'Cough' at speed

[RichardC]

Hi - I'm a new bod here so please bear with me...

My '88 Canadian Hawk has a strange problem I can't figure out.

If I open her up to 85-90 mph when overtaking (when the engine's good and warm) when I back off, it seems I can't balance the throttle. After I've backed off, it feels like one cylinder fires erratically. If I give it some more throttle (after backing off even more, perhaps), the problem goes away, but returns again as soon as I've backed off again.

The problem seems to have got worse after I changed the plugs (which are the standard type (NGK DPR8EA-9) but it happened before too.

Could this have anything to do with the air temperature? It didn't seem to happen during the winter.

Also, is there anything I should know about the air/fuel mixture, seeing as we have different fuel here in the UK (I run on 95 octane unleaded which is standard here). Could this have anything to do with it?

Any help will be greatly appreciated.

[Twinsport]

OK Key word here. Overtaking. Now this may sound strange at first. Are you using the stock airbox? If so under the tank on the right hand side is the air intake. When overtaking trucks in particular you can get a low pressure zone what in my epxerience would suck the air out of the airbox causing the stumble. The problem persisted for the first year i had the bike 1991 till i took the airbox top off. Never done it since.

cheers
brian
DOCC#612

[RichardC]

Thanks for the answer. I was wondering about just that: if it's an air starvation problem. I'm using the standard airbox.

It doesn't just happen when overtaking, but all the time - so you'd suggest removing the top of the airbox? Or maybe cutting some holes in it?

[LeftRightLeftThrottle]

I'd recommend finding a spare airbox top to cut up and hanging on to a whole one just in case you don't like the change. That being said, there are a couple of pics floating around that show you how to cut the top of your airbox off so you can get maximum airflow but the box top will still hold the filter element securely.

[Roberto]

there are a couple of pics floating around that show you how to cut the top of your airbox

Any links? Searching for "cut" & "airbox" doesn't yield results.

[HypnoToad]

not really related but did find this..in google.

http://www.cbr1000rr.org/HawkParts.htm

[RichardC]

I think I'll take the advice given elsewhere and replace the airbox with a K&N - and keep the standard one, just in case.

Has anyone tried these HifloFiltro filters? They look something like the bottom half of a standard one.

http://www.hiflofiltro.com/17.0.html?&L=0&sel_uid=286

[LeftRightLeftThrottle]

there are a couple of pics floating around that show you how to cut the top of your airbox

Any links? Searching for "cut" & "airbox" doesn't yield results.

try trolling through the member photos on hawkgt.com I think I saw it there.

[NT696]

I have a 'ventilated' stock airbox top I can take pictures of tonight if you are still interested. Holds down the filter and lets lots of air get to the rear. The 'snorkel' gets tossed in the ever bigger bin of removed and useless parts. I've since switched to pod filters, never liked the poor seal the K&N filter made with the airboox lower half.

[HypnoToad]

i am

I am interested,
I have the K and N air filter with the standard air box
The seal does not look so good but I do like the added sound compared to the stock air filter. also seems more powerfully.

[Hordpower]

The HiFlo is a nice quality, cheaper alternative to the OEM filter. Thumbs up from me. I haven't seen a K&N yet that seals worth a hoot to the airbox... unfortunately.

[RichardC]

Thanks for the info.

I've narrowed the problem down to a fuel starvation problem. It only happens when the engine's under high load for a protracted period, like on a motorway or fast road. It feels like one of the carbs is emptying (and the engine is only firing on one cylinder) or not delivering enough fuel, somehow, and it's not dependent on airflow (it happened anyway, even with the top of the airbox removed).

Any advice on this - as to causes? Is a carb rebuild that hard?

[lysholmvw]

same "cough"

I was searching for the same answer to the stumble at cruise. I have checked the float diaphrams and made sure they seal properly. My airbox have been altered for more flow. This only happens at a cruise. Wide open throttle works well and slow accelleration works well. Any more ideas out there?
Thanks

[NT696]

Another thought:
If it is a fuel starvation problem, it might be one of your fuel filters plugged up. As Hord mentioned in another thred, you actually have 3 fuel filters on your Hawk. The one inline between the tank and the pump is the obvious one, each carburetor bowl has one inside, (small and easy to plug up) and the petcock dip tube has one over it as well. These other ones may be the culprit of a fuel problem.

[PaullyBoy]

Also on the fuel starving problem. The petcocks themselves have a rubber seal in them that deteriorate. This can cause a slow trickel of fuel to reach the carbs. This will allow the bike to cruise fine at part throttle, but it will stumble at full, then go back to normal again once you've gone back to part throttle.