Took my bike to the bike doctor

[noobRider]

My Guzzi has been struggling to get any speed on ever since I picked it up in January. It accelerates fine up to about 60-70 mph but then just struggles. I didn't quite believe the guy that sold it to me saying that it was just an old plodder and that's all I could expect so I took it to a dyno today. Here's a boring video of the dyno run.

Basically it turns out that it's just horribly underfuelled at the top of the rev range: the jets are way too small, to the point that the Dyno guy said that if I ran it for too long at a steady 70mph then it would knacker the valves

The good news is that it should be an easy fix: I'm taking it in for new jets.

It makes me wonder about small bike shops without this sort of gear though. The guy I bought it from replaced the jets to cope with the non-standard air box on it but he was having to take it for runs every time he changed anything and basically had to guess whether he got it right or not. He advised me to take it for a blast which turns out to be exactly what I shouldn't do.
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Licence: Nov 2012, Bikes: Suzuki GN125, Moto Guzzi Strada 750, Triumph Sprint ST 955i x 2
AnPhonEh: I need plans, I need contingency plans also, I need back up contingency plans

[uberkron]

You have to be very careful when tuning with the ass dyno, the problem is people dont find good power and richen it, they just let it run hot and yes, valve damage ensues.

Always dyno with a gas analyzer. Anything else is unknown really.
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i ride a zip 50 2smoke. It overtakes a hundred times a day.

[uberkron]

You have to be very careful when tuning with the ass dyno, the problem is people dont find good power and richen it, they just let it run hot and yes, valve damage ensues.

Always dyno with a gas analyzer. Anything else is unknown really.
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i ride a zip 50 2smoke. It overtakes a hundred times a day.

[noobRider]

He had a gas analyzer hooked up during the run. Only had it on one exhaust though. He's supposed to be emailing me a graph showing the problem. This is why I took it there: I wanted to know what was going on rather than making guesses. This is why I'm wondering whether to stop taking the bike to the little shop where I got it and who looked after my previous bike. They haven't got the analysis stuff to quickly and accurately diagnose problems.
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Licence: Nov 2012, Bikes: Suzuki GN125, Moto Guzzi Strada 750, Triumph Sprint ST 955i x 2
AnPhonEh: I need plans, I need contingency plans also, I need back up contingency plans

[uberkron]

One exhaust for sampling is sufficient, smaller places could look at your plugs and tell you it was running lean, but unless lean running is caused by a leak or malfunction then yeah a dyno and other equipment is needed, but tuning and normal workshop work are worlds apart.

When i re jetted my ped i did near a hundred plug chops and had the carb apart every fifth time i pulled the plug. Thats the sort of method you follow without a dyno if you are chasing reliable power. It had 8000km when i did that and its past 18000 now. A few guys i know have changed the exhaust, fitted some random jet and had burnt pistons, albeit after a few months of caning. Mine spends alot of time at full power, so running white hot for silly power was not considered.

My understanding of engines is that they are an air pump, and if the amount of air and fuel are proper, the working surfaces dont get damaged by heat.
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i ride a zip 50 2smoke. It overtakes a hundred times a day.

[goodlifefarm]

Correct fueling is a science. Too rich, and you can get premature engine wear through 'bore wash', where the fuel washes the oil off the cylinder. Too lean, and the burn temperatures are too high, resulting in valves burning out or even pistons melting. Getting a modified carburetor fueled bike setup correctly on a dyno will take a lot of time and money.

[noobRider]

Just got it back from the doc. here's the graph:
https://i.imgur.com/7JXAqdr.jpg

Red is the before re-jetting. You can see that the fuel / air mix only sneaked onto the graph at one point, just over 50pmph, so it was horribly lean, and the power barely gets over 30bhp.

Blue is after: so it's running slightly rich 40-65 and slightly lean above 65. The power is significantly better, although I won't be racing my sons R6, and even the sv650 I did my DAS on would leave me in the dust.

I'm pleased with this. It felt more responsive on the way home and it should let me cruise the motorway comfortably. I won't be knackering the valves either.

I think I've decided that taking it to a garage with all the analytics gear rather than my very local small shop is probably the way to go. The small shop guy did bump up the jets a lot but not nearly enough and if I'd carried on without getting it properly checked I would have buggered the valves completely.
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Licence: Nov 2012, Bikes: Suzuki GN125, Moto Guzzi Strada 750, Triumph Sprint ST 955i x 2
AnPhonEh: I need plans, I need contingency plans also, I need back up contingency plans

[virus]

judging by the blue line on the a/f id wind the mixture screws in half a turn, and lift the needle height one notch.

that should make low speed response and idle a lot better, although its fueling a lot better than it was the idle is still quite highly lean.


Cheers
John
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own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

[noobRider]