Hi there!
It is been 18 month or so I 've not written news about my projec.
From the 327 SBC I questioned initially about the mixer sizing, I upgraded to a new 383.
By the time I also went through a partial but extensive restauration.
I've got now a new interior, new windshield, new electrical harness for entire car, lots of undercarriage prep and coating, and with the new engine I have all new accessories.
Car is finally ready and all bugs are solved.
Soon I'll go to the shop in the NEtherlands 1h45 from home for improvement on the propane fuel delivery system, new coil, addition of a ignition box Skyfire , monitoring and tuning.
Right now I want the car to run correctly. Mechanically it is ok as I said and I emptied the first tank checking mileage. THis is based on initial settings for advance 10 initial, 20-22 mechanical + 10 vacuum.
Vacuum reading is under manufacturer recommends @ 12 mmHg should be 14 for this engine.
I was measuring at idle AFR and I set the idle screws like I did with m old 327, a little bit rich at idle 0,95.
I didn't really drove the car , the engine was running fine in the garage and for test drive with these settings.
I then removed the AFR gauge because I installed another windeband sensor instead in preparation for the new propane system install in the NEtherland. the new sensor is not connected to a gauge reading.
So now I am driving the car with these settings and the mpg is very poor! I am @ 32l/100 km so this is about 8 mpg.
Today, with a vacuum gauge, I leaned out the idle screw. Impco 425 straight gas.
I was searching to maximize vacuum and RPM.
Guys I leaned out like 1 full turn!!
I am now @ a good 13 13,5 mmHG @ 875 rpm.
I stopped the setting but I wonder if I should lean out even further?
DUring my trials to lean idle setting I also went to smell exhaust gases. Only close to last final setting I don't smell propane gas anymore.
I am wondering however about my lambda sensor I used for initial setting....
I am also wondering about too lean conditions ... Am I at risk on the exhaust valves getting too hot using this method for setting the engine.
Car runs fine. No hesitation.
I am not beating her . No need for it . It is not tuned yet with the optimized system...
Your comments are appreciated.
New 383 SBC - question on mixer setting
Re: New 383 SBC - question on mixer setting
Your timings to late still. You need to have a mechanical of 27 to 30 degrees and another 10 of vacuum on top of that. You will end up with an idle of 15 or 16 but that's ok. No load at the point anyways. Unless you have a really high compression these are acceptable values without having access to fancy equipment like knock sensors or combustion pressure meters. Also if your concerned about your valves your current timing numbers will cause the exhaust to run hotter then if the mixtures are too lean. Its possible for the flame to still be burning when the ex valve open causing them to overheat. Adjusting the idle screw will have no bearing on mileage anyways. Three things cause hot exhaust valves. Late timing, incorrect mixtures and incorrectly adjusted valve lash (NA on engines with hydraulic lifter usually although it is possible to have these incorrect if they are adjustable). Any one of these items out of adjustment will not usually seriously impact the valve temperature. Having two of these things incorrect can be detrimental.
Although it does not apply to every case we found most chevy engines with normal compression (8.5 to 10.0:1) really liked the formula of 14+14+14. It usually resulted in a really nice running engine with good power and decent mileage. Also the mechanical should reach the stated 27 - 30 degrees by 2500 RPM. I believe once you advance your timing you will also find your idle vacuum increases.
Although it does not apply to every case we found most chevy engines with normal compression (8.5 to 10.0:1) really liked the formula of 14+14+14. It usually resulted in a really nice running engine with good power and decent mileage. Also the mechanical should reach the stated 27 - 30 degrees by 2500 RPM. I believe once you advance your timing you will also find your idle vacuum increases.
Re: New 383 SBC - question on mixer setting
Initial + mech = 10 + 22 = 32 so this is in the ball park of your setting 27 - 30...
I think you mention mech = my mention mech + initial.
Why do you day this :
"Adjusting the idle screw will have no bearing on mileage anyways. "
How then you control mileage
Impco 425.
Thanks;
Marc
I think you mention mech = my mention mech + initial.
Why do you day this :
"Adjusting the idle screw will have no bearing on mileage anyways. "
How then you control mileage
Impco 425.
Thanks;
Marc
Re: New 383 SBC - question on mixer setting
Hi, I was thinking again...
Pretty sure my timing is set close to correctly.
If you say idle setting doesn't influence mileage, what is the RPM below which it influence it? I am cruising often around 2000 RPM.
My gas valve did not change.
If the gas valve is giving the mileage there is maybe a change i did that influence badly? : I removed the restriction plate inside the gas inlet. This is the plate you see on the picture.
But I don't understand why it would influence badly.
Please tell me.
- Attachments
-
Re: New 383 SBC - question on mixer setting
Timing was OK. I checked again.
15 initial, 20 mech and 12 vacuum.
Went to a shop they measured CO, cannot be tuned lean from the mxer. When the idle screw is turned out it doesn't lean.
This must be a problem with matching evaporator model E spring setting?
15 initial, 20 mech and 12 vacuum.
Went to a shop they measured CO, cannot be tuned lean from the mxer. When the idle screw is turned out it doesn't lean.
This must be a problem with matching evaporator model E spring setting?
Re: New 383 SBC - question on mixer setting
Sorry for delay. I'm not able to check in as often as I used to.
I gave you timing numbers and you are running higher then those. As stated too mileage can be affected by too little timing and too much timing. Too little the flame goes out before the piston reaches the bottom of it's stroke. Too much can actually cause the flame to push backwards against the piston before it reaches TDC. Unfortunately you are in a trial and error situation. The 383 is a performance engine and perhaps someone on the site can recommend what they found best with the same engine.
OK. 425 mixer lesson time. Sorry, this is going to be a little long winded.
I posted this before but I'll run through it again.
The 425 has three areas of the gas valve that affect mixtures. The gas valve is a tapered bullet that is machined to deliver fuel dependent on air flow through the mixer. There are three areas on this bullet.
The first is the idle area. It is the largest step and supplies a preset amount of fuel. This amount is barely capable of keeping the engine running based on the air flow flowing through the mixer. To ensure adequate idle fuel and allow the mixer to be used on a wide variety of engines it has a circuit that allows propane to bypass the gas valve. The amount is controlled by the idle screw and the diaphragm in the idle circuit. Vacuum pulls the diaphragm down when the engine is running and the amount the diaphragm moves is limited by the idle screw. BY far the biggest mistake made is installing the idle screw incorrectly. If the plate is taken off the mixer for any reason first remove the idle screw and put it in your pocket or some other location. Why? The plate and diaphragm and the small spring under the diaphragm should be installed without the idle screw in place. The spring causes the diaphragm to locate itself in the proper position. The last step is to install the idle screw. It will now slide under the idle diaphragm lever naturally like it is intended to do. Failure to do this usually results in the idle screw being on top of the idle lever and when adjusted nothing happens. The idle has no major impact on mileage as most cars only idle a small percentage of their running time. Some may have an impact if they spend abnormally long times idling. The idle circuit typically has an affect up to approximately 1000 RPM. At that point the gas valve is lifted into the cruise area.
The cruise area of the gas valve takes up 80 to 85% of the bullet. The decreasing diameter allows more fuel as more air flow occurs. The bullet is machined to maintain an air fuel ratio of about 16:1. Maybe even 16.5:1. The mixtures are not really adjustable in this area. Some different gas valves (bullets) were designed to change the mixture for specific purposes. The 1644 valve was designed for MD to HD trucks. Another valve was made for electronic fuel control. Many operators have used the power valve to change the cruise mixtures. Closing the power valve will eventually override the cruise mixtures, however when the operator tries to accelerate or pull a heavy load the mixer cannot supply enough fuel because the power valve is not meant to be used in this manner on this mixer. The only way the cruise mixtures can be changed is to change the position of the bullet. IMPCO makes shims allowing this to be done on the 100 and 200 series mixers because they are used on fork lifts. They do not make them for the 425, however some clever design might allow this to be done but I've never has a need to do it. In the event such an experiment was tried were talking about a shim .003" thick installed between the diaphragm and the bullet causing the bullet to change it's position for the given air flow.
Last area is the power area. In this position the bullet is withdrawn between 90 and 100% out of the vapour inlet. At 100% this is where the power screw comes into play. Since it is now possible to supply too much fuel for a given engine the power valve has t be set to supply adequate fuel for power and adequate fuel for all cylinders. Many operators use gas analyzers or electronic analyzers to find the correct setting. Typically the air fuel ratio is going to be approximately 13.5 to 14.5:1. Many will say too rich, however an important item to keep in mind is the old intake manifolds were terribly inefficient. The old manifolds could be out vast amounts between the center and end cylinders no matter which fuel you are running. Setting the power mixtures too lean can end up starving some of the cylinders farther from the central point of the intake point. Lastly removing the plate was fine. The plate was installed due to the BTU differences between propane and NG. The plate was typically removed if a vehicle was converted to NG. Frequently it was also removed on propane for big block engines and higher HP units.
To summarize. You can adjust the idle and full power mixtures but you cannot adjust the cruise mixtures. Adjusting the idle mixtures below it's normal range can have a minor impact on the cruise mixtures but normally very little. Adjusting the power screw excessively lean can certainly affect the cruise mixtures but at the detriment of good performance and even possibly engine damage.
Lastly. You say adjusting the idle screw with a gas analyzer had no impact on the mixtures. In that case there is a strong possibility the idle screw is riding on top of the lever rather than under it. The only other thing that can cause this is either a worn mixer or oil blowby which is entering the intake through the PCV system. When adjusting the idle mixtures it should always be done with the PCV removed from the valve cover. This way you get a true fuel mixture. Once the PCV is reinstalled the fuel mixtures will read higher due to blow by. The average engine in good condition will have about CO increase of about .75% above the set mixture. The more blowby the more worn an engine is. I've seen engines with increased CO readings of 6 to 7% making it impossible to adjust the fuel mixtures unless you remove the PCV valve from the valve cover.
So in the final analysis which most people don't like, the cruise mixtures are not adjustable on most IMPCO air/gas valve mixers. Poor mileage is the result of other areas such as timing, regulator control issues, leaking vacuum lock offs, or worn out mixers.
One last thing. You cannot check cruise or full power mixtures in a no load state. The gas valve is designed for a specific lift under load. With no load it does not lift the same amount to maintain a no load RPM. In most cases the mixtures will lean out but in many cases they won't. The 425 is notorious for showing rich mixtures in a no load situation. The power valve also only comes into play at WOT and above 3800 RPM according to IMPCO. It takes that much airflow to raise the gas valve to it's fully open position.
I hope this helps you to understand the 425 mixer.
I gave you timing numbers and you are running higher then those. As stated too mileage can be affected by too little timing and too much timing. Too little the flame goes out before the piston reaches the bottom of it's stroke. Too much can actually cause the flame to push backwards against the piston before it reaches TDC. Unfortunately you are in a trial and error situation. The 383 is a performance engine and perhaps someone on the site can recommend what they found best with the same engine.
OK. 425 mixer lesson time. Sorry, this is going to be a little long winded.
I posted this before but I'll run through it again.
The 425 has three areas of the gas valve that affect mixtures. The gas valve is a tapered bullet that is machined to deliver fuel dependent on air flow through the mixer. There are three areas on this bullet.
The first is the idle area. It is the largest step and supplies a preset amount of fuel. This amount is barely capable of keeping the engine running based on the air flow flowing through the mixer. To ensure adequate idle fuel and allow the mixer to be used on a wide variety of engines it has a circuit that allows propane to bypass the gas valve. The amount is controlled by the idle screw and the diaphragm in the idle circuit. Vacuum pulls the diaphragm down when the engine is running and the amount the diaphragm moves is limited by the idle screw. BY far the biggest mistake made is installing the idle screw incorrectly. If the plate is taken off the mixer for any reason first remove the idle screw and put it in your pocket or some other location. Why? The plate and diaphragm and the small spring under the diaphragm should be installed without the idle screw in place. The spring causes the diaphragm to locate itself in the proper position. The last step is to install the idle screw. It will now slide under the idle diaphragm lever naturally like it is intended to do. Failure to do this usually results in the idle screw being on top of the idle lever and when adjusted nothing happens. The idle has no major impact on mileage as most cars only idle a small percentage of their running time. Some may have an impact if they spend abnormally long times idling. The idle circuit typically has an affect up to approximately 1000 RPM. At that point the gas valve is lifted into the cruise area.
The cruise area of the gas valve takes up 80 to 85% of the bullet. The decreasing diameter allows more fuel as more air flow occurs. The bullet is machined to maintain an air fuel ratio of about 16:1. Maybe even 16.5:1. The mixtures are not really adjustable in this area. Some different gas valves (bullets) were designed to change the mixture for specific purposes. The 1644 valve was designed for MD to HD trucks. Another valve was made for electronic fuel control. Many operators have used the power valve to change the cruise mixtures. Closing the power valve will eventually override the cruise mixtures, however when the operator tries to accelerate or pull a heavy load the mixer cannot supply enough fuel because the power valve is not meant to be used in this manner on this mixer. The only way the cruise mixtures can be changed is to change the position of the bullet. IMPCO makes shims allowing this to be done on the 100 and 200 series mixers because they are used on fork lifts. They do not make them for the 425, however some clever design might allow this to be done but I've never has a need to do it. In the event such an experiment was tried were talking about a shim .003" thick installed between the diaphragm and the bullet causing the bullet to change it's position for the given air flow.
Last area is the power area. In this position the bullet is withdrawn between 90 and 100% out of the vapour inlet. At 100% this is where the power screw comes into play. Since it is now possible to supply too much fuel for a given engine the power valve has t be set to supply adequate fuel for power and adequate fuel for all cylinders. Many operators use gas analyzers or electronic analyzers to find the correct setting. Typically the air fuel ratio is going to be approximately 13.5 to 14.5:1. Many will say too rich, however an important item to keep in mind is the old intake manifolds were terribly inefficient. The old manifolds could be out vast amounts between the center and end cylinders no matter which fuel you are running. Setting the power mixtures too lean can end up starving some of the cylinders farther from the central point of the intake point. Lastly removing the plate was fine. The plate was installed due to the BTU differences between propane and NG. The plate was typically removed if a vehicle was converted to NG. Frequently it was also removed on propane for big block engines and higher HP units.
To summarize. You can adjust the idle and full power mixtures but you cannot adjust the cruise mixtures. Adjusting the idle mixtures below it's normal range can have a minor impact on the cruise mixtures but normally very little. Adjusting the power screw excessively lean can certainly affect the cruise mixtures but at the detriment of good performance and even possibly engine damage.
Lastly. You say adjusting the idle screw with a gas analyzer had no impact on the mixtures. In that case there is a strong possibility the idle screw is riding on top of the lever rather than under it. The only other thing that can cause this is either a worn mixer or oil blowby which is entering the intake through the PCV system. When adjusting the idle mixtures it should always be done with the PCV removed from the valve cover. This way you get a true fuel mixture. Once the PCV is reinstalled the fuel mixtures will read higher due to blow by. The average engine in good condition will have about CO increase of about .75% above the set mixture. The more blowby the more worn an engine is. I've seen engines with increased CO readings of 6 to 7% making it impossible to adjust the fuel mixtures unless you remove the PCV valve from the valve cover.
So in the final analysis which most people don't like, the cruise mixtures are not adjustable on most IMPCO air/gas valve mixers. Poor mileage is the result of other areas such as timing, regulator control issues, leaking vacuum lock offs, or worn out mixers.
One last thing. You cannot check cruise or full power mixtures in a no load state. The gas valve is designed for a specific lift under load. With no load it does not lift the same amount to maintain a no load RPM. In most cases the mixtures will lean out but in many cases they won't. The 425 is notorious for showing rich mixtures in a no load situation. The power valve also only comes into play at WOT and above 3800 RPM according to IMPCO. It takes that much airflow to raise the gas valve to it's fully open position.
I hope this helps you to understand the 425 mixer.
Re: New 383 SBC - question on mixer setting
How frustrating. I just spent 30 minutes composing a detailed reply to your questions on mixtures just to get a this site is not working when I submitted it. I can't redo it at the moment. I will post it again in a day or two.
Re: New 383 SBC - question on mixer setting
I see my post made it through. Glad I didn't have to retype all of that post. Just a reminder. If the mixtures at idle do not change just remove the idle screw completely from the plate with the engine not running and then put it back in. This will ensure it is on the right side of the lever. Now start the engine and try adjusting it again. Still no change. It may be time to check the Regulator secondary spring height.
Re: New 383 SBC - question on mixer setting
I thank you very much C3H8 for your detailed explanation on the IMPCO 425 settings and functioning.
When I wrote end of august was just before a trip of a 100 miles to reach a well known LPG shop in the Nederlands that install build in ignition boxes and emission boxes.
I didn't really had the opportunity to play myself much more with the IMPCO 425.
So I drove with a very bad mileage to the shop (8 mpg).
Overthere they also could not get a satisfactory emission readings at idle. Always way too much HC / CO whatever the idle screw setting. Looks like however what I was reading on this forum the idle screw behave in this (other?) way : out = rich / in = lean???
I never understood why the mechanics at the shop told me that.
Anyway, I went back home and the car stayed there 3 weeks for system installation and tuning.
It turned out I paid a lot of many and I now I have a vacuum assisted spring regulation on the vaporiser, also vacuum connected to a electronic pulse device/valve wired to the emission box.
input 0.1 V lambda sensor is also going to this emission box.
On the more classical ignition box I have a vacuum signal too.
The ignition box does allow an electronic ignition timing curve mapping.
On the motor currently we have this ignition timing set :
12° initial
20° mechanical all in at 3200 rpm
at ~1100 RPM, the box gives an additional constant +10 degree throughout the curve.
the box also gives an additional cruising vacuum advance I guess another 10 or 12 degree this I don't know exactly.
So total at crusing speed ~2100 rpm timing is close to ~45 degrees.
The motor starts without manifold vacuum. There is no ported vacuum on my Holley base plate. Distributor vacuum is not anymore connected.
It was reported that with manifold vacuum too much air was pulled through the PVC getting bad HC/CO readings.
The mpg is higher than 13 mpg on LPG on a normal drive condition with warm engine. I have not driven the car very much so I cannot say a lot more about the economy.
I was pissed off by paying so much and by discovering other miseries with the car once I got over there by train to take it over. The miseries were about weird noises I am still fixing not related to the LPG. The shop has their know how and I was not going anyway to get full detailed explanations.
I was worried there is too much advance but they assured me no, on propane you have to run more advance.
I didn't argued and took my car away.
I still think at this shop they don't often have performance engine to deal with and I am believing that on an original low compression engine indeed high ignition advance is required for best overall LPG performance but in this case here I am not sure it's needed to achieve best performance.
I can play with activate or not activate (dash switch) both electronic boxes and I don't feet really a difference in power.
I still am driving always with the box on so i cannot report if the economy is improved or not.
When I wrote end of august was just before a trip of a 100 miles to reach a well known LPG shop in the Nederlands that install build in ignition boxes and emission boxes.
I didn't really had the opportunity to play myself much more with the IMPCO 425.
So I drove with a very bad mileage to the shop (8 mpg).
Overthere they also could not get a satisfactory emission readings at idle. Always way too much HC / CO whatever the idle screw setting. Looks like however what I was reading on this forum the idle screw behave in this (other?) way : out = rich / in = lean???
I never understood why the mechanics at the shop told me that.
Anyway, I went back home and the car stayed there 3 weeks for system installation and tuning.
It turned out I paid a lot of many and I now I have a vacuum assisted spring regulation on the vaporiser, also vacuum connected to a electronic pulse device/valve wired to the emission box.
input 0.1 V lambda sensor is also going to this emission box.
On the more classical ignition box I have a vacuum signal too.
The ignition box does allow an electronic ignition timing curve mapping.
On the motor currently we have this ignition timing set :
12° initial
20° mechanical all in at 3200 rpm
at ~1100 RPM, the box gives an additional constant +10 degree throughout the curve.
the box also gives an additional cruising vacuum advance I guess another 10 or 12 degree this I don't know exactly.
So total at crusing speed ~2100 rpm timing is close to ~45 degrees.
The motor starts without manifold vacuum. There is no ported vacuum on my Holley base plate. Distributor vacuum is not anymore connected.
It was reported that with manifold vacuum too much air was pulled through the PVC getting bad HC/CO readings.
The mpg is higher than 13 mpg on LPG on a normal drive condition with warm engine. I have not driven the car very much so I cannot say a lot more about the economy.
I was pissed off by paying so much and by discovering other miseries with the car once I got over there by train to take it over. The miseries were about weird noises I am still fixing not related to the LPG. The shop has their know how and I was not going anyway to get full detailed explanations.
I was worried there is too much advance but they assured me no, on propane you have to run more advance.
I didn't argued and took my car away.
I still think at this shop they don't often have performance engine to deal with and I am believing that on an original low compression engine indeed high ignition advance is required for best overall LPG performance but in this case here I am not sure it's needed to achieve best performance.
I can play with activate or not activate (dash switch) both electronic boxes and I don't feet really a difference in power.
I still am driving always with the box on so i cannot report if the economy is improved or not.