SU HiF6 weird problem on a 3.5 Rover. WITH SOLUTION - and you're not going to like it.

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SU HiF6 weird problem on a 3.5 Rover. WITH SOLUTION - and you're not going to like it.

Post by jagger »

Hi all.I really hope I can get a brain nudge here.This one is winding me up a piece.

Tired old 3.5 in my '66 Land Rover 88 was running fine after a sticky jet was replaced.Lovely 650 rpm tickover and a surprising amount of grunt when opened up.Unfortunately opens the oil leaks too.Good throttle response when cold.So,in the middle of a 500 mile road trip, the throttle started sticking when hot holding at around 1800-2000 for a good thirty seconds or so before settling back to 650.Nice and undriveable with drum brakes.No sticking linkages found and both butterflies returning fully.

I figured air leaks somewhere and so ordered a full rebuild kit from Burlen.The bushes were fine but butterflies and shafts were swapped with the new two part seals installed.The cold start was also cleaned out and resealed as per the book.I fitted a set of new red springs as well as new jets and fuel needle and seat.Cleaned everything spotlessly and reassembled.Put them on the car and got a nice 2-3000 tickover with both butterflies shut to the stops -no screws involved.I centred the butterflies as well as possible with just the tiniest amount of light getting through the odd crack on both sides.

I can force the tickover to drop and stall by winding the mixture screw anticlockwise (weakening) but if I go clockwise and richen it the tickover goes higher than I personally can tolerate.Over 3 grand at a guess.It just keeps going up as I wind the jet down and I back off before the revs do.This is with no tickover screw involved,shut throttles.I just cannot get my head around what could be causing this other than an air leak of some sort which I think I just eliminated with the new seals. A spray of carb cleaner around the carbs doesn't change the revs one jot.I've cleaned out the pistons and chambers and performed a drop test to check them for balance.I've tried higher rated piston springs which seemed to calm things a tiny touch but didn't make the problem go away at all.The engine is properly timed but a bit tired in the camshaft area.It was,as I say,running acceptably before this raised it's head.The dizzy isn't sticking and Powerspark ignition.Put a hand over the intake for two seconds and the motor stalls.One second and the revs drop and then recover to the high tickover.

Does anyone have any idea what could be happening in those carbs from those symptoms ? I wouldn't mind kicking a few ideas around before I go and remove them again for yet another inspection/talking to.
Last edited by jagger on Mon Jul 22, 2019 8:18 pm, edited 1 time in total.



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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

Not surprised there are no replies as I can't find a single reason for the carbs to behave this way after a rebuild.So I've ordered a new distributor.Although the fitted one( 35D with Powerspark trigger) seems not to stick it's a bit of a problem area for checking as my finger doesn't fit through the hole.The springs do seem a fair bit weedy,so I've got a few replacements coming for them too because I like fiddling with lost causes.The bob weight mounting has also been welded at some point so it's not exactly factory fresh.I shall update this when I get it running properly/good enough/badly/none of the above.

I've got a set (not a pair) of HiF44s that are a bit furry through non use.No wear,so a set of seals and a clean and fettle gives me another substitution candidate if the dizzy stuf fails.

Probably throw a rod next week.Or a gearbox. :t
Last edited by jagger on Tue Jan 22, 2019 9:28 pm, edited 1 time in total.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by unstable load »

Give Burlen a call and ask them if they have an idea what's going on.
Cheers,
John

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

I may just have to do that John.It's getting a bit old now.

I fitted a brand new dizzy today.Started it on full choke (cold start then) and it ran at about 1000 rpm and cold lumpy like they do but responded to the throttle and settled back.I quickly strobed the timing in to 6 degrees with no vacuum advance and then knocked the choke off.Revs immediately rose to 3000 rpm or just above with the tickover screws wound out.Tried richening the jets two full turns but the revs got too much for me that close up with no result so I killed it for the night (or a cup of tea).

For fifty odd years I've been preparing for a problem like this but obviously not well enough.I do hope it isn't a face-palm thing.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by unstable load »

jagger wrote:
Tue Jan 22, 2019 4:29 pm
For fifty odd years I've been preparing for a problem like this but obviously not well enough.I do hope it isn't a face-palm thing.
You may as well prepare yourself that it may well be just that... :lol:
Cheers,
John

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by minorv8 »

A couple of things: are the throttle plates fitted the right way ? Have you tried the idle with the throttle linkage disconnected ?

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by DaveEFI »

Do these carbs have emission control valves in the butterflies?
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

Yes to both.

New butterflies with emission valves.Fitted at the top (only way they fit) with the flat to the manifold.Same as the old ones that came out and centred as well as possible.New two part seals fitted.I've tried it with the the throttle linkage disconnected.

These are the questions I'll be perusing today.

1/ What can make a perfectly behaved engine start to hang at 1800 revs for twenty odd seconds when it gets warm,and then returns to idle @650 rpm ?

2/ How can a rebuilt carb (any carb for that matter) run the engine at 3000 rpm with the butterflies shut with the timing correct (I have a borescope) ?

3/Why does it idle at 1000-1200 from cold on full cold start and then race to 3000 when it's knocked off ?

I stripped,cleaned,checked,and rebuilt the carbs because I suspected an air leak around the cold start mechanisms for number one.And then fitted a brand new dizzy just in case.The problem got worse.

I have yet to form a hypothesis for question 2 as I haven't got a clue how that can happen.

I'm trying not to think about question three until I get going on question 2 but the two must be related in some way.I'll drink a bit of tea and see if I can come up with a logical engineering solution but I suspect there's a Gremlin in there somewhere.Staring at it hasn't worked yet.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

Jeez it's cold out there.Especially when you have to slap yourself around the face with cold hands.

Answers, on a postcard. :oops:

1/ Not sure yet,but a sticking throttle linkage could play a part.Haven't warmed it up yet.

2/ Easy if the throttle is actually stuck open.

3/ It doesn't if you stop the throttle from sticking open.

In my defence the throttle is several yards long.I've got a pedal which pivots an 18 inch shaft.On the end of the shaft is another arm acting on a pushrod about a foot long.This operates an arm on a cross shaft which goes over to the passenger footwell where it has another arm.This arm pulls on a cable which loops around the front of the manifold and right side of the engine to get back to the operating cam on the manifold.Then there's a push rod to the left hand carb and the link rod to the right side carb.I inherited it this way.There's a bit of friction somewhere in the cable which may get worse as it heats up (one to check) and I extended the small carb push rod to ensure free play.It does seem as if it were holding up somewhere in that Frankenmonster of a linkage.I'm not kidding about the yards long bit.

After getting rid of any tension or stickiness I found I started it on cold start,dropped that off and it stalled.It actually responded to the idle screws being moved at this point and I dialled in a stable 650 rpm tickover in proper bush mechanic style even with it cold.So now it appears that I have a set up that I may be able to actually tune with a screwdriver rather than a large hammer.

My face has been palmed ( cos I took a quick glance as gospel) and I'm drinking tea to build up the core body temperature required for outdoor mechanicking.At least I've got a flow meter to speed things up a bit.

I'll let you know if I get it running right before dying of frostbite.I've got a Vitesse engine I can build if all else fails but I think it will eat my gearbox.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

No is the answer to that.

It's not the throttle linkages as I disconnected everything to do a tune up.All seemed to be going well as it warmed up but then it started it's racing again.Hand over the throat and it settles back down to the set tickover.As it warmed up more I could set the tickover (after doing some rough mixture setting ) and stand back and listen to it rise to 3000 again.Hand over the throat slows it but it rises again as soon as the hand is removed.Unless I hold it there until it stalls.Connect up and blip the throttle and it just hangs high without settling back.

So it's back to square one.I thought I'd found some binding but with all linkages removed and both screws set to a 650 tickover it starts this racing off again.

I am really struggling to find any logic anywhere in this fault.Can the vacuum chambers/pistons cause the revs to rise with both butterflies closed/on the stops ? How are the pistons rising without the butterflies being open ? What can open up a big enough hole as it warms up to cause a massive air leak and then close up again as it cools ( the emissions stuff maybe) ?

Round and round and round we go,where it stops nobody knows.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by DaveEFI »

My guess is those emission valves on the butterflies opening when they shouldn't. Had that on a P6 3500S which resulted in a too fast idle.
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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by harvey »

DaveEFI wrote:
Thu Jan 24, 2019 9:21 am
My guess is those emission valves on the butterflies opening when they shouldn't. Had that on a P6 3500S which resulted in a too fast idle.
A very common problem on the SU SD1 as well. The way to prove it's them at fault is wait till the engine is running at the high idle speed, then turn the engine off, and then restart without touching the throttle. If it goes back to a normal idle it's the poppet valves causing the problem.

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

Thanks guys.That's worth investigation.Can't figure out why they would start giving trouble on the old butterflies and be even worse on the new ones though.I've just fitted a pair of new butterflies with the emissions valve.The originals had them too of course.The valves are actually fitted at the top despite me saying the bottom earlier.I had them upside down.

I've also just whipped them off again to have a torture session in the workshop and check my logic.

The HiF44s I have do have solid butterflies but I'm not sure they fit the HiF6 (I'll have a look tonight).I may have to rebuild the 44s and fit them.

The thing that is distracting me a bit is that I replicated the racing a while back at tickover with throttles shut by simply removing one of the emissions pipes to one of the carbs,thus weakining stuff a lot.This is sort of forcing me to look for an air leak that happens when the engine warms up a touch even though I can't find one.Or my brain is just blocked from properly investigating by that impulse.Dunno.I'm working on it.

This thread is turning into stream of consciencess rambling now I know but I'm trying hard to regain my logic.I have thought of a nickname for the engine though.Begins with B.......

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by jagger »

harvey wrote:
Thu Jan 24, 2019 1:23 pm

A very common problem on the SU SD1 as well. The way to prove it's them at fault is wait till the engine is running at the high idle speed, then turn the engine off, and then restart without touching the throttle. If it goes back to a normal idle it's the poppet valves causing the problem.
Good point.I've whipped them off again now but I could get the tickover to settle a while back by simply choking off the air at the throat on one carb.With the pre rebuild carbs it would stay low until the throttle was blipped.Since the rebuild the tickover is just going to 3000 when warm.So I've definitely introduced some user error there.Something I'll look for during my session this evening.

Would the progression of this fault indicate an incremental increase in manifold vacuum,pulling the valves harder than before ? I have the solid disks and several soldering irons so sealing/removing the valves is a possibility.I take it I have to remove the emissions pipes from the rocker cover to the carbs and fit a catch tank if I do that ?

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Re: SU HiF6 weird problem on a 3.5 Rover.

Post by Denis247 »

I'd remove and block all pipes coming from the carbs if I were fault finding this. I've even known a faulty vacuum advance to cause this type of problem, sucking in air, or even cracked pipe between the two. Also any vacuum feed to brake servos or other vacuum devices (my car has vacuum operated heater controls), one way valves or dodgy pipes. Basically, take everything off, and if fault persists check for manifold leaks, it may not be the actual carbs causing the trouble. Just my 2c.
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