Revs increase when brakes are applied

[jpjeep26uk]

Hi all
My revs have started to go up when I touch the brake pedal or when I stop with the brake and then pull away.
I've read a lot of things but mainly that it's the Brake Booster, probably got an air leak or the diaphragm is busted inside it.

If that's so, then i'll buy an new one and fit it.

But, and here's where I need your help, can it also be related to the Carburettor? I had 32/36 Weber clone put in to replace the EFI.
I've never been able to get it set right and although she runs really well, I have a lot of issues with cold starts, crappy choke mechanism, high idle, normal idle, what have you.
So frequent tweak with a screwdriver has been the only solution.
1. Could playing with the Carb settings affect the brake booster?
2. Anyone out there any good with carbs and can tell my what to do if I send a pic of the carb and point at the bits that I don't understand?
I know how to baseline the carb but within days it needs tweaking and I'm worried I've screwed the brake booster by playing around.

I've played around with this screw as I read it was the choke high idle adjuster???
As usual, any help would be hugely appreciated - thanks.

 

[Gilaguy23]

There is nothing you did or can do with the carb fiddling that damaged your booster. Id suggest disconnecting the booster vac line, plugging the intake and while sitting hit the brake pedal and see if you have the same thing go on. I drove my 73 FJ 40 for while with the booster unhooked because of a massive vac leak. It was not much fun and unsafe as you really have to smash the pedal to get any brake action. The basic idea on idle mix settings is to obtain the highest vac reading when turning the mix screw. Im no Webber expert but remember that single car type webber being a PITA that was OEM on one of my Ducatis. Once warmed up, everything normal,fuel pressure etc, there is no reason to have to be readjusting it all the time.

 

[jpjeep26uk]

OK, an update on the imaginary 'carb affecting the brakes' issue!

Here are my findings:
Engine warmed. Parked and idling at 1000RPM.
Took the hose out of the Booster and taped/blocked up the booster hole.
Taped up the end of the hose (it was sucking air like a dyson)
Brakes immediately felt firmer/ solid.
No 'air' sound when pressing on the brake pedal.
No change to revs.

Unblocked the booster hole.
The revs crept up by 200rpm and held but brakes still firm on the pedal and no rev increase when applying the brake pedal.

Plugged the hose back into the booster.
Revs went back down by 200rpms to 1000rpms and remained stable.
Revs went up 400rpm when brake pedal applied.

Very odd.

Came home after 30 mile drive and oddly, revs now don't go up when applying the brake pedal.

I can't explain it.
Maybe you folks have some advice??

 

[jpjeep26uk]

There is nothing you did or can do with the carb fiddling that damaged your booster. Id suggest disconnecting the booster vac line, plugging the intake and while sitting hit the brake pedal and see if you have the same thing go on. I drove my 73 FJ 40 for while with the booster unhooked because of a massive vac leak. It was not much fun and unsafe as you really have to smash the pedal to get any brake action. The basic idea on idle mix settings is to obtain the highest vac reading when turning the mix screw. Im no Webber expert but remember that single car type webber being a PITA that was OEM on one of my Ducatis. Once warmed up, everything normal,fuel pressure etc, there is no reason to have to be readjusting it all the time.

Hi Gila, don't get me started on my d4Mned Carb.
It's a cheap weber clone 32/36 I believe.
No matter how I tweak it I can never and I repeat NEVER just set it a go for week son end.
Good example was yesterday, running fine for 15 miles, arrived in the local town and the revs were dropping to below 500 and on the edge of stalling.
Eventually had to pull over and adjust the idling screw to make it sit on idle at 1000rpms which is the least vibrational idle speed i can find.
Drove home some time later about 15 miles, running nicely and lo! the revs are back up to 15/1600 on idle and at lights or junctions.
It never just gets set to idle at 1000rpm and stays there for weeks or months on end.
I tweak the idle screw, the mixture screw, the fact idle screw and there is a screw under the manual choke that seems to affct things too but not once have i found the 'right' setting and left it alone.
I'm thinking of paying a carb specialist to come out and set to it to base levels and then build from there!

A real PITA as you say.

 

[Torched]

Hi Gila, don't get me started on my d4Mned Carb.
It's a cheap weber clone 32/36 I believe.
No matter how I tweak it I can never and I repeat NEVER just set it a go for week son end.
Good example was yesterday, running fine for 15 miles, arrived in the local town and the revs were dropping to below 500 and on the edge of stalling.
Eventually had to pull over and adjust the idling screw to make it sit on idle at 1000rpms which is the least vibrational idle speed i can find.
Drove home some time later about 15 miles, running nicely and lo! the revs are back up to 15/1600 on idle and at lights or junctions.
It never just gets set to idle at 1000rpm and stays there for weeks or months on end.
I tweak the idle screw, the mixture screw, the fact idle screw and there is a screw under the manual choke that seems to affct things too but not once have i found the 'right' setting and left it alone.
I'm thinking of paying a carb specialist to come out and set to it to base levels and then build from there!

A real PITA as you say.

Is there a non cloned carb available? Just interested in this project and wonder if it is a cost thing or they don’t make them.

 

[jpjeep26uk]

Evening Gila, I think there is a 'proper' carb that can be bought but to be honest, I gave the Jeep to a Jeep Guy up country when I was ill in Nov 2024 and he told me the ECU was busted and the EFI was shot as a result of that, so a Carb was the only way to make the Jeep drivable again. So I told him to fire away and he ripped the EFI out and put the Carb in. To be fair, it fixed the problem but I've had problems with it since I picked it up in April last year and have also thought about replacing it with a 'proper' weber carb. I need to do some homework on the carb scene as I have no idea on what is a good one or how to fit it or tweak it.
Here are some snaps of mine ( without the filter top on)