The Stovebolt Site Forums The Shop Area Engine Shop Timing on a 235

Quick question. I have a stock 4 speed on my 235. Does the BB on the flexplate mark TDC or 5 degrees BTDC?

I think TDC is a vertical line between the U and the C like this: U!C

The BB is where you should be setting the timing according to the Shop manual.

bztguy - It also makes a difference on if your 235 is original to the truck and has the original flywheel or not. Definitely details we would need to know to help provide the best answer to your question.

It is original to the truck which is a 49'

Here's a very good and lengthy old thread on the subject of Flywheel Timing Marks. It does a good job of describing the different flywheels and markings. Hope this provides the answer you're seeking.

I understand the BB is where the shop manual says to set timing to. Shop manual also states it should be running 5 degrees advanced when idle is below 600RPM. So does that equate the BB to being 5 BTDC? Mine does not have the UIC stamping. It has the triangle for valve timing, 2 vertical lines, the BB, then another vertical line. I'm thinking the vertical lines are 5 degree increments. The reason I'm asking is my truck is stock but for the rear end has been swapped out from 4.56 to 3.73 gearing and when I set timing to the BB it idles and runs smooth as silk. But is gutless above 25MPH with the lower rear end gearing and I can't even take a small hill. I'm currently running by my timing light 3 degrees advanced of the BB which gives me better highway power but the idle starts to suffer. What I want is the smooth as silk idle below 600 per the manual but I need a steeper advance curve when I come off idle for highway driving. So I'm looking at the Pertronix billet distributor with the recurve springs kit, advance limiter, and vacuum advance. But I would like to establish baseline timing off of the BB so I know what my actual timing curve and all in advance is which from memory is around 24 to 28 degrees. I can't know where I'm at if I can't establish if the BB is 0 or 5 degrees advanced.

If you intend to switch to electronic ignition, there are better and cheaper ways to do it with off the shelf GM parts rather than with a Pertonix unit.

The BB is normally 5 deg BTDC.

What makes you think your dizzy/timing is responsible for the perceived lack of power? If your truck can't make it up a small hill, electronic ignition isn't going to solve your problem. I think you have other issues.

I have a 216 (unrestored), 3spd, 4.11:1 rear end with breaker point ignition and it drives 60mph all day long up and down any kind of hill you want to drive on.

Have you verified that your vacuum advance is working? What sort of compression do you have in the cylinders? Is your clutch worn out? What size tires are you using?

All of those things can affect the performance of your truck.

Mine couldn't climb hills either and could barely go over 30mph. It turned out the clutch was slipping and it had ridiculous 15" rims and small diameter tires on it.

Once those two things were changed, it could be driven in freeway traffic at 55-65mph.

Before I did anything else to the engine, I'd do a compression test on all cylinders. Your engine may be worn out, worn rings, burnt valves, or who knows what?

If that checks out, I'd statically set the timing at 0deg TDC on #1 cylinder and see where the BB is after that.

Otto I understand what you are saying. But my engine doesn't have any of those issues. Compression is great, valves are fine and lashed properly, vacuum advance works, no vacuum leaks, clutch is fine, tires are 16" rims with 29 total tire diameter. At idle the vacuum gauge is locked at 18-19 without the slightest quiver. I've been running pertronix in the stock distributor for 2 years. If I run around 5 degrees advanced I can run up to 65 in a heartbeat, accelerate up a steep hill and bob your head back if I punch it while going up that hill. Drop her back to stock timing and I lose higher speed RPM and torque because it's expecting a 4.56 rear end and smaller diameter tires topping out around 45mph by dessign. Her legs are too long now for the stock advance curve. I've been running it around around 5 degrees advanced for a couple of years but it makes it idle rougher and purely for personal preferances I want it quiet and smooth at an idle. The truck runs great but I want a smooth quiet idle at the factory spec'd 500 RPM. I can drop back to stock timing and she's smooth as silk on idle, runs and accelerates smooth as silk. No backfire running against a downshift. Just no power/torque past 30 MPH. I can get up to 60 but it takes 3/4 mile of flat road to get there at stock timing. The Pertronix distributor I'm talking about has recurve springs that will start in just off of 500 RPM and give a steeper advance curve and can be limited to the correct all in advance the engine is designed for.

There’s a lot of unknowns here. Valve adjustment, compression, vac leaks, carburetion, etc.

But taking your timing concerns at face value:

1) Make sure your dwell is within spec (or at least verify/adjust your points gap).

2) With the engine warmed up, and ignition timed to more/less the dot….hook vac gauge to vac advance port. Set idle SPEED screw to the point you JUST start to see the vac gauge lift off zero. Adjust idle MIX screw to max vacuum and then another 1/8-1/4 turn richer.

3) Whether or not it’s ideal….you now have at least a baseline to start playing with the timing. Advance the dizzy a bit at a time until you find a sweet spot where RPM almost maxes out, but things are nice and smooth and happy. Snug up the clamp. Shut off and try to restart. If it bucks against the starter, loosen clamp and back timing off a smidge until it starts easy again. Go for a drive and see how it feels.

—————

There are lots of ways to skin this cat. That is the way that works best for my well-worn engine. I am not convinced that a numerical approach to timing works well on a Stovebolt of less-than-pristine condition. It’s more of a feel game, IMO.

I can say with a lot of confidence that my 235 runs LOUSY timed right at the dot. It wants more timing than that.

Edit:

I typed all that before I saw your last reply. Ignore the points gap/dwell thing if you’re running Pertronix. All the rest should still apply. The Pertronix is a variable that’s hard to account for. Sometimes they work, and sometimes they don’t. Some people love them and some people hate them.

I am highly skeptical that you need a different distributor curve. My truck is even longer legged than yours….3.38 rear and same tire size. I can make it run nice at both idle and 70 mph.

I think something is amiss with your ignition or carb. Perhaps dizzy is worn internally and timing not stable at low RPM. Or could be carb….what carb are you running?

No point in wasting time or billions of electrons making any suggestions- - - - -I wish you the best of luck in getting the problem sorted out. Running anything made by Pertronix is equivalent to shooting yourself in the foot before a 5 mile hike!
Jerry

Bztguy, you might want to read this thread with a similar rough idle problem after installing Pertronix.

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1570835/1

The second thread is by the Stovebolt's resident mad scientist, Jon G who has deconstructed the Pertronix system to discover why many of them fail and many of them don't.

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1581649/pertronix-failure-solved

Thanks for linking, Otto. Been a while since I read the deep dive Jon did. Good stuff.

Shoot yourself in the foot! Hotrod you are something else... There still looking for your mold...

There are a couple of things you will want to sort out in your quest to get some accurate base line timing. First, confirm the year of the motor. Then, pull the cap off the distributor and grab the delco number.

Here is where it will get tricky. Different year flywheels were set up for different year motors. So if you read the above article, you will note the common uses of the same casting number on the flywheels, but different part numbers which reflects some variations in the bb placement and other marks.

So for example, 57 235 motors are set at tdc and the bb for that motor is at tdc. 59 is 5 degrees advanced and others can be 2 degrees. Most of those flywheels are marked for that advance, but not all. So you may have to draw out what’s on the flywheel and if it looks like no indication of anything other than tdc, you may have to make an assumption for the time being, that it is.

In terms of total timing, some of the distributors were set up for 12 degrees of distributor advance, others 14. Which means the motor will see 24 or 28 degrees, if the distributor is working properly. And there lies the rub. With these old distributors, they rarely come up to spec. I have had about 20 of them on my Sun machine before I take them apart and I will bet only three or four were advancing properly. One, an nos 2403 was so gummed up with old delco factory grease it would not advance more then a couple of degrees. Others were rusty, gunk laden, broken springs, or worn out with far too much end play.

So if you really want to see if you can build a little accuracy, maybe start with the year of the motor and the distributor part number and let’s eee where that takes us.

Dragsix - are the 235 dizzies limited in an absolute way to 24 or 28 degrees? As in, the weights won’t swing any farther no matter how gunk-free or what springs are used?

Or can a guy get more total timing with some sort of traditional mod like lighter springs, etc?

It’s an interesting question. What I have generally found is the 55-58 seem more likely to be 12, 59-62 more likely to be 14. But not always. And silly me, I never bothered to take a look at the weights and compare to see if there is any slight changes or other adjustments between the 12 degree internals and the 14 degree internals.

I am going to have to get my behind out to a swap meet or two and see if I can round up a few 2305 distributors and do something a little more detailed.

Dragsix you bring up good info for me to research on the dizzy. When I got the truck my research pointed me to determining the motor is a 56'. The rest of the truck as far as I know was original as far as tranny. The dizzy has been replaced with a rebuilt so I will gather the number. As far as the response about Pertronix. I did read the old post and they seem to all be talking about the original units that required the drop on ring around the lobe shaft. I don't believe that is in production anymore. Mine does not have that. It's a screw on plate that takes the place of the points/condensor and picks up from the shaft lobes directly. Do I carry an extra unit in the glove box? You bet. But I also carried a spare points/condensor set when I was almost left stranded on the way to a car show when the points burned up due to the condensor failing and I just barely limped back home. Point is anything can fail and points/condensor do not grant you immunity from failure anymore than a Pertronix. And I can swap the Pertronix along the side of the road quicker than a points/condensor change out. I've been running Pertronix in my 49', my 70 GMC, and a 65 Olds 442 for years. Not a hiccup.

If you don't happen to have a Sun distributor tester handy (I do) An Innova delay flash timing light will turn your engine into a field-expedient tester, and it will also tell you if the vacuum advance is working properly. Definitely start out with a thorough disassembly and cleanup of the distributor, and pay particular attention to the amount of shaft side play where the shaft exits the top of the housing. I install a Torrington needle bearing running against a hardened steel sleeve at the top and check to make sure the cam lobe lift and spacing is exactly correct. It's not uncommon to have a few degrees of error from one distributor cam lobe high point to another, and/or a few thousandths of difference in point gap (dwell) between cylinders. That equates to ignition timing errors from one cylinder to another.
Jerry

The Innova timing light is what I'm using. Currently set 3 degrees ahead of the BB. The Vacuum advance is working. I just replaced as it had a diaphram leak.

The variability in timing because of shafts being out of true, incorrectly machined cam lobes, worn bushings and anything else I've forgotten about, is why I don't try to get the last possible degree of spark lead out of my setup. It's a 261 with an 848 head, which going by published C/R figures should net out to about 9:1. I figure that's about the limit for 87 octane gas at sea level. I run regular gas. Off idle behavior tells me I could use a little more initial advance and the cam I use would indicate that. BUT I don't hear very well and I'm not confident I could hear spark knock. I have burnt a piston on the past engine so that's certainly possible and I don't want to push the issue. So, I static time the ignition using the BB and the U|C mark and the vernier scale on the octane selector. It runs just fine with the points triggering the MSD capacitive discharge ignition.

Even after all these years I still don't have a delay timing light nor a Sun distributor machine so it's largely a seat-of-the-pants affair but I've got away with it for 50 years. That said I doubt that the original poster's problems are related to ignition timing unless maybe the advance mechanism is totally frozen and since he's using a Pertronix dizzy I doubt that's the case.

I don't know if Pertronix units are capacitive discharge (CD) but my MSD is; and an artifact of this design is that the voltage rise time is much faster than a Kettering (battery and coil) system. That effectively means that there is more advance built into the system. So to have the factory total advance, you'd have to set the initial timing back a few degrees, which, If you have a long overlap cam, will cause a lag off idle as you're pulling off a stop sign or whatever.

Is it possible to assemble the advance mechanism backwords so that rather than advancing, the unit retards as RPM goes up? His discription of the symptoms sound like that might be the case. Were there any Delco Remy distributors that rotated the opposite way from Chevrolet? Could parts from such a dizzy, if one exists, have gotten mixed up on the bench or parts box and ended up in his dizzy. Ah, but it's a Pertronix so that possibility is out the window. I have to say that either there's missing information or the heat has gotten to my brain -- 100 again today.

Did this problem exist prior to replacing the vacuum advance canister?

I don’t think anyone would claim that points and condensers are dead-nuts reliable or maintenance free. Nor do I personally think your positive assessment of the Pertronix is flat wrong.

Problem with the current state of affairs is unmanageable variables the Pertronix introduces and that it’s darn near impossible to KNOW how it’s functioning or not. The measurables with points are very well understood and pretty binary in nature. The points are either working or not, they’re either adjusted or not. You either have proper voltage to the coil or you don’t. Much less of a yes/no decision tree with the conversion kit.

Thats where my head would go…simplify and eliminate variables, at least for this diagnostic phase. The same dizzy would have exactly the amount of internal wear and/or centrifugal advance movement if you temporarily swapped in some points. If the symptoms persist…cool, Pertronix wasn’t the deal and they can go right back in and you look elsewhere for issues.

Related note - did you ever share what carb you are running and what you know about it?

Improve my knowledge: what’s so different or better of the Innova timing light?

My knuckles have been well and truly rapped. I'm through commenting on this subject on the open forum. Let's take any further discussion to private messages, or preferably a phone call where we can discuss the pros and cons of ignition system diagnosis without any censorship. I've been building racing-specification distributors for over 50 years.
Jerry

Not much help but I know my timing is advanced far enough the bb (or mark my timing light is picking up) is out of the window.

Note to self, I do need to bring it back a smidge, running 100% gas with no ethanol lowered the octane enough to get a little pinging.

In return for less build up in the fuel system, at least I hope.

By the time you get to the point of hearing a detonation rattle the ignition timing is advanced far enough to start doing engine damage- - - -deflected ring lands on the pistons, piston crown damage, etc. There are much more precise ways to determine the optimum advance curve than using a "calibrated ear"- - - - -especially for us old geezers who can't hear thunder anyway. There have been too many jet engines, deer rifles, and racing engine open exhausts in my past!
Jerry