Carb / Distributor issue?

[myonnone]

I have a detonation problem that is apparent in the 1300 - 1800 rpm range.

427 / 390 HP engine with a Holley 3367 on it, stock manifold.

I just rebuilt the carb, because I haven't done it in more than 10 years and the choke wasn't closing for cold start. The rebuild fixed that.

Set the carb up after the rebuild with a vacuum guage on the line to the distributor. It seems to be running well except for 2 issues.

1. Detonation as noted above
2. Stumbling at over 3000 rpm. It will pop back through the carb at about 85 mph.

I took some timing readings as follows.
With the weights & springs removed and the vacuum connected to the distributor 12° at 500 rpm and above.

With the vacuum line disconnected and plugged and the weights and springs installed.
0° at 500 rpm
10° at 1000 rpm
15° at 1500 rpm
20° at 2000 rpm
20° at 2500 rpm

I am running it retarded to try to inhibit the ping, but what bothers me is that I'm getting about 20" of vacuum at idle. I thought about 5" or 6" is normal.

Any help from you distributor guru's is greatly appreciated.

Marty

 

[ctjackster]

hey Marty - I would urge a new, correct-spec vac can (not very expensive) for your application and, you might want to tear that dist down and freshen it up (rebuild) since you seem to have some odd advance issues, might be out of spec and/or it may be binding/sticking on advance . . . heck if you can tackle a carb a dist rebuild is easily within your Sat afternoon window of opportunity, just get a good collection of shims beforehand, and the various felt and other washers, make sure those little wire connections to the breaker plate, etc. are solid, etc.

 

[allcoupedup]

Myonnone,

Take some more measurements at higher RPM. If you have strong springs and a lazy advance curve, you may see additional advance over 2500... take some measurements until you see no additional advance.

I'm not familiar with big blocks but a small block should see about 36 degrees maximum mechanical advance.


Brian

 

[myonnone]

Jack,

Easy enough to put a new vacuum can on it, but as I understand it, the vacuum can starts to advance at about 6" of vacuum and is fully advanced at about 15". I'm getting about 20" at idle (500 rpm), so I'm in a full vacuum advance state all the time. With this scenario I can just plug the vacuum line and just use the centrifical advance with an initial timing adjustment made to make up for the initial (idle timing) setting. I have run the car in this fashion and it does not detonate, although there is some loss of performance. My fall back position will be to try a new vacuum can, if that doesn't work, run without vacuum advance and taylor the curve with new weights and springs.


Brian,

I do have some heavy springs in it right now, so perhaps there is a bit more advance available. My understanding is that the timing curves are the same for the small and big blocks.

My main issue here is, am I way off base on my theory of the the vacuum advance kicking in way too soon, or is there some other problem?

Thanks again for the suggestions.
Marty

 

[allcoupedup]

Marty,

You want full vacuum advance at idle. At WOT, there is no vacuum so there will be no vacuum advance. At partial throttle, you will see a slight decrease in vacuum upon initial acceleration , but it will climb back up -- along with your advance. The best way to check this out is to connect a vacuum gague and play with your motor. As a rule of thumb, you want full vacuum advance 2 inches of mercury BELOW your idle vacuum.

You will most likely experience idle quality and temperature problems if you abandon the vacuum advance set up. Lack of sufficient advance at idle increases exhaust gas temperatures along with the temp of your engine.


Brian

 

[ctjackster]

yeah what Brian said, the best proof of vac advance @ idle is the immediate loss of idle speed when you disconnect a properly set up vaccum can, such as when you are setting the timing. Based on our conversation yesterday, it might be that you have some sticking at the base plate or some other improper function of your can or advance weights, you said you have changed them (the weights) at some point, are you running correct-spec weights and springs? I am only familiar with the 327 SHP set up, but whatever your case it's all related.

 

[myonnone]

Thanks for straightening me out on the operation of the vacuum advance. I ordered a new correct can from LI Corvette. Should have it in a day or 2.

Any thoughts on where I can get the correct weights and springs?

Marty

 

[ctjackster]

same place

their distributor "spring and weight kit" has diff springs for use with diff applications, Section 48, pn 05 - $10.00. Also check to make sure your little rubber sleave that covers the pin that sits in the groove on the plate is still present or hopefully has been replaced with a brass one - that rubber sleave is also found in the same section, pn 06; a brass one is pn 06B, the better choice. Funny thing about the vac cans, they don't seem to come with the same sort of rubber sleave that fits over the little "hook" nub on the actuating arm of the can, and no one seems to list it, but my original vac can had it, so I moved it over to my replacement can, I suspect that little rubber sleave noted above as pn 06 is the same size and would work.

 

[myonnone]

Thanks Jack, I added the weight kit to my order for the vacuum can. I've got the brass bushing on the distributor now as well as an extra one if needed. I also have the rubber bushing on the can that is on the distributor now. How did you determine which weights to use?


Marty

 

[ctjackster]

as I recall the weights are not the thing that changes, it is the springs, those are color-coded (silver, brass) per application - this is from memory though

 

[JohnZ]

Someone has already been in the distributor - the original "smog" centrifugal advance curve was much longer, for use with 4* BTDC base timing, to give 36* total timing at 5000 rpm (long curve made up for retarded base timing):

8* @ 1250
17* @ 2000
32* @ 5000

The original vacuum advance can gave 12* advance at 12" of vacuum.

It should respond well to a curve that only supplies about 24* centrifugal (instead of 32*), all in by 3000 rpm, with 10*-12* base timing, and full vacuum advance at idle (connected to full manifold vacuum, not "ported" vacuum that's shut off at idle). Make sure the port on the carb that your vacuum advance line is connected to is full manifold vacuum, not "ported". If it's "ported", connect it instead to a tee inserted in the short hose that feeds the choke pull-off diaphragm - that has full manifold vacuum, which is essential to ensure that the vacuum advance is fully-deployed at idle.

If you have a dial-back timing light, you can experiment with weights and springs and limit bushings until you get the curve you want, but it's pretty time-consuming; better bet is to yank the distributor and take it to a shop with a Sun machine and have them set it up - takes about half an hour.

 

[myonnone]

Thanks for the info John.

I had this distributor rebuilt about 25 years ago although I've changed to some stronger springs more recently chasing this detonation issue. I know it was put on a distributor machine at that time, but I don't remember anything else about the rebuild. I do know that it now has a "B1" can on it now which should be 16° @ 18". I have the correct can on order along with a set of weights and springs. From my docs, the correct can should be 15° @ 12".

My vacuum line is connected to the correct port on the base of the carb and I get about 20" of vacuum at idle.

I'll post the results once I get the new parts installed later this week.