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Hello, have been following this forum for as while but this is my first post.
Working on a '65 C2 SMB with 350 HP (or that's what it should be).
Since a while I noticed that at a slow speed the engine 'rumbles' in the low revs range if you want to speed it up again. It's like it's not firing on all cylinders. So time to check out the ignition:
This is what I have for the shop manual for the 350HP engine:
BTDC is 8 degrees at 665 RPM idle
Centrifugal is 0 degrees at 700 RPM, 11 degrees at 1600 RPM and 24 degrees at 4600 RPM
Vacuum advance is 0 degrees at 8" and 15 degrees at 15,5"
Dwell angle: 28-32 degrees
This is what I have from the engine (using an Innova timing light):
Disconnected vacuum and set engine at 8 degrees BTDC at idle (all OK and stays very stable)
at 1600 RPM total advance (centrifugal kicking in) is 11 degrees (according to the manual and is OK)
at 2330 RPM total advance is 24 degrees (doesn't match the shop manual)
at 4600 RPM total advance is 30 degrees (doesn't match the shop manual)
Is the above normal? Looks that the centrifugal is not in line.
Note that during the whole process the engine is running very smoothly
Now with the vacuum link connected the total advance is 25 degrees at 1000 RPM. Is this normal?
During the whole process the dwell angle was recorded at 26 degrees and although it needs to be adjusted the angle stays very stable through the whole RPM range.
Assuming your engine is actually a real L-79 it does not have the correct distributor. The centrifugal should be 30 @ 5000, which is very lazy, and the original VAC was an eight incher, but a 12 would be best. If it still has the metal tag wrapped around the base, what is the part number on it?
Do an advanced search for threads started by me and look at the distributor blueprint overhaul thread. The subject distributor was an original from an original '65 L-79. We did a blueprint/overhaul, optimized the spark advance map, and it's been running fantastic ever since.
You should also look at my tuning seminar in the list of threads I started so you can understand how to set up a spark advance map for any SBC or BBC configuration. Peruse the two page list and you will likely find others of interest.
Ides - Welcome to the Corvette Forum. Sounds like you've done a pretty thorough job of identifying all the items associated with ignition. Along those lines, maybe you can fill us in on a couple other things such as the Vacuum Advance Cannister (VAC).
Looking down on it, while installed in the distributor, there should be some numbers or characters on there, can you post them? They vary as to how much advance they add and when it's all in.
Also, even though your focus is on the ignition side, anything to offer regarding the carburetor, such as model/list number, jetting changes, how many turns out the mixture screws are and lastly...when you hook up a vacuum gauge, what does it read at idle and is it steady? And lastly, not sure what you mean by 'it rumbles'.
Mike T - Prescott AZ
What I mean with 'rumble', is that when you go into a very tight slow corner and you shift down to first gear it sounds like the engine has troubles revving up again and it sounds like there is an ignition mismatch. There is no backfire but there is that deep growl of the engine not wanting to rev up (like missing many cylinders firing) when pressing (even gently) the pedal. And once it gets going after a couple of gentle nudges of the pedal the engine starts revving again.
By next time around I should be able to give you the details and the carb.
Your distributor is a kluge put together by bubba. I have no idea what what specs the VACs you ordered have. I guess you didn't search for the thread I recommended. Good luck. You'll need it, but luck won't get you an optimized distributor.
I'll check out your postings but I had the other VAC's already on order before I did my initial post. I'll make sure I'll do it differently next time in my quest for answers:-)
Thanks for the advice on the vacuum tester Powershift.
I'll keep on posting my findings in this thread over the next couple of weeks.
Happy that I had some time today to continue the testing on the timing and vacuum.
These are my findings on the vacuum test (vacuum dial connected to the vacuum advance port on the carb and vacuum can disconnected):
10" at 665 RPM idle
14" at 1040 RPM
19,5" at 2040 RPM
I've looked through shop manuals but couldn't find any reference to what is expected at specific RPM's except that one should try to set to the lowest possible vacuum reading.
What are the expected values?
The BTDC is at 8 degrees at idle (I'm waiting for spring kits to get the centrifugal back to the right values). Dwell angle is now stable at 30 degrees.
Now back to the 'Buba' vacuum can:-)
Hooked up the vacuum hand pump directly to the can while having the engine at idle:
At 4" measured 0 degrees extra advance
at 8" measured 0 degrees extra advance
The shop manual talks about 16,5 degrees advance with 8" vacuum. Which can can should be the best fit?
A real L-79 cam should pull 14-15" at 750. Your vacuum reading indicates that you have a higher overlap cam, so less vacuum at any idle speed, and you need a 8" B-28 to pass the Two-Inch Rule and have an acceptable stable idle. Set idle speed at the lowest commensurate with acceptable idle quality, which is your call, and my bet is that will be 850-900.
Your data indicates the installed VAC may be DEAD, but either dead or alive it is not functionally correct for you engine configuration. It's a boat anchor!
thanks for the feedback on the obtained readings. It seems that if one keeps on digging, less original the engine is (which heavy overlapping cam is in there?).
I've been looking around for the B28 can but no one has one for sale in Europe or willing to ship from the USA to Europe. I've found the following alternative parts for the B28 can. Below is what I found on various websites but for GM and also AC Delco in Europe I don't see availability of that part. Brand: GM
If someone can show me how to create a table and add pictures and description to cells it I can comment on the sequence of the pictures.
The above pictures show each time the RPM reading combined with the vacuum readings. Some pictures show the hand vacuum pump (8" and 4") but no changes on the advance on the light gun qualifying the current can out of spec for the engine.
Additional picture shows the dwell angle steady at 30 degrees.
The carb is a 4150 with secondary vacuum. Still need to check out the carb on adjustments and jets but first need to get the ignition timing right.
Was able to order the B28 vacuum can from rockauto.com (I didn't found a source in Europe) and the cans arrived yesterday. After reading this thread I was concerned of what I would receive but it all looks good so far. Now I will need to verify the specs of this can with the handheld vacuum pump. It has the 'B28' stamp on it.
Order number: 4V1053
Manufacturer: wvebrand.com
Alternative order number: 8511.90.0090
The 928G mechanical advance spring kit is on order at Summit racing and will be shipping soon.
Ignition advance going wrong?
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