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SU HS6 Tick over incorrect


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My TR7 tick over was running a bit fast (about 1000rpm), it seemed to be a problem with the front carb. The slow running screw didn't make much difference but loosen the shaft links and press down on the butterfly spindle soon slowed the tick over down. Seemed like the front carb was getting an air source via the spindles. Duly I purchased an HS6 repair kit from Burlen and changes the spindles and butterflies making sure the sure the butterflies were centrally located in the mouth of the carb.  Apart from that I put new gaskets on the float bowl and inlet manifold mounts. I didn't change the jets or replace the spindle bushes. The new spindles seemed quite OK in the body of the carb when installed.  

The problem is I now cannot get the engine to tick over. It either runs erratically at 1500rpm or hunts slowly between 500 and 1500 rpm. The obvious problem would be an air leak. I am running the car in the same manner as before, the servo hose is connected, as are the two pipes to the rocker cover and the small pipe to the air box is blanked off. I have also put a smear of grease on the four manifold gaskets to the carbs as well.

The carbs are fitted with Waxstat mixture adjustments on the underside.

I have had the carbs on and off four times now and have just about run out of ideas of how to sort this problem, so any help would be gratefully appreciated.

 

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What state are the carb rubber mounts in?  Not uncommon for them to split and leak air into the manifold.

http://www.leacyclassics.com/tkc1338.html

Cheers

Peter W

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The carb rubber supports were only replaced last season. The engine was running perfectly (tick over a little high) before I took the carbs off and put new spindles and butterfly in.Putting pressure up and down on the carb assembly makes no difference to the speed o=f the engine now which sort of rules out air intake at the rubbers.

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The repro rubber carb mounts are quite poor quality and can start leaking after a year. The other point is to ensure the carb linkages are correctly tighten and both throttles are turning together.  Also maybe lack of fuel to one of the carbs e.g. a blocked fuel pipe. Or problems with the breather pipes.

Forgot one last area, that is a sticking piston in the carb

73's

Dave

Edited by DaveR
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Posted (edited)

Hi Dave

No leak before carbs taken off and spindles and butterflies changed. So it would be a real odd coincidence that whilst carbs on bench, that the rubber perishes, so I think we can rule that one out.

I am tuning carbs with linkage removed, one carb at a time, just trying to set slow running. Both carbs have fuel in float chamber as the engine will run (roughly) when the mouth of each carb is blocked in turn. 

I have tried the engine with the breather outlets on the carbs blocked and also with hoses connected. No real difference.

The pistons seem free in the chambers. You can see the pistons rising and falling slight;y as the engine speeds up as it hunts from 500-1500 rpm. It doesn't hunt every time though, sometimes it is just stuck at 1500 rpm. Lift the pistons and they both fall easily.

73

Edited by steve redway
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Hello Steve,

Looking at any possibility and hence brain storming.

Air leak on the cam cover or oil filler cap. Blocked carb jets. Running too rich due to the float chamber valve not fully shutting off.

True the above may have no direct connection with your previous work, but gremlins can strike at any time!

73

Dave 

 

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Steve,

When I was looking to replace my carb rubbers a few years ago,the  Robsport mechanic warned me that all the available replacements were c**p so do not use them unless you absolutely have to. I have heard that they may only last a few months so pulling off the carbs after a year would be quite enough to finish them off.

If I recall correctly S+S were making a replacement metal set, not cheap but probably the best solution otherwise find a secondhand original set  of supports before you do anything more to the carbs.

Cheers

Howard

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Good point Howard, but I am not sure why there would be no change in the (****) tick-over whilst pulling backwards and forwards on the rubbers if they were perished? Indeed there is absolutely no movement whatsoever when you engage in this activity either. Prior to changing them a year or so back they would wobble all over the place. I have replaced the gaskets between in the inlet manifold, throttle spring bracket and the carbs, all four of them giving them a good smear of grease too.

Just as an aside i did find traces of silicon on the carb rubber faces which although I cannot remember completely, this would indicate that I had trouble sealing the rubbers to the throttle spring bracket in the past.

Dave looking at your points: Air leak on the cam cover or oil filler cap. Blocked carb jets. Running too rich due to the float chamber valve not fully shutting off.

Whilst the tick over was OK before pulling the carbs off and replacing the spindles and butterflies, the cam cover and oil filler cap both have new gaskets. If the float chamber valve was not fully shutting off then I would expect over flow from the drain tubes on the float bowl. I have incidentally replaced the fuel bowl gaskets.

I guess running too rich could be a possibility, however the richness nuts were not moved in the "repair" process.

Oil in the dash pot is circa SAE30 and you can see the pistons lifting slightly as the engine slowly hunts from 500-1500 rpm.

I think this hunting aspect of the tick over may be the clue. The rising and falling of the tick over in a 10 second period from 500 or almost stalling to 1500rpm then back again. I have never experience that one before?

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That hunting of the revs suggests it might not be an air leak as that would surely be constant. It does suggest fuelling, with an excess of fuel being used up then the revs dying until the excess builds up again. Float chamber level could be a prime suspect. 

If you pull the tops off the carbs where does the fuel level sit in the jets?

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3 hours ago, HowardB said:

Steve,

When I was looking to replace my carb rubbers a few years ago,the  Robsport mechanic warned me that all the available replacements were c**p so do not use them unless you absolutely have to. I have heard that they may only last a few months so pulling off the carbs after a year would be quite enough to finish them off.

If I recall correctly S+S were making a replacement metal set, not cheap but probably the best solution otherwise find a secondhand original set  of supports before you do anything more to the carbs.

Cheers

Howard

The repro's fell apart in 6 months. I too fitted the metal type.

73

Dave

Edited by DaveR
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OK I have approached from a different angle. 

The timing was rather retarded, so have tweaked that to 10 ish degrees before TDC. I have brought the jets right up to their maximum and then backed them down two full turns. Warmed up the car which sort of runs at 1200 rpm then lifted the dash pot lifters by 1/32 and tweaked the mixtures nuts to get minimum slow down of the engine when lifted. Then tried to get the tick over to about 800 rpm. This can be achieved for a short period 15 seconds before the tick over slowly dies and the car stalls. The slow 500-1500 rpm hunting has now been eliminated.

Am I getting closer to getting this car on tune I wonder?

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I have pulled the float chamber tops and removed the floats. The petrol level is 1" from the top in both carbs.

Currently I have got to the stage where I have managed to get the car to run at 900 rpm for a full minute before it slowly died on me

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I didn't mean the float chamber tops Steve - rather the suction chambers and pistons so you can look down the jet.  According to the SU book the fuel level should be between 1/8 and  1/4 inch down in the jet tube.  (Of course you will have to re-fill the float bowls again by running the engine before looking at that.) Sorry for not making it clear.

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Cant see anything all down the carb jets, they are just too small.

Currently getting the tick over to anything below about 1100 rpm  just causes the engine to very slowly die. Rev it, tweak the tick over and it goes back to 1100-1200rpm OK, reduce the tick over slightly and it slowly fades away and dies. This is becoming an annoying pain.

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