Low manifold pressure!

[OWD724]

Hello, I have a question to ask about my XK120. I feel the car is a bit flat in its running. I tried a manifold pressure gauge on the car by fitting it to the tube out of the manifold for the windscreen washer. The gauge would only read 10" instead of the correct 18" at 750rpm. So I looked for leaks and could not find any ? It would only get to 18" at about 2,500 rpm. If I flicked the throttle it would go to about a high of 20" and low of 8". The engine has 9/1 compression and I checked the compression today as 180 to 190lbs on each cylinder. The timing is set at 10 BTDC and runs an electronic system by 123 of Holland. It has D type cams, 2x2" HD8's with UO needles, jet set about 1/8th inch down, volume screws about 6 turns up to run at 750rpm tickover to keep the engine smooth with the lightened Flywheel that chatters the gearbox at any lower tickover revs. When I start the car from cold with electric choke manual switch, it runs but only at about 500rpm until I give it some throttle to raise it to 1,000rpm which it will hold after about two minutes. I have tried every position of the nut on the electric choke, but it will never run at more than 500rpm at the initial start.
So, does all this mean some lack of suck in the manifold system ???

 

[Deleted member 8987]

Just a WAG, based upon very recent experience.
My MK2 with a 2.4L was.....more than anemic. But having never driven a properly set up 2.4L, I had no idea.
I did know my power brakes were not good, even after rebuild and replacing of the servo booster. Vacuum was low at 15. Or less.
When the clutch lunched, and I pulled the engine, I tore the engine down to fix externally leaking head gasket...and other leaks...and ALWAYS checked where things are first.
The cams were 30 degrees advanced..so much the inlet valves had left marks in the top of the pistons.

Once I did rings, bearings, lapped the valves and set the cams right, the vacum was righty back where it was supposed to be.

If you don't have a cam setting tool, order one.
Pull the cam covers, find TDC, and check them, just to be sure.

MUCH easier on side draught engines than on downdraught 2.4's where you have to get the silly cabs off first...and the oil bath air cleaner which resides directly on top of the exhaust cam cover.

 

[OWD724]

In fact just tried the manifold gauge on my friends XK140 and it gave a manifold pressure of 15", which indicates retarded ignition but by how much I do not know ? I am going to borrow a cam setting flat plate tool, as I am told that I can adjust the cam timing with a vernier gauge. The 10" mine is showing does indicate incorrect valve timing.

 

[Deleted member 8987]

The real key is finding absolute TDC. You may need to probe #1 to find it. On mine, two timing mark sets, the one on the flywheel was so far off, they set the flywheel on wrong, maybe that was the issue with th 30 advanced camshafts.

 

[Deleted member 8987]

Check lower cam chain by plugs out, distributor cap off, rock crank back and forth. If it slaps and a whole lotta movement before the distributor rotor starts to move, gonna need some work.

Using the appropriate tools, set the upper timing chain tension.
Find TDC and mark.
Check both camshaft positions.
Re-assemble, check points dwell or gap.
Set timing.
Adjust carbs.
Road test.

 

[OWD724]

Yes, I am going to do all the suggested checks of tighten top chain, look at bottom chain. But what I have discovered today by talking to camshaft suppliers is that my D type camshaft which has a duration of 270 degrees and an overlap of 60 degrees will give a poor idle as there is just not enough suction at idle, but all the power comes in higher up. It is all to do with the overlap of the cam design ! I have looked up the specs for Rob Beere cams, Piper Cams and Newman cams and they are all suggesting unless I am racing these cams of mine are too aggressive. Basically the D type cam is 30/60
so twice 30 is 60 overlap. I should have a cam of 20/60 being 40 overlap of 25/65 being 50 overlap.
There is no doubt the engine is powerful for a 3.4 at 200bhp, but just not too tractable on the road !!!

 

[dklawson]

Not Jag related but from my experience with Triumphs, many non-factory cams will only develop 10 to 15 in Hg at idle. That seems to be consistent with what you are experiencing and have been told about your situation.

 

[OWD724]

Hello Doug, yes I think you are right, that is the conclusion I have come to. So nothing wrong with engine, just that the guy who built it made it a bit to racy for normal road use in heavy traffic like we have here in U.K. So I am going to change the camshafts for something more tracktable. The question is going to be, not loosing too much power considering I have 9/1 compression, large 2" SU HD8's and a lovely freeflowing stainless steel six branch exhaust system.

By the way I am a great fan of GT6's. I had my first 1967 wedgewood blue mk1 in 1973 when I was 19, then two more mk3's in the 70's and 80's. My last one, was very low mileage 1974 dark red car that I sold to the chairman of the Triumph sports six club in 1985.

 

[LarryK]

Whenever hot cams are used, you loose vacuum. That is why a lot of high horsepower engines for the street have vacuum pumps to maintain a vacuum level at lower speeds. I worked for a shop some time ago that had a V-12 XJ, shop foreman had the big idea for power and the owner OK'd the mod. Well, carbureted cams do not work well on an injected V-12. Lot of work, found it starved for air, figured the air problem, then had to try to modify ECU to accept the increase in air, which involved different injector, on and on and on. Shop closed and to this date I have not heard if the Jag ever ran right without putting in the stock cams.