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A question for the engine builders.

 
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1970 cortina GT



Joined: 05 Dec 2013
Posts: 303
Location: Saskatoon, Saskatchewan

PostPosted: Fri Aug 10, 2018 7:10 am    Post subject: A question for the engine builders. Reply with quote

I am trying to get my SU powered twin cam running better. I am pretty certain that I have the carbs balanced and the mixture good. I was aiming for around 12 BTDC, according to info I have researched. The only way I see to be able to get it to run half decent, though still under powered, is to have the timing set to around 22 BTDC? I also notice that with my twin cam MK2 I get around 3500 rpm at 60 mph but the MK1 gets around 4500 rpm at 60? Someone suggested maybe the timing chain is one tooth over? Is this something that is easy to verify?
If I had to compare it seems like the MK2 is like a Ferrari and the MK1 a Morris Minor, a little extreme but you get the picture. Thanks for any advice.
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enfoprefect



Joined: 04 Feb 2009
Posts: 172
Location: Oklahoma

PostPosted: Fri Aug 10, 2018 8:30 am    Post subject: Reply with quote

It sounds more like a distributor problem. Possibly the advance weights are not working. Or maybe it is supposed to have a vacuum advance and that is either not connected or not working properly. Cam timing being off a tooth makes it run really bad. I have made that mistake.
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Bob_S



Joined: 27 Nov 2009
Posts: 364
Location: Michigan, USA

PostPosted: Fri Aug 10, 2018 8:35 am    Post subject: Reply with quote

Hi,
I have never heard of a engine running 22 degrees BTDC. Are you sure that the pointer and pulley are correct ones? If something is off there with the pulley & pointer then you cam timing could be off also.

As for the RPMs at 60 MPH thats more related to drive ratio, I would think? Have to checked the car tach against a portable good tach? Are you running points or electronic ignition? That could cause a tach to read incorrectly.

Just some thoughts.
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First car 70 Cortina GT
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Bob_S



Joined: 27 Nov 2009
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Location: Michigan, USA

PostPosted: Fri Aug 10, 2018 8:41 am    Post subject: Reply with quote

enfoprefect may have something there, if the advance weights are stuck in the advanced position you would have to set the timing earlier and then there would't be any advance as the RPM increase. Use a timing light and see if the timing marks move as the RPM increase. if you have vacuum advance, unhook it and plug the line.
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1970 cortina GT



Joined: 05 Dec 2013
Posts: 303
Location: Saskatoon, Saskatchewan

PostPosted: Fri Aug 10, 2018 8:51 am    Post subject: Reply with quote

Thanks for the replies. The timing marks do move when the rpms increase. I am using a Lucas distributor with a vac advance but I have this just open, nowhere to attach it to, could this effect it? Is it better to plug this off? Interestingly enough, the rpms are lower on the gauge the more advanced the timing is set? It starts on the button and actually idles pretty decent but just seems hesitant when trying to get up speed. The timing I'm reading is when the car is idling at around 1000 rpm.
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Bob_S



Joined: 27 Nov 2009
Posts: 364
Location: Michigan, USA

PostPosted: Fri Aug 10, 2018 9:23 am    Post subject: Reply with quote

So you have a distributor with a vacuum advance but the vacuum advance is not connected to a vacuum source. Do you know if the vacuum advance is supposed to be port vacuum (before the carb throttle) or straight manifold vacuum. Since you're running 22 degrees BTDC and it starts OK I would guess that you need manifold vacuum. Is there any place on the manifold that you can tap off of? If yes hook vacuum advance to the manifold and retime the engine with it hooked up. Then see how it runs.

Thats my thoughts
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Brett Wilkie



Joined: 16 Sep 2012
Posts: 1409
Location: Vancouver British Columbia Canada

PostPosted: Fri Aug 10, 2018 11:09 am    Post subject: Reply with quote

Some early twin cams did have a vacuum distributor but did not have it connected, the later distributors had no vacuum canister at all. The normal twin cam used a distributor with 10 degrees centrifugal (20 crank degrees). The static timing should be about 12 to 14 degrees as I recall, the combination of the two should have you around 35 degrees at I think about 3500 RPM.
The cam timing can be tricky as there are 3 timing gears on that engine that could have been mixed up. I remember when I got my Lotus Cortina it ran very sluggish at lower RPMs and burst to life above 4500, when I checked the cam timing I found that the inlet and exhaust cam sprockets had been mixed up as they have the timing marks in different positions. That is one possibility of many so let us know what you find.
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JAN



Joined: 21 Oct 2007
Posts: 154
Location: Wigan, UK

PostPosted: Fri Aug 10, 2018 11:22 am    Post subject: Reply with quote

22 degrees b.t.d.c. suggests it's a diesel! A petrol engine should not be at that initial advance so something's wrong somewhere. If rpm reduces as the timing is advanced, it suggests that it is already very over-advanced.

The rpm at any given road speed is entirely dependent on the final drive ratio and wheel / tyre size; it's got nothing to do with the engine's state.

I would first check the timing marks. Place a screwdriver blade down No. 1 cylinder, then slowly turn the engine until the screwdriver is at its maximum height. It there is a 0 degree mark on the case, they should be near enough in line, otherwise the pulley mark should be slightly clockwise of the last mark on the casing.

At this point, either No. 1 or No. 4 valves should be on overlap, it makes no difference which, with both inlet and exhaust valves slightly open. If one or both are fully closed, it suggests that one or both cams are mistimed.

From memory, the Lotus engine used a distributor without a vacuum advance. The one fitted has a vacuum advance so is the wrong one. This will make a difference, but you'd probably need a stop watch to notice it. Blanking off the connection to the vacuum unit will not achieve anything.

Check all the parameters, and if any don't line up - fix it.
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1970 cortina GT



Joined: 05 Dec 2013
Posts: 303
Location: Saskatoon, Saskatchewan

PostPosted: Fri Aug 10, 2018 6:43 pm    Post subject: Reply with quote

Ok so with a screwdriver in number 1 piston my pulley alignment shows 10° with the screwdriver at its highest point. So that means my timing is actually at about 12° ? So whilst I am pleased that the timing makes more sense should I be concerned that the timing doesn’t line up to zero with the screwdriver test?
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JAN



Joined: 21 Oct 2007
Posts: 154
Location: Wigan, UK

PostPosted: Sat Aug 11, 2018 1:02 am    Post subject: Reply with quote

Yes. It shows that the timing is about 12 degrees retarded according to the marks, but that would be constant throughout the range. The question is why this should be.
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enfoprefect



Joined: 04 Feb 2009
Posts: 172
Location: Oklahoma

PostPosted: Sat Aug 11, 2018 7:41 am    Post subject: Reply with quote

The "screwdriver test" is really quite difficult to do with acceptable accuracy. Therefore it should only be used to ensure things are "in the ballpark." The problem is that at TDC, a few degrees of crank movement makes so little difference in the position of the piston.

Also, I don't know the Lotus engines all that well but if they used a vacuum distributor with the vacuum disconnected, you can be sure they recurved the mechanical advance. You really need to have somewhere around 35 deg total advance and you will typically get half that with a normal vacuum distributor disconnected. So set the initial timing where it should be then check the maximum as rpm increases.
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1970 cortina GT



Joined: 05 Dec 2013
Posts: 303
Location: Saskatoon, Saskatchewan

PostPosted: Sat Aug 11, 2018 6:04 pm    Post subject: Reply with quote

So if I remove the rocker cover I should be able to check the alignment marks on the cam sprockets when at tdc , from what I read they should be horizontal and pointing at each other if correct. I see what you mean by ‘ ballpark’ accuracy on the screwdriver test, room for error there. I’ll set it back to 12 ° btdc and check how far it rises. Thanks for the suggestions everyone.
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Brett Wilkie



Joined: 16 Sep 2012
Posts: 1409
Location: Vancouver British Columbia Canada

PostPosted: Sat Aug 11, 2018 9:37 pm    Post subject: Reply with quote

Yes! Horizontal and central between the cams. I can't remember 100% as it has been many years but I seem to recall the sprockets being marked for inlet and exhaust and the actual shafts being interchangeable, don't quote me on that as I will have to go into my old books to confirm that? Look for some sort of markings on the cam sprockets, (in or ex) ??
You will notice when you do the TDC/screwdriver test that there is a bit of a dwell period as the crank turns over TDC so be careful there, your pulley marks might be okay.
Here is more info to clog your head, it might help??
http://www.lotuselan.net/forums/lotus-twincam-f39/low-compression-t39266-15.html
The timing marks.

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1970 cortina GT



Joined: 05 Dec 2013
Posts: 303
Location: Saskatoon, Saskatchewan

PostPosted: Sun Aug 12, 2018 12:11 pm    Post subject: Reply with quote

So it looks like the sprockets and cams are correctly aligned, markers line up and I can just about make out an EX on the exhaust cam sprocket. Number 1 and 4 are slightly open so I think I’m good here, thankfully. I guess I’ll look up the correct valve clearances whilst they are accessible.

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