1/3 Scale Ford 289 Hi-Po
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Been pondering those symptoms. If this was a test question on a exam I was taking somewhere my best guess would be that there is air being drawn into the intake somewhere other than through the carb. The far open needle valve and nearly closed throttle plate are compensating for this.
I am not making suggestions to the master ! I just wonder if my reasoning is sound. I learn so much from your posts, thank you !
Arild
Active Member
Looking fantastic!!
Sparky's diagnosis makes just too much sense. If it hadn't been for the strong suction at the carb air inlet even at wide open throttle, I might have come to a similar conclusion. But it's well worth investigating.
When old-school troubleshooting an intake leak on a full-size engine, I'd typically spray some carb cleaner around the intake manifold and carb base while the engine was idling. If the idle speed was affected the carb cleaner, I'd be in the general area of a leak. With a model engine though it's hard to localize the spray, and keeping the engine idling hands-off is like balancing a razor blade on its edge.
Newer tools have been developed for troubleshooting similar problems in modern cars whose 'money lights' will turn because of leaks so small they would be unnoticed in this particular case. One of these tools I've used to find intake and evap leaks in my own newer cars is a 'smoke machine'. Basically, it creates a slightly pressurized stream of smoke (heated baby oil). For an intake leak test the stream would be injected into the throttle body through a sealed bladder. If there is any any leak at all, the tell-tale smoke will pinpoint its exact location.
For my test it was most convenient to plug the exhausts and inject the smoke into the Venturi opening through a conical rubber fitting stuffed in the throttle barrel. There was no smoke visible anywhere indicating a vacuum leak is probably not my problem.
I'm in new and territory here and will follow up on any suggestions others might have. Although my needle valve design is nearly identical to what I've used successfully in my last two engines, I'm beginning to wonder if I'm just not getting enough fuel to the Venturi when the throttle barrel is opened more than a few degrees.- Terry
When old-school troubleshooting an intake leak on a full-size engine, I'd typically spray some carb cleaner around the intake manifold and carb base while the engine was idling. If the idle speed was affected the carb cleaner, I'd be in the general area of a leak. With a model engine though it's hard to localize the spray, and keeping the engine idling hands-off is like balancing a razor blade on its edge.
Newer tools have been developed for troubleshooting similar problems in modern cars whose 'money lights' will turn because of leaks so small they would be unnoticed in this particular case. One of these tools I've used to find intake and evap leaks in my own newer cars is a 'smoke machine'. Basically, it creates a slightly pressurized stream of smoke (heated baby oil). For an intake leak test the stream would be injected into the throttle body through a sealed bladder. If there is any any leak at all, the tell-tale smoke will pinpoint its exact location.
For my test it was most convenient to plug the exhausts and inject the smoke into the Venturi opening through a conical rubber fitting stuffed in the throttle barrel. There was no smoke visible anywhere indicating a vacuum leak is probably not my problem.
I'm in new and territory here and will follow up on any suggestions others might have. Although my needle valve design is nearly identical to what I've used successfully in my last two engines, I'm beginning to wonder if I'm just not getting enough fuel to the Venturi when the throttle barrel is opened more than a few degrees.- Terry
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Dang ! A smoke leak detector, you have all the nice toys !
I have also heard of using small propane/butane torch, unlit, probing for leaks. Same effect as the carb cleaner method but cleaner. It might be worth a try just for good measure next time you are in running configuration. It wouldn't pinpoint a leak but might show a reaction. I am just wondering if there is any possibility of the smoke tester giving a false negative????? The tester is intended for engines that flow hundreds of CFM in operation, maybe it won't spot a tiny leak that could still cause problems on something this tiny? Beats me, just thinking out loud.
One other thought I had. What about fuel flow? I think of a engine that runs with the choke nearly closed but not when opened, common with a dirty/plugged carb. The nearly closed throttle plate and wide open needle could possibly point to possibly needing excessive vacuum to pick up fuel reliably. Just another of my hair brained ideas LOL Perhaps the same unlit torch near the intake when running to "richen" the mixture would yield some hints?
I have zero doubts you will find the problem soon. I learn much from your builds, the resolution to this problem will also be educational.
I have also heard of using small propane/butane torch, unlit, probing for leaks. Same effect as the carb cleaner method but cleaner. It might be worth a try just for good measure next time you are in running configuration. It wouldn't pinpoint a leak but might show a reaction. I am just wondering if there is any possibility of the smoke tester giving a false negative????? The tester is intended for engines that flow hundreds of CFM in operation, maybe it won't spot a tiny leak that could still cause problems on something this tiny? Beats me, just thinking out loud.
One other thought I had. What about fuel flow? I think of a engine that runs with the choke nearly closed but not when opened, common with a dirty/plugged carb. The nearly closed throttle plate and wide open needle could possibly point to possibly needing excessive vacuum to pick up fuel reliably. Just another of my hair brained ideas LOL Perhaps the same unlit torch near the intake when running to "richen" the mixture would yield some hints?
I have zero doubts you will find the problem soon. I learn much from your builds, the resolution to this problem will also be educational.
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Sparky,
The most popular use for a smoke machine is to find a leak an evap leak in a car. These leaks can be small pin hole leaks in the charcoal canister, vent valve, purge valve, or any of the interconnecting lines.
I've also used an mapp gas torch (not lit of course) with a rubber hose on the end, but I'm not sure with a model engine I could stay far enough away from the carb to avoid false positives.
By the way, if you ever have to pay a garage to find one of these leaks for you, you'll find you'll be able to buy a bi-directional scanner, smoke machine, and the replacement parts to fix it for what a dealer will charge you to 'maybe' fix it. - Terry
You have one of those little pencil butane torches in your toybox?
No, just your typical 'thaw the water pipes' torch.
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For me the most interesting part of a build can be the steps taken to get a newly finished engine running. Too often it's done behind closed doors, and readers who've been following the build miss out on an opportunity to learn about what is sometimes the hardest part of the build. Some of life's things have come up recently and are limiting the time I can work on the engine. But I'll sneak into the shop when I can and detail progress as it's made.
I discovered the problem with the fan belts (repurposed 1/8" o-rings) was that they were sitting in pulley grooves machined for 3/32" o-rings. I deepened the grooves as much as possible but not as much as needed because the crank and water pump pulleys didn't have the extra needed material. The belts were still problematic after re-machining, and the final solution was to order the correct o-rings.
The carburetor was removed from the engine and carefully inspected with the focus being on the needle valve assembly. I was hoping to find debris or a machining issue, but everything looked and measured as expected. While the carb was off the engine, a vacuum port was added to the rear of the high rise spacer below the carb so manifold vacuum could be measured later.
With an air leak ruled out, I next considered the possibility that the needle valve design was so far out of whack that the mixture was much too lean at any throttle setting. Although there's some interaction between the jet and the Venturi, I decided to work just one at a time.
The 289's Venturi size was derived using an equation that I've used before (see the posts on the 289's carb). This equation shows Venturi diameter being proportional to the square root of cylinder displacement. My Inline Six engine with its .382 cubic inch cylinder displacement has a .138" diameter Venturi and has been working reasonably well.
With a 1.076 cubic inch cylinder displacement, the 289's theoretical Venturi worked out to a diameter of .232". However, I elected to take only half of the increase and instead used .180". Right or wrong, I had my reasons. Unlike the large intake valves on the full-size engine my intake valves are identical to the smaller exhaust valves, and if I'm being honest their lift also wound up a bit shy of what's needed to create a theoretical maximum flow. The camshaft was also designed for low speed vacuum rather than a screaming top end because I'd never been comfortable with the sharp unblended runners that resulted from machining the intake manifold from a single piece billet. So, for now I decided to leave the Venturi alone.
The jet size however was another matter. The 289's needle valve assembly was machined identical to that on the Inline Six. The .0225" jet and .026" needle along with the 80 tpi needle gives the Inline Six's carb needle about 2-1/2 turns between full ON and full OFF. The Inline Six ran fine with only 3/4 turn, and I had assumed at least another full turn would be available to the 289.
In actuality, the 289 tried to idle with the needle fully open and the throttle within a flea's eyelash of being fully closed. Rpm was extremely sensitive to the throttle, and an additional degree or two of throttle caused the engine to surge before dying.
I decided to scale the new jet diameter up by the square root of the piston displacement ratio. The 289's new jet diameter became 1.678 x .0225" = .038", and a search through my wife's sewing basket turned up a .042" diameter needle. In a perfect world I'd only have to machine a new needle holder and reuse the existing needle valve body. However, the spray nozzle was too small to be re-drilled for the larger needle, and so I'd have to machine a whole new needle valve assembly.
The 8-80 and 10-56 threads on the micro-machined valve body make it the definition of a PITA part. After many, many hours and scrapped parts (same problems as with the original part) I finally had a new valve body. In the meantime the new fan belts had arrived, and testing showed they stayed in place. The radiator was finally installed and the system filled with 32 oz (1 qt) of 50/50 antifreeze.
I immediately happened across an operating point at about 1/4 throttle and a needle opening of 3-1/4 t (don't really know how many turns is full open with this new needle) where the engine would run at a few thousand rpm. With the engine now capable of sustained running, I didn't try any tuning but instead figured I'd better determine a proper running oil level.
It's surprising how even a little bit of sump oil gets whipped up by the crank windage and the pressure pulses inside the crankcase. For a model engine doing no significant work, I like to run with the highest oil level possible without an unacceptable amount of oil smoke in the exhaust. If everything in a V-8 wet sump engine is working as it should, this smoke will show up first in the starboard exhaust as the oil level is increased. This happens because the webs of the clockwise rotating crank tend to douse the undersides of the pistons in the starboard-side bank.
The sump was drained, and only 50 ml of the original 80 ml showed up in the pan. Hopefully there's 30 ml distributed throughout the engine. The good news is that even without an oil pump, oil has been collecting under the valve covers hinting that my PCV top-end oiling scheme may actually be working. The sump was refilled with 100 ml of 0-W20 and the engine re-started. The oil level was then increased 50 ml at a time until an unacceptable amount of exhaust smoke showed up at the current throttle and needle settings. The starboard side exhaust showed no smoke with 250 ml but 300 ml fogged up the shop. From now on, FULL is 270 ml.
The upper radiator hose is clear silicone and coolant can be seen flowing when the engine is started. The water pump seems to be working well, and the coolant temperature in the radiator after the oil tests was about 50F above ambient.
A quick check found a needle setting that will allow the engine to rev up at wide open throttle, but now idling is a problem. The air bleed doesn't seem to help, and so when I get the time my next focus will be there. - Terry
I discovered the problem with the fan belts (repurposed 1/8" o-rings) was that they were sitting in pulley grooves machined for 3/32" o-rings. I deepened the grooves as much as possible but not as much as needed because the crank and water pump pulleys didn't have the extra needed material. The belts were still problematic after re-machining, and the final solution was to order the correct o-rings.
The carburetor was removed from the engine and carefully inspected with the focus being on the needle valve assembly. I was hoping to find debris or a machining issue, but everything looked and measured as expected. While the carb was off the engine, a vacuum port was added to the rear of the high rise spacer below the carb so manifold vacuum could be measured later.
With an air leak ruled out, I next considered the possibility that the needle valve design was so far out of whack that the mixture was much too lean at any throttle setting. Although there's some interaction between the jet and the Venturi, I decided to work just one at a time.
The 289's Venturi size was derived using an equation that I've used before (see the posts on the 289's carb). This equation shows Venturi diameter being proportional to the square root of cylinder displacement. My Inline Six engine with its .382 cubic inch cylinder displacement has a .138" diameter Venturi and has been working reasonably well.
With a 1.076 cubic inch cylinder displacement, the 289's theoretical Venturi worked out to a diameter of .232". However, I elected to take only half of the increase and instead used .180". Right or wrong, I had my reasons. Unlike the large intake valves on the full-size engine my intake valves are identical to the smaller exhaust valves, and if I'm being honest their lift also wound up a bit shy of what's needed to create a theoretical maximum flow. The camshaft was also designed for low speed vacuum rather than a screaming top end because I'd never been comfortable with the sharp unblended runners that resulted from machining the intake manifold from a single piece billet. So, for now I decided to leave the Venturi alone.
The jet size however was another matter. The 289's needle valve assembly was machined identical to that on the Inline Six. The .0225" jet and .026" needle along with the 80 tpi needle gives the Inline Six's carb needle about 2-1/2 turns between full ON and full OFF. The Inline Six ran fine with only 3/4 turn, and I had assumed at least another full turn would be available to the 289.
In actuality, the 289 tried to idle with the needle fully open and the throttle within a flea's eyelash of being fully closed. Rpm was extremely sensitive to the throttle, and an additional degree or two of throttle caused the engine to surge before dying.
I decided to scale the new jet diameter up by the square root of the piston displacement ratio. The 289's new jet diameter became 1.678 x .0225" = .038", and a search through my wife's sewing basket turned up a .042" diameter needle. In a perfect world I'd only have to machine a new needle holder and reuse the existing needle valve body. However, the spray nozzle was too small to be re-drilled for the larger needle, and so I'd have to machine a whole new needle valve assembly.
The 8-80 and 10-56 threads on the micro-machined valve body make it the definition of a PITA part. After many, many hours and scrapped parts (same problems as with the original part) I finally had a new valve body. In the meantime the new fan belts had arrived, and testing showed they stayed in place. The radiator was finally installed and the system filled with 32 oz (1 qt) of 50/50 antifreeze.
I immediately happened across an operating point at about 1/4 throttle and a needle opening of 3-1/4 t (don't really know how many turns is full open with this new needle) where the engine would run at a few thousand rpm. With the engine now capable of sustained running, I didn't try any tuning but instead figured I'd better determine a proper running oil level.
It's surprising how even a little bit of sump oil gets whipped up by the crank windage and the pressure pulses inside the crankcase. For a model engine doing no significant work, I like to run with the highest oil level possible without an unacceptable amount of oil smoke in the exhaust. If everything in a V-8 wet sump engine is working as it should, this smoke will show up first in the starboard exhaust as the oil level is increased. This happens because the webs of the clockwise rotating crank tend to douse the undersides of the pistons in the starboard-side bank.
The sump was drained, and only 50 ml of the original 80 ml showed up in the pan. Hopefully there's 30 ml distributed throughout the engine. The good news is that even without an oil pump, oil has been collecting under the valve covers hinting that my PCV top-end oiling scheme may actually be working. The sump was refilled with 100 ml of 0-W20 and the engine re-started. The oil level was then increased 50 ml at a time until an unacceptable amount of exhaust smoke showed up at the current throttle and needle settings. The starboard side exhaust showed no smoke with 250 ml but 300 ml fogged up the shop. From now on, FULL is 270 ml.
The upper radiator hose is clear silicone and coolant can be seen flowing when the engine is started. The water pump seems to be working well, and the coolant temperature in the radiator after the oil tests was about 50F above ambient.
A quick check found a needle setting that will allow the engine to rev up at wide open throttle, but now idling is a problem. The air bleed doesn't seem to help, and so when I get the time my next focus will be there. - Terry
