Another 350 tuning thread

Let me start by saying I'm totally new to tuning carb/distributor engines but I'm an engineer so I know enough to get myself into trouble. :)

Have a 350 in a 70 Chevelle that needed an in depth tuneup. Purchased and installed new Holley street avenger 650, Accel HEI distributor, plugs, wires, removed/replaced intake manifold, and removed/replaced headers.

After all the installs and timing, I'm having an issue tuning out a backfire from the exhaust. At idle, it runs fine. With no load at partial throttle, I get a backfire if I throttle it quickly. As soon as I put it into gear, I get constant backfires and I'm severely down on power to the point of being undriveable.

I timed it to 10" base (vacuum disconnected and plugged). I was told to dial in total timing at 36-38". If I rev it up to 3k with no vacuum, my timing light is showing 44". This doesn't make sense to me because the springs installed in the distributor show in the manual they allow ~21" so wouldn't my total advance be around 10+21=31? With vacuum connected, I'm seeing about 47". A friend suggested I had too much advance and I should get a spring and weight kit. I did and put in a tighter spring that showed in the manual about 16". With that installed, my base timing was way off, so i reset it to 10" now without vacuum I was seeing a total of 25" at 3k rpm (still with backfire). With the vacuum reconnected I was seeing 50". I also tried turning down the vacuum advance by adjusting the diaphragm on the distributor but I saw literally no effect on the overall timing at 3k rpm.

I've double checked for vacuum leaks, exhaust leaks, sticking valves, improperly adjusted rockers, basically anything mechanical and can't find anything.

My questions are: with the backfire, which direction should i be trying to adjust: more or less advance?, Is there anything mechanical not listed that I could try that could be causing this (considering it had no backfires before the tuneup)?, and when you tune for total advance on throttle, do you leave the vacuum disconnected and plugged?

Thanks for any help. As I said I know enough to be dangerous to myself.

Are you certain you had #1 on TDC when you installed the distributor? A quick check is to turn the engine by hand to TDC on #1 cylinder, pull the distributor cap and see if the rotor is pointing at #1 plug wire...

I haven't verified since initial setup but we had the spark plug pulled on #1 and rotated the engine by hand to find TDC before installing the distributor. I'll double check tonight.

That reminds me, I also have a "corrected" cap installed to clean up the wires. I'll have to look up where the rotor should be for #1.

First, welcome to CHR from another gearhead engineer! I agree with Dave, it sounds like you might not have had #1 at TDC Compression Stroke when you stabbed the distributor. Another thing that I ran into was that my distributor wasn't seated all the way in the manifold bore when I set my base timing, and then when I tightened the hold down it pulled it down tight and retarded my timing 12*, but it sounds like you've checked yours after it's all tight. Are you using a quality dial back type digital timing light, or are you reading the damper marks at the pointer?

Another possibility is that your damper may have de-laminated from the core and slipped, throwing your "0" mark off; and there have been cases where someone has mis-installed the pointer when doing maintenance on the front end of the engine. You can verify your timing marks in a non-invasive way with some light oil, clear tubing and an old spark plug.
1) Break up an old spark plug and hot glue a length of clear plastic tubing to it (make it airtight).
2) Remove all the spark plugs.
3) Stick your thumb over the #1 cylinder spark plug hole. Rotate the engine using a breaker bar on the dampener bolt until you feel air pressure on your thumb for the compression stroke.
4) Screw in the spark plug with plastic tubing attached and insert the other end of the tube into a jar of light oil. Continue rotating the engine. Bubbles will appear until the piston reaches the top of its travel. When it starts down on the next stroke, the bubbles will stop and oil will begin traveling up the tube. Stop at a convenient point (don't go too far) and mark the liquid level in the tube and also mark the crank pulley at your pointer.
5) Rotate the engine backwards and watch the oil recede into the jar. Continue rotating. As the piston continues past TDC and downward it will again suck oil into the tube. Rotate the engine till the oil again reaches the mark on the tube. Stop at that point and mark the dampener where it lines up with the pointer. You should now have two marks on the crankshaft pulley, and the exact midpoint of these two marks lined up with the pointer is TDC. If you want you can iterate to smaller and smaller separation on the marks by re-marking your tube at lower levels and repeating the process.

If everything checks out you might suspect the new HEI distributor. There are a bunch of guys on here who wouldn't buy a hammer with the ACCEL brand. They seem to spend all of their money on advertising as opposed to quality design and control. Hope you'll post back what you find.

I agree with Roger about Accel, nothing but junk, IMHO. What you have is a symptom that used to be associated with a blown power valve, but your Holley should have blow out protection although it might not hurt to change the power valve out.

I found the source of the problem is either the new coil that came with the distributor or the corrected cap. I was troubleshooting tonight and when I took the old coil and installed it in the regular distributor cap everything worked great! Even took it for a test drive for the first time in a month!

I'm gonna narrow down tomorrow night whether it was the corrected cap or the coil and return whichever is the culprit. Thanks for your help guys. By pointing me to the distributor you narrowed my focus.

rspears, that's a rather ingenious way of finding TDC that I'd never heard of.

Thanks again.

You might want to put a new distributor from MSD, Mallory or even PerTronix on your Christmas list for some future peace of mind. Just sayin' I would always be wondering about that Accel dizzy, worrying more the farther I got away from home.

I can't claim originating that TDC idea. Found it when I was looking for a way that's 100% accurate while not having anything to get bound up sticking through a plug hole and have passed it on several times. Problem is I can never find it in a previous post with the way our search engine functions :rolleyes::(:mad:

Welcome to CHR, by the way! Hope you stick around and get into the discussions!

Corrected caps (the ones with all left side wires coming out the left side of the cap, etc.) are junk in my opinion. I had one numerous years ago, and it gave the same symptoms you are experiencing. I replaced it thinking I got a defective one, and it did it again. As soon as I replaced the distributor cap (and new wires, of course), the problem went away. I haven't spoken to anyone that has had good luck with those distributor caps.

Yup it ended up being the corrected cap. I'm doing a little research on them to see if I should bother getting a replacement or forget about it. They do look a lot better IMHO.

I'm still having an issue with timing though. I'm only getting about 8" of mechanical advance. Therefore to put my total timing in the range of 34-36, I'm turning the distributor until my base timing is around 27! I didn't feel comfortable with my base timing at 27 so I set my base timing to 16 (advance timing at 24), locked down the distributor, connected the vacuum, and took it for a drive. It seems to drive fine but I know those numbers aren't great. The springs I'm using are the stock springs which say I should get about 21" but I'm clearly not. Chalk it up to a crappy Accel distributor again? I didn't pull plugs but I'm sure I'm running rich.

If I were you, I would replace your distributor with a MSD unit. I like MSD equipment but do not like Accel stuff. I used an Accel distributor and coil 15 years ago with no problems, but it seems they have cheapened everything in order to maximize their profits and in the process, quality went out the window. I agree that the corrected caps have a nice look, and that is why I made my initial purchase. What annoyed me the most is the fact that I had to throw away a nice, new set of Taylor ignition wires when I replaced the distributor cap.

It might look better, but if it doesn't work....................

Imho, git rid of everything Accel, stock is better than that junk, and you'll be ahead of the game.

Been watchin' this thread from the side lines. First off, welcome to CHR! Lots of great people here and tons of technical expertise. I'll add my 2 cents re; Accel products have never been my favorite and I have had similar issues with an Accel distributor in a 400 SBC. I used it because the kid I was working with bought it and was real excited about it but it never timed well. Replaced it with a stock HEI and the problems disappeared.

IMHO - the stock HEI is good for all but the most extreme builds. If you can get a stock GM unit and install a new quality cap and rotor your issues may cure themselves. You're headed the right direction with 8-10 initial and all in at 32-36.

Summit has an OEM replacement for under $100 - http://www.summitracing.com/parts/sum-850001r/overview/ or an MSD for $120 - http://www.summitracing.com/parts/msd-8362/overview/

I hear ya on the corrected cap but have not had the best of luck there either. The OEM cap or an MSD unit seems to work the best

Good Luck,
Glenn