Carb upgrade for stock-ish qt50 as first mod.

Rob LaFreniere /

I just bought what I hope is a stock 1984 qt50 and want to upgrade it to eventually be fast enough to safely commute to work. I think that 40mph would be bare minimum.

I have had bad experiences making more than one mod at a time to previous vehicles that frequently resulted in frustration and un-met expectations. This time I will try to only do one performance upgrade at a time.

My first major step, after fixing the brakes and the faulty pet-cock and replacing the air filter with an aftermarket foam thing, will be a carburetor. Based on what I've gleaned from here, youtube, and Mr. Rutherford's site, I think the Mikuni vm20 + mlm intake would work for the long haul.

The one question I could not find an answer to is what jet sizes should I start with considering that the only thing I've done is added a less restrictive air filter?

regards,

Rob

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

Well the vm20 is nice because that main jet swap is so easy. That's a big carb for the stock bike. If you want to go with that carb, why don't you get a pipe first. maybe start with an 80ish jet in the stock carb. Get that tuned in and when you feel ready for the next step - go with the bigger carb. vm18 is nice too but doesn't have the easy main jet swap.

The pipe will get you much more speed initially than a new carb.(edited)

Re: Carb upgrade for stock-ish qt50 as first mod.

Rob LaFreniere /

Thanks for the response.

I've been struggling to choose an exhaust. My preference is for the MLM people's pipe but I have read that it's hangs low and is too easily scraped. I don't suppose it's been altered since its debut to fix that issue?

The circuit pipe looks like a constipated python.

Is the next level pipe sufficiently free-flowing to get me to 40mph with malossi 70cc kit + upgraded carb?

Re: Carb upgrade for stock-ish qt50 as first mod.

Bill Mazzacane /

> Rob LaFreniere Wrote:

> -------------------------------------------------------

> I just bought what I hope is a stock 1984 qt50 and want to upgrade it to

> eventually be fast enough to safely commute to work. I think that 40mph

> would be bare minimum.

Sounds fricken dangerous too me :)

If you don't increase the amount of air the engine consumes, you will create a lean condition because you increased the diameter of the carb. The carburetor's venturi reduces the air pressure inside the carb, so that atmospheric pressure can push the fuel (from the float bowl) and air (into the carb) into the engine.

But if you don't increase the engines demand for more air, or find a way to increase air pressure, the pressure inside the carb will increase, and less fuel will be pushed into the carb, and that is how the lean spot is created.

It has been 100 years since I worked on a QT :) They have a great stock exhaust pipe, and it works well the stock cylinders porting. A tuned pipe does not, or at least in my opinion, the power gains of a tuned pipe are not worth the money unless the exhaust ports blow down time (BDT) has been increased to 21 to 23 degrees. 18 degrees of BDT is the absolute minimum, but I wouldn't waste my time building or selling a tuned pipe unless the customer knew that changing BDT was essential to the performance of the engine.

I don't know these companies, but if you are under the opinion that you can bolt a pipe on to any stock cylinder and get decent HP, that isn't true.

With a stock cylinder, you are better off with a 1/2 pipe that only improves the engines scavenging abilities.

Bill M.

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

People's pipe doesn't hang that low.

Here's the circuit pipe in action. Wait for the qt50 to come into the video. 41.5mph with stock carb, mlm circuit pipe and 1.5mm spacer on stock cylinder. Circuit pipe is only $130 something. Sell your stock exhaust and get a much better pipe for peanuts.

What's top speed on a stock qt - 30mph? 38% better with changes above. It does have pw50 gears as well.

(edited)

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

more mlm circuit pipe goodness

Re: Carb upgrade for stock-ish qt50 as first mod.

Bill Mazzacane /

> Jack Rutherford Wrote:

> -------------------------------------------------------

> 41.5mph with stock carb, mlm circuit pipe and 1.5mm spacer on

> stock cylinder.

I am curious, you added the 1.5mm plate and raised exhaust and transfer ports, but you didn't increase the BDT? BDT is really important.

Did you at least remove 1.5mm / .060" from the top of the cylinder to compensate for the 40 to 60 psi in compression that was lost?

What was the final squish clearance?

Bill M.

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

Blowdown would not change as all ports were raised 1.5mm (the same amount) by the 1.5mm spacer although, come to think of it, since the purpose of the spacer was to expose more of the exhaust port (as the piston covers quite a bit of it at BDC), the spacer may actually have increased blowdown. I removed 1.5mm or thereabouts from the top of the cylinder. Just using stock head so squish would be the same assuming I took off the perfect amount from the top of the cylinder - which I probably didn't. If anything, you err on not taking off enough so you don't have any issue with the piston hitting the head - the minor loss of compression is actually helpful in fighting high engine temperatures. so if you lose 10 or 15 psi, no big deal.

You can always shave the head down to regain that lost compression if it's keeping you awake at night or take a little more off the top of the cylinder.

Anyhow, I haven't even tried to widen the exhaust port yet. I found a machinist who charges me $20 a cylinder to take material off the top of the cylinder. Beats the hell out of spending $200 on a bigger kit and matching head and I could probably get it to a top speed similar to that $200 kit.(edited)

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

Anyhow, I think a performance exhaust is the first upgrade anyone should consider. I mean what the hell were those Yamaha engineers thinking when they designed the stock pipe.

"Hey Asahi, this qt50 project is kicking my ass. It's doing 34 mph now and US moped regulations require us to keep it under 30."

"Can you turn up the oil pump anymore?"

"I don't think so. It smokes like a chimney burning pine wood in the dead of winter as it is and four strokes with the slightest tail wind."

"Throw an S curve into that 24" long header, that should slow it down real nice."

"Ah yea, why didn't I think of that, the S curve in the header trick."

And that's why your qt50 is so damn slow.

Re: Carb upgrade for stock-ish qt50 as first mod.

Rob LaFreniere /

Thanks for the advice. I've ordered the mlm exhaust.

Re: Carb upgrade for stock-ish qt50 as first mod.

Bill Mazzacane /

Assuming a 10:1 compression ratio = 150 psi. (the only compression related data was for a corrected CR of 6.0:1).

Also assuming that the 1.5 mm plate is sandwiched between 2 base gaskets, the actual increased height is closer to 2mm.

Recalculating the CR, it drops from 10:1 to a lower 7.17 CR and a new compression of 107psi. That is a loss of 43psi for using the plate.

A rule of thumb: engine requires ~100 psi to run. Obviously the engine with the plate & gaskets still runs, but a loss of ~43psi demonstrates the need for the cylinder to be decked ~2mm just to maintain stock compression.

At least raising the exhaust port to a 21' to 23' BDT can be easily accomplished.

Bill M.

On larger engines an increase .001" of deck height = a loss of 1psi. In this example .002" = 1psi.

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

material was taken off the cylinder. Extra gasket was accounted for. see my previous post. This is not my first rodeo.

Re: Carb upgrade for stock-ish qt50 as first mod.

Bill Mazzacane /

Jack Rutherford my comments are directed at the OP, not you. I used your posts to prove my point that adding only a pipe to a QT, will not provide decent HP.

You did a lot of extra work.

Too much work in my opinion.

> Bill Mazzacane Wrote:

> -------------------------------------------------------

> 18 degrees of BDT is the absolute minimum,

> but I wouldn't waste my time building or selling a

> tuned pipe unless the customer knew that changing BDT was essential to

> the performance of the engine.

>

> I don't know these companies, but if you (the OP) are under the opinion that you

> can bolt a pipe on to any stock cylinder and get decent HP, that isn't

> true.

>

> Bill M.(edited)

Re: Carb upgrade for stock-ish qt50 as first mod.

Rand Jacobson /

Simple pipe, 14.12 SHA and a little exhaust porting. 40s.

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

i went from 28 to 35 mph with just a pipe on my first qt. i think you did too much work on your posts bill.(edited)

Re: Carb upgrade for stock-ish qt50 as first mod.

Bill Mazzacane /

> Rand Jacobson Wrote:

> -------------------------------------------------------

> Simple pipe, 14.12 SHA and a little exhaust porting. 40s.

++1

Re: Carb upgrade for stock-ish qt50 as first mod.

Jack Rutherford /

that would be for a tomos

> Bill Mazzacane Wrote:

> -------------------------------------------------------

> > Rand Jacobson Wrote:

>

> > -------------------------------------------------------

>

> > Simple pipe, 14.12 SHA and a little exhaust porting. 40s.

>

> ++1

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