Nothing Special Posted April 12, 2021 Author Share Posted April 12, 2021 I like your theory on the old power valve or maybe it was leaking, as you say you will probably never know. Hopefully it runs better for you with the new jets. Bob I doubt it was leaking. This got last several posts back, but when I first started it this spring I could hardly get it to idle. Turned out (no pun intended) that the idle screws were turned way too far out. I'm guessing that was to deal with the vacuum leak at the throttle shaft. Now that I fixed the vacuum leak I needed to lean out the idle mixture to get it to idle. If the power valve was leaking I'd have gone lean at idle when that was fixed. And I didn't. But this is just bench racing. No way to really determine anything from it. Link to comment Share on other sites More sharing options...
Nothing Special Posted April 13, 2021 Author Share Posted April 13, 2021 I doubt it was leaking. This got last several posts back, but when I first started it this spring I could hardly get it to idle. Turned out (no pun intended) that the idle screws were turned way too far out. I'm guessing that was to deal with the vacuum leak at the throttle shaft. Now that I fixed the vacuum leak I needed to lean out the idle mixture to get it to idle. If the power valve was leaking I'd have gone lean at idle when that was fixed. And I didn't. But this is just bench racing. No way to really determine anything from it. Well I got the new jets, but the jury's still out. I didn't get much chance to test drive it last night. Our city had a 7:00 curfew yesterday so my evening was pretty short and I didn't get a chance to really get it up to operating temperature. And this morning I drove it to work, but it's pretty cold this morning (low 30s) and the temp gauge never got up to where it normally runs. That said, it's not a lot different. Maybe that's because it's not getting warmed all the way up, or maybe it's just not all that different. Last night it did feel like it was waking up pretty well with just a little more throttle. That might be accelerator pump, but I was trying to roll in slow enough that there wouldn't be much of that. So maybe it's that the power valve is about to open? Anyway, my plan today is to drill out my stock 48 jets to about 0.052" and see if that helps. But eventually I probably need to get an AFR and a vacuum gauge and really try to dial it in. Link to comment Share on other sites More sharing options...
Gary Lewis Posted April 13, 2021 Share Posted April 13, 2021 Well I got the new jets, but the jury's still out. I didn't get much chance to test drive it last night. Our city had a 7:00 curfew yesterday so my evening was pretty short and I didn't get a chance to really get it up to operating temperature. And this morning I drove it to work, but it's pretty cold this morning (low 30s) and the temp gauge never got up to where it normally runs. That said, it's not a lot different. Maybe that's because it's not getting warmed all the way up, or maybe it's just not all that different. Last night it did feel like it was waking up pretty well with just a little more throttle. That might be accelerator pump, but I was trying to roll in slow enough that there wouldn't be much of that. So maybe it's that the power valve is about to open? Anyway, my plan today is to drill out my stock 48 jets to about 0.052" and see if that helps. But eventually I probably need to get an AFR and a vacuum gauge and really try to dial it in. Hard to test w/o getting it fully warm. But if it felt like it was waking up you are on the right track. The power valve shouldn't be open at cruise unless you are climbing a pretty significant hill, so you need to get the engine happy with the jets. An AFR meter will help a lot, but you can do it with a vacuum gauge. That will let you know when the power valve is open and you can tune the main jets to keep it closed most of the time. As for drilling jets, that doesn't always work well in my experience. The jets are tapered to smooth the flow in and out, and if you take the taper out by drilling you may actually lose flow. But, it is worth a try. Good luck! Link to comment Share on other sites More sharing options...
Nothing Special Posted April 13, 2021 Author Share Posted April 13, 2021 Hard to test w/o getting it fully warm. But if it felt like it was waking up you are on the right track. The power valve shouldn't be open at cruise unless you are climbing a pretty significant hill, so you need to get the engine happy with the jets. An AFR meter will help a lot, but you can do it with a vacuum gauge. That will let you know when the power valve is open and you can tune the main jets to keep it closed most of the time. As for drilling jets, that doesn't always work well in my experience. The jets are tapered to smooth the flow in and out, and if you take the taper out by drilling you may actually lose flow. But, it is worth a try. Good luck! Looking at the 48 jets under a microscope, the output end looks pretty square, with just a simple chamfer on the input end. So I should be able to get it pretty close. And my intent is to just get it pretty close, then to order a range of "real' jets around wherever I end up. (And we'll see how many years it takes me to actually do that if "pretty close" is "good enough" ) Link to comment Share on other sites More sharing options...
Gary Lewis Posted April 13, 2021 Share Posted April 13, 2021 Looking at the 48 jets under a microscope, the output end looks pretty square, with just a simple chamfer on the input end. So I should be able to get it pretty close. And my intent is to just get it pretty close, then to order a range of "real' jets around wherever I end up. (And we'll see how many years it takes me to actually do that if "pretty close" is "good enough" ) I understand the "good enough". I have a t-case shift lever that's backwards to the pattern on the knob. Anyway, I think I've read that each jet size varies as to the input and output taper. And that some jets with different numbers have the same orifice size but with different tapers. I don't know if that is true, but my memory says I read it at Holley's site or in a book on their carbs decades ago. But if you have a microscope you can see if you have any taper left after drilling. If not, maybe use the next larger bit to add some? Or use a Dremel tapered grind stone, by hand, to at least take the edge off? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted April 13, 2021 Share Posted April 13, 2021 I understand the "good enough". I have a t-case shift lever that's backwards to the pattern on the knob. Anyway, I think I've read that each jet size varies as to the input and output taper. And that some jets with different numbers have the same orifice size but with different tapers. I don't know if that is true, but my memory says I read it at Holley's site or in a book on their carbs decades ago. But if you have a microscope you can see if you have any taper left after drilling. If not, maybe use the next larger bit to add some? Or use a Dremel tapered grind stone, by hand, to at least take the edge off? I don't think you're going to get a dremel stone into a jet. I have some micro diamond burrs for drilling FR10 PC boards. Maybe they would be small enough. But the place in Japan I got them from went out of business after SMD's took over. The power valve should only open when vacuum drops below it's set point, and older power valves (like came in the 4180's were two-stage. The best tuning instrument you can get (my opinion) is a vacuum gauge on the dash. This will tell you more about how you drive, and how your engine responds. Link to comment Share on other sites More sharing options...
Gary Lewis Posted April 13, 2021 Share Posted April 13, 2021 I don't think you're going to get a dremel stone into a jet. I have some micro diamond burrs for drilling FR10 PC boards. Maybe they would be small enough. But the place in Japan I got them from went out of business after SMD's took over. The power valve should only open when vacuum drops below it's set point, and older power valves (like came in the 4180's were two-stage. The best tuning instrument you can get (my opinion) is a vacuum gauge on the dash. This will tell you more about how you drive, and how your engine responds. You are probably right on the stone. I've not even been out to the shop since getting back, so haven't eyeballed a jet/stone combo. But hope to get out there in a bit and will do. And I agree on the vacuum gauge, at least for this application. Bob knows he's a bit lean given how it perked up with larger jets. So if he can get it to perform well off the main jets and just dip into the power valve when climbing a big hill or needing to pass, w/o WOT, then I think he'll be "there" on the mains. Then it is a question of when the PV comes in. Is there an area of throttle/low vacuum where the engine is "flat" just before the PV comes in? If so, then go to a PV that opens at a higher level of vacuum. If not, then it is "good enough" time. Link to comment Share on other sites More sharing options...
Nothing Special Posted April 13, 2021 Author Share Posted April 13, 2021 You are probably right on the stone. I've not even been out to the shop since getting back, so haven't eyeballed a jet/stone combo. But hope to get out there in a bit and will do. And I agree on the vacuum gauge, at least for this application. Bob knows he's a bit lean given how it perked up with larger jets. So if he can get it to perform well off the main jets and just dip into the power valve when climbing a big hill or needing to pass, w/o WOT, then I think he'll be "there" on the mains. Then it is a question of when the PV comes in. Is there an area of throttle/low vacuum where the engine is "flat" just before the PV comes in? If so, then go to a PV that opens at a higher level of vacuum. If not, then it is "good enough" time. I checked my 48 jets with plug gages before I drilled them out. An .047" gage would go through but an .048" gage would not. So I guess they were running slightly small. I ran a #55 drill bit through both of them and now an .052" gage goes and an .053" gage does not, so right where I was aiming. The chamfer in the jets is a lot bigger than the difference in hole size, so I'm not going to touch the chamfer. I think I have essentially the same lead-in now that these jets had before. I'll stick these jets in tonight and see what I think. On the power valve, I'm thinking it's probably pretty close. If it feels flat at steady cruise and wakes up with just slightly more throttle it seems likely that the power valve is opening just a little above steady cruise. I could imagine that once I get the jets dialed in that the power valve might be opening just a bit early, and going one step lower in vacuum to open might be better. But it's probably pretty close. Link to comment Share on other sites More sharing options...
Gary Lewis Posted April 13, 2021 Share Posted April 13, 2021 I checked my 48 jets with plug gages before I drilled them out. An .047" gage would go through but an .048" gage would not. So I guess they were running slightly small. I ran a #55 drill bit through both of them and now an .052" gage goes and an .053" gage does not, so right where I was aiming. The chamfer in the jets is a lot bigger than the difference in hole size, so I'm not going to touch the chamfer. I think I have essentially the same lead-in now that these jets had before. I'll stick these jets in tonight and see what I think. On the power valve, I'm thinking it's probably pretty close. If it feels flat at steady cruise and wakes up with just slightly more throttle it seems likely that the power valve is opening just a little above steady cruise. I could imagine that once I get the jets dialed in that the power valve might be opening just a bit early, and going one step lower in vacuum to open might be better. But it's probably pretty close. I think you are going to be "right on". I say that because the jets will work out nicely with the taper above the drilling. And the richer mix is going to increase the vacuum at cruise 'cause you'll have to close the throttle just a bit and you'll be farther away from when the PV opens because of that. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted April 13, 2021 Share Posted April 13, 2021 If not, then it is "good enough" time. Who are you???? And what have you done with Gary? Link to comment Share on other sites More sharing options...
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