Opel GT Forum banner
1 - 20 of 24 Posts

·
Registered
Joined
·
333 Posts
Discussion Starter · #1 ·
Hey all ya ex-spurts out there! Now that I've got the Opel cooling, vacuum and timing sorted out, it's time to start refining the carb tuning.

I took it for a drive the other day, and it worked pretty well overall, except for an apparent lean condition with the throttle barely cracked open. I could hear it backfiring and popping a little- open the throttle a bit and it was OK, tap the gas to get a squirt from the accel pump and it was OK for a minute.

I swapped the 45 primary idle jet for a 60, and the lean condition at light throttle went away.

It still doesn't really perform well, though. When you get into the throttle a bit and hear the secondaries open, there is no apparent power increase- it just pulls slowly up to about 4500 RPM with no real power.

Here's my current setup:
Local elevation, 6000 feet ASL. The car will rarely go lower than 4000 feet, and needs to be able to run at 11,300 if I want to take a highway to Denver or points east.

So, here's the current jetting.
Primary idle= 60
Primary fuel=132
Primary air=180

Secondary idle=50
Secondary fuel=145
Secondary air=195 (I think- the stamped number is so faint it could be a 185. Looks more than .005 larger than primary air, though)

The mixture screw is 2 3/4 turns out, and the emulsion tubes are F50's.

The extra parts I have on hand: 140 and 155 fuel, 165 and 175 air, 45 pri idle, 55 sec idle.

I also have two emulsion tubes, marked "15" and "16". These are from a Holley 5200- dunno if they're interchangeable.

I've been reading threads here and at various Weber sites, and I wanted to get the opinions of our local Opel experts on any suggested changes within the part grab bag I have on hand.

Anyone?

PS- low compression 1900 cc, 1972 ten bolt. Gasoline is 91 octane premium, 10% ethanol
 

·
Registered
Joined
·
11,747 Posts
....
Here's my current setup:
Local elevation, 6000 feet ASL. The car will rarely go lower than 4000 feet, and needs to be able to run at 11,300 if I want to take a highway to Denver or points east.


PS- low compression 1900 cc, 1972 ten bolt. Gasoline is 91 octane premium, 10% ethanol
I live at 900 feet with little opportunity to get high. :)
I would first get a wideband
Get the engine to run correctly at 6000.
Once thats done split the difference in elevation 6000-11,300 and tune for the mid-point.
lean the fixed jets out..add a slight amount of spark timing...keep the rpms up..
wider throttle angle.

Good luck

I love fuel injection BTW
 

·
Super Moderator
Joined
·
8,234 Posts
Best idle is supposed to be obtainable between 1.5 to 2 turns out with the correct idle jet.

Harold

P.S. Why high octane gas in a low compression engine? :confused:
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #4 ·
I live at 900 feet with little opportunity to get high. :)
Well, in Colorado, there's a lot of opportunity to get high. ;P

I'm looking into setting up an air fuel meter. I plan to start with narrowband (because I have a spare narrowband sensor) and see how close I am.

I did a minor tweak this evening. Changed the primary fuel jet to 140 from 132, and the secondary to 155 from 145. My thought is that the mixture can stand to be richer.

I hadn't realized that the primary fuel jet feeds the idle circuit, so I thought that enriching the primary a bit might also let me turn the idle mix screw in some.

It seems to have worked. The car idles smoother, with no tendency to die when I scream up to a stop sign and jump on the clutch. (As it wanted to do yesterday).

Also, it seems to have more power when the secondary opens, now making it close to 5000 rpm before it stops pulling.

I was watching in the mirror, and no black smoke at any combination of throttle and rpm, so I believe it's not too rich anywhere.

Pulled spark plug 2 and it's clean and slightly tan.

I forgot to count how many turns the mix screw is out, but I think it's at 2 1/4.

Maybe tomorrow's test will be to increase the primary fuel to 145 and see if there's any sign of excessive fuel...

Thanks, Gene. :)

Harold- It has to be better cause it says 'premium' right on the pump, and it costs more... :yup:
 

·
Registered
Joined
·
11,747 Posts
One of my tweaks
Get a 2.00mm air correction jet
Solder it up
Drill baby drill until you find IT
 

·
Registered
Joined
·
11,747 Posts
... What is your total advance?IF it isn't needed then it not only is a waste of money but can be detrimental to performance.
And what kind of ignition system?
Knock sensor/sensors can and will get you into trouble.
Another ? is the AFR
Narrow band is only good for a narrow range.

With High stuff there's less oxygen
Lets see the difference by the narrow band
More O2 =lean
Less 02 = rich
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #7 ·
It's a pertronix with high output coil, 10 degrees static advance, 10 vacuum advance, 35 total with centrifugal weights fully extended. (Distributor has been modded to limit mechanical advance).

I was having some pinging earlier, so I've been using premium. Now that all the vacuum leaks are plugged and the advance limited, I may be able to use regular gas. ( I think the pinging was excess advance but may have been lean due to vac leaks)

Maybe the fuel injection fairy will visit me some day. :no:
 

·
Registered
Joined
·
11,747 Posts
It's a pertronix with high output coil, 10 degrees static advance, 10 vacuum advance, 35 total with centrifugal weights fully extended. (Distributor has been modded to limit mechanical advance)...
High output coil...yah got my ear
40KV?
What does this mean?
With a super wide gap...say plug wire removed around a inch
Yep you'll get 40,000 volts over the secondary...the burn time tho will be very short.
In most cases you'll need 1.2-2.0ms of burn time to get the cylinder to light off.
This is at idle speed
as the rpms increase...just pop the spark plug to get her going faster.
The heck with burn times!
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #9 · (Edited)
Wrench, I had done exactly that, pulling the plug wire to make a gap, earlier on in the process. At that time, I had it idling so rich that the plugs were getting soot fouled, and pulling the plug wire a bit did help it keep running.

The coil will fire arcs about 1.5 inches long, so it's around 40kv. I use old ignition coils and lamp dimmers (rated for inductive load, of course) to make Tesla coil drivers and general HV.

I wonder if my Tektronix digital scope could get me ignition waveform traces. I could put the trigger input next to wire one, and the signal input by the coil wire. Hmm.

Worth playing around with.
 

·
Registered
Joined
·
11,747 Posts
..... earlier on in the process. At that time, I had it idling so rich that the plugs were getting soot fouled, and pulling the plug wire a bit did help it keep running.

.
Had a pull truck 390
that would not do it's job without the coil wire being slightly removed.
Whatever works right?
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #11 ·
Took it for a lot longer drive last night. Starts fine, idles smoothly on both fast idle (choked) cam and normal idle. Pulls well at partial or wide open throttle- up to about 4000 rpm.

At 4000 and above, if I have the throttle partially open, (secondary not open) the car pulls smoothly but not strongly up to 5500-6000 rpm (Second gear)

If I push the throttle and open the secondaries above 4000 rpm, the car 'roars' (maybe backfiring through the carb?) and loses power.

This makes me think I'm getting too lean with higher airflow and the secondary open, so I want to enrich the secondary circuit. The fuel jet is a 155, the biggest I have. The air is a 195.

So, since the primary circuit is OK or a little rich, how about I go with swapping the air correctors? Then I would have PF-140, PA-195, SF-155, SA-180.

That will lean out the primary and enrich the secondary.

My budget is $0.00 so swapping is, for now, the only option. Alternately, a 1/16 drill bit is 1.59 mm, so if I drill out the 132 fuel jet, will I then have a 159 jet?

Thanks, mateys.
 

·
Registered
Joined
·
1,494 Posts
Not necessarily a fuel supply issue, and viewing the spark plugs is no longer a definitive clue due to variation in additives supplied with fuel.

Baseline timing is too far advanced, which is in part why upper RPM response tops out early.
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #13 ·
I tried reducing the (static) advance to 10* from 16* and drove it to work, with no other changes. The problem seems the same-at 4000 rpm it's like a rev limiter kicks in. Unless you are in first gear and keeping the throttle from opening too far...

I drilled out the old 132 to 160, and will try putting that in the secondary this evening.
 

·
Registered
Joined
·
4,063 Posts
At altitude I'd have though you'd be running rich. I'm going to guess that going lean when you crack the throttle means the idle circuit is not doing it's job of providing a good transition from idle to running.

Get the idle circuit working first. If the number of turns on the idle mixture screw is too much then you need to try another primary idle jet. I think most of us at lower altitude are using 50s for the primary idle. If the mixture screw is out too far then you'll be past the transition and initial throttle may not be good.

If you went bigger on the idle jet and still have the screw too far out, it really sounds like you are compensating for a lack of a smooth transition by just dumping more fuel in.

Normally, the idle jet will be acting up to a couple thousand rpm. After that the mains take over.

Once the idle circuit is working you can play with the primary circuit. Here you have airs and mains and the mysterious e-tube. With a wideband you can watch your A/F as you climb through the rpm range. Use the mains to give you the desired A/F level and the airs to keep that level as rpm increases. If the A/F rises, go with a smaller air.
.
When the secondary comes on you should feel and hear it. If you get a bog you'll need to determine if it was caused by lean or rich conditions. Again, use a wideband and change the secondary idle get to fix things. If you get nothing, check the linkage.

Not sure how to advise on the secondary jets. Probably the same as the primary. I have no idea what to advise on the e-tube.

All this assumes timing is correct and the are no vacuum leaks.
 

·
Registered
Joined
·
333 Posts
Discussion Starter · #16 ·
Jet change helped a lot!

I swapped out the freshly made 160 for the old 155 in my secondary fuel circuit. Results were very satisfying.

Now it revs to above 5000 with no adverse effects, at WOT. Only once, pulling a steep hill in second gear at 4500 rpm and full throttle, did I get some backfire sound and a loss of power...

The timing is still at 10 BTDC, where I put it at lunch time today. I plan to advance it another 5 or 6 degrees to see what that does. (Following recommendations for a degree per thousand feet ASL)

Dan- solid core? The only mods I've done to the distro is an added stop screw to limit centrifugal advance to 25 degrees. Is there any advantage to tweaking springs in the advance mechanism?

I'm still looking at getting an AFM (wideband) but that's an item I'd have to save my pennies for.

But finally, the FireOpel feels more like a sports car than an antique...:drive:

Now it's time to look into the handling. How hard is it to change out bushings to new urethane types? :p
 

·
Registered
Joined
·
11,747 Posts
Oh, *that* rotor. As far as I know, it's an ordinary stock rotor- looks nothing like yours; it's solid plastic.

Does the pictured rotor have an integral spark gap or something?
Glass and plastic is pretty good insulators from my understandings.
 
1 - 20 of 24 Posts
Top