850 Dyno Run

Hi,

I made this dyno run with my 1973 850

https://1drv.ms/u/s!AvqKXnLrv2Bt8Op0NjF4SnXlG6V94A

The bike has twin sleeved Amals. Gas level is .25 from the lip of the bowl. New needles and needle jets. The clip is in the middle position. Based on it feeling really lean going up a hill wfo two up a few years ago, I went up 2 main jet sizes from a 260 to a 280. I raised the needles one clip because the pipes were glowing fairly noticeably going down a dark interstate a few years ago. Now the right side (the one they put the sensor in) glows slightly under similar conditions.

The folks operating the dyno said the jetting looked spot on for a street bike, but I'm wondering if it's a bit rich. They said the choppy curve at the high end was due to a worn sprocket or chain. I didn't replace the front sprocket when I last changed the chain, but I've got one ordered now.

I think 44.5 hp sounds pretty average for an 850. Any one with more experience in looking at dyno runs have any suggestions?

Thanks
Eric

For those who don't have a OneDrive acct.

Best way to ascertain carb settings on a dyno is to ask him to hold a steady 1/4 throttle, then 1/2, etc.

Most operators just go WOT which isn’t very representative of real use.

Having said all that, your AF ratio doesn’t look too bad to me.

Does a horsepower figure off a dyno mean anything, unless you can improve upon it ? The calibration of dynos in absolute terms, seems to me to often be very sus. The 1959 Triumph Bonnevilles were supposed to have 48 BHP and the best 500cc Manx Nortons - 50 BHP. You can do yourself a serious mischief with either of those. To me, my Seeley 850 which is on methanol, feels as though it has 80 BHP. However it is very deceiving. It might have loads of torque, because it pulls very high overall gearing - and with the close box, it accelerates like stink, once it is mobile.

Just curious, was the dyno an inertia dyno or a real dyno (one that can load the engine/drivetrain)?

In either case, I agree that the best use of a dyno is for before/after tests of specific changes OR direct comparison of different vehicles on the same dyno, preferably on the same day. A one-time run doesn't really tell you anything because there is no baseline for THAT engine on THAT dyno. IOW, a different dyno would show a different HP figure - could be higher, could be lower.

FWIW, 280 sounds awfully rich for a stock 850 but the thing to do would be to go back to the dyno and try different main jets and see what the dyno shows. It's easy enough to change the mains on an Amal without any disassembly. My 850 seems to run best with 240s though it "lives" at 6400 feet which correlates reasonably well with the expected difference between sea level and the OEM 260 main.

MexicoMike said:

Just curious, was the dyno an inertia dyno or a real dyno (one that can load the engine/drivetrain)?

In either case, I agree that the best use of a dyno is for before/after tests of specific changes OR direct comparison of different vehicles on the same dyno, preferably on the same day. A one-time run doesn't really tell you anything because there is no baseline for THAT engine on THAT dyno. IOW, a different dyno would show a different HP figure - could be higher, could be lower.

FWIW, 280 sounds awfully rich for a stock 850 but the thing to do would be to go back to the dyno and try different main jets and see what the dyno shows. It's easy enough to change the mains on an Amal without any disassembly. My 850 seems to run best with 240s though it "lives" at 6400 feet which correlates reasonably well with the expected difference between sea level and the OEM 260 main.

Click to expand...

Stock was 260. I only went to 280 because it felt so lean at WOT (like when you run out of gas) with me and my wife two up going up a really steep hill. Normally it's hard to really hold WOT for any length of time and keep your license on the road. I went back over the hill two up and it was much better with the 280s.

The dyno guy said if I was racing that weekend I could try dropping a jet but that he thought my jetting looked good for the street. It was a hottish 90 degree day and on a cooler day my bike would run leaner.

It was an inertia dyno but I was more interested in seeing what my jetting looked like than the hp number.

I was also concerned because my right exhaust pipe glows faintly at night on a dark highway if I cruise at a steady 80 or so. I did try rolling up to 100 and the glowing didn't get any brighter. From looking at the a/f mixture on this dyno sheet my mixture looks a tad rich at 80 which should be 1/2 throttle. Maybe the a/f mixture would have changed if he'd have lingered there. I think the big spike in the a/f ratio is where he applied power, probably an accelerator pump would help that.

My motto: Better rich than sorry.
Jaydee

MexicoMike said:

Just curious, was the dyno an inertia dyno or a real dyno (one that can load the engine/drivetrain)?

Click to expand...

Nowadays many dynos provide both features, ie., they have a large inertia roller for rips from low to high RPM as well as an eddy current brake for steady state testing under a wide range of load.

They had problems with my ignition (Boyer Mk III and no resistor anything) interfering with their data acquisition system. Eventually they were able to get a good run. I would think next time I do a dyno run i could use resistor plugs to alleviate this. Would resistor plugs change the timing any?

ewgoforth said:

Would resistor plugs change the timing any?

Click to expand...

No.

jaydee75 said:

My motto: Better rich than sorry.
Jaydee

Click to expand...

Using a dyno to adjust your main jets for maximum power is silly stuff. If you achieve it, the setting becomes weather dependent. If the weather turned very cold, you would probably burn a piston when you fanged the bike over a long distance. I use methanol fuel in my Seeley. I know what is too rich on the mains and I have never even done a plug chop. On short race circuits, you are not usually on full throttle long enough for too rich on the mains, to make much difference. Petrol fuel is probably a little more critical.
With dynos, there are two things - accuracy and precision. Accuracy is whether it give true values for horsepower and torque. Precision is it's repeatability - which is probably the only thing which is important. The Dynotec inertia dynos use a fiddle-factor to make the dyno read a reasonable value for a particular Yamaha superbike. It is a one-point calibration and does not mean the calibration is linear. When you use a dyno, all you need is one which is repeatable, so you can see whether you have achieved an improvement. The actual horsepower and torque figures are only about bragging rights. They are probably nowhere near reality.

The maximum torque figure is more important than the ultimate horsepower figure, because you use the gearbox to ride the top of the torque curve, when you fang the bike. With most bikes, if you improve the horsepower figure, you improve the mid-range torque. However that is not always the case. Commandos are different from a lot of other bikes.

We've recently dyno'd a 750 at Dynotech up near Basingstoke with standard premier carbs and settings, it managed a couple of horsepower more and more stable linear graph curves. The bike was there all morning and used different silencers to see if they restricted power. The A/F mixture was bang on all the way through the rev range, which was something else we were looking at as some customers said they found them to run lean, but this maybe a case of them using carbs that were worn out when new 40 years ago.

When I ran my 850 on the dyno, the sniffer up the exhaust pipe told me that I was running lean at low to medium throttle openings. When I fitted my O2 sensor and fuel/air ratio gauge, it told me the exact opposite with the sensor at the optimum distance from the head and the sensor calibrated. I would hazard an opinion that fitting an O2 sensor and a gauge will tell you a lot more than a dyno run about how youir engine performs. Mine went to 46bhp but feels much stronger at the low end and mid range than a standard Norton 850.