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1969 Bonneville 1986 Honda ns400r
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Discussion Starter · #1 ·
I have the premium Amals on my 69 Bonneville with 90 main jets; over all they run well and start easily. They do however run rich in the mid range. The throttle slide is a #3 and the needle is at the bottom. I happened to have one 3 1/2 slide lying around and based on a plug chop at mid-throttle it corrected the mixture. Then I noticed the side with the 3 1/2 ran hotter. After a 10 minute run I measured the head temperature with an infrared device and found the leaner side to be 50 deg F hotter (350 versus 300). This is not definitive as I did not measure both sides before the change but it does seem a large difference for a small change in the size of the cut away. For me the question is whether to order another 3 1/2 or run richer and cooler. Any thoughts?

Second niggle; The brake plate retaining block on the right fork leg rattles (sounds like a bucket of bolts on a rough road). The gap is .008". I tried shiming but could not find a way of keeping the shim in place. I am hoping that there is some way other than welding and re-machining.
 

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Bruce,
The shim: What material have you made yours from? I put the shim down the front gap so that it is held in place by braking pressure. But you also need to fold the shim strip down the inside edge aginst the brake plate and also across the top and bottom edges on the peg (like an 'S' shape) to stop it falling out. The gap on my Daytona is a bit bigger than 8 thou so perhaps easier to make the shim with very thin aluminium sheet.

By the way a 90 main jet sounds much too small for a Bonnie - are you sure its not a 190?. I can't remember exactly what size any of mine had back in the day but for comparison my current Daytona 500 has a 160 main jet in each carb.
 

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Hi Bruce1947, What is mid range? Mark your grip. Then use this tuning guide. It's the only way. For sure too lean runs hotter, way hotter. Print pages for road testing. Take some time & get the feel for this. Mark grip very accurately. Knowing slide position is key.
Amal+MK2+Carb+Manual.pdf (squarespace.com)

Make sure the float level is correct. Static setting is just above & level with gasket surface. So way higher that original carb would be. Liquid level in bowl is the best method. Do search on setting float level under my user name for photos.

Regarding front brake, the S bend BrianG describes is what many use. It works good. I've personally fitted one on '69 Bonnie. Trial fit pieces of tin until it's thick as possible to still get the tab in slot. You can use a feeler gauge blade. Cut to fit & bend. It's tough metal. Hard to work with but makes fitting simple as you trial fit until you get the best thickness. Cures problem. Also reduces loosening of the large nut that hold brake plate to hub.
Don
 

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Worn needle jets can make you try to compensate with a weaker slide and needle clip position. Then you’re in a confusing quandary!


If your needle jets are already new size 106, then in your place I’d try 3.5 slides and middle needle clip position (I think you’re saying the needle is dropped to its weakest position now).
 

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measured the head temperature with an infrared device and found the leaner side to be 50 deg F hotter (350 versus 300). This is not definitive as I did not measure both sides before the change but it does seem a large difference for a small change in the size of the cut away. For me the question is whether to order another 3 1/2 or run richer and cooler. Any thoughts?
Hi Bruce,
There is a point where all of the fuel and all of the oxygen is burnt, this is called the stoichiometric point. Too fuel rich and you have left over fuel, too weak and you have left over oxygen.
If you have left over fuel, this tends to cool the burn and slow the flame front as there is less oxygen molecules to combine the fuel with, the engine produces more carbon monoxide (1 oxygen atom) and less carbon dioxide (2 oxygen atoms).
If you have a fuel lean mixture you have the flame temperature goes up rapidly and the xtra oxygen combines with nitrogen to form nitrous oxide. The temperature rise has an exponential component so a slight difference in mixture weakness leads to a rapid rise in temperature, this is very dangerous for the engine.

Different fuels have a different stoichiometric point, the petrol that our carbs were tuned for ther ratio of air to fuel is 14.7:1, modern E10 Ethanol fuel is 14:1, so we might need to retune the carb to cope with modern fuel.

Although 14.7:1 is perfect for economy but it is a hot burn, peak power is richer than this around 12:1 for our air cooled engines (13:1 for warer cooled).

It is safer to go richer, lean burn destroys engines, with detonation, melting ofthe pistons and burning of the valves.

The mixture has to be set for all 4 conditions that control the mixture , full throttle (main jet), Tickover ( pilot jet/screw,) small throttle openings (slide cutaway), medium to large throttle openings (needle/needle jet).
All of these conditions overlap, so there is a lot of trial and error.

Tune for slightly rich mixture, never lean mixture.

Always start with new needle jets and correct float heights, the needle jets wear out rapidly and it is impossible to tell by eye if they are worn. The floats disintegrate in ethanol ethanol proof ones are available.

Regards
Peg.
 
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1969 Bonneville 1986 Honda ns400r
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Discussion Starter · #8 ·
Thank you all,

Great description Peg, I did not know that mixture and temperature were exponentially related. Are you implying that a 350 degree head temperature is verging on the dangerous level? For the first time ever the motor "ran on"l for a second after shut down which does imply carbon deposits now getting hot enough to ignite fuel. I should not have carbon deposits at all of course but back to the # 3 slide in the meantime . I will have to pull the head this winter.

This issue came about because my plugs read too rich. I should have explained that I use the BR8EIX Irridium plugs, not the usual choice but I find they allow one-kick starting and seem to last well. Except for a "heavy" feel on acceleration (compared to the original Amals) the bike runs well. I take the point that I should know at what throttle position this takes place but it is not like a definitive problem such as bogging down, hesitating or backfiring. I will go back and retune the pilot circuit. It is good to be reminded that all circuits overlap. I hope the needles and jet are still good as I only have about 1000 miles on these new Amals but will check. I cannot remember if I changed the float level from the factory setting but if I did so I would probably have used the lower original setting.(I did have trouble getting the machining swarf out of the various orifices). I can see that a low float setting would run lean but would a high setting run rich given that mixture is determined by the other circuits? (I have a car with Weber carbs that has multiple openings above the main jet in the float chamber - unlike our Amals).

With regard to the front brake block shim; not much luck, I tried using a feeler gauge blade but it broke rather than fold. I now know what to do so I will find some aluminum or tin and make a shim that will stay in place. It is a fiddly job!

Again thanks for the advice.
 

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Hi Bruce,

Yes, adding extra oxygen (weaker mixture) is like pumping the bellows on a forge, blowing on a fire, or turning up the oxygen on a welding torch to get a blue flame.-it get’s hotter rapidly.

The engine running on with the ignition off implies Pre Ignition.
An engine can live with a little detonation in the combustion chamber, it’s not nice but can be tolerated by the engine for a while.

But Pre-Ignition is very dangerous beast, it will blow a hole in your piston within seconds- be careful.

regards
Peg.
 
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