Running flat at high rev's

[Mad Max]

Just bought a 91 CB1, which needs a bit of TLC most things I have spotted and sorted, but I have quick question. It starts and runs well enough, but runs a bit flat at the top end I have a feeling it's due to the old dirty air filter, but any advice on this would be useful.

[a_morti]

Point 1 is it has only got 45hp so don't expect too much!

Other than that it may be the air filter may also be partially blocked main jets. Can't think of much else unless it's an electrical problem such as old plugs or failing coils but this mostly would go intermittent before failing.

[guntherrex]

if it's all stock air filter is your best bet, if someone modified the filter or the snorkels it might be that.

[Drewski]

If it's a fresh Jap import, I wonder if it might still have some restrictions on the speedo Or, if it's a UK bike, could it have the 33bhp restrictors??

[Mad Max]

OK an update, I've now fitted a new OEM air filter. It's running a bit freeer but it's still a bit flat in the mid range, which I thought was the top end I forgot this bike rev's so high. Took it out for a good run today did 110 miles up the coast and back.

Its a stock Jap import that been in the UK for a while, air box is standard and un-modified with the snorkel still fitted.

Going to order some new plugs, if that doesn't sort things then it will be carbs out an cleaned.

Cheers for the advice.

[guntherrex]

if you take the carbs out, also give the rubbers that connect them to the engine a good inspection. If those are cracked it will screw up the fuelling as well.

good luck