Advice sought on red six compression test/diagnosis

[Dave_EH]

Hi Guys,

Looking for a few opinions. Currently building a new red 202, but retaining existing induction, head and cam.

Have had a tuning issue for some time now, and hoping to clear up before engine swap.

Engine is modified, triple SU, waggott cam, modified YT open chamber head 47.5cc.

Issue is an inconistent vacuum reading at idle.  Engine isn't 'sweet' at low speed - slight burble, not crisp. Vac reading used to be somewhere around 13In/Hg from memory at 850rpm.

Although I don't drive the car too much, I drove it from Mt Isa to Brisbane at short notice a couple of years ago. At the end of this trip (say 10km from home) I noticed the exhaust note was a bit off down low.

I've worked away for long lengths of time since then, but at various stages have chased inlet leaks, overhauled carbs, checked ignition and checked valve clearance.  The only thing I haven't checked is lifter wear. I have done about 4 thousand miles in this time, and the problem has remained. Minor enough, but not running sweet.

My gearbox is due for an overhaul, so I haven't revved the motor hard in some time. Aye usually bumbles about at 2000rpm.

Today I got the chance to do a compression test. I didn't have a long enough reach on the thread, so was only a rubber boot job on the spark home.

Cylinders 1-4 all came in at 160. 5.&.6 seem to be where there might be trouble.  Plug 5 was a bit dirty, and actually seemed to come in high the first go set 180psi. 6 came in a bit low, with a reading of 115 and 145 - a bit hard to reach without the screw in tester. Wet test seemed to return these cylinders to 160 and 160.

When I restarted after wet test (5 & 6) only, she idled a bit higher and vacuum was steady and up around 15-16 for 15 seconds or so.

With a hose to my ear, carb 3 (cylinder 5/6) has a chatter to it and not a clean wsssssshhhhhhhh like 1 and 2, but airflow reading is same with jet and butterfly settings ok.


Head gasket is 2009 from memory. Say 6000 miles old? Head reco at same age. Block deck unsure of. Cam similar age. Suspect worn cam bearing though.

I guess head removal will show what's really going on, but could minor ring wear really play such a part?

Dave

[john253a]

What's the lob centre or the cam
The lower the number the less manifold vac
Anything below 110 will require a highstall or manual
With a high idle say 11-1400rpm
I'm a big fan of 107-108lc in a hot six, but I also run a vac pump when that low

[Dave_EH]

I would have to check my files for lobe separation. 108 rings a bell.

It's a manual. Duration is only 229 at 50 thou.

I understand the correlation with overlap and vacuum. It's happy idling around 850.

[CraigA]

Do a cylinder balance test.
Set a fast idle - say 1200rpm

Take cyl 1 and 2 leads off - measure rpm drop
then do three and four
then five and six

then do it one a time
Is it more related to a pair of cyl's (that carby) or just to one cyl?

Another thing to check is for a bent dizzy shaft. You can check in the car with a dwell meter or pop the cap off and watch while someone turns it over.
Dried grease on the lobes of the dizzy cam can also make a couple of cylinders weaker than the others (short coil charge cycle)

[john253a]

Never thought of the dizzy as when mine use to do it, it use to spin the cap and brake the plastic locator inside the cap that help it in the correct position

Side note, it may be a good excuse for a complete rebuild, if that's what u want
If you are 226@50 108lc that's pretty close to xu1 specs
2800-6000rpm, you should be running  9.5-10.5:1 comp, extractors/exhaust, triple 175's/465 or slimar, head work, re graphed dizzy/elect, 3.36 or 3.55 diff

[Dave_EH]

Thanks Craig.  Good idea on the cylinder balance test. Will try that. Will inspect dizzy shaft also. Yes I'm wondering about a leak between 5 & 6, but the wt test result seemed to go against that theory.

Vac gauge behaviour: fluctuating from 10In/Hg to 14 at idle, but smooth needle at higher rpm would suggest combustion chamber leak.

John253, yes cam is similar to xu1 but with improved drivability. Have all the necessary mods.

[ardiesse]

Dave,

To check how well the valves are seating you could do the "kero test" with the manifolds off -

As each cylinder in the firing order comes up to its compression stroke, squirt a couple of teaspoons of kerosene into the ports, and turn the motor over by hand.  Badly seating valves will hiss and bubble.

Just a thought.

Rob

[Dave_EH]

Hi guys,

Thank you for all the help.  Much appreciated.  I had a chance to test the compression again with the correct long thread adaptor. Turns out all cylinders are spot on 160-170psi.

At least the head seems ok.  Will try the leak down test next.

Do electronic dizzys have a dwell angle that can be measured?

I will check the lifters for wear next weekend.

Any tricks for picking up an intake leak? Something you can spray around the intake runner whilst running?

Dave

[ardiesse]

Dave,

WD-40 works quite well for picking up intake leaks.

Rob

[Dave_EH]

Great,

Thanks Rob. Will try. Would Ether be too dangerous to try considering manifold heat etc?

Headers are heat wrapped, so pretty safe.

Cheers Dave

[ardiesse]

Dave,

Ether.  Now that takes me back to flying model planes.  Are you thinking of something along the lines of "start-ya-bastard" in a pressure-pak?  That's probably ok, because I think the ether is diluted in a lot of carbon dioxide.  It's very volatile though, so you'd have to use it sparingly so the vapours don't get into the engine's air intake and affect the engine note that way.  Mind you, ether in your own air intake has its own, not entirely unpleasant effects too . . .

I used the WD-40 to diagnose an intake leak on an FX at a show and shine near Wagga a couple of years back (split manifold gasket on the 3-4 intake port).  Once I hit the spot the idle smoothed out instantly.

Rob

[Dave_EH]

Thanks Rob!

I got a spare minute this afternoon, had some Start Ya Bastard; you're right it is a 25% ether. Couldn't find any change in note or rpm sparing around runners.

I'm thinking that it's possible I've made an assumption... And we all know what that can lead to!

When I first got a miss, I had a sticky idle also which pointed to an uncentred jet on an SU. I was away for work for 3 or 4 months, but on a return visit I bought some jets and replaced the troublesome on on carb 3. Fixed the idle, bust still not running right.

About a year later, with only a few drives inbetween, I replaced the other jets. Now, maybe here us where the trouble is... I set the jets down from the bridge exactly where the old ones were: 0.087in. I knew this was right, as this is where it came up trumps with that needle profile on the gas analyser.

Now however, it appears to be responding to a leaner idle mixture. I'm at about 0.050 down from the bridge.

Maybe the only thing wrong is the mixture, and it's been so long between drinks that I've forgotten to reconfirm the basics?

Are there discrepencies between jets? Perhaps some are longer with a chamfer at the top?

To be continued...

P.S John the cam lobe centre is:
Ex 111 deg
In 109 deg

Duration at .050 227 In/Ex
Timings 38/78 symmetrical
Lift .457

Dave