TR6 TR6 Timing

[erstearns]

The old girl (1973) seems to run "best" with timing at 18 deg ATDC. Seem that it should be in the range of 6-12 deg BTDC. I had the dizzy rebuilt by Jeff and carbs rebuilt by the other Jeff (excellent jobs by both, and very helpful). The extra plumbing on the carb side has been removed (canister, anti-run on etc.). The rear carb bottom is plugged as is the dizzy.

Was thinking about setting timing using vacuum gauge and not worrying about what the timing mark indicated. However, the PO "rebuilt" the engine and I have some concern that the cam and crank are not quite in sync.

"best" as used above is defined as sounds good, runs smooth, idles a bit high, has less power than I think it should.

Any thoughts as to how best to proceed?

Thanks in advance, Eric.

 

[Brosky]

Eric,

Mine original engine also ran best at 18 BTDC, (not ATDC common error), but after I sent the damper out for repairs, I found that it had slipped about 1/4" and that made a big difference. Now I max out at 12-13 BTDC @1,000RPMS with a total advance of about 37 degrees. Jeff tailored my distributor to my new engine.

Best is like beauty, it's in the eye of the beholder. Just don't become a victim of the silent killer, detonation.

Some cars with Petronix will also run better at the higher base number. Why? I'm not sure, but I've seen it.

 

[poolboy]

Eric, try the Vacuum Gauge. As long as you have a reading between 17 and 18 in-Hg at 900 rpm with a stock cam you won't be overadvanced.
If your cam is stock and the gauge needle is fairly steady, the valve timing is just fine.

 

[Tinster]

Eric, try the Vacuum Gauge. As long as you have a reading between 17 and 18 in-Hg at 900 rpm with a stock cam you won't be overadvanced.
If your cam is stock and the gauge needle is fairly steady, the valve timing is just fine.

Paul/Poolboy: I am still in my 200 mile break-in phase of the
new rocker arm shaft and refurbished shaft components plus my
new crankshaft thrust bearings.

My valves are set at a not-tight .010", timing is 12* advance,
points at .015", carbs are close but not finally dialed in.
Warm idle was 1100 to 1200 rpm when I took the vac. test.

I get a very constant, non-twitchy 21 in-Hg at this idle.
Punch the throttle shaft and I get a fast drop to zero
followed by and equally fast surge to 25 in-Hg and then back
to a constant, non-twitchy 21 in-Hg.

Is the 21 in-Hg anything to worry about? or is it because of
the warm idle of 1200 rpm or the somewhat loose rocker arm
gaps? thanks,

Edit: my car has an exhaust header, if that makes a difference.

dale

 

[hondo402000]

Dale
I never heard of a break in period for a rocker arm shaft. If the clearance of rocker arm to shaft is ok then over time the rocker arm bushing or shaft will wear but dont think you need to drive easy to break it in. Now seating in new rings and a complete engine rebuild, Yes, break in period

Keep the faith

Hondo

 

[poolboy]

I'd say that it's a bit over advanced, Dale. 17 to 18 around 900 rpm is where most report finding a sweet spot.
This seems to confirm that.
https://automotivemileposts.com/garage/v2n8.html

 

[erstearns]

Paul was right, I had it backwards, it is set BTDC. looked at the manual and got sidetracted by the A & B on the diagram and did not think about which way the engine rotated.

Hooked up a vacuum gauge and achieved a peak of about 19.5" at about 24 degrees. Will work on carb tuning and fine adjustment of timing. Right now warm idle at that setting is about 1100-1200 rpm. Idle stop screws are all the way out, will need some carb fiddling to get idle down and redo the timing. The carbs are out of the box they way they came back from Jeff P.

 

[poolboy]

Eric,if you cut the advance back to around 17 or 18 the idle will drop some.
There's also the possibility that the Throttle Bypass Valve is open at idle. If it is you can count on that raising the normal idle speed by about 200 rpms, as well. The spring tension on the valve is adjustable.
Just remember when using a Vacuum Gauge, the object is not to advance the timing to the highest possible reading. Personally 18 is as high as I would go and my engine prefers a tad less, somewhere north of 17.5

 

[erstearns]

Makes sense, was going to start from here and work backwards. With timing back a few degrees idle is still high. Idle/throttle screw is all the way out and the mixture cam is all the way down and plenty of gap on fast idle screw. Seems like I should be able to adjust down to stall unless something else is amiss.

I have discarded the anti run-on valve and carbon canister. Maybe something is incorrect regarding current plumbing or a bypass valve is not seated.

Current plumbing is as follows:
No vacuum line to dizzy, dizzy nipple plugged.
Nipple underneath rear carb plugged.
Valve cover breather connected to top of rear carb.
Gas tank breather connected to top of front carb.
Bowl vents both carbs open.
Brake booster hose to manifold.
Manifold outlet adjacent to booster hose plugged.

If the current plumbing setup is ok then will tinker with bypass valve.

 

[poolboy]

2 things I see that may be holding up the idle.
First since you don't have the retard hooked up, the timing will be a little overadvanced at idle and the engine may speed up as a result. Connect the retard module to the bottom of the front carb and see how the engine handles that. Make your Vac reading with it hooked up as well.
Second, if the Throttle Bypass Valve is open, then excess fuel will enter the combustion chamber and the idle speed will increase.
Other than that, from your description the plumbing sounds OK.
Although Jeff rebuilt the carbs the TBV's may still need adjustment.
Turn the adjustment screws fully Clockwise, then CCW 8 turns.
The rear carb's TBV screw is hard to get to,but if you put a weight on the accelerator pedal, the linkage at the carbs will move a little more out of the way for the rear's adjustment.
Keep us posted.

 

[TR3driver]

One thing to look for is the bowl vent valves holding the throttles open. That was my problem.

 

[erstearns]

TBV's originally turned out about 11 turns CCW. Screwed "in" and backed out 8 turns. Screw on rear carb was no problem, used long driver and attacked from under and rear of front carb. Will hook up dizzy advance in the am and see where that gets me. Have lots to do tomorrow before leaving for Puerta Vallarta with family but should get that far. Anything else will have to wait until after turkey day.

Thanks for the help!

 

[erstearns]

Err......what is a bowl vent valve? Have to dig in the Big Blue Bentley for that one.

 

[poolboy]

Eric 11 turns is even better. That should have put enough spring tension on the valve. I was thinking that maybe there wasn't enough.
Sounds like Jeff went right in the middle of the adjustment range. There are 22 or 23 threads on the adjustment screw. So I guees we could rule out the TBV being open, 11 CCW would insure that.
One suspect eliminated at least.

 

[poolboy]

I see Randall went off line. He's talking about the L-shaped lever with the idle speed screw that opens and closes the float chamber (bowl) vent that uses the nipple on the left side of the carbs.
At idle the lever opens that vent, but when the throttle opens, the lever opens another port on the carb's air box mounting flange to vent the "bowl"
There is an adjustment screw for the L-shaped lever which can cause the idle adjustment screw to be ineffective at lowering the idle.
I know that might be hard to visualize by my description, but if you look at the carb and operate the linkage, you'll see it in operation and it becomes clear.

 

[TR3driver]

There are apparently two (at least) different styles of bowl vent valves. Here's what mine looked like (73 Stag motor). The red circle indicates the normal adjustment; however after disconnecting the vacuum retard, I had to remove the spring clip under the screw head, and bend it to get enough adjustment range.

 

[TR3driver]

However, here's the style that Nelson shows (which is probably more common)

 

[poolboy]

1973.I guess that does apply to you, Eric. Good thing Randall brought that difference up.
1973, the first year that Triumph put the Anti Run-on Valve on the TR6 which required access to the float chamber for the vacuum.
I believe that the ZS people used that arrangement first pictured for only 1 year, 1973. After that the set up Nelson illustrates on the Buckeye site was used.
For some reason that 73 set up has been troublesome; at least from what I read on the Forums and considering it was only on TR6 carbs for that one year.

 

[erstearns]

Hooked up the dizzy vacuum line, able to adjust idle down to 800-900. Hooked up vacuum gauge and could get 20+ and backed down to 18. Timing light indicated about 12 deg BTDC (therabouts, my memory is faulty). Took out for a test spin, seemed to run fine but throttle seemed to "stick" at about 2000 rpm (a few blips and would settle back).

Will turn TBV valves back to 11 turns. Maybe the high rpm is caused by a floating TBV valve, while turning in the front one pushed in against the spring accidently (could that have unseated the valve?).

I have the bowl valve arm that Rodney posted a pic of above (first one with page 22 reference at bottom). Will look to see if that is also causing a problem. Noticed it this am and will fiddle around there also.

 

[poolboy]

Probably a good idea to go back to 11.
I don't think pushing on the spring would mess up the valve, The valve is bonded to the diaphram which is sandwiched along it's perimeter between the 2 halves of the TBV housing.
Sounds like you are getting there.