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Power on the main jet

  • Thread starter Thread starter Oliverstrother
  • Start date Start date
O

Oliverstrother

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It's running a new 4->2 exhaust (Mac I think), and a stage 3 dyno jet kit.

I've finally had the chance to get the '82 gs650 on the highway to see how it performs when really pulling and it's struggle to get over 55mph.


Isles good, plenty of chunk when running on the needle jet. It appears the main jet isn't the right size or the fuel isn't getting through.

Any tips? New main jet? Bigger or smaller?
 
Oliverstrother said:
It's running a new 4->2 exhaust (Mac I think), and a stage 3 dyno jet kit.

I've finally had the chance to get the '82 gs650 on the highway to see how it performs when really pulling and it's struggle to get over 55mph.


Isles good, plenty of chunk when running on the needle jet. It appears the main jet isn't the right size or the fuel isn't getting through.

Any tips? New main jet? Bigger or smaller?
Click to expand...

Is it rich or is it lean?

Hard to tell from here.

I would guess it's too lean, but I have nothing to go on.
 
As tkent asks, "rich or lean?"

Are you running a stock airbox or pods?

If you have the stock airbox, you might be running a bit rich with Stage 3.

.
 
yes, its running pods. I'm not sure if its rich or lean. i think thats my main problem.

I'm not sure the intake pulse effect could be this dramatic, there really is very little power on the main jet.
 
Oliverstrother said:
I'm not sure the intake pulse effect could be this dramatic, there really is very little power on the main jet.
Click to expand...

The main jet is the one for high power, full throttle wide open operation. Try a bigger one. See if it's still too lean.
 
If it's lean you may burn valves in five minutes. Put in bigger main jets first.
 
any recommendations on how much bigger I should go? they come in increments of 2.5 right? It has 160s right now.
 
Last edited:
What size is in it now? I believe 110 is stock? are you running pods or the stock air box?
At which elevation are you mostly riding?
 
its running pods, and an aftermarket exhaust. ride around sea level all the time. it has 160s in right now.
 
Oliverstrother said:
any recommendations on how much bigger I should go? they come in increments of 2.5 right? It has 160s right now.
Click to expand...
exclaim_sign4.gif
CAUTION, WARNING, BE CAREFUL. :eek:

Those 160 jets are NOT the main jets that you need to be changing.

The main jets are the ones in the middle of the float bowl.

Your stock jets are 110. For pods, you might need to add 3-5 sizes. For your pipes, maybe another 1-3 sizes, both of them depend on how freely flowing those items are. That means that you might need anything from 4-8 sizes up from stock, or anything from 120-130.

Along with upping the main jets, you will probably need to shim the needles to raise them.

.
 
Steve, he's already got a Dynojet kit in it, but he hasn't mentioned which size jets.

Should have an adjustable needle anyway.
 
ok, quick update. The bike has been taken to the edge of the freeway. plugs pulled. scrubbed gently with a wire brush and the bike was then thrashed at full throttle for 5mins.

The bike pulls smoothly to aroung 45-50mph, then struggles to get much beyond 55mph. Kinda pulls a little, then retards a little. but very little power to go any further.

Pulled one or two plugs and they look lean.

The Dynojet kit <http://www.dynojet.com/pdf/3310.pdf> comes with two main jets (the one in the centre when the bowl is removed) options (DJ155 and DJ160)

Currently has the DJ160's in it.

As its running lean, I guess I need to enlarge the size of the jet a little.

How much bigger should I go for a first try? 162.5? 165? 167.5? 170?

As its not only running lean, but also not really making any power, I thought try the 170s?

Any advice would be much appreciated.

Ols
 
There is something else wrong.
That motor should pull to redline with those DJ 160s with no problem.
I'd check out valve clearance (may already of damaged valves) or ignition. I've seen poor ignitions do the same thing as with zero clearance on the valves.
Edit: could also be from leaky intake boots/o-rings
 
Valve clearances all within spec, but on the tight side. Book says 30-80microns is the spec.

Yeah, i heard about the leaky boot rings before, I took one off and it looked night perfect. not crumbled/cracked etc.

I could replace them all anyway, know a good supplier?

Thing is the problem isn't the whole running range, and its very very good on anything except the main jet.

Yeah, poor ignition will retard the top end, but it does gets a good spark, and has a new R/R, battery and stator.

At the minute, I"m sold on:
- Main jets too small, as the plugs look lean after 5mins on main jet. Order some 165's.
- Cracked boot seals, maybe. Cheap to replace so worth doing to eliminate as a cause.
- recheck valve clearances. Easy and simple to do. (and I have a new gasket waiting to be used.)
 
When you put a jet larger than a 150 mikuni (160 dynojet) in a 650 it will not be lean.
I don't even have 150s in my fire breathing 1100.
If your valve clearance is good I would replace the intake O-rings.
The ignition will not retard the timing but be weak and put the fire out. The plugs will look lean by being washed out with gas.
http://www.cycleorings.com/
 
That 160 sure sounds like overkill even factoring in your oddball pods. So it won't go 60 mph in fifth- will it go 40 mph in third?? Or are you hitting some wall at 5000 rpm? How about going downhill (less load)-is it happier?
 
Hey, thanks for the input all.

Thanks for the reference point on the jets for the 1100 fire breather.

Yes it will go 40mph in third easily. The only wall I hit is around 9k rpm, under heavy load perhaps in 3rd gear or higher. It kind of goes 8k, 8.5k, 8k, 8.5k, 9k, 8.5k, 9k then kind of top out there.

It was happily zing up to 12k rpm in first or neutral. I'll check to see if will do this in second on the way home tonight.

Sorry, didn't mean poor ignition would retard the timing, just retard the performance.

I'm trying to get a video on the go, to show what's happening.

New O rings on their way. At least that will be one potential cause eliminated.
 
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