Lord Flashart Posted April 5, 2014 Report Share Posted April 5, 2014 Noticed today when tuning the carbs using lifting pins and a colour tune that the rear carb needs the jet set way lower than the front to achieve the same mixture. Is this normal? Can't find any air leaks and compression on all four is the same. Also checked float levels and both are the same and correct. Any ideas? Paul Quote Link to post Share on other sites
ctc77965o Posted April 5, 2014 Report Share Posted April 5, 2014 Is this normal? ....err no I would guess airflow thru both carbs is unequal, with rich carb flowing more air and fueling the other carb's cylinders via the balance pipe. Quote Link to post Share on other sites
Lebro Posted April 5, 2014 Report Share Posted April 5, 2014 Check float chamber levels, Also check that one of the floats does not have petrol inside. Bob. Quote Link to post Share on other sites
Chris Hubball Posted April 5, 2014 Report Share Posted April 5, 2014 Hi Paul Check that both needles are seated correctly ie the shoulder of the needles are flush with the base of the pistons. Cheers Chris. Quote Link to post Share on other sites
mjdearing Posted April 5, 2014 Report Share Posted April 5, 2014 check overflows arnt blocked. Quote Link to post Share on other sites
BlueTR3A-5EKT Posted April 5, 2014 Report Share Posted April 5, 2014 (edited) Noticed today when tuning the carbs using lifting pins and a colour tune that the rear carb needs the jet set way lower than the front to achieve the same mixture. Is this normal? Can't find any air leaks and compression on all four is the same. Also checked float levels and both are the same and correct. Any ideas? Paul Are the needles a 'pair' ie both the same profile and set at the same height in the piston. Are the springs both the same. ie strength. If it is a pair of H6 carbs on a TR4 you are looking for SM needles with red springs Cheers Peter W PS and .100" hole jets Edited April 5, 2014 by BlueTR3A-5EKT Quote Link to post Share on other sites
Peter Cobbold Posted April 5, 2014 Report Share Posted April 5, 2014 A bent dash-pot damper rod can impair piston lift. Try with both removed. Quote Link to post Share on other sites
Lord Flashart Posted April 5, 2014 Author Report Share Posted April 5, 2014 Cheers chaps, I'll be back in the garage again tonight to investigate. Everything seems ok, the. Needles and jets are matched and seated correctly and springs etc are both the same. All a bit strange really. I'll try again and report back. Paul Quote Link to post Share on other sites
Badfrog Posted April 6, 2014 Report Share Posted April 6, 2014 Paul, Could you have a misaligned buttefly with wrong sealing of the venturi? Badfrog Quote Link to post Share on other sites
Lord Flashart Posted April 6, 2014 Author Report Share Posted April 6, 2014 Good to see you back Mr frog! Not sure what's going on. Looked through everything today and I can't find any thing wrong, but setting with a colour tune and lifting pin method, the rear carb need a much lower jet setting to achieve the same mixture. Very strange. Going to see if I can borrow another set of carbs from a mate and see if it still happens. Paul Quote Link to post Share on other sites
peejay4A Posted April 6, 2014 Report Share Posted April 6, 2014 I've never had much success with the lifting pin method. Last time I set mine up that way they ended up all over the place like yours. Then I set everything back to basics on both carbs. 22 flats down (I think) on both then road test. Fine tune by revving and letting the revs die away. Weaken until it stumbles as it approaches idle, the richen up until the stumble disappears. Same adjustment on each carb. Seems to work for me. Quote Link to post Share on other sites
Lord Flashart Posted April 7, 2014 Author Report Share Posted April 7, 2014 Thanks PJ I'll give that a go. The way things are the plugs are a similar brown colour with the rear carb with the lower jet setting. Initially set them both at 12 flats down and front two plugs were brown, rear two almost white suggesting too lean. Colour tune backs up this diagnosis. I'm also going to pull the fuel bowl off the rear car and make sure it's not partially blocked Paul Quote Link to post Share on other sites
peejay4A Posted April 7, 2014 Report Share Posted April 7, 2014 12 flats sound more like it. Good luck. Quote Link to post Share on other sites
TorontoTim Posted April 7, 2014 Report Share Posted April 7, 2014 Hey Paul - are you trying the traditional "plug chop" to get your plug colours? Find a nice piece of empty road, hold it at, say, 60 MPH/100KPH for a bit (I'm sure you can check for more accurate info than "a bit"), kill the ignition, coast to a halt and look at all the plugs. Apologies if I'm stating the obvious to you. I have to say, this sounds like an air leak - I can't think of any other logical answer. How are the carb spindles, for example? Gaskets all good between carb and insulator, insulator and manifold, etc.? A carb swap with your mate sounds like a good plan to eliminate a bunch of things. Good luck! Quote Link to post Share on other sites
Lord Flashart Posted April 7, 2014 Author Report Share Posted April 7, 2014 Cheers Tim. Is the plug chop that's really showing the lean running on 3&4. I can't find any air leaks and the spindles are all in good condition. I think next I'm going to remove the fuel bowl and have a look into the fuel route to the jet. The air flow is the same so suction is there, but for some reason it seems like it need a lower jet setting to draw a similar amount of fuel through as the front. I'm wondering if there is a slight blockage in the fuel tract. The fuel bowls themselves don't seem to have a problem as both are always full to the correct (and same) level whenever I remove the tops and fuel is freely flowing from the needle valves. Paul Quote Link to post Share on other sites
stuart Posted April 7, 2014 Report Share Posted April 7, 2014 Check your fuel pressure as well as the flow to the rear carb. Stuart. Quote Link to post Share on other sites
Lord Flashart Posted April 9, 2014 Author Report Share Posted April 9, 2014 Today I removed the rear carb, resealed everything and checked for evidence of air leaks. Couldn't see any. Refitted the carb with new gaskets to the manifold. I even fitted an inline electric fuel pump to boost the fuel pressure to both carbs. I set the mixture using a colour tune and the car ran well. On inspection, the rear jet is still considerably lower than the front to get the same colour on the colour tune. Interestingly, with the carbs set like this my eternal problem of the engine running on at shutdown is no present. So I'm going to run it for a while and see what happens. I'll do a few plug chops and try to see what's happening. In the mean time I'm going to look for another set of carbs. It would be very nice if I've inadvertently solved my run on problem. I'm wondering if the rear two have been a little lean and consequently running hot. There is also a chance that the leak is at the manifold to head joint, but I've fitted a new manifold this winter with new gaskets without the fault changing. Paul Quote Link to post Share on other sites
stuart Posted April 9, 2014 Report Share Posted April 9, 2014 With the air cleaners removed check the damper heights at various RPM when looking down the throats. Stuart. Quote Link to post Share on other sites
Lord Flashart Posted April 9, 2014 Author Report Share Posted April 9, 2014 Cheers Stuart. Will try that. Is there anything you can do to adjust the damper heights if they are differnt? Paul Quote Link to post Share on other sites
stuart Posted April 9, 2014 Report Share Posted April 9, 2014 Dash pots and pistons should be matched from new so if they have been mixed up at some time then the minuscule clearance between the two may not be correct so even with matched dampers and springs the rise rate will be different hence giving different mixture control. Stuart. Quote Link to post Share on other sites
ctc77965o Posted April 10, 2014 Report Share Posted April 10, 2014 Hi Paul, I think I mentioned before, if you wanna see what carbs are doing airflow-wise, then making up some dial gauge adapters to go in the top of the dashpots works nice: https://www.youtube.com/watch?v=3_1S-izfXMk I have used successfully on twin carb V8 and quad carb V12 Quote Link to post Share on other sites
Lord Flashart Posted April 13, 2014 Author Report Share Posted April 13, 2014 (edited) Quick update. Tuned the carbs with a combination of colour tune and lifting pins. The rear jet is a fair bit lower than the front. Drove the car and checked the plugs after switching off at different speeds ranging from 20mph to somewhere around the speed limit. (5000 rpm)! All plugs are a nice brown colour and the dieseling is gone to boot!!! I think Stuart is correct and the pistons are not opening in synchronization necessitating the lower setting on the rear, so I'm going to look for a new set of carbs as soon as funds allow. I'm very pleased I've finally stopped the infernal run on problem though, or at least found the cause. Thanks for all the help chaps. Paul Edited April 13, 2014 by Lord Flashart Quote Link to post Share on other sites
TorontoTim Posted April 13, 2014 Report Share Posted April 13, 2014 Good to hear that it's running well - even if the settings seem odd. I hope you get plenty of opportunities to enjoy it in and around the landscape there and the Okanagan area. Spring has finally arrived here in Ontario, so I was out for a good 9-hour run through the villages and back roads west of Toronto yesterday. Picked up a puncture and loads of mud, but had a good time. Quote Link to post Share on other sites
Lord Flashart Posted April 14, 2014 Author Report Share Posted April 14, 2014 Thanks Tim. I'll hopefully be driving it lots this summer. Not sure I'll ever manage a 9 hour trip though. Wouldn't be able to walk afterwards. It I guess you'll be doing at least 9 hour days on your forthcoming rally. Paul Quote Link to post Share on other sites
stillp Posted April 14, 2014 Report Share Posted April 14, 2014 Hi Paul, I think I mentioned before, if you wanna see what carbs are doing airflow-wise, then making up some dial gauge adapters to go in the top of the dashpots works nice: https://www.youtube.com/watch?v=3_1S-izfXMk I have used successfully on twin carb V8 and quad carb V12 A friend of mine used to have an SU tool kit, which included a couple of bits of bent wire like partly straightened paperclips. A U-shaped bit went in the carb dashpot, and a straight part stuck out to the side. The idea was to put one in each dashpot, align the straight parts with the engine off, then check as the engine was started and revved that the two 'pointers' went up and down together. It might be easier to make up a couple of bits of bent wire than to make up the dial gauge adapters and set up 2 dial gauges. Quote Link to post Share on other sites
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