Timing Problems

[jeffremj]

Having a further look at your photo, it seems that the lower part of the chain has been pushed inwards rather than allowed to go directly to the crankshaft sprocket - take a closer look as it may be that 148115 is fitted incorrectly.

[nick]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"> It appears to me that either head gasket is extra thick, cam has a different timing mark on the shaft or something else that I haven't thought of yet.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

I don't see how a thicker head gasket would make any difference. Assuming that is a stock cam, modified or not, with the mark in the correct position, the only way to have the timing mark off is if the chain was installed incorrectly and the sprocket is off one or two teeth. Take out the #1 plug. Insert long screwdriver in the hole. Crank the engine by hand until the screwdriver is at max height. This is very close to TDC. See where your timing mark is.

[img][IMG]http://i615.photobucket.com/albums/tt234/nickmi/TR7%201975/Yellow.jpg[/img]
nick
'79 TR7 DHC
'76 TR7 FHC

[Ianc]

Nick, I set the crankshaft position with the head removed and the timing cover fitted on the dowels, therefore could see exactly where No 1 piston was. I then fitted the fully assembled head with the chain over the camshaft sprocket. when I tried to fit the timing chain over the crankshaft sprocket it would not engage until the camshaft was rotated clockwise from its timing marks. The chain guides were not fitted at this point. I initially thought that the chain had stretched excessively, but discounted this as it was very tight just getting it over the jack shaft sprocket. I have now ordered a new German timing chain, new cam sprocket and tensioner, but can't see how they will make any difference.
If anyone has any ideas on this, I would greatly appreciate them.

[nick]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"> when I tried to fit the timing chain over the crankshaft sprocket it would not engage until the camshaft was rotated clockwise from its timing marks.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

At this point was the tensioner spring engaged or was it being held back in the cocked position?

[img][IMG]http://i615.photobucket.com/albums/tt234/nickmi/TR7%201975/Yellow.jpg[/img]
nick
'79 TR7 DHC
'76 TR7 FHC

[Stag76]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">I don't see how a thicker head gasket would make any difference.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
A thicker head gasket moves the cylinder head further from the crankshaft, which has the effect of turning the camshaft clockwise to compensate, which advances the valve timing. If a head is skimmed below its limit (112.4mm or 4.427 inches) it retards the valve timing. The thicker head gaskets are used to correct this.

TR7 Convertible
Sprint Motor
MegaSquirt EFI

[Ianc]

Nick,
The tensioner was not fitted.
As the engine is a complete new build I am going to take the whole thing apart and check everything including head dimensions.
Does anyone know how to check camshaft position without using the marks on the shaft or the bearing carrier.

[Stag76]

You can do it by determining TDC, attaching a Timing Disc to the CrankShaft, and checking the timing against the specs for the cam. Some specs. are based on the timing @ .050" lift, others on the centre of the highest point of the cam lobe.

You can measure the thickness of the head fairly accurately without removing it...use the depth gauge of a vernier caliper, measuring from the lowest point of the semi-circular cutout to the bottom of the head.

Are you sure the tensioner Part No 148115 that JefferyMJ mentioned is not touching the chain. The run from the crankshaft to the cam sprocket must be straight. If the chain is running on the tensioner and that is increasing the length of the chain slightly, it will advance the valve timing. I don't really know what the guide is there for...maybe to protect the timing case if (when) the chain breaks.

You could turn the crankshaft clockwise to TDC, then measure the position of the camshaft mark. If it's 4 Degrees advanced (for example), remove the sprocket and drill another set of holes that allow for the difference. This is easy to do using a rotary table to locate the existing hole as a reference point, then turning the wheel through 94 degrees, (making the adjustment for the 4 degrees retard), drilling a new hole, turning through 180 degrees, and drilling the other hole. I drilled 2 additional sets of holes in my Sprint cam sprocket at 2.5 degrees before and 2.5 degrees after the standard drilling. I've tried it on all 3 settings, and to be honest, can't really tell the difference.

If it's advanced, you can get away with elongating the existing holes to retard the cam slightly, as the rotational load on the stud is on the correct side, but you cannot do this to advance it if it is retarded. On a Sprint cam, the stud holes are 49.5mm apart, which translates to elongating by .431968...mm per degree, or .017".

TR7 Convertible
Sprint Motor
MegaSquirt EFI

[Stag76]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">With the cam sprocket lined up with the mark on the bearing cap, the timing mark on the crankshaft pulley points to 4 degrees before TDC when the timing chain is fitted.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

4 Degrees at the crankshaft = 2 Degrees at the camshaft. Ideally, you aim for <1 degree variance, but most TR7's, with worn sprockets and stretched chains would be out by at least 2 degrees.

TR7 Convertible
Sprint Motor
MegaSquirt EFI

[Ianc]

Stag 76
Thank you for all the information.
I am almost tempted to fit the engine "as is", as you say 2 degrees is not too far out, and as the chain wears it can only get better, but while the engine is on a jig, I think it deserves looking at again just to see if I can find anything obviously wrong.
Re- drilling the cam sprocket is definately an easy option.

[Ianc]

Just for information.
Today I checked TDC by putting a DTI on top ov No 1 piston,(head removed). After checking several times it appears that the timing mark on the crank pulley shows 1.5 - 2 degrees BTDC in relation to the timing marker mounted on the timing cover.
I fitted a new cam sprocket and German timing chain, the new sprocket had slightly larger mounting holes than the original, allowing me to gain 0.5 - 1 degree at the crank.
I gained possibly another 0.5 degrees by adjusting the timing marker as far as it would go on its clearance holes.
Finally I elongated the holes in the straight chain guide so that it was just touching the timing chain.
To sum up, after all af the above minor adjustment the crank pulley shows at approx 1 degree BTDC when the camshaft is aligned with the mark on the cam bearing cap.
Hope this is of some use to others.
One other thing that may be a factor in all of this is the head thickness measured 112.3mm, and the thick head gasket measured 1.6mm, therefore the distance between the crank and cam centreline is + 0.4mm from standard if I have got my calculations right.
Thanks to al for your contributions.

[Stag76]

That should be perfect

It's hard to judge when the camshaft marker is exactly lined up, so the 1 degree variance could be there, but is most likely due to the head being barely under the minimum thickness.

TR7 Convertible
Sprint Motor
MegaSquirt EFI