Flat spot off idle

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There have been dyno tests that showed the K&N reduced airflow over the standard Commando airbox. So you'd have to do some plug tests to really determine what the jetting should be if you switch to KN.
 
That is interesting, didn't know, but would it really vary significantly? I've used both stock filters and k & N and not changed anything in the jetting.
 
Well I'm dealing with what I was dealt when I bought the bike, so I don't know the difference. When you say you didn't have to change the jetting, what did you start with? 220 or 260?
 
I'm trying to find the specific test but I BELIEVE the test was either by Dunstall or Hemmings re the single KN filters vs the oem airbox. It could simply be a function of airbox volume which might better suit the Commando motor though I'm not sure if anyone back then paid any attention to airbox tuning so maybe it was just a factory "accident." In the late 60s Drag racers were learning that an airbox increased power over carbs that had no airbox but I'm not sure it was standard stuff in motorcycles yet.
 
Needle clip in top notch??, wouldn't putting it in the middle and richening up the lower end be a good place to start
 
cyclegeezer said:
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)
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74 850 with peashooters should have a 260 main jet and 3-1/2 slide. Start with the needle in the middle groove and report back.
 
Thanks Dave, that's where I'm headed. Today I'll check with the last of the die hard British guys in town and see if he has a pair of 260s on hand, if not, I'll call Old Britts and get a couple ordered. I've got #3 slides in now, but have a pair of 3.5s that aren't worn. Needle coming up to middle notch too.
 
Do one change at a time to isolate cause and effect.

I'm betting you see great improvement when you raise those needles...
 
Verify BOTH needles are the correct 4 ring needle for cutaway spray tube.
2 rings for straight spray tube.
 
MichaelB said:
Verify BOTH needles are the correct 4 ring needle for cutaway spray tube.
2 rings for straight spray tube.
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Both needles are new, as are needle jets, from Old Britts, they are the 4 ring needles.
 
BrianK said:
Do one change at a time to isolate cause and effect.

I'm betting you see great improvement when you raise those needles...
Click to expand...

I'll go raise the needles and try it before I pop for new mains.
 
When Amals are worn out the slides can have too much play even if they are new. When the throttle is lifted it tilts back first. When you slowly lift the throttle it will kill the engine. I have gotten used to this on my 71 Bonneville. You have to lift the throttle suddenly through this spot and then it is not a problem. You can set your idle for a very steady 1100 RPMs. When you want it to idle slower at a light you can gently open the throttle a bit to slow the engine down. When you are ready to go you have to close the throttle first and then lift it suddenly.

If you don't like this you can either get new amals or put a single Mikuni as I did on my 850 Commando. There are other alternatives to the single Mikuni that are better for top end power, but I doubt that for simpilicity and tractibility and ease of starting you can beat the Mikuni VM.


Nigel
 
Ok, I raised the needles one notch and my acceleration is much improved. Still burbles at steady throttle in the idle to 1/4 throttle range. Loses the will to go faster at wide open though, guess I will have to try larger mains after all.
 
You really should be on the top notch! 260 Main jet, 3.5 slide. Then it will go.
 
Well my trip to the last die hard British shop in town was in vain, no 260s on hand. He tried to tell me 220s were too big, I should be on 190s, then looked in his reference book and changed his mind. I'll call Old Britts tomorrow and get a couple of new mains ordered. You're saying the needle should be up as far as possible? Maybe I misread the intent of the service manual when it said needle on top notch, I took that to mean needle down as far as possible. Before the new slides, with the needle up as far as possible, I couldn't get a slow idle. I realize a combo of worn slides with needle up might cause that.
 
The needle should be "down" as far as possible, with the clip in the top notch.
 
Flo said:
The needle should be "down" as far as possible, with the clip in the top notch.
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Oldgeezer already sadi that he experienced a "great improvement" from raising the needles, why would you have him lower the needles now?

Stock jetting for the 1975 Mk III Commando is: 220 mains, 106 needle jets, with the needles in mid position.

I have a Web Cams 12a, 9.8:1 CR a flowed head with port matched mainfolds, new Amal 932s with 3 1/2 cutaway and chrome plated brass slides. I started with 260 mains and 'am now down to 220s.

I did have an initial problem with hesitation off idle and cured that with Burlen Fuel Systems "Stay-up floats", the stock plastic floats just weren't the right height.

Do revisit your floats.

RS
 
The classic test for main jets is to run WFO for a few seconds, then close the throttle slightly to about the 7/8 open position. If it runs better and faster at 7/8 open throttle than when throttle is fully open, your mains are too small.

Select a road for this test very carefully!

Good luck.
 
BrianK said:
The classic test for main jets is to run WFO for a few seconds, then close the throttle slightly to about the 7/8 open position. If it runs better and faster at 7/8 open throttle than when throttle is fully open, your mains are too small.

Select a road for this test very carefully!

Good luck.
Click to expand...


I'll try that tomorrow morning, I have the road I need for it, complete with slight incline.
 
New needle need to match the jets. Two types out there, over my head though.
 
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