Engine revs decrease when I brake

[islandcat]

I've recently had a new filler pipe fitted to my Maestro Mayfair 1.6, since then, its started to run unevenly when cold. The problem improved once the engine was warm. Its fuel consumption has increased a tremendous amount. While climbing a hill, sometimes the rev counter flicks up 1,000 revs then back down and this is then followed by the engine stuttering and 'kicking' for want of a better phrase. Also, the engine revs decrease when I brake. While the engine is cold, braking to stop for a junction stalls the engine no matter how much
choke or revs I put on. When the engine is warm, as I brake, the revs fall but not enough to stall the engine. Once I've stopped the revs return to roughly normal but still erratic. Can anyone shed light on this for me please? I've managed to get here to work but can anyone help me get home!! Thanks guys.

[E_T_V]

Two possible lines of enquiry are:

Electrical fault:
Or probably more likely since the recent work on the petrol tank,
Fuel fault:
My guess would be dirt in the carburettor and/or a blocked fuel filter. The work done to the fuel filler may well have left rust in the fuel tank leading to it being sucked through into the carburettor.

As you say fuel consumption has also increased check that your oil doesn't smell of petrol. If it does you'll need a new fuel pump (the seal may be leaking petrol into your engine). Also check that the choke part of the carb is clean i.e,., the bit inside that rotates when you pull the cable.

I'd recomend fitting a fuel filter and a complete strip and clean of the carb. You'll probably be suprised how much crud there is in there.

[SimonR]

Oh dear - Not you as well!

I've got very similar problems with my S-Series engine and as yet am unable to make it start/run properly when cold at all...

Do you find that when the engine is warm, the revs drop just to the point where you think it's about to stall and then it hesitates for a split second before idling properly again?

I'm sure others will be along with help v. soon but I'd start by looking at the rubber O-ring seals around the stepper motor barrel. You'll find the stepper motor on the right hand side of the carb (right hand, if you're sitting in the driver's seat, that is). Undo the three retaining screws and gently bend a metal bracket out of the way to withraw the motor.

These O-rings have been known to cause cold starting/running problems in the past as others will tell you. Sadly that hasn't fixed my car though :-(

Good luck and keep us posted when you fix it!

Simon R.

[G Force]

Hi there. I think the misfire you are getting as you climb up hill is most likely a low tension electrical fault as it affects your rev counter. You should look for a loose wiring connection on the ignition coil or the crankshaft sensor or the programmed ignition ecu. The wires in the back of the crank sensor plug can often break or the pins pull out due to the harness being so short and tight. If there is no apparent wiring fault the problem could be the crank sensor its self faulty or the ecu (less likely).

As for poor mpg and stalling, I take it that as you refer to giving it more choke, you have a manual choke conversion fitted. My guess is that the vacuum switch on the carburetter is faulty and causing both problems. To check it remove it from the carb and suck on the single vacuum port, if you can suck air through there then the switch needs replacing.

Cheers gary

[SimonR]

Originally posted by E_T_V
Also check that the choke part of the carb is clean i.e,., the bit inside that rotates when you pull the cable.

I might be wrong but I think this car has an auto choke - but the theory is the same (see my post above).

SR.

[islandcat]

Thanks for your responses, it looks like the fuel pump could be culprit. I'm now in the process of tracking down a new one. There is a slight smell of petrol on the dipstick and when I cranked the engine with the fuel pipe off, all that appeared was a rather half hearted dribble. After turning the engine over about 8/9 times, all that the pump put out was around 30ml. I'm pretty sure I'm right in thinking thats a bit on the low side. If I'm misguided, please tell me I've tested the vacuum switch and it seems good. I've also jiggled and tested connections breifly and all seem fine. I do have a manual choke and but the use of it at the moment is having little effect. Simon R - yes, the symptoms you describe are very similar to an ongoing problem I have. The fuel and associated vacuum system have long been a source of annoyance for me. I've not found a solution yet, but any progress I make, I'll let you know.

I will certainly clean out the carb of guk, it has been a while since I did that. I will keep you posted when I fit the fuel pump. Thanks for your help guys

[talkingcars]

Is the new filler pipe correctly sealed at both ends?

Was the tank removed for fitting?

James

[H48HPE]

Could the engine stalling on braking be an vacumm leak from the servo when the brakes are opperated?

Andy

[islandcat]

I've now replaced the fuel pump which has improved matters but not solved them. The fuel tank was removed when the
filler pipe was fitted so, after ruling out the fuel pump, the next task is to clean the carb. Bizarrely, the revs
dropping when I brake is now far less noticable. When starting from cold, it fired ok and with choke fully out
went straight to 3,000 revs for about 3 seconds. It then just died, and no amount of revving had the slightest effect.
This happened several times, and then at each junction. As the engine warmed up, it would idle but very lumpy.
Pulling up hills was a task, very little power at low revs, would only make progress if revs were kept around 2,500.
Putting more gas on only had the effect of seeming to choke the engine. I will definitely take your advice and check
the jets in the carb but will also check out the vacuum system. Can anyone give any advice on how to find a
leak in the brake servo?

[E_T_V]

I assume you mean a vacuum leak which could cause problems with the carburation.

A simple test is after leaving the car stand over night. Press the brake pedal. It should feel normal until about the third press where it runs out of vacuum assistance and it will become much harder to press. (Sometimes if you listen carefully you can hear the vacuum working) If it keeps the vacuum overnight then there is unlikely to be a vacuum leak. If it feels hard straight away to press the brake pedal then it could be a vacuum leak.

If a new fuel pump has helped it could well be dirt in the carb (the new pump will have more pressure than the old one so will reduce the problem as you described) It could also be a blockage at the fuel tank end on the fuel pickup but that is a lot messier to try and fix, so I'd have a go at doing the carb first.

[Wonko_The_Sane]

"Do you find that when the engine is warm, the revs drop just to the point where you think it's about to stall and then it hesitates for a split second before idling properly again? "

Yes, I get that, on a '91 S series. Frequently. My fuel use has also increased. I find it MUCH worse when using the heater blower...if I kill it before I brake, it's OK..if I kill it when it's doing it, it settles..

I know the loading of the alternator may contribute..but I did wonder about a power fault to be honest, affecting the PIC. When it does it's "stagger" act, if I turn it off and restart, it's immediatly fine again.

I just put it down to old age..

[E_T_V]

Could well be sticking O rings on auto chokes too as the voltage drop from heater and brake lights could exacerbate (ooh big word for me there!) a stepper motor that finds it hard to adjust the fueling because of sticky or swollen O rings.

Most cars revs dip a little when you brake because simultaneously you are adding a fairly hefty electrical load, upsetting the vacuum in the inlet manifold (as the servo needs "refilling") and also the fuel sloshes to the front of the carb altering the mixture.

[G Force]

Originally posted by islandcat
When starting from cold, it fired ok and with choke fully out
went straight to 3,000 revs for about 3 seconds. It then just died, and no amount of revving had the slightest effect.
This happened several times, and then at each junction.

Hi this symptom and some of your other symptoms, points to the ORFCO valve stuck open. Also when you strip the carb check you did'nt damage the smallest o ring on the choke when you fit the manual choke, I have seen quite a few that have and the slither of rubber always ends up in the main jet. How has the rev counter been behaving since you wiggled the wires on the ignition?

Cheers Gary

[islandcat]

I have manual choke conversion on the carb as the auto choke gave endless trouble. The last time I had trouble with the fuel delivery, just trying to get the choke mechanism back onto the carb showed up a piece of bad machining on the block that cut the O rings to shreds. A new packet of O rings has been purchased in anticipation I have an overhauled carb ready for swapping at the weekend. Thanks G Force for your suggestion - I'll check that valve out on the existing carb.

A temporary fix has been affected by the use of 2 fuses and some duck tape. I've removed the vacuum switch, blocked up the 2 channels into the carb with fuses and sealed the vacuum tube. This has resulted in a car that is far more drivable than before, unhappy but drivable

Since I've jiggled the wires on the ignition, the rev counter hasn't given a flicker like that one that I had when trying to climb the hill on the first day. I'm guessing theres a connection in there somewhere that needs attention.