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MG MGB Technical - Hard starting

I hope the combined wisdom of this BBS can help with a problem. I have a 1974 RB MGB GT that has a problem I am unable to rectify. When purchased it ran great but had twin hs6 carbs. In an attempt to put it back to stock (it came with twin carbs from the factory) I sourced a set of HIF4s, the correct intake and heat shield. I rebuilt the carbs and over the winter installed everything required to bring it back to stock. After everything was back on and the carbs set to 2 turns of the mixture screws ( turned to the right to start on the rich side) the engine started fine and ran great.

The car has an electronic ignition module. I pulled the plugs and no 1 was running a little lean and no 4 was a little rich, all cleaned, plugs gapped, rotor replaced adjusted mixture, checked timing and checked spark at each plug. The carbs were balanced and the engine ran beautifully when hot.

The problem is starting from cold, it will crank over for ages before it will fire once, then more cranking until it eventually will fire enough to get it started, once going it is sweet, no miss and a strong pull, it is just the starting from cold that is a problem.

I have searched the archives and checked everything mentioned but still no change. I look forward to any response that can get me running properly.

Sorry for the long post.

Brian.
Brian Smith

Does it re-start OK when warmed up?
Allan Reeling

I’m running HIF’s and my stating procedure if it’s been standing for a while, is to crank the engine with no choke. It won’t start like this but allows the oil to start flowing. As soon as I see a bit of movement on the oil pressure gauge, I pull the choke out about three quarters and it’ll start straight away.

Are you sure your choke mechanisms are working correctly?
R.A Davis

Allan, once it starts and warms up it starts first time every time.

R.A., I have just come in from my shop after taking the HIF4s off, stripping them down and ensuring that everything is set up as per the Work shop manual. Reinstalled without air cleaners but still doesn't work unless I spray a little carb cleaner in.

What exactly moves when the choke is pulled out? I could see no movement inside the float area or the jet tube.

Brian
Brian Smith

HIF carbs have a rotary valve that allows extra fuel in when the choke cable is pulled out.

Did you not remove the valves and renew the seals when you rebuilt the carbs?
Chris at Octarine Services

Chris, thanks for checking in.. I had to do a major clean out of varnish and will admit I did not remove the rotary valves or renew the seals as this was the first set of HIF carbs I had done. Each carb and all the bits were run through up to 5 sessions of cleaning in a parts cleaner and then cleaned out with carb cleaner ( a few more times). I had assumed the jet tube moved up and down but see I was very wrong.

I have fuel getting to the carbs but from a cold start it appears to not get fuel? Hot starts are great and everything is working correctly. I just pulled them apart and checked that the floats were set correctly, needles work and are not sticking but when put back on the engine it still just cranked over until I sprayed carb cleaner and it started immediately.
Brian Smith

It will be getting fuel to the jets, which do move up & down for mixture setting - not for starting, a cold start needs the extra fuel from the enrichment device.

The valves are fairly easy to get out and I bet you will find the holes in the valve body blocked with varnish.

Carb cleaner won't get in there without dismantling.
Chris at Octarine Services

Thanks Chris, any chance you know the size of the O ring as I have a supply here. At least I can start the rebuilding immediately while I await the balance.

My last B had HIF carbs but they just worked, I never had to work on them.

Brian
Brian Smith

Not wishing to hijack the thread, but is there any info available - links, etc., - to HIF carbs?

My B has a single HIF6 (long story) which always worked well, but after nineteen years off the road, then a R/R session last year, it seems to run very rich at idle.

I had read somewhere, in the dim and distant past, about an O-ring in the cold start which might cause issues, but I've struggling to find any info online.

Dave O'Neill 2

SU Carbs?

http://sucarb.co.uk/technical-hif-type-carburetter-dismantling
paulh4

Brian,
The HIF enrichment is very sensitive to getting full choke movement. Even a small decrease in movement i.e., too much free play, can lead to difficulty cold starting.
I pull the choke knob out 2 to 3 mm, then set the other end so it's just about to pull the choke.
That 2 to 3 mm is enough free play to prevent the choke barrels being "on" inadvertently.
Allan Reeling

Thanks Paul
Dave O'Neill 2

First pic showing the choke linkage

Second after removal of the spindle nut, lockwasher, plate, spring and seal cover - take a note of where the spring locates on the carb & plate.




Chris at Octarine Services

Third pic after the throttle spring is disengaged with the bottom plate and the two screws removed, the plate removed and the valve pulled out.

Fourth pic shows the valve partly pulled from its housing - there are two seals, one one the shaft and an O ring on the valve body.

Check the inner valve for fuel erosion channels from the holes out to the edge - these will give permanent choke!

Note the notch in the edge of the valve body - this aligns with the top screw.




Chris at Octarine Services

Thanks Chris, very informative. I am heading out to the shop shortly and will once again remove carbs and pull the starter valve/choke mechanism. Lots of time to work on the car right now as we are in self isolation, not yet lockdown but it means lots of time to Orkney on projects but unable to go out and pickup parts etc. Even groceries must be ordered and delivered to the door.


I pray that everyone remains safe and takes this seriously. Thank goodness for computers as we are able to remain in contact with our kids and grandchildren through FaceTime or Skype.

Appreciate the help, I will let you know how this turns out.

Brian
Brian Smith

It is surprising how many times we find that notch in the valve body out of place. Always worth checking. If it's not, the enrich doesn't occur when it should.
Paul Walbran

Thanks all, especially Chris! One of our local club members had everything to repair the starter valves. Once apart it became really clear why I had a problem, the O ring was like a rock, the viton seal was falling apart and the brass parts were really gummed up. Also the little hole in both valves was plugged solid with dried fuel. I cleaned all parts, ensured all holes were clear and once back together everything worked as it should.

Such a simple fix but honestly I don't think I would have even looked for those small holes of Chris hadn't mentioned them.

Thanks
Briansmith
Brian Smith

This thread was discussed between 24/03/2020 and 27/03/2020

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