20 replies to this topic

[seamistgreen]

Gday,

I am hoping someone can point me in the right direction, in solving my tuning issues.

My project ( Australian 1966 Morris Mini Van) has a 1989, imported 1275, large valve head, and I have put a set of twin hs4 su's on.

new throttle discs, jets, bushes on drivers side. Loosened all clamps, un wound screws so carbs are running independant.

Drivers side carb sucking in loads more air than other, and running rough, idle is up and down. Cant figure out why one carb sucks in more air than the other. Compression test was cylinder ( 1,2,3,4) , 135ish, 125ish, 135ish, 125ish.

Just ordered new needles and matching springs, ( dont know what old ones are).

Will try looking for vacuum leaks again, swapping existing pistons/needles,

Would love to know what else to check for.

Cheers, Richard

Attached Files

•  balancing.jpg   79.22K   96 downloads

[Dusky]

Did you balance the carbs in the first place?
match the piston to the dashpot ( nice guide on youtueb somwhere).

Then undo the throttle linkage when the car is hot, take a lenght of tube ( if you dont got the vacuum meter) and put 1 end in your ear, the other end in the carb. You then adjust the idle screw untill both carbs 'hear' teh same.

After that you tighten the throttle linkage again.

 

If the carbs are worn they'll suck air through the throttle spindle.

[nicklouse]

Well your first problem is your throttle cable is not fitted correctly.

Scrub that it might be but hard to tell.

But it looks like you have linkages not connected etc etc etc.

Presuming that that is actually a picture of your set up.

Edited by nicklouse, 13 October 2015 - 12:51 PM.

[jpw1275]

Your main problem is you have hs2 carbs not hs4 !!!!

Cheers james

[jpw1275]

Have you fitted a new gasket kit as it sounds like you have an air leak or you have worn spindles as said above ..... You really need to give them a full rebuild and take it from there ....

Cheers james

[carbon]

Do both pistons fall under own weight all way to the bridge? If this is fixed needle HS2 then centering the jets is critical. But that should not cause imbalance in air flow.

 

Check that both throttles are able to close fully, not catching on anything, may need extra return springs if not already fitted. Also there are adjustment screw on both the choke cams for fast idle, make sure this is not keeping the throttle open.

Edited by carbon, 13 October 2015 - 04:45 PM.

[madaboutcherry]

where's the throttle cable?

[jpw1275]

where's the throttle cable?

The red cable which goes to the bracket on the manifold

[dklawson]

Seamis, you have been given some good advice and things to look at in the posts above.  For clarity can you post the following additional information.

 

You listed a bunch of new carb parts and work.  

• Who did the work and have the carbs been operating properly since then?
• Since these carbs are rebuilt, did you confirm that each has the same type internal piston spring?
• When the needles were installed in the pistons, did you make sure their shoulder was flush with the bottom of the piston?
• Did you center the jets as Carbon mentioned?
• I know you said you did... but... when you made your air flow measurements did you have both throttle and choke linkages (not cables... linkages) 100% loose?
• As Carbon mentioned, when you made your measurements was the choke mechanism on each carb completely backed off and loose?
• Have you done the piston drop test as mentioned above by Carbon and Dusky?  If not, visit YouTube and look for the piston drop video by John Twist of University Motors.

[seamistgreen]

Thanks Carbon, Dusky, and Dklawson, good advice, and things to check for.

I put matching M needles, with matching Red springs,

I measured air flow with vacuum metre, whilst throttle and choke linkages loose,

the jets are centered, needles are flush with bottom of piston,

butterfly (throttle valve?) shuts fully.

I am thinking, vacuum leak, I have spare inlet manifold that I will try,

will also check valve clearances, noticed vac advance on electronic distributor not making a difference ,when connected or not??

I do appreciate the advice, thanks guys. :)

A process of elimination, I guess.

I do hope the fairly even compression rules out internal problems.

[nicklouse]

is that picture of your carbs?

[dklawson]

Before you start looking for vacuum leaks, take the carbs off one more time.  Back the idle screws all the way off so the throttle butterfly disks are closing off the carb throat.  Now look carefully at the disks.  They have a beveled edge to completely block the throat.  If the disks are put on "backwards" one edge of the bevel will contact the throat prematurely and hold the throttle open a bit.  Hold them up to a light and see how much light is "leaking" around each disk.   If the bevels are not seating properly in the carb throat that will mess up the airflow quite a bit.  

 

If the carbs have the stamped metal choke linkage, make sure that the jet returns to the "top" when the choke is actuated and released.  If that linkage binds, the jet will be held lower and that can affect running by quite a bit and make accurate tuning impossible.

[carbon]

Just to add to Doug's comments about getting the butterflies installed correctly. Yes, absolutely essential these are installed the right way round.

 

The other critical point is that the butterflies need to be correctly centred before throttle screws are tightened up. If they are not then there is possibility that one edge (upper or lower) may catch on the carb bore, leaving a small gap between the other edge and the carb bore. Then regardless of how far you screw out the throttle adjustment screw there will be gap and air flow.

 

So when installing butterflies into throttle here is suggested install sequence:

- put butterfly into spindle (the right way round)

- make sure throttle adjustment screw is backed right off or removed

- put in both butterfly retaining screws, but leave loose to allow butterfly sideways and up/down movement

- fully close the throttle. Should be no gap at either top or bottom of butterfly

- now tighen both butterfly screws, keeping throttle closed with light pressure

- check throttle opens smoothly and also closes all the way

- lock butterfly screws

[nicklouse]

Have any of you looked at the picture?

The carbs are not even connected correctly.

Again presuming they are the carbs in question.

[jpw1275]

Agree with Nick... I think we need some more pictures of this set up as it is , looks like the float bowl gaskets aren't new , I know it's not the problem but what are the rest of the gaskets like if these are meant to be rebuilt ? What fuel pressure are you running the twins on ?

Cheers james