1984 5.7L thunderbolt 4 timing chain question

[tfret]

I've spent a lot of time troubleshooting a bad hesitation, low engine vacuum, low WOT, and some backfire problem. Engine has new heads, new ignition sensor, and fully tuned Qjet. A friend suggested it could be a timing issue. I have the timing set right at 8* BTDC as it is supposed to be. However, for the heck of it I advanced the timing 3 degrees. Wow, what a difference! All the issues pretty much went away. I am assuming this is a timing chain issue(?). I check for chain slack by turning the crank back and forth while watching the rotor button and timing marks. I have 6 degrees of slack in the chain. I assume this is my problem(?). My questions are:1. Can I change the chain with the engine in the boat (please say yes). I don't want to drop the oil pan if at all avoidable. 2. Can I do this without any special tools? The manual mentions a special tool to remove the harmonic stabilizer which I don't have. 3. Why not go back with timing gears rather than chain? It seems like gears would last much longer than a chain. Please tell me if I'm off base here and should be looking at something else.

 

[achris]

Re: 1984 5.7L thunderbolt 4 timing chain question

1. You need to remove the timing cover to replace the chain. You can only remove the timing cover with the oil pan off....
2. Harmonic balancer removal tool is very cheap, or just hire it from an auto shop.
3. Chains spin the cam shaft in the same direction as the crank, gears would turn it the opposite way. On a V engine, balance issues come into play when you spin the cam in the opposite direction, also chains are generally quieter than gears.

As the spec for chain allows for up to 3/4" of 'slack', I suspect you probably don't need to change the chain.

Check that you have the right ignition module, and that it's advancing the timing as per spec... (timing curves are in the book)...

Chris.......

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

Its definitely good to know that I probably don't need a new chain. On the Thunderbolt 4 I think you just set the time at 8*BTDC at idle and that's it. Note; I did this with engine in neutral. I read somewhere that maybe I should do this with engine in gear(?). I have a timing light but don't know how to check the timing curve. Can you tell me how to do this?

 

[Scott Danforth]

Re: 1984 5.7L thunderbolt 4 timing chain question

I would replace the timing chain with a nice double roller setup.

summitracing.com will have them for about $30

also, you are checking the advance curve. If I recall, the advance is 22 degrees at 3000 RPM (however look up the exact spec). so at 3000 RPM, you should show 30 degrees total advance (8 base plus 22 advance)

 

[Don S]

Re: 1984 5.7L thunderbolt 4 timing chain question

6? isn't much movement at all. Don't recall ever seeing a marine engine wear out a timing chain. They just don't get that much use.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

I didn't think the grounding jumper was needed when setting base time on TB4. Is this correct? Also the manual says set base time in neutral, but I keep reading elsewhere to have it in gear. Which is it? Perhaps I just didn't get the timing set correctly like I thought.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

Well if my chain is good, and the time is set right, that puts me back to square one. Why does advancing the timing a few degrees make such an improvement in performance?

 

[04fxdwgi]

Re: 1984 5.7L thunderbolt 4 timing chain question

3. Chains spin the cam shaft in the same direction as the crank, gears would turn it the opposite way. On a V engine, balance issues come into play when you spin the cam in the opposite direction, also chains are generally quieter than gears.

Actually, a gear drive set up for a SBC has an idler gear (or 2 on some) on it so that it does spin the cam in correct direction.

Also, normally, a timing chain usually gives up the ghost when the engine is turned off. A timing chain jumping timing is actually pretty uncommon, as they usually break/strip off the plastic liner on the aluminum cam gear, leving not much for teeth. Most performance engines don't normally use the "silent" gear set (plastic lined aluminum cam gear), but a steel cam gear instead. I can not speak specifically of your gear setup. Sooooo, some slop in an older timing chain is not uncommon, as I have seen them so sloppy that they actually eat into the side of the timing chain cover.

As far as the extra 3 deg of advance makes a big improvement, in a ton of cases, that is so, but conceals what the actual problem is. But under load you may get a bunch of pre-ignition pinging, which will damage the engine if allowed to go on.

You need to do some more investigating. If I were you, I would pull the valve cover off over the number 1 cyl and test the cam timing with a degree wheel and dial indicator (or positive piston stop thru the plug hole) on # 1 Cyl to see if your timing mark is at TDC when #1 is at TDC. Only then can you be sure the cam timing / timing marks are correct and then go from there.

Google "Chevy V8 Cam timing set up" and there will be a ton of resources to show you how it's done. Hot rod sites are best ones for that. Or pick up a "Small Block Chevy Hot Rodding" manual and have fun.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

I have confirmed that the #1 cylinder is at TDC when the timing mark indicates 0 degrees. I did this when installing a fresh set of heads this Spring. I had the same problem before changing the heads.

 

[achris]

Re: 1984 5.7L thunderbolt 4 timing chain question

He's not talking about confirming the timing marks. He's talking about confirming the cam timing....

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

He's not talking about confirming the timing marks. He's talking about confirming the cam timing....

I don't know how to do this, can someone point me to a procedure for this? I guess this will tell me if the timing chain is installed correctly or if it has skipped a tooth(?)

 

[Scott Danforth]

Re: 1984 5.7L thunderbolt 4 timing chain question

you have to pull the timing cover and look at the marks, or use a dial indicator off of #1 intake valve if you have the cam card.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

I'll just plan to pull the timing cover to make sure the timing marks align properly. I'll probably just replace the chain since I'm already going to be there. Can someone tell me for sure; does the base time need to be set with the engine in neutral or in gear?

 

[Don S]

Re: 1984 5.7L thunderbolt 4 timing chain question

Neutral is fine for setting timing.

 

[04fxdwgi]

Re: 1984 5.7L thunderbolt 4 timing chain question

He could also have a harmonic ballancer that doesn't have the TDC mark in the correct position. Bad / moved ballancer outer ring would lead to an incorrect TDC initial timing. They do go bad and that's why they sell replacement ballancers.

I would install a double row roller chain if you are going to replace. But I would do a little investigating 1st, before ripping the oil pan and front cover off.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

yes I had already read about the "slipped" harmonic balancer problem. I checked it as best I could by looking into the #1 spark plug hole and rotating the engine with a 1/2" rachet. Not exactly the optimum way to do this I know, but it's the only option I had at the time I think. Anyway, as best I could tell, the timing mark hits zero when the piston stops moving up. I know there is a few degrees of dwell time at TDC as the piston rod transitions, and pulling with a rachet the motion is a bit jerky. But it still looks right to me, or pretty darn close. Someone told me that an "old timers" way of compensating for a worn cam is to advance the timing a bit. I'm not sure that makes sense. The motor has about 900 hours on it but has fresh heads and carb. I may just tweek the timing a degree or two each way between runs and try to find a sweet spot.

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

I want to check the cam timing to see what's going on. My service manual gives me the lope lift specs but that's all. Can anyone point me to the cam angle information for a stock cam?

 

[81 Checkmate]

Re: 1984 5.7L thunderbolt 4 timing chain question

The only way to check the cam timming is to remove the timming/oil pan cover and see if the dots on the gears are lined up!

Could be worn lobe on the cam for that matter.......900 hrs on a 1984 motor.
Did you do a compression test........Could be a tired motor that needs freshend up on the short block.

Is this a new to you boat or did you put the 900 hrs on it?

How does it run now after adjusting the timming?

 

[tfret]

Re: 1984 5.7L thunderbolt 4 timing chain question

Weve had the boat 6 years and put about 25 hours a year on it skiing and tubing. It has always had low vacuum and low WOT. Both symptoms of retarded valve timing. I've been all over the Qjet and all but disabled the secondary air valve, so I know its not a case of quadra bog. The compression isn't bad, around 150 on all 8. With timing set at 8* BTDC on hole shot it would stumble, hesitate, and sometimes back fire slightly. The harder I pushed it the worse it is. With time set at 11* BTDC the hesitation stumble backfire virtually are all gone. The harder I push it the better it does. But occasionally now while running 16mph pulling a knee boarder, around 2200 rpm, I'll get a bad backfire through the carb once or twice a day. That's why I feel the valve timing needs to be checked. It acts like the chain skipped a tooth, but I'm told this is very rare.

 

[NHGuy]

Re: 1984 5.7L thunderbolt 4 timing chain question

Take a look at your TB IV module. If it says V8 -22 on it that means it adds 22 degrees of advance at maximum. A lot of hot rodders and performance boaters will go to 34 degrees on an earlier (not vortec head) small block. If you are at 11 degrees idle and it adds another 22, that's 33 degrees all in.
Timing that is too advanced causes detonation. I think Mercruiser has us start at 8 degrees to take no chances with engine breaking detonation. That does leave some horsepower on the table. I'll probably get crucified for saying this but 33 degrees might be fine.
Boats run cooler than cars which is another helpful thing. A cooler combustion chamber can fire earlier without detonation because the ignition flame front moves slower in a cooler environment.
A difference that argues for a lesser advance is that boats run under heavy load compared to cars. The load produces lots of heat.
Mercruiser says 8 degrees at idle, that should get you to 30 at wide open. I run my timing a little higher than stock, but I never stay at wide open throttle for any more than a few seconds. I just do it long enough to get to full RPM, just a quick run up to top speed and back down. I think if my boat were really heavy and needed to run wide open for extended periods I'd be at a little more conservative timing because of the heat that can build up when running hard. Heat contributes to detonation.
Is that any help? Hope I don't start trouble by going against the book of Mercruiser.
BTW have you gone through a low vacuum troubleshooter? I will be right back with a link...