Carb Issues ... the saga continues

[F1loco]

So, I pulled out my Gunson color tune to try and dial in the A/F - seemed to running rich. After a few turns to lean out the SU6's, I got it to idle just fine, though a bit lean at idle - blue to white, and on full throttle a nice blue to orange color. Mid-range a nice burner blue glow.

 

So, I went to remove the fuel filter and put in straight lines in as it is a completely new system. (I know don't fix it if it ain't broken) However, now it will barely idle after starting. If I give the carbs a good half turn on each, I still have to work the throttle to get the gas to feed, and then it will take off and stay high for a good 20-30 seconds, but then return super low and start to stammer like it's going to stall. Again, working the accelerator to keep it going the idle sores up then back to nilch like it's fuel starved.

 

What is going on here? Ran the car at throttle, as once the gas feeds it runs like a champ, and hot killed. Plugs look nice and dry, not nearly as much soot as before. It's like some kind of strange fuel delivery issue all of a sudden? I may just put the fuel filter back in line where it was, but doesn't make sense. If anything, I should have more flow and better pressure.

 

Would retarding the timing a couple of degrees help since both carbs are leaned a bit than what they were?

 

So close to get this thing running correctly, but yet so far to go still ...

[ctc77965o]

Garbage in fuel line after filter disturbance...float valves blocked

[oldtuckunder]

I'd be checking the float valves for crap or sticking. NB you don't need much fuel pressure anything above 2.5-3 psi is too much, do you have a mechanical or electronic fuel pump?

 

When your waggling the throttle peddle all you are doing is opening and shutting the butterflies all this will do is put a depression pulse around the jet, if this is managing to pull enough fuel for it to pick up and then die back, it sounds like its a fuel level problem in one or both carbs.

 

NB stupid though it may sounds you can get a similar effect if one of the breather pipes to the carbs has come off and is leaking air in.

 

Alan

[F1loco]

Running a new mechanical pump.

 

I richened up a few flats on each carb and the idle/stall issue seemed to have disappeared. I know when I go to hook the Gunson back up it will be rich yet again. I know when I replaced the needles I used the SW (RICH) ones as was in place by the previous owner. Would going back to a regular metered needle help? When I had it dialed in with the Gunson I was only 5 flats out, but it produced the best blue to orange glow at throttle. Problem is I get flashes of white/blue at idle. Guess I'll live with 7-8 flats for now and see how it runs.

 

I previously set the floats and replaced the jets with triton tips. It had Grose Jets in it before and had some sticking issues.

[ChrisR-4A]

I have never had any luck with Gunson on older cars with twin SU or Strombergs.

Basic setting on SU jet is two turns down from up position (level with Bridge) This is 12 flats and richer needles may need a bit less so your 7-8 flats is somewhere near. I find it easier to start off a bit rich so at least it goes and then weaken off and check plugs after each adjustment.

When your nearly there it will hesitate on change up from 2nd to 3rd and another 1 or 2 flats should then fix it. Make sure engine is warmed up before you try this.

Chris

Edited April 11, 2017 by potts4a

[Peter Cobbold]

Check both pistons fall back rapidly with a light 'clunk' after lifting with a finger. One might be hanging up after 'working the throttle'.

Peter

[F1loco]

Check both pistons fall back rapidly with a light 'clunk' after lifting with a finger. One might be hanging up after 'working the throttle'.

Peter

Yep, both pretty much free fall, however, the front falls a bit quicker than the rear, but just a tad. I have reset the needle on the rear at least twice now trying to be sure it's centered and aligned. Damper oil was topped off in both. If I remove the damper piston, they both fall equally quick with a clank on landing. I did notice in playing with it that if the rear were to be completely lifted up, it has a tendency to hang up and not drop, though?

[iain]

Outside chance but......My father had a brand spanking new car that for months confused every garage it went to.....It would run fine then after an hour or so die.It was running on twin 40's. Everyone suspected the carbs.....All wrong, it was a piece of polythene in the fuel tank, would sink and get sucked over the outlet pipe. Stop for 10 mins normal service was resumed. Until 15 mins later and it all happened again.......This went on for 6 months.:-(

 

Is the fuel flow from the tank unrestricted either by obstruction or vacuum?

Is the fuel pump always full or does it show lots of air when the power drops?

Are the fuel bowls full when it cuts out?

 

Good luck

 

Iain

[F1loco]

I wish it were something that mundane. ;D New fuel cell, plenty of flow (my garage took a soaking when I swapped out the filter line) - they really should have a petcock in place to make it easier and more environmentally friendly - lol. Yes, bowls remain full and glass jar by the pump nice and full as well.

 

I'll take it out this evening to get a good read on the plugs, but seems like I'm heading the right direction. Ordered a couple new throttle springs as mine seem a bit warn out. In fact the previous owner has a larger spring on the main throttle linkage to the carb linkage hooked up to where the plate rotates on the firewall with the linkage to the black plate for the carbs - is this right? I would have thought the other two springs would provide plenty of spring to return everything? I don't think I've ever seen a third spring on the images I've pulled off the net.

[Geko]

In fact the previous owner has a larger spring on the main throttle linkage to the carb linkage hooked up to where the plate rotates on the firewall with the linkage to the black plate for the carbs - is this right? I would have thought the other two springs would provide plenty of spring to return everything? I don't think I've ever seen a third spring on the images I've pulled off the net.

It's not orthodox but it's ok. I did that too as I couldn't get my head around fitting the return springs properly

[Peter Cobbold]

Yep, both pretty much free fall, however, the front falls a bit quicker than the rear, but just a tad. I have reset the needle on the rear at least twice now trying to be sure it's centered and aligned. Damper oil was topped off in both. If I remove the damper piston, they both fall equally quick with a clank on landing. I did notice in playing with it that if the rear were to be completely lifted up, it has a tendency to hang up and not drop, though?

They are probably OK. The piston is unlikely to get fully up even at full throttle+max rpm. So stationary running -even blipping to high rpm- will barely lift the pistons at all. We can rule out sticking piston.

As the plugs are OK on the road I would also rule out fuel starvation. There's a lot more fuel flowing ( and air) when the engine is loaded than when in the garage.

Check fuel level in jet mouth is not set too high - look from above after lifting the dome/piston assembly. It should be a few mm down the jet, not level or overflowing The level is set by the float/needle valve.

Peter

Edited April 11, 2017 by Peter Cobbold

[F1loco]

Well, nice drive, idle a bit high, but if I go around 1K, and come off throttle it has a tendency to seek momentarily down to 750 then back up. Of course my self confidence on it dying at a red light and not starting doesn't help.

 

It is hot, but seems it is getting hot - well over 1/2 way to 3/4 the way to the white zone on the meter (assuming it is accurate).

 

Images attached. Still running rich, I think, but after dialing a flat back on each, it started to have some hesitation. Retarded it about a 2 degrees and seemed to have smoothed out. It's kind of crazy how one plug shows spot on timing while another shows a bit advanced.

 

Thoughts?

 

#1

 

#2

 

#3

 

#4

Edited April 11, 2017 by F1loco

[F1loco]

Check fuel level in jet mouth is not set too high - look from above after lifting the dome/piston assembly. It should be a few mm down the jet, not level or overflowing The level is set by the float/needle valve.

 

 

I take it this has to be done after I run it and rather quickly after killing the engine? I do notice when adjusting/sync'g carbs the back one has a tendency to splash on acceleration - meaning I feel more tiny particles of fuel blown out than the front - if that made any sense at all?

[F1loco]

So, it doesn't appear to over flowing out the jet when I remove the needle/piston.

 

I did go buy some carb cleaner, started it, and noticed when I spray around the brass color ends of the carb linkage located to the outside of the carbs (right of the front and left of the rear carb), I do get a very brief drop in RPM's? Is this something I need to worry about? Quick easy fix if so? No other areas seem to create a drop/lull in rpm's.

[peejay4A]

That indicates an air leak. How old are the carbs?

[ChrisR-4A]

Those plugs look too weak, should be light beige to light coffee colour or a mix of both but not white all over central electrode.

However if there is play on throttle spindles then getting tickover and initial pickup right will be nigh on impossible due to air leaks.

Chris

[Peter Cobbold]

Is the spark timing set right for this engine? I think USA engines had either:

advance and retard vacuum cpasules

or

only a retard capsule.

The retard capsules operate at tickover. And neither the adv nor the ret do much when the engine is driven under load, it will acclerate fine.

 

It may be a combination of air leak and tickover timing.

Peter

[F1loco]

Those plugs look too weak, should be light beige to light coffee colour or a mix of both but not white all over central electrode.

However if there is play on throttle spindles then getting tickover and initial pickup right will be nigh on impossible due to air leaks.

Chris

SMH ... guess I need to send them to Joe in NY or someone else to do a rebuild ...

 

When you say "too weak" do you mean too "cold" or something else? Would the leaking throttle rod cause this?

[ChrisR-4A]

Yes, Too much air, try another 2 flats on jets, if still white then 2 more. If spindles are worn different on each carb you may end up with jets open different amounts. Should still go ok but with uneven tickover.

[peejay4A]

I think we need to know a bit more about the carbs. What condition? How old? Have they been messed about? What needles installed?

[F1loco]

Carbs themselves are in good condition, clean and otherwise seem to function as normal. Running SW (rich) needles in them as that is what the previous owner had installed. Don't know much about the history of the motor, so not sure what if any modifications have been done to compression and/or head work.

 

The only problem on leaning it a bit more is I start to have that stumble issue on initial acceleration having to play with the accelerator to keep it going. Assuming this could be coming from the leaking throttle linkage possibly. Yes, the front is definitely more lean that the rear already on the same number of flats.

[peejay4A]

Are the jets centred?

 

http://sucarb.co.uk/technical-hs-type-carburetter-description-adjustments

Edited April 12, 2017 by peejay4A

[Peter Cobbold]

Are there over-run valves in the throttle butterflies? if so are they both sealing OK.

[ChrisR-4A]

Carbs themselves are in good condition, clean and otherwise seem to function as normal. Running SW (rich) needles in them as that is what the previous owner had installed. Don't know much about the history of the motor, so not sure what if any modifications have been done to compression and/or head work.

 

The only problem on leaning it a bit more is I start to have that stumble issue on initial acceleration having to play with the accelerator to keep it going. Assuming this could be coming from the leaking throttle linkage possibly. Yes, the front is definitely more lean that the rear already on the same number of flats.

Actuating lever is on front carb so more strain on spindle and more wear

[peejay4A]

What colour/color springs installed?