Carb conversion rough idle and misfire
Discussion
Hey guys, posted in the mini section but seems to be inactive. new to the forums so I’ll give a overview before I start my rant. Got a 1995 mini Mayfair around 2 years ago for my first car, restored it all and finally got it back to where I needed it to be. I started to learn to drive in it and straight away the spi started giving me problems, it wouldn’t start, it would cut out and had very bad hesitation when accelerating. The car has a high lift cam fitted along with a straight through decat exhaust and lcb manifold. So after all the problems with the spi I decided to do a carb conversion. I completed this about 2 weeks ago, I have a facet fuel pump delivering ~6psi to the carb which is a Nikki downdraught twin choke carb. The car runs worse than it did before frown it usually starts up fine but has a very rough idle at around 1200 rpm which I heard is about right for a mini fitted with a sportier cam, it hunts anywhere from 1100-1500 rpm no matter the temp of the engine. When just taking it easy around the car wants to stall, as soon as it gets up in the revs it’s perfect but revving it everywhere gets a bit tiring. Also the car seems to have a flat spot when accelerating. The ecu is still currently fitted to the car just running the timing although I have an A+ Dizzy to go on hopefully this weekend. Also the car overruns everytime I turn it off, dropping the clutch in 4th gear getting tiring. Any help of the problem would be hugely appreciated
Most carbs are supplied for a standard engine, so was yours jetted to take account of the mods that you made?
Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
catman said:
Most carbs are supplied for a standard engine, so was yours jetted to take account of the mods that you made?
Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
I believe the carb was taken off a standard engine but I’m not too sure, there wasn’t much of a description of it before I purchased it. Yeah I’ve done it all myself, I have a dizzy fitted but it’s still connected to the ecu controlling the timing as it was before the carb conversion, all the vacuum lines are still connected in the same way when it had the spi throttle body and manifold.Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
catman said:
Most carbs are supplied for a standard engine, so was yours jetted to take account of the mods that you made?
Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
A high idle speed will also cause an engine to run on. What cam have you put in? I don't know whether you can buy different jets for a Nikki carb, but I seem to remember that they used to suffer from icing up. I think that you would be lucky to buy a second hand one that would be right for your modified engine.Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
Your carb could also be worn, if second hand. You can have problems if the butterfly spindles are worn, for example. I used to use Weber 28/36 carbs back in the days of Mk 1 Cortinas.
They always ran beautifully and were easy to set up. You may need to have it set up on a rolling road.
Edited by catman on Monday 16th November 20:27
catman said:
A high idle speed will also cause an engine to run on. What cam have you put in? I don't know whether you can buy different jets for a Nikki carb, but I seem to remember that they used to suffer from icing up. I think that you would be lucky to buy a second hand one that would be right for your modified engine.
Your carb could also be worn, if second hand. You can have problems if the butterfly spindles are worn, for example. I used to use Weber 28/36 carbs back in the days of Mk 1 Cortinas.
They always ran beautifully and were easy to set up. You may need to have it set up on a rolling road.
I have a 264 Evo Fast Road Cam installed in the engine. Parts for the Nikki carbs seem to be almost impossible to come by, starting to regret my decision of purchasing one and wish I maybe stuck with a simple SU. The butterflies seemed to be okay when I rebuilt the carb and had little to no play in fully closed and fully open positions. I’m installing the A+ dizzy tomorrow so this will allow me to rule out any problems with the timing.Your carb could also be worn, if second hand. You can have problems if the butterfly spindles are worn, for example. I used to use Weber 28/36 carbs back in the days of Mk 1 Cortinas.
They always ran beautifully and were easy to set up. You may need to have it set up on a rolling road.
Edited by catman on Monday 16th November 20:27
oilrag1 said:
Im firstly thinking fuel pressure is too high ,from my own experience with an spi conversion the fuel pump we used was a facet cube 1.5 to 4 psi . im also assuming the brake servo(if still fitted) vacuum pipe has no leaks,and is the ignition vacuum advance working.
Yeah after looking around realised it was quite high, could this alone be causing my problems? I have a FPR but it’s not the best and ranges from 0-140psi and have read that these can give a quite large difference in the pressure set. Should I install this and see if I can get the pressure down a bit? The servo isn’t fitted at the mo so no leaks from there although will be fitted soon, at least I know that’s not leaking for now. The vacuum advance should be working fine as it is the same set up as what was in the spi just with a different manifold. Although even with the spi it seemed like it had timing issues so I’m hoping getting rid of the ecu and setting the timing manually may help me with my problemsYes, Su carbs are great and there are lots of spares around too. I agree that the fuel pressure is high, which may well give you a higher level in the carb. Didn't they have a see through panel to check?
Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Edited by catman on Wednesday 18th November 23:42
catman said:
Yes, Su carbs are great and there are lots of spares around too. I agree that the fuel pressure is high, which may well give you a higher level in the carb. Didn't they have a see through panel to check?
Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Yeah the float level in the carb is perfect, stays right on the line which is across the glass viewing window of the carb which is why I’m thinking it’s not the high fuel pressure. Okay I’ll have a look on how to set timing with a sportier cam, would this mean I need to advance the timing a bit more?Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Edited by catman on Wednesday 18th November 23:42
NMNeil said:
When you replaced the FI with the carb, did you have any electrical plugs left over?
If the PCM doesn't get an input from a sensor they will often go into 'limp home' mode, which could possibly explain some of your symptoms.
If the PCM doesn't get an input from a sensor they will often go into 'limp home' mode, which could possibly explain some of your symptoms.
Uh oh..... I had many electrical plugs left over
. I had read online though that the ecu controls the fueling and timing of the engine completely separately and only uses the vacuum from the manifold (which is still being provided to the ecu) to advance the timing. If it were in limp mode would this affect the timing in anyway?? Hoping to fit the new dizzy tomorrow so fingers crossed this will fix most of the problemsEdited by Callump on Thursday 19th November 00:08
Callump said:
Uh oh..... I had many electrical plugs left over ??. I had read online though that the ecu controls the fueling and timing of the engine completely separately and only uses the vacuum from the manifold (which is still being provided to the ecu) to advance the timing. If it were in limp mode would this affect the timing in anyway?? Hoping to fit the new dizzy tomorrow so fingers crossed this will fix most of the problems
As you have a different distributor on the way, that would remove the PCM from the list of suspects completely.
NMNeil said:
I know that most limp modes limit the RPM, and it's possible they do that by retarding the timing.
As you have a different distributor on the way, that would remove the PCM from the list of suspects completely.
Fitted the dizzy today, ready to try and start.... flat battery As you have a different distributor on the way, that would remove the PCM from the list of suspects completely.
will try and get her started tomorrow and get the timing sorted. PhilF329 said:
That’s a very high idle speed. I’ve had a lot of modified minis and never had one idle as fast as that, albeit on SU’s or Weber’s. Back to basics I think - hope you get it sorted
That’s what I was thinking, the only way it would idle was to have it that high, hopefully when I get the timing properly sorted with the A+ dizzy I’ll be able to drop the idle a bitGassing Station | Home Mechanics | Top of Page | What's New | My Stuff