Author Topic: Running in troubles - engine fade  (Read 633 times)

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gusmagicbus

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Running in troubles - engine fade
« on: 06 April, 2018, 20:45:22 »
Hello,

I've just rebuilt a 1964 BSA SS80 (after 7 years and quite a bit of help from this forum!). Its had new big end, small end, piston, rings, re-bored barrel, valves, valve guides, bushes.....the works. After fiddling with carb settings and timing, it was running ok for the first test ride (around 12 miles). However, on a test run last weekend, after 20 miles the power delivery became jerky and then lost power, I could get some power back by shifting down a gear but soon lost power again. It still idled. It was a bit better after cooling down while I checked all the settings on the road side, but had the same symptoms soon after. I changed nothing and took for a ride this evening, and exactly the same thing happened after 4 miles this time. I have been keeping to about 1/4 throttle and speed between 30 and 40 mph to run things in. It feels like the loss of power could be the engine tightening somewhere but I'm not sure. My first move was going to be to take the head off and see if I can see anything unusual. But would appreciate any advice of what to look for first.

It has a few other things going on that I don't think are related, and aren't my priority until I sort this engine problem out (but just in case you think this is effecting it). The charging circuit isn't working at the moment, but the 12v battery has not dropped below around 12.4v  on the test runs as I have been keeping it on an optimate when its in the garage. Lots of oil leaks, but I don't think that's unusual. The first time I fired it up the exhaust glowed red hot, but since dropping the needle in the carburettor and changing the ignition timing to 1/16 BTDC (as per the BSA service sheets), rather than 5/32 BTDC (as per RR book) it;s run a lot better and the exhaust has not glowed. The spark plug seems a good shade of grey.

Thanks!?


ducati2242

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Re: Running in troubles - engine fade
« Reply #1 on: 06 April, 2018, 23:23:02 »
recheck your valve clearance sounds like they are to tight and when everything gets hot the valves are slightly open.

AWJDThumper

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Re: Running in troubles - engine fade
« Reply #2 on: 07 April, 2018, 07:25:52 »
I would check all the settings again but I would definitely check out the carburation and make sure there are no blockages of the fuel flow from the petrol tank. You mention that you had to play around with the carburation but have you got the standard set up inside the carb and with the air filter?

JulianS

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Re: Running in troubles - engine fade
« Reply #3 on: 07 April, 2018, 10:04:29 »
I would check the timing on fully advanced setting - 33 1/2 dregrees BTDC.

If your bike is distributor type be aware that there was a difference in advance range between early and late models which would need different fully retard settings depending on which model you have fitted.

Sometimes rebores are too tight if the engineer used clearances for liquid cooled engines rather than air cooled.

gusmagicbus

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Re: Running in troubles - engine fade
« Reply #4 on: 08 April, 2018, 09:30:38 »
Thanks folks

Valve clearances - I've checked these and slackened them off a little, they were a little bit tight. Btw - The Rupert Ratio engine book recons that the valve clearances increase when the bike gets warm and advises reducing the valve clearance more. Anyway, it's certainly worth a try, as its is something changing when the engine gets warm. I did notice that my exhaust rocker is not very well aligned over the valve and there is some lateral play in the rocker so I can pull it towards the top of the valve to align it better, but it springs back to being off centre. I don't think this is particularly my problem but something to look at next time the rocker box is off?

Carburation - I think fuel flow is good, I have an inline clear filter which looks good and I can see fuel in their under running conditions and the one in the banjo is also good. It has a new hose. I have the standard wire mesh filter in the battery box. I have the correct carb (376/281) for the model and bought a refurb kit for it from Amal, so everything is new apart from the body. When I had the glowing exhaust incident I dropped the needle down, it is now on the second to lowest setting. I've also played around to get the idle ok (throttle stop and air screw). But that's it. I've looked in to the correct method to set it up, but because this starts with a wide open throttle test for the main jet, I thought i'd leave this until I have run the engine in for a few hundred miles. The thread at the top of the carb does have a piece missing, it still tightens up, and I have tape around it at the mo to stop any potential air leak. I have noticed that oil, which makes its way onto the spacer between the carb and the inlet flange tends to bubble a little (coz it's hot) but wondered if an air leak develops when its warm. I'm not sure the best way to check this though?

Ignition timing - Thanks for the service sheet, that solves one mistery. My model is one of the SS80s that has the 10 deg advance mechanism, and the correct 10 deg bits are in the distributor (I thought these were wrong until yesterday and it was on my list to get the 15 deg weights - but I won't bother now). I printed and made a timing disc last night, so i'll set it to 5 deg BTDC tonight. I think this is going to be much more retarded than it is at the mo. I will also check the valve opening and closing at the same time. I guess the deg points stated in the books for when the valve opens, is when is just starts to open, and when it closes is when the valve is fully closed? The spec I have for the SS80 states Inlet Open 51 BTDC, Close 68 ABDC, Exhaust Open 78 BBDC, Close 37 ATDC. Hopefully that does not change between years. I don't have a dial gauge, but can see when the valves open and close. Perhaps this is good enough to check the valve timing is on the right gear tooth?  Don't know how this effect things but I know for a fact that it doesn't have the correct high compression piston in for the SS80, it just has the std piston. I don't see how I can get 5 degs in retarded setting and 33 1/2 in fully advanced (because the difference is fixed based on the distributor mechanism), but I guess the service sheet you sent stated the retarded setting so i'll just set to this?

Barrel - Hopefully this rebore (+60 bought from ebay) is ok, I remember checking piston skirt clearances were as per the book when I put it together.  I think I had to file a smidgen off the edge of the rings to get the ring gap right though.


JulianS

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Re: Running in troubles - engine fade
« Reply #5 on: 08 April, 2018, 09:46:39 »
I would use the fully advanced position when timing, that avoids having it too far retarded due to wear in the mechanisms.

Checking the valve timing is probably easier just to check that the timing marks line up.

The other possible issue to consider is obstructed oil feed.

ducati2242

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Re: Running in troubles - engine fade
« Reply #6 on: 08 April, 2018, 13:22:56 »
Thanks folks

Valve clearances - I've checked these and slackened them off a little, they were a little bit tight. Btw - The Rupert Ratio engine book recons that the valve clearances increase when the bike gets warm and advises reducing the valve clearance more. Anyway, it's certainly worth a try, as its is something changing when the engine gets warm.
Yes that is usually the case but with engines that are cast steel , aluminium , and steel there can be differences . Just repaired a young lads 125 chinese honda copy . pushrod engine .  it would run fine for 20 minutes then it would stop and refuse to start . Increased the valve clearance slightly and off she goes running for hours of abuse . Worth a try .

MICKREEVES

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Re: Running in troubles - engine fade
« Reply #7 on: 14 April, 2018, 13:19:55 »
I also have the SS80, imported from USA in a rough state. It has the 8.5/1 piston. I rebuilt it but was disappointed that it would not reach the advertised 80 MPH.
One day, as I rode. the power started to diminish. I found that the distributor had rotated to retarded. I reset and tightened the pinch screw.
Then it did it again. This was repeated several times, until the clamp could no longer hold the destributor. I had to be towed home.
I was sent a copy of BSA service sheet which advised lock the distributor from rotating with a screw though the body.
During my work, I ran the engine and rotated the distributor as it ran. This gives you a good feel for the correct timing setting.
I checked the advance bobweights, and realised that the auto advance was the wrong way! Replacing the distributor would have been
expensive and not reliable, so I bought and electronic system.This powers the spark directly, and has two other outputs. I connected one
to the headlight (permanently on) and the other for charging the battery.
Now the bike will start first kick EVERY time, without touching anything, not even turning the fuel on!.
I decided that trying to reach 80 MPH at 8500 RPM would not be wise. These engines were not reliable or long lasting 'in the day'
I have fitted a bigger output gear (+ 2 teeth) and in now sounds like a big single, reach 65/70 at 5300 rpm.
Performance is not enough. I did a 25 mile run, against a strong wind, and it was very hard to get past lorries at 60 MPH.
I now have a B44.
Mick

Dabbist

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Re: Running in troubles - engine fade
« Reply #8 on: 18 April, 2018, 13:39:31 »
I'd be tempted to start with the really easy stuff. What's your battery voltage dropping to under load?
If the battery's a bit duff it can look fine when it's not doing any work, but fade under load.
If your charging circuit isn't working then it could be that the battery is giving up after a short while, increasing the revs will increase the output from the alternator, which might cover it up until you drop the revs again.
It's purely personal choice, but if I know there's something wrong I'll always fix that before looking for anything more complicated, even if I can't see it as the cause of the problem. If it doesn't fix the problem, at least it's done and eliminated from the list of possibilities.
rebuild blog at thebackyardbuilder.net

DEREKANDERSON8

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Re: Running in troubles - engine fade
« Reply #9 on: 18 April, 2018, 15:39:32 »
Hi gusmagicbus,

I would say get a second opinion, that's my advice.

What you have already said shows you have a reasonable knowledge and that should be enough for a british bike designed to be easy to maintain by a home mechanic, but you still have an issue and that needs somebody else looking.

btw the slot at top of monobloc carb the cap should fit in there and locking ring keep it all in place no need for tape to seal. When you say you dropped the needle did you mean you raised the clip position? That would weaken the fuel mixture.

cheers

derek

gusmagicbus

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Re: Running in troubles - engine fade
« Reply #10 on: 18 April, 2018, 22:58:58 »
Hello,

Thanks for all the new suggestions! I will pick this up again and feed back.  Dabbist - I think you're right, I have the chain case cover off currently so will sort the charging issue out. Derek - Yes raised the clip and weakened the mixture.

One thing which has just sprung to mind is I couldn't find any guidance on how to tighten the timing side nut at the end of the crankshaft. As per the photo below (hopefully it appears somewhere). So what I did was tighten the nut enough to take up as much lateral play as possible whilst ensuring the engine still turned (manually) without resistance. Is this how you would do it?

MICKREEVES

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Re: Running in troubles - engine fade
« Reply #11 on: 19 April, 2018, 10:55:30 »
I would put the bike in top gear, rear brake on, tighten the nut as far as you can, finish with some sharp hammer  taps on the spanner.
Mick

Dabbist

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Re: Running in troubles - engine fade
« Reply #12 on: 19 April, 2018, 18:35:36 »
Unless im looking at the picture all wrong, that nut is on the camshaft, not the crankshaft, it fooled me too at first. The hole next to it is for the timed breathing.
It's only there to take up end float on the camshaft and they didn't even bother with it on the B25.
I'd just use a spot of locktite 222 and tighten it up by hand with the bike in gear.
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ANDY HIGHAM

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Re: Running in troubles - engine fade
« Reply #13 on: 19 April, 2018, 19:32:17 »
There was no room for the nut on the B25, later C15 and later B40 at that area was full of points

Tony1951

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Re: Running in troubles - engine fade
« Reply #14 on: 25 April, 2018, 11:49:32 »
Thanks folks

 When I had the glowing exhaust incident I dropped the needle down, it is now on the second to lowest setting.

Sorry to but in here - I've been reading teh forum, and this sentence jumped out at me. Doesn't lowering the needle weaken the mixture even more? Sorry if I'm getting the wrong idea, but having ridden one of these forty five years ago, I think I had to raise the main needle to richen the mixture. My impression is that a red hot exhaust pipe means that burning mixture is getting past the exhaust valve and that could be caused by four things I can think of off the top of my head:

retarded ignition
badly seated valve
too tight valve clearances
weak mixture.

If I'm talking through my hat - I'm sorry, but maybe a few more thoughts from you up to date guys on the change to the mixture needle might help.

Cheers