TomMull Posted October 16, 2016 Report Share Posted October 16, 2016 SU choke issues: First, I am not able to pull hard enough o the cable to get full choke. The rear carb seems to operate properly but the front seems to have a distinct “hard spot” as the jet goes down. After disassembly of that forward carburetor, cleaning and installing new choke bearings and seals, I am still quite short of full choke and although much improved, it still has the hard spot. The cable seems to operate properly with no load, it has appropriate stops and the linkage is also correct. Second issue: I could find nothing visibly wrong with the jet in that forward carburetor and I had the intention of replacing it with a new one from my box of spares. However, I didn't like the appearance of the one from my SU963 kit so I polished and re-installed the old one The inlet orifice seemed much smaller that the original and there were no numbers on it. The part where the needle goes in did seem to be the same size as the old one but my means of measurement was not all that precise (drill bits). I still wonder if this is the correct replacement jet and also if there is some chance that the old polished jet could be faulty. Tom Quote Link to post Share on other sites
boxofbits Posted October 16, 2016 Report Share Posted October 16, 2016 Hi Tom I can only think you might have a bent needle, or it is not centralised properly? If bent or out of line it will tend to jam in the jet. Have you tried removing the dashpot and piston assembly on the front carb and then operating the choke. You will also be able to see from the top the jet pulling downwards, which might give you sight of where it is jamming in the bearing? Regards Kevin Quote Link to post Share on other sites
TomMull Posted October 16, 2016 Author Report Share Posted October 16, 2016 Thanks Kevin. The pistons on both carbs drop nicely ant it runs well after it starts. The jet drops marginally, only about .1 inch with the cable but will go down quite a bit more if I pull on the linkage by hand. I'm beginning to think the jet issue might be a red herring and the problem is in the cable or linkage. Tom Quote Link to post Share on other sites
iain Posted October 16, 2016 Report Share Posted October 16, 2016 Tom. I have a very similar issue. Talking to Martin Jay aka the Distributor Doctor,he too had the very same problem. His cure was to take the "Y" shaped linkage apart, thoroughly clean and grease...... problem solved. Mine I haven't had a chance to look at but I'm hopeful is as easy to fix! Iain Quote Link to post Share on other sites
jocklow@aol.com Posted October 16, 2016 Report Share Posted October 16, 2016 I've been round this one. Both the problem of wrong needles and also wrong jets - I guess over the years someone fits something they have rather than buying again - and also dodgy choke application. If you examine the choke linkage between the carbs you'll be depressed to see there are about eight connections in the linkage in total where you can lose movement and result in your original pull on the choke being dissipated. I replaced most of the pins, which got it as good as it'll get without replacing the weird pressed arms as well. I also did the valves in the head (separate problem) and with the engine easier to start now anyway ( better compression, correct needles etc) the choke is adequate to get started even in winter - the real test. Jock Quote Link to post Share on other sites
Ben Freer Posted October 16, 2016 Report Share Posted October 16, 2016 My choke is only connected to the rear carb. It works fine and is easy to operate; perhaps you can save yourself some anguish and do the same? Quote Link to post Share on other sites
Lebro Posted October 16, 2016 Report Share Posted October 16, 2016 Ben. You photo does not show the front carb end of the choke "rod" it would normally rotate when choke is applied, & work on both carbs ? Bob Quote Link to post Share on other sites
Ben Freer Posted October 16, 2016 Report Share Posted October 16, 2016 You are quite right Bob, Here is a better picture.....(taken by torch light!) Quote Link to post Share on other sites
TomMull Posted October 17, 2016 Author Report Share Posted October 17, 2016 Ben and Bob, thanks for the replies. It would appear, however, that Bens carbs are HS6s. Mine are H6, different choke, or perhaps I've missed something. Ian, Thanks also. I'll take the linkage apart and clean as you suggest and fit new pins also. That whole setup seems a bit loose and shaky. Jock, I'm not sure the jets in my kit are wrong or not but they do look different and it would be nice if they stamped the size somewhere. Tom Quote Link to post Share on other sites
TomMull Posted October 22, 2016 Author Report Share Posted October 22, 2016 Tom. I have a very similar issue. Talking to Martin Jay aka the Distributor Doctor,he too had the very same problem. His cure was to take the "Y" shaped linkage apart, thoroughly clean and grease...... problem solved. Mine I haven't had a chance to look at but I'm hopeful is as easy to fix! Iain Thanks Ian. I took the Y piece off and found it to be slightly misshapen, so I replaced it with one from another set of carbs, cleaned and greased. I'd say about 80% improvement. I suspect a new cable might help with the other 20% but I do have some choke now. Tom Quote Link to post Share on other sites
Peter Cobbold Posted October 22, 2016 Report Share Posted October 22, 2016 (edited) I'm not sure the jets in my kit are wrong or not but they do look different and it would be nice if they stamped the size somewhere. Tom Those cross drillings look very different. The fuel flow might be restricted at high horsepower loads by the smaller bore drilling. I would drill out the smaller bore to match the bigger one. The holes dont do any fuel metering, but must allow fuel to flow as fast as the main jet/needle dictate. Which means,as a rough guide, the area of the drillings must be at least as big as the area determined by the needle in the main jet. The difference in position along the jet of the cross drilling wont be so important as long as it is below the fuel level in the float chamber. Assuming the main bore along the jet are identical. Peter Edited October 22, 2016 by Peter Cobbold Quote Link to post Share on other sites
TomMull Posted October 22, 2016 Author Report Share Posted October 22, 2016 Thanks Peter. Yes, the drillings were quite puzzling but the jets did come from kits for the H6 carbs so I wondered if anyone else had noticed the difference. I haven't done the math but the two smaller holes might equal the volume of the jet orifice. I've new kits coming from Burlen Ltd. so it will be interesting to see what they look like. For now, the old jets, polished up, seem to be fine. Tom Quote Link to post Share on other sites
Graham Harris Posted October 23, 2016 Report Share Posted October 23, 2016 If you can get some really slippery lube one the outside of those jets it will make a huge difference to the ease of pull on the choke button. Graham Quote Link to post Share on other sites
TomMull Posted October 23, 2016 Author Report Share Posted October 23, 2016 If you can get some really slippery lube one the outside of those jets it will make a huge difference to the ease of pull on the choke button. Graham Particularly true with the Viton seals. The jet bearing kit I used had seals (cork?) that appeared to have been coated with graphite. Tom Quote Link to post Share on other sites
TomMull Posted October 26, 2016 Author Report Share Posted October 26, 2016 Just a quick follow up: The new kits I got from Burlen have jets that match the originals in every respect, except that the size is not stamped on them, so I can only assume those in my other sets are incorrect or inferior. Tom Quote Link to post Share on other sites
Richardtr3a Posted October 30, 2016 Report Share Posted October 30, 2016 I moved the choke cable to a new position on the dashboard. It was curling round the heater and was arched up in the air over the carbs. I moved the pull knob to a new position on the dash board beside the speedometer. The original location is now used for the spot lamps switch I also made the cable shorter so that it is a more or less straight run. It now is easier to pull and seems to have full movement on the carburetors. I still have to use the clothes peg to keep the choke knob "out". Richard & H. Quote Link to post Share on other sites
TomMull Posted October 31, 2016 Author Report Share Posted October 31, 2016 Thanks Richard. Seems logical to check for poor leads on the cable under the dash but I haven't done that. I will today, and I do have a clothespin. Tom Quote Link to post Share on other sites
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.