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Diagnosing miss at tip in. New trick?

Fuel System: Gas (Petrol) tanks, Carburators
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redblk63
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Posts: 100
Joined: Mon Oct 15, 2007 5:16 pm
Location: pasadena, california

Diagnosing miss at tip in. New trick?

Post by redblk63 » Fri May 15, 2020 12:35 pm

I decided to post this since I have now discovered this same problem for both a 1963 CB77 and a 1971 SL350 K1. The symptom is when first letting out the clutch at a start and twisting the throttle the engine misses. I also noticed this during steady low-speed riding. This does not keep you from riding, but is not the smoothest way to start off, as more throttle is needed, with associated clutch slippage. No biggy, but this does add clutch wear and is not the smooth Honda experience.

I assumed the problem was most likely retarded timing. No, timing was fine. I next suspected a bad battery, and this was the villain for the same symptom many years ago, although it showed up more with a heavy throttle. My mantra is that 95% of carburetor problems are electrical. Well, I hit two 5%s in a row.

Next on my list was too lean. Wrong again.

How I trouble shoot:
1. Fiddling with idle mixture had little results. That is, changes were less than the noise of start to start variations.
2. A useful approach I have used in my prior career is to see if I can make things worse, so I tried shutting off the fuel tap so the carb bowl would have a lowering fuel level. I expected this to lean the mixture, making things worse. Lo and behold, a magic cure!

My solution for both bikes was to lower the float adjustment to the leanest side of the factory adjustment, and then make it a skosh (Japanese "sukoshi" = tiny bit) leaner.

The lesson: This is an easy test you can do for free. BE SURE YOU ARE IN A SAFE AREA, and turn the fuel tap off. See if rideability gets better or worse and tune floats accordingly if needed. For reference I can usually ride about 1/4 mile with the tap off.

Cheers,
Larry

DJM
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Posts: 512
Joined: Thu Oct 09, 2003 1:54 pm
Location: Chesterfield UK

Re: Diagnosing miss at tip in. New trick?

Post by DJM » Sat May 16, 2020 6:12 pm

For quick and easy testing you can add a second, and perhaps a third fibre washer under the float valve seat, this has the effect of shutting off the fuel sooner or lowering the fuel level in the float camber.

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Muddy
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Posts: 195
Joined: Wed Mar 30, 2011 11:03 pm
Location: Sunshine Coast, QLD, Australia

Re: Diagnosing miss at tip in. New trick?

Post by Muddy » Sun May 17, 2020 2:23 pm

That is a good tip!
Thanks.

Regards

Muddy

'64 C72

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