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Author: Subject: classic mini 1275 engine/rebuild/compression problems
anthony1

posted on 21/1/16 at 05:36 PM Reply With Quote
classic mini 1275 engine/rebuild/compression problems

Busy with the rebuild of a classic mini engine for son...... we have had the block rebored to give 1310cc, with new pistons etc. etc. I rebuilt the engine and got it running , but it seemed to be running on 3 cylinders , or 3 1/2 anyway. fiddled with the timing....new Aldon distributor, that was fine......had already balanced the carbs , but did them again.......made no difference! Then decided to check the compressions.......even though it had new pistons/rebore and new cylinder head!!! This is where the fun starts....no.1 was 12 bar, no.2 was 6 bar, no.3 was 12 bar as was no.4 .....no wonder it would not run properly. Injected some oil into no. 2 this brought it up to 11 bar. I have spent countless hours checking and re-checking swapping components around etc. but can't get any different reading for no.2 cylinder. Back to the engineering company who did the rebore.......pressure test of block(porosity) fine , measured bores , fine....... WHAT DO I DO NOW????? Your ideas would be greatly appreciated.......I have leak tested the head , even tried another head , re set the valve clearances again and again , tried another cam.......swapped pistons around just to check them.......still have low compression in no. 2. Am I missing something obvious??????
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BaileyPerformance

posted on 21/1/16 at 05:49 PM Reply With Quote
If the engine is a new build, you must use mineral oil to bed it in, the cheaper the better.

DO NOT let the engine idle or run with no load, set the timing as best you can and mixture on the lean side.

Then drive it, dont thrash it but dont pussy foot it ether.

You may just find you have a poor ring seal just because of starting and stopping it.

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hizzi

posted on 21/1/16 at 05:51 PM Reply With Quote
if putting oil in the bores bring its up then i would think that cuts out the head . gasket and valves. you say swapping pistons round doesnt make a difference? that where i would have looked bad rings or something. have you checked the bore is round?
have the bores been cross hatched/honed to aid bedding in?

[Edited on 21/1/16 by hizzi]

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BaileyPerformance

posted on 21/1/16 at 05:51 PM Reply With Quote
Also did you check your ring gaps when you built it?

And did you set your rings in correct position? (gaps not directly in-line)

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garyt

posted on 21/1/16 at 05:52 PM Reply With Quote
rings on no 2 piston check that these are free and seating properly as when you oiled it the comp came up so my first place of call would be here,but as you have already eliminated all other sources head , valve and bore intolerance , it really doesn't leave much else. remove the rings checking that they are not gripping the piston, they can stick in the grooves when compressed , it doesn't take much, even clean the grooves and a new set of rings again making sure that nowt is overly tight when reassembling.
good luck
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BaileyPerformance

posted on 21/1/16 at 05:54 PM Reply With Quote
just to reiterate doing a compression test on a new engine can be a red herring. The rings will not be bedded in so you can get some random reading due to poor ring seal with the bore.
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anthony1

posted on 21/1/16 at 06:01 PM Reply With Quote
Wow....fast replies....

Rings were correctly spaced at 120 degrees , ring gap was 15 thou. Would it be worth running engine some more to aid the bedding in of the rings in no. 2 cylinder ?? as well as the others obviously.
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BaileyPerformance

posted on 21/1/16 at 06:03 PM Reply With Quote
quote:
Originally posted by anthony1
Rings were correctly spaced at 120 degrees , ring gap was 15 thou. Would it be worth running engine some more to aid the bedding in of the rings in no. 2 cylinder ?? as well as the others obviously.


I have been here before!

Make sure you are using crap oil, and go drive it :-) Do another comp test after 30 miles. Normal driving, dont pussy foot!!!

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BaileyPerformance

posted on 21/1/16 at 06:06 PM Reply With Quote
What carb(s) are you using?

An explanation why only cylinder 2 is low could be the fact that this cylinder is running richer than the rest. Fuel distribution is can be poor even on a stock SU.

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mark chandler

posted on 21/1/16 at 06:09 PM Reply With Quote
As Dale says, do not pussy about, after its run for an hour at various revs you want full load for a couple of seconds then coast at differing engine speeds, it snaps the rings out then draws up oil.

Synthetic oil is so good it will struggle to bed in.

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02GF74

posted on 21/1/16 at 06:42 PM Reply With Quote
quote:
Originally posted by BaileyPerformance
What carb(s) are you using?

An explanation why only cylinder 2 is low could be the fact that this cylinder is running richer than the rest. Fuel distribution is can be poor even on a stock SU.


I was gonna suggest something along those lines but both b and a series have siamesed inlet ports so cylinder 1 should behave like 2.

Different head, different pistons, what else can it be.?, camshaft but you would have set valve clearances, else has to be block if the problem stays in same place.





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02GF74

posted on 21/1/16 at 06:47 PM Reply With Quote
Run compression test without inlet manifold fitted and report figures?

Is it possible no. 2 is not getting oil? Did you swap rods or just pistons, checked crank oil driilings are clear?

Its such a big difference in psi that im find it hard to believe its due to bedding in but i may be wrong here.





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BaileyPerformance

posted on 21/1/16 at 06:49 PM Reply With Quote
I was gonna suggest something along those lines but both b and a series have siamesed inlet ports so cylinder 1 should behave like 2.




Thats the point, if the cylinder head has been modified (incorrectly) then the siamese fuel distribution issue can be made worse. Just a guess, not the best cylinder head in the world.

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rusty nuts

posted on 21/1/16 at 07:05 PM Reply With Quote
quote:
Originally posted by 02GF74
quote:
Originally posted by BaileyPerformance
What carb(s) are you using?

An explanation why only cylinder 2 is low could be the fact that this cylinder is running richer than the rest. Fuel distribution is can be poor even on a stock SU.


I was gonna suggest something along those lines but both b and a series have siamesed inlet ports so cylinder 1 should behave like 2.



Charge robbing, A series suffer from it.

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britishtrident

posted on 21/1/16 at 07:16 PM Reply With Quote
The inlet arangement on A & B series engines is a pain when setting the carbs but it shouldn't affect compression test results if only one cylinder affected , also it can't be the old A series brass plug in the head issue.

I would agree compression tests a newly rebored engine can be leading but the difference is huge.

I would opt for a cylinder leak down test.

[Edited on 21/1/16 by britishtrident]





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rusty nuts

posted on 21/1/16 at 07:30 PM Reply With Quote
Just a thought, I once had a K series with low compression , the piston on the offending cylinder wasn't coming anywhere near to the top of the bore due to a bent con rod.
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owelly

posted on 21/1/16 at 10:42 PM Reply With Quote
You've swapped the pistons? Did you swap the rings too or keep the rings with the same bores?





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02GF74

posted on 22/1/16 at 09:03 AM Reply With Quote
Checked no. 2 rod is same length - im guessing the smaller capacity a series engines had shorter rods.

Shorter rod would mean piston at tdc leaves a larger combustion chamber reducing the cr.

... but im guessing youve compared piston tdc pistions at tdc.

Have you measured bores yourself? Using same piston ring in bores, measure the gap if you having a bore gauge.

Most unusual!!





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anthony1

posted on 22/1/16 at 11:00 AM Reply With Quote
Let the rebuild begin again........

Collected the block this morning after the pressure test. Many thanks for all of the replies and the constructive suggestions. I am going to take up Dales (Bailey Performance) suggestion of using cheap oil as an aid to the bedding in process , cover about 40/50 miles and see what happens.........Break open the tool boxes..............
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mark chandler

posted on 22/1/16 at 11:10 AM Reply With Quote
Also make sure the rings are the correct way up!
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rusty nuts

posted on 22/1/16 at 07:08 PM Reply With Quote
I once had customer with an A series with an intermittent misfire, compressions checked , cylinder leakage test OK , another distributor, leads, cap, rotor and spark plugs all changed, still played up . Eventually it came in misfiring, pulled the leads off one at a time and isolated the offending cylinder, rechecked compression, none ! Leakage test showed a leaking exhaust valve so off with the head , stripped it down to find the valve stuck open due to a loose valve guide floating about .
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