Ok ,when you say Full Manifold....Can I use the same source as the automatic transmission and power brakes source? I only have one spot where I can tap into and this is what I used to read the vacuum originally at 15hg at idle. Thanks
Yep will recheck. I adjusted the back throttle blades to give more air ,but not enough to expose the slots, helped some but not a lot.
Great, that is what I have been doing. will get new readings and recheck timing. I read somewhere that a guy had the same issues that I am having and he adjusted his "back lash for his valves" , solved the problem. Coarse he knew what type of modified cam he had and knew what adjustments to make.
Probably better off leaving the valves alone ! If your curious about the condition of your rings and valves , run a compression test and a leak- down test , otherwise ...see above
A 4160 does not have idle circuits in the secondarys by opening the throttle blades you just caused a vacuum leak and leaned the mixture. You need to figure why the front is rich.
A 4160 does have secondary idle circuits , most are .026-..031 IDLE feed orifice discharged through the constant idle discharge ports ( keeps the fuel from getting stale,)
Well, installed a lower power valve and cleared up the running rich issue. Replaced idle mix screws...no difference in adjusting. Put a spring on the back of the throttle assembly to pull the throttle plates closed more to cover transition slot, this helped. Set timing with timing light and by vacuum gauge...... can not get the spread narrower on the idle at park and idle in gear, still about 450 to 500 rpm apart, clunks hard when putting into reverse or drive, replaced transmission mount and all other engine mounts are good. If I close the rpm gap it stalls at traffic lights. New small high torque starter, works great, hits first crank. Wish it had a normal stock cam.
I'll admit I'm no expert with Holley carbs, but there seems to be more idle adjustability than just the idle mixture screws. https://www.musclecardiy.com/perfor...dle-and-transition-circuit-calibration-guide/ Make sure the idle air bleed circuits aren't plugged, or when trying to adjust the idle mixture temporarily block the idle air bleeds with your fingers, or a wooden toothpick. If the idle starts to respond to adjusting the mixture screws you may need to adjust or modify the air bleed circuit. If the carb doesn't respond with the air bleeds blocked you may have to do some detective work to make sure the idle air circuits aren't blocked or restricted.
Thanks, Everytime I go back to this article I learn something new. Two of my idle air bleed screws look to have been drilled out. Will need to do some experimenting here. It also said 1000 rpm at idle for a modified cam is normal, mine is right at 1000 in park, in drive 600/550. More tinkering.
Make sure you're looking at the idle air bleeds. I think the two inboard air bleeds are for the main jet circuits and you probably don't want to make any changes to them. The air bleeds perform two functions. First, they act as an anti-siphon vent so fuel can't be siphoned out of the bowl after the engine is turned off. Secondly, they provide air to the emulsion tubes which mixes the air with the fuel before it reaches the idle mixture screws. This makes it easier to make fine adjustments to the idle mixture with the idle mixture screws. If the idle air bleeds are too big or have been improperly modified or adjusted they can make the air/fuel mixture too lean before it even reaches the idle mixture screws. As a result you can end up with a situation where adjusting the mixture screws fully open to fully closed makes little or no change to idle speed. That's why temporarily blocking the air bleeds with your fingers or wooden toothpicks can suddenly richen the idle mixture and make the mixture screws act more responsive to adjustment. Also, some carbs can develop internal vacuum leaks that can cause the same kind of lean idle condition. Here's a general view of what's going on with air bleeds and emulsion tubes.
Plug them with a couple of wooden toothpicks or even a small blob of modeling clay and see if it improves the idle quality, or at least makes the carb respond to adjusting the idle mixture screws in or out. That would at least tell you if your problem lies somewhere in the idle air circuit.
Sounds good will try this this weekend. Runs good but wish I could get the idle in park and gear closer together. Pulled a plug and it looks great.
Keep an eye on the manifold vacuum while you're making all these little adjustments. Try for the highest, steady vacuum reading you can get. You can increase the vacuum by getting the idle mixture right which might allow you to to back down the idle speed a bit which should also make a little more vacuum by closing the primary throttle plates a bit more. Also make sure the secondary throttle plates are fully closed at idle. Putting in a little more initial timing will increase manifold vacuum too, up to a point. You'll just need to make sure you don't end up with too much total timing.
Hmmm. . ? Now supposin' a fella could wire up a micro-switch or two on the shifter somehow . . . So that when the shifter moved from Park to Reverse or from Neutral to Drive . . . So that it would open the ignition circuit for maybe a quarter of a second or so and momentarily kill the engine . . . And then close the circuit and feed power back to the ignition once the shifter was placed in gear and keep the engine running. Do you think it would drop the idle enough to reduce the clunking and banging of putting the trans into gear at the higher idle speed? Though it might just be easier (but not cheaper) to put in a looser converter.
It'd be easier cause it would work ( changing the converter) .Given the throttle plates are in the best position , as the r.p.m. decreases do does the vacuum ....The op should not have a timing reduction ,/ vacuum change ( at the ported vac port) between 1000-550 rpm , that's why there's so much variation in rpm. What does it do if you disconnect the vacuum ??
If I disconnect the vacuum it idles down. After reading and searching I was wondering about the converter.
The two middle ones when plugged...no change, when the outer one's we're plugged the motor revved up noticablely.
That richens the mixture plus you don't need as much pressure to initiate flow, I thought you were fighting rich ?
If the butterflies are closed as they should be , at idle , you should not have any discernable ported vacuum. Disconnect and plug the vacuum , adjust idle so its stable in gear ,then put it in neutral and see what it does.....
Did the engine respond to idle mixture screw adjustments when the air bleeds were plugged? That would at least tell you that the idle mixture circuit is working. Though your mixture will probably be on the rich side, as mentioned above by 2OLD2FAST. You won't want to leave the idle air bleeds plugged. That eliminates the venting that prevents fuel siphoning in the idle circuit. And the carb is designed to work best with an emulsified air/fuel mixture at the adjustment screws. But it could be a chore to get the right sized air bleeds installed. It looked like some Holleys had removable, threaded and/or adjustable air bleeds. But I suspect a lot of them are just pressed into place. Removing them and pressing in new ones isn't a job for the faint of heart.
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