question for phil

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kennatt
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question for phil

Postby kennatt » Sun Aug 05, 2018 8:47 am

my son has a midget 850 twin carb,which has had( Prior to him buying) a complete engine rebuild.It starts but runs rough until full throttle then it pulls well and runs ok until easing off, Comp ok on all pots,new dissy coils swapped about,carbs are new ,when pulling off plug lead 2 no change in engine revs,pull any other and you get the usual result. Timing checked ,when pulling leads 2 and 3 car will start on 1 and4 pull 1 and 4 and no start. The car is in france so I.ve no access to it ,initially I was confidant that it was a valve burnt ,or gasket problem but he assures me that the compressions are equal at 122 and that it has been checked over several times by a garage well known in the area as a specialist with classics. There is a strong spark at the plug lead and at the plug. It has the usual three branch inlet manifold,and he assures me that the carbs are set up ok. Bearing in mind your expertise with performance on A series is there anything else you could suggest he checks,I still suspect a vale /gasket problem. But compressions don't confirm this ,I do doubt his ability, but they have been checked a couple of times by other mechanics. Would a fuel mixture cause this, bearing in mind no start on 2 and 3(standard su carbs) . Also says tappets are gapped correctly. If all above is correct I'm at a loss as to what else he can check . Any input welcome .

philthehill
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Re: question for phil

Postby philthehill » Sun Aug 05, 2018 11:31 am

Kennet
The compressions at 122lb are a bit low I would have expected a higher compression figure especially as the engine has recently been rebuilt.
Pulling No:2 plug lead and no change indicates something wrong with No:2 cylinder.
Pulling No: 2 & No:3 and no start usually indicates a gasket failure between cylinders 2 & 3.
Cylinder No: 2 is served by the front carb and cylinder No: 3 is served by the rear carb...………..

Whilst the carbs have been set - have they been set correctly with a balanced mixture and a balanced air flow?

What does spring to mind is the possibility of one or more sticking valves.
What gap have the tappets/rockers been set too? With unleaded fuel 0.015" is acceptable even if a little noisy. 0.012" is too tight for unleaded fuel.
A bottle of Redex put in the fuel tank would not go amiss. I always put a dash of two stroke mixture in the tank of my M/C as the valves tend to stick if used with unleaded petrol which is too dry i.e. it has no lubrication qualities.
I hope that the above is of help.
Please come back on here if you are still having problems.
Phil


RobThomas
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Re: question for phil

Postby RobThomas » Sun Aug 05, 2018 11:42 am

Silly question, but leads are 1342, anticlockwise? He hasn't got 2 and 3 in the wrong order and thus firing on the wrong part of the stroke?
Cardiff, UK

philthehill
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Re: question for phil

Postby philthehill » Sun Aug 05, 2018 11:55 am

Rob
The firing order is 1342 clockwise when looking at the distributer end on.
Phil


kennatt
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Re: question for phil

Postby kennatt » Sun Aug 05, 2018 6:40 pm

hi phil and rob I also think gasket but the even readings,if they have been checked correctly,and as said no access to car myself would tend to discount that. What would you say was an acceptable comp for a decent 850?.He tells me that the mechanic in france has a good knowledge of A series ,but don't know how he has acquired that.
Rob thanks.. but again no way for me to check,he says the car runs well at full throttle so would think leads are ok .And again so called mechanic has been over the engine two or three times so SHOULD have spotted that one???.

I've told him to pull the head to check the gasket so we will see what lies underneath.,might get him to stick some lubricant in tank,don;t know if the rebuild was with unleaded head,apparently cost previous owner two thousand euro with all new components and carbs .will report back....eventually things tend to go slowly over there .

philthehill
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Re: question for phil

Postby philthehill » Sun Aug 05, 2018 7:01 pm

The Midget is not 850cc (which was the original Mini engine) but if fitted with the small bore block will be 948cc.
I would be looking for around 150psi on the compression gauge.
Pulling the head will be a good move.
I would suggest that the valves need to be taken out and the valve stems and guides examined as well as the head gasket.
Photos of what is found would be appreciated.
Phil


kennatt
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Re: question for phil

Postby kennatt » Wed Aug 15, 2018 7:30 am

An update......he has now had the comps tested ,by someone who knows what they are doing ,and get readings of 155 approx. on all pots so eliminates head gasket, but looks like he has cured the fault, on my suggestion he has replaced the rotor from the NEW dizzy with an old one from a spare ,and the misfire on no 2 has cleared ,just can't trust so called new components ,still can't work out why only one cylinder was effected by a dodgy rotor but fault gone. Thanks for input.

pgp001
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Re: question for phil

Postby pgp001 » Wed Aug 15, 2018 8:13 am

I would also replace the distributor cap based on what you have said.
The different rotor arm is now probably masking the original fault on N° 2 post in the cap, and will probably return in time.

Distributor Doctor is your man for a good quality rotor arm and dissy cap.

Phil


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