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Why timing mark floats by and oscillate ?

10K views 22 replies 11 participants last post by  k-star 
#1 ·
I was helping a friend with his 76 chevy pick up with 305 and HEI and when he was setting up the timing, timing mark was just floating by . Engine was sitting for about year but runs now. Has a new distributor with pick up coil and module, cap and rotor is new.
What causes timing mark not stay on one spot ? Mark is jumping back and forth and keeps floating by at the same time. Never seen anything like this.
 
#3 ·
timing

This could get to be a long list of thing that cause that to happen.....

A few thoughts.

I have seen cars with HEI'S do this when the charging system is not working right. I would put a meeter on the battery when the engine is running and make sure you have 13.5 / 14 volts or so....

If the distributor is worn out and the shaft is loose in the body or the pins that hold the weights are worn it will act up....

Could be a bad timing light ......

Worn timing chain.......

Keith
 
#6 ·
Re: timing

k-star said:
This could get to be a long list of thing that cause that to happen.....

A few thoughts.

I have seen cars with HEI'S do this when the charging system is not working right. I would put a meeter on the battery when the engine is running and make sure you have 13.5 / 14 volts or so....


Keith
Battery was 12.5, is that enough ? Will new alternator give more volts than that ? Alternator is about 4 years old.
 
#7 ·
I have never seen low battery voltage cause the timing to float. Either there will be enough voltage to trigger the HEI or there won't. Battery voltage of 12.5 is perfectly normal and indicates a full charge on the battery.

There are only a limited number of factors that can cause timing to not be steady. Given the crankshaft is connected to the camshaft which drives the distributor options for problems are few. Given the distributor is new (and you are sure it is in good condition) and your vac advance was disconnected the only other logical choice would be the timing chain. If it is loose and worn it will cause this symptom.
 
#9 ·
Erratic timing (jumping around) could be chain too loose. The floating around sounds like a bad pick up coil on the timing light. Try turning the pickup lead around and/or move the lead to the # 6 cylinder plug wire. Also clean the clamp and make sure it is closing completely around the plug wire.
 
#10 ·
Olds 69. was that 12.5 volts at the battery with the engine running??? If so there is something wrong with the charging system. If it was without the engine running then it's ok. But you really need to check it with the engine running to tell.....

Rick wi. No disrespect but i have seen problems with the HEI cars when there is a problem with the charging system. I am not saying this is the problem in this case, but i have had it happen to me twice. Both times when i replaced the altenator and got the charging system voltage back up the timing jumping around was corrected...

For some reason the HEI's seam to pull alot of amps to work correct. Even though you might have 12 volts to them. There might be enough draw in the system to still the proper amount of amps for them to work correct.....

Keith
 
#11 ·
Never seen the low voltage issue, but it makes sense. In rush current would take higher amps or longer time to charge the coil if the voltage is low. If the main power wire to the distributor is too small, amperage is limited. Low coil charge could somewhat delay the spark travel I guess........dont know, just thinking out loud.

Chris
 
#17 ·
Re: Re: charging

Olds 69 said:
What do I need to do ? New alternator or something else ?
I would double check the battery connections( clean and tight). make sure the belt is in good shape and adjusted correct. If it still does not work then it's probably the alternator. You can hook your meeter right on the large post of the alternator and see if it's 12.5 there. If it's at 13.5 volts there it could be a break in the wire. But i would guess since the truck was sitting it's the alternator.

Keith
 
#18 ·
The harmonic balancer is made from two parts: an inner and outer piece that are basically vulcanized in rubber. As they get older and oil soaked, the rubber deteriorates letting it wobble, slip etc. You need to look to see if the rubber is missing chunks and use a large screw driver or some pry bar to attempt movement of the otter band (the part that has your timing mark on it). Next, I would make a chalk mark across the front of the balancer in line with the timing mark. Then start the motor and hold the timing strobe on the front of the balancer. If it is bad, you will see separation in the line you scribed. Probably will have to do all this from the underside.

Trees
 
#20 ·
I have never seen the timing bounce around because of low battery/charging system voltage. (Not to say it couldn't happen but probably not)

When i was running my stock car I used HEI GM ignition system stock except for lighter springs in the distributor and disconnected vacuum advance.

The stock car had no alternator I would charge the battery up prior to race night then go race. By the end of the night the battery would be down to about 10 volts. I checked the timming quite often with this set up and never ever had a bouncing timming mark.


Ric
 
#21 ·
You didn't mention the state the engine is running in. Is it idling perfectly or is it loping? Is it stuttering, raising and lowering? If the engine isn't dead smooth the timing light will pick up the problem and move all over the place. Up until a few days ago my vette was doing the same thing. I recently put the engine back together after timing chain adventure and the timing was doing what yours is. A vacuum leak on my engine caused it to run extremely rough and when i put a light on it , it was all over the place. I couldn't get the mark to stay still. After i stopped the vacuum leak and the car was running smooth, the mark stayed in place. Make sure you remove the vacuum advance hose and plug up the end of it with a pencil or something until it's timed. If it's a later model engine be sure to disconnect the EST wire (electronic spark timing) until it is timed.
 
#22 ·
I want to thank everyone for help . Problem was a crack in vacuum hose . When we disconnected it on distributor and sealed advance with A/C tape timing mark was working OK. Also engine was running a bit rough because choke was not adjusted properly and car was hard to start.
As far as low voltage problem, alternator is putting 12 at idle and 13.8 at 1,500rpm on alternator terminal to battery so it looks good too. If this voltage is too low, let me know so we can replace alternator .
 
#23 ·
fixed

Glad to here is fixed,,,,,

You might be ok with the charging system the way it is. It should still have over 12.5 at idle. If you don't have many power accessorys and you don't let it idle much it could work out. I think it's a sign of a up coming problem though ,,, IMO

Keith
 
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