Dwell Tachometer for Carburator Synch Job

Just finished the valve adjustment & carb overhaul, now it's time to synch the carbs. I am going to make my own 2-channel vacuum comparison setup from long tubing filled woth tranny fluid & whatnot so that part is covered.

I need a sensitive tachometer to do the synch job (the book told me this) so I have been looking at dwell tachometers.

I came across a Belden PowerPath & a Fox Valley E986 on Ebay, both are digital dwell tachometers. Anybody ever heard of either of these tachs? I can't find jack squat for technical info about them on the internet.

What do you guys use for a tach when synching carburators?

Thanks.

 

A DWELL tach, as i remember, is primarily used to check for proper points gap on a old-fashoned ignition system, but let's wait for our #1 old-timer and perpetual grouch with belt AND suspenders ("you kids get off my damn lawn or I'll call the cops !!") Don Branscomb to comment.

A very accurate tach (must indicate + or - 50 rpm) is useful for setting mixture screws using the idle-drop proceedure ( which really isn't all that critical), but not used in actual carb synch, vaccum gauges ONLY for that.

 

I refer to them as "dwell tachs" but they function as both. Yes, they are for older cars with point-type ignitions but they are pretty sensitive tachometers too. And yes, I was meaning to use them to determine the RPM drop from air screw adjustments. Thanks.

 

Impossible to do the job right without vaccum readings from all 4 cylinders simultaneously.

 

I have an old Allied MAC tach/dwell meter my Dad gave me, Squirrel is right, you will want a four chamber vacuum reader like a Morgan Carbtune.

I think Actron sells a decent tach at Sears for around $30US.

Sears: Online department store featuring appliances, tools, fitness equipment and more

 

Thank you for all the input.

I guess I'm going to call the local bike shop & see what they charge to synch my carburators...

VERSUS

$85 for a Motionpro 4-chamber vacuum gauge + $30 for the tach from Sears. ($115)

I would love to do it myself but how often will I really use these tools?

Tojo's revenge in full effect.

My other bike is a 75' HD Shovelhead. Tuning the S&S Super B carb on it doesn't require special tools & the valves need to be set every 10 years.

 

my garbage can doesnt need its valves adjusted either...its made in america also

 

If I could suggest, purchase a manometer for your tool collection. It will pay for itself in short order.

www.carbtune.com

 

^ This (invisible cities). They really are handy, and very accurate. I generally just make sure the idle speed is close to what it should be, and adjust the carbs at that RPM. If they are really out of whack, balance-wise, the idle speed might have to be raised or lowered during the syncing process, but ultimately the goal is to get them to flow evenly at a low RPM.

Then again, if the pilot screws are way off, adjusting them will affect how the carbs flow, so you'd possibly see a difference. If they are adjusted to some base setting, like 1 1/2 turns out, syncing the carbs first and then fine-tuning the pilot screws shouldn't affect the overall sync that much.

Does your manual say in what order the carbs need to be adjusted and synced?

 

+1 on your +1 Lgn.

I'm not a proponent of the Idle Drop Procedure. This is more an issue of politics and performance. Adjust the pilot screws until the idle is perfect and leave there. This most likely will be 2-1/2 to 3 turns out (about 3% CO on an EGA).

 

Thanks for the usefull information (you know who you are) & also the shitty remarks regarding my Harley (you know who YOU are.) I was just making an observation, didn't mean to get anybody's panties in a bunch. Also, I ride my Shovelhead a hell of a lot farther than "down to the bar".

 

Re. the Idle Drop Procedure, this actually came from a requirement that an EGA be used when setting the idle.

Being creative, Honda came up with an alternate method of using a 25-50 RPM electronic tach. The objective of this procedure is to lean the mixture very slightly from the position the engine likes best.

This was for emissions purposes, i.e. lowering the CO level below the mandated requirement. The rub is that the engine would most likely pass emissions without this procedure.

Once you have the idle dialed in, about 2.5 to 3 turns out on a VF500F, leave the pilot screws where they are. Your engine will perform better.

Hope this is of help!

 

++ on what the other posters have said - a "Dwell" tachomter is used for adjusting old-style points -type ignitions. Contrary to what the manual says, your bike's tach is accurate enough for carb synch - I wouldn't worry at all about the idle speed drop procedure, that's really more of an emissions/politics thing & your bike should pass emissions without setting that anyway.

I also agree with setting pilot screws to 2-1/2 to 3 turns out as other posters have suggested. You might have to touch up the carb synch afterward, but not usually. Not sure whether there's an "official" or "Honda recommended" way to do this but I always start with setting pilot screws to 2-1/2 turns out; then a carb synch; then re-set idle to factory spec +50-100 rpm or so; then fiddle with the screws slightly to get fastest idle; then re-set the idle speed screw; then re-check the synch. Not as complicated as it sounds, especially after you've done it once.

Setting idle 50-100 rpm above "stock" setting doesn't affect gas mileage or engine wear since you're rarely at idle speed anyway. That's more for old-time bikes with points that may go wonky in between adjustments, so you have a little cushion to keep you running at stop lights. May not be needed on a bike with more-modern ignition.

Motion Pro makes a really good manometer. The manometers with fluid in the line seemed to last longer and be more accurate than the gauges, for me at least. They're cheaper too.

Synching your own carbs pays for itself after only 1-2 times, and you should do it after taking the bike out of winter mothballs anyway.

 

Borrow a tachometer to find out what your true RPM's are on your bike and use that for future reference, I have found that Honda tachs can be off as much as 300 or more RPM's.

That will make a difference on your synch job if your are 200 or more RPM on your adjust.

The motion pro gauges are only designed for small displacement bikes that are max 500cc's- I can't tell you how many people (including me) I have heard complaining about the blue fluid getting sucked out of the manometer- Dial gauges will work fine if you are doing infrequent synchs.

 

It's easy to make your own manometer set for about $5 in materials, and you can find directions on-line.

 

If going homemade, more intel can be found here:

http://vfrworld.com/forums/1st-2nd-generation-1983-1989/30292-homemade-carb-sync-tool.html#post268596

 

Cool, thank you. I will just build a manometer myself then. You say I need a 4-channel manometer though?