20 replies to this topic

[bwi_bwi_12]

hey guys, im in some serious need of help at this point from some mini boffins.

details about the car: piper 270 cam, 10.3ish : 1 compression, standard valves, standard bore and stroke, weber DCOE 40 on cast aluminium 5" manifold, valve timing is standard (dot to dot) using non adjustable duplex chain , ignition timing is at 10' with no vac advance, ignitor electronic ignition kit. I think thats all...

the problem: it seems to be running mainly on the inner two cylinders, with cyl 4 running a bit better than cyl 1. i know this from removing various HT leads. removing cyl 1 lead does nothing to the engine, removing cyl 2 (with all others back on, obviously) causes the engine to run worse, same effect with number 3, while with cyl 4 the effect is far less dramatic but noticeable none the less. if i remove 1 and 4 simultaneously, the engine continues to run almost as it does with the leads attached.

i have considered that these two leads should be swapped, but they are in the correct place on the cap etc. i have tried swapping them but to no effect.

there seems to be no problem with the leads themselves because i have swapped leads around and the spark continues on cyl 2 & 3. also, there is spark on all four HT leads when i earth a plug on each of them. so to me it seems that i am getting the right spark, which is as strong on each lead. perhaps something wrong with the electronic ignition causing incorrect spark timing?

it seems as if there is unburnt fuel and oil coming out the exhaust. this would mean i am risking damaging my newly re-honed cylinders by washing the oil from them!

im confident that my valve timing is correct and that my tappets are adjusted correctly. they have been done twice but i will re-do them again. im considering swapping out the electronic ignition for standard ignition again to see if it has an effect. could the dot to dot method for timing the cam be incorrect? at this point im not bothered about it being a degree or whatever out...

please help! im really disappointed at the moment. ive just spent two weeks pulling the engine out and rebuilding it top to bottom with new rings etc in hope that it would be sorted out. the old rings weren't damaged and i couldnt find the problem internally, so the problem persists!

thanks in advance

[Dan]

Have you set up the carb?

[racingbob]

you got the distributer in correctly !!

[semag]

Try new plugs or swap the plugs from the two firing cylinders to one & four. Plugs can appear to fire correctly in free air but then fail when under compression. I am suggesting this after my recent experiences with my own mini, I spent two weekends trying to diagnose my very difficult to start and then running terrible engine. Finally in desperation we took the plugs from my mates car and it started and ran perfectly.

[mk1leg]

Have you got the dizzy timmed in properly......and the leads in correct order........1-3-4-2 in anti-clockwise rotation....NO:1 lead at approx 1-2o'clock....and NO: 1 cylinder is rad end..........

[bwi_bwi_12]

Have you set up the carb?

the jets have been chosen as an approximation, so it is possible that they are not 100% correct. surely the engine would still run even though it may be a bit too rich/lean? the guy who rebuilt and jetted the carb consulted a number of people regarding the spec of the engine and the sizing was done according to that. It hasnt been to the dyno. i want to sort out any other possibility before i waste time on a rolling road.

as for the distributor, how could it be wrong? as far as i can tell i have put it in correctly, along with the distributor drive shaft. it was done according to the haynes manual...

also, the plugs are all brand new. not that there was anything particularly wrong with the old ones... ive swapped them out and mixed them all arould but still no improvement. they are Champion N9YC (i think i remembered that correctly?)

forgot to mention, there is compression throughout. cyl 1: 10,5 kPa, cyl 2: 12 kPa cyl 3: 13 kPa cyl 4: 10,5kPa
the breather on the crank case is not blowing excessively, which i have been told indicates that there is not any serious blow-by past the rings. this is obviously confirmed by there being decent compression...

[bmcecosse]

10 degrees static advance is too much - you NEED a dizzy with vacuum advance. Then you can set 2 or 3 degrees static. And the 40 carb is less than ideal - the settings could be anywhere. Honestly - far better with a single HIF44 than a DCOE 40 which is too small for a 1275 engine anyway.
The compressions show the problem - why are the two end ones much lower than the centre ones ????????
And N9Ys are fine for now, but when you get it running well you will want N6Y plugs.

Edited by bmcecosse, 06 February 2012 - 10:47 PM.

[bwi_bwi_12]

Have you got the dizzy timmed in properly......and the leads in correct order........1-3-4-2 in anti-clockwise rotation....NO:1 lead at approx 1-2o'clock....and NO: 1 cylinder is rad end..........

yeah timed it with a strobe to 10'. plugs are all connected correctly in the anti-clockwise direction.

earlier i re-checked the valve clearances (all were fine... set to 0.30mm) and ive stripped and rebuilt the dizzy and replaced the electronic ignition with the original points. as far as i can tell the points arent burnt. but maybe im wrong...

[Dan]

Well a difference of 1.5 kPa is 0.2 psi so it's not much of a difference. But since 13 kPa is 1.88 psi either those results are wrong or the head isn't torqued down at all. It should be 100 times that, so assuming it is an error the difference is actually around 20 psi which is not a lot for an engine that isn't run in.

The center two cylinders in a Mini always run richer due to port robbing. It's a side effect of the way the head is designed. A Webber manifold makes this worse, which is why you use the longest one possible. If it's set quite lean it is entirely possible it would only just be able to fire on the center two and leave the outer two dangerously lean. And if it is the case that a 40 is a little small for it (I don't deal with Webbers) then the effect will be exagerated. Surely it will run? No, not at all. Carbs have to be vaguely right to start with, within a certain range of tollerance so to speak. Also, as said 10 degrees might be a little aggressive as a static timing. Try 8.

Hold on, just noticed you said you strobe timed it. That's not static timing, leave it as is for now until its running.

[bwi_bwi_12]

10 degrees static advance is too much - you NEED a dizzy with vacuum advance. Then you can set 2 or 3 degrees static. And the 40 carb is less than ideal - the settings could be anywhere. Honestly - far better with a single HIF44 than a DCOE 40 which is too small for a 1275 engine anyway.
The compressions show the problem - why are the two end ones much lower than the centre ones ????????
And N9Ys are fine for now, but when you get it running well you will want N6Y plugs.

the engine ran like crap when i had a single HS4 SU carb set up... i had a specialist select the main jet and needle but it never ran nicely. the weber, by contrast, is super responsive and very smooth. You say a 40 is less than ideal and too small? please explain...
And as for the dizzy, i was told to leave the vac unit as it is and not worry about it. should i reset my timing to be lower? around 5'?
another mechanic has said that i have plenty compression on the first cylinder (which reads the lowest) and that the difference is probably as a result of it being a newly built engine which hasnt been run in yet. they seem to say the difference in compression is not really anything to worry about. im not sure yet. i dont understand why the compression would be different? it really doesnt make sense.

[Dan]

It takes really very little difference in the physical properties of any two cylinders to get a difference of 20psi at 10.3:1.

[bwi_bwi_12]

Well a difference of 1.5 kPa is 0.2 psi so it's not much of a difference. But since 13 kPa is 1.88 psi either those results are wrong or the head isn't torqued down at all. It should be 100 times that, so assuming it is an error the difference is actually around 20 psi which is not a lot for an engine that isn't run in.

The center two cylinders in a Mini always run richer due to port robbing. It's a side effect of the way the head is designed. A Webber manifold makes this worse, which is why you use the longest one possible. If it's set quite lean it is entirely possible it would only just be able to fire on the center two and leave the outer two dangerously lean. And if it is the case that a 40 is a little small for it (I don't deal with Webbers) then the effect will be exagerated. Surely it will run? No, not at all. Carbs have to be vaguely right to start with, within a certain range of tollerance so to speak. Also, as said 10 degrees might be a little aggressive as a static timing. Try 8.

Hold on, just noticed you said you strobe timed it. That's not static timing, leave it as is for now until its running.

so even with a carb that has two chokes such as the weber, compared to a single SU with one barrel, the robbing by the inner two cylinders is still in effect? why would it be then that i have been told that it seems to be running too rich? it literally seems that unburnt fuel is passing down the exhaust port (mostly cyl 1 but also assuming cyl 4 is doing the same) carrying oil with it.

[bwi_bwi_12]

okay so can we eliminate the compression as being the fault?

[Dan]

Well just because its not rich enough to burn doesn't mean there isn't fuel there at all and it has to go somewhere, it would get blown through and it would condense and pool. From what you've said I can't see what else might be wrong. Webbers and their manifolds are worse than the SU for port robbing yes, they are also bad for fuel condensation. That's why split Webbers were developed. They are also quite clunky carbs by comparison, quite fuel thirsty and unrefined. As I've said though I don't really get to have much to do with them, I expect in the hands of an expert they can be made to sing. Many people seem to love them, it's just I'm not one of them!

Assuming you have just written the kPa results wrong here by a factor of 100 then yes, I'd say the compression is fine. Compression tests are a little pointless really in my opinion. They only ever tell you something you can already tell by other means or by elimination. But if you did a wet compression test you would probably find the small difference went away.

But just in case, and for the sake of asking, have you gapped the plugs?

[bwi_bwi_12]

the kPa i read without remembering the x100 at the bottom (silly me) and no, i havent gapped the plugs. nut im sure it has worked previously, so i cant see that as being the problem. i cant understand why it would run mainly on the inner two cylinders! i understand the whole thing about robbing and now everything about what you say about webers...