Woodray
I have used colour plugs before as I had a couple of Ducati motorbikes during my twenties and they (colour plugs) are the greatest thing for tuning idle mixtures on engines, mainly because you can actually see the colour of the flame inside the cylinder. Once you know that you have the engine running on the idle mixture circuits, turn the mixture screw/s out until the flame starts to turn orange (rich) then turn them back in until the orange goes away and you are left with a nice bunsen burner blue flame, job done. Then set your idle level screw to the desired rpm and recheck idle mixture. When I had my motor bikes I had two of them and a set of vacuum gauges. Once a week I would ride the Ducati home from work and when it was in the garage, in with the colour plugs and set up the vacuum gauges, make sure that both carbs are reading the same vacuum level and in ten minutes perfect idle. However they are not designed for WOT power runs. This is best achieved with dyno tests or now that these things are available, Air/fuel ratio and EGT pyrometer gauges. I now have both and as I have previously mentioned the air/fuel ratio gauge constantly reads lean as the pipe has an air leak. My cousin (a machinist), is currently trying to design a temporary fix for this. The exhaust system is on its last legs and a new one is now on the agenda which will fix this issue completely.
Your comment about the exhaust water is valid as any sort of moisture will destroy the oxygen sensor and I take the water off that pipe when I am running the oxygen sensor. This involves taping the water hose on that side of the boat out over the deck. In my case the water in the exhaust is only a sound level aid to try and take two or three dB off the top of the noise level.
The big problem with the new (870) carb was figuring out what we had to do to ensure the carb was only running on the idle circuits, as mentioned previously the power valve in a holley can flop open if the value of the power valve doesn't sit below the vacuum level of the engine at idle. Once I sorted that out the colour plug sorted the idle mixture in a few minutes. The holley carbs have an idle mixture screw on each side of the carby and the final bit of the puzzle was to make sure that the two idle mixture screws were correctly matched.
The base level tune is now sorted, the next step is to work on the mixtures in the top end which is controlled by the primary and secondary main jets, in conjunction with the power valves. The 870 comes with 80 size main jets standard and a power valve in both metering blocks, this proved to be too rich and I have now dropped the main jets front and rear down to 75's which my mate and I feel is more realistic for this engine. As the air/fuel ratio gauge is constantly reading lean (due to the exhaust air leak) we are relying completely on the EGT pyrometer gauge (current research tells us that about 1300 degrees F is about right with good top end mixture) to tell us how we are going at the moment. The new exhaust will fix that issue, but not before Naranderah.
By the way both the air/fuel ratio gauge and the EGT pyrometer gauge both have memories which means that I don't actually have to watch them while I am driving the boat, I can come back to the bank and press the memory button on the two of them after I have pulled the engine kill switch cord and shut the engine down. A ten minute session on the water, about five laps to warm the engine up, about three laps at 3/4 throttle and about three laps at WOT and we will know what the top end EGT is and whether we need to change jetting. My mate thinks that the main jets should be 76's and I have put 75's in, he is a little more conservative than I am, but Naranderah is 568 feet above sea level, we will see.
The reason that I am writing about all this on here is in the hope that the information I am providing may help some other owner/driver with knowledge that will be of some help to them in sorting the tune in their boat.
Bob
Winter fixes
Re: Winter fixes
Thanks Bob, that's a great read. Your efforts to pass on some knowledge and experience are appreciated.
The work on my boat (and car) have been about successfully getting a single side draught DCOE to work on Holden sixes (red and grey). I've spent numerous hours and dollars in jetting components and chokes trying to get these to run correctly all through the range. As some will know these set ups are plagued with flat spot problems. My last find was what I thought to be the solution with the acquisition of a real early Italian built 40mm DCOE with ported vacuum take off (discontinued on later models). I was hoping the lean out during the flat spot would be cured with a little more advance applied by the ported vacuum when the throttle was cracked. No luck there. Small chokes have been a mild solution (now down to 24mm) with no loss in top end, but still the flat spot exists. Probably the hardest has been working the black art of emulsion tubes as the Webers rely on these for mixture curve/s under different loads as they don't have a power valve to fill the hole. Without an O2 sensor it is impossible to get these right. Either way the emulsion tubes are a fine tune to the main jet and air correction i.e. lean or range adjustments low down or high up. My end conclusion to the flat spot issue lies not in the carby but manifold design i.e. valve size to port relationship and induction velocity. No amount of tuning will prevent the air slowing to such a rate when the throttle is slammed open the fuel drops out of suspension. Redesign to smaller manifold cross section and reworking the plenum would solve it but be too costly. I have gone to SU carbs on my car and this is the fix. I intend the same on the boat but I still need to fine tune the needle profiles hence the O2 sensor.
I'll post more on this probably next year when I start on the new engine.
Thanks for the info Bob. Looking forward to reading more on yours. I have learnt something on yours. I never thought the power valve circuit had any influence on the idle circuit when idling with reference to additional full flow. Its been many many years since I've played with a Holley though, no doubt they have changed a bit too since then. I'll have to dust off the books when I get home to rehash I think. Just goes to show you can still learn. Great stuff.
Cheers. Craig
The work on my boat (and car) have been about successfully getting a single side draught DCOE to work on Holden sixes (red and grey). I've spent numerous hours and dollars in jetting components and chokes trying to get these to run correctly all through the range. As some will know these set ups are plagued with flat spot problems. My last find was what I thought to be the solution with the acquisition of a real early Italian built 40mm DCOE with ported vacuum take off (discontinued on later models). I was hoping the lean out during the flat spot would be cured with a little more advance applied by the ported vacuum when the throttle was cracked. No luck there. Small chokes have been a mild solution (now down to 24mm) with no loss in top end, but still the flat spot exists. Probably the hardest has been working the black art of emulsion tubes as the Webers rely on these for mixture curve/s under different loads as they don't have a power valve to fill the hole. Without an O2 sensor it is impossible to get these right. Either way the emulsion tubes are a fine tune to the main jet and air correction i.e. lean or range adjustments low down or high up. My end conclusion to the flat spot issue lies not in the carby but manifold design i.e. valve size to port relationship and induction velocity. No amount of tuning will prevent the air slowing to such a rate when the throttle is slammed open the fuel drops out of suspension. Redesign to smaller manifold cross section and reworking the plenum would solve it but be too costly. I have gone to SU carbs on my car and this is the fix. I intend the same on the boat but I still need to fine tune the needle profiles hence the O2 sensor.
I'll post more on this probably next year when I start on the new engine.
Thanks for the info Bob. Looking forward to reading more on yours. I have learnt something on yours. I never thought the power valve circuit had any influence on the idle circuit when idling with reference to additional full flow. Its been many many years since I've played with a Holley though, no doubt they have changed a bit too since then. I'll have to dust off the books when I get home to rehash I think. Just goes to show you can still learn. Great stuff.
Cheers. Craig
Re: Winter fixes
Woodray
The power valve isn't supposed to have any influence on the idle circuit. However the issue of whether it does or does not is dependent upon the relationship of the power valve value and the idle vacuum of the engine. A 65 (6.5 inches of vacuum) value power valve needs an engine vacuum above 6.5 inches of vacuum to hold it closed. When it is closed it does not flow any fuel. This is the situation which is supposed to be in action at idle and up to about 2500/2750 rpm whilst driving around town and on the highway at speeds up to 100 to 120 kph. the power valve is supposed to open when you put your foot on the floor and the engine vacuum drops below the value of the power valve. The power valve when it opens delivers additional fuel to the main jet circuits to allow the engine to have enough fuel for maximum power and acceleration at WOT. A standard engine will likely have an idle vacuum of 12 to 13 inches of vacuum, so a 65 power valve wont be able to open until you have your foot firmly planted on the floor.
When you heavily modify an engine as I have done with mine the vacuum at idle can and does drop to very low figures. So if you leave a 65 value power valve in your holley and the idle vacuum of your engine is only 5.5 inches of vacuum the power valve is going to be open at idle. This will allow fuel to flow through the power valve into the primary throat main jet circuits at idle and into the throats of the carburetor and hence into the engine creating a massively rich fuel mixture at idle which you will not be able to correct with the idle mixture screws. Technically speaking it doesn't have an influence on the idle mixture circuit, ie the fuel it provides does not flow through the idle mixture circuit.
In my case I had to get down to a 35 value power valve in the primary's (3.5 inches of vacuum) to get the power valve below the manifold vacuum level at idle and stop the power valve from feeding fuel into the primary carby throats via the main jet circuits. Once I was able to stop the flow of fuel at idle via the primary power valve the engine was then idling on the idle mixture circuits alone and then I was able to tune the idle mixtures and the idle rpm.
I hope this clarifies the power valve and idle relationship and the fact that the power valve is only supposed to open at WOT to top up the fuel requirements of the engine for its maximum power and rpm performance. The mixture at WOT when the power valve is open is produced by the main jets and the amount of fuel through the power valve added together in the main jet circuits. If you take the power valves/s out then all of the fuel required for WOT has to be provided by the main jets by themselves which requires that you increase the size of the main jets to compensate for the lack of power valve fuel input, this will cause a rich mixture at cruising speeds on the highway. So in effect having the power valve in means that when driving at cruising speeds around town and on the highway the fuel for the engine is only provided by the main jet.
Bob
The power valve isn't supposed to have any influence on the idle circuit. However the issue of whether it does or does not is dependent upon the relationship of the power valve value and the idle vacuum of the engine. A 65 (6.5 inches of vacuum) value power valve needs an engine vacuum above 6.5 inches of vacuum to hold it closed. When it is closed it does not flow any fuel. This is the situation which is supposed to be in action at idle and up to about 2500/2750 rpm whilst driving around town and on the highway at speeds up to 100 to 120 kph. the power valve is supposed to open when you put your foot on the floor and the engine vacuum drops below the value of the power valve. The power valve when it opens delivers additional fuel to the main jet circuits to allow the engine to have enough fuel for maximum power and acceleration at WOT. A standard engine will likely have an idle vacuum of 12 to 13 inches of vacuum, so a 65 power valve wont be able to open until you have your foot firmly planted on the floor.
When you heavily modify an engine as I have done with mine the vacuum at idle can and does drop to very low figures. So if you leave a 65 value power valve in your holley and the idle vacuum of your engine is only 5.5 inches of vacuum the power valve is going to be open at idle. This will allow fuel to flow through the power valve into the primary throat main jet circuits at idle and into the throats of the carburetor and hence into the engine creating a massively rich fuel mixture at idle which you will not be able to correct with the idle mixture screws. Technically speaking it doesn't have an influence on the idle mixture circuit, ie the fuel it provides does not flow through the idle mixture circuit.
In my case I had to get down to a 35 value power valve in the primary's (3.5 inches of vacuum) to get the power valve below the manifold vacuum level at idle and stop the power valve from feeding fuel into the primary carby throats via the main jet circuits. Once I was able to stop the flow of fuel at idle via the primary power valve the engine was then idling on the idle mixture circuits alone and then I was able to tune the idle mixtures and the idle rpm.
I hope this clarifies the power valve and idle relationship and the fact that the power valve is only supposed to open at WOT to top up the fuel requirements of the engine for its maximum power and rpm performance. The mixture at WOT when the power valve is open is produced by the main jets and the amount of fuel through the power valve added together in the main jet circuits. If you take the power valves/s out then all of the fuel required for WOT has to be provided by the main jets by themselves which requires that you increase the size of the main jets to compensate for the lack of power valve fuel input, this will cause a rich mixture at cruising speeds on the highway. So in effect having the power valve in means that when driving at cruising speeds around town and on the highway the fuel for the engine is only provided by the main jet.
Bob
Last edited by The Spook on 19 Oct 2015, 19:11, edited 2 times in total.
Re: Winter fixes
Yep that's how I understand it totally Bob.
My statement was based on that I wouldn't have thought you would have enough air flow through the venturi to draw fuel from the main nozzle with the throttle closed. So I'm assuming your engine is fairly worked and at idle and you are drawing fuel from the idle circuit below (throttle plate) and fuel from the main discharge nozzle above the throttle plate at the same time (either enriched or not as dictated by whatever power valve is used). Do you have drilled throttle plates?
My statement was based on that I wouldn't have thought you would have enough air flow through the venturi to draw fuel from the main nozzle with the throttle closed. So I'm assuming your engine is fairly worked and at idle and you are drawing fuel from the idle circuit below (throttle plate) and fuel from the main discharge nozzle above the throttle plate at the same time (either enriched or not as dictated by whatever power valve is used). Do you have drilled throttle plates?
Re: Winter fixes
That's it in a nutshell as I understand it. The power valve can only flow fuel into the carby via the main jet circuit and apparently that's what happens if the idle vacuum can't keep the power valve closed.
Yes I do have the primary butterflies drilled with a 2.5 mm drill and that helps maintain the relationship between the edge of the butterfly and the idle slots which are in the bottom of the carb, without that small hole you have to open the butterflies more to achieve the higher airflow required for the higher idle and this can cause a stumble off idle when you blip the accelerator pedal. You can use a 3mm or even a 3.5mm drill but never start out with too big a drill bit as if the one you use is too big you can't put the metal back. I am fortunate as the 2.5mm drill bit was enough and there is no stumble off idle when I blip the pedal.
As for the state of tune of the engine, it is a stroked Holden early model 308 block and is now 357 cu in. The same engine in 308 mode with a 750 cfm carb and low rise single plane manifold has spun 8300 rpm. We, my mates and I are unsure what it will spin in 357 mode and we currently have the ignition system rev limiter set at 7800 which so far, due to carby size and tuning issues we have not been near yet.
It has 4 bolt mains
ARP main cap stud kit
Bis-alloy main cap girdle
Scat, steel stroker crank
Eagle 5.85 inch "H" beam rods with ARP L19 conrod bolts (rated @ 1400 hp per set)
Forged pistons
Bottom of the block has been grouted
Yella Terra dash 3 alloy heads - ported
2.080 I/L valves 1.6 inch exhaust valves all stainless extreme duty
380 lb spring tension on the valve at .600" lift which is max lift
260 deg duration I/L and Exh @.050"
alloy high rise air gap inlet manifold - port matched to the heads
a one inch alloy spacer on the top of the inlet manifold
11.5 to 1 compression ratio
now has a 870 carb as the 750 wasn't big enough
currently using 102+ unleaded from Powerplus fuels
I'm pretty comfortable that the bottom end should handle 8500 rpm
As one of my mates said recently "Bob you've done everything that can be done to that engine, that can be done to ensure that it has as little vacuum at idle as it possibly can have"
Bob
Yes I do have the primary butterflies drilled with a 2.5 mm drill and that helps maintain the relationship between the edge of the butterfly and the idle slots which are in the bottom of the carb, without that small hole you have to open the butterflies more to achieve the higher airflow required for the higher idle and this can cause a stumble off idle when you blip the accelerator pedal. You can use a 3mm or even a 3.5mm drill but never start out with too big a drill bit as if the one you use is too big you can't put the metal back. I am fortunate as the 2.5mm drill bit was enough and there is no stumble off idle when I blip the pedal.
As for the state of tune of the engine, it is a stroked Holden early model 308 block and is now 357 cu in. The same engine in 308 mode with a 750 cfm carb and low rise single plane manifold has spun 8300 rpm. We, my mates and I are unsure what it will spin in 357 mode and we currently have the ignition system rev limiter set at 7800 which so far, due to carby size and tuning issues we have not been near yet.
It has 4 bolt mains
ARP main cap stud kit
Bis-alloy main cap girdle
Scat, steel stroker crank
Eagle 5.85 inch "H" beam rods with ARP L19 conrod bolts (rated @ 1400 hp per set)
Forged pistons
Bottom of the block has been grouted
Yella Terra dash 3 alloy heads - ported
2.080 I/L valves 1.6 inch exhaust valves all stainless extreme duty
380 lb spring tension on the valve at .600" lift which is max lift
260 deg duration I/L and Exh @.050"
alloy high rise air gap inlet manifold - port matched to the heads
a one inch alloy spacer on the top of the inlet manifold
11.5 to 1 compression ratio
now has a 870 carb as the 750 wasn't big enough
currently using 102+ unleaded from Powerplus fuels
I'm pretty comfortable that the bottom end should handle 8500 rpm
As one of my mates said recently "Bob you've done everything that can be done to that engine, that can be done to ensure that it has as little vacuum at idle as it possibly can have"
Bob
Last edited by The Spook on 20 Oct 2015, 13:45, edited 2 times in total.
Re: Winter fixes
Ok, now I get it. Thanks Bob. I could talk this stuff all day!
Cheers
Craig.
Cheers
Craig.
Re: Winter fixes
Craig
Are you coming to Naranderah?
Bob
Are you coming to Naranderah?
Bob
Re: Winter fixes
Sadly no Bob. I'm at work that week. Not sure what else is on that'll I'll get to. I'm thinking Yarrawonga and Goolwa hopefully.
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