2.25 Petrol - Odd Symptoms

Lately I've been chasing a wobbly vacuum needle and intermittent misfire on my Rochester equipped 2.25. I had the Carb rebuilt professionally and have fiddled to a steady vacuum reading and eliminated the misfire.

I ran the Rover around the block last night - it ran perfectly. Lots of power, steady vacuum needle, great idle. I decided to run it up to approx. 3200 rpm and listen for any problems. I ran it up to 50 mph and came to a stop at a red light. The vacuum gauge dropped to 12 or 15 and the needle was very erratic. The truck would not idle but shifting up through the gears the needle was steady and there was lots of power. Engine breaking up to a stop light produced lots of backfiring at the exhaust and coming to a stop - again the vehicle wouldn't idle.

I pulled the truck into a side street and shut of the ignition. The engine ran on for a few revolutions before coming to a stop. I let it sit for a minute or two - started it up - and everything was fine. Steady idle, lots of power. The minute I run it up to higher RPM's the symptoms return. Again - shut it off, restart - the truck is back to behaving itself.

I've switched the coil, checked the timing, ignition parts are all pretty new. There doesn't seem to be any fuel issues. Lots of fuel in the bowl - no sign of air bubble revving the engine in the driveway. Any ideas?

Thanks again,
Henry

Fluctuating vacuum readings are usually because of sticking valves or weak/broken valve springs. On modern (96 - 04) Rovers this was caused by carbon buildup on the valve stems. They did multiple re-designs in an attempt to correct this (none of which was very successful) When I worked at the dealership replacement of the heads with rebuilt units was standard when doing head gaskets.

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Originally Posted by Shearpin

Lately I've been chasing a wobbly vacuum needle and intermittent misfire on my Rochester equipped 2.25. I had the Carb rebuilt professionally and have fiddled to a steady vacuum reading and eliminated the misfire.

I ran the Rover around the block last night - it ran perfectly. Lots of power, steady vacuum needle, great idle. I decided to run it up to approx. 3200 rpm and listen for any problems. I ran it up to 50 mph and came to a stop at a red light. The vacuum gauge dropped to 12 or 15 and the needle was very erratic. The truck would not idle but shifting up through the gears the needle was steady and there was lots of power. Engine breaking up to a stop light produced lots of backfiring at the exhaust and coming to a stop - again the vehicle wouldn't idle.

I pulled the truck into a side street and shut of the ignition. The engine ran on for a few revolutions before coming to a stop. I let it sit for a minute or two - started it up - and everything was fine. Steady idle, lots of power. The minute I run it up to higher RPM's the symptoms return. Again - shut it off, restart - the truck is back to behaving itself.

I've switched the coil, checked the timing, ignition parts are all pretty new. There doesn't seem to be any fuel issues. Lots of fuel in the bowl - no sign of air bubble revving the engine in the driveway. Any ideas?

Thanks again,
Henry

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I would do a compression check. Its possible that your oil rings are good but your compression rings are worn. At low speeds and revs the engine has good power but when things speed up the compression gets passed the rings and you have your symptoms. What are you running for oil and when was the last time you changed it?

Compression test is my next step. Power is great at the upper RPM range and the needle is steady - even up to 3400 rpm. I suspected valve problems early on due too the vacuum gauge reading but the power is great throughout the rev range and the needle settles down and stays steady with the throttle on.

The part that's confusing me is the Rover runs perfect until I run the revs up. I suspected vapor lock at one point. If you shut the engine off and wait a few seconds it will start, idle, and run fine until you flog it again.

Thanks for the responses.

Henry

Classic rust flakes\junk in the fuel system symptoms. Unfortunately, also classic signs of failing ignition components. Just because they're newish doesn't mean they're still good.

Check fuel pump output. Start with the simpler (cheaper) stuff first. I have seen dodgy fuel pump delivery mimmick many other problems.