Danwedges Posted November 20, 2017 Report Share Posted November 20, 2017 Hi everyone I've recently became the owner of a mk3 gt6 fitted with a 2500 running twin su hif carbs (unsure exactly what carbs and unsure how to tell) and it drives fine below 3500rpm but won't pull at all above 3500 and will only creep towards 4000. I have the mixture running slightly rich and the timing set to 13deg btdc temperature stays around the half way mark and the only way to get it to pull past 3500 is to run with the choke fully on which is what has me confused as I have the mixture running slightly rich to start with I would expect choke to bog it down. Anyone experienced aanything like this or have any ideas how I can fix it? Any help or suggestions would be greatly appreciated! Thanks Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 20, 2017 Report Share Posted November 20, 2017 Think the first job is to pull the needles so we know what specification is fitted What air filters are fitted Any other modifications away from standard Is there any brass spec. tags on the carbs float top screws Pete Link to comment Share on other sites More sharing options...
Danwedges Posted November 20, 2017 Author Report Share Posted November 20, 2017 It has k&n filters and no tags the only markings I can find is pcd1 on one carb and mcd1 on the other I will have a look for a strip down guide and make a crack at getting the needles out tomorro after work Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2017 Report Share Posted November 21, 2017 are there any fuel filters fitted? With a KN filter it needs very different needles to a std filter box. If the fuel, lines have been disturbed recently then the dreaded sliver of rubber hose that floats about in the supply pipes and gets stuck in the back of the float needle valve, this will reduce fuel flow and these little sods can be a real menace theres a good few Tee shirts out there for these critters. Pete Link to comment Share on other sites More sharing options...
Danwedges Posted November 21, 2017 Author Report Share Posted November 21, 2017 It has 2 inline fuel filters one before the pump and one after, I replaced the one before thinking it could be restricted (one after looked okay) but no change. I'll pull the tops off the carbs tonight and look at the needles and clean out what I can then the next step would be strip and rebuild the carbs any idea what needles I should have or would I be able to adjust my needles to suit if they are the issue? Link to comment Share on other sites More sharing options...
dougbgt6 Posted November 21, 2017 Report Share Posted November 21, 2017 Also, if everything else is correct, timing at 13 degrees BTDC is too much these days. It's in the manual as 13 but that's for old 5* fuel, modern stuff doesn't like it, mine's set around 7. Easy enough to adjust, turn the distributor for maximum revs, then back a gnats. Unsure about needles, I've got Strombergs and there are only three, early, late and pancake filter. I believe there are loads for SUs Link to comment Share on other sites More sharing options...
Danwedges Posted November 21, 2017 Author Report Share Posted November 21, 2017 Pulled the tops off the carbs and took some photos they seem quite dirty to me so will get some carb cleaner and give them a blast off any ideas if these needles are suitable or if the problem could be from elsewhere Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 21, 2017 Report Share Posted November 21, 2017 The carbs are from a Dolly Sprint. They are fitted to a standard GT6 Mk3 inlet manifold with a Triumph Tune adaptor kit. It is difficult to know what needles and springs you should have without knowing the compression ratio and cam. As a rough start point can you tell me what the engine prefix. e.g CR, ME, MG etc. Dave Link to comment Share on other sites More sharing options...
Guest Posted November 21, 2017 Report Share Posted November 21, 2017 Dave. The photo of Dan's set-up has an additional hose in front of the rocker breather hoses compared to your photograph; could it be that Dan's carbs are emission controlled SU's which may be part of the problem. I cannot say for certain, but just guessing from his photo. Regards. Richard. Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 21, 2017 Report Share Posted November 21, 2017 Thanks Richard for spotting that. You may be correct. In which case they have Sprint tops on emission carbs. This is too fit under the GT6 bonnet. Why not just fit the Sprint carbs? Unless the pipe is from the float chamber vents? Thanks, Dave Link to comment Share on other sites More sharing options...
Danwedges Posted November 21, 2017 Author Report Share Posted November 21, 2017 The pipe is fuel from one carb to the other and the engine is mg51303ebw feel like we're making progress now we have identified the carbs haha Link to comment Share on other sites More sharing options...
Clive Posted November 21, 2017 Report Share Posted November 21, 2017 Have you looked at the other forum (s?) you posted the question on? You may find useful information there! BTW have you checked the throttle is fully opening? 1 Link to comment Share on other sites More sharing options...
Guest Posted November 21, 2017 Report Share Posted November 21, 2017 Dan. I have numbered the various pipes as I think some clarity is required, ideally a photo from the other side. Other than pipe 3 (rocker breather), please can you say what each of these pipes are doing and also where they connect to on the carbs or other junctions ?? It may give a better insight (well to me at any rate) and promote a more detailed response. Thanks. Richard. Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2017 Report Share Posted November 21, 2017 From the photos of the carbs I hope the offset dashpot tube is down to camera angles Do not like the look of the jet , it should be a few mm below the bridge, not level with it, the top pic looks like a different jet bore And the holder is set lower You ask abour needles but you have to remove them from the air piston to see their letter number Its marked on their ends Pete 1 Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 21, 2017 Report Share Posted November 21, 2017 The MG prefix is for a 2.5PI Saloon engine. BDC needles with yellow springs would work on that engine. BAE and green springs will given better performance with less MPG. But one point worth exploring is if the throttles are in sinc and fully opening when you operate the throttle pedal. Thanks, Dave Link to comment Share on other sites More sharing options...
Danwedges Posted November 21, 2017 Author Report Share Posted November 21, 2017 Clive I completely forgot about that other post thanks for reminding me! 1 And 5 are overflow pipe I think 2 is fuel in and 4 is fuel to 2nd carb I'll pull the tops off again and see if I can find the numbers on the needles and have another look at that jet to see if they are the same size I'll let you know how I get on might be tonight but it might have to wait for some daylight tomorro after work Link to comment Share on other sites More sharing options...
Danwedges Posted November 21, 2017 Author Report Share Posted November 21, 2017 Okay so I've pulled the tops off and measured the depth of the jets back carb is sitting .06' below flush and front is .03' below flush both running bbs needles the look to be ground down one side how do i identify the springs? I don't have a way of measuring the jets without stripping the full carb but one jet definatly looks newer than the other Link to comment Share on other sites More sharing options...
Guest Posted November 21, 2017 Report Share Posted November 21, 2017 Dan. Spring identification is normally via their colour. Link to comment Share on other sites More sharing options...
Pete Lewis Posted November 21, 2017 Report Share Posted November 21, 2017 The spring wire diameter is another way . Use a drill to gauge the jet hole Burlen fuel systems have most of the spec showing sizes Pete Link to comment Share on other sites More sharing options...
Guest Posted November 21, 2017 Report Share Posted November 21, 2017 Following on from what Peter has mentioned, this Burlen link may assist: http://sucarb.co.uk/piston-springs.html You may also want to have a look at their ethanol proof "Stay-up" floats amongst other things on their site. Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 22, 2017 Report Share Posted November 22, 2017 On the MG prefix engine the needles run from weak to rich starting with weak, BDB- BDM -BDC-BAE, ending with the rich. Yellow spring are the normal but fitting green will improve the low end torque. Air filters are free flow e.g. K&N's. Spent many an hour on HS6's fitted to both 2 and 2.5 litre engine. I was never happy with them on the 2 litre. I am aware they were used on the later 2000TC but they had more to do with emissions than performance. For performance they were OK on 2.5 with right cam and head. Dave Link to comment Share on other sites More sharing options...
Danwedges Posted November 22, 2017 Author Report Share Posted November 22, 2017 I've had a look at the springs and can't see any identifying colours needles are bdb with modifications I'll upload a picture I'm going to try stripping and cleaning the carbs this weekend any advice as to what height the jets should sit? The jets seem to be the same size 2.5mm drill has about the same play in both sides If a good clean doesn't work I think I'll order some bae needles and hope for the best Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 22, 2017 Report Share Posted November 22, 2017 The needle profile looks in a poor state. It should be round. Also BDB needles are to weak. Plus as Pete has pointed out are both the jets the same. They should have a 0.10" bore and have a single grove at the top to identify this. Dave Link to comment Share on other sites More sharing options...
Danwedges Posted November 22, 2017 Author Report Share Posted November 22, 2017 I have checked the jets using a 2.5mm drill shank measuring 0.097' and both had similar play but can't see any grooves on the top of the jets but I haven't taken them out just looking in from the top I think needles are going to be a must by the sounds of things I assume springs will be standard too would these need replacing to go with the richer needle? Link to comment Share on other sites More sharing options...
dave.vitesse Posted November 22, 2017 Report Share Posted November 22, 2017 Sounds like the bores are the same. The grove can only be seen when the jets are removed. See attached 3b. Your carbs are HS6 types Link to comment Share on other sites More sharing options...
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