Nit Picky Tuning Questions to throw out to wiser people

[6PAK72]

So, this weekend offered me enough time to put about 300 miles on the TR6, in 20-mile increments as I fooled with just about every electro-mechanical specification on the engine, trying to get rid of a mis-fire around 4500 rpm under load. Lee suggested it might be fuel supply, and I thought I heard detonation.

The valve lash got adjusted extremely carefully, the entire secondary electrical has been replaced and checked, air filters are new, the fuel filter and pump replaced, carb diaphragms replaced, jets set...you get the idea. One change at a time.

The engine is running MUCH smoother, but here's the issue:

When I retard the distributor timing (Still has a vacuum advance in place) to 34btdc, the idle is terrible, and accelleration from 1,000 rpm at any pedal pressure causes a bog. BUT, there is NO missing (detonation??) above 4200rpm, and the engine revs powerfully to 5500rpm.

BUT, the engine temp indicates hot all the time. A lasar thermometer indicates 190f at the sending unit, 155f at the bottom of the radiator. Not good for the TR.

When I advance the distributor timing to about 44btdc, the engine idles really smoothly, does not bog accellerating, and pulls very strongly to about 4200rpm, where it begins to miss. It misses all the way to 5000rpm, and the power just dies off at 5000, does not pull to 5500. As for temp, the cockpit indicator is at neutral, and the lasar reads 166f at the sending unit, 123f at the exit side of the radiator.

Mfr. spec is 34btdc, though with the 276deg duration cam, and 1.55 roller rockers I know I've moved that off the spec. Pistons are .03 over, deck is dropped .02, spec compression was 9.5:1, with the Carillo pistons I think I'm up around 10.6:1. I know full race engines running 13:1.

Anyone have ideas? Spark plug heat range? Richness?

Give up and drive a Honda?

BTW, Lee and Stephan, I have 5-points installed

Thanks, see you in KaBek,

[John F]

Longer duration cams do funny things to vacuum operated devices. You are probably seeing the bog at 34 btdc because of it and bumping the advance masks it. You may be seeing some detonation with the 44 degree advance. At full throttle the vacuum does nothing. Go back to 34 degrees and remove and plug the vacuum hose and see what happens. Since there was no issues at 5000 rpm, the jets don't seem to be an issue.
I don't know about wiser, but this is just old school.

[breakaway500]

I agree about setting the timing back to 34.
190 F. into the radiator is not really hot for any car. Does your car overheat at this level? What degree thermostat are you running? What pressure cap? Advancing the ignition will make the engine run "cooler" but detonation is a very,very bad thing.

If the engine runs fine at 34,with the exception of a poor idle,I would look for a vacume leak,or possibly an obstructed idle circuit in the carb.

The bog....it's either too rich or too lean. Too bad you don't have access to a wide band air fuel meter. Try putting the choke on just a bit and see if the bog is better or worse.

Sure,you could drive a Honda..but think of all the fun you will miss tinkering with your classic!

[TroyV]

What is a choke?

[DanB]

Mechanical advance probably adds too much.
What is your ignition advance at idle to arrive at 34* BTDC with mechanical advance "all in"?

I'd definitely compromise with the "right" advance up high (presumably ~34 - 36 degrees) and suffer with too little down low.

I filled in the slots in the centrifugal advance mechanism on my 240Z with JB Weld in order to run ~18* advance at idle and still limit it to 35* max (stock setup requires 10* idle advance to arrive at 35 max). You might consider doing something like that with the TR. Fill in the slots at the lower-rpm end, so that if the JBWeld wore down or fell out, you're still limited to ~35* max.

[betelgeuse]

Could be a bad spark plug. It happened to me when I was running with COM in the 90s. At the time I was running a 4 cylinder BMW with side draft webbers. I alway put new plugs in before every event. The motor ran great untill I got it on the track. I went thru those carbs 100 times trying to find the problem. I didn't figure it out untill the next event when I changed the plugs.

Just a thought,

Greg

[6PAK72]

So this just proves that I am the least smart of all the people I know. Thanks all - I have one more weekend to chase the issue.

New plugs about 600 miles ago, but they were new at NHIS and that's where I noticed the issue, though I had never run it at redline for so long a duration, so it could have been other things. The plugs are NGK, for whatever that's worth. Also, I've checked spark from each wire, and sprayed water on the ignition cables at night to check for arcing/cracks - no issues.

The factory spec is 34btdc at 850rpm (idle). When I had it on the dyno the real-time readout was 51btdc at 5500rpm.

Dan, that's Brilliant! Nice idea about filling the slots! The Grp44 race manual (we have a customer who drove for them in the V12 Jag and the Porsche) details eliminating the vacuum advance, going to all centrifigal, and the weight target is 1.2 ounces each with "reduced throw" but the manual never details how....

My assumption has always been that removing the vacuum advance makes it pretty unhappy on the street, right?

As for "overheating", the spec is a 165 degree thermostat and a 3.5 pound cap. That's what I am running, on an aluminum radiator with 2x the volume of coolant, 1.3x the frontal surface, and 2x the depth of the stock iron/copper radiator. I have not actually overheated, even in summer bumper-to-bumper traffic - the indicator (with no numbers) when indicating neutral is a perfect 165f.

When I get the 190f reading, the indicator is 15% up the hot side.

And for the driver who wants to know what a choke is, I bet you have a coffee pot with a self-timer....

[DanB]

Factory spec is 34* at idle? And you were running 51* at 5500 on the dyno?!
Holy crap, that's outrageous! Talk about "slow-burn" chambers

The dyno is the perfect place to find the right amount of advance. FWIW, I gained 18hp by reducing advance from 43* total down to 35* total. It actually made the same power from 34* up to 38*. For L-series Z engines, that's the usual range, I had foolishly just set it to 18* initial advance and let max total go too high.

There shouldn't be any ultimate performance benefit to removing the vac advance. If it works properly, keep it. It's function is quite different from the mechanical advance. Mechanical gives you more advance with rpms. Vacuum gives you more advance at part-throttle cruise. At WOT, there's no vacuum, and the (properly functioning) vac advance doesn't do anything.

[mr2sc]

My old GT6+ (um, 30yrs ago) had a vernier adjustment on the side of the distributor for advance. I seem to recall that this was on a distributor that having obtained from some other 6cyl Triumph application, as being the 'hot setup', along with Pertonic breakerless setup.
At the time, the best advice I got (the car had a hot cam and larger 1.75 carbs) was to advance till I got a slight pink, then back off a wee bit

[DanB]

My old GT6+ (um, 30yrs ago) had a vernier adjustment on the side of the distributor for advance. I seem to recall that this was on a distributor that having obtained from some other 6cyl Triumph application, as being the 'hot setup', along with Pertonic breakerless setup.

I'm using a Pertronix in my Datsun! Works great.

At the time, the best advice I got (the car had a hot cam and larger 1.75 carbs) was to advance till I got a slight pink, then back off a wee bit

I see that recommendation given all the time on the Z-car forums, but IMO it's a piss-poor method. That method put me at 18* static/43* max advance. I didn't get any "pinking" (or pinging for that matter), yet I was advanced way beyond any benefit. Backing it down gained me 6% more power! More advance is NOT always better, even in the absence of detonation/pinking/pinging. Also, insidious high-rpm pinging often goes undetected.

[mr2sc]

yeah...but I got that advice 30yrs ago

I recall that car had an incredibly tall first gear (2.65:1 i think), in an incredibly fragile gearbox. Getting it rolling, with acam, light flywheel and Paddy Hopkirk sport clutch assembly, always involved a flurry of revs, wheelspin...followed by pinking/lugging when it hooked up.

[DanB]

I love the *idea* of the GT6
Small, lightweight, coupe/hatch

Perfect candidate for a rotary swap

[mr2sc]

I couldn't afford a Z at the time

Once when I had the body off the frame, I stepped on the left rear corner, it flexed down about 1", but the opposite end didn't lift up. Then there were the brass trunions in the front suspension ...
but I admit, I sure do miss that car sometimes...lots of good times with it and learned an awful lot while sitting on the front tire with the bonnet in the air

[6PAK72]

A long time ago in HS, I had a 64 f85 Olds Cutlass with the Police engine in it - 330CI, Stromberg 4-barrel, factory duals, about 315 horse. I DID love that car!

Anyway, there was this really cute girl.....she drove a white 240Z. Needless to say, all sorts of antics occurred but the Olds could never catch the Z on the back roads of central MA.

A few years later, I ran into her (not literally) at some event, she still had the car, somewhat worse for wear, and I got to drive it. First thing I did was spin it - the front-rear balance was WAY unfamiliar to an American Iron driver.

I would love to build a Z-car.

So, thank for more of the tips. I have the dyno print out, and KTR tuned it from 92rwh, to 108rwh at 5200, where the cam just completely dies. They had it set on the idle side at 32*.

I'm messing with it right now, I'll keep you posted.

See you!

[6PAK72]

So, with another day to mess with it, I got it.

The engine was rebuilt with the OEM 22r distributor and a Pertronix kit.

Since I have an endless supply of cash I figured I'd spend yet another $100, and replace the Pertronix. What the hell, I've replaced everything else...

All problems solved. No missing anywhere in the rpm range, pulls strongly to 6,000, I set the advance to 34 at idle which runs it up to 48 at 5,000, and there's not a bit of missing or detonation. Richened the carbs until the exhaust manifolds were the same temp, the bog is gone, and all the cooling is spot on.

Man, that was a trial!

Thanks everyone for the advice - I executed a bunch of suggestions shared, and the engine runs like a top now.