of the kick starter should it take to fully rotate the 4 strokes? I suspect I need to expose the lifters on the front cylinder to really know what is happening

 

[Chris Launders]

Just take the rear plug out and see how the front cylinder compression feels, then swap over, I doubt your decompressor is holding the valve open as you would have no compression on the front, it may be you simply have the carbs out of balance and the rear is doing all the work at tickover.

 

[Bill Thomas]

Does the firing order have some effect ?,
The gap between the two is strange, Not like a BSA Twin etc,
Number one, The rear, has a long way to fire number 2 the front ?.
Cheers Bill.

 

[Garth Robinson]

I don't use the valve lifter at all to start the bike, and yet I feel like one compression is higher than the other. How many downward cycles of the kick starter should it take to fully rotate the 4 strokes? I suspect I need to expose the lifters on the front cylinder to really know what is happening

You would help things if you got used to using the valve lifter to start,when things get sorted out.You will eventually wreck the kick start crank splines,and possibly your knee when things let go.Both are expensive repairs.

 

[rcheli32]

Just take the rear plug out and see how the front cylinder compression feels, then swap over, I doubt your decompressor is holding the valve open as you would have no compression on the front, it may be you simply have the carbs out of balance and the rear is doing all the work at tickover.

This was great advice, nice and simple. After doing this I can confirm that the front is about the same compression as the rear, and certainly not leaking much. The compression holds my bodyweight (over 100kg) and very slowly eases down as compression leaks, about the same on both front and rear.

I did notice the rear plug was very wet with hard carbon buildup compared to the front which was clear of carbon and rinsed clean.

Somebody mentioned balancing the carburetors, but that seems like a job for a pro with the right vacuum tool?

 

[Bill Thomas]

You could do it by ear !, With a plastic tube,
Lots of Fun, Cheers Bill.

 

[chankly bore]

Balancing carburettors, as Bill says, can be easily done. Depending on what carburettors you have there are different problems and solutions. So, for a start, what carburettors, and in what condition? The original Type29's are sensitive to float level, for example. We can go on with more information from you. I used a plastic hose on the S.U.'s of my Mk. 2 Jag. Seemed to work fine- though I never got more than 15 m.p.g.