Idle Drop tool

[Jeff Dean]

According to the manual, a tachometer, capable of detecting a 50rpm change is required to properly perform the carb. sync. Referred to as the Idle Drop method. The question is: What tool do you use for this? A digital automotive timing light, a multi meter with RPM's ? I know where the "general" setting on turns out is, so I'm not looking for a "set em' all" at 2 1/2 and go with it answer. Thanks in advance. Jeff Dean #4112

[sr71cbx]

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[Jeff Dean]

Yes, I want to take the time to get it right. Where it "feels happiest" would end up being all screws set the same. All at 2 1/2, or all at 1 3/4, etc. Is what is correct for one correct for all, or is it a compromise? I would think a INDIVIDUAL account and fine tuning of each one would be a benefit. The bike in question is not running its best at that portion of the throttle opening. I know someone out there has done this correctly.

[EMS]

Jeff: If you set everything so it runs happiest at idle, it will do just that. Run happiest at idle, not necessarily where you need power. I have not used this "idle drop method" when synchronizing carbs. I am not sure what instrument would give you a 50 rpm increment reading and how you would connect it? Mark says he runs an electronic tach on his Turbo. Maybe he has another word of wisdom?

[Jeff Dean]

Mark, EMS, Thanks for your words of wisdom. I have seen this done with an automotive tach with digital rpm. Its rather pricey and I was looking for a cheaper alternative. Thanks again guys.

[sr71cbx]

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[Guest]

Jeff,



Just a little insight into what a tach would need to be to accurately register a 50 rpm drop. I work in a lab and typically "instrument" grade things like this have a +/- 1% of full scale accuracy, that means that a tach with the full scale reading of 10,000 rpm would be accurate to +/- 100 rpm or a 200 rpm range so you'll have to find a tach with a 2500 rpm scale and a advertised 1% accuracy, very pricey I'd suspect. There is some merit to setting each cylinder individually, I just setup a friends Valkerie with 6-6 Cobra pipes and did this with a lamda exhaust gas analyzer and there was a considerable difference from the initial setting of all at 2 turns out but and this is a big one, I don't know how you'd do this w/o individual pipes and having them all collected together does have a direct affect on the fuel mixture settings. One thing that I'm considering is a laser thermometer to measure the individual pipe temperatures at the head but I haven't done this yet and am not sure how it will work, I'll let you know. One thing I didn't see is exactly what your having problems with and if it's just an issue of a flat spot that is usually either float or needle settings,or possibly main jet which just about covers it all. Idle mixture has an effect on the whole rpm range but usually won't cause a flat spot although if it's not to severe you can sometimes cover up the real problem that way. I'm with Mark & EMS on this, I usually find a happy compromise on a couple of cylinders and then set them all the same. Good Luck, Steve P.

[Mike Nixon]

The idle drop procedure is not necessary. It is nothing more than an alternative to using the correct tool, the exhaust gas analyzer. Back in the 1980s, some manufacturers were forced by the feds to require their dealers to use an EGA. Honda was able to opt out by borrowing a procedure from their car division, the idle drop test. The objective of the test is to lean the mixture slightly from the position that the engine likes best, for emissions purposes. That's why the 25~50 rpm electronic tach, a rather unusual tool. But the procedure is redundant. Simply adjust the mixture screws to two turns out and after warming the engine explore with another 1/4 to 1/2 turn more. Or, if you must, use an EGA and adjust to 3% CO (carbon monoxide).



Oh, one more thing. Even at three (3) turns out, the engine will still pass emissions at the local state check station. Honda is that anal about the original settings...

[cshores]

I have an electronic Tach I got from Sears back around 1987, and it has a scale 0-2500 rpm in 20 rpm increments. I agree, it may not be extremely accurate but I would expect its precision to be good enough to do the 50 rpm idle drop procedure. For example the Tach may read 960 RPM then drop to 900 RPM in your adjustment procedure when the true RPM was 1000 RPM to 940 RPM.



I am confused by the wording in the 1979 Honda shop manual for the idle drop procedure, it sounds like at one point you are supposed to completely turn in the screw for the number 1 cylinder while the engine is running. Does anyone have a good procedure written for the idle drop method ?