How do you set correct idle speed on the carburetor?

[central52]

I finally got my 47 Plymouth started and running nicely. With rebuilt carb it idled fast, so I turned it down a little. Runs great, but I'm curious as to how to set it exactly to the manual's setting of 450 to 500 rpm. I lost count after one when looking at the fan go around. Does the amp gauge help? When it idled fast the amp reading was close to +35, when I lowered the idle, the gauge went down to half the distance between 0 and +35. I have a new voltage regulator and rebuilt generator. Ed P.

[Plymouthy Adams]

only accurate way to read the rpm is with a tachometer..the current indicated on the amp meter shows the current being returned..it will increase per rpm until the load/charge of battery has been restored..with a good battery the amperage used to start the engine has to be restored..again this is just an indication of current flow.

[grey beard]

Your ammeter tells you only if current is flowing at all, and in which direction - charging or discharging. It may change with idle speed and be affected by it, but has no organic relationship, otherwise.

The only way whereby you can be certain of idle speed in regard to exact specs is to use a tachometer. After many years of tuning engines, the average skilled tech can get it pretty close by ear. Normally this adjustment is the very last one made to a carburetor, after all ignition adjustments - timing, advance mechanism and point gap - have been made and the idle mixture adjustment has been made to the point of maximum manifold vacuum at idle. THEN is the time to adjsut that final hot idle speed down at your throttle shaft stop screw. JMHO

Lots of Luck

[oldmopar]

As the others stated you need at least 2 pieces of equipment a tachometer and a vacuum gauge . When the engine is warm set rpm with tachometer and then set carb idle to get highest vacuum reading.A timing light would also be a useful item

[greg g]

Well I got a way that uses the speedometer, and a flat piece of road. With the stock rear end and 27.75 inch diameter tires, you shuld be able to let the clutch out and idle along at just under 10 mph. this will vary a bit with the different rear ratios and tire sizes, but a properly tuned and running flat 6 should hold 8 to 12 mph and be idling between 450 and 550 rpm. This may be different for a gluid drive eqquipped car, but it is true on my p15.

[Tom Skinner]

Central52,

Trying to recollect the proper order of tuning lets see:

1. Change the Points. Condenser, and Plugs.

2. Check the Dwell with a Dwell Meter. to see if Points are correctly set.

3. Set the Timing. using a Timing Light.

4. Check your Lean/Rich Mixture at the Carburator

using a Vacumn Gauge at the Intake Exhaust Outlet/Opening

5. Finally using a Tachometer set your RPM's at the Throttle Screw at the

Carburator.

Or just set the Timing with the Dizzy Loose using a Vacumn Gage then set Lean/Rich Screw at the Carb, each time obtaining your highest reading, then Use a Tach to check RPM's. With a Fluid Drive seek 7mph, in gear from a stop.

Tom

[Powerhouse]

I usually just do it by ear...if it sounds good and itsn't gonna race or stall..that works. a friend of mine can run his engine so slow that it hardly looks or sounds like it's running...and it's an original engine. The fan is so slow you can almost follow the blades...but he drives rather slowly...which is not a bad thing. I go slow to...well...it is a 39 Roadking...with a well needed valve job.

Anyway...how does one check the vacuum at idle? Do ya disconnect the vacum line to the vac advance and put a gauge on it? If so what should it read for a 53 dodge 218?

[Jim Saraceno]

A vacuum gauge will tell you many things, go here to see how to read one: http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

I have tried to set the timing with a vacuum gauge, but it would be advanced by 10-15 degrees if I set it at the highest reading so I stick to the timing light.

[greg g]

vacuum for a stock engine in good condition should be in the 18 to 21 inch range at idle witha steady needle. Needle movement direction and tpe of mvement will tell you a lot regard the engine's condition. Probably as noted on the linked page.

[Don Jordan]

Do you need a 6v tachometer? Where do you get a vacuum gauge? I finally got a 6v timing light. This is all very interesting.

[Jim Saraceno]

Sears has both

[David Mac]

Does anyone know if having a reground (3/4) cam has an effect of vaccum readings at idle?

[greg g]

performance cams will have a tendency to reduce vacuum at idle, as will extra carbs. You hould have a steady needle and the reduction should be no more than 5 or 6 inches. I run a stock cam but dual carbs, seems my dormal reading is 16 inches.

[Plymouthy Adams]

Not only will the cam have an effect on the vacuum..it will also a have a severe effect on idle quality and for the most part require a higher baseline idle though by ear it will sound like 600 rpm..electronically it will behigher. Set your vacuum to highest steady reading..if you see needle bounce..check for vlave timing..(electrical to mechanical ie move the distributor) 3/4 cam will have an effect here also..the grinder of the cam should have included a timing tag with the cam for these new baselines.

Don't assume a cam grinder is on the money...I had one here from a national know cam grinder...the cam when V-blocked showd lobes as much as .030 off on lift and about so much off on the centerline you could not say for sure it had a centerline...it would make a good stick to whack a rabid dog with only...the entire engine was overbuilt for the street..vlaves was hitting the pistons..this so called top line machine shop did not do their homework for sure...I rebuilt the newly rebuilt engine including relieved pistons..Erson 1966 vintage cam (sweet) better lifters..that was 17 years ago..a well built cam will drop in from the box without need to degree..but if you going for the peak ..suggest dialing it in..

[randroid]

Central,

Before you are finished, you might also want to polarize your generator and regulator. The amp readings you mentioned indicate that perhaps you haven't done that yet, but it's a simple process described lots of places.

-Randy

[Don Coatney]

Central,

Before you are finished, you might also want to polarize your generator and regulator. The amp readings you mentioned indicate that perhaps you haven't done that yet, but it's a simple process described lots of places.

-Randy

If the generator were not polorized it would not show any charge. Polarizing of the generator is done at the connections on the regulator but the voltage regulator itself needs no polarization.

[David Mac]

Tim:

Thanks for the info. Do you have a good cam grinder?

I had a local machine shop, who came highly recommended, but thru the course of his work (over a year) we ended up on not so good terms. I was told to come pick up my stuff. The cam came from Precession cams in Calif but no info with it. On the box it said 3/4 grind, thats all. I tried calling the cam shop with no success as they say the machine shop who ordered the cam has that info. I'm not calling the machine shop again. I suspect what you are saying about the centerline is my problem as I had a difficult time adjusting the valves. I still have a noisy valve that I can't seem to get rid of. When putting the Vac Gauge on I get a reading of about 16 (duel carbs) but it bounces. What would you recommend I do next?

[Jim Saraceno]

I still have a noisy valve that I can't seem to get rid of

One thing that can throw your valve adjustment way out of wack is when the tappets get hollowed out on top.

This was was hollowed out .012" which threw the adjustment out by a factor of 2.

[Plymouthy Adams]

David,

As Jim pointed out, the condition of the lifters is the key to easy even adjustment of the valves. Did the machine shop do the install of the cam and lifters? If not and you did the assembly this would have fallen on you to check closely. A 3/4 cam will have a tad of uneveness in the idle..but should be very smooth a few hundered RPM higher..

Without the timing tag and baselines you are pretty much left on your own but by keeping in mind a few extra rpm for idle, a few degree advanced on the timing and of course remembering that SMOOTH will be higher yet in the RPM band you can pretty much tune your car by ear and perfornace runs after the fact..when you have it the way you like it..check your timing with a timing light as your new baseline, also benchmark your rpm. Please ensure that the advance both mechanical and vacuum in your distributor is working so to take full advantage of this cam. If you are running dual carbs..be sure they are blanced both at idle circuits and run circuits..use a good flow meter and tach..tuning by ear and a tube in the ear is close but does not lite the cigar so to speak.

I highly suspect that the cam grinder in your case has the specs but will not give them to you as he suspect they may be a problem and will not enter as third party from the machine shop. But to be brief, it should run out of the box if done right..V-blocking it you can retrieve the specs..symetrical in each cylinder is the main issue..

I have no known good local type cam grinder...surely in the A-town area you live there has to be a speed shop or two..I am not sure if Clifford would have specs for you or not for the flattie..they deal in the likes of four cyl running like a six and a six running like an eight..their specailities. Maybe an e-mail there will net answers.

[David Mac]

Thanks Jim & Tim:

The machine shop installed the cam bearings. I installed the cam along with new lifters, springs & valves.

Tim

Explain a flow meter. I know where the air fuel mix adj is. How do you adjust the run circuits? I have the 2 - 2 Barrel carbs from Langdon. I spoke with Tom a while back about adj the carbs and he said he didn't have time to teach me all about carbs that I should go to my carb guy. I don't have a carb guy. Was a bit disappointed in his reply after spending as much money with him as I did, but thats the way it goes. Maybe when I am closer to getting my car on the road I should give you a call.

Merry Christmas to you and the wives,

David Mac

[Plymouthy Adams]

Very curt reply I would say...if you have the time to sell and accept a man's money you should at least have the courtesty to answer a question. the answer from him should have been that a flow meter will be required to sync the airflow of both carbs to be equal at low speed while using the idle circuit and then at a higer speed...when the run circutis are in play. The flow meter has to be able to measure the air passing in to the throat of the carb and thus has tomake a good seal..oft times an adapter is needed depending upon the meter used...velocity stacks are handy here...I use the item below..quickest picturre I could find of one.

http://cgi.ebay.com/CARB-SYNC-DUAL-CARB-ADJUSTING-TOOL_W0QQitemZ250199805180QQihZ015QQcategoryZ33550QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[martybose]

While Tim's answer is technically very accurate, I don't think it is as much a factor with our manifolds, which always have both carbs feeding a common plenum. It's really important when there is no connection between carbs, as you want all of the engine running the same.

On my Langdon setup I just backed the idle screws out all of the way, played with my linkage until both primaries opened simultaneously, then set the idle speed by adjusting both equally; seems to run fine.

Marty

[Plymouthy Adams]

I think that you will find a number of dual carb owners owning and using sync meters here on this forum...if you are content with "seems to be fine" I am tickled pink...

[Don Coatney]

Marty and Tim;

Using the "sync" tool on Carter B&B's is easy. Not so easy on the Carter Webbers. I used the tool to adjust the throttle linkage on both of my Carter B&B's until it was even. I found that adjusting the idle mixture screw had little or no effect on on the flow but linkage adjustments had a big effect. Once I got the linkage set so both carburetors had an equal flow I then adjusted the idle mixture and throttle stop for the best idle not using any tools but my ears. Once I made this adjustment I have driven several thousand miles with no further adjustments required. And my #1 race car runs great!

When I was recently in San Francisco I had the thrill of riding in and hearing Marty's engine. I was impressed to say the least. It appears that the adjustments Marty made work well.

I also had the thrill of hearing Pete (blueskies) Andersons #2 engine run last summer. He has dual Webber carburetors on his engine and it also runs well. At the time I heard it run I do not believe he had used any flow tools to set it up. He may have done so now and hopefully he will chime in with comments.

I will be installing a pair of the Carter Webber two barrel carburetors with progressive linkage on my engine. As I mentioned the flow meter I have will not work on these carburetors as is. So I will do the same as Marty and eyeball the linkage to dial it in until I figgure out how to make the tool work.

[Plymouthy Adams]

I will admit I am in the dark on the style and construction of the carbs in question but when using this tool on my webber duals years ago I could only use it with the velocity stack mounted..(see comment in first post) I also admit I did not find said list of adapter for the Uni-sync in a quick web search..though I am sure they must exist. I have also used this tool to sync 4 carb motorcyle on a single mainfold common pull linkage, off the engine...for a perfect balance..you really don't need an engine if you have the carbs on a common manifold and common point linkage, but it is much much easier