Carb conversion rough idle and misfire

Carb conversion rough idle and misfire

Author
Discussion

catman

2,490 posts

174 months

Saturday 21st November 2020
quotequote all
Yes, the carb adjustment should be the last thing to do. You're wasting your time if the timing's out. Too much advance could make it run on too. The new cam may require different tappet clearances from standard to run properly, as well, if you haven't done that already.

Because of the standard jetting in your carb and the mods, you may not get it to run perfectly, what ever you do. if it doesn't run how you want, maybe revert to an SU set up. There are a lot of them around and you should be able to get advice on the correct jetting for your application.

Darkslider

3,071 posts

188 months

Saturday 21st November 2020
quotequote all
Lotobear said:
Little Pete said:
Did you fit the camshaft? Is it timed up correctly?
...my first thought too
Think you should go back to basics on this one, check compression hot and cold, valve clearances (if they're adjustable) timing, fuel pressure at the carb, spark strength and condition (accuspark make a set of testers you can bridge the cap to plug with to eyeball while you're revving the engine) exhaust rich or lean? (Presuming no lambda on these despite the SPI?)

Too much chance of you chasing a more and more obscure seeming fault because you've overlooked something basic that you weren't to know as the car obviously has an interesting past!

Callump

Original Poster:

24 posts

40 months

Saturday 21st November 2020
quotequote all
Darkslider said:
Think you should go back to basics on this one, check compression hot and cold, valve clearances (if they're adjustable) timing, fuel pressure at the carb, spark strength and condition (accuspark make a set of testers you can bridge the cap to plug with to eyeball while you're revving the engine) exhaust rich or lean? (Presuming no lambda on these despite the SPI?)

Too much chance of you chasing a more and more obscure seeming fault because you've overlooked something basic that you weren't to know as the car obviously has an interesting past!
Cheers for all the help guys pretty much sorted now car was running earlier but timing is a bit off, just done to the point where it’ll run atm. Compression i know is fine as car was running a few weeks back. Valve clearances were set maybe a month or two ago when rebuilding the engine. Timing is the thing I need to set now. Pressure is ~6psi which is too much from what I’ve heared although the fuel fills to the float line and stops perfectly, is this okay or could a too high pressure do any other things than force the float open? Spark seems quite strong took all four out yesterday and checked. Spark plugs seemed perfect Sandy colour although 2 were quite white on parts of the spark plug so possibly a bit hot??

Darkslider

3,071 posts

188 months

Saturday 21st November 2020
quotequote all
To be honest I don't think 6 psi would be enough to overcome the spring on the metering valve, but as a test you could pressurise the carb inlet up and see whether it holds or not. I've got a mityvac pump that's great for stuff like this well worth having. Though by the sound of the plugs it might be running a little lean possibly standard jetting with the high lift can/high flow air filter and exhaust to blame?

PhilF329

235 posts

237 months

Saturday 21st November 2020
quotequote all
I would start from scratch on this just to make sure you don’t miss something, as you could end up chasing your tail and spending money on parts you don’t need. I’d check compression, cam timing, valve clearances, static timing, ignition etc before moving on to fuelling.

catman

2,490 posts

174 months

Saturday 21st November 2020
quotequote all
You may find that if the timing was too advanced, that could cause the overheated plugs. The standard jetting in the carb could also be a factor, as mentioned previously.

NMNeil

5,860 posts

49 months

Sunday 22nd November 2020
quotequote all
High idle and white spark plugs (lean mixture), is often the indication of a vacuum leak.

Callump

Original Poster:

24 posts

40 months

Sunday 22nd November 2020
quotequote all
Did some more work today, no luck, the car will start after a while but cuts out straight away and is definitely not firing on all 4 cylinders despite having strong spark in all 4. I checked out my dizzy and I think it has a 45d point in it, but the dizzy is a 59d dizzy, I think this could be causing my problems.... I checked out the electronic ignition dizzy after this and it was gross inside, rotor arm and points were badly visible under all the cr*p on top of them. I’m thinking I’m going to put the ecu back in, clean up the electronic ignition dizzy and see how it runs as it’s fairly easy to install. I have read online that the timing and fueling on the ecu are not linked in anyway, so not having the fi throttle body connect won’t throw any issues with timing from the ecu. I will try and get either a 59d dizzy rebuild kit or buy another dizzy with the correct points and try to set it all up without the ecu soon but due to it atleast being close to driveable before when the ecu was controlling the timing I’m going to use this in the meantime, hopefully how gunked up the dizzy was was the main cause of my problems. Will report back tomorrow with my findings.

MB140

4,027 posts

102 months

Sunday 22nd November 2020
quotequote all
Erm your post at 19:16 yesterday implies it was running okay and the fault was fixed. Which is it.

Or has it started having problems again since yesterday.

Callump said:
Darkslider said:
Think you should go back to basics on this one, check compression hot and cold, valve clearances (if they're adjustable) timing, fuel pressure at the carb, spark strength and condition (accuspark make a set of testers you can bridge the cap to plug with to eyeball while you're revving the engine) exhaust rich or lean? (Presuming no lambda on these despite the SPI?)

Too much chance of you chasing a more and more obscure seeming fault because you've overlooked something basic that you weren't to know as the car obviously has an interesting past!
Cheers for all the help guys pretty much sorted now car was running earlier but timing is a bit off, just done to the point where it’ll run atm. Compression i know is fine as car was running a few weeks back. Valve clearances were set maybe a month or two ago when rebuilding the engine. Timing is the thing I need to set now. Pressure is ~6psi which is too much from what I’ve heared although the fuel fills to the float line and stops perfectly, is this okay or could a too high pressure do any other things than force the float open? Spark seems quite strong took all four out yesterday and checked. Spark plugs seemed perfect Sandy colour although 2 were quite white on parts of the spark plug so possibly a bit hot??

Callump

Original Poster:

24 posts

40 months

Sunday 22nd November 2020
quotequote all
MB140 said:
Erm your post at 19:16 yesterday implies it was running okay and the fault was fixed. Which is it.

Or has it started having problems again since yesterday.
It was running yesterday same as today, but yesterday I got it running to the point where it sounded normal and then it went bad again, I’m not too sure if the points moved in the dizzy or what but it was cranking, fired up nicely for maybe 5 seconds then cut out and wouldn’t really start again. So I thought it was just something I have to mess about with a bit now to get it all good but after finding it has the wrong points in the dizzy I think it would be better to wait and get all the correct parts. Please correct me if I’m wrong and if the 45d dizzy points can be used in the 59d dizzy but they just need a bit of tweaking, any help to get it running properly now smile

GreenV8S

30,149 posts

283 months

Monday 23rd November 2020
quotequote all
Does it have a vac advance on the dizzy? If so, confirm it's connected to the base plate properly. If that has come adrift you will have random and inconsistent timing.

Callump

Original Poster:

24 posts

40 months

Monday 23rd November 2020
quotequote all
GreenV8S said:
Does it have a vac advance on the dizzy? If so, confirm it's connected to the base plate properly. If that has come adrift you will have random and inconsistent timing.
All the vac lines were connected securely, I tried to check it was working by sucking on the advance and the plate shifted, I didn’t really know any ways to test it. The plate inside was all secure so I assumed it was okay.

GreenV8S

30,149 posts

283 months

Monday 23rd November 2020
quotequote all
Callump said:
I tried to check it was working by sucking on the advance and the plate shifted,
That's OK then.

Callump

Original Poster:

24 posts

40 months

Monday 23rd November 2020
quotequote all
Okay, cleaned up the electronic dizzy today, fitted, fired straight up no issues running great. Was idiling a bit high so took it down to maybe 1200 which is still high but it hunts quite a lot. Also when accelerating it pulls to maybe 3000 rpm then there seems to be no power after this point. Any suggestions on what my next move should be? I’m still going to fit an A+ dizzy eventually, but anything I can check out in the mean time to help sort the hunting idle and lack of power would be great. Cheers guys

catman

2,490 posts

174 months

Monday 23rd November 2020
quotequote all
Hi, regarding the idle, are you adjusting the mixture screw as well as the throttle adjustment?
The loss of power higher up could be many things, including having the wrong jets in the carb, incorrect timing for the new cam, incorrect advance for the new cam.

Fairly basic, but have you checked how much the carb opens when you floor the gas pedal? How does the engine actually behave higher up?

Callump

Original Poster:

24 posts

40 months

Monday 23rd November 2020
quotequote all
catman said:
Hi, regarding the idle, are you adjusting the mixture screw as well as the throttle adjustment?
The loss of power higher up could be many things, including having the wrong jets in the carb, incorrect timing for the new cam, incorrect advance for the new cam.

Fairly basic, but have you checked how much the carb opens when you floor the gas pedal? How does the engine actually behave higher up?
I’m only adjusting the idle screw, should I be doing both? My colour tune got delivered the other day so I’m going to get that on ASAP and see how it’s looking. Problem is spare parts for the Nikki carbs are hard to come by and the timing is all controlled by the ecu frown does anyone know anywhere I can get some Nikki jets or would any other jet work aslong as its the correct size? When the throttle is fully open both chokes on the carb are fully open. It just seems to have no power, like after say 3000 rpm I put the clutch in by carry on building the rpms of the engine, but less noticeable.

Edited by Callump on Monday 23 November 17:26

Callump

Original Poster:

24 posts

40 months

Monday 23rd November 2020
quotequote all
One thing I found on the drive is that when coming to a stop the car will continue to drop rpms sometimes untill it stalls itself, sometimes it drops to say 1000 and then picks back up and idles fine

anonymous-user

53 months

Monday 23rd November 2020
quotequote all
Callump said:
One thing I found on the drive is that when coming to a stop the car will continue to drop rpms sometimes until it stalls itself, sometimes it drops to say 1000 and then picks back up and idles fine
1-The car has a high lift cam fitted / check
2-Nikki downdraught twin choke carb / check
3-The car runs worse than it did before / check

1+2=3 every time!

Your answer is here:-
Tuning the A-Series Engine: The Definitive Manual on Tuning for Performance or Economy by Vizard, David (1999)

If the cam was designed for an SPI mini then it will never work properly on a carb setup. To be fair SPI shouldn't work either but they did some very clever things to overcome the intake port design issues. David covers this (and much more) in the book. You might get away with retarding the cam but it depends on the overlap you have.

catman

2,490 posts

174 months

Monday 23rd November 2020
quotequote all
Getting the idle right can only be done by using the mixture screw and the throttle stop screw. When the engine hunts, slowly turn the mixture screw in or out until it runs smoothly, then slow the engine down with the throttle stop screw. Keep repeating until you get a steady idle at sensible revs.

As mentioned though, you have put several parts on that don't really match. You may spend hours trying to find a missing fault, but it may just be that the parts fitted don't work together. It may be worth talking to a Mini specialist, to find the best way to achieve what you want.

Callump

Original Poster:

24 posts

40 months

Monday 23rd November 2020
quotequote all
catman said:
Getting the idle right can only be done by using the mixture screw and the throttle stop screw. When the engine hunts, slowly turn the mixture screw in or out until it runs smoothly, then slow the engine down with the throttle stop screw. Keep repeating until you get a steady idle at sensible revs.

As mentioned though, you have put several parts on that don't really match. You may spend hours trying to find a missing fault, but it may just be that the parts fitted don't work together. It may be worth talking to a Mini specialist, to find the best way to achieve what you want.
Will do this tomorrow thank you, sure I’m pretty sure the cam was for a carb’d engine as I was planning to do a carb conversion as soon as my car was running due to all the things I had heard about the spi. Sometimes a tight budget comes back to bite you in the a*se. Although this is my first car and I haven’t even passed my test yet, as much as i want it to be running the best I can get it I’m not looking to get any crazy figures from the car just want a fun (and hopefully reliable) car.

Edited by Callump on Monday 23 November 21:09