Fluctuating RPM's
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Fluctuating RPM's
I saw another thread where someone was talking about their RPM's fluctuating and it got me thinking about mine. My RPM's fluctuate by around 50 or so when engine is idling and you can hear a pick up and a drop. Since I never have done this before I thought this was normal. If it is not does this mean incomplete combustion and if so could this be causing the smoke in the exhaust? When I was checking the compression the #8 plug was discolored.
Also, when idling, especially upon start up, the RPM's slowly raise and to get it to drop I have to tap the gas pedal which seems to unstick something and make the RPM's drop back down.
Also, when idling, especially upon start up, the RPM's slowly raise and to get it to drop I have to tap the gas pedal which seems to unstick something and make the RPM's drop back down.
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From: Angier, NC
You did not say what the RPM is set to and what it goes up to.
As for the idle going up on start up you are running a carb with a choke.
The choke & high idle cam comes on because the motor is cold. Also it is hard for the air/fuel mix to light/fire off till the motor has some heat in it.
As the motor builds heat the RPM will go up.
The choke/high idle cam should start to pull off as more heat builds.
Thing is the high idle cam can not move as it is held in place by the fast idle screw. When you hit the gas the screw moves away from the cam and the idle will lower.
That's my story and I am sticking to it!
Dave ----
As for the idle going up on start up you are running a carb with a choke.
The choke & high idle cam comes on because the motor is cold. Also it is hard for the air/fuel mix to light/fire off till the motor has some heat in it.
As the motor builds heat the RPM will go up.
The choke/high idle cam should start to pull off as more heat builds.
Thing is the high idle cam can not move as it is held in place by the fast idle screw. When you hit the gas the screw moves away from the cam and the idle will lower.
That's my story and I am sticking to it!
Dave ----
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From: Northeast, OK
I don't think anything was said about ignition timing. However, I wonder if that may be the issue on the fluctuating RPM's. Where is your vacuum advance connected? If it is to the manifold then that can cause "hunting" of the R's. Check to see if there's vacuum to the advance at idle.
On the other hand, a rich idle air/fuel mix can cause an RPM fluctuation. So, look at the vacuum advance first and, if that isn't it then maybe the turn the idle mix screws in 1/4 turn each and see what happens.
On the other hand, a rich idle air/fuel mix can cause an RPM fluctuation. So, look at the vacuum advance first and, if that isn't it then maybe the turn the idle mix screws in 1/4 turn each and see what happens.
FTE Legend
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From: Angier, NC
Ok there is 3 parts to the timing total.
Base, Mechanical, Vacuum.
Base: you check at idle vacuum disconnected & plugged. if checked that way you have 12 BTDC
Mechanical: with everything just like checking base timing bring the RPM up to 2500 and check timing. Take this reading remove the base timing (12*) and this is mechanical timing.
Total: now hook up the vacuum, bring the RPM back up to 2500 and note timing. Remove Base & Mechanical and this is what vacuum adds to the total.
So base of 12*
Say mech. of 15*
say vacuum of 12*
total of 39* = 12*+15*+12*
Most spec books should list what each part should be from the factory.
I would use them as a starting point and adjust 1 or 2 or all 3 to get the best performance.
Dave ----
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From: Northeast, OK
If we are looking for fluctuating RPM then it isn't likely to be either initial/base timing or mechanical advance. I agree that he can adjust them to dial in the performance, but I would expect only vacuum advance to make any difference in fluctuations.
Laughing Gas
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If it idles fast cold and you can kick It down once it warms up a bit the choke on your carb is working correctly. Check.
950 in park and 750 in drive is about right for idle speeds. There might be a sticker under the hood that specifies the correct idle speed.
Generally, if it doesn't stumble when you tip in the throttle coming off the line, pulls good through the RPM range with smooth power, gets reasonable mileage, and starts okay hot or cold, then the carb should be okay.
Now about this "fluctuation"; is it a smooth increase and decrease in idle speed or is it more like your tach needle is bouncing.
Because if it's bouncing you should check for vacuum leaks and bad wires. It's either a lean miss at idle or an ignition miss.
Did you say spark plug #8 looked different? Start by inspecting that wire and work your way back to the dizzy cap and rotor. If plugs, wires, cap and rotor are old and questionable replace it all as a maintenance item and go from there.
To check for vacuum leaks I like starting fluid. With the motor running spray a LITTLE bit here and there around the intake, base of the carb, and vacuum lines. If the idle speed suddenly picks up and smooths out momentarily you may have found a leak!
950 in park and 750 in drive is about right for idle speeds. There might be a sticker under the hood that specifies the correct idle speed.
Generally, if it doesn't stumble when you tip in the throttle coming off the line, pulls good through the RPM range with smooth power, gets reasonable mileage, and starts okay hot or cold, then the carb should be okay.
Now about this "fluctuation"; is it a smooth increase and decrease in idle speed or is it more like your tach needle is bouncing.
Because if it's bouncing you should check for vacuum leaks and bad wires. It's either a lean miss at idle or an ignition miss.
Did you say spark plug #8 looked different? Start by inspecting that wire and work your way back to the dizzy cap and rotor. If plugs, wires, cap and rotor are old and questionable replace it all as a maintenance item and go from there.
To check for vacuum leaks I like starting fluid. With the motor running spray a LITTLE bit here and there around the intake, base of the carb, and vacuum lines. If the idle speed suddenly picks up and smooths out momentarily you may have found a leak!
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It is pretty bouncy, as in it will go from 950 to 920 with only a couple of numbers in between. I am using a digital tach connected to the #1 plug wire. All of my plug wires are new and the distributor cap and adapter is new as well. I will try the starting fluid test.
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To check full timing you need a dial back timing light or timing tape on the crank to read as much as 45*+.
Ok there is 3 parts to the timing total.
Base, Mechanical, Vacuum.
Base: you check at idle vacuum disconnected & plugged. if checked that way you have 12 BTDC
Mechanical: with everything just like checking base timing bring the RPM up to 2500 and check timing. Take this reading remove the base timing (12*) and this is mechanical timing.
Total: now hook up the vacuum, bring the RPM back up to 2500 and note timing. Remove Base & Mechanical and this is what vacuum adds to the total.
So base of 12*
Say mech. of 15*
say vacuum of 12*
total of 39* = 12*+15*+12*
Most spec books should list what each part should be from the factory.
I would use them as a starting point and adjust 1 or 2 or all 3 to get the best performance.
Dave ----
Ok there is 3 parts to the timing total.
Base, Mechanical, Vacuum.
Base: you check at idle vacuum disconnected & plugged. if checked that way you have 12 BTDC
Mechanical: with everything just like checking base timing bring the RPM up to 2500 and check timing. Take this reading remove the base timing (12*) and this is mechanical timing.
Total: now hook up the vacuum, bring the RPM back up to 2500 and note timing. Remove Base & Mechanical and this is what vacuum adds to the total.
So base of 12*
Say mech. of 15*
say vacuum of 12*
total of 39* = 12*+15*+12*
Most spec books should list what each part should be from the factory.
I would use them as a starting point and adjust 1 or 2 or all 3 to get the best performance.
Dave ----
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I don't think anything was said about ignition timing. However, I wonder if that may be the issue on the fluctuating RPM's. Where is your vacuum advance connected? If it is to the manifold then that can cause "hunting" of the R's. Check to see if there's vacuum to the advance at idle.
On the other hand, a rich idle air/fuel mix can cause an RPM fluctuation. So, look at the vacuum advance first and, if that isn't it then maybe the turn the idle mix screws in 1/4 turn each and see what happens.
On the other hand, a rich idle air/fuel mix can cause an RPM fluctuation. So, look at the vacuum advance first and, if that isn't it then maybe the turn the idle mix screws in 1/4 turn each and see what happens.
Also, my acceleration is still horrible. The 0-60 last time I tried was in the upper 20's. I haven't tried 0-60 since because I think that is what caused my pushrod to break, but I can still tell there would be no burn out if I was to floor it.
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From: Angier, NC
Now as Gary pointed out the only thing that my cause the issue you are having would be vacuum advance but is easy to check, just pull the hose off and plug it and see if the idle stops bouncing. If it does then you know the vacuum advance is at the "tip in" point.
Now think it was Gary that posted some of the vacuum advance can be adjusted with a small allen wrench in where the hose goes on.
If the idle still is bouncing with the hose off then need to dig a little more.
You said you are using a digital tach and is the numbers bouncing why you said the idle is moving or you can hear it in the motor speeding up and slowing down.
That is an issue with digital gauges, they work faster than the analog ones and people think there is something because the gauge is bouncing.
Dave ----
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Yes it is bouncing, but it is not going just up like an increasing idle, but both the motor increase in speed does not seem to effect the amount of variance. I am using a digital tach so maybe this is normal.
Oh, and Gary, I have the vacuum plugged to the middle front of the carb. I will check that soon.
Oh, and Gary, I have the vacuum plugged to the middle front of the carb. I will check that soon.
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I pulled the dizzy vacuum hose off and hooked it to a vacuum gauge (it was $10, but I can suck it to -10). I plugged the carb hole. The gauge did not move so I have no vacuum from the dizzy. Is this causing the fluctuating RPM's? How about the acceleration or possible incomplete combustion and therefore the smoke?
FTE Legend
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From: Northeast, OK
You have it backwards. The carb provides the vacuum, not the distributor.
The test is to start the engine, pull the vacuum line to the distributor off the carb, and see if there is vacuum at the carb with the engine idling. If so you don't have ported vacuum, you have manifold vacuum. I would recommend that you find a port on the carb that doesn't have vacuum at idle, meaning it has ported vacuum, and use that for the vacuum advance.
The test is to start the engine, pull the vacuum line to the distributor off the carb, and see if there is vacuum at the carb with the engine idling. If so you don't have ported vacuum, you have manifold vacuum. I would recommend that you find a port on the carb that doesn't have vacuum at idle, meaning it has ported vacuum, and use that for the vacuum advance.
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From: Angier, NC
I pulled the dizzy vacuum hose off and hooked it to a vacuum gauge (it was $10, but I can suck it to -10). I plugged the carb hole. The gauge did not move so I have no vacuum from the dizzy. Is this causing the fluctuating RPM's? How about the acceleration or possible incomplete combustion and therefore the smoke?
That would be good.
Did the idle stop hunting? If not does the vac gauge still 0?
The thought was with the idle hunting the vac advance was kicking in & out causing the hunting.
If the idle still hunts with out the vac line hooked up the hunting is caused by something else.
With the vac gauge still hooked to the carb dist. port if you give it a little throttle and hold it up to say 1500 rpm what does the vac gauge do? It should go up if you have the right port on the carb for dist. vac advance and it works as it should.
Just wanted to rule out the vac advance the cause.
Dave ----