Author Topic: Before I play with jets, how big is too big?  (Read 1606 times)

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Offline Soos

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Before I play with jets, how big is too big?
« on: May 12, 2007, 12:59:47 AM »
Wheehee!!!!!!!!!!!!!!!

My sets of jets FINALLY arrived today  :o :o :o :o :o ;D ;D ;D ;D ;D ;D

Sizes 100, 102.5, 105, 107.5, 110, 115, 120, 125, 130, and 135.

So far, I have tried running on my '79cb650 with pods and 90 mains... WAY lean.
I now want to go to the other end of the extreme(i'm running a old set of 98's right now).

What jet sizes would you put in to experience RICH...(i have a set of used 165's as well, but those seem WAAAAY to big)
What jet would you recommend to have it run just right?(or get me in the ballpark anyway)


l8r


thnx
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline 750goes

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Re: Before I play with jets, how big is too big?
« Reply #1 on: May 12, 2007, 02:57:15 AM »
you have got plenty to choose from - start at 135 then drop three sizes for the first change (120's) then two sizes the next (110's)... hopefully you will find the happy medium - you also need to check for each throttle position for the mains from 1/2 to full throttle ???

hope it works easy for you... :)

Offline jabbadeznuts

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Re: Before I play with jets, how big is too big?
« Reply #2 on: May 12, 2007, 12:14:07 PM »
I'm glad you FINALLY got your jets.  :o I don't know why it took so long  :-\
'75 CB550
'82 Suzuki GN125 - glad to be rid of that thing.

Offline Soos

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Re: Before I play with jets, how big is too big?
« Reply #3 on: May 12, 2007, 03:30:04 PM »
They said they were still waiting on 2 sets, when i contacted then last thursday, i told them to hold off shipping until fri afternoon to see if the rest shows up in their freight, and they thankfully did!

Anyways....
I started with 135's, and scared the holy sh*t outta me on the highway....
ran fine through 3400rpm, stuttered a bit up to 5000rpm, then like a freiking rocket to 9500rpm!   :o ;D ;D ;D
after a mile and a half of 8000+riding, the bike would act like it was out of gas.
I switched to reserve, no better.
I would back off on the RPM's(upshift a gear) and it would go away after 1/2 mile or so.
The third time it did this, it basically died.

Good thing I was in the far right hand lane.
Puled over, removed the key, and started checking for loose wires, blown fuses... ANYTHING.
I was thinking i just fried my new coils or something with the RPM's.

Didn't find anything. went to start her, and she fired right up.
took off, and no problems 'till about 2-3 miles down the road, same effects as before, bogging(slow acceleration) from 3500 to 5000rpm, then a bat outta hell.
The first exit i could get off on, I immediately found the nearest gas station.
Started looking again, and found.... haha, a kink in the fuel line.  :D
hahaha... I didn't re-run the fuel filter where i normally do, and it caused a kink right after the filter.

Fixed it, and WHOOOHOOOO!!!!!!!!!!!!!!!!!!!!!!
the entire ride home, no more hesitation between 3500 and 5000, and no more dying at high RPM.
Never knew I could do 75 in third gear before.

Got home, checked the plugs, and suprisingly not too bad. A bit on the sooty side, but tolerable if i cleaned the plugs once a month i'm guessing.
But i was affected with twist-itus on the ride back home, that may have had something to do with not too bad a layer of soot.(6000rpm+ for 6-7 miles, more than half of that at 8000+)

Changed to 125's and tried the same route.
This time with no kinks in the fuel line, in fact i re-routed it so it would be near impossible to kink it again.
Not too much difference power wise, still pulled like a bat on fire.
At the end of the ride(this time with 12-13 miles of 6000+rpm) the plugs were not sooty but a shade darker than i want to run them.


I just got back from a ride on the same route same RPM's with 115's, and a noticible difference in power, mainly at the RPM range of 7000-9000.
Had to roll the throttle along with the increasing RPM's, or the acceleration was flat, compared to the runs with the 125's, and 135's where going from 3/4 throttle to WOT made me feel the acceleration.

I'm going to run with the 115's for a week and track my MPG, and next weekend I am going to try 120's.
As long as I stay within the 30+(preferably 35+) range, i'll be willin' to sacrifice milage for fun.

ARGH, I hate removing the whole carb assembly just to change 4 small bits. >:( >:(
Although I can do it without spilling the gas in the floats now, and in under 20 minuites to take 'em out, change jets, and install them.

l8r
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline eurban

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Re: Before I play with jets, how big is too big?
« Reply #4 on: May 13, 2007, 11:11:16 AM »
A few things to consider here.  The proper test for main jet size would be to do a true plug chop at WOT in top gear.  The technique that you describe will not really distinguish between issues with the main jet needle position/profile and main jet size.  Both of these carb systems overlap and while changes to the main jet size will effect mid throttle settings, these changes are not really the correct way to alter the mix at mid throttle.  If you really want the best performance you will need to at least isolate and tune the idle circuit, the main jet needle position and the main jet size.  This will help to give you carburetion that performs well in the wide range of conditions that you encounter on a street bike.  On your bike you may need to use shims on your main jet needles as they may not have multiple clip positons.  Also tune your carbs only after all the bikes systems are in top working order.  If for instance your idle jets are plugged or your acellerator pump is not working then you will be wasting your time swaping jets and reading plugs.
« Last Edit: May 13, 2007, 12:24:40 PM by eurban »

Offline number13

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Re: Before I play with jets, how big is too big?
« Reply #5 on: May 13, 2007, 12:54:18 PM »
I was poking about the interweb last week and found this excellent general guide to
jetting,hope it is of some use.

http://www.xs11.com/forum/showthread.php?s=2146e8c3cd3da67d492403c0b1bad069&threadid=382


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Offline Soos

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Re: Before I play with jets, how big is too big?
« Reply #6 on: May 13, 2007, 02:15:10 PM »
If you really want the best performance you will need to at least isolate and tune the idle circuit, the main jet needle position and the main jet size.  This will help to give you carburetion that performs well in the wide range of conditions that you encounter on a street bike.  On your bike you may need to use shims on your main jet needles as they may not have multiple clip positons.  Also tune your carbs only after all the bikes systems are in top working order.  If for instance your idle jets are plugged or your acellerator pump is not working then you will be wasting your time swaping jets and reading plugs.

I have timing set right on, have recently re-checked valve clearance, Idle jets were cleaned 2-3 weeks ago, Idle circuit has been dialed in before I had started playing with my new jets, Emulsion tubes are good on the main jets, and the accel pump is working, plugs are gapped correctly, double checked and greased advance unit before riding season(it was working, but while i was in there..),
And I have been verifying/adjusting carb synchronization with each time I remove the carbs....

I can change needle height using shims like you said, but I can only shim it higher(richer), I cannot shim the other way unfortunately.
I figure if I get to the point where i am very close to where I want the plugs to look, I can then start playing with the needle shims for a fine tune.
Wish I could replace the jet needles with adjustable ones.

I have done a 3/4 throttle, and WOT plug chop this morning with the 115's,  it does show a bit rich on a plug chop.
Seems richer at WOT throttle than 3/4 throttle, making me think that a shim under the needle would help even it out a bit.

After riding it last night, and a bit today, plus the plug chop I am thinking that a drop in jet size next weekend is in order instead of going to the 120's..
107.5 or 110's might be geting me closer to where it needs to be.
May have to break down one day and save for a lambda tesrter though.
I would imagine that it would be WAY easier than doing plug chops, but I have never seen or used one.


Nice link there #13!
by those calculations I should be running 2 sizes over stock.(4 individual filters, stock 4-2 exhaust)
doing thatt that, i should be perfect at 95's.
With 98's it read lean while doing plug chops, and norm driving conditions.
That could be due to mods I have made to the carb bodies themselves though. Whether they were for the better or not, only time will tell, because I am by far not a carb expert... I just hate leaving well enough alone. :)




l8r
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline eurban

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Re: Before I play with jets, how big is too big?
« Reply #7 on: May 13, 2007, 03:02:27 PM »
Rather than spend for a fuel mix sensor why not take a trip to a decent local dyno shop?  They should be able to sniff your exhaust while the bike is under load.  Your current thinking of dropping the mains and slightly shimming the needles sounds look a good direction to go. 

Offline Soos

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Re: Before I play with jets, how big is too big?
« Reply #8 on: May 13, 2007, 04:08:13 PM »
slightly shimming the needles sounds look a good direction to go. 

Yeah I would go to a dyno shop if i had the spare cash right now.
I figure after the riding season(or towards the end) i may try that out to see.
I know a shop about 20 miles away that charges only $65.00 for 3 pulls, they will do the exhaust gas analysis as well as a HP test at the rear wheel.


Any ideas as to what shim thichness should be?
I am thinking I will be making my own shims(unless i find some small washers that fit right). So any thickness above 0.015 is no problem at all, under 0.010 and I think i would have problems making it without tearing/crumpling/seriously distorting it while parting it off.
But I really have no idea of the steps to go in...
I will check what the spacing is on my cb750 carb needles on monday at work.
Does anyone think that the clip steps on 750 carbs are spaced too far apart, or too close together?
I have forgotten the model# of those carbs, they are stock '74cb750 carbs(i beleive)

l8r
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline eurban

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Re: Before I play with jets, how big is too big?
« Reply #9 on: May 13, 2007, 05:51:28 PM »
On my 78 750K I have used small electronics washers available from Radio Shack as shims.  I think they come in a variety pack that has different size washers.  The problem was that the material thickness wasn't always exactly the same so I had to pick and choose till I found a set that was consistent thickness.  I have since replaced my stock needles with a multiple position set from a 77 model so I haven't been experimenting with shims in a while.  I would think that  using the distances between clip positions from a adjustable 750 needle as a guide would make some good  sense although if you get really good at doing plug chops and reading the plugs accurately you might want to use shims that are the equivalent of a half step.

Offline brandon

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Re: Before I play with jets, how big is too big?
« Reply #10 on: May 13, 2007, 09:46:19 PM »
OP: Where did you purchase your jets from. Reason I ask is that on my 400 I bought the bike used and it came with pods only and no original air box. I just want to make sure the jetting is correct and by the link the other guy provided (nice link btw) I think I may be a bit on the small side.

Thanks in advance.