SB chevy timing retarding

[toclub]

Nobody has mentioned anything about the coil. MSD requires a certain type of coil for each box. i.e. 6al,7al. Maybe you need a new coil.

[sdroadster]

Today was a long noisy day. The points distributor didn't work out well. The timing was erratic and jumpy because the distributor is worn. I gave up on that.  A friend who has built many motors, and is a land speed guy came over today with a trunk load of MSD boxs, coils, and distributors. We installed a analog MSD, a digital 7al box and a associated coil. We also installed new ignition circutry, and another new coil. It still retards. The timing is kind of consistant, but does change somewhat. The motor lays down usually between 5000 and 5500 rpm. There is an audibale change in the exhaust note. At that same time the timing typically retards 8 to 10 degrees.  BUT, one time it ran up to 6000 with no retard, and one time the timing began to retard at 4000 when the motor was cold. We made 12 tests today and have pretty much decided the problem is not electrical. I have borrowed a Vertex mag that I will install after New Years. If it retards with the Vertex installed I'm going to pull the engine and take it back to the owner/builder. (I own the Studebaker, he owns the motor)  Thanks Guys I'm going to give the neighbors a rest for a week or so.

[manta22]

What kind of cam drive are you using? The distributor is driven by the cam so if the cam drive is erratic, the distributor will be as well. If you are using a timing chain, it or its sprockets may be worn. If it is gear-driven, look for a bad bearing.

Regards, Neil  Tucson, AZ

[sdroadster]

The cam drive is a chain. Also I forgot to mention we tested several valve springs and they were in the 180 to 200 pound range. thanks

[desotoman]

Have you thought about checking the end play of the camshaft to front cover? Sounds to me like you could be getting cam walk.
 
You should have some kind of button on the front of the cam to limit movement to .005-.010, and make sure you have a sturdy timing cover that does not flex. Cam walk is common when using roller cams, since they are not ground on a taper to keep the cam pushed back in the block like hydraulic and solid lifter cams are.

You might want to pull the timing cover and take a look, As little as .040-.050 cam clearance could be causing your problem.

Tom G.

[sdroadster]

Thanks, I agree with you. In order to retard timing does the cam have to back into the block, or forward into the timing cover? Doesn't the helical cut gears, and the resistance of the oil pump want to draw the cam back?


Unfortunatly this is not my motor. I have to convince the owner because he disagrees that there could be anything wrong mechanically. Soon I'm going to pull it, and send it back to him Yellow Freight.

[POPS]

We had the same type of problem with the 6AL. Anything below 12.5 volts and it wouldn't make power. Put a volt meter on the battery to see if its the problem.

[sdroadster]

14.45 volts. Thanks

[desotoman]

Thanks, I agree with you. In order to retard timing does the cam have to back into the block, or forward into the timing cover? Doesn't the helical cut gears, and the resistance of the oil pump want to draw the cam back?

I am not an expert on SB Chevy's by any means of the imagination so I would have to look at a block to determine whether forward of backwards movement of the cam would cause the timing to retard.

Spring pressure coupled with non perfectly machined blocks or possibly cams will cause cam walk when using roller cams. So if the button on the front of the cam is worn or damaged and the cam wants to walk out of the block the spring pressure will keep the cam pushing out towards the timing cover. It would only be a guess on my part to say whether the the oil pump and helical cut of the gears would be able to pull the cam back into the block. Sorry.

I am not saying cam walk is the issue, just saying that is something else I would look into.

Tom G.

[JimL]

You probably already tried this, but, put the negative probe of your voltmeter on the battery Positive post, and put the positive probe of your voltmeter on the 12V input to your coil or MSD (maybe try one at a time).  Rev it up and see if the voltmeter starts showing voltage (positive or negative volts).  It basically tests your supply circuit under load.  Reading voltage from negative ground to the supply point will only show battery voltage (minus a tiny bit), but reading voltage drop will show if there is resistance under electrical load.  

You can have every strand but one broken in a length of wire, and it'll still read full battery voltage and show zero ohms resistance on your voltmeter doing traditional test methods.  This way, anything over .1 volt is a pretty big problem (except when testing a starter cable....those are still ok up to about .5 volt with this test method).  Its also a good way to see if your battery cable is maybe too small for the run length.

If I have these numbers wrong, dont throw rocks....I am old, forgetful, and dont duck as fast as I used to!

JimL

PS:  This is another dumb question, but you dont have "heavy fuel' by chance, do you?  If you see sooty exhaust pipe ends, and the spark plug porcelain is very white, you might have heavy fuel that wont vaporize fast enough.  It works ok, if the engine is hot enough, but not when you get the revs up  and the time to vaporization becomes longer than the combustion events can handle it.  Wont rev worth a darn on a cold engine, but always makes high exhaust temperatures (EGT).

SpeedWeek 2000 we fueled up (I forget which gas) and took the roadster to the line.  Car ran 102 mph (after previous years speeds had been 150-170).  We didnt touch a thing, just went back to the fuel truck and changed to A19A, and went straight back to the line.  Next pass was 183 mph.  No change but the 90% distillation temp of the gasoline choice.

[sdroadster]

I just got a call from the fellow that owns the engine. He said we have tried everything, and he want's it back for repair. Thanks to everyone! I'll keep you informed if he discovers anything.

[JimL]

Me being a bad one for "showing my dumb side", I am still thinking this could be a false timing read.  As I read the literature, these MSD can double or triple strike each spark event.

I had a bad experience with that method, while working with a fellow who wrote the double-strike spark code used in some early OBD II Ford software.  When our supercharged engines (with add-on spark processor he wrote code for) had high cylinder pressure, and a lean mixture, the "first spark" didnt make the jump.  The second one did, which was late in time-event, and then caused our fast 32-bit engine ECU to detect the change in crankshaft revolution as a misfire event.

This shouldnt be an issue with a race engine, of course, unless that spark plug is surrounded by highly compressed air with the gasoline not yet vaporized enough to lower the electrical resistance of the compressed air.  If that happened on a multistrike system, your timing light will only flash on the following spark and the engine wont rev cleanly.

This late vaporization can be really ugly.  I have had this "heavy fuel" discussion with a number of folks experiencing lack-of-rpm, misfire, low-power, black sooty-exhaust, high EGT, and burn in the cylinder so bad that they smoked pistons.  These are very experienced people that are really good, but got bit by this issue as I did.

I will butt out now, but this sure seems similar to what happened to us, (and some really top notch teams).  Sometimes the old "its the gas" story can be true.  When the only thing youve changed is the gas...well,  thats how my 76 year old Mom figured it out for me at Speedweek '98.  After two days of frustrating inspection and attempted repair, she was the only one with the right answer in our pit!

If it ran fine, and you didnt change anything.....    ....and if you already tried some fresh, lighter gasoline, my apology for wasting everyones time.

JimL

[Crackerman]

I also partially agree witih what JimL is saying.

Have you tried narrowing the gap on the plug, making it easier for the spark to jump?
usually it will misfire instead of retard timing, but with a multiple spark, it could be just firing on the last spark thrown, resulting in retarded timing.

I am now very curious as to why this is happening...

[Ron Gibson]

If you don't know what is causing the problem, how does the engine guy know what to repair Hate to see him replace all of the parts and have the same problem.

Ron

[sdroadster]

Thanks. I don't have an ECU. The tank has new VP C12 in it. The engine builder is going to have a look at everything, but concentrate on the cam. I'm sure he'll lay hands on the gears, chain, cam button, and end play. He's going to run it on a engine dyno when finished. I'll keep you informed if I learn anything.