Carb tuning with a vacuum guage

So I've picked up a harbor freight vacuum pump/guage setup (the one that comes with the brake bleeder kit), and I'm attempting to tune the stock carb on my '77 Celica and avoid spending $400 for a CARB-exempt Weber 32/26.

I connected the pump to a source of manifold vacuum, and got a reading at idle. It was vibrating between ~17 and 20 inches of mercury. A blip of the throttle would get the needle to settle down at a higher value, then decrease with rpms until the motor was idling at which point the needle would resume it's oscillation.

Adjusting the mixture screw out (which I'm assuming is leaning out the mixture, as I know this car was running rich previously), would increase the overall vacuum at idle. First it increased the high end, then would bring the low end up, sometimes it would "tighten" the variation, sometimes increase it, but always moving either the low end or high end up. I got it to where it was going between 20 and 24 inHg, continued backing out of the mixture screw would increase the overall vacuum, however the engine would also start missing (maybe I still have a vacuum leak somewhere?).

Until recently I had the timing bumped up to around 12-14 degrees, however with the VERY low rpm nature of this motor, I bumped it down to factory spec (~8 degrees). When I did this, I also switched from running 91 octane (required according to the manual... no, seriously) to 87 octane (due to my own understanding of octane requirements based upon compression, timing, and rpm).

So, my questions... 1)Is the vacuum fluctuation at idle normal? 2)When tuning this way, is the goal maximum vacuum, or is there a spec you're shooting for? 3)The misses that started to occur, would those go away with increased timing, higher octane fuel, or some combination of the above?

For the record, it does seem to be running better/smoother since making this adjustment, however it's not a night/day difference. Also, this car seems to get HORRIBLE fuel economy for what it is. I get 13-14mpg doing the exact same commute/driving style that gets me 16-17mpg in the Cobra or 12-13mpg in the Bronco.

Thanks for any/all info!

Reading some stuff online, I'm starting to question my readings... 24 inHg seems WAY high... I'm gonna double-check this tomorrow, maybe play with the timing some to see how that affects things.

I've used a vacuum gauge many times to help optimize ignition timing, but never the carburetors. I tune the carburetors more typically by ear, listening for smoothness and such.

Pulsing of the gauge is normal. The closer to the plenum you are, the less the pulsing, but it will always pulse. Remember, the vacuum in the manifold is coming from pistons going down. That's not a steady state action.

I don't understand what you're saying when you describe increasing the high end and then the low end of the idle.

Were it my engine, I'd play with optimizing ignition timing with the vacuum pump, always understanding that I'd be optimizing the timing at idle, no load, which doesn't do diddly for going down the road. Then I'd adjust the idle mixture for the smoothest idle range, with decent throttle response, mostly lack of bog.

Lastly, you're right, that gas mileage is terrible.

Well a few things..

1) Get a real vacuum gauge. The one on the mini-vac is junk.

2) To tune with vacuum you need to be at the plenum right under the carb. If you’re off on the end of the plenum or one leg you'll get bounce in the needle. Some units come with "pills" a small plug with an orifice to smooth out the hysteresis in the gauge.

If you are under the carb in the pelum and have huge bounces still your ither idleing to low or have a bad intake valve or an stuck PVC! Bump the timing up and keep adjusting the carb untill you get max vac and MPG.

You may want to look in to a "color tune" plug. there a glass plug so you can see the burn. They work great in the dark.

44

I do not have the instructions here with me, but I seem to recall that a fluctuating needle on the vacum gauge is indicative of valve guide wear.

If the valve guides wear, you will get an air leak into the intake tract. In order to counter-act this leak, the carb will be set a bit rich and all is well at idle.

The problem is that the rich condition will be applied to the entire RPM range, and you now are running rich off idle.

24 inches of Hg seems pretty high. I expect to see 18-19 out of a healthy engine with very little flutter.

If you have big cams, all bets are off.

Four bangers tend to vibrate the needle, I wouldn't be concerned about that. If it makes big swings then that would concern me. 18-19 at idle is a good reading.

On the vast majority of carburetors, turning the idle mixture screw out richens the mixture at idle meaning that turning it in leans it out. A general rule of thumb is to turn the screw all the way in till it's lightly seated, then back it out 1 3/4 turns, start the engine and use a tach to get the highest smooth idle. Most wind up somewhere between 1 1/2 to 2 1/2 turns out. Of course, that is dependent on whether the float level is correct. That is the most important adjustment in the whole thing; if the float level is wrong nothing else will be right.

Vacuum doesn't necessarily tell you all there is to know about how an engine is tuned. Plug readings are a better indicator. The exhaust note is also a reasonable indicator, the idle should be smooth and even. If it's labored and has a rhythmic beat, that indicates too rich. If it sounds erratic and 'splashy' that's too lean.

foxtrapper wrote: I don't understand what you're saying when you describe increasing the high end and then the low end of the idle.

My apologies for being unclear. What I meant was the range that the needle on the vacuum guage was bouncing between.

For instance, starting off it would bounce, say, between 17 and 21. Adjusting the mixture screw out 1/2 turn would bring up the low end of that range such that it was bouncing between 18 and 21. Another half turn would then push the top end up, so that it was indicating 18-23. The next couple half turns would bring up the bottom, so the range would be at 19-23, then 20-23. Then it would push the upper end of the range with the next half turn. It never seemed to just move the whole range up, i.e from 18-20 to 19-21.

The stuck PCV may be an issue - the car still has the factory PCV valve, and I did just replace the PCV grommet the other day, as it was causing a vacuum leak there.

The point that I'm pulling vacuum from is below the carb - I didn't feel like pulling the air filter housing, and none of the four ports on the side I had access to would give me a reading at idle. A picutre would speak 1000 words here, I'll try to get one tomorrow morning.

Interesting. Don't know what to make of that.

Do know that every piston engine I've ever hooked a vacuum gauge to would set the needle to bouncing and trembling. The lower the cylinder count, the more the needle would bounce. Hence the need for dampers on things like synch-sticks and such for motorcycles.

Screwing a mixture screw out usually rich-ens the mix. But I have not worked on the one that you are specifically playing with. I have never used vacuum to tune I have always used it to diagnose a problem.

Okay, so I messed with it some more today.

I bumped up the timing the redneck way (loosened the tiedown, shifted it oh, about that much, tightened it back up) before starting, so from a scientific perspective, I've invalidated all my evidence. So be it.

I hooked the vacuum guage up to the PCV hose, and initial readings were spot on what I said from yesterday, wild vibration from indicated 20-24 inHg. I first cranked up the idle rpm, every time I did this seemed to tighten down the range, 'til it "min"ed out from a variance of 4 inHg down to 2. That seemed more to spec with what I've read for a healthy engine. I then adjusted the mixture screw, ad was able to bring it up to 21-23 inHg, and that's where I've left it.

I still don't think it's "right", and I've noticed a stumble that happens right around 4200 rpm. Most likely that was always there and I never noticed it before, but all the

I will say this, though. The 20R is like the AK-47 of motors. No matter what I do to it, it runs. I may have the carb tune grossly off, but it still runs, and runs alright. I'm thinking the cat is plugged, but the car doesn't care, it just drinks more gas to make up for it. It doesn't go fast, it won't turn over 5k rpm, it makes all of about 80hp at the crank (giving an awe-inspiring 36 hp/L, lol), but it just keeps going.

Er, a vacuum gauge was the old school way of setting ignition timing. You adjust the timing to maximize the vacuum reading. Then you hope the advance curve works for your engine.

Of course the pulses are going to slow down as the engine is reved up. That's how a piston pump works. Rev it up high enough and the needle will barely move.

The R series engine was a long stroke engine, they aren't rev happy.

foxtrapper wrote: The R series engine was a long stroke engine, they aren't rev happy.

?

How are you defining "long stroke"? I agree with you in that they act how I understand long-stroke motors should, but the 20R is square (okay, it's bore is ~.5mm smaller), and the 22R has a larger bore than stroke.

I'm just curious, as I always thought it was the relative size of the stroke versus the bore that defined how "long" or "short" stroked a motor was, but empirical evidence is elsewise.

Is there a particular problem you are trying to fix, like a CO2 reading for emissions or etc?

Jensenman wrote: Is there a particular problem you are trying to fix, like a CO2 reading for emissions or etc?

Just trying to get the car running as best as I can with the everything-that-I-read-says-it's-total-crap stock carb.

It'd be nice if that helped with the atrocious fuel economy I'm seeing. I would expect this car to get same or better mileage than my '89 pickup did with a stock carb'd 22R and a 4-speed, which was ~22-24. Guys on the Celica board supposedly get 25-30mpg with a stock motor. For the record and ease of someone reading this, I'm currently seeing about 14 city, 18-20hwy.

If I can swing it, I'm gonna get the cat replaced tomorrow, and if that happens, I'll be putting 2.25" pipe all the way back (and throw the cheapest glasspack they'll sell me). There's a good chance a lot of this is due to the car still having the stock pellet-style cat, which is probably bad after 32 years. I'm honestly not sure.