Author Topic: Timing with lower compression pistons.  (Read 452 times)

Offline UncleD

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Timing with lower compression pistons.
« on: 03.01. 2021 10:20 »
58 Super Rocket...all standard.

Recently had a rebore (+60) and fitted flat top (8:1) pistons instead of the domed (9:1) which were in there.

Only 100 miles on the run-in but a noticeable power drop.  She has a larger than standard piston-bore clearance (I live in the tropics) and even still, I would expect some tightness ....but what was surprising is that she will not pink under any circumstances, even from a start with no manual retard.  I had to use the manual retard at take-off and under load with the former pistons....and she went like a rocket.

These is causing me to wonder whether decreasing the compression requires me to advance the timing?  A quick look today seemed to indicate it is currently at 3/8" advance.

Thoughts?

Northern Territory, Australia

Online KiwiGF

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Re: Timing with lower compression pistons.
« Reply #1 on: 03.01. 2021 10:38 »
58 Super Rocket...all standard.

Recently had a rebore (+60) and fitted flat top (8:1) pistons instead of the domed (9:1) which were in there.

Only 100 miles on the run-in but a noticeable power drop.  She has a larger than standard piston-bore clearance (I live in the tropics) and even still, I would expect some tightness ....but what was surprising is that she will not pink under any circumstances, even from a start with no manual retard.  I had to use the manual retard at take-off and under load with the former pistons....and she went like a rocket.

These is causing me to wonder whether decreasing the compression requires me to advance the timing?  A quick look today seemed to indicate it is currently at 3/8" advance.

Thoughts?

My GF with flat tops (7.25:1 from memory) and 356 cam has std timing (albeit its electronic advance) and runs on 91 and never pinks, if that helps.

I’ve personally not found altering the timing makes much difference to performance on engines of this type, it can be out quite a bit before you’d notice!
New Zealand

1956 A10 Golden Flash (1st finished project)
1949 B31 rigid “400cc” (2nd finished project)
1968 B44 Victor Special (3rd project,in progress)
2001 GL1800 Goldwing, well, the wife likes it
2007 KTM 950 Adventure, cos it’s 100% nuts

Offline UncleD

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Re: Timing with lower compression pistons.
« Reply #2 on: 03.01. 2021 20:30 »
some additional information that may help.

When reassembling the carb, I forgot which needle settling it had been on so reset it in the middle slot (3rd down), recommended being second down (slightly leaner).  Plugs are dark sooty rather than brown soot, so the needle could drop a bit.

When riding yesterday, I pushed her up to 60mph but this seemed to be the limit.  Prior to piston change she would do 70mph with plenty left.

The restricted to end and need to lean the mixture seem contradictory (I would have thought higher speed requires more fuel).

This leaves me thinking that the sluggishness at the moment is just a tight engine (despite large clearances) and new valvesand rods settling in.

Northern Territory, Australia

Offline Seabee

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Re: Timing with lower compression pistons.
« Reply #3 on: 04.01. 2021 01:19 »
Have you done a compression check? It might be interesting to see what the difference is now vs before rebuild. I'd re-check valve clearances too. Easy stuff first.................
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Offline muskrat

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Re: Timing with lower compression pistons.
« Reply #4 on: 04.01. 2021 07:27 »
.but what was surprising is that she will not pink under any circumstances
That's not such a bad thing!
You could try advancing till it does then back it off a couple of degrees.
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Online KiwiGF

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Re: Timing with lower compression pistons.
« Reply #5 on: 04.01. 2021 08:54 »
I don’t think a engine that is running in is going to be down on power (unless its about to seize, or has seized) so your problem lies elsewhere.

What else apart from the top end end and carb were overhauled? Could the cam timing be out? Or valve clearances?
New Zealand

1956 A10 Golden Flash (1st finished project)
1949 B31 rigid “400cc” (2nd finished project)
1968 B44 Victor Special (3rd project,in progress)
2001 GL1800 Goldwing, well, the wife likes it
2007 KTM 950 Adventure, cos it’s 100% nuts

Online Triton Thrasher

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Re: Timing with lower compression pistons.
« Reply #6 on: 04.01. 2021 09:58 »
some additional information that may help.

When reassembling the carb, I forgot which needle settling it had been on so reset it in the middle slot (3rd down), recommended being second down (slightly leaner).  Plugs are dark sooty rather than brown soot, so the needle could drop a bit.

When riding yesterday, I pushed her up to 60mph but this seemed to be the limit.  Prior to piston change she would do 70mph with plenty left.

The restricted to end and need to lean the mixture seem contradictory (I would have thought higher speed requires more fuel).

This leaves me thinking that the sluggishness at the moment is just a tight engine (despite large clearances) and new valvesand rods settling in.

It requires an approximately correct mixture of fuel and air at all speeds.  An over-rich needle position can restrict speed at about 2/3 throttle opening.

As already said, there isn’t really such a thing as a tight engine.

Online RDfella

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Re: Timing with lower compression pistons.
« Reply #7 on: 04.01. 2021 16:49 »
"there isn’t really such a thing as a tight engine."
Afraid I'd have to disagree. Last engine I rebuilt (couple of years ago now) was a 3.8 Jaguar. Must have been about 20% down on power until it'd done several hundred miles, then really came alive. Found the same with many engines. In fact that's why I've always built competition engines to almost worn-out clearances (because there's little opportunity to run them in).
But getting back to the original question, might be worth trying more advance. A lower compression ratio can certainly take the shine off an engine - over the years I've found the majority of petrol engines seem happiest around 8:1 to 9:1 comp ratio (side-valves excluded).
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Online Triton Thrasher

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Re: Timing with lower compression pistons.
« Reply #8 on: 04.01. 2021 17:37 »
What “tight” components in Uncle D’s BSA engine are restricting his top speed to 60mph?

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Re: Timing with lower compression pistons.
« Reply #9 on: 04.01. 2021 17:48 »
TT - it's in the second line of the question
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Offline Joolstacho

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Re: Timing with lower compression pistons.
« Reply #10 on: 04.01. 2021 23:00 »
Isn't it a myth that advancing the ignition can improve performance?
I thought that many racing engines can work with less advance because of the more efficient combustion space.
(Obviously at 'given' revs).
Discuss...

Online Triton Thrasher

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Re: Timing with lower compression pistons.
« Reply #11 on: 04.01. 2021 23:18 »
Isn't it a myth that advancing the ignition can improve performance?
I thought that many racing engines can work with less advance because of the more efficient combustion space.
(Obviously at 'given' revs).
Discuss...

Advancing the ignition timing improves performance if the timing is retarded.

Offline Joolstacho

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Re: Timing with lower compression pistons.
« Reply #12 on: 04.01. 2021 23:23 »
Obviously.
But the OP was surmising that the change in compression ratio might require a change in timing advance.

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Re: Timing with lower compression pistons.
« Reply #13 on: 05.01. 2021 01:27 »
What “tight” components in Uncle D’s BSA engine are restricting his top speed to 60mph?

I think I know what you are getting at here and agree, if a bearing (or piston) was “tight” enough to absorb over 50% of the engines hp (which it would have to, to be limiting the top speed to 60mph) then it would mean impending disaster as things would get very hot very quickly.

This engine has done 100 miles and 60mph so will almost certainly already be “broken in”, I used to think this meant the bore and pistons are “worn” together but nowadays I go with the more accepted view it’s really just the rings that bed down during “breaking in”.

I read an article (similar to that below) by Wiseco a short while back, and they recommend a quick “break in” process, reading that and similar advice on here has stopped me doing hundreds of gentle miles as I used to, after a rebore.

http://blog.wiseco.com/proper-engine-break-in-after-a-rebuild



New Zealand

1956 A10 Golden Flash (1st finished project)
1949 B31 rigid “400cc” (2nd finished project)
1968 B44 Victor Special (3rd project,in progress)
2001 GL1800 Goldwing, well, the wife likes it
2007 KTM 950 Adventure, cos it’s 100% nuts

Offline Joolstacho

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Re: Timing with lower compression pistons.
« Reply #14 on: 05.01. 2021 02:08 »
UncleD can answer his own question.
If he sets the mag timing at the recommended advance degrees, but with the lever set a halfway of it's travel, then he can test on the road, seeing if advancing or retarding the lever makes any difference to the performance.
(I do wonder why when he started the bike at full advance he didn't get any sign of a kickback. Maybe it IS a bit retarded at full advance).