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PrOpeller
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707 Posts
Discussion Starter · #1 ·
Previously posted on this site was something about modifying a Weber to work with a hot cam/big valve setup (I can't find the old thread).

I believe it involved something like cutting an internal spring and drilling throttle plates to get the car to idle with the hot cam.

If this rings a bell, could someone please describe the correct procedure to me?

I think this is what I need to do to get my car to idle after installing a big-valve head (by Roger Wilson) and a hot cam. I've already ported the intake and rejetted the carb according to prevalent Opel wisdom.

Thanks,
Walter
 

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Senior Contributor
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903 Posts
It would be best to make sure you need to mods before performing them.

What are the specs of your cam? Where's the ignition timing set? Have you swapped the emulsion tubes? Where did you end up in terms of jets?
General engine specs? The more info the better...

-Travis
 

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Yeah, what Travis said.

Make SURE the distributor is setup correctly for your cam before messing with the carb or it'll never be right....won't idle, big flat-spot off idle, detonation at higher rpms...etc. Pretty much as soon as your cam surpasses the 220-225 degree @ .050" duration point, you need to eliminate the vacuum advance/retard unit, and redo the total timing 'curve'. See my FAQ on the distributor curves for some basic advice. I can get you close to where your baseline timing needs to be if you supply the cam specs and compression ratio.

Bob
 

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PrOpeller
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707 Posts
Discussion Starter · #4 ·
Okay, Guys...

The cam is a solid .298 cam lift / .430 valve lift / 284 duration.
The timing right now is set at stock with retard disconnected.
The jets on the DGAS are F66 emulsion, 180 air, 50 idle, 145 main, and using the needle and seat from a 32/36
I think the compression ratio is at 9.0:1 (8.45:1 pistons with a decked block).
Sprint manifold with 2" exhaust.
Don't have the exact valve specs on me but I think they're 1.84" / 1.50" (will doublecheck this afternoon).

So what do you make of it? Timing needs adjustment, huh?

Thanks,
Walter
 

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PrOpeller
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707 Posts
Discussion Starter · #5 ·
Now that I think of it, the compression may be at 9.5:1 considering that the larger valves probably bump it up a bit.
 

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PROPEL said:
Okay, Guys...

The cam is a solid .298 cam lift / .430 valve lift / 284 duration.
Duration @.050" and lobe seperation would help give a better representation of the cam. Do you have these numbers?

-Travis
 

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PrOpeller
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707 Posts
Discussion Starter · #8 ·
.020" lifter rise

Intake Timing
open BTDC = 32
close ABDC = 72

Exhaust Timing
open BBDC = 72
close ATDC = 32

Intake Timing
open BTDC = 14
close ABDC = 54

Exhaust Timing
open BBDC = 54
close ATDC = 14
 

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Uh, you need some serious timing added at idle!
For a 248 degree/ 110 lobe separation angle camshaft, you'll need about 17-20 degrees timing @ idle, depending on where the cam timing is actually set at (advanced or retarded, straight up?) If it's straight up, try 20 degrees, if it's advanced 3-5 degrees, drop the ignition timing back to 16-17. Now, the key is to restrict total ignition timing advance to 35-36 total. So if you start at 20 degrees, you need 16 degrees of advance, which is a lot less than stock.

Eliminate the vacuum advance mechanism, and rivet or screw the upper breaker plate to the lower plate to eliminate timing 'walk' or variance. You then have to determine the year of your distributor, and from that and my distributor FAQ, you can determine the existing total timing advance. You have to restrict your distributor's advance, possibly to half of what it is now. If you need more info Walter, let me know.

Bob
 

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PrOpeller
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707 Posts
Discussion Starter · #10 ·
Thanks alot, Bob! I'll read the FAQ, go home, ponder a bit, and see if I can get this thing set up right now. You guys are great.

Walter
 

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Hey Travis, when I retire from Opels, do you want to take over as 'RallyTravis'? Would save me a lot of writing!!

Bob:D
 

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I just figure the less you have to answer the more time you'll have to answer my questions:)

RallyBob said:
Hey Travis, when I retire from Opels,
Don't say such things!

do you want to take over as 'RallyTravis'?
Wouldn't I need a Rally Car or something? No wait, I guess I don't:p

-Travis
 

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It would appear that Travis is the Liaison between us gearhead wanna bees and the Zin Master. RallyBob imparts his knowledge upon the masses... We all sit with a blank stare on our face... Travis actually understands and completes the modification and and we think WOW... I can do that TOO!:D

We all appreciate your wisdom, Bob, and Travis your insights on how you accomplished it.

Paul
 

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Senior Contributor
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Paul said:
It would appear that Travis is the Liaison between us gearhead wanna bees and the Zin Master. RallyBob imparts his knowledge upon the masses... We all sit with a blank stare on our face... Travis actually understands and completes the modification and and we think WOW... I can do that TOO!:D
Paul
If TRAVIS can do it, anyone can....

I think that's what that says:)

-Travis
 

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Contemplate these:

'What is the sound of one Solex dropping in the trash?'

'If a Solex gets thrown into the forest, and there's nobody there, do I even care?'
 

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RallyBob said:
Contemplate these:

'What is the sound of one Solex dropping in the trash?'

'If a Solex gets thrown into the forest, and there's nobody there, do I even care?'
Or the classic..... 'If a man speaks in the woods and there is no woman around to hear. Is he still wrong?'
 

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PrOpeller
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707 Posts
Discussion Starter · #18 ·
So I think I'm getting on track here...

I eliminated the vacuum advance completely and bolted a metal strip over the hole in the distributor where it had been attached. I drilled a hole through the breaker plates and screwed them together. I hope I did this correctly... with the plates separated apart from each other as far as they'd go (left and right).

I set my timing at idle (~900 rpm?) to 20 degrees advance (straight up cam timing). Then I figured my total advance (at ~3500 rpm) to be 50 degrees, leading me to believe that I have a 1970 distributor.

The car feels much better at low rpm now (I can finally hold it there at part throttle). She still won't idle on her own, however. Granted, I haven't fiddled too much with the carb yet, but it seems like it'll only idle while on the throttle circuit.

I know that I still have to recurve my distributor so that there's only 36 degrees total advance. But would that be keeping the car from idling now?

Also, where can I buy an assortment of springs needed for trial and error distributor testing? Don't know of any local junk yards to collect these from.

Walter
 

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PROPEL said:

I know that I still have to recurve my distributor so that there's only 36 degrees total advance. But would that be keeping the car from idling now?
The springs do not affect the total advance. Total advance is controlled by a mechanical stop. No, the total advance will not affect its idle.

-Travis
 

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Travis is correct. The springs in the distributor merely control how fast or how slow the centrifugal weights move out. They don't affect the total advance however. And regarding where you riveted the breaker plates.... I choose to rivet them at their neutral position (center the two notches under where the round steel ball is located). You may have inadvertently added full-time 'vacuum' advance to your total timing.

Have you performed a vacuum test? The only way to determine how much to clip from the power valve spring in the Weber is to perform an idle vacuum test with the engine running at it's best. If, for example, your engine idles with 14" of vacuum at idle, you will need to have the power valve open roughly 2" below that, or at 12". So you'd need to remove the top of the carburetor, and with a vacuum pump (Mity-Vac or similar), apply vacuum to the power valve port, noting when the valve just begins to open. This will probably be at 17" with a stock spring and power valve assembly. This means the power valve will be wide open at idle with 14" of vacuum, and will dump fuel down the carburetor. Carefully clip one coil of the spring at a time, checking with each cut what the value of the vacuum is required to open the power valve. Usually, it's between 3-4 coils, but the only way to be sure is by checking!

If you cannot get the idle circuit mixture screws to have any affect on the air/fuel ratio, then you may need to drill small holes in the throttle plates to allow a little air to bypass. This will allow the throttle plates to be closed somewhat, bringing the idle circuit back into operation while allowing the engine to idle at the higher rpms need with a hot cam.

Bob
 
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