Does higher top end = less low end?

gufazi /

Apr 26, 2008 03:29PM

I finally got past the break in period for my Tomos 2008 Streetmate R and installed a BiTurbo pipe and a Uni air filter. Since I'm a mechanical and moped newbie, I'm going to do some plug chops and change the jets next.

Anyway, I gained 5-7 mph top end. Because there are so many hills where I live it's hard to get an accurate flat ground reading.

But it seems that I've lost some low end torque from a dead stand still and I swear I'm climbing hills a tad slower than before I put on the pipe and filter.

Is this typical?

Will it improve with correct jetting?

Also, where do I make adjustment to the A55 carb so I can fatten the mix a little before I get to upjetting it in a couple weeks?

Re: Does higher top end = less low end?

justin turner /

onlythebest131

Apr 26, 2008 03:47PM

umm, jet properly first then see what your low end is like. and if it is still bad, think about changing up those sprockets yo.

Re: Does higher top end = less low end?

David Ciao /

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davidw

Apr 26, 2008 03:51PM

yes, if you are changing things, like gears and timing.

but also a pipe should add to both when you tune properly.

a top speed pipe doesnt give you as much torque and vice versa but they should all increase both when compared to a stock set up

Re: Does higher top end = less low end?

Slocketman /

slocketman

Apr 26, 2008 03:59PM

Quote = 'gufazi'

"But it seems that I’ve lost some low end torque from a dead stand still and I swear I’m climbing hills a tad slower than before I put on the pipe and filter.

Is this typical?"

With the Biturbo, yeah, that's typical. It's a pretty good pipe, but not the best, and it is known to take a bit of your low end. Does put a few more mph up top though.

Quote = 'gufaz'

"Will it improve with correct jetting?

Also, where do I make adjustment to the A55 carb so I can fatten the mix a little before I get to upjetting it in a couple weeks?"

Power throughout the range should improve a bit with correct jetting. What brand, model carb comes on the A55? The way to change the mix on basic carbs is usually to change the clip setting on the jet needle for midrange, and change the main jet for WOT.

Re: Does higher top end = less low end?

gufazi /

gufazi OP

Apr 26, 2008 05:12PM

Thanks for the input. The carb is a Dellorto PHVA.

Please excuse my ignorance (I said I was a newbie) .....

With the carb intact and the motor running I can locate idle screw. To the left of it is another (flathead) screw. I can turn it clockwise (screw goes in) a few turns and it kills the motor. I can also turn it counter clockwise (screw goes out) with a few turns and it kills the motor. I assume this the clip/mix adjustment setting.

What direction does what and which way should I go?

Re: Does higher top end = less low end?

.Allen Schlung /

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allens

Apr 26, 2008 05:41PM

Yes, very typical. Pipes are tuned for maximum efficiency for a specific RPM (usually higher), so If you bolt on a pipe with a larger expansion chamber, your bike does not run as efficiently in the lower RPM range.

Re: Does higher top end = less low end?

Slocketman /

slocketman

Apr 26, 2008 05:41PM

Don't assume :-)

That idle screw just adjusts your idle, so the engine doesn't die at idle, or so it doesn't idle so high that it tries to move forward.

All the adjustments that you need to make to lean/richen the mix are inside the carb, has to be taken apart to get to them.

Attached is a diagram of a Dellorto PHVA, I wish it was labeled, can probably find better in a Dellorto manual. But anyway, the main jet is part 4, adjusts your mix at WOT. The jet needle is part 2, changing where the clip is on the needle adjusts your midrange mix.

Re: Does higher top end = less low end?

gufazi /

gufazi OP

Apr 26, 2008 06:12PM

Slocketman,

Thanks - trying not to assume.

I am familiar with the idle screw. I was talking about what I assume (gulp!) was the air mix screw to the left of the idle screw in this pic http://www.dellorto.co.uk/editor/uploads/images/phva.jpg

Re: Does higher top end = less low end?

Slocketman /

slocketman

Apr 26, 2008 09:45PM

Sorry, not too familiar with the PHVA. It looks to me like you have one idle screw that adjusts the idle rpm, and one that adjusts the idle mixture. Regardless, adjusting the idle mixture screw isn't going to affect anything other than the idle really.

The adjustments to the jet needle (slide needle) and main jet you need to make when you put on an expansion chamber exhaust and high flow airfilter will affect your half throttle to WOT performance.

In addition to changing the clip setting on the slide needle, you can get different needles as well if you can't get enough adjustment from the different positions.

"This site":http://www.dellorto.co.uk/merchandise/products.asp?CategoryID=2&PartsectionID=26 shows all of the parts in that diagram, and

"this site":http://www.thisoldtractor.com/gtbender/dellorto_slide_needle.htm#slide_needle_specifications_for_phva-phbn_carburetors_-_-a_slides- shows all the different needles that you can get.

Re: Does higher top end = less low end?

Slocketman /

slocketman

Apr 26, 2008 09:47PM

This site:

http://www.thisoldtractor.com/gtbender/dellorto_slide_needle.htm#slide_needle_specifications_for_phva-phbn_carburetors_-_-a_slides-

shows you all of the different needles you can get for the PHVA.

Re: Does higher top end = less low end?

gufazi /

gufazi OP

Apr 27, 2008 09:22AM

Thanks everyone. Looks like I'm going to have to open up the carb sooner than I thought.

Re: Does higher top end = less low end?

gufazi /

gufazi OP

Apr 28, 2008 12:12PM

On a hunch, I took off the Uni filter and put the stock box back on. I left the biturbo on and did some running around. Lo and behold my take-off and hill climbs were back to stock if not a little better. My top speed was about 3-5 mph better than stock, 2-3 lower than with the Uni on.

I also did a (my first) plug chop soon after just to see. It looked pretty good, kinda dark brown and dark gray, no pitting, and white area was white. No much different than the older plug I took out. So I'm thinking I won't need to upjet if I only leave the biturbo on. It was very cool yesterday (50s). Should I expect different results when the hot humid air drops on DC latter this summer?

Re: Does higher top end = less low end?

coco /

cocoloco105

Apr 28, 2008 02:01PM

when it gets hot outside you may need to down jet

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