15 replies to this topic

[Jacko-lah]

Mayfair 998
Year: 1988
Description of problem (please be as in depth as possible):

Okay :

1) Starts Perfectly
2) Revs in Idle perfectly
3) Points are clean and gapped.
4) Spark plugs a little sooty, (checked after running with choke for a couple of mins, but gap is fine and no wear.
5) As you accelerate away it develops what appears to be a flat spot at about 2,500 RPM.
6) If you put foot on accelerator it immediately misfires
7) If you are gentle you can keep the majority of the misfire at bay, but it still appears to be misfiring.
8) Won't go much above 3,500 RPM without misfire.
9) There is fuel in float bowl, and I ran it with top off and it goes down.
10) At the same time I sent fuel from fuel pump into a container and it certainly delivers fuel. (ie fills container much faster than the reservoir goes down.
11) I found a manufacturing fault with the newish dizzy cap so replaced with old one.
12 ) I found the HT lead to coil wasn't far enough into coil.
13) I've attempted to check timing with strobe leant by my brother inlaw but it's dire. Hardly able to see a bloody thing even in dark garage. But Appears to be about 8 deg with vacumm disconnected.
14) Vacumm advance definately works (checked by sucking)
15) Noticed the Exhaust to manifold joint was leaking, had a go at sorting it, but there's still a bit of a leak.

Coil not checked yet
Carb not disassembled yet

ANY Ideas what next to check ?

Wife saying "take it to a garage"


Any non-standard parts that might be involved with the problem? Unleaded head ?

Edited by Jacko-lah, 11 April 2011 - 02:32 PM.

[tieclubbyestate]

Mayfair 998
Year: 1988
Description of problem (please be as in depth as possible):

Okay :

1) Starts Perfectly
2) Revs in Idle perfectly
3) Points are clean and gapped.
4) Spark plugs a little sooty, (checked after running with choke for a couple of mins, but gap is fine and no wear.
5) As you accelerate away it develops what appears to be a flat spot at about 2,500 RPM.
6) If you put foot on accelerator it immediately misfires
7) If you are gentle you can keep the majority of the misfire at bay, but it still appears to be misfiring.
8) Won't go much above 3,500 RPM without misfire.
9) There is fuel in float bowl, and I ran it with top off and it goes down.
10) At the same time I sent fuel from fuel pump into a container and it certainly delivers fuel. (ie fills container much faster than the reservoir goes down.
11) I found a manufacturing fault with the newish dizzy cap so replaced with old one.
12 ) I found the HT lead to coil wasn't far enough into coil.
13) I've attempted to check timing with strobe leant by my brother inlaw but it's dire. Hardly able to see a bloody thing even in dark garage. But Appears to be about 8 deg with vacumm disconnected.
14) Vacumm advance definately works (checked by sucking)
15) Noticed the Exhaust to manifold joint was leaking, had a go at sorting it, but there's still a bit of a leak.

Coil not checked yet
Carb not disassembled yet

ANY Ideas what next to check ?

Wife saying "take it to a garage"


Any non-standard parts that might be involved with the problem? Unleaded head ?

sounds like your dizzy is in the wrong place

[Jacko-lah]

sounds like your dizzy is in the wrong place

Where should it be ?

[tieclubbyestate]

sounds like your dizzy is in the wrong place

Where should it be ?

i dunno i think i have the same problem :/

[dklawson]

Your description and list are excellent. Thanks for being thorough and descriptive

There are only a couple of things I can suggest that you have not already tried.
1) If you have not replaced or cleaned the air filter, by all means give that a try. I have a friend whose Mini would misfire and would not go over 45 MPH. It turned out to be a very blocked/dirty K&N air filter.
2) You have worked on a lot of the ignition system, but consider taking the dizzy out and removing the breaker plate and examining the bob weights and springs underneath. If the springs are broken, bob weights are frozen in place, OR the 4-lobe dizzy shaft cam is not free to pivot on the dizzy spindle, the timing will not advance. Under load/speed this may cause all sorts of running problems.
3) If you have not replaced the condenser, I would try that. I rarely suggest replacing ignition coils. However, if you can BORROW a known good one I certainly would consider trying a loaner.
4) At night time, start the engine with the car in a darkened area. Lift the bonnet and look to see if there are any sparks or other electrical effects visible around the plug wires.
Regardless of what your instincts are, change only ONE item at a time and watch for the effect of the change.

Is this change in your engine sudden or did it come on slowly? Are there other items you have worked on recently... even seemingly unrelated items?

[Jacko-lah]

Your description and list are excellent. Thanks for being thorough and descriptive

There are only a couple of things I can suggest that you have not already tried.
1) If you have not replaced or cleaned the air filter, by all means give that a try. I have a friend whose Mini would misfire and would not go over 45 MPH. It turned out to be a very blocked/dirty K&N air filter.
2) You have worked on a lot of the ignition system, but consider taking the dizzy out and removing the breaker plate and examining the bob weights and springs underneath. If the springs are broken, bob weights are frozen in place, OR the 4-lobe dizzy shaft cam is not free to pivot on the dizzy spindle, the timing will not advance. Under load/speed this may cause all sorts of running problems.
3) If you have not replaced the condenser, I would try that. I rarely suggest replacing ignition coils. However, if you can BORROW a known good one I certainly would consider trying a loaner.
4) At night time, start the engine with the car in a darkened area. Lift the bonnet and look to see if there are any sparks or other electrical effects visible around the plug wires.
Regardless of what your instincts are, change only ONE item at a time and watch for the effect of the change.

Is this change in your engine sudden or did it come on slowly? Are there other items you have worked on recently... even seemingly unrelated items?

Tomorrow, I'm going to try a new set of points - these are old ones that have been cleaned up and regapped more than once. Condensor was new about 1500 miles ago, but for the price I'll get another one.

[liirge]

Are you sure its firing on all four cylinders? sounds rather similar to when my Moke refused to run on 4.

If you are sure it is running on 4 then maybe the valve clearances need checking.

Edited by liirge, 11 April 2011 - 04:59 PM.

[Jacko-lah]

Are you sure its firing on all four cylinders? sounds rather similar to when my Moke refused to run on 4.

If you are sure it is running on 4 then maybe the valve clearances need checking.

Do you know I was waiting til I got it running okay before doing that. DOH.

[liirge]

Yer it might be worth checking them, you have to do it with the engine cold (well you don't but most people do) and then set them to the clearances specified in your haynes...off hand i believe that it is 12 thou, some people like to put the exhaust valves at 15 thou, and if your running roller rockers and/or uprated cam 15thou across inlet and exhaust valve.

[backstreetclassics]

does the carb have oil in the dash pot mine ran low causing it to missfire under load

[Jacko-lah]

Okay, Just took the air filter out and filled up the dash pot.

For a few moments I thought I'd almost cracked it. Drove up the bypass, and all seemed okay, and I was thinking - just sort the exhaust leak and get a ne air filter and adjust the tappets and we'll be sorted, but then it developed the same symptoms all over again.

[liirge]

seems to be as it gets warm....i think clearances are the next step matey

[Jacko-lah]

Yesterday I managed to fix it


1) Repaired the leaking exhaust to manifold joint with GUM and Tape under the clamp.
2) Reset the tappets (never did them when I serviced it. Why ? lazyness?)
3) NEW points and condensor.
4) New Coil.
5) New Plugs which are a slightly different grade to that fitted but I went with what the Unipart/partco man said matched my Reg Number. Was ngk bp8es now ngk bp6es.
6) Refitted OLD Dizzy Cap and LUCAS rotor arm - noticed a manufacturing fault on the MOSS supplied cap.
7) Set the timing the Dave Sexton way - by ear. Does it sound right at idle, does it sound right when reved. them checked with CRAP strobe that you can hardly see
8) 20/40 in the Dash pot

All sorted now - works a treat, will rev to the red line in 2nd without a hiccup. Will pull from 1,500 rpm in 4th to erm 70 mph with no flat spots. Then went to Southport and back to buy a McD's and make sure it's all sorted.

Now sure which of the 8 things above finally did the trick as I did them all at once, but I'm happy again

[haggz]

i know its a bit late to add to this but i had same problem. ended up being the coil after i had changed everything else.

[glyn]

glad you sorted it, but Derbyshire to southport for a McD's