TC carb balancing

Bullet 1, yes. Bullet 2, there is a tolerance for how fast the piston should sink in the dashpot, without spring or damper. I don't know what that number is off the top of my head, but suffice it to say both carbs should be within the tolerance. Bullet 3, as long as the springs aren't inches different should make no difference. Bullet 4, yes on the unequal opening of the front & rear carbs. Bullet 5, yes that sounds like the lumpy idle.

Yours
Vern
 
Richard Zahra said:
So I have overhauled the carburetors, balanced them and adjusted the mixture. I am fairly happy with the result but will fine tune after a run. At the moment I am still on the rich side with the mixture.
Now question time! The carburettors are HS8s and the engine is a 9:1 CR version.
  • Are the jet assemblies the same for 10:1 and 9:1 CR engines?
  • The 2 pistons do not travel down at the same speed. I tried the pistons on their own, with springs only and with springs and damper. The inside of the domes and the pistons do not have any scraping marks and are practically undamaged. Should this be a problem? They both stop with a thunk!
  • The 2 piston springs are not the same length. One seems stretched. These carbs were never overhauled, and the springs never changed and nobody except my father has ever worked on the car. I could not see any colours on the springs to confirm if they are black/blue. Thoughts?
  • I balanced the carbs by means of a stethoscope with fittings in each throat so one can hear the hiss from each side at the same time. When the accelerator is pressed slowly the rear carb opens before the front one. I think that this is due to the clearance required in the front carb at the lost motion connection. Right?
  • The car idles at around 800rpm . If the engine is raised to above 1500 rpm , kept there for some time - 10 seconds or more- and then the pedal is released, the idling speed will initially start at 1000 rpm for a couple of seconds and then go down to 800 rpm and remain steady but will occasionally falter to 650 and the back up to 800 rpm. Is this the lumpy running mentioned in other posts?
I have taken the car for a ride and performance wise it is healthy but will fine tune when I find the time.

Regards
Richard
Click to expand...

Hi Richard,

1) Yes, the jet assemblies are the same between the 9:1 and 10:1 CR engines. Needles may differ though.
2) They should drop at the same rate. Best way to test is remove the springs and refit damper. Raise both pistons and release at same time. They should drop at the same rate.
3) The different length springs seems like a smoking gun to me. I've never been able to see the colour markings on the springs either. It might make sense to order fresh springs from Burlen under the circumstances.
4) The two carbs should open precisely together. The lost motion linkages can be adjusted after the carbs are balanced at idle. Make sure the linkage from the pedal is in good order and the bushing is still in place. Then loosen the clamp bolt on the shaft nearest the front carb and adjust the finger so it just touches the lever on the throttle spindle, retighten and test.
5) Cars for the US market had a damper fitted to help emissions. Not sure if they were used on UK cars though. Have you check valve clearances? They can close up and give lumpy idle, poor performance and make it impossible to balance carbs. Idle should be fairly smooth at 750rpm if all is well.

Good luck and hope I was some help,

Steven

Quick edit to say I missed Vern's reply. Mostly agree with his except for 4 (I may have misunderstood Richard's description) And 2) as they should be similar with dampers fitted.
 
Thanks Vern and Steven for your relies.
Going through my points and reading your comments -
The car has only 57500miles to date and my father, who bought the car new and was an automotive engineer by profession, always did his own maintenance and never let anyone work on the car . I think i was in my early thirties before he relented and let me start to take over. So whatever has been done to the car is documented from day 1 and I know about it.
  • I changed the jets but the needles are the original ones and do not have signs of wear. I wanted to make sure that I installed the right jets.
  • according to the manual they should drop within 2 to 4 seconds and they do but one is quicker than the other.
  • One spring is a good 4 inches longer than the other. I am thinking that the piston dropping speed in point 2 manifested itself in my fathers time and he might have tried to correct it by stretching the spring to overcome the extra friction wherever it is coming from. That said I am quite skeptical that he would have distorted a spring.
  • so when all is set, tightened and done , should the 0.8mm clearance at the lost motion lever taken up to have equal opening of the butterflies?
  • I have not checked the valves as such but there is no chatter from the valve gear and they were last set in Solihull or wherever the engine was put together in 1969.
I hope to find some more time this weekend for fine tuning.

Regards
Richard
 
To be clear, the carbs should not open at exactly the same time, because of the .008" (0.2 mm) clearance in the lost motion link. However, the delay in the rear carb opening should be very small. You saying 0.8 mm, if you didn't misspeak, suggests you may have 4x the amount of play specified.

The length difference in the springs does suggest problems, I was thinking more like a half inch making no difference.

Yours
Vern
 
Richard Zahra said:
I have not checked the valves as such but there is no chatter from the valve gear and they were last set in Solihull or wherever the engine was put together in 1969.
Click to expand...
The valve clearances close up with age, valve chatter only manifests itself when clearances open up. Worth a quick measure with feeler gauges at least.

The variation in piston drop speeds is certainly a concern. I would check to see that someone hasn't accidentally swapped pistons in the past. I've seen it happen with other cars believe it or not. As Vern says, test the drop without dampers and springs first.

Good luck,

Steven
 
Vern Klukas said:
To be clear, the carbs should not open at exactly the same time, because of the .008" (0.2 mm) clearance in the lost motion link. However, the delay in the rear carb opening should be very small. You saying 0.8 mm, if you didn't misspeak, suggests you may have 4x the amount of play specified.

The length difference in the springs does suggest problems, I was thinking more like a half inch making no difference.

Yours
Vern
Click to expand...
I was following the amount specified in the workshop manual supplement specific to HS8 carburetors which states 0.7 - 0.8mm or .025 - .030 "

"The .025 to .030 in. (0,70 to 0,80 mm) clearance
in the lost motion linkage is extremely important
and is designed to allow the rear carburetter to
operate by a pre-arranged amount before the
front carburetter, to allow individual setting of
the carburetter."
 
sdibbers said:
The valve clearances close up with age, valve chatter only manifests itself when clearances open up. Worth a quick measure with feeler gauges at least.

The variation in piston drop speeds is certainly a concern. I would check to see that someone hasn't accidentally swapped pistons in the past. I've seen it happen with other cars believe it or not. As Vern says, test the drop without dampers and springs first.

Good luck,

Steven
Click to expand...
Will check valve clearance next week. On start up when cold the valves are not audible but once warmed up I can hear the start of a chatter so I don't think they are far off.
 
Richard Zahra said:
Will check valve clearance next week. On start up when cold the valves are not audible but once warmed up I can hear the start of a chatter so I don't think they are far off.
Click to expand...
I have tested the pistons on their own and it doesn't make a difference, it seems to be the air resistance in the dashpot.

Richard
 
Regarding matcing carb pistons and bell chambers:
First you remove needles, springs and dampers. Then with some tape, you block the two holes that are at the bottom of each piston.
The you do the drop test for each pair, and measure the time each piston takes to drop out of its bell chamber. If the times are within a second, all is well. If not, swap over pistons to the bell chambers. Of course before you do the test you have to make sure that both pistons and bell chambers are properly clean.
If you cannot match the falling times by swapping, then take some 2000 grade wed & dry paper and polish carefully the piston of the slower falling pair. Continue to do the drop test, until you get even times. When you have a pair with equal falling times, assemble the carbs again, fill them with oil, fit the dampers and check them again in order to be sure that they fall evenly. If there are differences, swap the dampers and see if this improves things. If not, you can play a little with the viscosity of the oil in one of the carbs, in order to make them have equal falling times.
I hope that it is clean enough, but if not, do not hesitate to ask again.
 
So an update.
I have removed the suction chambers, springs, pistons and piston dampers and mixed and swapped concluding that the problem, if there I one, is with the rear suction chamber since either pistons are always slower when installed with it. They are both within the 4 second time allowed but there is a difference of about 1.5 seconds between them. I will be leaving them as they are for the time being.

The actual difference in length between the springs is around an inch and cannot understand where I got the 4 inches previously stated.

I went for a test drive and am quite happy with the performance. It will potter around smoothly at around 1400 rpm in 4th gear and will accelerate cleanly with small accelerator openings. When accelerating through the gears using 2nd and 3rd gears and limiting myself to between 5000 and 5500rpm, the acceleration is constant. The only niggle is a solitary misfire on 3rd gear at around 3800/4000 rpm and will not always do it.

One question – I remember clearly that before I had these problems with the carbs, the car would accelerate up to around 4000 rpm and then accelerate more aggressively. Is this how it should be or should the acceleration be constant?

I have removed the spark plugs and they are a nice biscuit colour with plugs 3 and 4 slightly darker than 1 and 2. The mixture seems to be about right.

I will continue to fine tune along the way and keep you updated.

Thank you Steve, Vern and Demetris for your advice and remarks.
 
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