I know there are a lot of people that can tune by ear. Well, I'm just not that good. ......yet...... So, I use a vacuum gauge and rpm gauge. Now I understand that you set your base timing and then using the vacuum and rpm readings, you tweak the idle mixture until the vacuum reading is peaked at the desired rpm for idle. This weekend, I changed out carburetors on my truck to see how it would run. The carb I was using had a main jet measuring .068 and the rod measuring .039 and the tip being .028. The replacement carb measurements are .070, .043 and .025 respectively. But I do have the following questions that are related to this and the main jet - needle combo for cruise and wot. I notice that my cruise speed vacuum is lower than the original carb. Would a higher vacuum at cruise indicate the mixture provided by the main jet and needle combo is better? Note that the vacuum readings are coming from the manifold below the carb base.
If you mean a higher vac number, like 18 instead of 15. This is the way you want to go. The less opening of the throttle, the more efficient you are. Frank
Tune by ear: Advance timing until it detonates (pings), then back it off. By sight: no black smnoke at WOT, clean plugs.
A few questions here for you this will help the fellas diagnose your differences. Did you switvch from a stock carb to an after market carb? like day a carter SFB to an Edelbrock AFB type. Or did you change CFM ratings or carb types like say a 600 CFM Q jet to a 125 CFM Carter AFB? Differences in ventury size or profile are going to affect your vacumm numbers more so that jet size. Does that make sense?
Yes, makes sense and apologies for not specifying the engine/carb setup. 2.8l gm v6 60 degree bored over .010, (OH YEAH!...LOL), with 2 barrel varajet. I had a problem with the varajet that I rebuilt 8 months ago. Just recently, the primary choke pull off went bad, so I had a spare rebuilt varajet that I put on it. This is after spending some time with it and a choke angle gauge verifying the linkage was set up properly. This current carb has a needle jet combo with a slightly larger metering area than the carb I had on it. The first actual drive at cruise speed for longer than 5 miles, it ran o.k., although, when I got on it, it seemed as if the response was not that great and the cruise speed vacuum reading seemed lower than what the other carb had, BUT, I don't recall the exact number, I just think it was at least an inch to two higher. In any case, I've not been able to figure out what needle/jet size would be typical for 2800 feet, and I have no reason to think that the carb/engine is original due to the fact it's been rebuilt by a previous owner. Oh, and I don't know if the truck is originally an arizona truck. Not going to run a carfax to figure that out.
You really need to know the angle of the throttle blades on the two carbs at a constant Rpm to answer your question. You have bigger jets, and now need more air to keep AF ratio the same. Opening the throttle blades furtheralows more air and decreases vacuum.
after a 15 mile drive at 60 mph: acceleration seems a little better, but the cruise seems a little more sluggish than what it was. when i get in it, such as to pass, it doesn't seem as responsive. going to have to check the idle mixture again and think about this a little more.
Idle mixture settings shouldn't matter at all to mid range or WOT. To read plugs and establish WOT jetting . . Fresh or cleaned plugs, high way WOT run and shut the engine off while at WOT and coast to the side, pull plug and look. The seat of the pants feel is a pretty good judge for improvement or falling of performance. Bigger jets slowed you down.
When dealing with a rod/jet combination, you have to figure areas of both the jet and rod to determine effective area. The effective metering area is the area of the jet less the area of the portion of the rod then in the metering portion of the jet. Without doing the math, it would appear that the new set is leaner at high vacuum (0.070/0.043) and richer at WOT (0.070/0.025) than the first set. One also needs to look at the internal venturi area, and the calibration of the air jets when determining absolute calibration. Jon.
I have just sent off an email to a fellow that I hope knows the markings for the needles and jets for these Varajets. Also thinking more about getting a Weber carb and being done with it, but setting up the tv cable linkage and concocting a solenoid bracket for a/c operation seems like just as much work as finding the best needle/jet combo. Would a wide band o2 sensor - fuel mixture gauge be helpful?
If you are even thinking about considering the possibility of calibrating a Weber on an engine other than the original, you need to be answering questions on this forum, not asking them! Jon.
Fuel delivery and ignition advance requirements vary a lot as the csrt is driven. Adjusdtments made at idle do effect calibrations at other throttle openings, but what is perfect idle may not translate into the perfect settings for other situations. Point being, carb calibration and advance curve is about more than just idle.
A wide band o2 sensor is the fastest way of tuning an engine that I have seen, but the final wot setting will still need to be done with a stop watch to get the best jet needle combo.
If the distrubutor curve is already dialed in, any deviation from the correct initial advance will adversely effect the entire operating range. In some carbs the idle circuit contributes or effects fuel over the carbs entire operating range. Calibration of those carbs starts with idle and works from there. The point of my post was that good idle settings don't mean the entire operating range is dialed in.
O.K., so I've put the previous carb on that I had set up and had been running decently. This carb is not the original carb I pulled off the engine, that one is the carb with the adjustable power piston with the jet measuring .070 and needle measuring .025 tip and .043 wide end. I think I need to start over from that point and it might be easier for me to figure where I need to go. Tomorrow, I will start with verifying the timing without vacuum and peak the vacuum at that point. I am going to read up and make sure I understand where I want the mechanical timing to max out. I did forget that the last thing I did to the engine prior to starting all this with the carb was putting lighter springs on the weights so that the mechanical would hit 24 degrees @ 2500 rpm with a base timing of 12 degrees for a total of 36 degrees. I did not tweak anything other than the springs at that time. Now, having the engine warmed up and pretty smooth with the vacuum sitting about 17" at idle, I disconnected the pump and then blipped the throttle slightly and noticed a pretty snappy response. I then gave the pump a little blip and noted that the engine seemed to want to die out, which I understand as a rich idle condition. As it stands, this carb has the .068 jet, and needle measuring .028 at the tip and the wide end being .039
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