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MG MGB Technical - HIF carb woe

1972 MGB.
I had posted before on this, had good help, but i'm still missing something really obvious i think. I'm trying to adj the carbs, car is nearly ready for the road. Found i had fuel pissing into the engine, everytime i would lift up the piston i would see petrol squirting up past the needle. Now, i thought i had cleaned the carbs, no dirt, sticky bits etc. I've watched the University Motors video on tuning the HIF carbs, but i can't get the engine warm enough as it just floodeed the plugs.
The carbs are now apart, and i want to do this right.
Everything seems to be intact, clean, no leaks.
Questions
1/The needles, are they supposed to be a bit floppy or rigid?
2/ The needle valve for the float. Which way up should it be, rubber or metal baqll at top?
3/ How do i know if the floats are leaking, i've emersed them in petrol but see no bubbles etc ?
4/One of my dashpot dampers has a hole in the top, so when the piston is lifted up, oil comes squirting out the top?
Any help please, please. I just need this problem sorted out then i'm on the road for the first time in five years.
Gerry

Gerry - Check the overflow/vent tube from the carburetor to the charcoal canister. If it is clogged, the fuel in th float bowl will be pressurized and the fuel will be pushed through the jet and into the throat of the carburetor. Cheers - Dave
David DuBois

Dave, you always seem to be there with help.
I've blown through the vent tube and that seems fine. But when this first happened the charcoal canister did fill up with fuel so it would seem that everything was being pressurized. I've just found that the breather hose, its about 4 ins long, is split badly, plus that dashpot damper with the hole. Would that give so little pressure (unable to keep any oil in it, as it just squirts out the top) that the piston would rise at the slightest provication causing a overtly rich mixture ?
Gerry

The damper should have a hole in it. If you push the piston up manually too fast then yes the oil will squirt out but if it rises on its own the way its supposed to, very little or no oil will come out.

The needles are supposed to be a bit floppy as you say in the newer style carbs.

The float valves should have the pointy end inside the valve body and the flat end with the pin sticking out should be what the float pushes up against.

If your float isnt full of petrol, it should be alright. Are you sure your float is adjusted correctly? When the carb is upside down with the bottom cover off, place a ruler or something flat and level across the face of the carb body so it goes across the middle of the float where the U shaped indentation is cut out (You need to have the jet removed for this or it will be in the way). When left resting under its own weight upside down with the valve in place, the float should be 1mm below the ruler. If its not, then you need to gently bend the little metal tab up or down so that it is sitting properly. This does need to be fairly accurate. I know John Twist explained this in his video, but do it again just to be sure.
Ross Kelly

Thanks Ross.
I've checked the float height, using the University Motors Video to help.
I'm putting everything back together really carefully. Its fine when you find a problem, sort it out and bingo things work again. But when you can't see anything obvious, it’s not as easy. Only one dashpot damper has a hole. Both floats seem fine although the float valves don't seem to come down immediately with the float when the carb is turned upright.
How quick should the piston with the needle in it return down?
Please bear with me guys, its British Car day at the Larz Anderson Museum, in Boston next weekend. If only...
Gerry

Gerry,



To verify that you do not have a problem with your carbs overflowing or your carbon canister, leave the over flow tubes disconnected from both carbs initially. Not sure if the overflow tubes to the carbon canister is part of the North American requirements or not but there should be not problem with venting the carbs to the atmosphere like other SUs. That way you can determine if you have a problem with your vents and carbon canister separately from solving your carb issue(s).



As for you needles and needle and floats, Ross Kelly already provided you information. However, I would like to add that the “floppy needles” should point/lean towards the intake manifold(if my memory serves me right). There is a tab on the piece that holds the needles that forces the needle in this position. You will note an arrow on the piece holding the needle, that should be flush with the base of the piston, that shows the direction of the tab.



Finally, I learned years ago that the floats, with the arm off one side, in the HIF carbs were changes to make them significantly stronger than originally designed. You will probably note that your floats can be twisted significantly clockwise and counter clockwise while secured in the carb body. The new one will not twist like this and am sure will change the amount of fuel allowed in your bowl when adjusted correctly. As the front and rear carb floats are sided you will need to purchase one of each. I would suggest you replace them. They are available from Moss and suspect they are available from other souces as well.

HTH

Fred

Fred Wright

Missed the British Car day. Thought i had it sorted then,flooding again.
The float is NOT sticking on anything
The float needles are free
The carb is v clean inside and out
when i have the problem and take the vent tubes out, the ones that go to the canister, the fuel is comming out there, left carb only.

Not run the car without the vent tubes connected, that is next. The vent pipes are not blocked. Would a blocked carbon canister have any effect?
I tried to see if the floats had any leaks, left them in a bowl of petrol and nothing. plus if they were bad i'd have seen fuel in them when held to the light. I realy don't want to spend $30 on each float to see no difference.
If it helps the flooding only seemed to happen after the choke was put in.

Please help, i'm ready to sell the bloody HIFs for a weber, anything...
Gerry

Gerry, Were are you located north of Boston ?
SJ MacKAY

If it's always just one carb then it must be a problem with that carb. I had repeated intermittent problems with one carb over a long period until I noticed the float did have fuel in it. Squeezing and heating and cooling showed no bubbles, so the leak must have been minute.

A blocked vent would cause massive flooding into the *engine*, as the vent needs to be open to allow the float to rise and shut off the fuel. If it can't because the vent is blocked the pump forces fuel up through the carb jet and into the inlet manifold.

The vents are connected to the canister, along with the tank vent, to prevent fuel vapours from expansion getting into the atmosphere.

Not all damper caps have a hole in them, the later ones have the upper part of the damper chamber vented internally (via a passage in the rib down the outside of the neck) to stop dust being sucked into the carb through the hole in the damper cap. Oil shouldn't squirt out of the hole in the damper cap anyway, I can't see how it could be so overfilled as to cause this.

It is the piston and needle being held *down* by the damper that gives a richer mixture for acceleration, it is when it rises too quickly that you get stumbling from a weak mixture. The piston and needle are only damped when they rise, they should drop smartly back to the bridge with a metallic 'clunk' when released.

Whilst a choke problem could cause excess fuel in the inlet manifold it shouldn't cause the float chamber to flood. What float valves have you got? Grosse 'Jets' use a metal ball and these days they are said to be no good, whereas the Viton-tipped (pointed) valves are an improvement over the brass ones.

It's one of the bug-bears of the HIF that you can't simply remove the float chamber lids and compare fuel levels, so see if you do have a float or valve problem. Whilst the floats can't be swapped over the valves can, which may help with diagnosis.
Paul Hunt

SJ, i'm in Billerica. Paul, i have the regular needles, i did get a new one and they don't stick, ie holding the carb with the bottom off, the float falls down and the needle goes with it.
I'm going to put the carbs back on tonight, God i can do this in my sleep now, and run the car without any connection to the canister.
OneDamper cap has a hole and the other, well you can hear a hiss when pushing the piston up so it must be internally vented.
Gerry

Gerry,

I don't know that it would cause the problem you are seeing now, but since one of your damper caps is vented and one is not, there is a good chance that one of your dash pots is internally vented and the other is not. So, be sure that you use the vented damper cap on the non-vented dash pot, and vise versa. See Paul's note if you don't know how to tell the difference.

Charley
C R Huff

Also mis-matched carbs is not a good thing, although it wouldn't have a bearing on one leaking and one not.
Paul Hunt

Coming late to this thread.... Gerry, I'm in Paul Hunt's camp on this. First suspicion is the float needles, assuming - as seems the case - that the floats are properly adjusted and not filled with gasoline. I had the same problem recently. It turned out that one carb had a float full of gas, and the other had a bad float needle. Both were overflowing. Like to made me (more) insane.

Swapping the float needles is easy, and you can actually do it with the carbs on the car. You can't see what you're doing, so you have to fiddle around a bit - spilling a cupful of gas on the exhaust while you're at it - but it's still a lot easier than fighting the linkage yet again.

Rick Stevens

I,too had a problem with fuel overflowing. first problem was the new style Grosse jets. went back to Viton tipped needles and it became better but occasionly it would flood heavily. I installed a pressure regulator and turned it down to 2.5 lbs and all was well. The customer had installed a new pump that was a lookalike but was putting out 6 lbs.
Since regulating no further leakage.
Sandy
conrad sanders

wELL I HAD HER RUNNING THIS MORNING WITHOUT THE PIPES TO THE CARBON CANISTER. gOT WARM, EVEN GOT TO TUNE THE CARBS A little. No leaks, this is it i thought. Well took her out the garage tonight and she was running rough, and there was the petrol coming out the carbon canister outlet on the LH carb. Hit the side of the carb with a hammer, lighlty mind you, and the petrol pretty much stopped coming out. Took off the carbs and as best i could see, nothing in the float, no shadow inside, no sloshing sound, nothing. The float needle and housing has already been replaced, after the problem started, after i over tightened the housing and broke it, so you could say the housing and float are matched. so thinking of a stuck float, i'm going to look for any areas it could stick open.
Thanks everybody for your input, this car has come so far, new floors, body work, inc new door skin, Sebring front end, all done, and now this one thing holding the whole thing up.
Gerry

The float needles do seem to have problems with modern fuels. I had this problem and got to the point where i could stop the fuel coming out of one overflow only to see it break the seal on the other. Viton tips worked fine in the short term, in the long term I fitted a pressure regulator set to 2.5 psi which works fine with the standard needles. I also changed over to a new solid state SU fuel pump while this was ongoing , it had no effect.
Stan Best

Also coming to this late, but no mention of fuel pump or which carb is flooding. Is it a standard, SU pump? Some after market pumps deliver too high a pressure and need regulating with a filter king or similar.Is the flooding carb the first to get the fuel? Does the problem occur when the engine is hot? If either or both of the latter it could be the pressure from vopourising fuel in the inlet pipe overcoming the inlet valve in the float chamber. Check the routing of the fuel pipes. Is the heat shield across the heater box, in place? And even try insulating or shielding them further.
Allan

Allan, the pump is standard and in good order. I originally had a post about the pump thinking that was the problem, but everybody seemed to think that it would only pump when required. Its the first carb that is flooding. Heat shield across the heater box ? There is like a channel at the bottom of the heater box that all the pipes, wires fit in. That is fine.
Gerry

How is the heat sheild under the carbs? Check that the "material is still attached if it's a metal sheild, but i think the later ones were all fibre, and prone to losing the odd corner!!
Allan

The heat shield is pretty much rubbish. The "material" has come off from one side, and it looks like one end was cut off by the previous owner !
Gerry

I will share my HIF experience in hopes it could be helpful. I too could see raw fuel coming in past the needle on one of my carbs. Mind you, this car was running, albeit very obviously rich (sounds like you're getting yours to run a bit). I had thought I had these carbs tuned, but since I could tell they were rich due to the liquid fuel I could see, I cranked the jets up (which is leaner)a lot, perhaps four or five full turns. I watched inside, they went up to flush with where they're seated in there, then I went up around 1 1/4 to 1 1/2 turns of the screw higher. This is where it began to fail to idle well. It made a real difference in my car, they were way too rich. My only thought is maybe the jets are way too low in your carbs, making it so rich that liquid fuel comes in past the needle causing or contributing to your problems. Good luck.
Greg
G Holmes

This thread was discussed between 22/06/2008 and 14/07/2008

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