Crossflow timing

[DavidL]

Hi all,

My car, 1990 Caterham with the 1700 Supersprint crossflow, has developed a bit of a stumble at about 3k rpm. It was running perfectly until I adjusted the valves, so I figured OK, time to reset the carbs (Weber DCOEs) and check the timing. Although I had rebuilt and set the the carbs before, this was my first time attempting to set the timing. I found the timing marks on the front cover and the crank pulley, but at idle my timing light claims that there is about 45 degrees of advance, which is of course silly. So, a couple of questions.

- Is cylinder #1 is the one closest to the front of the car? I was assuming that.

- Could the crank pulley be positioned incorrectly?

- Are there timing marks on the flywheel? I didn't see any obvious way of seeing them if there were.

 

My plan was to follow the Sevens and Elans instructions that came with the car, which says to set the total advance at >3500 rpm (36 degrees) using a timing light with a variable advance feature.

 

Thanks for any advice -David (Menlo Park, California)

[TheDingo8MyBaby]

Since you said the problem started after you adjusted the valves, are you sure you adjusted the valves correctly? There are hot/cold values.

 

As far as the timing goes, it should be around 10 degrees at idle and 36 degrees after 3500rpm. Make sure you've got the correct mark as the pulleys do tend to chip. An easy way to find TDC is to stick an extension down #1 and rotate the crank until you reach the top of the stroke. Have you tried adjusting the timing back to the standard position? It could actually be 45 degrees at idle (though you would expect it wouldn't run that well period).

 

Here's my take on your questions:

 

1 - Yes.

2 - Not likely. I've only seen this happen on cars with a harmonic balancer. More than likely the mark is not correct on the pulley.

3 - I cant remember if the standard crossflow flywheel has a timing mark on the flywheel.

 

3000 rpm is a trouble spot for super sprint crossflows. Do you have an aldon distributor? The advance on the aldon is really late which tends makes the car bog around 3000rpm. Additionally (although it's related to throttle position, not rpm), it could be that the webers are transitioning from the idle to main jets.

Edited February 24, 2012 by TheDingo8MyBaby

[Kitcat]

Do a search for "advance" and amongst the threads will be a few on this.

[DavidL]

Thanks for the help. Re the valves, I had contradictory info on the settings. The factory assembly guide says: "valve clearances (cold), inlet 0.022", exhaust 0.024" ", whereas the assembly guide supplement from Sevens and Elans (came wit the car) specifies the same clearance, but hot. I did the adjustments cold out of convenience. The pulley has matched notches on both rear and front faces, so I think I'm looking at the correct marks. I think I should make sure the timing light is working correctly, find TDC as you suggest to check the markings, and reset the valves hot. Not sure if the distributer is an Aldon.

[bball7754]

Do you have the Kent 234 or 244 cam? If so, the valves are set cold. I had also seen conflicting advice, so emailed Kent. They replied that the valves should be set cold.

 

If you check for TDC you might want to leave the valve cover off so you can be sure you're on the right stroke.

 

Steve

[DavidL]

Just to complete the thread, I fixed the problem by replacing the plugs. Number 1 was oily, seem to remember the previous owner mentioning it did that sometimes. Car just hums now!

 

Thanks for the suggestions.