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Author: Subject: Obscure reasons why an engine might be down on power?
smart51

posted on 16/11/23 at 10:12 PM Reply With Quote
Obscure reasons why an engine might be down on power?

I have a 1975 Fiat 500 with a 594cc air cooled twin. It should produce 22.6 BHP giving a 67 MPH top speed. The most it will give is 62 MPH, suggesting about 18 BHP. I've no interest in suping it up, I just want it to run as it should.

I've been playing with carburettor jetting and have tried various combinations of main and air corrector around the two different book settings. Some are slightly worse, others quite a bit worse. I found that 'the other' idle jet works better, which is a plus. I completely disassembled the carb not long ago and ultrasonically cleaned it to food grade standards. That carb is clean.

The car has a new exhaust, since a bit of the old one fell off. It's also had a new coil, distributor, HT leads and spark plugs. I've tried different spark timings. I can only lose top speed by adjusting that.

I've been over the induction system and that is all in top condition. The valve clearances are all set.

Compression is 110 PSI on one cylinder (spot on) and 90 PSI on the other. That could be costing me 0.6 BHP. 4 horses doesn't sound like a lot but that's 18% of the total output. It would be nice to have them back.

What else could be a cause of lost power?

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perksy

posted on 16/11/23 at 10:59 PM Reply With Quote
Your getting full throttle?

Fuel pressure ok?

Ignition advance working ok?

Brakes free and not binding?

Had a head skim and compression ratio is wrong?

Compression test is more than 10% different, not by much but as you say its a fair bit of the horsepower...

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Mr Whippy

posted on 17/11/23 at 07:48 AM Reply With Quote
Without seeing the car, this reminds me of someone at work who despite my warnings bought a S3 Landrover. He later comes over to me and asks to borrow my compression gauge as he thinks its got no compression since it will only do 45mph flat out. So at lunchtime I take it for a spin and promptly get it up to almost 70mph which is the proper top speed. His comment when I said there's nothing wrong with it was 'but your thrashing it!'. Well welcome to the world of Landrovers I told you not to buy it!

Your car is essentially an Italian BEC and so needs to be driven hard.

Youtube linky

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rusty nuts

posted on 17/11/23 at 08:40 AM Reply With Quote
Iíve had new exhausts that have been too restricted causing lack of power. Did you do a wet test when you found the compression down? As already suggested are you getting full throttle? From memory there is two hollow bolts , one at the front of the engine the other at the rear, screwed into the cylinder head horizontally, are either of them allowing pressure out? If so it has a head gasket failure. If the head has to come off I think it is worth stripping , lapping in the valves etc. I have seen engines that have severely restricted ports due to carbon build up , admittedly not on a 500 though. Dragging brakes, incorrect tyre pressure , slipping clutch will all have an effect on performance. A very underrated but useful diagnostic tool is a vacuum gauge connected to the inlet manifold , unfortunately the 500 doesnít have one but it was possible to fit a carb spacer with a vacuum port to be able to take a reading even when the car is being driven . Mind you I have an even bigger loss of performance than 18% than I had 50 years ago
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gremlin1234

posted on 17/11/23 at 10:22 AM Reply With Quote
I presume you have nice, fresh, hi octane fuel
I know you said you had been all over the induction side, but is the air filter flowing enough air?

I also found a pdf manual for the car, I guess you already have it but here it is anyway

https://www.revoracing.cz/userfiles/downloadencyklopedie/8%20Workshop_Manual_Autobooks.pdf

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smart51

posted on 17/11/23 at 10:26 AM Reply With Quote
quote:
Originally posted by perksy
Your getting full throttle?

Fuel pressure ok?

Ignition advance working ok?

Brakes free and not binding?

Had a head skim and compression ratio is wrong?

Compression test is more than 10% different, not by much but as you say its a fair bit of the horsepower...
It wasn't getting full throttle when I bought it. Bending the accelerator pedal fixed that. The ignition is advancing 1į more than the book says. I'll take that. Brakes seem fine. Standard compression is 7.5:1. You'd have to skim it an awful lot to go too far.

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smart51

posted on 17/11/23 at 10:29 AM Reply With Quote
quote:
Originally posted by Mr Whippy
Without seeing the car, this reminds me of someone at work who despite my warnings bought a S3 Landrover. He later comes over to me and asks to borrow my compression gauge as he thinks its got no compression since it will only do 45mph flat out. So at lunchtime I take it for a spin and promptly get it up to almost 70mph which is the proper top speed. His comment when I said there's nothing wrong with it was 'but your thrashing it!'. Well welcome to the world of Landrovers I told you not to buy it!

Your car is essentially an Italian BEC and so needs to be driven hard.

Youtube linky
It doesn't take much at all to be driving it flat out I was reading a tuning guide from a lotus owners group. They struggle to test WOT to the red line on public roads. I can do it in all gears and still be holding up traffic

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smart51

posted on 17/11/23 at 10:35 AM Reply With Quote
quote:
Originally posted by rusty nuts
Iíve had new exhausts that have been too restricted causing lack of power. Did you do a wet test when you found the compression down? As already suggested are you getting full throttle? From memory there is two hollow bolts , one at the front of the engine the other at the rear, screwed into the cylinder head horizontally, are either of them allowing pressure out? If so it has a head gasket failure. If the head has to come off I think it is worth stripping , lapping in the valves etc. I have seen engines that have severely restricted ports due to carbon build up , admittedly not on a 500 though. Dragging brakes, incorrect tyre pressure , slipping clutch will all have an effect on performance. A very underrated but useful diagnostic tool is a vacuum gauge connected to the inlet manifold , unfortunately the 500 doesnít have one but it was possible to fit a carb spacer with a vacuum port to be able to take a reading even when the car is being driven . Mind you I have an even bigger loss of performance than 18% than I had 50 years ago


Kudos for knowing about the hollow bolts. There is nothing blowing from them. The new exhaust is the same style as the one that was taken off. Record Monza since you're in the know. It is much better made than the old one. The internals remain a mystery unless you cut it open mind.

A few drops of oil down the spark plug hole made next to no difference to the compression. The cause of the loss of compression is above the piston. Since it doesn't seem to be the gasket, it is likely to be valves. I don't think this is the bigger part of the power loss though.

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smart51

posted on 17/11/23 at 10:49 AM Reply With Quote
quote:
Originally posted by gremlin1234
I presume you have nice, fresh, hi octane fuel
I know you said you had been all over the induction side, but is the air filter flowing enough air?

I also found a pdf manual for the car, I guess you already have it but here it is anyway

https://www.revoracing.cz/userfiles/downloadencyklopedie/8%20Workshop_Manual_Autobooks.pdf
Yup. Lovely Esso Synergy Supreme+ E5, because it has zero ethanol (outside the west country).

Thanks for the link to the manual. It looks better than the Haynes manual I've got.

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smart51

posted on 17/11/23 at 10:57 AM Reply With Quote
My thoughts are that it could be something to do with the valves. Either the timing chain is on one tooth wrong or they're not opening enough.

The fuel pump is driven by a push rod which is pushed by the cam shaft. It shares the exhaust cam with the cylinder with low compression. I replaced the fuel pump recently. Part of the procedure is to measure the protrusion of the push rod at each end of its movement. You fit either a thick gasket, a thin gasket or both to accommodate the length of the push rod. Mine has the thin gasket and a lightly sanded spacer to make it fractionally smaller. Part of the check is that you measure the stroke of the rod. The stroke on mine is a couple of tenths of a millimetre too short. The only possible cause of that is wear to the cam lobe. That being the case, perhaps I'm not getting enough lift on the valves. You can't alter the rocker arm ratio.

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gremlin1234

posted on 17/11/23 at 12:49 PM Reply With Quote
quote:
Originally posted by smart51
Yup. Lovely Esso Synergy Supreme+ E5, because it has zero ethanol (outside the west country).


unfortunately esso no longer guarantee it's zero ethanol :-(

" please note that from September 2023 our Synergy Supreme+ 99 will transition to contain up to a maximum of 5% ethanol at all Esso pumps irrespective of which part of the country they are located."

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Slimy38

posted on 17/11/23 at 04:25 PM Reply With Quote
You asked for 'obscure' reasons, are the tyres the right size compared to what it needed when it was new? I can imagine it's pretty easy to lose 10% by using the wrong tyre size.

I suppose that does depend on how you're measuring your speed, I'm just wondering whether the issue lies away from the lump of metal producing the power, and more with how that power gets to the tarmac.

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smart51

posted on 17/11/23 at 04:33 PM Reply With Quote
quote:
Originally posted by Slimy38
You asked for 'obscure' reasons, are the tyres the right size compared to what it needed when it was new? I can imagine it's pretty easy to lose 10% by using the wrong tyre size.

I suppose that does depend on how you're measuring your speed, I'm just wondering whether the issue lies away from the lump of metal producing the power, and more with how that power gets to the tarmac.


I had a puncture at the weekend. It was only when fitting the spare that I realised my car has 145/70 R12 tyres on 4J rims, where the spare is an original 3.5J rim with a 120 R12 tyre. The rolling circumferences are the same. With the same tyre pressures, the contact patch will be the same area, just wider rather than longer. Rolling resistance will vary from tyre to tyre of course.

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rusty nuts

posted on 17/11/23 at 07:58 PM Reply With Quote
quote:
Originally posted by smart51
My thoughts are that it could be something to do with the valves. Either the timing chain is on one tooth wrong or they're not opening enough.

The only possible cause of that is wear to the cam lobe. That being the case, perhaps I'm not getting enough lift on the valves. You can't alter the rocker arm ratio.



If you have the figures for the valve lift and cam timing it should be easy enough to check both without major dismantling, just need a degree wheel and DTI A slack timing chain would retard the valve timing causing lack of power , would probably show up on measuring the inlet vacuum

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scudderfish

posted on 17/11/23 at 09:43 PM Reply With Quote
Is your speedo under-reading? Verify with a gps speedo phone app
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smart51

posted on 17/11/23 at 10:17 PM Reply With Quote
quote:
Originally posted by scudderfish
Is your speedo under-reading? Verify with a gps speedo phone app

Yep, GPS speeo on a phone. The speedo itself claims to be in km/h, but it's not. 70 km/h on the speedo is exactly 40 MPH. Each unit of 10 km/h is 6 MPH.

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smart51

posted on 17/11/23 at 10:23 PM Reply With Quote
quote:
Originally posted by rusty nuts
quote:
Originally posted by smart51
My thoughts are that it could be something to do with the valves. Either the timing chain is on one tooth wrong or they're not opening enough.

The only possible cause of that is wear to the cam lobe. That being the case, perhaps I'm not getting enough lift on the valves. You can't alter the rocker arm ratio.



If you have the figures for the valve lift and cam timing it should be easy enough to check both without major dismantling, just need a degree wheel and DTI A slack timing chain would retard the valve timing causing lack of power , would probably show up on measuring the inlet vacuum


I have digital vernier callipers. Tomorrow I'll have a replacement battery for them too. In the mean time, I've used the callipers to gauge the distance between the rocker arm and the head and measured the callipers with a ruler. It looks like the cam is lifting the rocker by 5.5mm. The book says 6.1, which sounds like a lot of wear to me. I'll double check tomorrow when I've got a new battery.

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smart51

posted on 18/11/23 at 12:48 PM Reply With Quote
Confirmed with the verniers, 5.7mm. Depending on which of 3 slightly different cams I have, that's 0.4mm or 0.5mm less than the spec.

That's 8% less valve opening. If that corresponds to 8% less air and 8% less power, that is costing me 1.8 BHP.

The slight loss of compression is costing 0.6 BHP. I've still got 2.5 BHP to find. When you started out with 22.6, that's a lot!

[Edited on 18-11-2023 by smart51]

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BenB

posted on 18/11/23 at 03:18 PM Reply With Quote
Knackered battery / charging issue and the alternator is on full chat constantly? Just a thought!!
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coyoteboy

posted on 18/11/23 at 03:38 PM Reply With Quote
quote:
Originally posted by BenB
Knackered battery / charging issue and the alternator is on full chat constantly? Just a thought!!


That's a good call actually, nice thinking.

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Mr Whippy

posted on 20/11/23 at 12:52 PM Reply With Quote
Is this the point when you tell us your only 45 stone?

[Edited on 20/11/23 by Mr Whippy]

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rusty nuts

posted on 20/11/23 at 06:06 PM Reply With Quote
If it is an exhaust lobe itís likely to be the cause of your lost power , if the exhaust valve is closing too early there may be pressure still inside the cylinder when the inlet valve opening causing a reversal of the inlet charge. Time for an updated cam ? You never know ,you might get 25 horsepower?
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Mr Whippy

posted on 20/11/23 at 07:38 PM Reply With Quote
quote:
Originally posted by rusty nuts
You never know ,you might get 25 horsepower?


OMG!



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Prof_Cook

posted on 21/11/23 at 10:23 PM Reply With Quote
I am lacking technical skills in BHP to MPH correlation and grateful for any clarification on what the theoretical relationship is.

But assuming 22.6 BHP gives a 67 MPH max speed. The most you are getting is 62 MPH (this is 92.5% of 67MPH), then assuming a linear relationship this (92.5% of max power) suggests about 20.9 BHP so 1.7 BHP is what is missing.

[Edited on 21-11-23 by Prof_Cook]

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gremlin1234

posted on 22/11/23 at 12:44 PM Reply With Quote
quote:
Originally posted by Prof_Cook
I am lacking technical skills in BHP to MPH correlation and grateful for any clarification on what the theoretical relationship is.

But assuming 22.6 BHP gives a 67 MPH max speed. The most you are getting is 62 MPH (this is 92.5% of 67MPH), then assuming a linear relationship this (92.5% of max power) suggests about 20.9 BHP so 1.7 BHP is what is missing.

[Edited on 21-11-23 by Prof_Cook]

it is not linear, drag goes up by the square of the speed.
so about 16% difference

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