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Bike Carbs, what am I missing?
SJ - 29/6/23 at 09:29 PM

I've been fiddling around with some mikunis on my MGB while I rebuild the SUs. I was interested to see if 2 ZX6R mikunis would work. Problem is I can't get them to stop overflowing. The float valves seal perfectly if I blow through with them whilst lifting the float or connect them to the fuel pump upside down so gravity closes them.
I've set the float height correctly but despite this petrol sprays out of a vent at the front of the carbs and floods into the Venturis. Obviously the floats must not be closing the valves but I can't work out why. The floats appear in perfect condition and move completely freely. I have the same carbs on my Zetec with zero issues.

Any suggestions?

Thanks

Stu

asn163 - 29/6/23 at 09:40 PM

What fuel pump are you using?
Electric bike pumps cut off when up to pressure.

adithorp - 29/6/23 at 09:42 PM

Too much fuel pressure.

ReMan - 29/6/23 at 09:43 PM

Yes definitly need very low pressure bike pump or similar else it will force past needle valves

SJ - 30/6/23 at 05:50 AM

I'm using a ZX6R pump so definitely not that. Also gravity closes the float valves perfectly against the pressure of the pump

Mr Whippy - 30/6/23 at 07:12 AM

Probably one of those that needs a return to the tank and a T-piece used with a restrictor on the outlet to the carb. Regardless of how well it seems to seal there can only be a few options -

1) Float doesn't float
2) The float does not not close the value when it floats.
3) The pump pressure is greater than the closing force on the needle.
4) Aliens.

SJ - 30/6/23 at 08:25 AM

quote:
Originally posted by Mr Whippy
Probably one of those that needs a return to the tank and a T-piece used with a restrictor on the outlet to the carb. Regardless of how well it seems to seal there can only be a few options -

1) Float doesn't float
2) The float does not not close the value when it floats.
3) The pump pressure is greater than the closing force on the needle.
4) Aliens.


Agreed, it's definitely one of the 4.
I had another play this morning and checked the floats in water.
1. They are perfect and definitely float.
3. With the fuel pump connected and up to pressure and the carbs upside down so gravity is causing the floats to close the valves there are no leaks so can't be pump pressure.
4. I haven't seen any alien activity.

That leaves 2. Something is stopping the floats from floating up to close the valves. Just need to work out what.

I'm going to try setting the float height very low so hardly any fuel can come in before the floats close the valves and see what happens.

Thanks

40inches - 30/6/23 at 09:08 AM

Just my thoughts, have you mounted the carbs at the same angle as they are on the bike or parallel?
There should be a mark on the float bowl indicating carb fitting angle. Some bike carbs are ok mounted parallel, others not

SJ - 30/6/23 at 09:48 AM

I thought of that so tried them not mounted on the manifold and the angle they sit at makes no difference

SJ - 30/6/23 at 12:25 PM

I Tried again after looking at where the floats were on another set of carbs I have that haven't been changed. Seems the instructions for measuring float height I found online aren't right, but in the wrong direction meaning my floats were set too low.

I set them the same as the other carbs and after a bit of a leak initially they settle down and were barely leaking at all, so definitely progress. Still can't get it to run right though. I think I'll wait until my SUs are rebuilt

johnemms - 1/7/23 at 12:58 PM

I would have thought most bike carbs could be gravity fed..
If they flood from gravity feed - you got a problem

adithorp - 1/7/23 at 01:13 PM

quote:
Originally posted by johnemms
I would have thought most bike carbs could be gravity fed..
If they flood from gravity feed - you got a problem


Where do you propose mounting the tank so it can gravity feed the carbs in an MGB?

Mr Whippy - 1/7/23 at 01:55 PM

quote:
Originally posted by adithorp
quote:
Originally posted by johnemms
I would have thought most bike carbs could be gravity fed..
If they flood from gravity feed - you got a problem


Where do you propose mounting the tank so it can gravity feed the carbs in an MGB?


In the skuttle like a Model T...

SJ - 1/7/23 at 09:13 PM

quote:
Originally posted by johnemms
I would have thought most bike carbs could be gravity fed..
If they flood from gravity feed - you got a problem


No idea, bit tried that, just used the pump that was designed for them

cs3tcr - 4/7/23 at 02:01 PM

Can't really help on the fuel leak, but you may want to get one of these BMC Replacement Units for the car. It works great on bike carbs:


SJ - 4/7/23 at 02:39 PM

quote:
Originally posted by cs3tcr
Can't really help on the fuel leak, but you may want to get one of these BMC Replacement Units for the car. It works great on bike carbs:




Funny you should sat that as I already have one of those in my MK. The temptation to put a bike engine in the MK and the Zetec and type 9 in the B is quite strong.

Also seen few MX5 engines and gearboxes for less than the cost of a carb rebuild kit for the SUs

Deckman001 - 4/7/23 at 03:27 PM

Just a thought, are you using a complete set of the carbs, ie: 4 of them all connected as they should be ? seeing as you have another set of them on a zetec and work correctly, how about using that set up instead to see if any part of that can be ok'd so that you can move onto the next question part of the setup?
Jason

SJ - 4/7/23 at 03:49 PM

quote:
Originally posted by Deckman001
Just a thought, are you using a complete set of the carbs, ie: 4 of them all connected as they should be ? seeing as you have another set of them on a zetec and work correctly, how about using that set up instead to see if any part of that can be ok'd so that you can move onto the next question part of the setup?
Jason


No, I'm not, just using 2 as an experiment. I have a spare set of 4 but would need to make an inlet manifold to fit 4 whereas 3 re-spaced fit on the standard manifold easily enough.

I've replaced all the seals in the SUs so just trying to get them setup now though I'm struggling to get the B series to tick over properly and can't work out if it is fuel of ignition. It was running fine when I first got it back on the road but now tickover is very erratic.

So far in addition to rebuilding the carbs I've changed every part of the ignition system and checked the compressions (all between 170-175).

I need to check the tappets next - I did them a couple of weeks ago but maybe I forgot to tighten one up.

Otherwise I'm fairly stumped at the moment.

Deckman001 - 4/7/23 at 04:41 PM

Oh ok, if your using just half the set of four carbs, the pump may be set to pump enough fuel for the full set of carbs, have you tried to block the overflow part in case it's used by the set of four carbs to fill the other carbs of the set?
I am using a set of gsxr600 carbs on my newly installed zetec, the carbs were previously used on my x/flow for the IVA test and about the next year and a half. the same setup works perfectly on both engines and I am using the std bike fuel pump fitted along side the fuel tank and it pumps the fuel from the back to the front of the locost and fills, and stops perfectly.

SJ - 4/7/23 at 06:16 PM

quote:
Originally posted by Deckman001
Oh ok, if your using just half the set of four carbs, the pump may be set to pump enough fuel for the full set of carbs, have you tried to block the overflow part in case it's used by the set of four carbs to fill the other carbs of the set?
I am using a set of gsxr600 carbs on my newly installed zetec, the carbs were previously used on my x/flow for the IVA test and about the next year and a half. the same setup works perfectly on both engines and I am using the std bike fuel pump fitted along side the fuel tank and it pumps the fuel from the back to the front of the locost and fills, and stops perfectly.


Thanks, the pressure is what matters rather than flow rate. I managed to get the carbs to seal - seems like the float level info I found on line was wrong.

I need to get it working on the SUs first then I might start playing about with the bike carbs again.

Dingz - 4/7/23 at 07:03 PM

Have you checked all around for possible air leaks, could try a heavier grade of oil in the dash pots?

SJ - 4/7/23 at 08:32 PM

quote:
Originally posted by Dingz
Have you checked all around for possible air leaks, could try a heavier grade of oil in the dash pots?


I have, and I can't find any though I probably need to look again. Doesn't seem like an air leak though. possibly leaks could be either side of the heat shield on the inlet manifold - it still has the original fibre heat shield so it's n not the best surface to get a seal, so I may take it off and test without. Only other thing is the brake servo which I can easily test by plugging the house.

Thing is the car hadn't been run for 20 years and when first started was using really well. It was after driving for a couple of days that the idling issues started.

Annoyingly my timing light seems to have died after being used a handful of times so can't check the timing now.

Thanks

Stu

SJ - 5/7/23 at 02:19 PM

Update, looks like it could be an air leak between the manifold / heat shield / carb spacer so I'll make or buy a metal heat shield and see how that goes.

The timing light is actually fine - seems like the MGB ignition system is marginal in it's ability to induce enough EMS if the timing light pick up to make the light flash. Not sure if that implies a fault in the ignition system or just dirty plugs. Every single ignition component is new though so should all be OK.

Stu

Deckman001 - 6/7/23 at 12:49 PM

Well done Stu, as you have a nearly new full ignition system, just a thought, make sure the plug gaps are correct as they would also cause an issue with a weak spark making the engine run badly.

SJ - 6/7/23 at 06:26 PM

quote:
Originally posted by Deckman001
Well done Stu, as you have a nearly new full ignition system, just a thought, make sure the plug gaps are correct as they would also cause an issue with a weak spark making the engine run badly.


Already checked that one multiple times!