'81 495 jetting...

[pmcburney]

I took the big girl down to Biddaddaba on Sunday (QVMX practice day) and had a ball, except for the heat and getting the bike to perform at its best (in that heat).


I've got an original power-jet Bing fitted (54/40/107) to a pretty-much standard engine with one of Brad's lovely pipes helping things a long (the big bertha 'size' I think).

I've gone as big as I can on the pilot and wound the air-screw in to about 1/2 a turn and it was still running-on into corners when low in the rev-range, then as I'd accelerate out it would bog at about 1/3 throttle - lean and rich all at the same time?

I dropped the needle down to its lowest position and that helped the bog a fair bit, but I couldn't seem to get rid of the run-on, which is concerning on a bastard-big twostroke with no real brakes to speak of just as you tip it into a corner.

It was suggested to me that the heat was going to play havoc with it regardless, so I shouldn't be too worried about trying to get it jetted right with the Bing, and that I should instead fit a Mikuni VM38 - ostensibly a much better carb than the Bing and more adjustable.

My question is, with a basically stock, non-reed 495 running an LBR pipe, what jetting combo would be pretty much right for a VM38?


Thanks
Paul

[Slakewell]

You will lose top end fitting Mikuni but on this bike it dosent really matter, Fitting a new 38 should fix your problems ( buy a new one it's not worth useing an old one ) Bring you beaut 495 to Slakewell motodrome on the 25/26 March and get the KTM Guru Mr Lewis to help in person with the jetting.

http://ozvmx.com/community/index.php?topic=16246.0

[evo550]

The run on sounds like an air leak somewhere, and the bogging as you accelerate sounds like a lean running motor. Check for air leaks and you may find that bothof your problems are solved.

[pmcburney]

You will lose top end fitting Mikuni but on this bike it dosent really matter, Fitting a new 38 should fix your problems ( buy a new one it's not worth useing an old one ) Bring you beaut 495 to Slakewell motodrome on the 25/26 March and get the KTM Guru Mr Lewis to help in person with the jetting.

http://ozvmx.com/community/index.php?topic=16246.0

Slakewell,


Would love to but that is a QVMX race weekend.

So, I will be riding, just not unfortunately at your place.

[Slakewell]

How rude of them to put a race on the same day    I guess it comes down to what is more important.
But yeah look hard for air leak or even barrel gaskit. Ill ask Daryl who made your pipe if he has any spec's for a 38 mikuni

[ozktm]

Check the head, as they don't run a head gasket.

[pmcburney]

Check the head, as they don't run a head gasket.

Hmmm...


The engine has been completely refurbished top to bottom - the mating surfaces for the head and barrel looked very clean and smooth on assembly.

If air was leaking in at that point, should I also see fuel/oil leaking out? I've not noticed anything leaking from that juncture...

I'm going to go over the whole intake tract carefully on the weekend and see if I can find a leak anywhere.

I'm guessing I could also have a leaky RHS crank seal...?

Thanks for the ideas so far guys.

[pmcburney]

How rude of them to put a race on the same day    I guess it comes down to what is more important.
But yeah look hard for air leak or even barrel gaskit. Ill ask Daryl who made your pipe if he has any spec's for a 38 mikuni

Daryl made the pipe on my bike himself...


I bumped into him at CD7 - he was walking past where I had set up my pit and noticed it and came over for a good chat.

Actually, he's ridden my bike. I encouraged him to take it out for a few laps after lunch - he should remember, he was having a ball.

Say hi for me and yes please pick his brains about VM38 jetting.


Ta
Paul

[evo550]

....a change of exhaust systems can also effect your jetting. Having to rejet after fitting a new pipe or silencer is not uncommon. Although the run on sounds like classis air leak symptons. Base gaskets also can be the offender. Does it run fast on cold start up with the choke off?

[pmcburney]

....a change of exhaust systems can also effect your jetting. Having to rejet after fitting a new pipe or silencer is not uncommon. Although the run on sounds like classis air leak symptons. Base gaskets also can be the offender. Does it run fast on cold start up with the choke off?

The bike actually doesn't idle faster when cold, and because I'm using the standard Bing, I don't have a choke as such with which to compare cold running too much.

The run-on really manifests itself when the engine is hot - it runs nicely while it'd still relatively cold.

The day in question was very hot on the track and the bike would run well for the first two laps, then start to run lean.

Maybe I've got a lean condition once various parts start to expand?

[Slakewell]

With the bing enduro carb check the slide has the choke hole blocked off. This is in the slide itself were the old cable choke would attach, The bike will run odd till this is blocked off its a square shaped hole.
Air screw should be 2.5 turns out.

[evo550]

Hmmmm. Usually if there is a leak between two surfaces as the motor heats up and metal expands, the gap is closed sealing off the air leak, Because it changes I doubt it's a jetting otherwise it would be a constant issue, hot or cold. Is it a metal to metal intake flange (Maico type) ? What's the plug look like?

[pmcburney]

With the bing enduro carb check the slide has the choke hole blocked off. This is in the slide itself were the old cable choke would attach, The bike will run odd till this is blocked off its a square shaped hole.
Air screw should be 2.5 turns out.

I've got a few slides, some with a small round hole in the bottom of the choke recess, others without.

There are also two other small round holes in the bottom of the slide either side of where the needle carrier protrudes through.

So, am I supposed to be using a slide with no hole at all in the choke recess, and am I supposed to have any other holes in the base of the slide?

[pmcburney]

Is it a metal to metal intake flange (Maico type) ?

No, there is a paper gasket between the intake manifold and the barrel. I may pull this apart and put some sealant on the gasket in case air is sneaking in at this point.

What's the plug look like?

Dunno, I haven't pulled them out for a look yet. I'll get on that.

[Slakewell]

Not explaining this well.
If it had a cable choke the slide has like a square hole thats like almost half the slide if this is there you will have problems.
Your slide should have a white plastic thingy under the spring that locks the cable in place and fills a hole next to the cable