Timing a Weber-based engine

[Steve Toner]

Here is what I am being told regarding setting timing on my engine (289 w/4x44IDF Weber carbs & no vacuum advance):

Weber-based engines like a lot of initial timing, and of course every engine is different so you don't just set it to a fixed reading with a timing light. The suggested method is to advance the initial timing until the engine becomes hard to start when hot, then back off about 3 or 4 degrees from there.

Total timing is again going to be engine-dependent, so 36 degrees, 38 degrees, even 40 degrees may be appropriate depending on the engine. In my case, I have an MSD 8479 distributor that uses an advance stop bushing to set the maximum mechanical advance. The factory-supplied bushings are color-coded, and it comes with the blue one installed, which is supposed to provide 21 degrees of mechanical advance (though I measure 18 with a dial-back timing light for this bushing). Other supplied bushings and their sizes (as measured by me) are:
<ul type="square"> [*]Red (28 degrees): .280 inches [*]Silver (25 degrees): .310 inches [*]Black (18 degrees): .370 inches [/list]
I'm guessing the blue one is .340 inches in diameter, though I have not measured it.

Bushings are .180 inches thick with a 3/16 inch center hole. I have a mini-lathe, so can make up any size bushing I need (and the above diameter measurements can be used as a guide to determine the advance that any given diameter would provide).

The procedure would be to set the initial timing as described above, then adjust the total timing by changing bushings if detonation occurs (NOT to back off initial timing).

The advance curve is set in the normal manner with springs.

Any comments on this procedure? Also, I'm curious if anyone here has played with the MSD 8680 adjustable timing control - this allows the timing to be retarded from the cockpit in the event the ambient temperature or altitude or some bad gas gets you into a detonation condition...

 

[Trevor Booth]

Steve,
Personally I have not noticed any great difference between webers and any other carburettor as far as ignition advance is concerned. The amount of total advance is more a function of combustion chamber shape, compression ratio, plug location and type. The maximum advance is best established on a dyno. This can be done with any dist and then work back from max for the curve you want. Dont forget to deduct the static from the total. ie If you have a max setting of 38 and a static of 8, the dist advance is 30. this is crankshaft degrees . the actual in the distributor will be half ie 15

 

[Steve Toner]

Trevor,

Perhaps the issue is not the carburetors themselves, but running without vacuum advance. In that case, you don'r get the advantage of the timing advance at high-vacuum conditions, so must dial in more initial advance. For instance, what I am fighting right now is a slight stumble at tip-in from light throttle cruising (a high-vacuum condition). The suggestion was that I needed more timing, and focused mostly on how to give maximum initial advance. And yeah, if you know how much total advance you have, you're going to have to reduce the maximum mechanical advance when advancing the initial timing - that's what all the bushing talk was about... Finding the best total advance without a dyno will presumably require some experimentation.

 

[Trevor Booth]

Steve,
The vacuum advance is not related to performance. Its purpose is to advance the timing at light throttle (ie freeway Cruising ) to give an improvement in fuel economy. The amount is usually in the range 3-4 deg. One of the basic fundamentals of an engine, vacuum highestand velocity lowest at throttle closed . Vacuum lowest and velocity highest at open throttle . Fuel is not sucked into the engine, air is, and as the air passes the throttle venturi it creates a negative pressure at the fuel delivery point in the carburettor throat. Fuel is pushed in from the carburettor bowl by the difference in pressure. You can see the fuel delivery tube in a weber, it is located inside the secondary venturi. If you open the throttle whilst looking down the throat you will see fuel gushing forth. I would suggest that your problem is fuel related, a faltering as you open the throttle can be caused by to lean or to rich mixture. With weber carbs the accel pump jets could be to large or the pump bypass jet could be too small giving too rich a mixture as you open the throttle. The throttle venturi size can also affect this ie too large. Tuning weber carbs is not for the fainthearted because there are so many combinations. Your local performance shop or weber dealer should be able to give you a very close setup. The other alternative is to purchase some books on tuning and jetting weber carbs. Jetting these things yourself can be expensive as you need to buy 8 of everything and still not right. As far as your total advance seek some advice from your local performance shop for your engine build. Trying to find the best max advance by trial and error "on the road" can lead to engine problems if you inadvertently run too much advance. I am not familiar with setups on V8 so I cant give you any clues but 30-35 is probably not too far out. Have you done a search on the forum for weber jetting, someone surely has gone through the exercise.

 

[Steve Toner]

I'd like to keep this thread on topic and not have it turn into a tutorial on carburetor operation, or the fine points of Weber jetting, or whatever. My jetting is close. I'm working with one of the major "hot rod" Weber suppliers here in the states, and we're working through the jetting and timing.

I did check my timing, and there was very little initial timing in it (the guys who set it probably believed the pointer was in the right place, but I've got a 302 cover on a 289 so TDC is not where it appears to be). I cranked it up to about 18 degrees initial/36 total and it woke up the engine pretty good. It seems like it can take more initial timing, but before I do put in any more, I'd like to hear if anyone else has any comments on the above procedure and any advice beyond "ask somebody who knows" about getting the total timing correct. OK, we have the suggestion from Trevor to run it on a dyno - are there other methods that can get me there without involving a dyno and that will not result in destroying the engine?

 

[Tim Kay]

Steve,

Good suggestion on keeping this thread alive, I am paying close attention to all the tips on jetting for I too am tickering with 4 48IDF's.

Tim

I got a new shadow. I had to get rid of the
other one -- it wasn't doing what I was
doing. -- Stephen Wright

 

[Trevor Booth]

Steve, the reason for the dissitation on carby fundamentals was that your previous post indicated a lack of understanding given your comments about vacuum advance operation. Given your comment about TDC have you accurately located it. IMHO you are on the wrong track with 18/36 if your TDC mark is accurate.

 

[Steve Toner]

That's interesting. I thought your response about vacuum advance showed an incomplete understanding, but I really didn't want to dwell on it. Vacuum advance does play a significant role at part throttle, and on these engines, you will typically have way more than 2 or 3 degrees of vacuum advance. The stumble I described was in a high-vacuum situation, and when I said "tip-in," what I meant by that was light additional throttle application - I'm running on the progression circuit here, I haven't given it enough throttle to engage the shooters.

I really do appreciate your desire to help, but I think we're operating at completely differnet levels here. Manufacturers will provide numbers that are guidelines, safe numbers that can be used across engines in various states of tune and different environmental conditions. What I'm trying to get at is how do we find an optimum setting for a given engine (which understandably may change over time). And you're really distracting from this goal.

FWIW, my engine builder (who supplied the long block but knew the Weber setup I was going to use) specified 15 degrees of initial timing for my engine. Again, remember that's probably a safe number to be used as a starting point. Whether it wants this much timing because it is not running a vacuum advance or for other reasons is really irrelevant to the discussion - I suggested the vacuum advance theory as one possible reason because it made sense to me.

 

[Ron Earp]

My experiences with tuning Ford engines is give them as much initial timing as possible. This has been shown to be useful time and time again on the track and dyno, regardless of theory.

With the last one I tuned, a 5.0L with AFR heads, it liked an initial timing setting of 18/19 degrees - and it was a correct 19 degrees too - not a guess. Max advance ran to 36 if I remember correctly with it all in by 2500, but my memory isn't so good. We basically gave it advance and did the pulls. I think it would have liked more, but we ran into pinging with any more. You can easily try it out on the road too - advance it til it pings, pull it back.

I know this isn't what you're looking for, but maybe it'll help a bit.

 

[Howard Jones]

Hummmmm. I wasn't aware that low compression,&lt; 10 to 1, SBF's would like that much advance. I set mine,9.5 to 1, to static 10BTDC and total 34BTDC @ 2400 RPMs. It ran so good I just never fooled with it further. You're right about running it up until it pings and back off 1-3 degrees, Been going it that way since high school auto shop class.

Without a dyno, and without a timmed drag strip, you are forced to use a change in speed/ time method to gauge results. This will take some imaginaton. One way is to run the car in top gear at about the middle of the rev curve. Floor it and stopwatch the lenght of time it takes to increase from say 2500 revs to 3500. Longer time=less power. Flat road, same direction, no wind, same time of day, air temp, fuel load, etc. Watch out for cops.

 

[Steve Toner]

Ron,

It's a thought. That's pretty much what an ECU does (on a constant basis). Perhaps a piezo knock sensor driving an LED woudl provide a more reliable knock indicator than ears. Probably need a high-pass filter on the output &amp; a one-shot to widen the pulse enough to see the LED flash...

 

[Steve Toner]

[ QUOTE ]
Watch out for cops.

[/ QUOTE ]

Yeah, that's the hard part, isn't it? /ubbthreads/images/graemlins/smile.gif

 

[Ron Earp]

Some ECUs do that. Before 1994 the Ford EEC-IV system on 5.0s didn't have a knock sensor. So, you did it the old fashion way and gave it as much as you could stand. Low comp or not, they definitely made more power, in general, with a lot of advance. You're running carbs and I assume no knock sensor. However, should you want to add one there are a few stand along systems with an adjustable filter so you can change the knock threshold.

For my system I'm going to rig the MSD-6 Digital Ingnition retard to a switch so I can retard it when I like for bad gas, hot weather, etc.

 

[BenL]

[ QUOTE ]
That's interesting. I thought your response about vacuum advance showed an incomplete understanding, but I really didn't want to dwell on it.

[/ QUOTE ]

Is that a joke? As they say, the essence of humor is timing.

 

[Steve Toner]

Hey, c'mon Ben, I'm trying to make some serious points here /ubbthreads/images/graemlins/tongue.gif

 

[Trevor Booth]

Steve,
IMHO the settings posted by Howard are the more likely scenario. 18/36 or similar indicates an advance curve not suited for your engine setup. If the vacuum advance works as you believe, then why not fit one. If you want to find the OPTIMUM setting for a given engine, trial an error on the road will not necessarily do that. What it will do is provide a reasonable setting. At the same time as you are doing your trial runs check the A/F ratio as you open "tip in" the throttle. FWIW the technical manual published by Edoardo Weber, Bologna, Italy is a good read.
You did ask for an optimal setting ))

 

[Steve Toner]

I think we can do without the pedantry, smileys or not. And the flamebait as well.

The question is not what you think the correct timing is, but what procedure would you use to determine it? Suppose it's 1966, and you've blown an engine in practice at Sebring. You put in a new engine and you need to get it ready for qualifying. What's your procedure? (And yes, I know that you're probably running with the mechanical advance locked out in this particular case, but try to answer the question in the spirit in which it is presented.)

 

[Trevor Booth]

Steve,
Forgive me if you think the reply is pedantic.
Assuming that you have established that TDC is accurate.
Mark crank pulley in 5 deg increments to say 45 deg if not already done.
Set static timing at what you think is appropriate.
With vehicle stationary and using a timing light check max advance figure at the RPM that you want maximum advance to occur. Take the car for a drive and do acceleration runs at varying throttle openings, in varying gears and varying speeds. Flat out is not necessary. The object is to hear or feel any pinging. After those runs either advance or retard the max as above. Do More runs. repeat until you are happy with peformance/ timing. This will give a reasonable max figure. You will then need to check static and adjust till engine starts easily. From the max and the static so obtained set the dist to give the reqd advance, (max-static)
If you can obtain a portable gas analyser I would also observe a/f readings as you do the accel runs.
Sebring 1966-seeing that I started racing in 65 I would not have had to guess.
cheers

 

[Steve Toner]

Thank you, Trevor. I think your procedure will work better than starting with the initial timing as the procedure I had been given did.

 

[Howard Jones]

Steve, What octain rating do you have in Utah for premium. Here in in the PRC (peoples republic of Calif.) we have 91 and it's 10% alcohol. More than likely the worst gas that anybody on this forum is getting where you live. Fred are you reading this. I wouldn't be suprised to find out we are loseing 10% from the gas. My Audi A4 with the boost turned up to 1.1 bar with bigger injectors goes like hell on Nevada gas but spends most of the time on the knock retard in Calif. unless it's raining and cold. I bet it makes 20 more hp on the 93 octain Nevada gas. By the way, I bought some 100 octain race gas and mixed it up to get 95 octain blended mix with the crap we get here and picked up 5mph on the top end. 138 to 143-145. The little bugger goes all the way to the new rev limmit of 7000.

So.... the starting point of 10 degrees btdc static and max at 34/2400rpm is what works for me. I know that many of you will be able to best this with better gas. But I think if you start here the thing will run pretty good and you will avoid starting your tuning program from a way off track posistion. The AF ratio meter would be my first addtional piece of test equip/infromation. 11.5 to 1 would be a good place to start here.