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ET: Engine (Twin) Fuel and Carb Settings

MartynG

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With modern fuel and a standard motor I understand the optimum timing is 4 BTDC fully retarded to give easy starting and 32 to 34 BTDC fully advanced. ATD springs must not be sloppy - in the fully retarded position there needs to be sufficient 'preload' on the springs to hold the weights fully home.
 

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Bobv07662

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Today was the day to check the ATD with the mag synchronizer and got a good result. Timing on the rear cylinder starts at 3' BTDC and is all in at 38' BTDC, the suprising thing is that the front cylinder was almost exactly the same at 2 1/2' BTDC and is all done at 37 1/2 BTDC. Close enough for government work. I am running the bike on non-oxygenated, leaded 110 octane fuel so I don't think I will need to back the timing down to 34-35 .
The banging continues in the silencer but is quite reduced after fresh plugs. I rode the beast almost 10 miles this evening and it seemed to be getting better as it warmed up. 80'F temperature here so it wasn't too hot out.
Even after riding the 10 miles the oil tank was not warm yet. These are some cold blooded machines!
Running Champion N5C plugs to start with but while learning the bike I did foul up the first set.
Any suggestions for plugs and diagnosing the banging in the silencer? It is the straight through type muffler. Since it fouled a set of plugs running around the shop, I'm hesitant to richen the pilot settings, but I believe it ran a bit smoother with the air levers pulled on a small amount. Perhaps different plugs might make a difference? Also, is any sealant required where the carbs clamp on the intake spigots?
Ahh yes the clutch...a bit grabby but much more controllable after increasing the play in the cable to 3/16.
Any suggestions would be appreciated. Thanks for all the help so far!
 

bmetcalf

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What compression pistons are you running? My 8:1 pistons are fine on 87 octane E90.
 

Bobv07662

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What compression pistons are you running? My 8:1 pistons are fine on 87 octane E90.
According to the engine build sheet 7:1 flat tops are listed. I did look at them with a bore scope and they are indeed flat tops with 2 valve relief pockets.
 

Robert Watson

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You should realize that pulling on the air levers a small amount only effects what happens when the throttle slide is pulled up past the bottom of the choke slide. I see several people, on starting a cold engine, insist it just needs a little bit of "choke" and then attempt to start with no or very little throttle.

As for the banging in the muffler, and if you are convinced that your idle circuit is clear ( A # 86 drill in that tiny hole in the jet block) then you need to either richen the pilot screw or look for air leaks. Did you seal the spigot joint when fitting the clip fit carbs, are the carb to head stubs sealed tight to the heads, they can get distorted over the years, are the exhaust nuts sealing the pipes at the head, and are the head to piston liner sealed with the correct clearances on the head joint. Min of 2 thou shallower for the counter bore in the head than the height of the liner above the muff.

I run NGK B6 series on both original mags and a newer BT-H so basically t he same heat range.

My B does pop a little now and then, especially on a cold motor but otherwise it behaves reasonably
 

MartynG

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banging in the muffler, especially on overrun, is often caused by air leaks where the exhaust pipe connects to the cylinder head.

The ignition advance is good for starting but seems a bit high on full advance. 34 is much safer - I have melted pistons running 36! with 7.5:1 piston and 100 octane fuel.

It can be an expensive lesson!
 

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Bobv07662

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You should realize that pulling on the air levers a small amount only effects what happens when the throttle slide is pulled up past the bottom of the choke slide. I see several people, on starting a cold engine, insist it just needs a little bit of "choke" and then attempt to start with no or very little throttle.

As for the banging in the muffler, and if you are convinced that your idle circuit is clear ( A # 86 drill in that tiny hole in the jet block) then you need to either richen the pilot screw or look for air leaks. Did you seal the spigot joint when fitting the clip fit carbs, are the carb to head stubs sealed tight to the heads, they can get distorted over the years, are the exhaust nuts sealing the pipes at the head, and are the head to piston liner sealed with the correct clearances on the head joint. Min of 2 thou shallower for the counter bore in the head than the height of the liner above the muff.

I run NGK B6 series on both original mags and a newer BT-H so basically t he same heat range.

My B does pop a little now and then, especially on a cold motor but otherwise it behaves reasonably
Thanks for the insight Robert, I will add more air lever on the next ride.
I did not put any sealant on the spigot clamp area as there was none there when I disassembled them. Should there be? The manifold to the head gaskets are new and not leaking. I did seal the exhaust pipes quite well with new gaskets and a small amount of sealant after reassembly due to the Dave Hills stand install.
 

Robert Watson

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That could well be where you are getting the leaness from. Permatex #2 (noe some loctite number but the same stuff) and just a light smear will do the trick, especially if you give it a bit of a twist as you slide it on.
 

Bobv07662

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That could sell be where you are getting the leaness from. Permatex #2 (noe some loctite number but the same stuff) and just a light smear will do the trick, especially if you give it a bit of a twist as you slide it on.
Will definitely check that out thanks!
 

bmetcalf

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According to the engine build sheet 7:1 flat tops are listed. I did look at them with a bore scope and they are indeed flat tops with 2 valve relief pockets.
I don't think the avgas is worth the money. The bikes were designed for wartime rationing low octane pool petrol.
 

Bill Thomas

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Have you an oiling problem that's wetting the plugs, I would not want to go softer / hotter than N5.
Does it smoke at all ?.
Once a plug has got wet etc, I throw them in the bin.
 

greg brillus

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As soon as the engine fires pull both choke slides to the rear of the machine.........probably banging in the exhaust due to an overly rich mixture..........N 5 C plugs should be fine and not soot up easily unless it is over rich. the snatchy clutch will be broken springs in the shoe plungers most likely, but that's another job for another day. It sounds like either your slides are wrong, cut away too small or the float levels are too high.
 

vibrac

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I stoped giving advice when I read this bit "I am running the bike on non-oxygenated, leaded 110 octane fuel" all us limeys can say is no wonder the US is called 'land of the free'
 

greg brillus

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they've banned it here in racing now........unless you have a big compression engine, there is no point in using avgas, in fact long term use will do more harm than good, very corrosive on valves, guides and so on, plus it will readily block up a muffler as well.
 

Bill Thomas

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Mine was fine for years on 12 to 1 on the track and same bike on the road at the same time,
But needed 100 octane and 26 degrees advance, "D" Distributor.
 

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Bobv07662

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Thank you all for your input, it's been very helpful. I do apologize for the good old fashioned race fuel we have available here in the States. (Not Av-gas).
The bike does not smoke at all and after last night's ride I checked the plugs and they look fine.
I had previously made up an inspection tube to look at the float level on the pre-monoblocs, they are about 1/4 inch below the jet and not leaking either. The bike does respond to adjustments to the pilot circuit and appears to be happier at idle the richer I make it, so I believe lean due to air leaks around the unsealed spigots as suggested by Robert Watson will be my next thing to look at.
Today was fine tuning the rear ride height for the Dave Hills center-stand.

Fuel I'm presently using...https://vpracingfuels.com/product/motorsport-105l-ms105l/?c=217
 

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