Plug gap for 907?

[Ray Brandes]

We are working on a 907 out of a Jensen-Healey.
I can't find the points or spark plug gap anywhere.
Please help! Thank you for your consideration.
Regards, Ray
Panama City Beach, FL

 

[LarryK]

I looked this up in a 80s tune book. 68-70 Lotus Twin Cam, Dwell 57-63, Gap .015, Plug gap .025, Timing 12 deg BTDC. Looks typical of the British 4 cyl points and plugs. Hope this gives you a target to get or a starting point. Maybe a Jensen owner can send a picture of the owner manual page or something more accurate. I assume this is for carb engine as fuel injection has different timing, which I cannot find, except on Jenson Healy Forum ( sorry Basil).

 

[Ray Brandes]

Thank you LarryK. This gives me a starting point. It is a Stromberg carb engine.

Regards, Ray

 

[LarryK]

Carbs done yet?

 

[Esprit2]

I realize this is a little late to be helpful, but file it away for next time.

0.014" - 0.016" = Points gap for a Lotus or Jensen-Healey 907
(0.36 - 0.41 mm)

Champion N7Y Spark Plugs
0.025" (0.64 mm) = Gap for Stromberg carb'd Federal engines.
0.023" (0.58 mm) = Gap for Dellorto carb'd Euro engines.

*~*~*~*~*~*
Ignition Timing:
Distributor Mechanical Advance:
. . . . . . . . . . . . Stromberg . . . . Dellorto
Below. 1000 . . . No Advance . . . No Advance
. . . . . 1200 . . . . 2.0°. . . . . . . . . 2.0°
. . . . . 1400 . . . . 4.4°. . . . . . . . . 4.4°
. . . . . 1600 . . . . 6.8°. . . . . . . . . 6.8°
. . . . . 1800 . . . . 9.0°. . . . . . . . . 9.0°
. . . . . 2000 . . . 11.4°. . . . . . . . 11.4°
. . . . . 2200 . . . 13.6°. . . . . . . . 13.6°
. . . . . 2400 . . . 16.0°. . . . . . . . 16.0°
. . . . . 2500 . . . 17.0°. . . . . . . . 17.0°

Static Advan . . . 10.0° BTDC . . . 12° BTDC
Vac RETARD. . . . 5°-7° BTDC . . . None, all mechanical
Total Idle Adv. . . 1°-3° BTDC . . . 12° BTDC

There is a bias spring between the distributor shaft and the auxiliary shaft. If you just loosen the distributor clamp, the spring will pop the distributor out of the housing. Apply inward pressure with one hand, while loosening the clamp with the other. Maintain the inward pressure until after the timing is adjusted, and the clamp is re-tightened.

*~*~*~*~*~*
The Stromberg carb'd 907 is tuned for emissions, and it's performance is weak. If the engine doesn't have to pass an annual emissions inspection, then re-tune it a bit. Early Strombergs used adjustable jets, later ones used adjustable needles. Figure out what you're working with, then adjust accordingly to set the mixture for best running instead of low emissions. Then set the static ignition timing to 12° BTDC, and disconnect & plug the vacuum line to the distributor's vacuum capsule.

Time both cams to 110 MOP. Early J-H 907s used 115 MOP on both cams (emissions... cost 6-8 Hp). Later Federal J-H 907s used 100 Intake / 110 Exhaust MOP. It's 'that' cam timing, plus really lame ignition timing that earned the 907 the name, "Torqueless Wonder". It's a great little engine, it's the Federal emissions tuning that sucked.

*~*~*~*~*~*
BLACK OEM timing belt TENSION (Gates T104 belt):
Don't use the tension spec given in the Jensen-Healey Workshop Manual. It's TOO LOW.

Using a Burroughs BT-33-86J, or BT-33-73F tension gauge,
98-100 for a NEW belt being tensioned for the first time, to allow for initial stretch.
95 . . . .for a Used belt being re-tensioned during a routine service.

Using a Krikit KR-1 gauge, reading the POUNDS scale:
53 - 55 for a NEW belt being tensioned for the first time, to allow for initial stretch.
52 . . . .for a Used belt being re-tensioned during a routine service.

Replace the BLACK timing belt every 24,000 miles or 24 months, whichever comes first. Most Jensen-Healeys don't pile on the mileage fast enough to hit the 24k mile limit first, so just plan on every two years.

Replace the tensioner bearings with every other BLACK belt, or at 48k miles.

Gates Racing BLUE HNBR timing belt TENSION (Gates T104RB):
83-87 on a Burroughs BT-33-86J, or BT-33-73F tension gauge,
34-35 on a Krikit KR-1 gauge, reading the POUNDS scale.
The Blue HNBR rubber belts are very stable, and essentially do not stretch. Continue to periodically check the tension during routine servicing as called for in the manual, but the Blue belt's tension will rarely require re-adjustment.

Replace the BLUE timing belt every 7 years. That's conservative, but you're playing with an interference engine that is expensive to rebuild. The Gates T104RB BLUE belt costs more, but requires less maintenance, and lasts longer.

The tensioner bearings need to be replaced every ~48k miles, so there's really no point in trying to stretch the BLUE belt's change interval any longer. The belt has to come off in order to change the tensioner bearings anyway, so 'why' re-install an old belt?

Regards,
Tim Engel
a Lotus fan