1968 911L timing/running problem

[helidoc]

I have a newly rebuilt 2 liter Weber carburetor motor that I am starting up for the time but am having problems with timing and running. The Webers and distributor have been rebuilt as well. I am using a Pertronix set up and did have to replace an initial bad pickup unit but all is fine on the Sun distributor tester now.

With the static timing at 5 degrees I can get the engine running with a reasonable idle with light coughing intermittently but cannot get the engine above the 3000 RPM range where there is backfiring, coughing and the intake sounding like it is starving for fuel. If I rotate the distributor to about 35 degrees BTDC the engine idles at a much higher RPM, has less backfire and coughing, and will rev to at least 5200 RPM (briefly) and possibly more, but I am reluctant to take it much higher until I am sure everything it correct. I have checked the timing light to make sure it is on #1 spark plug wire and checked the distributor rotor multiple times for static timing and location relative to the distributor cap and drive gear. I have checked the timing marks on the pulley and replaced the spark plug wires twice. I meticulously followed the carburetor set up with various literature and videos. I have triple checked the firing order and wires to the appropriate cylinder. I was thinking maybe I set up the cam timing wrong but discussing with the engine builder where I had the machine work done, he did not think this would be the issue as the problem would be different in not being able to get the engine to run well at any set up.

I am at quite a loss as where to go from here. Anyone else had this problem or any thoughts?

[ed mayo]

You need to verify the tdc mark by bring #1 piston to TDC and then if pulley TDC lines up with case mark.

[helidoc]

I have verified TDC and rechecked valve timing which was 3.22 mm on the left and 3.04 on the right for valve opening at TDC. The manual says for this motor 3.0 to 3.3 mm with .1 mm valve lash. So from a mechanical viewpoint every thing looks ok. I am not sure what to try next. I suppose I could try another coil and maybe put the points back in instead of the Pertronix. Maybe I will try a leak down test as well and go through and check all the other valve lashes again.

[Walter_Middie]

helidoc,

Can you post a picture, with the #1 cyl at TDC on the compression stroke? Take a picture of the distributor with the cap off and the rotor installed, so we can see the position of the distributor relative to the engine. Also, take a picture of the timing marks on the crank pulley so we can see that Z1 lines up with the engine parting line.

When you rotate the distributor to 35 deg BTDC, are you turning the distributor clockwise or counterclockwise from the idle setting?

If your centrifugal advance is not working, you might see something like this. Does the timing change as you rev up the engine?

The fact that the engine starts and runs makes me think you have a very simple problem that will be easy to fix.

[helidoc]

Thanks for your thoughts Walter. Attached are the pictures you requested. I am turning the distributor counter clockwise to advance towares 38 degrees. Centrifugal advance is working fine, both in the engine and on the Sun machine. I have on picture with a mirror so you can see the mark better as someone may have added some additional ones.

[Walter_Middie]

Thanks Helidoc. I was just trying to eliminate problems I have seen before.

You are not using an RPM limiting rotor, and I have seen those timed incorrectly. Your rotor looks to be pointed in the correct location.

I have also seen the wrong crank pulley installed, or the locating key sheered off. Your pulley looks correct and is pointing at Z1. So this also looks good.

I would move on from timing. You mentioned the coil. It would be nice to try a known good one to see if there is an improvement.

[Jeff Higgins]

It might be time to look at your freshly rebuilt Webers. Are they getting adequate fuel delivery and pressure? Are the floats set properly? I'm assuming jetting is at least in the range for a 2.0, even slightly off it wouldn't behave like this until you get it on the road, under load. It just sounds to me like you have exhausted everything ignition related, and like maybe it's "running out of fuel" when reved up a bit.

[Jeff Higgins]

Another thought, being a '68: Does it have a CD box? If so, that might bear looking into.

[helidoc]

Thanks for your thoughts Jeff. I have checked the fuel pressure and that is within spec. Floats are fine. Air flow is ok. I don't think fuel delivery is the problem because it only acts like it is starving with the l the static set timing but if you advance the timing it can get to the higher RPMs and not have that starving behavior. Not saying it couldn't be the carbs but there is nothing to rebuild or set on them at this point. I just checked the valve lashes again and all are at the .1mm mark. I think I will try another coil next after a leak down test, and if that does not work will take out the Pertronix and put the points back in. If that doesn't work I guess I will have to pull the motor and decide whether to re-time it again or send it off to an "expert" with maybe a dyno to play with it. What a bummer.

[ed mayo]

Try it with the vacuum hose removed and plugged.

[Walter_Middie]

Helidoc,

Is this the first time you've done cam timing on one of these?

How do the spark plugs look? Are #1, #2, and #3 OK and #4, #5, and #6 look different?

I am just wondering if you set the left bank correctly at TDC in overlap, and set the right side at TDC on the compression stroke. I've seen that one a few times too. Some repair shop manuals go into great detail on how to set the left side, then simply say to repeat that on the right without telling the reader they have to roll the engine over to get #4 on TDC in overlap.

[helidoc]

Yes this is the first time I have done cam timing for this type of engine so anything is possible. The spark plugs all look the same, a little dark and moist when removed. At first I thought the car was running rich because of that. I closely followed the book by Wayne Dempsey but it has been several months ago. I did look at it again last night and did notice that you set up the timing differently in the later engines with the shorter duration cams. He discussed the shorter duration cams first so I may have used the wrong method (believe my vintage would be the longer duration cam). I did note last night that the onset of #4 valve opening seems to be earlier than I would expect using the dial method but did see elsewhere on this forum that the specs were based on 1 mm opening as the start point (as opposed to the first change on the dial indicator).

[helidoc]

Thanks for your thoughts ed. I have tried it both ways, with and without the vacuum hookup but no difference except the vacuum affect with the retard seen on the timing light. Runs the same either way.

[ed mayo]

Just to be clear, when timing to the overlap method the valve clearance is .1 mm(.004") and you read the toral valve lift at the overlap point(TDC) of the exhaust stroke. When timing by opening and closing points using a degree wheel the valve clearance is set to 1.00mm. You are timing by overlap so you should read the total indicator movement. You may have set the cams way retarded which would cause your problem.

[helidoc]

For the right side I set the valve clearance (lash) to .1mm in the non overlap TDC, then rotated 360 degrees to the overlap point TDC, placing the pin in the sprocket hole (best fit) with the dial gauge reading between the 3.0 and 3.3 mm range but as close as possible to the left side reading (spec for this motor). The tip of the gauge had been on the top of the valve spring retainer in the non overlap position where it was zeroed out. I hope that was the correct way giving total indicator movement.

[Jeff Higgins]

Silly question, but did you read your dial indicator correctly? One full 360 sweep of the needle is 1mm, so your needle would have to make three full turns. The smaller "telltale" needle will tell you how many turns the large needle made.

[ed mayo]

To check timing on the 456 side ,since that is the right side that you started at,,,crank pulley at TDC,,,rotate cam to the both valves closed point, set valve clearance to .1mm. Now position dial indicator and zero both large and small dials, rotate crank one revolution you are now seeing lift at overlap.

[helidoc]

I used a digital dial indicator so don't have to worry about how many times the needle goes around. ��

Originally I started on the left side, got that set then went to the right but I get your point. I did recheck it the same way in the car but not sure the chain is tight enough as I used the Stromski mechanical chain tensioners with the initial set up and relied on the existing tension from the hydraulic tensioners for in car testing. I have enough concerns that I might have to pull the motor an redo everything.

[ed mayo]

Using the tensioner against the chain will not be a problem. I realize it's easier to work on a removed engine, but you can do this in the car.

[Walter_Middie]

but you can do this in the car.

Ed - I was thinking the same thing. On my car (1975), I can check the cam timing in just a few minutes. Removing both upper valve covers is the hardest part, which is easy on these older models. I also use a "Z" block and metric dial indicator (reading in mm), which is a different setup than Helidoc is using - but should still be easy, and could save Helidoc from removing the engine if he doesn't have to. If the cams need adjustment, there should be room, just not as convenient with the engine installed.