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  Calling all carb Gurus!

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Author Topic:   Calling all carb Gurus!
Salmon Tub posted 06-07-2002 06:58 PM ET (US)   Profile for Salmon Tub   Send Email to Salmon Tub  
Here is the deal, as I recall, Nissan wants the pilot screws set between .75 and 2.-- turns open. I spent a lot of time getting my baby to idle nice and smooth so as to troll with it. I had to go down from the factory pre-set 2 turns to approx. 1.25 open (each carb is a hair different). As a result, I have lost some acceleration. Now the top end is fine since the jets take over by then. If I hammer it now, you can tell that it is not as zippy as before. I wanted to know if jack-rabbit starts at this setting can do any damage to the engine? I want to take it out a few times for tubing/skiing and really don't want to open them up again since it will be a bit## to get them back at where they are at. Also, small 2-strokes are made to fire up and gun, such as weed whakers, lawn mowers..., is this also the case with bigger blocks, or will constant hammering cause more damage than gentle acceleration?
Bigshot posted 06-10-2002 09:41 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
constant hammering will ALWAYS cause more wear than being gentle, on ANY kind of engine. But it does not mean that it won't last a long time, just not as long as if you never hammered it. I would richen it up some.
Ed Z posted 06-13-2002 05:00 PM ET (US)     Profile for Ed Z  Send Email to Ed Z     
I'm not sure just what kind of engine you have (2 or 4 stroke)... Remember that a 2 strokes fuel mix can also be a function of lubercation (if not a pumped oil mixer)... Most carbs have the idle mixture curcuit still functioning at above idle speeds (just the high speed cuts off at idle, not the other way around)...You might be running a little lean at high speeds... I would first check into giving the carbs a good cleaning (possibly a rebuild) first and then put new spark plugs in too... Then proceed with teaking the idle mix screws... The reason you may need to run the idle screws toward the lean side could be as a result of the floats set to high...If all the above checks out and there are no vacuum leaks and you still find the origonal settings (lean) to work the best at idle, then I would look into going maybe 1 step richer on the high speed jet as a precaution to having lean high speed running... Just my 2 cents...
bsmotril posted 06-17-2002 10:19 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
You want your low speed jets calibrated to supply adequate fuel for the worst case load they would encounter in the RPM range in which they are active. That worst case would be while accelerating up onto plane. By leaning the motor our for a smooth idle, you are likely running lean while powering onto plane. Hammering it is probably preferrable because you spend less time in that lean transition zone before the other jets kick in. Easing onto plane causes longer running in that lean zone. If it were my motor, I would set the jets for a good smooth acceleration transisition with no stumble, meaning you are tuned for adequate fuel for the max load, and live with a stumbly idle knowing that my motor was going to ultimately live longer.
BillS
Salmon Tub posted 06-18-2002 01:39 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Here is the deal, the 90 was actually set at a little under 2 turns open. The 1.25 open I got from my note pad and was a setting I had on my 40 hp. Thus the original post I placed had was incorrect as far as the facts.
The factory calls for the following, set to 1.5 open, adjust +/- .25 until you find the point where the RPM's are the highest, then back out 1/8 of a turn for cold starting. So, 1 5/8 (+/-) 1/4. I set them yesterday at 1 7/8 and it runs smooth, also, this puts me on the richer side to keep the cylinders from getting too hot. I am still not satisfied though.
I know what the problem is now. I have three carbs and three pilot screws. When fully closed, the slots in the screws do not line up with each other, making it difficult to get an exact point of reference. Now, it is simple enough to open them up 2 full turns, then back in approximately 1/8 to achieve 1 7/8 open, the only problem is, I start dwelling on the fact that I can not figure out a way to be 100% exact. In other words, I can be (+/-) 10 degrees on each screw and it is just too dificult to eyeball that. There are no real good points of reference that can be used due to the shape of the carbs. Anyone out there have any "tricks of the trade" that can help me, or am I getting to the point where I am spliting hairs when I am trying to be so exact?
Bigshot posted 06-18-2002 02:06 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
It aint ever gonna be perfect but just because the top runs great at 1 7/8 does not mean the middle will....might like 1 3/4, etc. You have to do each carb seperate.
lhg posted 06-18-2002 02:12 PM ET (US)     Profile for lhg    
I have three carbs on my Mercury in-line 6's and they have low speed idle adjustment screws as you are describing. Too lean of a mixture adjustment does cause the engine to stall out on acceleration as you go past 1600 RPMs. Too rich of a mixture causes heavy black oil to drip out of the prop exhaust after a day or so. It also puts a lot of crud in the water on start up.

Mercury's recommended procedure is as follows, and I don't see why this wouldn't apply to your engine also:

Put engine in gear, at idle speed. It should be turning about 600-700 RPMS, as manual recommends. Starting with the top carb, SLOWLY turn adjustment screw counterclockwise until engine starts to load up and run rough, because of too rich mixture. Then, counting turns, SLOWLY turn back clockwise until engine begins to run rough and misfires from too lean mixture. Then set mixture screw half way between these two extremes. So, for instance, if the distance between between too rich and too lean is 2 1/2 turns, back off 1 1/4 turns for half way. Do this for each carb. After all three have been done, do it again for additional fine tuning. Then run engine up to 4000 prm's to clear it out, and see if it hesitates on acceleration. If so, you're still too lean.

This method is not based on screw slot positions, which can never be accurate, but rather lets the engine tell you where it's "happy".

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