My XBR500 will not go over 60 mph

[guest1778]

Has anyone out there had experience of a seamingly restricted 1988 XBR500, if so HOW was it restricted and what did you have to do to de-restrict it.  I have replaced the CDI with a used one which has had no effect at all.  Going to re-check the entire fuel and air intake system tomorrow.

Any Ideas

Trevor

[SteveC#222]

Is the carb slide lifting fully? if not check the diaphragm isn't perforated. Is the ignition advancing fully?  Air filter/ intake not blocked or restricted?

[Furry John]

Hi tttwh,

I am pretty sure Jethro had to fit Honda restricting kits to one or two when they were to be ridden by BHP restricted learners, I can't remember what was done but it did involve a Honda supplied "Kit". I am sure he will be along soon and fill in the details.

Furry

[SteveC#222]

Found this as part of a reply on another site

"There is a alternate throttle linkage assembly for the XBR500 that replaces the standard UK one. This prevents the throttle from opening fully and restricts it to 33hp. This is for the 25kw german model, i presume youll be able to get it from a honda dealer, i'll keep an eye out for the part number."

Don't know if this applies to your bike but a good place to start looking.

[Propellor]

I'm no expert on xbr's but surely 33hp will get you wa a a ay past 60mph. A 125 with 12hp will do 60mph. (Ok, skinny tyres, but even so).

As a guess, wouldn't 33 hp get you to between 90mph and the ton?

Just a thought.

[blew]

I bought an xbr that refused to go over 70 and burned oil like a two stroke.The piston was damaged.

[guest564]

I'd check that the air filter isn't clogged or the carb diaphragm damaged.

[guest1778]

Hi Guys, many thanks for your feedback.

With your help I have solved the problem

With no throttle responce, not even a change of engine note; Like Steve I suspected the main diaphragm. I removed the carb and drained out the petrol. I then connected a vacuum cleaner to the head side of the carb, switched on and opened the throttle, the vacuum slide did not move. But it was very free to move by finger.

I totally striped the carb and found the air valve cover had bowed by 0.015 inch. I lapped this out with a piece of wet and dry on a flat surface and then carfully rebuilt the carb.  I then did the vacuum cleaner trick again and the vacuum slide rose to it top position. Bingo!

I test drove the bike again today and the difference is quite substantial.  Even without trying it was up to and over 80mph. On the way home i diverted to the M27 and waited for a large gap in the traffic. The bike almost touched the max speed of the official bike test before I had to close down.  I don't normally go that fast!

So, the bike was not restricted in the first place.
How many other XBR500 are there out there with this simple but debilitating fault.

Thanks to you all

Trevor

[500 T C]

excuse my ignorance, but what is air valve cover? Is that the chrome cover at the top of the carb?

[guest1778]

Hi, 500 TC.

The air cut off valve is on the throttle link side of the carb. It is a small round cover with two projections cast into it to take two securing screws to the carb body, it also has a small peice of pipe at its centre which takes an external rubber tube to the vacuum source; usually a drilling in the top of the carb where it attaches to the cylinder head.  Some times the vacuum pipe is case internally in the barb body, in which case it will not have the small pipe.

This plate deforms and bends due to the presure of the screws.

Regards  Trevor

[500 T C]

Thanks for that, I had a carb stripped down at the weekend, and guessed that was it. Did the same thing, and flattened it on some wet  and dry.

[Propellor]

What is the function of the air cut off valve? Why is it needed?

Many thanks.