SU HiF6 weird problem on a 3.5 Rover.

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jagger
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger » Wed Jul 10, 2019 12:25 pm

After a slight break I have returned.Sorry the video for the broken engine has gone missing as I shut down my channel hosting it.

The slight break was caused by me building an engine to put under the carbs.Vitesse bottom end with some stock carb heads and the usual 3.9 grind replacement cam.It's all run in and working nicely.Except that when ..............

Yup.The carbs are still playing up. It seemed to work ok for a bit on book settings (for a carb rebuild) as I got the timing sorted after running in the cam and it drove rather nicely with gobs of grunt up to that 3000 mark. And then I tried to tune the carbs into the new engine. Drivers side no problem. Mixture screw lean too much and the revs drop,richen it too much and ditto. But on the other side the moment I screw the mixture screw clockwise from the 1.5 turns starting point the revs rise.And keep rising as I turn the screw.Adjusting the tickover screww until it's off of the cam has no effect lowering the revs and they can hit 3000 before I crack and kill the experiment.It will stall if I weaken the jet.I have been able to set a steady tickover but only before the motor warms up fully.Once warm it will rev away from the tickover set up to it's favourite 1800-2000 point and sit there until it just drops back to tickover.This is with the throttle cable disconnected and the interconnect between carbs off as well.Fast idle adjusted properly,or a bit slack if anything).Just a pair of carbs operating independantly on tickover with zero physical inputs one of which revs the engine seemingly at will,or randomly at least.

I've got a logic block somewhere. The throttle disk isn't a gas tight seal with the stop screw all the way out.There are a few points of light that squeeze around the edges but it's the same on both carbs and the disks were out of the packet.This should restrict the airflow so much the engine will stall and it needs to be opened a turn or so on the idle screw to tickover. So where's it getting the air from to suck that fuel inside when I richen the jet with the butterfly closed ? I can't work that one out.I'm told that the air draws the fuel out of the jet as the air pressure drops above it but not much air is getting by the throttle disk when it's revving up.But somehow by enrichening that carb I can get the revs up way past the point of comfort.Spraying WD40 all over during the revving makes everything stink of WD40 and nothing else.

I am at the point of junking this set of carbs and maybe the manifold (I could do a manifold swap I suppose but I can't see where the problem might be).I'm looking at a complete replacement Weber kit from RPi for a bit of mental relief.I've looked at the birdsnest of wiring on the EFi set I have on the shelf and really don't fancy that much despite the benefits of fitting it.It would mean ripping the exhaust off for welding and experimental wiring stuff, which I normally do in the winter.I would much rather drop something on that works properly on it from the start and go bloody driving for a bit rather than fault finding without finding the fault.

Last ditch attempt this morning was to replace the copper washers on the manifold take-offs and blanking plug.It didn't work.I'm stumped.



DaveEFI
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by DaveEFI » Wed Jul 10, 2019 12:50 pm

Have you looked as the fuel flow past the jet at idle with them both set approx the same mechanically? A decent torch will be needed. Might give you a clue if it is very different. There must be extra uncontrolled air getting in somehow. Are they solid discs or fitted with over-run valves?

BTW, the standard EFI system, but with a MegaSquirt ECU, works very well indeed, and since you have most of the bits I'd guess would cost a lot less than a new Weber carb and manifold. And I'd say much easier to tune yourself.
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger » Wed Jul 10, 2019 1:21 pm

I do know the benefits Dave,and can see the attraction (fuel costs). I'm just a bit miffed at the minute with all the work I've put in to be for nowt.It's called sulking I think.

Brand new disks from Burlen with overrun valves in them.I tried flat disks but they jammed stuff up in the linkages.The carbs both look the same when ticking over and set up with a flow meter to be balanced.Throttle response is pretty good and driveability too but the hanging/sticking/sucking/leaking/other throttle makes it undriveable.

I had a bit of a read up on Megasquirt a while back,and did a bit of rough pricing up,and it seems the Weber conversion and Megasquirt are in the same ballpark.I'll maybe have a check back on that score as I have a mate with experience of it (he built his own from the boards up). I'm about done fiddling with it currently. This has been going on since the turn of the year and it's dragging on and dragging me down now.So a drop on kit that doesn't involve drilling the exhausts for sensors or a rewire on the old Land Rover it's in (1966) in which the wiring loom is what I'd describe as bush mechanic interesting. A carb set up would be more in keeping with the old bus too,although I've always wanted a rootes blower like the old hot rods had I must admit.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger » Wed Jul 17, 2019 7:00 pm

I'm wondering aloud here.

I've just checked/rebuilt/factory set the carbs for the umpteenth time and this is still happening and it appears like a valve opening as the engine hits normal running temps.And the way I got it set up to drive is so fiddle-with-it that it ran weak on the 250 mile drive I took it on.Got hot a bit too obviously.The rebuild has been with a friend as overseer (only has 4 v8 Rover engines).

So,has anyone heard of a cracked manifold messing up the mixture on one carb ? Or some other valve like air leak appearing when hot ?

I've got other manifolds here available to try but it does seem like a bombproof bit of casting. But as my wits have also gone on holiday I'm just about at the point of insanity as I still have a 250 mile drive home to do.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by DaveEFI » Thu Jul 18, 2019 9:09 am

My first guess would be an over-run valves opening when they shouldn't. I similar - but not so bad - problems on my P6 3500S. Some solder them closed. I fitted plain discs.
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by ChrisJC » Thu Jul 18, 2019 10:46 am

Check that the inlet manifold is tightly screwed to the heads. It's quite easy for it to come loose and leak air.

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