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Hi all I am curious about the time it takes a tr3 to come down to say a normal 800 rpm idle. I know the common problem for hi idle is usually a vacuum leak in the shafts or a sticky piston, but I am more curious about the time it takes a normal tr3 to come back down to idle. Mine sometimes takes about 8 seconds to come to full rest. I am thinking that the problem is piston drop in the chamber because the front piston drops a lot slower than the one in the back carburetor. I read that article, by I think, Taylor and found it interesting when he talked about piston drop being roughly 5 seconds or something. However, please forgive my ignorance and tell me does he want the spring and oil filler rod dealie out when one does the testing. And thank you doc for Joe’s number, I purchased some glad washers from him a couple of days ago.
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TR2/3/3A tr3 idle rest
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Don Elliott
Obi Wan
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I would estimate it takes between 8 and 12 years for the idle to get back down to 800 RPM. The tops of all my right shoes are deeply marked where I pull back on the pedal.
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I guess I don't know for sure, but 8 seconds seems a little long, what are you using for your dashpot oil?, should be about a 20 weight or equivalent.
Jets should be centered so pistons go up and down easily, throttle return springs strong enough to bring shut the butterflys quickly.
On the other hand if everything else is OK probably not a cause for concern, but you ight want to look at how the pistons rise as you accelerate, as if one is slower (like you noted on deaaceleration) you may be getting a little too lean a mexture on accleration.
Jets should be centered so pistons go up and down easily, throttle return springs strong enough to bring shut the butterflys quickly.
On the other hand if everything else is OK probably not a cause for concern, but you ight want to look at how the pistons rise as you accelerate, as if one is slower (like you noted on deaaceleration) you may be getting a little too lean a mexture on accleration.
The oil is 20W and I centered the jets ok and still the float of the pistons varies. The back carb shoots up and down easy and the front carb comes up and down nice and steady. I am thinking the front carb is the more desirable. However, I guess if I want the idle to drop quickly then perhaps the fast dropping piston would make that happen quicker. Although, the fast lifting and dropping carb would probably make the engine run richer thus more rpm at idle until the gas burns, I guess. I am just not sure how to look at it or where to look. I want the engine idle to respond back quickly, but maybe I am asking too much. Perhaps the engineering of the SU system does not work that way.
Yes that is what this one does. It hangs at about 1200/1100 then drops of to about 800/700. In the past, I have had it hang at about 1000 and drop to 600 and I can live with that. After this leak and thus glad repair it wants to idle a little higher and my OCB is not having that.
MGTF1250Dave
Jedi Knight
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Aloha,
Here is an extended quote from Jim Taylor describing the drop test:
XII. VACUUM DROP TESTS -SU CARBS
SU carburetor suction chambers and pistons are furnished as matched assemblies from the factory. There is a controlled clearance and thus controlled air leakage between the piston and suction chamber bore. A convenient means of checking this is a vacuum drop test. The proper damper should be in place. Chamber and piston should be clean and dry .Check piston for any spots of drag or interference over full travel. Spray piston rod lightly with WD-40. Turn assembly upside down with piston against top of chamber. Plug holes in the bottom of piston with windshield caulking (commonly referred to as Dum-Dum). Measure the time for the chamber to slide down and fall off the piston. Specified times are as follows:
1 1/4" and smaller carb = 3-5 seconds
1 1/2" and 1 3/4" carb = 5-7 seconds
2" carb = 7-10 seconds
If vacuum drop time is too fast, carb will tend to run rich. If drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct. Chances are someone has sanded inside of chambers to clean them which is a no-no. If they are too slow you can polish the chambers or pistons or both. very lightly and recheck frequently for compliance. If one carburetor is fast and one is slow there is a good chance pistons have been interchanged. Try switching them. I recently had a pair of 1 1/2" TF carbs where the front was 5 seconds and the rear was 8 In seconds. Switching pistons gave me 7 seconds on the front and 6 1/2 seconds on the rear -just lovely. It is ideal if both are alike and right in the middle of specification. It doesn't happen very often. Minor disparities from specified drop times can be accommodated by tuning adjustments. If I had a 1 1/2" carb with a drop time of 4 seconds I wouldn't fret about it. If it was 1 second or 1/2 second (which I've seen) I'd hunt for some different pans. It should be noted that if you do interchange pistons. it will be necessary to re-center both jet assemblies. Ideally this type checking should be done at the time of a major rebuild.
Here is an extended quote from Jim Taylor describing the drop test:
XII. VACUUM DROP TESTS -SU CARBS
SU carburetor suction chambers and pistons are furnished as matched assemblies from the factory. There is a controlled clearance and thus controlled air leakage between the piston and suction chamber bore. A convenient means of checking this is a vacuum drop test. The proper damper should be in place. Chamber and piston should be clean and dry .Check piston for any spots of drag or interference over full travel. Spray piston rod lightly with WD-40. Turn assembly upside down with piston against top of chamber. Plug holes in the bottom of piston with windshield caulking (commonly referred to as Dum-Dum). Measure the time for the chamber to slide down and fall off the piston. Specified times are as follows:
1 1/4" and smaller carb = 3-5 seconds
1 1/2" and 1 3/4" carb = 5-7 seconds
2" carb = 7-10 seconds
If vacuum drop time is too fast, carb will tend to run rich. If drop time is too slow carb will tend to run lean. If both carbs are too fast there is nothing you can do to correct. Chances are someone has sanded inside of chambers to clean them which is a no-no. If they are too slow you can polish the chambers or pistons or both. very lightly and recheck frequently for compliance. If one carburetor is fast and one is slow there is a good chance pistons have been interchanged. Try switching them. I recently had a pair of 1 1/2" TF carbs where the front was 5 seconds and the rear was 8 In seconds. Switching pistons gave me 7 seconds on the front and 6 1/2 seconds on the rear -just lovely. It is ideal if both are alike and right in the middle of specification. It doesn't happen very often. Minor disparities from specified drop times can be accommodated by tuning adjustments. If I had a 1 1/2" carb with a drop time of 4 seconds I wouldn't fret about it. If it was 1 second or 1/2 second (which I've seen) I'd hunt for some different pans. It should be noted that if you do interchange pistons. it will be necessary to re-center both jet assemblies. Ideally this type checking should be done at the time of a major rebuild.
I have one comment, one question and one other thing to say about this one.
The comment is that I had that problem way back when and it was a leak in the gasket where the manifold mates with the head. I found it by spraying something volatile in the area until the RPM went up.
The question is what is the glad repair kit?
The thing I have to say is I think (and I may be wrong but I am pretty sure about this) some of you may have the rich and lean function of the dashpots backwards. The oil slows the pot from going up and that makes the mixture richer. Keeping the dashpot down keeps the mixture richer (it is like putting the choke on). It seems counter intuitive because the needle is getting smaller so you would think that the jet is bigger when the pots are up. This is true but the air flow is such that the mixture is richer if the dashpots are held down artificially. This oil is there to provide the same function as an accelerator pump in American carbs. The SU is a constant depression carb and with no oil the mixture would stay more or less correct through the entire RPM range, and in fact it does, without the use of multiple jets for different engine RPM. The oil just lets it get richer for acceleration. I ramble too much, must be the lack of food - darn diets!
The comment is that I had that problem way back when and it was a leak in the gasket where the manifold mates with the head. I found it by spraying something volatile in the area until the RPM went up.
The question is what is the glad repair kit?
The thing I have to say is I think (and I may be wrong but I am pretty sure about this) some of you may have the rich and lean function of the dashpots backwards. The oil slows the pot from going up and that makes the mixture richer. Keeping the dashpot down keeps the mixture richer (it is like putting the choke on). It seems counter intuitive because the needle is getting smaller so you would think that the jet is bigger when the pots are up. This is true but the air flow is such that the mixture is richer if the dashpots are held down artificially. This oil is there to provide the same function as an accelerator pump in American carbs. The SU is a constant depression carb and with no oil the mixture would stay more or less correct through the entire RPM range, and in fact it does, without the use of multiple jets for different engine RPM. The oil just lets it get richer for acceleration. I ramble too much, must be the lack of food - darn diets!
MGTF1250Dave
Jedi Knight
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Aloha Adrio,
I concur with you. The oil in the damper is to keep the dash pot from raising too quickly on acceleration to temporarily richen the mixture. Air is less dense than fuel and therefore has less inertia. If the dash pot was not dampened the air entering the engine would increase faster than the fuel, thus leaning the mixture below the desired 14:1 air to fuel ratio.
I concur with you. The oil in the damper is to keep the dash pot from raising too quickly on acceleration to temporarily richen the mixture. Air is less dense than fuel and therefore has less inertia. If the dash pot was not dampened the air entering the engine would increase faster than the fuel, thus leaning the mixture below the desired 14:1 air to fuel ratio.
MGTF1250Dave
Jedi Knight
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Aloha,
I do the drop test with out the spring in place or oil in the damper, but the damper and cap in place. I remove the dash pot (piston and cover) from the carburetor. Hold it vertical with wide lip of the piston even with the bottom of the cover. Place a finger over the hole near the bottom of the piston and let the cover drop. Time the drop.
The purpose of the test is to make sure the gap between the piston and cover will allow engine vacuum provide for correct carburetor operation. If piston and cover gap is too wide,engine vacuum can not raise the piston adequately.
It is desirable to have the pistons raising and falling at a similar rate. Perhaps the covers on your carburetors have been inadvertently switched at some time. Try swapping them to see if that corrects or improves your situation.
I do the drop test with out the spring in place or oil in the damper, but the damper and cap in place. I remove the dash pot (piston and cover) from the carburetor. Hold it vertical with wide lip of the piston even with the bottom of the cover. Place a finger over the hole near the bottom of the piston and let the cover drop. Time the drop.
The purpose of the test is to make sure the gap between the piston and cover will allow engine vacuum provide for correct carburetor operation. If piston and cover gap is too wide,engine vacuum can not raise the piston adequately.
It is desirable to have the pistons raising and falling at a similar rate. Perhaps the covers on your carburetors have been inadvertently switched at some time. Try swapping them to see if that corrects or improves your situation.
T
TRDejaVu
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I found that the idle still hung up on my TR4 even after replacing the butterflies and worn carb shafts. On mine it appears to be a worn throttle rod bellcrank that is creating some linkage slop. I have experimented with shims around the pivot bolt, but the high temperature from the exhaust can make it bind up. Until I get it right, I will continue to give a short sharp blip followed by the shoe-ruining technique. After that it idles nicely at a steady 700.
vivdownunder
Jedi Warrior
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One thing needed for the car to promptly return to idle is to ensure the ball joints from the accel pedal to the carbies are well lubed and not binding. The bell crank under the front carbie needs to be lubed too.
If it's worn like TRDejaVu's, repair or replace, as it will move sideways before rotating.
Make sure the return spring isn't missing from the accel lever near the firewall. The angle of this spring isn't really ideal for maximum effect.
Many cars I've seen are fitted with an additional custom made throttle return spring. From bell crank to generator bracket is a common location for this.
Also, a drop of oil on the spindles adjacent to the carbie bodies will work its way in, and minimise friction loss. This also reduces spindle wear.
Although awkward to get at, lube the two throttle pedal rod bushes.
Together, these things pretty much take friction loss out of the equation as a cause of delayed return to idle.
Cheers,
Viv
If it's worn like TRDejaVu's, repair or replace, as it will move sideways before rotating.
Make sure the return spring isn't missing from the accel lever near the firewall. The angle of this spring isn't really ideal for maximum effect.
Many cars I've seen are fitted with an additional custom made throttle return spring. From bell crank to generator bracket is a common location for this.
Also, a drop of oil on the spindles adjacent to the carbie bodies will work its way in, and minimise friction loss. This also reduces spindle wear.
Although awkward to get at, lube the two throttle pedal rod bushes.
Together, these things pretty much take friction loss out of the equation as a cause of delayed return to idle.
Cheers,
Viv
One other thing I just remembered. I have read that the pistons and the dashpots are matched to each other and the fit is to exacting standards. As a result the pistons must always be kept with their matching pot. I wonder if in your case the pistons may have been swapped and if the sized were in oposite directions one would end up faster and one slower there-by making the difference between the two even bigger. I imagine this would show up in the drop test that is being talked about.
TR3driver
Great Pumpkin - R.I.P
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I agree completely, with one minor caveat : Without oil in the damper, the mixture actually goes quite lean when you first open the throttle.Adrio said:
The story I've heard is that because it's much more dense than the air, it takes longer for the fuel to start flowing more quickly through the jet. Don't know if that's true or not, but I do know the SU carbs go quite lean with no oil (or even very light oil) in the dampers.
I agree that without the oil it will go quite lean when you first open the throttle. The comment I made about the steady state at any RPM rage. I should have been more specific.
These carbs are ingenious devices when you think about how simple they are to achieve such a complex result.
Then again the word "engineer" does come from "ingenious" and not from "engine" Did I just start a whole new "debate"?
These carbs are ingenious devices when you think about how simple they are to achieve such a complex result.
Then again the word "engineer" does come from "ingenious" and not from "engine" Did I just start a whole new "debate"?
TR3driver
Great Pumpkin - R.I.P
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Adrio said:
Also, the pistons have relatively little effect on airflow, especially at idle. If you are having idle problems, and the pistons/needles are not actually sticking or binding, then the problem is likely somewhere else.
However, having the needles bind in the jets at idle (due to improper centering) is quite common. The test for this is to lift the piston (with carbs on the car but engine not running) and let it fall. You should hear a distinct 'click' as the brass stop contacts the bottom of the carb throat. I like to do this test with the mixture nut removed and the jets as high as they will go, to ensure there is actually some clearance with the mixture set.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:] The fall speed is controled by the fit between the piston and pot.[/QUOTE]More by the size of the air passages that vent the chamber to the throat, IMO. Those passages must be sealed for the "drop time test" mentioned earlier in the thread.