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2500 gt6 mk3 no power after 3500 revs


Danwedges
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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 

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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

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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?

 

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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 

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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  

100_1506.JPG

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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.

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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.

Pipes Galore_LI.jpg

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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

 

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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 

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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

 

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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

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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

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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 

1511364733904-1202681509.jpg

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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?

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