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MG MGA - Carb Jets

I replaced my old jets with Grose jets because gas was coming out of the front overflow pipe. I also got new floats. I put in the new jets and floats and engine runs very rough...it's like the gas pedal has no effect on rpms. I put the old jets back in and the engine runs fine...still pouring gas from both overflow lines now. Then I put the new Grose jets in again and the engine ran fine for about 30 seconds and then so rough you can't keep it going. Whats going on? Please help if you can.

KC
Kelly Clifton

Hi Kelly. I have been running grose jets for years, and have had wonderful results with them. The fact that you have fuel pouring out the carb overflows, indicates that the fuel supply is not shutting off when the floatbowls fill. Check to see that you have proper replacement floats, and that they do not have a tiny hole in them. You may also need to adjust the needle valve shut off lever. (float lever) Your manual should tell you how to do this. Question: did your MG run OK before you changed the floats? If so, it could be that the new floats are a slightly different size, and are not closing the needle valves or grose jets properly. Perhaps try putting your old floats back in the carbs. Good luck! Glenn
Glenn

Glen, The car was running fine, then, one day I had trouble starting it That is when I noticed gas pouring from the front overflow pipe. I looked in the archives and found that it might be the jets. I also checked the floats and found that they had gas in them.
But before the problem showed up, the car was running great. I found a float level adjustment number in the archives, but where is it measured? I'll check the new float size to see if it is the same as the old one. I don't have a manual to refer to, so thanks for your help! Kelly
Kelly Clifton

Hi Kelly. The float lever adjustment is checked as follows: Remove the float chamber lid. Invert the lid.
When you look at the float lever, the part that hits the needle valve or grose jet ball is flat. There is a curved part of the float lever also. Take a 7/16" metal bar, and place it diametrically across the float chamber, so it passes under the curved section of the float lever. With the bar positioned as stated, the curved part of the float lever should just touch the top of the test bar. If the clearance is too great, or too small, then carefully bend the float lever to get the proper adjustment. Note that the float lever needs to be bent ONLY where the end of the flat portion meets with the curved portion. The flat portion of the float lever MUST remain flat to open and close the needle valve or grose jet valve properly. Hope these instructions are understandable! Cheers! Glenn. PS you might want to get an MGA manual. Sometimes a picture really is worth a thousand words!
Glenn

When I replaced the SU jets with Grose Jets I used two of the fiber washers on each jet body (they came with the jets)since this was necessary to raise the jet end to the point where the flat part of the actuating lever is horizontal when the valve shuts off. Without the washers it was not possible to get the correct geometry and achieve proper shutoff.
Tom Heath

Thanks for your help guys, here's an update. Adjusted float levers as per Glen's instruction. Car now starts fine and idles fine, but will not take gas. When I depress the gas pedal the engine trys to rev but runs really, really rough. I have had this car for a couple of years and have never had any carb problems until this overflow issue. Again the car was running great before fuel line overflow and switch to grose jets and new floats. Please help if you can.
Thanks
KC
Kelly Clifton

Hi Kelly. If you fooled with the float lever adjustment, the carbs could now be totally out of adjustment. I suggest that you set up the carbs from scratch. Read this entire post first please. That means disconnecting the linkage between the 2 carbs, disconnecting the choke linkage, ensuring that both main jets are properly centered, and then adjusting fuel/air and idle speed. The fact that your car starts and idles well now is encouraging. You might want to remove both carbs and give them a thorough cleaning inside and out. The pistons need to be able to rise freely in the suction chambers. The insides of the suction chambers often get dirty or gummy, and need to be cleaned periodically anyway. I suspect that after a thorough cleaning, and proper setup, that your carbs will again work correctly. SU carbs are pretty simple and reliable. There isnt too much that can go wrong with them. You might also want to check that all other tune up type issues are addressed BEFORE fooling any more with the carbs. If it has been a long time since your last tune up, then I would run a compression check, replace plugs, points and condensor, check distributor cap and ignition cables (replace as necessary) check valve adjustment and ignition timing. Check oil level in carburettor dampers. If all these items are OK, and the engine still doesnt run right, THEN fool with the carbs as instructed above. Good luck, let us know how it all turns out! Cheers! Glenn
Glenn

Kelly
As Glenn sugests you must check all ignition components as wll as the carbs. I had similar problems twice with different solutions. Both times my car started and idled fine but would not run under load. The first time it was fuel starvation caused by a broken float bowl gasket that blocked the fuel flow to one carb. The last time was a bad coil.
Kris
Kris Sorensen

Kelly,

Your engine runs fine with the old floats and needle and seat except for gas coming out of the over flow, yes?

Your engine now idles but will not rev when the throttle's are open and your using new floats and gross jets, yes?

Your original problem was the float(s) sinking in the bowl. To test the needle/seat hold it closed with your fingers and no fuel should escape when the pump is running.

If fuel does escape then the needle/seat are bad too. (maybe obvious eh?)

You have manipulated the original configuration of the float lever and this has caused the car to idle but not rev.

If the lever was moved (bent) too far then the carbs are not getting enough fuel through the gross jet to allow the engine to run at higher rpm. Raise (bend) the lever a bit (toward the float bowl cover).

Also despite some opinions - moving the float levers will not affect the rest of the carb settings. The gross jet is like a faucet- it's on or off or maybe leaking a bit.


My $.02.
jb
John

Kelly,
Have you checked to make sure that the new floats are nice and free on their pins and free to go up and down in the chamber and not rubbing on the sides?
FWIW
/ gil
gil

Many thanks to everyone for your input! I didn't make it out to the garage last night, but plan on it this evening. I've learned alot about this car over the last 2 years, but I've had zero problems with the carbs. I don't have any experience with carbs at all so I'm very anxious about this problem. Thanks again for your help!

KC
Kelly

Hi John. Regarding moving the float levers not affecting carb adjustment: I see your point about the grose jet being like a faucet, either on or off. However, I thought that if the float bowl was overfilled, that it might flood the main jet, which would cause an over rich mixture. I remember years ago, that a sticking needle valve caused the flooding of the front carburettor on an old volvo that I owned. That was with an HIF model carb, and not an H series. Now I am wondering if the H4 carbs could flood due to an overfilled float bowl? Or would the overflow tubes be able to drain off all the excess gas? Just curious. Glenn
Glenn

Well I checked the ignition system, I had changed plugs night before last hoping for a miracle, wires are good, dist. seems fine, dwell right around 60. I checked the oil in the dash pots and found the front carb was low. Topped it off with Marvel Mystery Oil. When I tried to start the car gas started pouring from the front carb overflow. I adjusted the levers back to 7/16. The car started fine then, but no change in performance. As I let it run a few minutes, I noticed condensation forming on the front carb. I felt it and found that it was cool compared to the back carb. Should this tell me anything? Pulling on the throttle cable very slowly the engine would rev smoothly up to a point and then it would start sputtering.If anyone has any more ideas, please let me know.
Thanks,
KC
Kelly Clifton

Hi Kelly. I am certainly NOT an SU carburettor expert, but I have rebuilt several sets of H4, HS4, and HIF6 carbs over the years. Since you have checked the tune up items and have not noticed an improvement, I think you will likely end up having to remove the carbs, and give them a thorough cleaning. Dismantle and soak the parts in carburettor cleaner, and then use compressed air to (gently) blow out all passages and orfices. The fact that the front carb is still shooting gas out the overflow tube indicates that there is still a problem with fuel shut off, probably relating to dirt in the grose jet. It may be that there is excessive gum or dirt in the front carburettor that keeps fouling the grose jet. Dirt, gum or grit would likely be trapped in the float bowl. I have seen some SU carbs with an eighth inch of sediment in the bottom of the float bowls! It might also be that your gas tank, or fuel lines are dirty, or rusty. If this dirt or rust gets pumped up to the carbs, it can make the needle valves or grose jets stick, which makes the carb or carbs flood. You could try cleaning the carbs, and before starting the engine, install a good quality (racing type) inline fuel filter just before the carburettors. An good inline fuel filter (with a clear glass body) would trap any dirt or rust, and allow you to see it, when it gets trapped in the filter. I changed to grose jets on my own MGA because of rust particles jamming the needle valves. The carbs would then flood and pour gas out the overflow tubes. Grose jets helped with the problem considerably, and combined with the installation of a good inline fuel filter, ended my flooding problems. If you decide to remove and clean the carbs, you need to know that the carbs come off as a pair, not individually. The carbs should be disassembled and cleaned one at a time to avoid mixing parts from one carb to the other. Be VERY careful not to bend the metering needles on the pistons! The throttle shafts would not have to be removed from the carb bodies. You should have a good manual or at least an exploded view of the carbs, to aid in disassembly and re assembly. A digital camera is handy for taking photos as the carbs are disassembled. If you remove the jet assembly as a unit, it will HAVE TO BE RECENTERED, which can be tricky. You can remove the jet tube ONLY, while leaving the rest of the jet body assembly in the carburettor. That way you would not have to recenter the jet assembly, but could clean it out. Photos sometimes help make it easier to reassemble things. If you are not mechanically inclined, you might want to seek assistance from someone who knows H4 carbs, and/or SU carbs in general. Perhaps other readers might have a better suggestion, or greater insight as to what the exact problem and cure might be. Good luck! Let us know what you find. cheers! Glenn
Glenn

Well...perhaps I may take a stab at this one. Rather than start from scratch right away, maybe you could check over a few things first. It ran before and it does idle, so why go too far?
Some may (I believe yes) have been mentioned. The float level levers are a bit tricky, as I've recently installed a few of the grose-type valves. You do definitely have to play with them, as they seem to be perfect for the original type valves and NOT for the shorter grose-type. There is a balance between the 7/16" setting and the amount of lever drop. It is possible that the outer/pivoting end hits too soon and the lever has too little drop...the valve remains partially closed and this starves the carbs of fuel oon acceleration. If the outer end has two slits, leaving a moveable tang...adjust this for a definite clearance of the ball. If there are no slits (mine had none) you can remove the lever and create enough of an adjustable-tang so that the lever won't be stopped prematurely.
My other thought was to check the dynamic timing. If someone will rev-er-up slowly while you check the timing...you can see if the mechanical advance is working fine too. I mention this because I've seen gummy oil/grease down in there before that inhibited free movement of those cams. This check requires no special effort other than hooking up the timing light and having a friend ease the revs up for you.
Anyway...just a couple things I'd take a look at...Cheers
Jon Bachelor

Forgot to mention that I do agree that you should remove the carbs' upper chambers (one at a time...don't switch them) and use carb spray cleaner to remove any buildup/debris. Use a soft, clean, lint-free cloth to wipe the inside walls of those chambers and around the rings of the pistons (you can leave them in place). You can temporarily remove the piston return spring, put a dab of oil on a finger and coat only the outside of the piston's damper tube. Then just reassemble...This will help assure that the pistons aren't getting stopped on the way up while accelerating. Check for free movement of the piston in both directions. If it still feels like it's hanging up, you may need to pull the pistons too and clean the carb body and piston where they might contact. Cheers
Jon Bachelor

Problem solved! Thanks to everyone for the help! I probably should have caught the problem sooner and maybe some of you were trying to steer me in that direction, but I didn't catch on. I removed the float cover on the carbs to adjust the float levers (again) and noticed that the rear bowl had no gas in it and the front bowl did. I filled the rear bowl with a little squirt bottle and fired the car up and it ran great...until the bowl emptied. I found that the new grose jet was stuck closed. I put the old jet back in and it works great. Took the car for a spin and no problems at all.
Thanks again!
Kelly
Kelly Clifton

This thread was discussed between 03/04/2005 and 10/04/2005

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