Any ideas wot this could be? SU carb starts racing high as engine gets warm. It's a Rover 3.5 V8 engine with twin SU's (only 1 carb is doing it). The tickover screw is just touching the end stop and starts off lovely at tick over. But once the engine gets warm it starts racing and makes the engine run at very high revs. A couple of turns of the tick over screw brings it back down nicely but then it wants to conk out. There is no throttle cable or choke cable attached at this point, so I'm a bit flummoxed. Anyone had this before? Any ideas? Thanks a lot P. Sent from my SM-G780G using Tapatalk
Dashpot air vent line blocked? .. friend bought shiney version with chromed tubing for his twin SU TR3.. .. car conked out on/after hills. Discovered the people who made it had soldered up the end of the thin tube as it went into a banjo connector on the rear carb .
I'm going to check for air leaks and worn spindle bushes tomorrow, and of course top up the dash pot! I remember thinking ill do that last in case I need to take it off again, and then forgot to do it! Sent from my SM-G780G using Tapatalk
I'd start by checking the linkages to make sure that one butterfly isn't being held slightly open. Then check that both carb pistons are falling at the same rate and one isn't getting stuck, either because it is gummed up or catching in the bore, or the cap is distorted, or that the jet is off centre and catching on its bearing.
So after lots of heartache it turns out that the floats, needle valves and settings were all spot on. But on these H15 SU’s they had some sort of over run valves that are widely thought of as bad news. They get soldered over and new discs put in. This solved the racing revs problem. Happy days , but increased the overflow problem on both carbs. Facet fuel pumps (which constantly pump) often have a return pipe from the carbs to the tank. I took off the return pipe and found a bloody great screw blocking it up! With said screw removed and pipe back on, hey presto ticking over lovely and no overflow what so ever! Again happy days. But no, when I test drove it there wasn’t enough fuel pressure to feed the carbs when under load and it was simply flowing back to the tank via the return. So what people do is fit an Amal monobloc or concentric type main jet from a motorbike carb into the return line to generate the correct pressure. Does anyone have one of these knocking about in their shed/workshop/garage I could nick? Cheers p.
Look up Bypass fuel pressure regulator. Not cheap but stops you getting hot fuel in a dead headed (Malpassi style regulator system) as unused fuel goes back to the tank to cool off, if the pressure is too high. Same as an FI pressure regulator. At least one model on eBay has 0 bar as the low end setting so might also be a better solution for Bay Window applications requiring low pressure and variable flow, where heat soak into carburettors and fuel lines may also be an issue.
Thank you chaps. Will try a few different sized main jets or even a migwelder tip first to see if I get any joy. Them I’ll go down the regulator route.
A cheaper idea which might take some fiddling with springs, and need a pressure gauge teed off the fuel line: Some metal one way check valves can be unscrewed and you can play with the spring pressure on the ball bearing to control the pressure needed to lift the ball off its seat.. these only cost a few pounds.
