Runs rich or lumpy

[GrahamFountain]

I'm having a bit of bother setting up the carbs on the Sprint and I wonder if anyone's had a similar issue.

If I set the jets so the car feels right when running, the plugs say its rich and it smokes a little bit when booted hard. But if I set them so the plugs are a good colour, its hesitant setting off - though it seems to run reasonably okay else. The other thing is that, if I set the jets flush with the step and turn down 12 flats, I find I have to come back about 8 or 9 flats to get to the sweet spot on idle speed, and that seems to last pretty much back to flush with the step - the full 12 turns back up. Also, it seems that both carbs are reaching the sweet spot together, as in turn either back down, and it goes off about the same for either. Also, using the pins to lift the pistons manually has the same effect on front and back - drops revs and comes back in a couple of seconds.
I've checked idle screws on both, and they're about the same. I've checked the oil in the dashpots, in case that was causing the hesitancy, and that's not changed aught. And I've checked the points - the gap was a bit small, but setting that back right hasn't changed anything. I have yet to recheck the timing, but I'm not expecting it to be far off from opening the points 5 or 6 thou. I'm reasonably happy there're no air leaks, but I intend to go round with the WD40 and check tomorrow.

I'm wondering if there's an issue with float levels being high, if the idle speed sweet spot is close to the jet being flush with the step. But it seems a bit odd that it's the same on both carbs. Same issue with a worn main or needle.

Graham

[Bumpa]

Has it run properly before? Have you done any work on it and changed something? If the carbs are properly assembled with the right needles etc, 12 flats down should be close to the ideal mixture.

[GrahamFountain]

Those carbs have been on for 4 years ish, since I swapped the engine out of my old DHC TR7 Sprint, the body of which had largely dissolved. As far as I remember, the last time I did the job properly, which will be a couple of years ago, it went okay.
It did occur that I didn't know exactly which needles I have, so I checked and they're the BDQs. I looked in my stash and all I can find is 3 other BDQs. I thought about swapping those in for two the same, but decided not. I had a good look at them while they were out, and I can't see any wear or scuffing or signs they've been filed - I was told the engine was blueprinted, but I don't think there's any mods to the cam or aught.
The other thing is its got K&N pan filters, not the standard, which might affect the airflow and the pressure drop at the step a bit, but they've been on since forever.
I don't know what the dashpot springs are - I guess, if they were over strong, the piston would lift less for a given inlet vacuum. That would mean a higher air speed over the step and so a bigger pressure drop. I can see how that would mean the jet would need to be higher to stop the mix being richer. But they don't feel especially strong. Do stronger springs make the mix weak on big throttle openings at lower engine speed? It feels to me like the mix is going weak at low speed/high load, and setting it rich overall corrects that flaw.
Graham

[dollyman]

Might be a silly question Graham, but have you loosened the clamps that join the two carbs together?

Tony.

[GrahamFountain]

Might be a silly question Graham, but have you loosened the clamps that join the two carbs together?

Tony.

Er, which clamps?
My carbs are bolted separately to the inlet manifold. There's a plate on the TR7 Sprint joining the two carbs that mounts the aircon full throttle cut-off switch. If you have one of those, I'll buy it.
I've set the two arms clamped to the throttle bar to open both carbs at the same time. I put a lot more effort into that then I ever put into balancing them on the tickover screws. The latter only affects tickover, and as long as there's nothing annoying about it ticking over, I don't care that much if the balance is imperfect. All I do is make sure that for a 1/4 turn, both carbs on their own lift the tickover the same amount. But once the cable is operating the butterflies, I care a lot that both carbs are playing their parts equally.
Graham

[dollyman]

With the clamps slackened you will be able to hear which carb is responding to adjustment (more important than setting tickover) this is more so if using a co meter when adjusting. It is of course entirely your choice Graham, but in nearly 50years in the motor trade that is how i was taught and had no problems.

Tony.

[GrahamFountain]

I've seen that method of setting the carbs separately, but I find it's such a pain to set those clamps so the butterflies are opening the same off the cable that I don't like to mess with them once they're right. It wouldn't be so bad if they both moved on the spindle the same when you tighten the clamp bolts. But they never seem to. So I end up doing it by trial and too much error to get it as right as I want. I probably obsess about that too much. I've even looked at fitting a cross link like used on Webers, where there's a paddle gripped between a sprung pin and a screw and you can adjust the offset between them either way. But I've not got very far with that idea.

So I usually rely on checking both carbs are set effectively the same with the pins that push the pistons up manually, and fine tune on the colour of the plugs. But I'm not sure how well those methods work in a commercial workshop environment.

As a result, I'm fairly sure both carbs are peaking on tickover with the jets only about 3 flats below level with the step. I just don't know why that is, and why that than leads to the car faltering when setting off. I'm not even really sure how long it's been like that.

It does seems to me that if the springs are less strong than standard, that would let the pistons rise higher for any given vacuum. And that might mean the jets needing to be higher to compensate. Then, maybe, when the damper slows the piston's rise, that makes it stay weaker than otherwise. I was told that the action of the damper made the mix rich for a while, when the throttle is first opened. If I understood correctly, that is because until the piston gets to where it should be, set by the spring rate, there's a greater airspeed over the step causing a greater pressure drop and more fuel flow for a given amount of air. That was from a lecturer in vehicle mechanics at tech collage (when I was doing my City & Guilds back in the 70s). But it could be simplistic or I misunderstood.

I've some other carbs that may have different piston springs in - I believe the Sprint ones are coded yellow for AUD 663 carbs, but the ones in have no obvious markings . But they're at a garage where the TR7 Sprint is. So I can't get to them till that re-opens.

Graham

[dollyman]

Hi Graham, i agree that my method is a pita, but it's the only way i can them to run right. I don't have a Sprint sadly But my Dolly 1850, 2500S and a Stag have all been set up my way, and all being automatics need to set off right and not stall. I must have to much time on my hands

Tony.

[GrahamFountain]

Well I've got too much time at the moment anyway.
Graham

[GrahamFountain]

I found the specs for the standard piston springs on line, 8oz at 2.75 inches compression. And they're both spot on, as near as I can tell. So it's not that someone put lighter springs in.
Graham

[GrahamFountain]

Just to say I solved this problem.

It turns out it was the air filters, which I hadn't cleaned properly, just given a wash with water. I do the mixture with filters on, because I want the mixture set how the car is set-up to run. Instead I take the dashpots and pistons off (and clean them), making sure not to swap them over, so I can use a vernier to set them the same initial distance down.

But this time, that shortcut of not removing the filters turned round and bit me in the ass. At least I'll know what this problem is if I see it again.

Now I've invested in a proper K&N cleaning kit, it all seems okay. I probably could've got the filter cleaner and the oil on their own for less, if I knew what they were: the user instructions for the cleaner read like it's red fuming nitric acid; though I doubt it's really even soviet rocket science.

Graham

[dollyman]

Ahhh, that makes sense Graham. I missed the bit about K+N filters, i would have said go back to standard set up. A pal had a similar problem on his Spitfire only he didn't tell me about the filters until i saw them when i went round to have a play. Put standard set up on..... End of problem Anyone want to buy a set of Spitfire 1500 pancakes?
Good that you sorted it Graham

Tony.

[GrahamFountain]

Oddly I've been thinking about putting a standard set back on - I have the front box spare from making up a set for the TR7 Sprint with a new TR7 front box (if it ever arrives). But the one back plate I have is going to be resprayed to match the 1978 y.m. TR7 front - the difference is the welded on P clip for what may be the longest dipstick in the world.

I'm also thinking about going back to a mechanical fan. The electric one and its sensor/controller is now sorted, but the reasons it was fitted don't apply anymore - I was too strapped to fork out for a 3rd vicious(ly expensive) coupling and I really didn't think when I though the mech fan was drawing power when it wasn't needed, i.e. when the car is moving through the air (which isn't exactly the same as moving on the road). But I think I need a new fan boss as well, cos if it weren't duff when I took it off, it will be by now.

Graham

[GrahamFountain]

Further to this issue:
While cleaning the filters made things a lot better, and the jets aren't so near the step when running right, it was still a bit rich, and yet needed lots of choke for longer than seemed reasonable, even at like 12C ambient. Though, otherwise running ok when hot.
So, thinking the problem is still related to the air filters, I looked for the cheapest option for me to replace and found these:
https://www.ebay.co.uk/itm/AeroLine-Sta ... 2749.l2649

They look a bit shallow compared to the K&Ns, but they still offer more area than the originals, and the sponge filters can be washed or replaced reasonably cheap. But at £18, I though I'd just get a pair. I've yet to do a full reset of the carburation, but the need to run with the choke too far out for too long seems to have been solved. There's a bit much throttle lift off the choke now, but that's an easy fix (but why do I need a 7mm spanner to do it?).

I can't say there're entirely simple to fit; at least, not as simple a swap as I expected:

The edges of the expanded steel round the foams are razor sharp, so the latex gloves were shredded and I did get cut a bit.

The holes for the screws that hold the front and back together are like a millimeter too close together, so one head or the other binds on the edge of the carb's front face.

Also (presumably for cost) these slotted pan head screws are not captive, like on the K&Ns. That means that, to tighten the front nuts, you need a screwdriver no more than about 25mm long to get in between the plate and the carb - it's too tight for even one for a socket driver.



So I've ordered some M6 thin nuts to lock the screws to the back plate. That will mean there'll be nuts inside the filter enclosure, but they'd have to run down 30mm of screw to come loose and get in the engine. So I'm not over worried about that.

Graham

[Bish]

Oddly I've been thinking about putting a standard set back on - I have the front box spare from making up a set for the TR7 Sprint with a new TR7 front box (if it ever arrives). But the one back plate I have is going to be resprayed to match the 1978 y.m. TR7 front - the difference is the welded on P clip for what may be the longest dipstick in the world.

I'm also thinking about going back to a mechanical fan. The electric one and its sensor/controller is now sorted, but the reasons it was fitted don't apply anymore - I was too strapped to fork out for a 3rd vicious(ly expensive) coupling and I really didn't think when I though the mech fan was drawing power when it wasn't needed, i.e. when the car is moving through the air (which isn't exactly the same as moving on the road). But I think I need a new fan boss as well, cos if it weren't duff when I took it off, it will be by now.

Graham

Hi Graham

If you’re in need of a viscous coupling I have a brand new one plus a new tolerance ring for sale. £69.99 posted to your door. I bought it recently from Wins so (it’s a decent one) and no longer need it because I found a complete new; boss, coupling, and fan. So fitted that instead. it’s on eBay if you want to take a look at a couple of photos.

With regard to the carbs, I had similar problems to you until I fitted the club solid mountings. My rubber ones didn’t look too bad until I took them off and found how perished, and obviously leaky, they really were. I guess you’ve given them the WD40 leak test though already. Just a thought.

All the best, Bish.