Thread: Symptoms of incorrect cam timing
19-01-2010, 02:17 PM #1
Symptoms of incorrect cam timing
Hi guys are gals,
can you tell me the symptoms of incorrect cam timing?
- advanced cam timing = will move the torque band down as comp is increased at lower RPM as the exhaust valve is closed sooner (in relation to rotation of the crank)
- retarded cam timing = will move the HP higher in the power band as the enigne is not working against compression as the exhaust valve has closed later (loosing charge)
But what are the detremental effects of advanced or retarded cam timing?? - ie will an engine with retarded cam timing be harder to start? Foul the plugs? If retarded cam timing closes the inlet valve sooner will the CFM requirement be reduced because the cylinders are not filling the chambers (as the valve is being shut prematurely in relation to the position of the crank/pistons)?
Last edited by Bill355; 19-01-2010 at 02:27 PM.Some days you are the dog . . . some days, you are the tree.
19-01-2010, 02:31 PM #2
I think you can see where I am going here . . . .
My symptoms are that the engine wants to make power outside of the advertised cam powerband - the engine tends to be a little hard to start - power is down - it appears to be fouling plugs - 750 carb is jetted down as if the engine is massively overcarbed . . . . something is not right and I reckon it's a duck - it sure quacks and waddles like one!
Gonna take it apart this weekend to dial in the cam but thought I would ask you guys first . . . .Some days you are the dog . . . some days, you are the tree.
19-01-2010, 04:01 PM #3
if an engine is overcarbed u need bigger jets as the metering signal is lost. my 750 annular is 1 number off stock jetting.mine has cam advanced 4-6 deg and is fine.although to far retarded can be soggy down low and not real responsiveI love Prius drivers cause I need their share of the fuel supply!
19-01-2010, 05:12 PM #4
I you move your cam timing you will have the effect of the power band lower or higher in the rev range, BUT it is not necessarilly going to just be ok. i broke a cam pin on my old engine and when we reset the timing the degree wheel moved and it was retarded 4 degrees.
we lost power top end, torque, responsiveness and ran in general crappy
it did have a small sweet spot around 6 but no worth writing home about
The same thing can happen the other way, except it would be really torquey but all over before the rest of the combination could get moving.
Carby could be an issue, as i said in another thread, a 650 would probably take the lazyness out of the engine and bring up the wind speed in the carby and hence a bigger and more positive pulse to meter the air fuel mixture.
Evl253 is correct if you have made the wrong choice of carby to big you need to not lean it out with smaller jets, but you are masking a problem that wont go away.
without creating a arguement i still find it hard to beleive that a 750 annular is working correctly on a 253. when im sure a 650 dp or vacuum sec will give it more torque and power, but only my opinion.
ISOThe more power you make the more you want!!!!!!!!!!!
19-01-2010, 05:13 PM #5
On a another note maybe a little more initial advance might bring it to life
what comp and timing are you running
initial and totalThe more power you make the more you want!!!!!!!!!!!
19-01-2010, 05:41 PM #6
Bills initial and total is now 14-27 was 6-27
He told me that it runs alot better down low but still seems to run a little off he mention!
Bill have you adjusted your lash settings yet?
19-01-2010, 06:19 PM #7
Advancing a cam will DECREASE the Piston to Valve clearance on the INTAKE valve, and of course INCREASE the clearance on the exhaust valve too (Retarding will do the opposite )... If you go too far, be mindful of that, if you didnt have too much clearance to play with in the first place ..
Going 4-6 degrees from where you first measured could result in clearances changing by about 30-40thou ...
19-01-2010, 06:59 PM #8
- Join Date
- Sep 2009
- Inala by the sea
Totally agree with Paulzig. If running FT pistons, be careful. (nett lift dependant obviously)
I'm only assuming here, but I'm guessing your running an IWIS chain? If the cam timing was dialled to spec, then I would imagine your cam timing is about a day late by now. But, like I say, only guessing. This weekend will tell.
Anyhow, the reason for the post is, if you require a positive stop to determine TDC with the heads on, then call me and I can express post one down to you. Just post it back when your done.
0418 194 647
19-01-2010, 09:46 PM #9
20-01-2010, 09:59 AM #10
Man that is frustrating!!! I was just finnishing my typed comments and my computer crashed/updated and I lost it all - bloody computers!
I have no idea what is happening with my jetting then!
The engine was run in and tuned on an engine dyno. Primary jets were taken from 75 stock to 68 (!) and secondary jets from 80 stock to 71 (!!).
For sh*ts and giggles, yesterday when I got home from work, to see wht would happen I put some 73's in the primary and 78's in the secondary.
Now you would think haven gone from 68 to 73 and 71 to 78 I would be followed by a black cloud of smoke - but nup, she felt ok to drive, perhaps even a little more torquie and when I pulled the plugs they looked ok (actually they looked really good!)
Once I have sorted the cam timing I will put her on a rolling dyno.
I suspect the builder has no dialled in the cam at all and the cam is retarded.
Once I have the cam timing sorted I will borrow my Cuz' 650 hp street and see how she feels with that bolted on.
If I find that the cam timing is currently retarded, my intention is only to move the cam timing to where the spec sheet recommends (and return my lash from .30 to .26)
If the cam timing is spot on (which I doubt) I only intend on moving it 2 degrees advanced (and set lash to .26)
I reckon that is the case - I'll let you know.
Thank you very much for the offer to borrow your positive stop - appreciated. Fortunately I still have one kicking around the shed that I bought when I dialled in the 360's cam.
Thanks again mate - nice of you to offer.
I intend on making the engine run as well as I can with what is in there atm - then if I still want more I will decide what to do.
Don't get me wrong, the engine performs well, very well, but I still reckon there is more in her. And as long as there is, I will be playing with her to find it.
So far the following matters have been addressed:-
- the results from the first dyno run where terrible - gave the engine back to the builder and told them to fix up the heads - had them reported to 220cfm = 450hp
- needed to replace the valve stem seals while the engine was bolted to the dyno as the ones installed by the builder either slipped off or could be removed using fingers!
- Seated spring pressure was only 100lb resulting in valve bounce on the dyno - shimmed the springs to bring pressure upto 135lb
- when the engine left the engine dyno workship it was fitted with a 40 squirter - went down to a 35 - it is now running well with a 31.
- dizzy cap was stuffed
- one lead was stuffed
- plugs gapped to 60 thou - was dynoed this way! Now reduced to .45
- Carb jetting was 68/71 - currently 73/78 (still not sure what is going on here!? - Time on a rolling dyno will tell what jets are required
The lash is still currently set a .30 - it should be .26. The engine made more torque/power by 6,500 and by increasing the lash.
As I said, the engine runs well but I just feel it could run that bit better.
If after she is tuned properly I am still not happy with how the engine feels, I may sell the heads and port matched TP DP manifold (great for a 308 or 331 stroker), and step the engine up and fit ported -3's and port match another TP DP manifold. All about the $$$$.
Last edited by Bill355; 20-01-2010 at 10:52 AM.Some days you are the dog . . . some days, you are the tree.
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