Carb and Timing Help! - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 07-02-2018, 02:45 PM Thread Starter
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Carb and Timing Help!

Hi... I have been using Cliff Ruggles book on Rochesters and Tuning by Lars notes but was hoping to get some direction. I have created a stock 1969 RAM III 400 top end with correct cam and heads ...lower is still 1975 9.5 to 1 compression. I have used Cliffs book starting with the stock rods and jets for the 1969 400 (72 41) but discovered I had to drill the idle bypass out to .110 to get it to idle in park. Now I am afraid I have exceeded my idle circuit and I have to tilt the primaries past the idle slots to get it to idle...that was problem 1...after selecting a power piston spring 2 inches lower then my vacuum in D it begins to pull on that spring in D and will now finally idle in drive. I tried Lars suggestion of disabling accelerator pump and tapping the throttle..this test indicated a rich condition so I have now tried several combos of jets and rods.. currently at 71 and 43B ... does not like to idle out of park until fully warm ...good performance off idle..spins tires...but now getting exhaust popping under WOT. So I was hoping to get idle suggestions and is it too lean at WOT or is it too rich? As well I am using manifold vacuum with a B26 can so my vacuum advance was in my range of 15" idle park and 10" idle in drive...that can is all in by 8" as I recall.. finally initial timing is about 16 deg...mechanical is only giving me 10 deg from stock 1969 400 dist...and about 16 deg from vacuum advance... help?
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post #2 of 5 (permalink) Old 07-02-2018, 09:33 PM
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I never completely rebuild a carb with that many changes to jets but the APT screw always gets me on later Rochester carbs
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post #3 of 5 (permalink) Old 07-03-2018, 06:09 PM
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Agree with doing too many changes at a time. Do 1 change and if no results, put it back and do the next one.

There were 2 cams used in the RAIII engine depending on built date. If you have the "068" cam, you would be doing better. If you opted for the "744", then I would say you might be in trouble and explain your problems.

Is your cam a "factory duplicate" or an aftermarket grind with a different Lobe Separation Angle and/or lift? Factory LSA was 116 for the "068" and 115.5 for the "744." This will affect idle quality if you went with a different LSA.

10" of vacuum is Drive is extremely low and no doubt contributes to the engine stalling when cold and needing to warm up. Might want to adjust the choke to be slightly closed and fully open as it warms up to compensate - at least for now.

No numbers on your idle speed and RPM drop when in gear. Normal idle speed in Drive is typically 650 RPM's. Try bumping that up to 750 RPM's - BUT, my fear is that with your primaries open as you mentioned, when you shut the car off hot, the engine will continue to draw air/fuel and it will "diesel" on. This will DAMAGE your engine. This was kinda a common thing in the 70's and you either had to turn your engine off in Drive to stall it, or stall it out by releasing the clutch in 4th gear. The fix was an electric solenoid that when adjusted correctly would close the primaries completely off when you turned the key off.

I also think your timing could be part of your problem as well due to adjusting too many things at once. The exhaust popping could be too rich, too lean, a timing issue or a combination of these.

With the carb, I would go 2 sizes up from stock for a RA III carb on the primaries for now. Ethanol, if using it, tends to run a little leaner and needs to be richened up. Idle mixture screws probably need to run a tad bit richer as well. I screw them in to seat, then back out 2 1/2 turns each for my initial setting. Then back one side out until it begins to smooth out or run better. Then stop. Turn the screw back in until it runs poorly. Then one more time turn it back out until it runs smooth and stop. Repeat for the other side. This will now be your initial setting. You can do this anytime you change your carb jets/rods or timing so as to get the carb "right" again at idle. From this setting, you might road test, and then experiment by turning each idle mixture screw out to richen up the mixture. Go 1/2 turn each side, the another 1/2 each side for 1 full turn. If you notice any improvement, back them out another 1/ turn each. If no change, turn them back in the 1/2 turn. Probably the most yo want to go would be 4 turns. You can use a vacuum gauge, but sometimes "old school" works better in my opinion and it is how I had always done it. So you can do it either way if you like working with the vacuum gauge.

If I recall correctly, the B26 can begins to operate at 8" of vacuum. Factory spec shows 9 degrees initial. Mechanical advance then begins to kick in at 850 RPM's and is shown to be between 3-7 degrees @ 1400 RPM's and 16-20 degrees @ 5100 RPM's. (These degrees would be added to your 9 degrees initial).

Vacuum advance is listed as starting at 8-10" Hg and having a total of 20 degrees at 15-17" Hg.

Let's look at this from a "stock" point of view. So if you only have 10" Hg in Drive, you are really getting no timing advance help from the vacuum advance can. So the 9 degrees of initial would be it and a good reason why the engine might die when cold and put into gear. Now with 15" in park, you would be getting 9 degrees PLUS up to 20 more degrees advance for a total of 29 degrees advance. If this could be kept up, the car would most likely pull away when cold and then of course the vacuum advance would immediately drop once the carb was opened and engine vacuum drops (normal/typical occurrence) and mechanical advance would come into play as RPM's increased. The vacuum advance only comes into play under light load or coasting. It can aide in engine cooling and gas mileage. Once the engine is under load, engine vacuum drops, and vacuum advance is no where to be found.
Drag cars and many hi-performance cars did not use vacuum advance - only mechanical advance.

If you search through past posts on this forum using the Google Search in the upper right, there are some great explanations and exacting steps in setting up distributor timing. Some guys use ported vacuum, manifold vacuum, or no vacuum advance at all as each engine build will respond differently and "one size fits all" is not for every car.

With that, I see both areas that need to be tackled individually - carb & timing. Trying to go back and forth and make adjustments to one and then the other, neither clearing up the problem, will be like a cat chasing its own tail and never catching it. My suggestion would be first to get your timing issue somewhat fixed or dialed in first using the carb to set idle mixture and idle RPM's to coincide with your timing. Get your advance curve dialed in as well as your total advance. You can then fatten up, lean out, or make what ever other adjustments you feel necessary to the carb to dial it all in.
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post #4 of 5 (permalink) Old 07-07-2018, 10:27 PM Thread Starter
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Thanks everyone... especially Jim...yes too many changes at once. So an update...the cam is a Melling SPC3 which is supposed to be an exact replica of the 744 cam as far as my research shows. I also goofed.. it is a B28 can so it starts at 3 to 5 inches and all in at 8 inches so even my 10 inches in drive should have it all in at idle. I definitely had a timing issue...a stock 1111946 distributor was only giving 10 deg at crank for mechanical at 2500. A spring kit helped but I had to compromise as I was getting too much mech right at idle...i now have a total of 9 deg at idle with base and mech. at about 30 deg at 2500 rpm. Was hoping for 34 to 36 for max horsepower at WOT but had to compromise with base timing. With light springs I could achieve 36 at 2500 but had an extra 10 deg from mechanical right at idle...decided that was not good. That's all without vacuum advance of course.. idling at 1050 in park I get 27 deg timing with base and vacuum..drops to 900 rpm in DRIVE. Took it for a long drive and idles really well after been driven for a few minutes..the popping and hesitation are gone under WOT. It was nearly 95 F outside and no issue with diesling or temp increases. In fact it ran nice and cool. I was able to close the primaries more at idle with the new timing while still maintaing same idle RPM. The only trouble is it will stall in drive after first start even after letting it warm up. I think it might be loading up? I think I will try a fatter set of rods to lean it up... at ..and off idle. I am using ported vacuum but with my primaries open that much it is moot.
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post #5 of 5 (permalink) Old 07-07-2018, 11:03 PM Thread Starter
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Too add to Jims comments...after reading your post looks like my stock 1111946 may have been working correctly ....but for my application, and Timing discussion by Lars, I was trying to get 34 degrees at 2500. I have significantly reduced my base timing from the first post so that may have been the only culprit in my popping as I went to WOT.
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