ignition timing crazy

[Jonny B]

I got my 1974 GMC c1500 with 350 sb and 350 trans,edelbrock 4 barrel.I checked TDC on compression stroke and timing marks lined up correct.Replaced HEI distributor cap and sprung coil to rotor contact with ac delco parts(old contact was burnt out and cap looked poor),truck was running.I decided to check distributor was positioned correctly,No1 at TDC and rotor pointing at at no1 cylinder,all good.Initial timed at 16 degrees BTDC and it runs crap,no power and chugs along.Used timing light to set.Anything less than 16 degrees tdc and it barely moves and backfires bad.If i advance it further it to 20+ degrees it is responsive and best power at 24 degrees btdc BUT if i set it this high it starts pinking and rattling bad(as you would expect).Carb runs manifold vacuum to distributor,new pipe,tried ported vac and it still ran same.Other carb ports are at rear of carb -vacuum to brake booster,front ported vacuum capped and sealed.Front large centre port to rocker cover good.Ive ordered new rotor,cant understand why it cant run at correct initial timing?Any ideas welcome,thanks Jon

 

[Bextreme04]

You are setting it warmed up with the vac advance disconnected and plugged (the side towards the source) right? 16 is WAY too advanced in my experience. That is where I would expect to see it with the vacuum advance connected.

You will probably also need to adjust the carb idle mixture and rpm and then go back to the timing again afterwards.

Next step if you still can't get it going right would be to check for intake leaks and then compression test and leak down test. That will tell you what the mechanical health of the motor is outside of the timing/carb settings.

 

[Jonny B]

Thanks ,yes engine warm and vac plugged.Fitted new plugs and they look good colour.Yes it runs real smooth at 16-20 degrees initial at 750 rpm,at 10 degrees it runs real lumpy and doesnt wana idle.Ill check compression on all cylinders next then,what psi range is ok?carb idle mixture seems good,i have all new fuelsystem too.Does intake manifold leak cause these symptoms?carb has new gaskets and is torqued correctly.Could poor rotor cause problem or maybe distributor is bad,what is best way to check intake manifold leak?cheers,Jon

 

[mtnmankev]

I strongly suggest you check for timing gear/chain wear.
Start with the mark on the harmonic balancer at 0 TDC no. 1 compression stroke.
Remove distributor cap.
Make a mark somehow or take note exactly where the rotor is pointing.
Turn the crankshaft the opposite direction of rotation and see how many degrees it turns before the rotor moves.
A few degrees is acceptable and to be expected on an engine that isn't freshly built.
If the engine has anywhere close to 100,000 miles and the timing chain/gear set has never been replaced, I would strongly suspect that.

 

[mtnmankev]

Also, perform a vacuum gauge test.
Hook the gauge to full intake manifold vacuum, take careful note of the needle's actions and readings.
Check at idle, fast idle (holding at a steady RPM) and compare the two.
I would suggest try it at 16 degrees where you say it runs smoothly, and then try it at about 10 degrees where the timing sounds to me like it's running late.
There are numerous sites with excellent charts and videos of what the different vacuum gauge readings indicate.

 

[Jonny B]

i will try and see if chain is worn as you say,if the slack is causing real timing to retard afew degrees then wouldnt i make up for the retarding by advancing it more?Could it be that previous owner modified engine and increased compression ratio to maybe 10-1 and added a cam causing the need for higher octane fuel?Ill put some 99 octane in and try her.Ill check chain,vacuum and compression etc,she runs great in park at all rpm with real smooth ,clean revving,no pinking/rattle and at fully warm temp,once in drive she starts pinking bad?Thanks for the good ideas and info

 

[75Monza]

So, starting from the beginning...this was an okay running truck and you decided to do some tune-up items or a not running for crap that you were trying to make run good? Bottom line, what was condition before it was touched and how many miles are on the engine (assumed miles). On a well used engine, I've done a dizzy replacement before and got it a tooth off which caused a backfire under load that jumped the chain a tooth...then I was screwed.

 

[mtnmankev]

If the timing chain is stretched, it will cause both cam timing and the distributor to run late, so simply advancing the distributor cannot correct the issue.

Yes, higher compression will require a higher octane fuel, but do you know the history of the engine ?

The distributor timing can be adjusted for less advance to help with the pinging issue, but performance will suffer.

If you do in fact have a high compression engine, you are stuck having to feed it premium grade gas to insure longevity of engine life (pinging can do a LOT of expensive damage) and to enjoy the power and performance it's built to produce.

 

[Jonny B]

was running ok for a daily driver since i bought her ,then after sorting fuel system after it stopped running i decided to check timing and get it running great and fully serviced etc.Found dizzy cap and contact had burnt out and also noticed rotor was pointing at front centre of truck,not no1 cylinder at tdc on compression,so i replaced cap and contact and reset distributor .It is now all reset as per spec.I wonder if PO set it due to chain being off?It sounds like id better go through everything eh ,miles???shes old but no leaks and no smoke,thanks for helping

 

[mtnmankev]

Another thing you will want to verify since the engine does have unknown miles on it is verify the timing mark on the harmonic balancer has not moved.
Being it's a 2 piece balancer, sometimes the rubber does slip from age and throws the timing mark off.
Find a way to get #1 piston to TDC (without damaging anything of course) compression stroke, and verify the mark lines up with 0 on the plate.

 

[Jonny B]

Yes i checked tdc and timing marks,all correct,i will start further process of elimination this week,the secret is to do all the cheap easy stuff first eh,nothing worse than spending big money when its a two dollar ,ten minute fix.Comp,chain and vacuum checks cost nothing,timing checks done,its getting high octane fuel first.i will let you know what i find,lots of posts on this subject ,ive read em all but its still better to ask and understand,i think it also motivates us get stuck in and fix it eh,nothing better than finding the culprit,fixing it and driving it again with help from this great forum,thanks

 

[mtnmankev]

I was VERY fortunate in High School to have had the best auto shop teacher one could want to learn from, and 48 years later, I still work by his teachings.
He drilled automotive theory and the laws of physics into us in a way we would never forget them, and could apply them to just about anything we encounter in life.
His favorite saying was "Test, don't guess".
It costs nothing (or very little) to test suspected problems, and diagnose from there.
Replacing parts just because one thinks it's the culprit is a stupid idea as well as expensive.

There are many wonderful members of this forum with vast amounts of knowledge and experience willing to share what they know and help others.
It's a great forum to belong to and a good place to hang out.

 

[AuroraGirl]

I was VERY fortunate in High School to have had the best auto shop teacher one could want to learn from, and 48 years later, I still work by his teachings.
He drilled automotive theory and the laws of physics into us in a way we would never forget them, and could apply them to just about anything we encounter in life.
His favorite saying was "Test, don't guess".
It costs nothing (or very little) to test suspected problems, and diagnose from there.
Replacing parts just because one thinks it's the culprit is a stupid idea as well as expensive.

There are many wonderful members of this forum with vast amounts of knowledge and experience willing to share what they know and help others.
It's a great forum to belong to and a good place to hang out.

In a similar vain, my crankshaft pulley seal is starting to.,.. uh... push out? Like, its squished out of the valley it sits on in the rear of the pulley. In that case, wouldnt it make sense to just replace the thing since its likely leaking anyway and then my timing mark is 100% for sure proper

 

[mtnmankev]

Taylor - Are you talking about the rubber that bonds the two pieces of the harmonic balancer?
It's normal for it to be squished out, and it does not serve as a seal for pulley.
The pulley does not require a seal.
Or are you talking about the crankshaft seal in the timing cover?
A good pic of the item in question would help too.......

 

[AuroraGirl]

It was on the rear of the crank, here ill go see if i cant grab a pic