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Only once that is done should you proceed with tuning the carb. You may need to have the primary and secondary throttle blades open equal amounts to not overexpose the primary transfer slots. And in some cases you may have transfer slot exposed in the primary and secondary. Which is not a problem, you will just need to have the mixture screws turned in more.
If you are using a PCV valve, it also must be one for low idle vacuum. The standard V173 works well in low idle vacuum engines.
Always sort out timing, then carb.
Statistics: Posted by hipockets72 — Thu May 27, 2021 4:14 pm
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This is ordinarily not caused by being rich. chief by-product of rich exhaust is CO, which is colorless and odorless along with being poisonous.
the eye-watering part is caused by mis-fire, pumping unburned mixture out the exhaust. less misfire = less odor. connecting the vacuum advance directly to manifold vacuum is a good way get enough advance to help the idle without mechanical mods to the distributor.
Statistics: Posted by RFSPHPbb — Tue May 25, 2021 4:47 pm
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You only went half way with what I suggested in my last post. But the engine behaved as I predicted: idle rpm & vac increased with more idle timing, & idle got smoother.
Do you realise what is happening here? I think not. The extra timing at idle is what the engine WANTS. Just increasing timing increased the HP at idle, indicated by the rpm increase. The engine needs less air because it is more efficient with the extra timing. Less air means less fuel.
Idle timing is the FIRST thing that should be addressed when dialling in the idle cct of a carb. NOT THE CARB FIRST, timing first. Don't worry, you are not the first person to get this sequence wrong.....
Also, I think you are confusing total [ centri ] timing with timing at idle. It is entirely possible that the engine needs more timing at idle...than it does at WOT [ 36* centri in your case ]. The easiest way to achieve this is to use a dist with an adjustable vac adv unit. In the old days, hot rodders quickly found out their engines ran better with a lot of idle timing. So they 'locked' the dist. That was great when we had good petrol, but all that timing might cause pinging with some engines & today's crap fuel. Some engines will tolerate locked timing, some will not. You have to try it. Vac adv connected to manifold vacuum is the answer for those that will not tolerate locked timing. More on MVA in the link below.
Unless you get idle timing sorted out, you will never get the carb to deliver the best idle & off idle response [ Yes, off idle too ]. Many factory engines left showroom floors with MVA. Pontiacs were one, & idled with 26*. 6* init + 20* added with MVA. When you consider these were high comp engines [ 10.75:1 ] with mild cams, & bearing in mind big cams & low CR needs more timing, suddenly 36-50* doesn't seem that big, does it....
Scroll down to post #6:
www.hotrodders.com/forum/vacuum-advance ... 47495.html
Statistics: Posted by GTO Geoff — Tue May 25, 2021 8:27 am
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Due to your cam overlap that will create recycled exhaust gasses diluting the intake charge, inefficient combustion that makes for excessive HC at idle will always be present and therefore always a degree of eye watering effect. AFR meter will tell lies as an inefficient combustion means un-burned oxygen is being sent out the chamber and read as leaner than it is. Let the engine tell you what it wants, not the exhaust.
Statistics: Posted by Right hand drive — Tue May 25, 2021 7:56 am
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While I was typing, I thought about my old nemesis Hot Restart from my Ram Air III days. That car worked best at 26° initial, but in warm weather hot restarts required jump starting despite having a 1,300CCA truck battery. Backed off to 24° I only had to jumpstart it if ambient was 80+. Embarrassing. So, with the extra timing added I wanted to see if the Valiant was affected. It was not. The car sat for about 20 minutes and started perfectly. However, the idle had suddenly decided to settle closer to 950. I touched nothing under the hood. Multiple throttle stabs brought it back to the same 950-ish idle. Back up to full heat, I let it soak for about 10 minutes and tried again. No issue--plenty of cranking speed. The idle seems to now stay at ~950 and while choppy, is perfectly acceptable. The nose-burning mixture, not so much. She'll clearly suffer more initial timing without making me look a fool in parking lots mid-August, anyhow.
Should I lock out the advance and just give 'er all the lead she'll take? Open up the PIAB to .070" like the secondary side? Both? I feel like I'm closing in on this--thanks to everyone's input, which I really appreciate--seeing the improvement in vacuum amount and steadiness, and in spite of it still being so fat its cereal bowl needs a lifeguard.
I'll probably get banned once I get into tuning my 340 Six Pack...
(For the record, the RA-III engine that wouldn't start hot had a 110°LSA)
Statistics: Posted by Sinatra — Tue May 25, 2021 1:43 am
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Try this simple 5 min test: With the engine idling, loosen the dist & slowly turn it CCW [ advances timing on SBM ]. I will bet rpm will increase & idle will get smoother. Vac will also increase. Keep going CCW until you get maximum idle rpm. Then check what the timing is. Do not be surprised if it as above.
Reply with the results.
Statistics: Posted by GTO Geoff — Mon May 24, 2021 8:24 am
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Edit: I just quickly skimmed over the thread again. Has initial timing been mentioned. Probably want 25*+ depending on a few factors.
Statistics: Posted by Right hand drive — Mon May 24, 2021 2:55 am
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