Ignition Timing vac/idle

[RRTEC]

I have been battling my truck in regards to power and drive ability.

289 with a Holley 600.

Idles and starts great, was running ok, but was slow. I have been working through this and decided to change all related parts to rule them out. I have done that, just still don't have this thing fully tuned and need some help.

I connected a vacuum gauge and was not happy with the results. The truck does not run well with the initial timing set at 6-8deg advanced. It runs much better at 12deg but still sometimes stalled when punching it.. I was going through some threads where people said to give it as much initial timing as it could handle. So I cranked it up to 24 and the truck hauled but did have spark knock. I pulled it back to 20deg and the truck runs great but the idle is a little high. I could work out the high idle with a carb adjustment.

What are your thoughts? Picture is the vac gauge at 12deg. Shows lateing ignition. @ 20 degrees it pushes up into the green indicator showing "good engine"

[RRTEC]

I'm really starting to suspect that the previous owner clocked the cam incorrectly....

[bruceandersson]

What are the symptoms of it not running right? Does it stumble on acceleration? There could be a lot of things wrong, more information would help a lot. Stumbling can be caused by a bad acceleration pump - doe sit squirt gas when the throttle is opened? It could also be caused by problems with the distributor advance - this can be examined with a timing light. The cam is possible, but would not be high on my list to start with. The old motors/chiltons manuals all have a good trouble shooting section that takes you though a logical process to find the problem.

[RRTEC]

Symptoms:

Feels low on power, very sluggish to accelerate
Timing must be over advanced to run well
Vacuum shows low on gauge
Stumbles sometimes on acceleration mostly in reverse.
Has backfired through carb several times
Idle drops very low when put in gear

Replaced:

Fuel pump, lines, and filter
Spark plugs, wires and distributor cap
Harmonic balancer/ checked TDC
Coil
Ignition module
Accelerator pump and riser gaskets
Checked screen on carb
All vac lines and PCV valve

Fuel squirts out when throttle applied, idles fantastic other than a very small shake. No leaks or odd noises.

[William-in-St George]

Crane sells a vacuum restriction system that prevents excessive vacuum advance and spark knock. You should be able to set you initial advance to 6-10 with 20 centrifugal and about 4-6 degrees vacuum. What you are doing here is matching the distributors mechanical, vacuum and initial advance to your engine. I'd almost bet you have higher than stock compression. Try this...disconnect the vacuum advance hose and plug it off. Set your initial timing at 6 BTDC and go for a ride. See how the engine performs. I'd filler up with premium for this test. If it is still a slug run a compression test (hot) and see if there is a problem with the engine internally. MOFWIW.

[BarnieTrk]

RRTec,

How old is the fuel you're trying to feed it?

Is the fuel 100% gasoline -or- 90% gasoline/10% ethanol?

My 390 likes to live at 12* advance timing (measured with the vac line to the distributor plugged) at a 900 RPM idle. I feed it 100% gasoline.

BarnieTrk

[charliemccraney]

Any idea about the cam specs?

To a point, it's normal for the idle speed to increase with an increase of initial advance. You simply need to readjust the carburetor.

[BarnieTrk]

Crane sells a vacuum restriction system that prevents excessive vacuum advance and spark knock. You should be able to set you initial advance to 6-10 with 20 centrifugal and about 4-6 degrees vacuum. What you are doing here is matching the distributors mechanical, vacuum and initial advance to your engine. I'd almost bet you have higher than stock compression. Try this...disconnect the vacuum advance hose and plug it off. Set your initial timing at 6 BTDC and go for a ride. See how the engine performs. I'd filler up with premium for this test. If it is still a slug run a compression test (hot) and see if there is a problem with the engine internally. MOFWIW.

RRTec,

Since there was not a 289 factory-installed in your Slick and assuming your 289 has been rebuilt, do you know what the compression ratio is in your 289?

BarnieTrk

[RRTEC]

The motor appears to have been rebuilt (has been painted and has all new gaskets, also doesn't smoke or leak.

I am going to check compression. I have no idea on cam, although it has a mild lope but I wasn't sure if that was because of the poor tune. It definitely like lots of advance.

When I unplugged the vac advance and ran it at 6-8degrees it was so slow I thought the parking brake was stuck. It also hesitated and stalled... 12 degrees is "ok" up close to 20 degrees the thing screams.

[RRTEC]

The fuel is new, and I have a clear filter the fuel is ethanol mix right now. I plan to run only pure from here forward.

[William-in-St George]

Have you verified that the timing mark on the vibration damper has not slipped? 20 initial is a lot. Pull the valve cover and run no 1 up to TDC on the compression stroke and then see where the timing mark is on the damper.

[Toyz]

Same old answer! Check your centrifugal advance. It sounds as if it may not be working; thus all you are getting is base timing.
Paul

[RRTEC]

I have verified TDC and the damper is new.

Anyone have thoughts on the HEI conversion? If I swap distributors might be a good upgrade?

[Toyz]

Other than being burt-ugly, IMO, the HEI was a good system in the '70's IF you weren't concerned about performance. The days of the cheap knock-offs of a flawed design have not resulted in any improvement. There was a reason why GM dumped them in favor of such questionable designs as the Opti(no)-spark! The Duraspark is a superior system. BTW, I taught courses in both in the seventies.
Just my opinion again, but I would consider FIXING the problem rather than dumping more new parts at it! I still have not seen an answer to the advance question. It is easily checked as well as easily repaired. If you have in fact checked it; then tell us exactly what was found so we may save you some effort and money. Or, hell, throw a new engine at it; that might solve your otherwise easily diagnosed problem!
For the benefit of anyone else who has a Duraspark and has a spark knock issue, or would just like to be able to run considerably less expensive fuels, there is a much less expensive and more helpful fix for that without the expense and questionable results of a vacuum limiting kit. If for some reason one wanted to do that, Ford has made it extremely easy to do just that at much less expense! And yes, a judicially applied hammer will do the trick at the cost of future adjustibility, LOL!
Paul

[RRTEC]

I am working my way through the process. This is my first "classic". My previous cars have all been tune by laptop via wolf 3D, haltech, mega squirt etc or my last hotrod LS project was on the atomic LS. Back to basics here for sure, but still getting the hang of it.

I plugged the vacuum advance and revved the engine to 3k and set the total timing to 36 deg. Which had me close to 12deg.. Mechanical Seemed to function fine. Truck was just a dog.

[Toyz]

`Your original query on this post was "what are your thoughts?" Here is a continuation of mine. Not knowing any of the specifics of your setup, I may be far from accurate! Too much carburetion; too much timing advance, if you managed to accurately set total advance at 36 degrees. Not certain as to how you went about this, unless the new balancer is equipped with a degree wheel! I don't know if you have a single diaphragm distributor,or a dual. All these factors and many more can influence diagnosis. Some modules were set up to retard timing under certain conditions. Lots of variables there; only major difference is that you don't get the benefit of the use of a laptop to tell you what is going on. As you said, get back to the basics! The old "modern" adage applies to the "classics" as well, GIGO! What makes a computer car run well is absolutely no different than an old school setup. Check all functions, correct and tune!
Paul

[RRTEC]

Thank you for your insight. I honestly don't know the answers to the dual diaphram question. There is much more I don't know about this setup than I do know which is why I have been prone to replacing parts so I know my baseline.

My balancer does have degree marks all the way up to 40 btdc.

I put a buddy in the truck and had him put the truck in gear under load and I checked timing and carb set up, and vac under load too. Under partial throttle the vac needle moves into the green "normal engine" segment of the dial. The timing goes past 40deg on the dial. My distributor vacuum advance is "adjustable" by my best guess via the diaphram design although I am still searching how.

I agree that my carb may be too big, but there are lots of people running a 600 on the 289. I have considered jetting down and putting a lower power valve in.

[Toyz]

OK, that pretty well rules out advance issues whether from module or distributor itself. Vacuum readings would seem to eliminate exhaust restriction issues. I really don't see a cam timing issue, but that can also be verified. A dual diaphragm distributor simply has two chambers to provide advance AND retard with separate vacuum lines. The adjustable advance indicates you have the advance-only setup. The vacuum line should come from ported vacuum ( above throttle blades).
There still may be an ignition issue, but it sounds as if baselines are correct. Plug condition may give a clue as to fuel mixture roblems.
Paul

[charliemccraney]

Many of the symptoms aren't out of line for a mild cam that is used in conjunction with a stock tune. That will also explain why it runs well with more advance. The vacuum you observe is to be expected with that, as well. And stumbling, particularly at lower rpm, is also expected as the cam gets bigger because they simply don't function as well until a higher rpm. You didn't say if the stumbling is at all rpms or only a particular range.

It might be worthwhile to pull the valve covers and measure valve lift. You can also use some masking tape to mark the damper every 5 degrees or so, to get a good approximation of the duration. Those together will give you a good idea whether it is a stock or performance grind.

The backfire and significant drop in RPM in gear probably indicates another issue.

Regarding the carb, a vacuum secondary 600 will most likely work ok but if it is mechanical secondaries, maybe not. Without knowing all of the details about the engine, it is hard to say either way.

[RRTEC]

The truck does really well at the upper end of the rpm range (over 2500 rpm) no stumbles or hicups. The vac advance is on the ported nipple.

The Holley is a next gen with vacuum advance and I verified that it operates as well.

I think I am going to keep tweaking the timing until it spark knocks and then back off a degree at a time until it stops. Basically tune by the seat dyno. Untill I decide to swap the cam for a low end torque cam. I don't drive very fast (MPH) but love a little get up and go..