Flat spot off idle

cyclegeezer · Aug 22, 2010

I'm about at the end of the easy fixes for this problem I'm having with a 74 850 Roadster. Flat spot off idle, new needle and needle jets, new #3 slides, 220 mains, K&N filter, peashooter exhaust (probably EMGO). I can't seem to shake the flat spot off idle, and it seems to be concentrated in the right cylinder. I've double checked the idle circuits and they are clear. The timing is fine. Float levels set to the usual, I forget what measurement that is. When the flat spot occurs while riding, flooding the carb with the tickler helps. Are my mains sufficient for the K&N and fairly open exhaust? I also switched the coils and the problem remained on the right side. Boyer Bransden ignition. Any ideas? Thanks.

BrianK · Aug 22, 2010

I'm not an Amal guy (I run Keihins), so don't know the internals of those carbs, but I'd try getting a little extra gas into your low-speed circuit, first by richening the idle circuit (bigger idle jets or slow fuel screw out or slow air jet in) or if that doesn't work, possibly raising the needles one notch.

Best of luck.

britbike220 · Aug 22, 2010

I thought a flat spot off idle or throttle on is a lean condition, which could be several things. slide cutaway, possible needle jet, main jet if up on the throttle range or float level which you have eliminated. If you are running 932's shouldn't your main be a 260?, maybe different for 74, but my 73 is running a 260 main, 106 needle jet, needle on second slot and I believe a #3.5 slide. I never had good luck with dual carbs and often had this condition along with many other running issues. I just run a single 932 amal with stock set up for a 73 850 with stock air filter and have pretty decent luck. An occasional maintenance hiccup, but generally great running bike. If your ever interested in trying a single amal set up I have a MAP manifold I just bought and do not use.

Hobot · Aug 22, 2010

I"m learning stuff on this annoyance too. As assumed above, sure sounds like initial lean condition until air velocity can draw up enough gasoline. Float level or base line rich vs lean setting can be judged by how many turns out of air screw for best idle. 1.5 out is Amal ideal to aim for. Clear flow paths, spark on time, though a tad advanced helped off idle response, up to a point, then can act like yours.

Sprays tubes matter most at just off idle. Only 850's and 2 strokes use a cut down spray tube d/t lack of piston air draw momentum off idle. Peek at them to see what's in there. Can also just trim to lower length for easier-faster spill out of gas into slow air stream. I was told cutting a slope on 750'a spray tube had an accelerator pump like effect on initial power up. New gasoline is oxygenated which means leaner to begin with using traditional jetting.

I've had examples enough now to know that Cdo's should be pretty eager to take right off with thoughtful fast initial throttle opening. Hope you joy is close at hand.

jseng1 · Aug 22, 2010

It has a flat spot because its an Amal and that's typical. If you can afford it - get a better carb.

The Keihins are cheap and work 100% better.

get them at www.JSmotorsport.com

ewgoforth · Aug 22, 2010

I've seen a flat spot off idle when my carburetor bodies were worn.

A good test for worn carburetor bodies is with the bike warmed up and idling at a nice 1000 rpm or so, turn the twistgrip just enough to take the clack out of the throttle cables. If everything is good, the idle speed won't change at all. If the idle changes, or the engine actually dies, the slides are a loose fit in your carburetor bodies and need to be sleeved or you need new carburetor bodies.

-Eric

cyclegeezer · Aug 22, 2010

Good ideas all. Some forgotten details I should have put in the first post. New slides fit snugly, no rattle like old ones which were visibly worn. Needle clip is in the top groove of the needle. Spray tube is the cutaway type specified for the 850. I wondered about the mains, have seen both 220 and 260 or is it 280 in spec books for that year.

Rich_j · Aug 22, 2010

I started writing a twin carb setup 101 but it was getting really long and I reckoned everyone would lose the will to live by half way.

The shortened version.

Yes a change from standard to K&N could affect the mixture and mean a change in main jets.

I suggest an early morning (less traffic) dose of full throttle plug chopping followed by a brisk session of carb balancing.

My carb balancing essay is stored for future use - be afraid

Flo · Aug 22, 2010

First of all, make sure your pilot jet holes are clear.
Then set exactly to book.
260 Main jet, 106 needle jet, top pos. on needle, 3-1/2 slide cutaway, make sure you have the right needle, & the cutaway spray tube. These settings run properly, when set up they run very very well. My float heights are different & it doesn't make any difference.
They should pull cleanly from 500 revs right up past the red line.
Of coarse, you can try some foreign carbs, apparently they are ----- oh I think I will leave that one alone.

cyclegeezer · Aug 22, 2010

Flo said:
First of all, make sure your pilot jet holes are clear.
Then set exactly to book.
260 Main jet, 106 needle jet, top pos. on needle, 3-1/2 slide cutaway, make sure you have the right needle, & the cutaway spray tube. These settings run properly, when set up they run very very well. My float heights are different & it doesn't make any difference.
They should pull cleanly from 500 revs right up past the red line.
Of coarse, you can try some foreign carbs, apparently they are ----- oh I think I will leave that one alone.

I'll have to get a couple of 260 main jets and try all this again. You say 3.5 slides? I thought they were 3.0, well at least I have a pair of 3.5s around here if needed. Pilot jet holes are definitely clear, I double checked that Friday night. Regarding balancing, I'll put the mercury sticks back on again and double check that too.

MexicoMike · Aug 22, 2010

I agree totally with Flo. Be SURE that the carbs are stock and they have the correct parts in both carbs - mine had different jets and needles in them!. Your mains are much to small; I run 240s at 7400 ft.

Now...re the flat spot; if you are talking about being at idle or just barely above and suddenly wicking it to WOT, there will be a flat spot because Amals are neither COnstant Velocity carbs nor do they have accelerator pumps. So it is impossible for them to react to that level of throttle change from idle/off idle without a hesitation. No carburetor in the world can do that if it is not a CV carb OR have an accelerator pump - when wicking to WOT, the air accelerates a lot faster than the fuel in the bowl can catch up. OTOH, if you roll the throttle on briskly but not immediately to WOT from idle, the Amals will pull like a train when properly set up (and idle smoothly at 5-600 RPM.

slimslowslider · Aug 22, 2010

Flat spot cured by operating the tickler?
Had the same issue (on an Amal Mk2 though, operated the choke to get away).
Just changing the slide from 3.0 to 3.5 cured it.

Flo · Aug 22, 2010

It seems you have set your carb's for a 750 Combat. I hope you have the correct needle, 928/104.
I think the needle has 4 rings above the clip grooves to show which one it is. Hopefully somebody here will either agree or put me right on this one.

Rich_j · Aug 22, 2010

cyclegeezer said:
Flo said:

First of all, make sure your pilot jet holes are clear.
Then set exactly to book.
260 Main jet, 106 needle jet, top pos. on needle, 3-1/2 slide cutaway, make sure you have the right needle, & the cutaway spray tube. These settings run properly, when set up they run very very well. My float heights are different & it doesn't make any difference.
They should pull cleanly from 500 revs right up past the red line.
Of coarse, you can try some foreign carbs, apparently they are ----- oh I think I will leave that one alone.

Click to expand...

I'll have to get a couple of 260 main jets and try all this again. You say 3.5 slides? I thought they were 3.0, well at least I have a pair of 3.5s around here if needed. Pilot jet holes are definitely clear, I double checked that Friday night. Regarding balancing, I'll put the mercury sticks back on again and double check that too.

I wouldn't bother with gauges for synchronising slide carbs, fine for CV's but for slides it's important to get the carbs mechanically operating identically especially to get the idle and pickup operating right.
A lot of folks have given up on twin carb's because they couldn't get them set up right (they're usually the ones giving advice on how to do it), it's worth spending plenty of time on this part and have patience.
Been having a look at Bushman's guide and it's pretty much the same as I do and saves a lot of typing so have a look at
http://www.jba.bc.ca/Bushmans Carb Tuning.html#6SYNCH

Flo · Aug 22, 2010

ludwig said:
slimslowslider said:

Flat spot cured by operating the tickler?
Had the same issue (on an Amal Mk2 though, operated the choke to get away).
Just changing the slide from 3.0 to 3.5 cured it.

Click to expand...

That's odd .. 3.5 is leaner than 3 ..
Size of main jet shouldn't matter at small throttle openings .
Is the transition from carburettor to manifold smooth ( no step ) ?
You could try to raise the float level a bit ?

If the needle is too high (or the wrong one) coupled with too rich a slide, then it could richen it up too much to run properly.

cyclegeezer · Aug 22, 2010

Rich_j said:
I wouldn't bother with gauges for synchronising slide carbs, fine for CV's but for slides it's important to get the carbs mechanically operating identically especially to get the idle and pickup operating right.
A lot of folks have given up on twin carb's because they couldn't get them set up right (they're usually the ones giving advice on how to do it), it's worth spending plenty of time on this part and have patience.
Been having a look at Bushman's guide and it's pretty much the same as I do and saves a lot of typing so have a look at
http://www.jba.bc.ca/Bushmans Carb Tuning.html#6SYNCH

I've got them pulling simultaneously and opening full throttle the same. Thanks for the pointer for the Bushman's guide, I looked there the other day and then lost the link. I've had good luck with the 3 on my Trident, and was thinking that these 2 would be easy. I'll probably try moving to larger main jets, it's the only thing I haven't changed in all my testing.

britbike220 · Aug 22, 2010

My two tridents were a walk in the park compared to dual carbs on a norton. You can't miss on a trident since set up is mainly done from the gantry. If you insist on using dual amals at least have them sleeved otherwie you are going to continually have issues.

Rich_j · Aug 22, 2010

Unfortunately if you've changed from the stock filter and don't have any data from the filter vendor are no shortcuts, you have to be systematic and work through the process start to finish.

Main jet is step 1.

Flo · Aug 22, 2010

I am not sure the K & N will cause ny trouble. I have used the standard jetting with Bell mouths, without Bell mouths & with Standard filter & it has always been alright. At the moment I am running standard filter with the top & bottom of it cutaway.
Had a few bikes in the past that have really worn carb's, so much so, that it caused them to have their own tickovers due to air leaks. But they have run alright.

britbike220 · Aug 22, 2010

Wouldn't the K&N give more air flow and cause a bit leaner condition? I agree w/ Rich_j somewhat except I'd certainly make sure you are not dealing with worn out slides and bodies first, then I'd go to jetting.

Flat spot off idle

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