Help! engine spitting and backfiring.

[williamraff]

Hi all, I am in need of further guidance to keep my ailing 950 running. It has never been particularly smooth and has had various intermittent misfires but now it will barely run when warm and spits back through the carb and backfires through the exhaust, alhough it runs a little better whilst cold. Is there a quick rule of thumb way of adjusting the mixture on the carb and doing a static timing check as I do not have a dwell / multimeter. I am looking to check all the peripheral things first before resorting to taking the head off.
Many thanks ,Will.

[PSL184]

Roughly set the mixture by turning the mixture nut fully upwards and then winding down 2 full turns. To set timing, with engine running, slowly swing the dizzy until you get the best (smoothest/fastest) idle. Then set the idle speed screw to give the correct idle. I would suggest that you fit new plugs, points and condensor before doing this and make sure you gap the points correctly. If it still runs rough after all this you need to adjust the valve clearances which is easy to do and described in the workshop manual in detail. Good luck - The A series is very reliable and tolerant and usually just needs a good service to get running nicely.

[williamraff]

Many thanks ,I will get on to Bull Motif tomorrow. Assumingal the problem persists, are we loking at a burnt out valve / head gasket trouble? I just want to be prepared for the worst.
Cheers, Will.

[PSL184]

Yes possibly - Head gasket is cheap enough and easy to change so if you are ordering parts stick one on your list (along with a dizzy cap and leads). With the head off you can asses the valve condition - they may just need grinding in to reseat them. Don't worry about the "worst" as there is nothing that can cost a huge amount to fix - just a bit of your time

[williamraff]

Affirmative, I will do just that. Hopefully that will do the trick.I will advise of progress and thanks again - excellent forum!
Will.

[bmcecosse]

I would start with checking/re-setting the valve gaps! Also - a compression check would be useful - even if it's just with the Starting handle. Just turn the engine over (ign off) on the handle - you should feel 4 'even' pulses as you turn - if one is 'easy' - then that suggests a problem - probably with a burned valve. If 2 are 'easy' then possible head gasket blown between two cylinders - but this results in a car that can barely move.

[williamraff]

Good thinking, I have a compression tester, I will make that my first job but I will order the pats from Bull Motif anyway. It wont hurt to replace the service items at the same time.
Thanks again, Will.

[bmcecosse]

I'm sure you can get the parts you need locally! Otley is not exactly remote - have a mate who lives in Ilkley, was heavily into Minors at one point and had a large box of spares - which he gave to me! Compression tester is ideal - hot engine, fully charged battery, all plugs out, throttle held partly open - crank it till the reading steadies. Repeat for each cylinder. Ideally - you are looking for ~ 160 psi on a standard engine, but anything over 140 is ok. Anything less than 100 is a problem!

[autolycus]

Compression tester is ideal - hot engine, fully charged battery, all plugs out, throttle held partly open -

Why only partly open? Surely any throttling of the air flow will result in an indeterminate pressure at the start of the compression stroke, and therefore an unpredictable final pressure? I always go for a fully open throttle, so although you won't get complete charging, at least it will be fairly consistent from cylinder to cylinder and engine to engine.

Kevin

[bmcecosse]

Fully open if you wish! But the amount of air being sucked in at cranking speed will easily get past a part open thottle. The point is - it should be OPEN - not sitting in the idle position!

[williamraff]

Hi, just a quick note to advise of progress. Valve clearances checked and adjusted, points removed and cleaned, condensor changed, rotor cap and leads cleaned,mixture set and timing advanced to smoothest / highest idle,vacuum pipe renewed, compression test reads 150 psi on all cylinders.
The engine is transformed to such an extent that I have splashed out on a duplex timing set and all new service items.
It is difficult to know if mixture and timing are correct without appropriate equipment but the engine starts first time and has a smooth steady idle and picks up the revs well. I have not been on the road yet so may need to fine tune and I have my doubts about the distributor advance mechanism -not quite sure how to test this but it does not seem to make much difference whether the advance pipe is connected or not.
The spark plugs are very sooty and soot up straight away after being cleaned - not too sure on that one, does turning the jet screw down weaken or enrichen the mixture?
Anyway, I am pleased with the results so far and would like to express my thanks for all advice offered.
All the best,Will.

[MarkyB]

Turning the jet adjuster down make the mixture richer.
Wind it up a bit then try the lifting pin under the carb piston.
The ideal if if the engine revs rise a little but then settle down.
If they just rise the mixture is still too rich.

[mogbob]

Will
The plugs "sooting up" are an indication of an " over rich " mixture setting, wasting fuel.

With the distribuor cap off ( and ignition off ) you can see the movement of the base plate if you disconnect the vacuum pipe at the carb end and suck.The tensioning spring and base plate will move.....if it is working.
( At " tick over " nothing moves..it's only at high revs that the vacuum advance works. )

With access/ use of a timing light you can see the timing "move" as the vacuum advances in step with increased revs of the engine.

Bob

[bmcecosse]

Compressions are good for a standard 948 - no worries there. At 'tick-over' there is reasonable manifold vacuum - and the vacuum advance DOES move, pulling the advance up to give a nice idle but with low static advance for easy starting. But anyway - suck the pipe and see if anything moves! If you can suck air through the pipe - the diaphragm is bust - you need a new unit. Does sound like good progress though - wouldn't bother with the Duplex timing unless going for super high lift cam and/or super strong valve springs! As for the plugs - after a good hard run the plugs should be light tan/beige colour.

[williamraff]

Oh dear I have already bought the duplex kit. I thought this was the best way to cure a noisy timing chain and stop it from becoming noisy again. Am I incorrect in this assumption? If so I could have saved myself £25.00
Anyway, I will check the advance as described and try a leaner mixture and see how she pulls.
One thing I am slightly worrried about is the oil pressure; I have been checking a few of the posts and technical guides on here and it seems that my pressure of 25 -30psi (hot tickover) (45 -50psi) at speed is not within parameters for a healthy engine; should I be worried?
Thanks,Will.

[bmcecosse]

That oil pressure is ok - not perfect, but not desperate either. Are you using 20W50 oil ? Since you have the duplex - you had better fit it! You need to countersink the two little holes holding the front plate to the front main bearing housing - should be possible in the car, although may be easier if you take the whole grille panel off to get decent access.

[williamraff]

Good, I will put some decent 20/50 in and see how we go. I am not sure what is in it at the moment but it does not look very good. I do have an allegedly good 1098cc supplied with the car but I think that will be a job for next Autumn / Winter and I beleive the duplex will transfer directly on to this anyway.
On another matter, I recently bid and won a hif 38 carb on ebay for £1.24! once stripped, cleaned and jetted would ths be a good match to a heated MG / metro inlet manifold? there are currently a couple of these on ebay at the moment.
Many thanks again -Will.

[Stig]

On another matter, I recently bid and won a hif 38 carb on ebay for £1.24! once stripped, cleaned and jetted would this be a good match to a heated MG / metro inlet manifold? there are currently a couple of these on ebay at the moment.

Indeed it would. That's what's on my (1098) engine. You got a bargain there, I bet the manifold will cost a fair bit more! Did you get a Metro air filter and elbow too, or are you going to fit a K&N?

[bmcecosse]

Don't bother with the HIF38 on that 948 engine - keep it (and the duplex) for the 1098 engine! ASDA 20W50 is plenty good enough for a Minor engine - don't waste money on fancy oil! MUCH more important tis to make sure the oil filter is doing a job - fit a new one, and make sure the support plate and spring are present and correct - they are often thrown away with the old filter.......

[williamraff]

Understood, Asda it is then and I will check the oil filter spring etc. I was looking to couple the the hif 38 and heated manifold with a K&N style filter, am I right in thinking this will allow for beter engine breathing? I assume from the previous comment that I will acheive better gains on the 1098 engine than on my present 950?
Cheers, Will.