Author Topic: Engine misses and loses power  (Read 1776 times)

Offline chaterlea25

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Re: Engine misses and loses power
« Reply #30 on: 20.02. 2020 22:48 »
Hi Jim,
Are the guides cast iron or bronze?
I came across an iron head A10 that had bronze guides fitted  (it had a so probably working hard)
Anyhow the bronze expands more than the iron and if clearance to the stem is tight will expand inwards and trap the valve stems  *problem* *problem*

John
1961 Super Rocket
1963 RGS (ongoing)

Online beezermacc

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Re: Engine misses and loses power
« Reply #31 on: 20.02. 2020 22:50 »
Here's a 'left field' thought and I'm not even sure it's relevant..... A few years ago I built an A10 engine for a mate. When first fitted it started straight away and ran beautifully and quietly. We rode it round the block and it was great, except that it didn't feel very lively. After a few miles I gave a bit of extra welly and it lost power and stopped with little or no compression.  We trailered it home by which time compression had returned but we didn't want to push our luck by restarting it again so we stripped the top end and found that both the exhaust valves had picked up and then freed off again. We cleaned the scuff marks off the stems and made a flap wheel out of a long stick and some 1000 emery and 'honed' the guides, put it all back together and it started first kick and sounded great again. So why did the valves pick up? I think because I had set the ignition timing a bit too retarded and the bike was consequently running too hot. I'd retarded the ignition because I'd had problems with another bike pinking and I was blaming the cr4p modern fuel. Anyway we retimed the bike and it has run fine ever since, and when I say 'ever since' I mean the last twelve years!
Priory Magnetos Ltd - A10 spares, magneto and dynamo refurbs. www.priorymagnetos.co.uk

Offline BigJim

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Re: Engine misses and loses power
« Reply #32 on: 22.02. 2020 15:00 »
Guides are iron Chaterlea25.
Can't find info anywhere so please let me know can i use t cut as a grinding paste?
Jamie,  Supporter of Distinguished Gentleman's Ride

Offline berger

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Re: Engine misses and loses power
« Reply #33 on: 22.02. 2020 16:06 »
errr yes jim if your grinding paint layers off *conf2* what are you wanting to grind? have you got new valves and tried them in cleaned out guides to see how they fit? if its an ally head [ED] I think they should be bronze, when my oil was getting black very quickly the engineer said my cast guides in the exhaust had worn really badly and they should have been bronze in my ally head regarding the two metals close expansion rates and cast iron in iron ed.

Offline Swarfcut

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Re: Engine misses and loses power
« Reply #34 on: 22.02. 2020 17:20 »
Jimbo.. Just read through from the beginning. I understand you had a head rebuilt with new guides and valves, had trouble and had to sacrifice the new exhaust valves.  The original problem may be down to the too close running clearance between the guides and new valve stems. Tightish inlet valves are usually OK as they are cooled by incoming mixture. The poor old exhaust valve sticks its head into the fiery furnace draught, hence a tendency to stick. Usual rule was cast guides in a cast head for a standard motor.

 If you can pluck up the courage can you compare how an old valve slops in the new guide and vice versa. Also whether there is any appreciable difference between old valves, new sacrificed valves and your latest replacements as regards stem diameter. There needs to be just a little bit of rock when cold, as it is a compromise between allowing free movement and adequate lubrication versus oil being drawn down the valve stem.

  Fine grinding paste is what you need to grind in the valves. T Cut is much too fine. Whoever fitted the new guides should have re-cut the valve seats to ensure concentricity of the seat in relation to the new guide. With just a few miles covered, and with a bit of luck, the new valves may seat just fine, and do not need attention. See if an assembled valve will hold back liquid, petrol or paraffin was the usual standby, but turps or any other oil based product you have handy will do.  If it does, you can't improve on that. Always good practice to make sure valves are straight by spinning them in a handy cordless drill. This will show bent or bowed stems.  Check that there are no nasty rough edges at the collet end of the valve to gouge the bore of the guide. Assemble with grease on the stems to give lubrication in the first few minutes of dry start up.

 Swarfy.

Offline BigJim

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Re: Engine misses and loses power
« Reply #35 on: 22.02. 2020 21:06 »
Thanks Swarfy for that. The seats were re-cut, will check seating as you say with petrol when the replacements arrive. Lots of good advice there and much appreciated. Was asking about t cut as it's been a long time since i needed to re-seat any valves and this was to hand.
Do you think an excess of oil due to inadequate tension on the head bolts could have caused the valves to stick open? Having said that none blew out over the head gasket.
Jamie,  Supporter of Distinguished Gentleman's Ride

Offline chaterlea25

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Re: Engine misses and loses power
« Reply #36 on: 22.02. 2020 22:28 »
Hi Jim,
I have found that new cast iron (or bronze) guides "shrink" after fitting and need a 5/16in. reamer run through them
Fancy "colisbro" and other  exotic bronze guides should be honed to size
Unfortunately I have not fallen upon a Sunnen hone for free so PB and cast iron for me

John
1961 Super Rocket
1963 RGS (ongoing)

Offline Beeza

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Re: Engine misses and loses power
« Reply #37 on: 23.02. 2020 07:54 »
Hi Jim, I’m just checking, on your first post you never mentioned new valve springs, but you did make reference to reinstalling the old springs. Do you have new springs in this head mate ?
Valve spring pressure is important for a good engine, many old (original) valve springs do lose pressure/ability to preform well.

Cheers Thomas 

Offline JulianS

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Re: Engine misses and loses power
« Reply #38 on: 23.02. 2020 09:39 »
For info during 1957 BSA changed the guide material for light alloy heads from Hidural 5 to cast iron.

Offline BigJim

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Re: Engine misses and loses power
« Reply #39 on: 23.02. 2020 10:17 »
Iron head and new springs. new guides etc. Had fitted expensive SRM valves. Will be replacing with cheaper ones in the exhaust only. Top of inlet valves show signs of hammering from rockers. The engine showed signs of over run which i assumed was due to poor cable routing. Beginning to think inlets binding too.
Who thinks giving them a bit of sliding up and down with t cut to loosen them a touch is a good idea? I do not have access to reamers or whatever is the correct method. Have thrown so much money at this bike that i am loathe to spend a lot more.
Jamie,  Supporter of Distinguished Gentleman's Ride

Offline Swarfcut

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Re: Engine misses and loses power
« Reply #40 on: 23.02. 2020 11:01 »
Jimbo.
     Head gasket should have no bearing on your problem. Make sure threads are clean, lightly lubricated, no muck or oil at the bottom of the blind holes. Centre head bolt is at a slight angle, always loose off first, tighten last. Someone should be along with a suggested torgue figure, BSA just say "really tight."  Clean matched surfaces and the correct clamping force on gaskets is the key.

   Replace valve springs with new, and  yes you can try T-Cut, but you may find a high silica whitening type abrasive tooth powder or paste a little more aggressive. Next up the scale is CIF bathroom cleaner. See how your new valves fit before you start polishing inside guides...and to state the obvious, check the guide bores are nice and clear.

 Hammering to the top of the valves is interesting. Soft? To much clearance? Too tight in the guides?  Coil binding?  Always thought mushroom adjusters a good idea as the original parts seemed to only have a small contact area to the valve tip..all my valves have had a slight hammered dip, so setting a clearance is a bit of a guess.

 Swarfy.

Offline BigJim

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Re: Engine misses and loses power
« Reply #41 on: 23.02. 2020 11:14 »
 Thanks for that Swarthy , will try to post a picture of inlet valve tops. This is a pre-unit engine and head. Is that centre bolt angled in both semi and pre-unit?
Jamie,  Supporter of Distinguished Gentleman's Ride

Offline Swarfcut

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Re: Engine misses and loses power
« Reply #42 on: 23.02. 2020 12:08 »
Jimbo
    The design is basically unchanged from Plunger/Semi Unit to the end of S/A.  That centre bolt is only slightly inclined to the rear, so much so it is often overlooked, but for true perfection it's this one first out, last in. Also there are variations in the lengths of the  head bolts.

 Centre and Front   1  1/2"        QTY  3

 Front Outer           2 11/16"     QTY  2

 Rear Outer            2 3/8"         QTY 2

 Rear                     2 11/16"      QTY 2     All bolts 3/8"

 Source: BSA Service Sheet 204  These variations accommodate the different positions within the cylinder head casting, but lengths are useful to establish you have the correct size bolts, not some spurious assortment. All have washers.

 You can also try cleaning up the top of the valve stems with an oilstone. This area is normally hardened, and the hardness should extend deep enough to allow a light refinish of the top surface. Be careful, keep it square with the stem. If you find the material relatively soft, I would suspect the quality of the valve. A good, old genuine item is to be preferred over a modern repro, as you have a greater confidence that the metallurgy of the old component is at least up to the required standard as originally specified. We like to think newer is better, but it's not always so.

 Swarfy.

Offline BigJim

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Re: Engine misses and loses power
« Reply #43 on: 23.02. 2020 14:09 »
Not really in focus sadly but you can see the mark which is a slight indent. 30 miles approx. Also pic of writing on bottom. What is the difference between exhaust and inlet valves, can they be swapped?
May dig out some old ones from my leftovers box and see how they slide into the guide.
Cheers
Jamie,  Supporter of Distinguished Gentleman's Ride

Offline berger

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Re: Engine misses and loses power
« Reply #44 on: 23.02. 2020 14:21 »
jim quick test is inlets are magnetic, and won't stand the heat as exhaust valves *beer*