Opel GT Forum banner
21 - 33 of 33 Posts

·
Super Moderator
Joined
·
14,713 Posts
Just an FYI, but a naturally aspirated CIH always makes its best power between 12.8 - 13.2 AFR on the dyno.

You can be a little leaner at cruise of course, and at idle I’ll go as high as 14.2-14.5.

But at that AFR you need a very good ignition system to fire the lean mixtures.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #22 ·
My experience back in the day switching vacuum sources causing pinging, so I switched back. I hook mine up the way I suggested. According to most articles I've read your recommendations are correct. Who knows, I may try it again. Goodness knows, I don't like having to use 89 octane in an low performance car.

Opel was using vacuum retard before 1971 in the US models.

Harold
Thanks,, Didn't know that ,, thought they added it when they went to the lower compression pistons in the 1.9,, in 1971.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #23 ·
Carbs do not run well at 14.7 idle on any car (unless it’s a severely undersized carb). Opels like 12.0-13.0:1 in my experience. 12.8:1 is the sweet spot for me! The Opel CIH is notorious for over scavenging and needing significantly more fuel than stoicomeric at all times.

I aim for around 12.5:1 everywhere with my DCOE.

With a carb that has smaller venturies and more velocity you can afford go leaner, but you aren’t going to get good cruising or idle performance leaner than 13.5:1 even with a 32/36 in my experience.

There’s more to it than just tuning to 14.7. There’s also emulsion, atomization, temperature of fuel (and exhaust/ combustion temps), and varied mixture requirement of center cylinders vs outer cylinders on a CIH to consider.


Also I noticed you are talking about vacuum advance during hard acceleration. During hard acceleration you will not have ANY vacuum advance unless your carb is undersized. Full throttle should have close to zero vacuum at the manifold and carb port. Vacuum advance only operates at partial throttle. Never at idle, and never at WOT.
Thanks for the info,,,

Not sure on the last comment however. If your referring to the vacuum ported line off the Carb only, then, yes, no vacuum at idle should be seen, as long as your throttle valves are closed. I guess I'm not sure what happens at WOT. You definitely have air flow through the throat of the carburetor, so, I would think you would still have a decent vac. off the carb port since ported line operates on a venturi effect, but I guess I really can't say here. I believe the point of the discussion was simply, if you remove the retard function side of the distributer vacuum canister, and operate only a vacuum advance, where do you hook the the advance vacuum line to, the ported or manifold? Doing this has several advantages as was discussed earlier, but the main thing that is very noticeable, is to help with low end idle on a built or slightly built engine. Since at idle you have no Vac on the carb port you hook the advance side of the distributor vacuum canister to the manifold, where their is vacuum at idle. This pulls the timeing up a few degrees at idle helping your engine run smoother, but since the vacuum drops in your manifold during acceleration, the few deg. advance also goes away and your back on your performance settings in your distributor mechanical advance.

As far as the distributor and the vacuum advance set-up goes, I'm sure you already know this, I just repeated some things as to not confuse some readers who may not, or jumped in at the middle.. :eek:)

Thad
 

·
Can Opeler
Joined
·
4,041 Posts
Thanks for the info,,,

Not sure on the last comment however. If your referring to the vacuum ported line off the Carb only, then, yes, no vacuum at idle should be seen, as long as your throttle valves are closed. I guess I'm not sure what happens at WOT. You definitely have air flow through the throat of the carburetor, so, I would think you would still have a decent vac. off the carb port since ported line operates on a venturi effect, but I guess I really can't say here. I believe the point of the discussion was simply, if you remove the retard function side of the distributer vacuum canister, and operate only a vacuum advance, where do you hook the the advance vacuum line to, the ported or manifold? Doing this has several advantages as was discussed earlier, but the main thing that is very noticeable, is to help with low end idle on a built or slightly built engine. Since at idle you have no Vac on the carb port you hook the advance side of the distributor vacuum canister to the manifold, where their is vacuum at idle. This pulls the timeing up a few degrees at idle helping your engine run smoother, but since the vacuum drops in your manifold during acceleration, the few deg. advance also goes away and your back on your performance settings in your distributor mechanical advance.

As far as the distributor and the vacuum advance set-up goes, I'm sure you already know this, I just repeated some things as to not confuse some readers who may not, or jumped in at the middle.. :eek:)

Thad
Ported vacuum is not developed by the Venturi effect. This is a common misconception. If you put your vacuum gauge on that port and rev up to full throttle you will find that despite the velocity being the high there will be almost no vacuum at the port.

That port is basically manifold vacuum with an on/off switch (the throttle plates).

If you remove vacuum retard you can move the advance to either port depending on how your car reacts. I find running advance of the manifold causes excess stumbling because if you slam the throttle you go from 10° or so vacuum advance to zero instantly. I’d rather already be a 0 so I can tune around the issue.

My solution was to advance the distributor to 10° static advance and remove the vacuum advance completely. This also requires placing adjustable stops on the distributor weights to limit max advance. My car hits max advance very quickly since I made the resisting springs for the weights smaller. Part throttle performance/efficiency is the only negative of removing vacuum advance entirely.
 
  • Like
Reactions: kwilford

·
Registered
Joined
·
43 Posts
Discussion Starter · #25 ·
Just an FYI, but a naturally aspirated CIH always makes its best power between 12.8 - 13.2 AFR on the dyno.

You can be a little leaner at cruise of course, and at idle I’ll go as high as 14.2-14.5.

But at that AFR you need a very good ignition system to fire the lean mixtures.
Thanks very much Bob,, ,, Your power range for aspirated engine seem to fall inline with other charts I have found on line, some go as rich as 12.0. Although I do not know you personally, I do know,,, you know, what you are talking about,, Ill target your numbers.

I do have a question concerning the AFR read out,,,, With an engine set up like mine (see my first post) Is it common for them to be very erratic in their digital readings? I mean idle and even cruising down the road the engine seems smooth but the AFR reading is not very steady . You can watch it and get s sense of the mid point but just was wondering. I'm running a 2" exhaust with a shorty header, a Crain Fireball XR700 ignition with a hotter bosh coil. O2 senser is set about 4" back of the header mixing point at about the 1 o-clock region on the pipe. just wondering if I have something else going on.
 

·
Can Opeler
Joined
·
4,041 Posts
Thanks very much Bob,, ,, Your power range for aspirated engine seem to fall inline with other charts I have found on line, some go as rich as 12.0. Although I do not know you personally, I do know,,, you know, what you are talking about,, Ill target your numbers.

I do have a question concerning the AFR read out,,,, With an engine set up like mine (see my first post) Is it common for them to be very erratic in their digital readings? I mean idle and even cruising down the road the engine seems smooth but the AFR reading is not very steady . You can watch it and get s sense of the mid point but just was wondering. I'm running a 2" exhaust with a shorty header, a Crain Fireball XR700 ignition with a hotter bosh coil. O2 senser is set about 4" back of the header mixing point at about the 1 o-clock region on the pipe. just wondering if I have something else going on.
Mine is very erratic at cruise as well on all my cars. I tune by the mid point of the numbers. At WOT it is pretty steady though. Idle is fairly steady. Between 12:1 and 13:1 all the time but a bit bouncy between them.
 

·
Super Moderator
Joined
·
14,713 Posts
The bigger the cam, the more erratic it will be.
 
  • Like
Reactions: Nitromancer

·
Registered
Joined
·
43 Posts
Discussion Starter · #28 ·
Mine is very erratic at cruise as well on all my cars. I tune by the mid point of the numbers. At WOT it is pretty steady though. Idle is fairly steady. Between 12:1 and 13:1 all the time but a bit bouncy between them.
Thanks just wanted some confirmation, since I haven't used an AFR before. Just watched a video where and AFR was hooked up and it to,, was jumping around... Thanks
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #29 ·
In reading your posts it looks like you have a very good understanding about what you’re doing. I have a 2.0, I set up my cam with a 4° advance bushing using 405 lift/256 duration cam, less aggressive than yours. I can get my 38/38 to idle nicely at 14.4:1 and that’s about it, no higher. My only problem with this set up was at closed throttle on fairly aggressive deceleration was way too lean going above the 18:1 burn limit and too much stinky sputtering from the exhaust. I’ve got it at its happiest at idle between 12.6-13:1 and it runs about 13-16:1 closed throttle deceleration the engine sounds fine with no little sputters and a much cleaner burn.

On tuning, I too had a struggle with everything let me just put it that way. I prefer the f-66 e-tubes, but if you make that change be sure to raise the float level, I recommend using the brass float. I didn’t get the good consistency on the plastic float. The main jets do have plenty of effect on the progression/transition circuit. My 2.0 seemed to like it between the 47-50 idles, I tried many, many combinations on the mains and ended up using the 145 mains with the 185 air correctors. The 47 idles were a bit too lean and the 50’s were just a bit on the rich side. I ended up drilling the 47’s to 49’s and she purrs like a kitten with smooth transition and good power all the way through. I tended to start lean, find the happiest setting on the idle mixture screws for the smooth idle and transition & work my way slowly to being just enough on the rich side to get the good performing afr numbers.

It also likes a lot of ignition timing, just over 13° of mechanical advance at 800-850 rpm idle with 35° all in at 3,500 rpm’s. Add the 7° vacuum advance brings it right at 42° all in on the highway. I discovered that moving the vacuum advance to the manifold offers a much smoother stop & go. With that much initial advance, a zero to 7° of additional advance going up & down with the throttle opening and closing at low & higher speeds the cylinder filling is just a lot smoother. I’m averaging 29 mpg combined. My happy place with this little car is to get its best power and gas mileage as both priority one, Having the afr gauge lets me look at its performance, driving at 13.5:1 is where it’s frequently at. in the 14’s on the highway.

Here’s what took me so long to understand, with the Weber there’s a lean spot towards the top of the progression stage, that’s normal, it’s when you’re cruising on the highway and saving fuel. The CIH likes fuel so what I’ve found, the way I have it set up, I do most of my driving with plenty of power 12:1-14:1 driving under that lean spot. I used to get hung up on trying to stay lean all the time. That’s where the afr numbers were tricky, jetting leaner gave me good afr numbers 13-16:1 the engine ran smoothly but because I was driving more on the mains on or past the lean spot with more throttle my performance and mileage didn’t fair as well. As previously mentioned there’s a fine line in giving it too much fuel, you can be doing 12.4:1 with a spongy pedal and using too much throttle. The air fuel ratio gauge doesn’t do it all, especially when you’re on the rich side. Here’s how the 38/38 I have is set up. I use 91 octane California e-10 gas.


Idle 49’s Mains 145’s Air’s 185’s

F-66 emulsion tubes

Pump jet 65

Brass Float 38/48mm (with gasket)

Idle 12.8:1 turns: 7/16 inner screw 1 full turn outer screw

¾ turns in on speed screw

Cruise 50 mph steady/tip in 12:5 to 13.5:1

part throttle decel 11.2-12.5 mostly 11.8-12.3:1

Cruise highway 14.5-15’’s mostly upper 14’s occasional 16-17:1 blip on tip in

part throttle highway decel 12.6-13:1

Light acceleration 12.5-15.5:1

Aggressive acceleration 12.6:1- 13.5:1

Hard decel throttle closed 13.5 to 15.8:1

WOT 12.6:1

Good all the way around

5/14/22 changed distributor bushing from 5.56mm to 5.4mm, not much of a change. 34° total 12.5° idle

7/5/22 re adjusted distributor to 13.5° at idle & 35° total add 7° vacuum advance for 42° total part throttle. Moved from ported to manifold.
Thanks CUB a lot of good info,, You must have micro drilled for fine tunning those idle jets..... I am running F-66 emulsion tubes as well,, plastic float though. I set the floats about level maybe a touch high.. just enough to cover the the enrichment tube ports in the upper portion of the float bowel by about 1/8" maybe a little less.. Funny though, I have not seen the enrichment tubes pull any fuel into the carb throat when I momentarily go full open ,, I'm assuming they need a longer duration to generate the negative pressure to pull fuel up and in...
 

·
Registered
Joined
·
1,237 Posts
Yes, I’ve found that when tuning the 38/38 one jet size, (example 140 main jet to a 145) is like jumping two jet sizes on the 32/36. I believe that’s the reason Weber has the 142 main jet & the.47 idle jet. I just like taking it a step further.

I found once I got in the the float level right & my mains close, getting the idle circuit set up right as Harold mentioned on the speed screw throttle plate adjustment not pulling any vacuum on the port. Once the throttle plates open the progression holes add more air to the fuel in addition to the fixed idle air correctors, which reeks havoc on the transition. This is why proper mixture screw positioning is so critical. A lot of guys give up and can’t get past this point with the stumble. Too far in or too far out with those screws.
There’s multiple ways of tuning the idle/progression circuit
Here’s one of the best jetting threads I’ve read:


(See post 76) one of the guys pulled and tapped his idle air bleeds with a 10/32 tap. He has an assortment of allen headed brass plugs from 1.50 up to 1.90 in .5mm increments (1.9 is the factory supplied idle air bleed size) not at all necessary just pretty cool educational stuff.

Once I found the spot on the mixture screws that offers the smoothest transition instead of doing that or enriching or leaning my idle afr by moving the mixture screws off their sweet spot to fine tune, I decided to drill out the idle jets, I’ve got a factory .47 (14:1), .48 (13.5:1), .49 (12.9:1) & the factory .50 (12.2:1) and my mixture screws are sitting in the sweet spot. The car has smooth transition with any of the above the .49 is where I’m getting the best overall performance.

I got frustrated with the plastic float, you may have far better success than I did, if you seem to be doing everything right and still are frustrated it’s money we’ll spent to pick up the brass float. Maybe Gil at OGTS has a few used ones lying around. I battled with the plastic one for months, take your time enjoy the experience, it’s a lot of fun tuning with the afr.
 

·
Registered
Joined
·
2,144 Posts
I got frustrated with the plastic float, you may have far better success than I did, if you seem to be doing everything right and still are frustrated it’s money we’ll spent to pick up the brass float. Maybe Gil at OGTS has a few used ones lying around. I battled with the plastic one for months, take your time enjoy the experience, it’s a lot of fun tuning with the afr.
That's interesting you had tuning problems with plastic floats. I ve had no issues but recall the recommended Weber plastic float height setting would make my car run out of gas in 3rd WOT. I adjustment the float height to keep a slightly higher fuel level in the bowl and never had the issue again.
 

·
Registered
Joined
·
1,237 Posts
That's interesting you had tuning problems with plastic floats. I ve had no issues but recall the recommended Weber plastic float height setting would make my car run out of gas in 3rd WOT. I adjustment the float height to keep a slightly higher fuel level in the bowl and never had the issue again.
Good information! Can you please post the final float measurements? Closed being the most important. If I remember correctly you also are using the f-66 e-tubes and very happy with your set up. It should help the OP
 
21 - 33 of 33 Posts
Top