| Author | Message | ||
     Barry Brears Frequent User Username: turney_2009 Post Number: 74 Registered: 8-2009 |
Re Bentley T1 SBH17291,when the engine is either hot or cold it idles quite nicely thankyou but every so often it goes lumpy as if the choke is on.Now I have replaced the automatic choke with a manual one would that be the reason this has only just happened since replacing needles and seats also jet needles also filters and float levels have been checked as per the workshop manual.The fuel pump has also been replaced by an electronic one.Can anyone help ? Barry | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 695 Registered: 6-2006 |
How long does it run lumpy for and how often? | ||
| Barry Brears Frequent User Username: turney_2009 Post Number: 76 Registered: 8-2009 |
Hi Paul ,About 1 minute and all the time. Barry | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 697 Registered: 6-2006 |
Barry, if it's running lumpy all the time it can't be doing it for 1 minute! Please elaborate! | ||
| Jan Forrest Prolific User Username: got_one Post Number: 166 Registered: 1-2008 |
How low have you set the idle at? Anything less than 650rpm would be borderline for a smooth idle and could occasionally show up as 'lumpy' idling. A slightly lean mixture could do the same which is why the factory often richened up the mixture at small throttle openings as it also stopped the tendency of the car to shake or bounce at low revs. | ||
| Barry Brears Frequent User Username: turney_2009 Post Number: 78 Registered: 8-2009 |
Hi Paul and Jan,It runs lumpy for about 1 minute and then it runs smooth and then after about2 or 3 minutes it starts to run lumpy again. Jan what is meant by richening up the mixture at the small throttle openings (where are they?) Barry | ||
| Jan Forrest Prolific User Username: got_one Post Number: 167 Registered: 1-2008 |
I may be wrong, but I believe that this function is carried out by (just to confuse the issue) the 'weakener' valve. As it has it's own filter unit which is also attached to the evaporation loss canister, both of which are service items even though they don't usually need changing often, a fault with either could possibly result in poor idling. Unfortunately non of the weakener parts can be adjusted or overhauled and can only be replaced with new units. However I would expect it to be all the time and not just for part of it. On the other hand it could be due to an accumulation of 'varnish' in the main jet which is restricting fuel flow due to the main jet not allowing a smooth fuel flow. A quick check for this is to use the 'lift pin' just under the right hand side of each carburetor. With the engine not running you just push the pin upwards and release it by moving your finger to one side. the piston should drop with a barely audible click as it hits the bridge inside the piston. If you do it (without dropping it this time) with the engine running the revs should rise for a moment before settling back to their original level. This will tell you if the main mixture is correct or not. Although mixture setting - and carburetor balancing - are fairly simple tasks they do require a partial strip down of the system to see what you're doing and a 'manometer' or half way decent vacuum gauge make things a lot easier and much more accurate. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 698 Registered: 6-2006 |
Could be rich or weak. Speed control or vacuum pipe leaking. Air blend flap or switch or hoses leaking. Weakener can be adjusted but should definitely be cleaned, Filter not usually a problem. It's hard to know what's wrong without hearing it and making some adjustments. How did you set the carbs up? Why is the choke manual? Although it shouldn't make a difference. Is the choke air way blocked off? | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2260 Registered: 4-2003 |
Manual choke: that takes me back to the first Holdens. Even my 1954 R-Type has an automatic choke, and it does the job so well that I hardly know that it exists. Pull-out choke knobs were invented to hang handbags conveniently. Jibes aside, the manuals are fairly good on setting the carburettors, once you have done Paul's recommended checks of course. So often, the volume screws are mistaken for needle position and so on, but once set up the beast should never be lumpy whatsoever. The Technical Library section on postwar engine tuning may be a useful guide http://rrtechnical.info/enginesettingspost1946.pdf where your car, Barry, is tabled to idle at 600RPM.: see Table 2. However, idle speed is, to me anyhow, set as a compromise between smooth idling (a bit faster is easiest) and gentle engagement of the transmission when selecting reverse (the slower the idle the better). Our 1972 T does not have all the later emission controls, so it is happy to idle around 580RPM. Putting it above 620RPM makes the bump when selecting R noticeable, and with the idle lower than 550RPM it is not smooth especially while warming up (note, it is not lumpy, just not smoothed off totally by the flywheel inertia). The choke on the T has never given the slightest problem. So, I'll bet, Barry, that your mixture setting is at best marginal. Once it's all set up properly, the systems are stable for decades. RT. | ||
| Barry Brears Frequent User Username: turney_2009 Post Number: 79 Registered: 8-2009 |
Thanks Gentlemen,the reason for the manual choke is there was no automatic choke the linkage where the fast idle screw is located was held up by a piece of wire the rest of it had been disconnected.There also is no fuel weakner device that also has been disconnected.As far as checking RPM I am waiting on a gauge so as I can check that.So it seems it may just be carb adjustment I set the volume screw by screwing it in and then backing off 1.1/2 turns and then take the dampers out and listen to the hiss and adjust accordingly.The slow running or mixture jet I screw right in then screw out 1/8 of a turn and keep doing that until the revs increase.I have yet to locate vacuum pipe and I beleive I don't have speed control fitted. Regards Barry | ||
| Jan Forrest Prolific User Username: got_one Post Number: 168 Registered: 1-2008 |
Oh dear, oh dear. As we say in Blighty "You've had some right cowboys in here"! Considering what's been disconnected it's surprising that the car runs at all much less reliably. Although you are following the basic tuning procedure you don't mention whether you've checked the carb balance or main jet settings, which must be done first. Also the procedure depends on all the factory fitted devices being present and in full working order. It seems to me that you need to return the fuel system to its original configuration before you have much chance of getting your 'Old Girl' running reliably again. I can only suggest that you acquire the missing components (any linkages obtained used should be adequate) and reinstate them before wasting any more time and effort in trying to bodge the job up any further! |
