flat spot

[mickwinf]

story so far, 500 four 1974, rebuilt and on the road. I have an issue with a flat spot as I pull away from a standstill or low speed, it hesitates badly unless I give it lots of revs, sometimes even cuts out. carbs have been cleaned and new gaskets and o rings, all jets clear and correct numbers. Timing has been checked, valves are correct gap, new clean petrol. otherwise it runs well, over 3000 revs it goes very well, goes better when colder, worse when hot. I have synchronised the carbs and tried the air screws at various settings from half a turn to two turns out. standard aircleaner with new filter. no air leaks.So where do I go next, should I try changing the needle settings?

[hairygit]

Float levels would be my first thought to check. Does it start from cold without choke? Do you get sooty looking plugs? Any sign of dribbling from the overflow tubes? Any of those would make me suspicious of float levels or float valve leaksge, especially as it gets worse with the engine warm/hot

[mickwinf]

float levels have been set using special tool, no leaks, plugs are slightly sooty so I suspect its a bit rich, it does tick over ok but when the throttle is opened it hesitates or cuts out.

[Bitsa (Ralph Wright - RIP)]

Defiantly rich
Cheers
Bitsa

[mickwinf]

I agree but adjustment of air screws does not improve it, should I be looking at needle position?

[Bitsa (Ralph Wright - RIP)]

Mick
Its a start mate back in the days bozos thought more lift more fuel goes faster they did not realise it screwed most up.Honda put them in the middle for a reason
Best of luck and also the last 400 I did all the o rings were knackered leaking more fuel again.
Cheers
Bitsa

[Ashdowner]

What colour are the plugs?

[mickwinf]

plugs are slightly sooty but not real bad, haven't had time to do anything further yet.

[deltarider]

Always start with the simplest and check the ignition (all items) and doublecheck before you move to the carbs. What plugs do you use and what's the gap?
https://www.youtube.com/watch?v=t3cuvGVwnjI
Left condensor (1+4) is bad, right condensor (2+3) is good. Condensors seem to go bad when warm.

Floatlevels have to be checked. Where's your CB500 from? Yanks had the needles in 4th (combined with the airscrew 2 turns out, at least that's what an American Honda booklist says). In Europe we had the needles in 3rd and airscrews 1 turn out. What main jets are in? Is there a cover over the airfiltercase? A forgotten cloth?

[mickwinf]

its a UK bike, main jets 100 as per Honda, pilot jets 40 and have been cleaned, float levels checked and are correct, all rubber o rings replaced, all air filter parts are original and good, no air leaks, I have tried without filter but no better. ig timing spot on and all four cyls firing fine on tickover and higher revs, so the only thing left to try is the needle position, I don't think I checked them so that's the next step, thanks for the imput guys.

[deltarider]

Just curious. What's the stamped on number on your carbs?

[mickwinf]

carbs are 627b stamped on all 4.

[mickwinf]

still having problems, finally had time to check the needles, they were in the second groove from bottom so I put them in the middle groove, with a slight improvement, but now having issues with ignition timing and points gap.I have new points plate complete but pattern part and quality seems poor, when I set timing properly it runs badly, then when I fiddle with gap it improves somewhat but wont tick over now. spent hours yesterday, now fed up, thinking of selling it!! Should I go to electronic ignition?

[totty]

I've read loads of posts about pattern points and plates, none of them positive, but one post (on the US site I think) gave tips on how to shim the plate to make it usable. When I had similar issues to yours I fitted a Boyer kit, as it included coils and does away with the mechanical advance it was the cheapest way of replacing the entire system. All genuine parts is the other alternative.

[Johnwebley]

still having problems, finally had time to check the needles, they were in the second groove from bottom so I put them in the middle groove, with a slight improvement, but now having issues with ignition timing and points gap.I have new points plate complete but pattern part and quality seems poor, when I set timing properly it runs badly, then when I fiddle with gap it improves somewhat but wont tick over now. spent hours yesterday, now fed up, thinking of selling it!! Should I go to electronic ignition?

 I can advise you on Boyer ignition,used it since late 1970's,been wonderful,nice fat spark,I run a 33thou plug gap,and have a nice
tickover and revs to 9k,

 well worth investing in,