Too much mechanical advance

[Josh D]

I have an '80 RoW SC with the "optimized" 188 hp 3.0 930/09 big port, low compression (8.6:1) and K-basic CIS.

Problem: I seem to have too much mechanical advance. the only timing spec I have found on this one year only engine is 5* BTDC static and 24-29* @ 6000 rpm dynamic. This was found in Waynes engine rebuild book.

When I set the static at 5*, I'm getting 35* at 6000 rpm. Problem is I get detonation under heavy throttle in second gear at about 5800 rpm. This is on 91 octane. It will only tolerate about 30* total with summer temps and running A/C, and about 32-33* in the cooler winter months.

What I've done so far to counter this is to run it as rich as it would tolerate with off throttle idle dip of about 400 rpm with the drag of the A/C compressor on. This had some, but not much affect on the high rpm, high load detonation. Plugs are dark brown.

I tried an octane booster and that did make a difference, however this was more to test than a bonified solution.

Finally I bit the bullit and just dialed back the ignition timing to 30* max for the summer. This pretty much solved the detonation problem, but created others. 30* max advance puts my static timing at 0* BTDC. In this configuration, the car is noticeably down on power between off idle and 3500 rpm. It's also slow to restart when warm. Residual fuel pressure was tested and well within spec.

When set to the 5* BTDC static, throttle reponse is very good and the car pulls like a frieght train from off idle until the 2nd gear WOT detonation event.

So, what I would like to do is take 5* out of the mechanical advance. This would allow me to run the factory spec of 5* BTDC static and the 29* (+/-) @ 6000 dynamic in the summer. In the winter, I could advance it a bit (32-33* max) for a bit more power.

My question is, whats the best way to accomplish this? My thought is to bend the advance weight stops in towards the weights, taking away a bit out of their travel. Has anyone done this? What would the factory have done if the Mechanical advance was out of spec?

BTW, I have already done the dissassemble, clean and lube of the distributor. The plug wires are new Clewetts. The plugs are about a year old and are the factory recommended heat range (W4). The factory coil was replaced with an MSD Blaster II to rule out the coil as a possible suspect.

I've determined that this is clearly a timing issue.

What say you Pelican Brain trust?

[ClickClickBoom]

Stop using large throttle inputs with A/C running, add the hot air from the condenser to large throttle inputs, I would be surprised if you didn't get detonation. Mucking about with the advance will probably just yield a full time compromise. If you read what you posted the answer is there, it's an intake air temp issue, take the heat out of the cabin and put it into the intake and this is what you will get. What you are trying to achieve is really only possible with modern ECUs, with antiknock sensors ect. With dark brown on the plugs, you probably reached the maximum benefit of fuel cooling.

[ClickClickBoom]

[T77911S]

if you are getting 30-35 degrees, you have a problem with the dist.

do you still have the vacuum pots attached tot he dist?

are they working?

best thing to do is to connect everything up original and see what it does.

fix the dist first.

send the dist off to be checked/repaired


if you have big port heads, i would find the big port runners and if possible, i would use the CIS off the 78-79. have the dist curved for that motor.

i dont know how much advance the 3.0 can handle, but my 2.7 kinda had the same issue. detonation under load and WOT. turns out i had an injector line that was partially clogged and only allowing half the fuel to one cylinder.

[Josh D]

Originally Posted by ClickClickBoom

Stop using large throttle inputs with A/C running, add the hot air from the condenser to large throttle inputs, I would be surprised if you didn't get detonation.

I agree, and I do do this, for the most part. However, I still get detonation, even without running the A/C, when max advance is 35*. My quest is to get to the factory spec.

[Josh D]

Originally Posted by T77911S

if you are getting 30-35 degrees, you have a problem with the dist.

do you still have the vacuum pots attached tot he dist?

are they working?

Mine only has vacuum advance, and yes it is working.

best thing to do is to connect everything up original and see what it does.

This is the problem. If I set the timing to spec, I'm getting too much mech advance. Everything is connected originally. This is a RoW motor with factory K-basic, not a US spec lambda motor.
fix the dist first.

send the dist off to be checked/repaired

The distributor isn't "broken", it's just exceeding it's spec for mechanical advance. Per spec, it should be about 25*, but I'm getting 30*. I've even followed the curve at varying rpms to assure it is functioning correctly. If there is a way to fix it, I'll do it myself.

if you have big port heads, i would find the big port runners and if possible, i would use the CIS off the 78-79. have the dist curved for that motor.

[COLOR="Red"]Again, this is a RoW motor, not US spec. It already has the big runners and K-basic CIS as the 78-79 US motors.[/COLOR]
i dont know how much advance the 3.0 can handle, but my 2.7 kinda had the same issue. detonation under load and WOT. turns out i had an injector line that was partially clogged and only allowing half the fuel to one cylinder.

This is something that I have wondered about, but have not yet checked. I need to check fuel volume at each injector.

Still looking for more suggestions. Keep them coming!

[Bob Kontak]

Check this thread out. It may provide some insight.

Seems the earlier SC motors had a few more degrees on top of the 25.

SC Distributor Curves - Little Long

[911pcars]

Don't have an SC or specs handy, but are you sure the initial timing is BTDC instead of ATDC?

Regardless, if detonating, retard the initial timing immediately, then advance until no more symptoms.

The next step is to determine why the engine is in detonation mode (incorrect plugs, cooling issues, carbon buildup, fuel octane, etc.).

Sherwood

[Josh D]

Originally Posted by Bob Kontak

Check this thread out. It may provide some insight.

Seems the earlier SC motors had a few more degrees on top of the 25.

SC Distributor Curves - Little Long

Thanks Bob. I do recall coming accross that thread in a search I did awhile back looking into my timing issue, but couldn't find it recently.

It confirms my thoughts that I'm exceeding the factory specs for mechanical advance, if only by a few degrees. I just want to maintain the 5* static for throttle response and low end power while limiting max to 30* to prevent detonation. Question is, can it simply be done by "adjusting" the weights limit stops?

[cmonref]

are you sure the initial timing is BTDC instead of ATDC?

For a ROW 930/09 engine, 5* BTDC is the idle spec. There are some US versions -- don't know what years or specific engines -- that had vaccum advance at idle to obtain an ATDC spec.

if you have big port heads, i would find the big port runners and if possible, i would use the CIS off the 78-79

The 930/09 has big port heads and runners and no lambda. The US version had small ports and runners and lambda.

Brian

[jpnovak]

Originally Posted by Josh D

TQuestion is, can it simply be done by "adjusting" the weights limit stops?

Yes, Since you know how many degrees you want to limit advance this is easily done.

remove the distributor and clamp it in a vice. Use wood blocks to also lock the distributor gear from turning relative to the body. Then attach a protractor on the distributor body and pointer on the to the rotor/mechanical advance mechanism. Measure your existing angle of advance and then bend the tabs in until you reduce the 2-4 degrees you are targeting.

[Josh D]

Originally Posted by jpnovak

Yes, Since you know how many degrees you want to limit advance this is easily done.

remove the distributor and clamp it in a vice. Use wood blocks to also lock the distributor gear from turning relative to the body. Then attach a protractor on the distributor body and pointer on the to the rotor/mechanical advance mechanism. Measure your existing angle of advance and then bend the tabs in until you reduce the 2-4 degrees you are targeting.

Excellent! Thank you. That was what is was looking for. I just needed some confirmation it had been done before successfully.

[jpnovak]

Use Gunter's thread on distributor disassembly. You will have to take it most of the way apart on the top end (reclutor wheel, coil pack, vacuum pods, etc) to get to the advance mechanism. You will find two stops.

Each weight has a lower pin that rides in a slot on the advance plate. Additionally, the weights hit the side tab to stop their advance.

When you use the pointer, pull the weights out as compared with rotating the rotor shaft. This is a more accurate representation of advance "pull" compared with rotor "push". I hope that makes sense as written.

Once you have it apart it should make sense.

[JFairman]

With a 930 distributor which is similar to an SC distributor (they both use the same cap and rotor) you don't have to take any of the distributor apart to do this and there's no need to clamp it into a bench vise.
I know because I have done it.
I've also completely disassembeld and rebuilt these and also old 6 cylinder BMW distributors. FWIW, they both use the same advance weights even though they rotate opposite directions.

There is a sort of oval shaped steel plate about 3/4" long that is pressed/snapped into a hole in the side of the cast aluminum distributor body right at the level of the advance weights and the two steel tabs that limit how far they swing out from centrifical force while spinning.

Remove the steel plate and visually locate the steel tabs, stick a medium size flat blade screwdriver in there and twist the screwdriver to bend them both inwards a little. Use the protracter to figure out how far to bend them or just reinstall it and check it with a timing light till you are satisfied.

Thats it.. simple, reversable, and done.

[Josh D]

Yes, I follow you in regards to the weights creating the movement rather than rotor moving the weights.

I've also done the distributor dissassembly for clean and lube per Gunter's thread (dreaded little dowel pin!) so I'm good there.

Might be a week or two until I get to this. I'll report back on my success.

[Josh D]

Originally Posted by JFairman

With a 930 distributor which is similar to an SC distributor (they both use the same cap and rotor) you don't have to take any of the distributor apart to do this and there's no need to clamp it into a bench vise.
I know because I have done it.
I've also completely disassembeld and rebuilt these and also old 6 cylinder BMW distributors. FWIW, they both use the same advance weights even though they rotate opposite directions.

There is a sort of oval shaped steel plate about 3/4" long that is pressed/snapped into a hole in the side of the cast aluminum distributor body right at the level of the advance weights and the two steel tabs that limit how far they swing out from centrifical force while spinning.

Remove the steel plate and visually locate the steel tabs, stick a medium size flat blade screwdriver in there and twist the screwdriver to bend them both inwards a little. Use the protracter to figure out how far to bend them or just reinstall it and check it with a timing light till you are satisfied.

Thats it.. simple, reversable, and done.

Cool, I'll look into this as well. I'm assuming the distributer is removed from motor to do this?

[JFairman]

yes...

R&R is cake. One 13mm nylock snugged down holds it in place.

Mark the distributor shaft body where it inserts into the case and also rotor position on the distributor body where the cap fits on with a fresh pointed sharpie pen so you can reinstall it in the same position.

Use a timing light to check everything when done.

Nice thing about this is you can shorten the timing curve so final centrifical advance is the same as you had it before or stock... While at the same time static timing at idle will be advanced around 10-15 degrees or whatever you settle on so the motor runs a heck of a lot better and stronger at idle and during acceleration through the lower midrange speeds.
It might not pass nitrogen oxide emission level testing with advanced idle timing with no catalytic converter.. thats hard to say.
CO levels will stay pretty much the same.

I'm in Florida where there is no emission testing so thats not an issue.

[Bob Kontak]

Originally Posted by jpnovak

Measure your existing angle of advance and then bend the tabs in until you reduce the 2-4 degrees you are targeting.

Jimmy Neutron is da bomb!

Cool beans.

[Eagledriver]

Remember that the distributor turns at half the speed of the engine. If you want to reduce your advance by 5 degrees, you only need to make the distributor advance 2.5 degrees less.

-Andy

[Josh D]

Remember that the distributor turns at half the speed of the engine. If you want to reduce your advance by 5 degrees, you only need to make the distributor advance 2.5 degrees less.



-Andy

Good point. It shouldn't take much adjustment.