SU HIF6 carb problems

Hi all,
I'm having problems setting up the twin SU HIF6 carbs on my Rover P6 V8 engine. The car drives fine when cold on choke but when warmed up it's reluctant to pull above 3000 rpm. Pulling out the choke again at this point and the car runs fine suggesting its running too weak? However no matter what I do with the carb mixture screws (turning them clockwise to run richer) it makes no difference. I have checked the jets in both carbs and they do raise and lower when the mixture screws are turned. The fuel pump is electric, ECCO model 1.5 litres/minute, 3 - 4 psi which appears to be correct for the carbs. Any suggestions on what the problem may be would be greatly appreciated as its driving me nuts at the moment!
Cheers, Rob


Well-Known Member
If the jets move upwards and no matter how far you go it's still weak then check for an air leak, but check the plugs first as it may only be weak on one carb, and once you know which one if that's the case then that will narrow the field a bit.
Thanks for your reply. I have checked all over for air leaks but can't find anything. This problem has only started after removing both cylinder heads during the winter months for an overhaul and then refitting them. So I'm wondering whether the inlet manifold gasket may be at fault causing an air leak and the weak mixture?


Well-Known Member
Did your idle speed increase after the head work? That can indicate an intake leak.

You can try spraying starter fluid around gasket joints to help you find an intake leak also. Listen for a change in engine speed as you try areas.


Active Member
What exactly did you do over the winter on your heads, manifold, carbs, fuel supply ? What has changed compared from situ car running fine and now car not running fine ? Did you check spark plugs as Harvey mentioned ?
During the cylinder head overhaul I did also fit a spare set of HIF 6 carbs to the original P6 inlet manifold. These carbs had been stripped down, checked over, cleaned out and new gaskets etc fitted and according to the ident tags on them they are off a 1979 Rover SD1. They needed to be set up when the engine was started so I can't tell if there was any change in idle speed. New spark plugs were also fitted but because I have only driven the car a short distance on the initial test run they were only very lightly sooted up. I have driven it again since so I will recheck to see if the weak mixture is only on one carb.
I've now removed all the spark plugs and they are all the same indicating that the engine is running on a weak mixture. Trouble now is it won't start to check for any air leaks - starter motor solenoid clicking, engine not turning over. Battery removed and tested okay, starter motor removed and tested okay and ignition relay replaced! What size battery and alternator (amp/hr) should be fitted just to check that these are correct?


Well-Known Member
If you cannot richen up the mixture at all you need to check the needle is in the correct position in the piston, it may be that the needle is installed too low.
The sintered needle holder needs to be flush with the trough on the base of the piston and not flush with the very bottom of the piston.
The info is on this section on Burlen fuels site.
Tuning - General - SU Carburetters
Thanks for your replies. Fitted a different starter motor and that cured the fault (faulty solenoid causing the starting problem). Re the carbs problem I am now in the process of rebuilding the original HIF6's to refit them to see if that cures the weak mixture issue as I have not been able to find any air leaks.
Regards, Rob
Have you managed to cure yet? I'm having problems with mine too. I have rebuilt carbs checked timing distributor changed cap rotor and leads. Only thing not changed is fuel tap seal screw wont undo. I am now considering changing camshaft and followers.
I haven't found a cure yet, the original SU HIF6's have been rebuilt and fitted and the problem is still there! The only thing that I haven't changed is the distributor so I might as well replace that to eliminate it from the proceedings. Interesting to see if changing the camshaft and followers cures your problem.
I have just been reading on the internet the effects of a worn camshaft and one possible symptom is weak mixture under load which is what I am experiencing. Are there any checks I can do without having to strip down the engine to inspect the camshaft i.e. measuring the distance travelled by each valve stem when operated by the rocker shaft? If so, what would be the normal length of travel?
I am at a loss as to where to go next. I have also rebuilt HIF carbs changed inlet manifold gasket tried electronic ignition and reset points condenser and timing. I know my vacuum advance isn't working but I am told this will not have any effect. I have changed servo hose and rebuilt fuel pump. So apart from camshaft I have no other answer unless is it the fuel reserve tap seal which for the life of me I can not get screw to budge to change. I don't see any leak from tap but you never know. Hope we get to bottom of this issue.


Well-Known Member
I know my vacuum advance isn't working but I am told this will not have any effect.
It will have no effect on performance other than if the diaphragm has failed it will make the LH carb run weak unless the mixture has been adjusted to compensate for it.

unstable load

Well-Known Member
Are there any checks I can do without having to strip down the engine to inspect the camshaft i.e. measuring the distance travelled by each valve stem when operated by the rocker shaft? If so, what would be the normal length of travel?
Whip off each rocker cover, pull the plugs to make turning the engine easier and have a look at the rockers as the engine is turned through at least 2 revolutions per rocker. Measurements of travel is dependant on cam type/model, I don't have the numbers.
A worn cam should be relatively easy to spot, as the lobes go almost round when the engine starts showing symptoms.


Well-Known Member
Another thing you can do to check the condition of the engine is look at the intake manifold vacuum. Gunson's sell a gauge that you can plug into one of your vacuum lines with indications of good and bad engine condition at idle.
only problem with this is it wont idle but thanks for reply. I do have a vacuum gauge/pump though this is how I know vacuum advance is siezed