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tuning HEI for 4spd 350

I have the dark spring installed with
15" vac at idle as per cliffs recommendation. He also states that with as much vac you have @ 21 any of the springs would work. The varying cut in rates keep the power system closed at Lower vac levels at idle due to larger cams.
At steady speed/rpm power system should be closed and not in play.
That's where the APT comes in to play to provide proper cruise fuel and also effects off idle performance as you have seen recently.
Remember you are dealing with a heavy truck even tho it's a 4 spd I believe your coming in to soon with the mechanical advance , 10 deg initial should be plenty IMHO.
I would assume the brown spring that was discontinued due to not being needing 11.5 cut in after substantial testing.
 
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I agree, the 11.5" APT spring would be too much - when I wrote that it might be applicable I thought 11.5" was the opening point. On the timing and APT adjustment, I will try a heavier mech adv spring and add more APT.
 
It has been 6 months and I am happy with the current HEI tune. No pinging on 87 octane regardless of the outside temperatures and grade of the highway.

I had a two core radiator and now have a three core radiator, read more https://ck5.com/forums/threads/upgrade-radiator-on-74-k5.325832/ The result is my temps are more stable now so I can begin to play with different thermostat settings and get more agressive with lean carb tunes and timing curves. Before I had to leave a factor of safety for that rare hot day and steep grade that pulls the temps up and then begins the detonation-heat death cycle.

Has anyone played around with thermostats and looked at the effect on the timing curve? I'd like to try thermostats 180 deg (have now), 195 deg, and 205 deg.

My goal is to improve drivability with the higher temperature. If my cooling is now adequate I no longer have a reason to run the 180 deg thermostat. My reason for the 180 deg is the engine would cool down to a lower temp and then on steep grades or while accelerating on hot days would heat up to wherever it went (210 deg F), once I got to a flat or let off the gas, things would cool back down to be ready for the next hill or acceleration. I don't need all that bs anymore.
 
I went from a 160-180-195 on my 72 but that was an auto. I can tell you that having a good working fan clutch and good cooling system the temp was much more stable, I tow also so I know that working them makes the cooler stat less effective, because now there isn't as much heat exchange.

I'm not sure how much timing you run at cruise, I usually shot for the highest number possible while not pinging, if that was 50 then that's what it was.
I was getting 12mpg with a k20 on 31 inch tires and 4.10s and no overdrive. And all this on a Holley 750dp.. I thought that was decent and I have to assume keeping the combustion temps up some and the radiator doing its job helped. That was a tight motor so quench volumes where tight, .040. I bet that had more to do with the detonation resistance than all of the rest.
 
sreidmx said:
that was decent and I have to assume keeping the combustion temps up some and the radiator doing its job helped.
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  • I don't recall my combustion chamber size but this motor should be around 9:1; the machine shop guestimated and it was sort of +/- based on the headgasket. We have similar motors at .040 over; different trans and gears but similar milage, tires, etc.
  • Same here, no idea what the curve is but it does not ping. My timing is set the 'old' way, set base timing to just before ping with no vacuum adv. and heavy mech adv springs. Then lighten mech adv springs until ping. Then adjust vacuum adv. until ping.
  • No fan clutch - I use a flex fan that cools well at low rpm (I have used three or four different flex designs before I found one I liked).
Sounds like from your experience that now that I have my cooling under control it is worthwhile to try a 195 thermostat and see what my gas milage does.
 
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There is an issue with low speed bucking. I thought was related to too much Q-jet idle bypass air but my father-in-law thinks it is the timing curve. So I will post it here.

At low speed (idling in 2nd gear around the parking lot) when making a turn, unless I give just the right amount of throttle (not too much and not too little) the engine will start lean bucking.

I have slop in the drivetrain and no give; SM465 with a worn out coupler sleeve to the transfer case and a locker - once it starts bucking it gets out of control quick and the only solution is to push in the clutch.

My father-in-law thinks I have too much timing for my mild cam and this is the problem. At 600 rpm five pistons are fired per second, that is 0.2 seconds per piston fired. I think at some point in those .2s the air passing through the carb bypass air tube stalls so no fuel is drawn that instant and forms a lean pocket of air. If I give too much throttle things are not much better since I am just creating a larger venturi and the conditions for the air to stall increase.

Has anyone solved this?
 
Is there any chance your float isn't set high enough? After all your carb work, I'm guessing no, but it sounds more like fuel starvation. I ran into that with my car and it was neck-breaking.

Having an issue during turning without high G-forces doesn't seem to be a common issue. I don't see why turning should affect fuel delivery to that extent.
 
dyeager535 said:
Is there any chance your float isn't set high enough? After all your carb work, I'm guessing no, but it sounds more like fuel starvation.
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It is possible: Recently I added a pressure regulator to resolve intermittent mild flooding due to high fuel pressure (~10psi). When this thread started I had set the float to the book spec but felt like there was flooding (wet upper gasket) and lowered the float twice during teardowns until I was happy with it. I think I was lowering the float to offset the high pressure. Now that the pressure regulator is on I should be able to raise the float.

All that said, I agree that idling through a parking lot it is hard to think there is a fuel delivery problem. I can floor it right through all the gears onto the freeway without any fuel problems. Give me a couple days and I will post a video of the dash (speed, gear, rpm, & vacuum) during the lean buck. I don't drive offroad much but I'd imagine if I was in rocks and idling in too high a gear while crawling I would get a similar buck. I will also disconnect my vacuum advance and see if that has any effect. I should be able to post something later this week or early next week.
 
Have you used an A/F gauge to look at what ratio's you're actually getting on your exhaust? That would probably tell you a lot about if you're having a fueling problem or timing problem.
 
blazer74 said:
Sure sounds like a lean buck, or it just doesn't like being in low gear that slow.
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I agree. Is there a low speed adjustment other than idle fuel on the QJet? At that low speed both the power valve is not enriching quick enough and the accelerator pump shot is not enriching quick enough.
 
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JoshHefnerX said:
https://www.amazon.com/dp/B004MDT8M...UTF8&colid=3RBJIUZNNLGYR&coliid=IPZUSSHP0OO9H
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Thank you - in one of these threads I had a discussion about installing wideband in my truck. The limitation is most of these wideband kits don't have data loggers plus there is no record of throttle position, vehicle speed, and gear. I more or less need to drive around and stare at the gauge and take notes.

There is an article in hotrod magazine http://beta.hotrod.com/articles/hrdp-1210-on-the-road-holley-carb-tuning/ where they added wideband to some 300HP holley cars; they more or less found 10% horsepower gains at 4,000rpm and 1 mpg. I was super surprised that was all they found. Some of these cars had the timing set with the vacuum connected and the tuning crew was adding 10 degrees of advance before they even started on the carb. They reported no gains under 4,000rpm. I never get above 3,000rpm so I am guessing my current tune is probably right about on it.

In my opinion, for a simple issue like my lean buck, I don't need a wideband to tell me I am lean at 600 rpm in 3rd gear at part throttle (if I wasn't lean there would be something to talk about). Even if I want to be double sure, an exhaust analyzer in the tailpipe on a shop roller will get me the data I need to validate my tune. A fulltime wideband gauge is overkill. I am fairly certain the solution will involve a drill and adding adjustable idle air bleeds if I want to find a 'foolproof' solution or I can just live with it and know to use the clutch.
 
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600RPM is AWFULLY low for a manual vehicle under load, probably equally so for an auto. If that is what you are trying to work around, I'd probably suggest just avoiding that situation. I know that sounds simplistic, but I don't know that it's a reasonable expectation for a carb to work at that low of an RPM range, generally.

Every once in awhile I'll drag mine down that low, only negative I see (being injected) is that the oil pressure starts to get really bad.
 
dyeager535 said:
600RPM is AWFULLY low for a manual vehicle under load, probably equally so for an auto. If that is what you are trying to work around, I'd probably suggest just avoiding that situation. I know that sounds simplistic, but I don't know that it's a reasonable expectation for a carb to work at that low of an RPM range, generally..
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I agree. For me the oil pressure is good at low rpm. It is more of a parking lot turn around (2nd gear) and right turn without stop (3rd gear) issue. The locker and worn t-case couple makes it worse. With an automatic it would just coast and then downshift and the torque converter would pick up so it really would not get into the 600 rpm range. My 3.08 gears creates a bad spot on SM465 2nd/3rd split. If it is what it is then it is no problem. As it is if I go easy on the gas it is drivable - by itself not bad. I just want it a little better so I can pull out of the hole without risking it starting to buck.
 
It could be slight lugging that is made a lot worse by the worn out couple and locker. Because I can tune it out with richening the idle screws a half turn I know it can be solved; at least at the expense of fuel economy. Let me see if I can make a video this evening, I will take the family for a drive and have them hold the camera
 
forest_peterson said:
It could be slight lugging that is made a lot worse by the worn out couple and locker. Because I can tune it out with richening the idle screws a half turn I know it can be solved; at least at the expense of fuel economy. Let me see if I can make a video this evening, I will take the family for a drive and have them hold the camera
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Expense probably wouldn't be just MPG either, but increased noxious exhaust fumes. Sounds like you've experimented with it already, perhaps you found different. But IME richer at idle makes being around the vehicle nasty.

The primaries are reliant on vacuum to keep from fueling, correct? Vacuum keeps the piston (and thus primary rods) pulled into the primary jets right? Won't any air bleeds affect that? Either holes in the plates or cracking the throttle plate a bit, don't you end up with the same thing? Just spit balling. How about changing the primary rods to a thinner tip profile?
 
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