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Setting up HS6 Carbs


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I have a pair of HS6 carbs on a TR4a manifold on my TR3a and recently when I had the engine out I stripped them down and gave them a good clean plus reset everything.  I also replaced the springs with new Red ones because I discovered the ones that were fitted weren't the right ones and fitted TW needles.  Now with the engine running and the carbs balanced and the mixture showing bunsen blue on my two colortunes, the jets are are at different depths below the bridge.  The front one is 0.90mm below the bridge while the rear one is 0.76mm down.

This is nothing new for these carbs they have always been like this but I had hoped that by resetting everything including the float heights they would have been closer together.

Has anyone else had a similar experience?

Rgds Ian

PS the bushes etc. are all good.

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Did you set the float heights to be the same gap for both ?  I thought about this some time ago, & because the HS6 carbs have the float chambers arranged so that the front one is ahead of the carb, but the rear one is behind it, & because the engine is not level, but slopes downward at the rear, then to get the same level of fuel in the jets you need the floats to be set differently. I did this on my '3, measuring the fuel height below the bridge (easier to do with full choke on).

Whether it makes any difference - who knows !

Bob.

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1 hour ago, Lebro said:

Did you set the float heights to be the same gap for both ? 

Bob.

Yep.  And it's a good thought Bob but I'm afraid I'm not about to dismantle the carbs to do the same unless this lockdown extends to the middle of July or somesuch and I get really, really bored!

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Couple of points in addition to Bob’s

Are the needles both set with their shoulders level to the base of the piston?

Did you check the piston drop rates are equal   ?   This made a big difference on my carbs that I joyfully stripped and rebuilt without check what went with what.

 

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Yes to both the above Peter. But thanks for the suggestions.  I have also checked that both pistons rise together and by the same amount when I blip the throttle.

Rgds Ian

PS When I acquired the carbs (many years ago) they had indeed been mixed up and it was a long time before i realised why I couldn't get them set up.  It may have even been the same video that got me checking.

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How standard is your engine Ian?  Hotted cams will change the fuelling requirements and the distributor advance curve.  I have diddled for ages to get my mixture sort of right and ended at red springs & RH needles. TW caused a flat at 3k. SM also but not quite so distinct.   A rolling road session is well over due now the running in is over and done with.

Peter W

Edited by BlueTR3A-5EKT
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Hi Peter, It has a Newman fast road cam and a Phoenix tubular exhaust manifold, apart from that it is pretty standard.  I had it on a rolling road last year but that was with the wrong springs.  They still got it running really well. 

As and when I have run it in after it's recent strip down - i.e. when the weather improves and we are allowed out again, I will book it for another session.

Rgds Ian

Edited by Ian Vincent
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2 hours ago, BlueTR3A-5EKT said:

How standard is your engine Ian?  Hotted cams will change the fuelling requirements and the distributor advance curve.  I have diddled for ages to get my mixture sort of right and ended at red springs & RH needles. TW caused a flat at 3k. SM also but not quite so distinct.   A rolling road session is well over due now the running in is over and done with.

Peter W

Hi Peter.

I am trying to best guess what needles I should be using. SM at present, seems to be running ok with no carb changes since before rebuild. now running 87mm, PH1 cam, & 9.5 - 10:1 CR. I'm not surprised TW caused a flat spot, it is much weaker than SM at higher throttle openings. I'm thinking RH or RG.

Ian Vincent is on SY, which is much richer, following a rolling road session at Revingtons. I used to get up to 41 MPG before, I would not wish to be reducing that by a huge amount !

http://www.mintylamb.co.uk/suneedle/

Bob.

176858996_SUNeedlecomparison..jpg.1bd97a797cd1e52d4b2744f4e75604b4.jpg

Edited by Lebro
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1 hour ago, Ian Vincent said:

Hi Bob,

 I’m on TWs now in an attempt to improve my fuel economy because to date it has been pretty dire. Not that fuel economy is a major issue with the the number of miles I cover annually. 

Rgds Ian

Out of interest, just how low was it? On RH’s I was getting 17-19mpg.... currently on SM’s and around 22mpg - no touring-type driving, just commuting and 10-15 mile runs.

....... Andy 

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9 hours ago, AndyR100 said:

Out of interest, just how low was it? On RH’s I was getting 17-19mpg.... currently on SM’s and around 22mpg - no touring-type driving, just commuting and 10-15 mile runs.

....... Andy 

Hi Andy,

Somewhere in the mid twenties if I was touring. For short journeys less than that, probably 20/21. 

Rgds Ian

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1 hour ago, Ian Vincent said:

Hi Andy,

Somewhere in the mid twenties if I was touring. For short journeys less than that, probably 20/21. 

Rgds Ian

Thanks Ian, much appreciated

...... Andy 

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These figures seem very low, before my rebuild I was getting up to 42 on long motorway (mainly) journeys (cruising at around 60 - 65)  round town nearer 30.

Bob.

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17 minutes ago, iain said:

I run RH a get an average of 30mpg even with a 4.1:1 rear axle

Iain

I get more than that on twin 40 Dellortos.

Stuart.

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Good going Stuart, my figure is a tad conservatives on the Liege Brescia Liege, which was from Home to Home 3000miles we achieved 32mpg with some fairly spirited Alpine driving. 

Iain

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22 minutes ago, iain said:

Good going Stuart, my figure is a tad conservatives on the Liege Brescia Liege, which was from Home to Home 3000miles we achieved 32mpg with some fairly spirited Alpine driving. 

Iain

I get a nominal 32  and on a long run a lot more, when running with PI cars on long tours I normally fill every other time to them as they didnt like to run below 1/4.

Stuart.

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On 1/13/2021 at 10:15 PM, Lebro said:

Interesting, TW is much leaner than SM. I'm surprised it runs well.

Bob.

Yesterday we had some sensible weather in this neck of the woods and with enough rain having fallen to wash the salt of the roads, I risked a few miles to check things out.  First off it was a pig to start and I had to replace the fuel in the float chambers with new fuel to persuade it to start, then when I took it for a drive there was a huge flat spot at low revs.  It would idle OK but as soon as you tried to accelerate at low revs the engine just died.  So after about 10 - 15 miles I put the car back in the garage and went for a beer.

This afternoon I pulled a plug and sure enough it was running lean so I changed the TW needles back to SY.  Because it is now raining again - and we are in lockdown and it isn't a quiet Sunday afternoon I haven't taken it for a test drive but just idling and blipping the throttle in the garage it sounds better.

Moral of the story, if it ain't broke don't fix it.  And TW needles don't appear to work in my car.

Rgds Ian

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Ian.

It seems to me you are going from one extreme to the other. TW dips below the SM curve by quite a margin, & SY climbs above it by an equaly large margin.

why not try something nearer to SM, maybe RG or RH which are a little bit richer.

Bob.

Needles.thumb.jpg.52b05864f4d32b60bba802845e1d46e4.jpg

Edited by Lebro
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I hadn't seen the Burlen screen but I have seen the Mintylamb one.  I have my own excel spreadsheet which I have populated with the needle sies for 5 needles based upon data in my Tuning SUs book.

You are right Bob I have gone from one extreme to the other and it is possible that the SM would be the answer but for the moment I've gone back to what I know works albeit thirstily and when the weather improves and we are allowed out again will probably try the SMs that I have.  I went for the TW because that is the stock TR4a needle and apart from the extractor exhaust and cam my engine isn't that far away from a TR4a.  `clearly little things make a big difference.

I'll be interested to hear what you end up with.

Rgds Ian

PS I haven't got a pair of RH needles otherwise I might be tempted to try them.

Edited by Ian Vincent
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Been exploring the Burlin spreadsheet a bit more. If you select the advanced screen, you can compare two needles in terms of percentage change in  area.

It's worth reading through all the text which is on the first page of it, a very versatile tool.

This screen shot compares SM with RH needles, showing that the flat out area of RH is 111% that of SM.  The "F100" is the jet size.

Bob.

2048111488_Needles2.thumb.jpg.0b2479786afa84e9ae55cb2bb9bc9d23.jpg

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Are the oil-filler caps drilled with a vent hole. It is possible with bitsa SUs to have no vent holr and no cross drilling at the top of the damper tube under the inside of the dome. That combination traps  air in the tube that compresses wehn the engine is underload and will richen the mixture at cruise, lower mpg.

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