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Post by 3litrekiwi on Sept 26, 2021 7:42:32 GMT
My engine is running nicely, recently I rebuilt another carby including new spindle bushes and seals. I used the economiser from the spare carb which had a 0.9 mm jet. I assume this is the original size. A slight surge returned which was cured by fitting my modified jet which is 0.8mm. When I was trying to sort this out I blocked the economiser completely and the engine ran fine. The reason I rebuilt the spare was to improve the reliability of the idle without a heavy spring.
I think the economiser may cause a lean running condition, perhaps due to changes in fuel over the years.
My suggestion is to unscrew the economiser jet, whittle a match so that it can be used as a plug and refit the jet fully blocked. If the problem disappears then you are on the trail of the problem. If not then time to think again.
Cheers
Martin
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Post by Ken Nelson on Oct 2, 2021 19:02:53 GMT
Sorry to take a while getting back to you Martin. I tried using a tapered toothpick to plug the hole in the economizer jet (picture below of the attachment on top of reserve bowl that I assume is the jet opening you mean). When driving from a cold start, the car doesn't seem to stall as much and uses less choke. But I still have trouble re-starting again after 10-20 minutes with a hot engine. It cranks over a lot, and finally starts best when I push the throttle fully down and hold it. The engine also surges off and on sometimes at idle. Removing the toothpick doesn't change the surging; but if I remove the toothpick (which I put in tightly and assumed was plugging the jet drilling fully) and then plug the opening with my thumb, the engine dies completely. So once again I'm not at all sure what's happening. Further thoughts from any and all appreciated! Attachments:
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Post by Phil Nottingham on Oct 2, 2021 20:33:37 GMT
I tend to agree with Enigmas - dispense with it if it will resolve the surging which so far even with E10 fuel I have not encountered apart from when the vacuum advance diaphragm failed and surging occurred on the overrun
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Post by 3litrekiwi on Oct 2, 2021 20:46:56 GMT
Hi Ken, it seems that my problem was running lean due to the economiser whereas you are having a problem with flooding. It could be that surging can be due either to running rich or lean. My next step would be to pull the float out and give it a shake to see if it has a leak. I have had this problem with one of my spare carbs and small engines. The needle valve may aso be starting to leak a bit. My car still had the oiginal steel seat type that leaked a bit even after lapping. A more modern rubber tipped needle fixed it and there are also more effective ball types available IIRC.
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Post by Ken Nelson on Oct 2, 2021 23:46:32 GMT
Thanks Martin and Phil. I'll try your suggestions and report back my results.
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Post by Ken Nelson on Oct 3, 2021 18:39:17 GMT
OK, so I tried just disconnecting the line from the carb plate (where vac advance and economizer take off from) to the float bowl. That didn't work and the car ran badly. So then I checked in the float bowl, and the shutoff valve was the old steel type, so I replaced it with a viton tipped one. I checked the brass float and there were no leaks, but I replaced it with a solid one (viton type I think). Perhaps it restarts a little easier when warm now, but I need to drive it some more to clearly find out. But, as has been suggested, how can the economizer be down away with on this carb completely? Even when plugging the tube to the float bowl it didn't run well. I did check the orifice in the economizer jet, and it is significantly smaller than 1/16" so it appears the proper size. Any more advice?
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Post by enigmas on Oct 3, 2021 22:27:54 GMT
Even with the added complexity of these carburettors most of the functionality of a CD type carburettor (SU or Zenith Stromberg) is down to the metering needle or unwanted unmetered vacuum leaks. Any vacuum leak, leans off the mixture. Consider that even worn inlet guides and valve stems introduce unmetered air into the fuel stream. Fuel (petrol) today has quite a different formulation to the product made for primarily carburettored engines of the 40s - 80s when Efi started to dominate.
Given these variable parameters then why stick with a mixture needle that was developed for these engines when they were first designed and built to specific tolerances.
Has anyone considered fitting a wide band oxygen sensor and gauge to monitor the air fuel ratios in real time. This would be a good Club initiative. Fund the components to an appropriate member who then presents the info and any changes to fuel metering of standard 3 litre 6s or P5Bs.
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Post by Ken Nelson on Oct 3, 2021 23:08:04 GMT
Thanks Vince, yes there are a lot of variables to consider, but the engine has been completely rebuilt within 5,000 miles, and carburetor refurbished with new throttle shaft bushings. It seems a bit better, so I'll keep fiddling with it for awhile. It still seems to start best when warm by cranking it over with a wide open throttle. I have the float set for 7/16" in the carb as suggested, but I will likely pull the dashpot to again check for the level of fuel in the main jet. I agree that it would be nice if someone could evaluate the carb needles again in view of modern fuels, etc but I'm afraid I don't have the skills necessary for that.
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Post by enigmas on Oct 4, 2021 4:29:45 GMT
Hi Ken, if the engine in your car starts best with a wide open throttle hot or cold then it's either running rich or the idle circuit is out of action. Although the air slide won't lift significantly during cranking speed (350 rpm?) by opening the throttle the engine is pulling as much air as it can (and drawing fuel) via the metering needle and jet. That indicates an issue with the idle circuit. Engine's fitted with SUs or CD Stromberg carbs (including 3 litre 6 cyl IOE types) once warmed, start without throttle and at the merest push on the starter button.
I read that the throttle shaft was replaced on the SU but that doesn't mean it was setup correctly. Unless you have a concise understanding of how each circuit in that particular carburettor works and interacts with other circuits then the fix will remain elusive.
During normal driving if there's a slight flat spot off idle as you apply throttle then the transition circuit or what passes for it is faulty/non operative in some way.
PS. Ken, if you're up to it try this. Remove all the paraphernalia comprising the air cleaner and air intake system so that the SU and its intake throat is in plain sight and unencumbered by any bolt on additions. Get hold of a small spray bottle (empty windex bottle or similar.) Place some fuel in it. Lift the air slide up and spray 2 or 3 squirts of fuel into the carb throat. Return to the driver's seat, turn the ignition on, depress the throttle about a quarter and crank the engine over. It should fire immediately.
If it were mine I'd leave all the air filtering paraphernalia off until the issue is sorted.
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Post by Ken Nelson on Oct 4, 2021 13:19:20 GMT
Thanks Vince, I'll give that a try and report back. I have a spare carb, so I'll try to examine it also.
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Post by djm16 on Oct 5, 2021 0:25:59 GMT
When you say you reset the float to 7/16", do you mean you checked the closing point by putting a 7/16" drill bit under the float lever?
You should double check the float bowl level as follows: first turn on the ignition and wait for the petrol pump to stop pumping, then turn off the ignition and pull off the carb elbow and dashpot. Remove the carb piston and needle so that you can eyeball directly down the needle valve jet.
You should see a petrol level a millimetre or two below the top of the main jet. It certainly should not be over the top o the jet and wetting the floor of the carb piston chamber.
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Post by 3litrekiwi on Oct 5, 2021 9:18:54 GMT
I agree with Vince that the transition from idle to acceleration is something worth investigating. The reason I started looking at this was the idle with a slightly worn, less than 100% sealed throttle shaft was a bit erratic. When I bought the car there was a second spring fitted to the accelerator linkage. There wasn't much wear and the seals were still ok, but not quite right.
Anyway once the shaft and bushes were a nice fit and new seals fitted, I found it took a bit of fiddling to get the butterfly seated so that there was no light around the edges when it is fully closed. The holes are pretty generous so it is possible to not quite seat the butterfly correctly.
Once this was done, the idle screw could do its job reliably as the only other source of air flow is through the hole in the butterfly. Getting the butterfly right also lets the throttle snap shut without any sticking.
It might be worth pulling the carb off and checking that there is no light visible around the butterfly and if so, backing the screws off a bit and getting the best fit possible.
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Post by Ken Nelson on Oct 5, 2021 13:25:32 GMT
Thanks Martin and djm. Yes I did use the drill-bit method of setting the float level, but I am going to check the fuel level by taking the dashpot and piston off to see whether it may be too high and resetting it if necessary. I did set the butterfly pretty carefully on the carb, but if the fuel level is OK I'll have a look at that again as well. Report to follow, but I'm on a trip for a week or so first.
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Post by Ken Nelson on Oct 21, 2021 18:31:56 GMT
Here's my next update which is disappointing. I have the viton tipped shutoff valve in place now, and finally took the air chamber and piston off to look for possible flooding when starting the engine warm. I found no flooding. The fuel level is slightly below the level of the bridge of the carburetor after the fuel pump has quit ticking over. The carburetor piston and air chamber were also heavily carboned up (I cleaned it up before the picture below), so it seems unlikely that fuel starvation is an issue. I'll keep searching for a solution, but it remains elusive. Attachments:
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Post by Ken Nelson on Oct 23, 2021 20:46:45 GMT
Here's my next updated attempt to make my HD Carb work properly. More investigation led me to realize I was missing a small O-ring on the idle circuit adjustment screw. So I got one and replaced it. Does the car run better now with it? Perhaps, but I need a bit more time driving it to be sure. Pictures of the rubber O-ring are below. Be sure to keep tuned in for what I hope may be the exciting and final conclusion to this elusive search! (the rubber O-ring is #12 in the diagram)
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Post by Ken Nelson on Oct 28, 2021 4:24:16 GMT
Well, I would say that the car does seem to run and restart warm better now. It is still much more sensitive and finicky than my MGTD, MGA, and Riley RME with H2 and H4 carburetors. I went over the Rover carb again and made sure that 1) the 5th hole in the intake manifold is open and not blocked by the gasket, 2) all the carburetor passageways are clear-including the small vacuum hole (next to the ignition advance hole on top) for the idle circuit is clear, 3) the butterfly plate closes fully at idle and has a 1/8" hole drilled through it and, 4) the Economizer venturi is clear but slightly smaller than 1/16" as designed, 5) the rubber O-ring on the idle airflow needle is in place, 6) the correct 20W oil is in the dashpot for the piston damping, 7) the float level is set correctly so flooding does not occur, and 8) the ignition system is set up properly. So for now I'm going to try to get more driving in before snow sets in, and hopefully feel reassured that no further trips will end with the Rover coming home on a 'hook'! It does seem that all the issues mentioned leave many areas for the unwary to be caught out on, and I continue to feel that the idle circuit and Economizer for the SU HD8 carburetor leaves much to be desired in the complexity of design. Once warm, and on the highway, it functions very well though.
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Post by Phil Nottingham on Oct 28, 2021 5:58:29 GMT
Well, I would say that the car does seem to run and restart warm better now. It is still much more sensitive and finicky than my MGTD, MGA, and Riley RME with H2 and H4 carburetors. I went over the Rover carb again and made sure that 1) the 5th hole in the intake manifold is open and not blocked by the gasket, 2) all the carburetor passageways are clear-including the small vacuum hole (next to the ignition advance hole on top) for the idle circuit is clear, 3) the butterfly plate closes fully at idle and has a 1/8" hole drilled through it and, 4) the Economizer venturi is clear but slightly smaller than 1/16" as designed, 5) the rubber O-ring on the idle airflow needle is in place, 6) the correct 20W oil is in the dashpot for the piston damping, 7) the float level is set correctly so flooding does not occur, and 8) the ignition system is set up properly. So for now I'm going to try to get more driving in before snow sets in, and hopefully feel reassured that no further trips will end with the Rover coming home on a 'hook'! It does seem that all the issues mentioned leave many areas for the unwary to be caught out on, and I continue to feel that the idle circuit and Economizer for the SU HD8 carburettor leaves much to be desired in the complexity of design. Once warm, and on the highway, it functions very well though. I agree the economiser circuit is overly complex but it must be fully in order to avoid problems. Our P4 has this set up and it had occasional flat spots on take off. This was eventually found to be merely because said volume control O ring was in the wrong position and not in the groove. Miniscule amounts of air must have been occasional drawn in down the threads of the screw weakening the mixture
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Post by enigmas on Oct 28, 2021 8:21:17 GMT
Goes lean when you take-off and accelerate...use a heavier viscosity oil in the dashpot.
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Post by Phil Nottingham on Oct 28, 2021 14:40:57 GMT
I use 20/50 now
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Post by Ken Nelson on Oct 29, 2021 15:42:39 GMT
Thanks Phil. I've been using 5W/20 and will try switching to 20W/50. Perhaps that might cure the stumbling on acceleration from idle when cold that requires so much choke and throttle. Otherwise the car idles and accelerates fine when fully warmed up (at least as much as a 3-litre in that car can manage to accelerate). If that were to be cured I might actually be content with the carburetor! Report later-perhaps the final one?
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Post by Ken Nelson on Nov 7, 2021 0:19:42 GMT
Well, I keep whittling away at all the bits on the carburetor issue and MAY be at the end of the trail...but who knows. This time when I returned to the car after a 6 day layover I again found gas dripping from the carb inlet. After partial disassembly I think it may have been coming around the shutoff valve threads, or else the valve had grit in it (but it is a brand new viton tipped valve and the fuel is clean). It perhaps wasn't tightened fully though, so I put a fibre washer under it and tightened it more securely. I also took Phil's and Vince's advice and put 20W/50 oil in the dashpot and bent the float shutoff lever further down to lower the fuel level in the bowl a bit more. Now it started well in the cold (40º F) and restarted much better when warm without requiring the throttle to go to the floor. So PERHAPS this has cured my flooding issue. PLUS the car doesn't seem to stumble on takeoff now, once partially warmed up, and the intake inlet has stayed dry. I am surprised the dashpot needs oil that heavy, but it seems to make a big difference. Dare I call this a success? Time will reveal the answer to that question. Meanwhile, my starter motor bendix (inertia) gear is sticking occasionally, and I hope it will free itself on its own since the motor weighs 22 lbs and is a devil to get at.....
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Post by enigmas on Nov 7, 2021 5:18:08 GMT
Looks as though it's problem solved Ken. As for the heavier oil in the dashpot...all that it does, is to hold the air slide down longer before it can lift. So the engine then imposes a greater vacuum on the jet, pulling more fuel and providing a richer mixture. It parallels the function of an accelerator pump on a more traditional carburettor. John Twist would approve.
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