Throttle response at high speed - just carb icing?

[ali]

Hi all,

Only had the @ a few weeks, but I've been noticing a pretty severe lag in throttle response at higher speeds. If I roll off the power at anything over 60 and then roll in a little throttle nothing happens for some time. The higher the speed the worse the effect. Anything close to three figures and it feels like I've run out of petrol!

Could this just be carb icing, or is there something more sinister going on??

Cheers,

Ali

[africajim]

I've ran @T's in the winter up here in the north and never had issues with carb icing so can't believe it would be that. Definitely sounds like starvation though!

I'd take a guess and say either the fuel pump or the filter? Check the inline filter and replace if need be. If that's still no good you may need a new pump. They're prone for that, quick search on here and you'll find all the info you need for replacement.

One last thing though, it could be the opposite, too rich, this would cause power loss, check the plug colours, simply remove one of the easy ones and check for sooting. If it's black check the air filter then the choke cables. The chokes are prone for sticking which would cause the bike to be lethargic.

Best of luck mate.

[ali]

Thanks for the reply Jim. It does feel fuelling related, but not necessarily down on power. It's just the yawning gulf between opening the throttle and acceleration...

I only really notice it about 4-5 times a day as most of my commute is spent at 30mph.

[icenian]

Checking fuel supply sounds like good advice. Legend has it you can disconnect and bypass the fuel pump, and the bike runs fine off so long as you have over half a tank of fuel! If the pump has gone, the OE ones are expensive and likely to break again, check out the shop.

If it's carb icing then the bike should stall if you stop (or ride very slowly) after going fast for a few miles; then after a minute or two it would run fine, as the engine heat will have thawed the carbs out. My AT doesn't suffer badly, unless it's very damp and cold; bear in mind that petrol varies in volatility (?) so you could get carb icing that disappears after you fill up again. If this really is the problem, buy from Silkolene Pro-FST fuel additive.

If the choke is stuck, you should suffer from poor idling and "lumpy" power at lower revs, but it would probably feel fine over 4-5k rpm. If you suspect this, do a search for threads that mention "choke" and "idling problem", and be amazed/appalled at how may AT suffer this problem! (Worst case scenario: about 120 quid on OE parts and 2 hours guddling about under the tank and swearing a lot; best case scenario 12 quid for pattern choke cable and an hour guddling about under the tank and swearing nearly as much).

Some people have reported problems where the airbox isn't properly sealed to the carbs (this is a bit tricky to get right, and over tightening the "collars" can make them pop off). I've never experienced it, but I think if you throttle off after running at high speed, it produces a lag in the power coming back in. Search the old threads on "carbs", "airbox" etc. and you should find the info...

[YEN_POWELL]

Some people have reported problems where the airbox isn't properly sealed to the carbs (this is a bit tricky to get right, and over tightening the "collars" can make them pop off). I've never experienced it, but I think if you throttle off after running at high speed, it produces a lag in the power coming back in. Search the old threads on "carbs", "airbox" etc. and you should find the info...

I go with this one, definitely a lag when closing the throttle at 60/70ish then opening it again. Usually when following a dipstick in a car with a yoyo for a right foot.

[mladen]

I had the exact same problem, and couldn't get it fixed. I read the thread about the airbox and checked it but it was fine. When I returned from the Sahara trip, I noticed that the problem was gone and it is still working OK.
I have no idea what was wrong, but it was pretty annoying when I was lying in a curve at 120km/h, open the trotle and for some time nothing happen.

[ali]

I had the exact same problem, and couldn't get it fixed. I read the thread about the airbox and checked it but it was fine. When I returned from the Sahara trip, I noticed that the problem was gone and it is still working OK.
I have no idea what was wrong, but it was pretty annoying when I was lying in a curve at 120km/h, open the trotle and for some time nothing happen.

This is excellent news! I've been looking for a reason to go to the Sahara, and I'm sure telling the wife that I have to go because of poor high-speed throttle response will go down well!

Many thanks for all the replies. Sounds most like the airbox mounting issue to me, or a variation on (perished/split carb rubbers). Will have a good look at the weekend.

Cheers,

Ali

[Reevie]

I had the same problem check the carbs, air filter ect.

then a month ago comming back from a rally she died totally-------Fuel punp.

Fitted a facet pump and its a different bike ---responsive, starts well, and ticks over.

[ali]

Just a quick update on this.

I took the bike to PDQ in Slough last week to check out the fuel system and tweek her on the dyno. Nick replaced the main jets (118s), adjusted the needles a notch, replaced the fuel filter, new plugs, set the idle and balanced the carbs.

Bike now feels much better off the throttle, but there is still a definite lag over 80 when rolling back on. It's nowhere near as pronounced now but I doubt it'll ever go away.

Money well spent though and the bike rides as it should.

Ali

[ali]

Final update on this!

Have now tested this in conditions where the problem was woerst (high speed / cold damp air) and can confirm that the bike is definitely much better on part throttle and rolling on from no throttle.

Cheers,

Ali