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| Running issues? https://auszookers.com/forum/viewtopic.php?f=37&t=55959 |
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| Author: | hypo_thallamus [ Thu Jul 25, 2019 3:30 pm ] |
| Post subject: | Running issues? |
So I just rebuilt my su carby and replaced a few ignition parts on my 1.0 sj40 due to starting issues, now I have noticed when I first start my car it idles too low and I have to be real careful not to stall at the lights but after 15 Min of driving the idle seems to be to high, for example 4th gear cruises at 50kmh without touching the accelerator. Not sure what the issues could be maybe either worn out accelerator cable or something. The carby, not being stock had a fairly crap looking bracket for the accelerator cable and some pretty bad looking u bolts to attach it to the throttle leaver on the carby. I'm not really sure how the su carbs linkages are supposed to look but I'm guessing it's not like they are set up in my car. Any ideas on how I can make the set up a bit more solid? |
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| Author: | hypo_thallamus [ Thu Jul 25, 2019 3:35 pm ] |
| Post subject: | Re: Running issues? |
Also it starts first time I start it in the morning but then after 5 minutes of driving, if I stop it's impossible to start |
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| Author: | 303zuke [ Fri Jul 26, 2019 6:17 am ] |
| Post subject: | Re: Running issues? |
Have you had a good look at how the SU carbs are supposed to operate? Like all things British, they've got their eccentricities and if they're not set up right will give a world of problems. For example, the sliding pot/needle assembly is supposed to have a precise amount of a special lubricating oil.Wrong amount or grade of oil, and they play up like nothing else. Get the oil right, they begin to run like a Japanese Carb! |
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| Author: | fordem [ Fri Jul 26, 2019 12:37 pm ] |
| Post subject: | Re: Running issues? |
SU carbs are not that fussy about the oil in the dashpot - neither the grade nor the quantity is particularly critical. Top it up with the same oil you're using in the sump (20w50 was common on the BMC A & B series motors) to within 12~13mm (1/2") of the top and it'll be fine - the problem is more likely to be that the needle and spring combination is poorly matched to the vehicle, especially given the fact that the SU was never a factory option on that engine, and it's probably running a needle/spring/jet intended for something like an 850 or 998 cc mini. There may also be some degree of unfamiliarity with the carb and consequent misadjustment/operator error - there's no auto choke, cold start enrichment is done by manually pulling the choke knob which simultaneously opens the throttle plate to raise the idle speed and lowers the jet assembly so that the annular gap around the needle is larger allowing more fuel to flow - what this translates to is that it is the driver's responsibility to control the cold idle speed using the choke knob which should not be returned to the fully off position until the engine has warmed enough to run without it. Set the idle speed/mixture correctly with the engine warm and then for a cold start, pull the choke somewhere between 3/4~full on, once the engine has started push the choke back to approximately 1/4 out and that should hold the idle high enough to prevent it from stalling - as the engine warms gradually push the choke the rest of the way in. If you have a genuine SU choke cable assembly it should be possible to rotate the knob about 1/4 turn clockwise to "lock" it, so that it does not move by itself. Incidentally the oil is not there for lubrication, it is intended to "dampen" the movement of the "piston" in the chamber, preventing it from rising too rapidly - there is no accelerator pump on an SU so a sudden throttle opening would result in a weak mixture and subsequent hesitation, if the piston were allowed to rise unchecked - the oil slows the rise of the piston causing an increase in air flow velocity which in turn increases the vacuum or draw on the jet, richening the mixture, as the piston gradually rises to the correct level the mixture returns to what it should be. You can fiddle the "throttle" response to some extent by playing with the oil viscosity, but this is similar to playing with the accelerator pump jet, too small a jet (or too thin oil) and you get a flat spot, too large a jet (too thick oil), and you waste fuel. |
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| Author: | hypo_thallamus [ Sun Jul 28, 2019 2:27 pm ] |
| Post subject: | Re: Running issues? |
OK so apart from the carby is there anything else that would affect starting, taking into consideration that first thing in the morning it starts instantly no worries but if I stop for petrol 5 minutes down the road it won't start? |
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| Author: | fordem [ Sun Jul 28, 2019 11:47 pm ] |
| Post subject: | Re: Running issues? |
What are the symptoms of this "won't start 5 minutes down the road"? Can I assume it's NOT a "no crank" scenario? Have you checked for spark when it won't start? Have you tried starting fluid? Is 5 minutes down the road long/far enough for the engine to be warm enough to be run without the choke? What happens if you make the stop 10~15 minutes down the road when it has warmed more? Trying to troubleshoot this remotely is likely to be immensely challenging - I don't want to condemn the carb out of hand, but it is my first suspect, and at this point I have no idea what SU you have, what it came off of, what needle/spring/jet combination is currently fitted, and what condition (wear wise) it is in. Older SUs are notorious for wearing the body where the throttle butterfly spindles pass, and will suck air through the gap when the butterfly is closed upsetting the idle mixture, and on a cold or cool engine it can cause start problems. It might be an idea to replace the carb (once you have verified that you have spark) with something known to work with that engine - I can't see myself struggling to get an SU sorted unless I was working on a classic on which I wanted to maintain authenticity, something like an MG or Austin Healey. If you want to continue with the SU, here's an explanation of how they work https://zparts.com/index.php/resources/su-carburetors-explained/ Please note - items 8 & 9 are reversed in the diagram - 8 is the needle, and 9 is the jet. |
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| Author: | shakes [ Wed Jul 31, 2019 1:17 pm ] |
| Post subject: | Re: Running issues? |
hypo_thallamus wrote: OK so apart from the carby is there anything else that would affect starting, taking into consideration that first thing in the morning it starts instantly no worries but if I stop for petrol 5 minutes down the road it won't start? It's an SU, requires a specific combination of throttle pumps, holding your tongue correctly, holding the throttle at specific amount of imperial but sometimes metric and often a combination of the two, almost certainly always needing a couple of swear words thrown in, and then it might run ok. Regardless of the amount of tuning. (can you pick I'm not a fan of SU's) I'd go back to the factory carb and linkages. You will get the SU running pretty well eventually, but it will require alot of time getting the jetting and other intricasy's correct. What do you mean by 'rebuilt' the carb? |
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| Author: | losfer [ Mon Aug 05, 2019 12:18 am ] |
| Post subject: | Re: Running issues? |
why dont you just spend like $100 and buy a new sj410 carb , jesus im a mechanic and i shudder at the thought of working on su's they were meant for old unreliable cars like a mini , lol. |
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| Author: | Gwagensteve [ Wed Sep 04, 2019 6:56 am ] |
| Post subject: | Re: Running issues? |
What’s the correct diagnostic connector for an SU carb? |
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