400/4 struggles to pick up when warm

[adriangsmith]

Hi All,
Nearly finished finding my way round some of the problems with my 400/4 build.
Just had carbs professionally set up and balanced and the timing set correctly.
However i still have the issue that once the bike has warmed up (10 minutes of good riding) the bike will struggle to pick up from low revs.
open the throttle too fast and it will bog down and die.
needs to be slowly feathered up to around 3000 rpm before it will run smoothly again.
It's fine once going but around town or when stopping it makes it a right pain to ride.

Any ideas where i start looking for the cause of the problem?
something electrical would be my first guess but don't know where to start looking and checking.

Cheers
Adrian

[Rob62]

Whenever i read “professionally set up” i suspect it was done by some hairy arsed cowboy... 

My advice would be to start with the easy stuff... new plugs first, then look at fuel filters, breathers, fuel flow, air filter, go through all the standard settings, valve clearances, ignition timing, points, condensers, air screws, carb settings etc... test the compression ... Somebody may be able to point to a particular fault they have experienced in the past but if that involves some kind of major strip down or big expense i would still go through the standard settings first... Nowt worst than spending hundreds of pounds and lots of hours only to find you’re no further forward and then later on discover you have a partially blocked fuel pipe or something similar...

[hairygit]

Also check that you do NOT have resisted plug caps AND resisted plugs. One or the other is fine, but both together will cause problem like this. Are you running standard airbox?

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[paul G]

I would check the plug colour first as it sounds like it is running weak.
Could try adjusting the air screw a 1/4 turn at a time and having a run.
More than one full turn and the carb settings will need looking at.

[Nurse Julie]

+1 with Mr Hairy and Paul. Check plug type and sounds like its running lean to me.

[adriangsmith]

cheers all, will get on that today. plugs and caps sound like a possibility.
Been through most of the simple stuff already.
The guy who set up the carbs is an ex Honda mechanic and SOHC 4 and CBX1000 specialist so i trust him explicitly as its now running a lot better than it was before.
What are everyone's recommendations for plugs?

[Nurse Julie]

D7EA 

[Northy]

Watch out Adrian I'm  on here now ! :-), Is still not running better , best check all those HT leads and caps.   

[ttr400]

D8EA. Std.

As a rule, Run the coldest plugs possible, without fouling.

For fast track full throttle running i use D9EA or even D10EA's

Kevin

[K2-K6]

If it's running lean it should respond to a little choke when not accelerating,  which would confirm a lean diagnosis.

As Kevin has said,  I'd be careful with running a lower number NGK plug if you suspect it's lean at the moment to avoid overheating the plug.

Did the engine come to you built or have you worked inside it at all?

[Nurse Julie]

I use D7EA as mine is running spot on, not too lean and not too rich, just perfect. 

[adriangsmith]

Ok. So found the problem was down to the needle position in the carbs. I had fitted new cruizin image needle and jet kits during the rebuild. Despite fitting them on the given slot they were too high. Dropped them down a notch and it has improved no end. Just wont idle but guess that'll improve if i balance the carbs. If not I'll try dropping the another notch.

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[ogri211]

A thing to be careful off is if your bike is not idling when warm recheck your valve clearances if they are too tight the valves wont close properly when heated up and will burn the seats, a good indicator of this problem is the bike will idle when warming up but will not idle at full temperature.

[adriangsmith]

Thanks Ogri. Will recheck these today

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