Jota Valve Train Questions

[tom3c75]

Could those of you more experienced in these matters help with some valve train questions for a Jota that I?m restoring?

The bike
1980 Jota
Mileage is approx 60,000kms.
I have not run the engine since purchasing the bike. 
The engine does not appear to have been previously opened. 
Cams are 4C.
Pistons are high compression. 

Valve train components
Inlet valves are 39.5mm and exhausts 36.5mm.
Shim buckets are the short type.
Has short valve springs, ie. approx 34mm.
I haven?t dismantled the valves yet, so not sure yet if they are single or double springs.

Current valve clearances & shim sizes:  
Inlet clearances are .20mm.
Exhaust clearances are tight: outer cylinders are .20mm and the centre cylinder is .10mm.

Shims are in the 2.30 to 2.40mm range, except for the centre exhaust, which is 1.85mm.


Ques 1 ? Valve Sizes 
Should I retain the 36.5 exhaust valve, or should i fit the slightly smaller valve as used on the series 2 Jotas?   
There is no evidence that the existing valve sizes have caused a problem.

Ques 2 ? Valve Springs
Should I retain the shorter springs, or is it advisable to fit the longer 43mm springs, together with the appropriate retainers?   
The tension on the current springs don?t seem to have the problem described in Stevio?s post.  http://www.laverdaforum.com/forum/index.php?topic=48199.0 

Ques 3 ? Shim Sizes
Am I correct in thinking that the short springs provide a use of a wider shim range?
This because the spring retainers of the short springs are lower than for valves fitted with the longer springs.
If this is the case, and if the current valves and seats are serviceable, I should be able to use a shim of 1.70 for the centre exhaust valve without the risk of the bucket fouling the valve spring retainer. 

Many thanks

Tom

 

[tom3c75]

Forgot to attach to previous post

 

[sfcpiet]

Tom,

Shouldn't be much of a problem fitting a 1.70 shim, but I'd rather investigate why it should be necessary.  That's a lot of wear for 60000km!  Has the seat recessed so far or is the valve stretching?  Seat recessing is bad for the flow, doesn't help performance.  Valve stretch is bad for the wallet, when things go pear-shaped, it's gonna hurt!

By all means, check the springs, I think I'd just toss them in the scrap and replace with new ones together with suitable retainers.  You can keep the short buckets, but judging from your pics, they could do with an hour or 2 of elbow grease on a flat plate and a sheet of 240 grit emery...

Both types of retainers offer the same amount of total valve stroke, ie, the collets would be the first to hit the stem seal or guides.  The longer springs need the "long" retainers to prevent spring coil binding at full travel.  Overall valve lengths were the same throughout, so the usable shim range is also similar.

The valve configuration is not the most efficient, but changing means spending quite a handfull!

Once you get it all back together, check float heights in the carbs, primary reason for valve stretching.

piet

 

[tom3c75]

Thanks Piet.

The centre exhaust does seem to have been running quite a bit leaner than the other two.
Next step is to dismantle and check the valve and seat condition which will give more direction for what to do.
I do have a set of Phil's valve springs in hand for the change.