ET: Engine (Twin) Front cylinder cooler

[Chris Harrison]

Afternoon all,

My twin was unusually difficult to start today but once running I noticed the front cylinder carb was overflowing. I stopped the engine, removed and blew out the float chamber and the overflow was resolved. When I started the bike again it just didn't sound right, the tick over was uneven, I quickly checked the cylinder temperature (IR thermometer - brilliant!) and the front cylinder was significantly cooler than the rear, 30c versus 80c. OK, I thought, pop a new plug in, this had no effect on the problem. Next job was to sit both plugs next to each other on the front head and kick it over, spark intensity looked about the same. I've just stripped the carb and made sure all of the passages were clear, I think they are but the temperature difference remains. When the engine cools I'm going to check the valve clearances.

It may be that at tickover the temperature difference was always there but I hadn't noticed it. The bike has a BTH mag fitted. The front cylinder air screw is 2 1/2 turns out.

Am I missing anything?

Thanks,
Chris.

 

[greg brillus]

2 1/2 turns out sounds very lean...........normally 1 to 1 1/2.........but what carb's are you running. Perhaps the front slide is closed more than the rear, if the carb's are old and worn more air will bypass the slide which makes setting them up difficult.

 

[Chris Harrison]

Hi Greg,
I'm not entirely sure which carbs are fitted, I can't see any identification marks. However looking at the Burlen website I think, based on the intake design, 29/289 is likely. I measured the bore of the carb on the engine side and this was 1 1/16". Both carbs are set to 2 1/2 turns out which, as you suggested, looks to be too lean.

Now I have to publicly shame myself, the only mitigating circumstances I can offer in my defence are being aged, of poor eyesight and limited flexibility. To check the carb balance I took the time to put the bike on the hydraulic workbench and get it up to eye level, the absence of a throttle stop screw on the front carb was there plain to see (or not in this case).

I'll pop down to Burlen tomorrow and pick up 2, yes 2, Thottle & Pilot Air Screw Kits, one for the carb and one for the tool tray, no doubt this will fix the problem. I'll then adjust the pilot air screws as you've suggested.

I've not had the bike a year yet but intuitively I knew something wasn't quite right when I tried to start it. Isn't it great how on these older machines you become 'connected'. This part of the motoring experience has been lost with modern bikes and cars, the only real connection is when you plug in the laptop........
Cheers,
Chris.

 

[Bill Thomas]

Hello Chris, When it's on the road, I was always told, The rear cylinder runs cooler ?, The air goes around the front one to make the back one cooler, Could just be an old Story ?.
Can you tell us about the IR thermometer. Cheers Bill.

 

[Chris Harrison]

Hi Bill,
If you search eBay for 'IR Thermometer', or look for this auction number, 292149459584, you'll see the gadget. Very handy tool for working on engines and cheap as chips. You can easily spot temperature differences between cylinders, without melting your vinyl gloves. However the most important use of the thermometer is entertaining the cat!

 

[Bill Thomas]

Hi Bill,
If you search eBay for 'IR Thermometer', or look for this auction number, 292149459584, you'll see the gadget. Very handy tool for working on engines and cheap as chips. You can easily spot temperature differences between cylinders, without melting your vinyl gloves. However the most important use of the thermometer is entertaining the cat!

They say Vin's run cool, But mine don't, They are as hot as hell, On a warm day, They are ex race engines but I don't think the L/ning and Flash are highly tuned. Cheers Bill.

 

[Bill Thomas]

That could be FUN, Riding down the road, Pointing the gun at the engine !! Hope no Po lice are looking. Cheers Bill.

 

[van drenth]

Try to measure at the sparkplugs, should be both the same +/- 114 Degree centigrade at idle
or make some M5 Threadhole in both manifolds and check the vacuum and readjust your carbs at 800-850 RPM. than you can really tune .
Regards, van Drenth JC

 

[greg brillus]

Hi there Chris, It sounds like your bike has the original carbs, either 289's or 276's ( 89's on Shadows and 76's on Rapides) My suggestion for an initial set up would be to install the new throttle stop screw, then remove each carb off it's stub manifold and set the throttle slide height as close as you can on both carbs. Generally so the gap at the very bottom is about 1/16th of an inch or where the crescent is about 10 mm in width. If you set both carbs like this as close as you can, this will give you a good starting point, be sure and check that the throttle cables are not hanging up and/or holding the throttle on (there should be some slight slack in each cable). Refit each carb back onto the manifolds and check that the float bowls are vertical before tightening the clamp bolt (stand in front or behind the bike and visually look at the angle of each carb to get the float as vertical as you can and ideally with the bike sitting on its wheels or an the rear stand. Once you are happy with that, then synchronize the carb slides so that when opening the throttle that both slides lift at the same time. Put your index finger of your left hand in the mouth of the rear carb, then whilst leaning over the seat, visually look into the mouth of the front carb and open the throttle. Adjust the cables until they lift at the same time, then set each mixture screw 1 and 1/4 turns out from fully in. Start the engine, warm it a minute or so and check the idle speed. You may need to go for a spin around the block to warm it up better, then adjust more to fine tune the idle and mixtures on each cylinder. I do this with a fan on the engine so it does not get too hot, and make the adjustments quickly. Try to aim for an even beat on both cylinders, remembering it is better to keep the engine on the rich side rather than too lean. Good luck with it all...............Cheers...........Greg.

 

[erik]

i heard that an infrared Thermometer is not useful on Aluminium alloy. i works only good on steel.regards erik