Left cylinder running hotter than the other side

[rhysbud]

Okay, so a bit of back ground on this before i start. I have rebuilt this TX650 motor, inside and out with .25 over pistons, new wiring with pamco and brand new VM34's mikuni carbs.

Now, i've had it running a few times and i immediatly noticed that the left exhaust pipe is hotter than the right. To the point that its tarnished the new stainless on the left. So i thought it might have been a lean mixture and pulled the carbs down, even though they're brand new i blew out all the jets and passages. Reassembled with these settings, Air mix screw 1 turn out
pilot -25
main - 220
needle - 6F9 middle clip position
Needle jet - 159 0-2

I started it again and used a thermal pointer, the left is 210 degrees where as the right was around 150-160. I've double checked valve clearances to be sure, gone back to BPR6ES plugs as the iridiums died on me and ive never had much luck with them anyway. To top all this off, when i was trouble shooting the bike wouldn't start anymore I have spark, plenty of fuel and used another set of plugs to no avail. The only think i can think of is the pamco has somehow played up and the ignition timing isnt right? Anyway, any help is appreciated.

 

[peanut]

well the most common reasons for one cylinder to run hotter than the other are exhaust valve not seating properly or long enough, the mixture being too lean in that cylinder or a poor spark in that cylinder.

I'm not sure if the pamco operates on the wasted spark principle but if it does then pamco is unlikely to be at fault as the other cylinder is working fine. if it is wasted spark then try swopping the HT leads over on the plugs and see if the problem resolves on the hot cylinder. If the problem switches to the other cylinder then the coil, HT lead ,Cap and plug are possible culprets.

I would check the compression and compare ,they should be fairly close .

Try loosening the valve settings to .006 inlet and .012" exhaust until you find the cause. At least you can then eliminate the valve clearance being the source whilst testing other stuff.

Then you are left with air leaks to carbs or inlets .

If the exhaust is burning that much my money is on the exhaust valve not seating properly which should show up on the compression test

 

[rhysbud]

Thanks for your reply peanut. I double checked the valve clearances and made sure they were set at .002" on the inlet and .004" on the exhaust, those specs are correct? I have a friend with a high output coil that is brand new so ill be using that to eliminate spark.

 

[peanut]

.002" and .004" is what is recommended in the manual by Yamaha for the TX650 but its common knowledge that they are too tight and almost impossible to achieve proper valve seating and gas flow . The early XS650 engines had a recommended valve clearance of .006" and .012"

I set my valve clearances at .006 inlet and .012" exhaust on my 79 XS650 to ensure that the valves do not stay open longer than necessary and they seat properly and for as long as possible to dissipate heat from the exhaust valves into the head

Look at how miniscule that gap is .002" the slightest rise in temperature and thermal expansion or a weak mixture could leave your gap at nill or worse leave your valve not seating properly.

I would try the larger gaps to see if the is a difference . if there is no improvement then go back to the smaller lash settings at least you have eliminated the valve settings being the source of the problem.

 

[gggGary]

yes 2 is too tight. IIRC Yamaha changed that setting with a service bulletin. I used 2.5 and 6 for years but recently changed to 3 and 6. What Peanut said sounds good but I am kinda slow to change and not sure I could deal with the noise on settings that big. My guess has always been the valve settings were an engineering vs sales force war. engineering wanted them big, for better cooling dealers couldn't sell bikes that sound like a hail storm on a tin roof at idle.

 

[peanut]

yes the clatter is a little worrying isn't it but it drowns out the clatter of the rings and cam chain bit like turning the radio up in the car lol

I guess I'll experiment a little but no matter how many times I adjust the valve lash when I turn the engine over and recheck there always seems to one or two valves that have changed a thou or two

 

[hardrock785]

what gary said

 

[gggGary]

As far as I can tell once the tappet ends are pitted the clearance is impossible to set accurately. I guess in the end engineering wins, no matter what you set; the tappets and valve stem spall and the clearance gets larger... Once the surfaces are rough you can't set them right because the feeler bridges the rough spots and holes giving a false reading. Don't try to beat this because both parts move around, some times it's hole hitting hole with large clearance other times it's ridge on ridge and small clearance.

 

[peanut]

exactly ! I keep trying to convince folk that these machines are incredibly crude in design and construction compared to modern fuel injected 12k red line bikes of today. They were not made to fine tolerances .

You cannot expect to run 35 year old engines to 2016 standards of precision.

Why people go to extraordinary lengths to establish TDC to 15 seconds of a degree is completely beyond me when you consider that the whole of the drive chain and camchain have such huge working tolerances .

 

[rhysbud]

Okay. Well valve clearances have been re set to the larger settings but it still won't start. Found it has no spark, luckily for me a friend has an XS project that's running behind schedule and he has a Brand new pamco and coil I could use. Still no spark hmmmm. Also when setting the clearances, is it normal to set one side, rotate the crank 360 and the same side go's tight? Go another 360 and the clearances are where I just set them... hmmm

 

[peanut]

yes you should adjust one cylinder with the piston at TDC on its compression stroke then rotate the crank 360 and adjust the other cylinder on its compression stroke .
The camshaft lobes are not symetrical for both cylinders if you look along the camshaft

The engine ran before so its got to be something really simple probably something you have disturbed in the wiring .
Run through the areas that you have been working near and check the wiring and connectors etc carefully