Racing engine, high idle. Where to begin.
Posted: Fri Dec 27, 2024 9:41 am
Good afternoon, long time lurker first time poster here from the UK.
I picked up a £300 XJ600 back in October and there began my sorrows.
Lately I’ve managed to get it running, sort of.
I pulled the carburettors, cleaned them, put them back to stock jetting as the PO had run pods, checked float levels and after dropping the engine; reinstalled the airbox and bolted everything up.
It started fairly easily and would respond to small blips of the throttle. That is until it revved to 2300rpm and then the engine would just rev itself out to 6/7k before I bottled it and hit the kill switch.
The balance of the carbs is spot on, they are surgically, forensically clean.
I had a look at the carb rubbers, there are no cracks.
However, the gasket that should be under them is missing. When I removed them prior to taking the engine out, I removed them, cleaned off whatever black gunk was there, slathered them in silicone gasket and put them back on. I later hit the boots with some carb cleaner and the revs picked up ever so slightly from idled, but only if I direct it towards the engine side of the boot at carb 3.
My hypothesis is that at idle there isn’t enough vacuum from the engine under 2000rpm, but once it gets above that, there is enough of a leak to “overrule” the airflow being governed by the throttle butterflies. At which point the engine is allowed to draw extra air and thus rev high.
So, before I order new intake rubbers and gaskets, is there anything else that I should be looking for? Or is this a classic example of a vacuum leak?
Thanks everyone.
H
I picked up a £300 XJ600 back in October and there began my sorrows.
Lately I’ve managed to get it running, sort of.
I pulled the carburettors, cleaned them, put them back to stock jetting as the PO had run pods, checked float levels and after dropping the engine; reinstalled the airbox and bolted everything up.
It started fairly easily and would respond to small blips of the throttle. That is until it revved to 2300rpm and then the engine would just rev itself out to 6/7k before I bottled it and hit the kill switch.
The balance of the carbs is spot on, they are surgically, forensically clean.
I had a look at the carb rubbers, there are no cracks.
However, the gasket that should be under them is missing. When I removed them prior to taking the engine out, I removed them, cleaned off whatever black gunk was there, slathered them in silicone gasket and put them back on. I later hit the boots with some carb cleaner and the revs picked up ever so slightly from idled, but only if I direct it towards the engine side of the boot at carb 3.
My hypothesis is that at idle there isn’t enough vacuum from the engine under 2000rpm, but once it gets above that, there is enough of a leak to “overrule” the airflow being governed by the throttle butterflies. At which point the engine is allowed to draw extra air and thus rev high.
So, before I order new intake rubbers and gaskets, is there anything else that I should be looking for? Or is this a classic example of a vacuum leak?
Thanks everyone.
H