Setting up on the dyno

Just a quick post to cover the importance of setting your 2 stroke on the dyno. I don't mind admitting that up until a few weeks ago I was a dyno virgin.. Most of my bikes are either standard or near standard and so I had convinced myself that I didn’t need to use a dyno even though I did understand the benefits of doing so. My NSR 250 seems to run well and I have never had a problem even when on a track day but there is always the niggling doubt that the mixture is correct all the way through the range and as such I am always wary of using wot especially for more than a couple of second...just in case. My RD250 also runs fairly well but is a bit sluggish to pick up revs, but she runs well when on the road... So when I bought the 350 YPVS I had no intention of putting it on a dyno, afterall its standard except for the exhausts so a couple of sizes up on the main jet and a plug chop to check and surely she will be ok....
Well she sounded ok and picked up revs really nicely... but when I finally got out to do the first test run she wont pull more than 3k revs and stutters like hell....all the way to the MOT and back.. So I had the carbs on and off looking for problems...is it the carbs? Is it electrical? Is there a fault in the motor... I couldn’t get it to run. In steps Dave (muttsnuts) and offers to run her on his dyno to save guessing... so off I go. Well we soon discover that she is massively rich before even putting the gas analyser on...just removing the airbox lid makes a huge difference. But once we get the gas analysers going we discover another issue, she’s not just too rich but its only on one side, the other side is ok and the difference is huge! about 6 jet sizes and when we try to compensate the readings don’t do what we expected.... so the conclusion is that one or both of the carbs are faulty and to prove the point we stick on a set of spares from a similar bike, hey presto suddenly she is making 53 hp at the rear wheel... Dave asks what chance would you have finding that fault without a dyno.....
So now I need some more carbs which I find on ebay from Germany..They arrive and I fit them and she runs ok... I need to go back to Daves to return his spare carbs and to double check that she is now running ok, i expect it will be just a simple run up and all will be well, but no, the top end power has gone and she is massively rich at the top end, both cylinders. Even though she seemed ok when i tested her briefly.. Off with the carbs and change the main jets from 185 (standard) down to 160. Now she runs better but is still way too rich.... off with the carbs again and down to 150... now she runs better, makes 53 hp at the back wheel but is still on the rich side, we decide that she is running well and being a little rich is on the safe side so we leave it at that. But Dave reckons that we could go down another size and still be ok and probably get more power....but as we’re not chasing performance we decide not to.
Now I would never have considered dropping the main jets down from 185 to 150 but the dyno reveals exactly what is happening and it s what it is, so the message is, without a dyno you are just guessing.

Here’s a movie of the first test run after sorting out on the dyno

ive just seen this nobby, for some reason it just got flagged up as i was surfing some gentlemans material website.

its a good job that video was filmed on private roads as some of the speeds would put you in a prison cell.

i suppose it was squires on the isle of man on mad friday?

ive just tuned my paloma's dellorto by riding it on the roads near to me. a dyno would make it miles easier but i doubt i have the power to even turn the thing!!!

my rd350 carbs are all over the place apart from main jets which like yours are set too rich.

i like to go one or two sizes down off four stroking

which on a dyno i'm guessing would be massively over rich and prompt you to lean out a bit.

i remember crando setting his bike up on a dyno a few years back and then holing a piston while tootling along. i think they are scary because you are tempted to chase the power.

power which on the road is impossible to detect. there isnt a rider on this earth who can detect a 1-2 hp drop at a certain revs.

so i believe being blissfully unaware of lost power at the expense of being over rich is something i can live with.
4 yrs ago i tuned my rd 350 bang on for max revs and it was indestructible. once did the bardney tt - 24 miles flat out and blew off my mates (on their litre sportbike zimmers and mobylettes.)

but twice cruising i holed pistons.
so now im happy for it to run like a pig massively over rich and detuned low comp heads but never hole again. (famous last words)

but to detect your carbs were out that much would have been hard/ pissing in the wind without a dyno.


funny thing tlc's r1z had been on a dyno recently and set up perfectly but once home it just ran like a pig low down.
only some fettling from an experienced tamperer (me) got it running ok.

more luck than owt but ive know all along my intuition pales in comparison to a dyno.

for the first 6 months of my relationship with my missus i stuck with her thinking her family was rich.

then i found out i was wrong and too late i was hooked..

if only i'd put her on a dyno...