Technical Why could my 594cc engine be underperforming?

[smart51]

Most of you will have followed my journey working on a 500R I bought last year. It has a 594cc 126.A5 engine with a 28IMB carburettor. I bought this version deliberately so that it would have a little more go than a 499cc car, but everything I've done so far has drawn a blank. The car has a top speed of 60 MPH as measured by GPS.

The fan is solid and blows air well. The air intake tinware is clear and in good condition. The air filter was new less than a year ago and has about 500 miles on it. All the ducting and carburettor is clear. The throttle opens fully. The cylinder head has the 30mm diameter inlet port and the cylinder head looks good.

I have a fuel filter in next to the tank which is full of fuel and looks very clean. The fuel pump is new, the carburettor was stripped and cleaned meticulously. I've tried various jetting combinations and the standard ones work best.

The engine has 130 PSI of compression on 1 cylinder and 120 PSI on the other - fair figures.

The exhaust ports on the head have been lightly opened up - mostly softening the inside of the curve where there was quite a lip. The exhaust elbows have had similar light treatment. I have a sports exhaust which is new.

It has a 126 wasted spark ignition system. The coil, ballast resistor, distributor, condenser, HT leads and spark plugs are new. The points are gapped and the timing set with a strobe.

It has a new 35/70 cam, new followers, new timing chain and gears. New gaskets everywhere except the front of the crank shaft. I've had the sump off and inspected the inside - all good. I've had the flywheel cover off and inspected - all good.

The brakes don't drag. The gearbox has oil in it and changes gears smoothly. It sounds good and drives well.

Yet on a level road, it will not go over 60 MPH according to the GPS app on my phone, which is just short of 110 km/h on the speedo. It suggests that the engine is making about 17.5 BHP at 4200 RPM. The 126.A engine should make 22.8 BHP at 4800 RPM and give a top speed of about 67 or 68 MPH in a 500. Is there a difference between the 126A and 126A5 engines that I have missed? Is there something about the engine I've not checked? I can't think of any reason I've not checked why my car is underperforming. Any ideas?

 

[Noah500]

60 does sound a bit low. My '71 L has a (I think) standard 499 engine and it shows a GPS 64-ish mph on the flat with a long enough run-up. Good luck!

 

[fiat500]

I have occasionally been stupid enough to push my car (currently standard 594 with standard 126 box) to close to its limit and the 60mph marking has been passed. From comparison with the other engine and drivetrain combinations I have previously fitted, the speedo reading is very accurate with this setup. So I do have a bit of leeway to go past 60; but on a level road and without a helpful tail wind, probably not much.

Presumably, the engine performs well on acceleration and has a bit more of that kick of torque that the 499 engine lacks? If so, I would stop looking at the satnav speed, which isn't consistently accurate in any case, and glory in the car's performance where it really matters in a 500, ie. it's ability to chug up hills and for the engine to pick up as you pull out of sharp bends etc.

But maybe that's a bit suspect as well? Can you get to drive another 500 for compaison?

 

[smart51]

I don't know why I typed 60. It consistently does 62 MPH at the same point on the same road. Late last year, (before the cam swap) I tried a number of combinations of main jet and air jet. Lots of combinations gave 59, 60 and 61 MPH. Going back to the original settings gave me 62 MPH again. I've tried advancing and retarding the timing. Same story.

62 MPH is about 4300 RPM in a 500R. The shift markers on the speedo are at 4700 RPM. Even when pressing on, I shift before the markers. In typing this out, I wonder if there is an engine speed related thing going on, rather than air flow or fuel flow. I wonder if it will rev higher in lower gears.

 

[smart51]

For clarity, I don't want to do speeds above 60, I want to be able to maintain 50 MPH on a typical B road, which means doing 50 uphill. Top speed is just a measurable proxy for BHP

 

[fiat500]

I don't know why I typed 60. It consistently does 62 MPH at the same point on the same road. Late last year, (before the cam swap) I tried a number of combinations of main jet and air jet. Lots of combinations gave 59, 60 and 61 MPH. Going back to the original settings gave me 62 MPH again. I've tried advancing and retarding the timing. Same story.

62 MPH is about 4300 RPM in a 500R. The shift markers on the speedo are at 4700 RPM. Even when pressing on, I shift before the markers. In typing this out, I wonder if there is an engine speed related thing going on, rather than air flow or fuel flow. I wonder if it will rev higher in lower gears.

The 500 is so low powered that even a few Kg weight difference could affect performance and keeping up the revs is the challenge for getting best perfomance out of it. When I have sub-nine-stone Sheila in the car, performance is affected. 62mph is not a bad achievement in a 500, whatever the road and weather conditions. With a standard 499 engine it would be screaming.

For clarity, I don't want to do speeds above 60, I want to be able to maintain 50 MPH on a typical B road, which means doing 50 uphill. Top speed is just a measurable proxy for BHP

You will never keep up a speed of 50mph on a country road in a 500. When Sheila and me tried a bit of rallying, where the objective is to maintain an average speed of 30mph, we failed every time, despite having foot to floor whenever possible.

I have a steep, (for a main road) test hill that is about two miles long, without any significant bends or changes in gradient. With the 594 I can just manage to stay in third gear without labouring the engine. With the 499 i'm down to second. It's totally different from the modern car where cruise contol will just take it up and over without hesitation.

On a 500, the gearstick is rarely in one place for long, and I think that is part of the fun.

 

[Goldnrust]

I don't know why I typed 60. It consistently does 62 MPH at the same point on the same road. Late last year, (before the cam swap) I tried a number of combinations of main jet and air jet. Lots of combinations gave 59, 60 and 61 MPH. Going back to the original settings gave me 62 MPH again. I've tried advancing and retarding the timing. Same story.

62 MPH is about 4300 RPM in a 500R. The shift markers on the speedo are at 4700 RPM. Even when pressing on, I shift before the markers. In typing this out, I wonder if there is an engine speed related thing going on, rather than air flow or fuel flow. I wonder if it will rev higher in lower gears.

There's a valid point to be considered here. Due to the 500 being so low geared, to get to those peak speeds in 5th you are talking about revving the engine as hard as possible. It's possible you have an issue that's preventing the car from reaching max revs, rather than being down on power elsewhere.

Whilst my engine is a 499, it will pull just past the red lines in the lower 3 gears if I push it. Maybe that's worth testing on your car?

@Fiat500s test hill idea, might be a better way to gauge the mid range power?

I don't think I'm far from you, seeing some of the pictures you post, so happy to compare hill speed notes sometime for my stock 499cc car, haha

 

[fiat500]

Looking back at some of my old posts where I have no reason to believe that I was hallucinating or telling fibs.....by then, I think I had a 652 engine and the 9:39 BIS crownwheel and pinion fitted.

Murphio's Meanderings | FIAT 500 (Classic)

To give a better 'lead' for my No. 2 ignition lead, I found a slighter longer screw for holding the carb top-cover on, and used this to hold a small plastic covered 'P' clip onto the top of the carb. I lead my No. 2 lead through this 'P' clip---I have not suffered any chafe problems. Can I ask...

www.fiatforum.com

 

[smart51]

I did a quick test drive today and took it up to 5000 RPM in the first 3 gears. It revved cleanly and smoothly all the way up and only started to sound a bit gruff right at the top. The power dropped off noticeably over the last 500 RPM, particularly in 3rd where more of the power is being used to maintain the speed so there's less left over for acceleration. My guess is that the fuelling is a bit off at the top end. Perhaps I should take it to a rolling road and get them to measure the AFR over the full range.

 

[Goldnrust]

Certainly changing the cam could well upset the jetting in the carb, so having it checked and set-up on a rolling road isn't a bad idea.

As you seem to enjoy the challenge of the problem solving, you could alternatively buy and install your own Wideband lambda sensor in the exhaust, then you can monitor your AFR yourself, and make jetting changes for just the cost of the jets rather than paying for expensive rolling road time.

I have an AEM branded one that I have used in several vehicles now, a slightly older model of this: https://www.demon-tweeks.com/uk/aem...-air-fuel-sensor-controller-gauge-aem30-0300/

 

[smart51]

Some progress at last. After a bit of thinking, I've devised the Poor Man's Dyno(tm); a hill and counting. I drive up the hill in 2nd from 20 to 40 km/h on the speedo, then in 3rd from 30 to 60. Sing a song at 120 BPM before you start, then count zero, half, one, half, two, half until you finish. The carb came with 115 + 190 jetting, which is right for a 654cc engine. Today I tried 120 + 200 jets. It is a few seconds faster in each test. Also, for the first time ever, I saw 65 MPH on the GPS, which is a nice improvement on 62. The next test will be with 125 + 215 jets, which are the book settings for a '74 on 594cc Fiat 126. The last time I tried this jetting it was worse than the 115 + 190 setting, but I had the old, worn camshaft at the time. It looks like I might finally be getting somewhere.

 

[Bleeding Knuckles]

Hi Smart, having gone through much the same problem solving with a complex Solex twin choke I found that although I had achieved efficient running and that the car set off like a scalded cat when pushed I never got to that sweet spot I was working for. In the end I decided to invest in a rolling road session and this did indeed improve my settings several fold and made the engine better to drive in all respects. We did have to tinker with jetting over the full range and a slight adjustment in the ignition advance improved things. My point here is that you may adjust your jetting and expect a certain book value result, But, the only way you can possibly know exactly what is going on is to have the engine performance measured under load and adjusted to suit.
Ian.

 

[smart51]

Hi Smart, having gone through much the same problem solving with a complex Solex twin choke I found that although I had achieved efficient running and that the car set off like a scalded cat when pushed I never got to that sweet spot I was working for. In the end I decided to invest in a rolling road session and this did indeed improve my settings several fold and made the engine better to drive in all respects. We did have to tinker with jetting over the full range and a slight adjustment in the ignition advance improved things. My point here is that you may adjust your jetting and expect a certain book value result, But, the only way you can possibly know exactly what is going on is to have the engine performance measured under load and adjusted to suit.
Ian.

There is a rolling road in the area that I've used a long time ago. Their MO is to change one thing and say "that's a bit better, if you want us to do more you can book another session". I want to get in the ballpark first, then book it in for fine fettling.