Carb balancing after refitting - which tool?

[Ralph Morgan]

I have at last got round to replacing the "Misab" spacers between the inlet manifold and carbs (40DCOE) with the "old fashioned" (but these days moulded plastic, not cast alloy) spacer blocks with O-rings in grooves. This was in an attempt to get rid of the "whistle" on hard accelleration that's plagued my Zetec installation since I did it 5 years ago. Enquiries here suggested that this was due to the venturi or"reed" effect of the Misabs, since they are of a larger internal diameter than the carb and manifold bores each side and lead to sudden pressure changes as the air passes through.

 

Now I've fitted the new spacers, which are of the same I/D as the carbs and manifold (and a 🙆🏻 of a job with the engine in place and clamshell wings!) the engine sounds like it's running on 3 (even 2!) cylinders. I'm guessing that I've disturbed the balance between the 2 carbs when taking them off and refitting. It was running OK before, although was occasionally a bit lumpy on tick-over and low speed running.

 

I assume it's just a case of adjusting the connecting linkage between the two units, using some sort of balancing tool to equalise the flow on each carb. Am I right or could I have disturbed something else? What's the best sort of balancing tool? Searching the web gives a rotary flowmeter type that fits in the trumpet, and a 4-pipe manometer item that seems to connect directly into threaded fittings on each carb. There also seem to be rather expensive electronic units.What's the best to go for? I'm reluctant to spend a lot on something that I'll probably only use once in a while (I've had this car for 16 years and never used one before...) but obviously want something that works!

[ECR]

If you have reasonable hearing you can use a length of small bore plastic tube to listen to the induction hiss from each cylinder and then adjust so that the hiss is the same on all. I use the rotary type of balancer and find it very good.

 

Incidentaly, I'm not sure that you have made a good move replacing the moulded on misab washers with the seperate o ring type. If the engine is at all prone to backfiring through the carbs this can suck the o ring from its seat and give an air leak (I know cause it happened to me !)

[DaveMorris]

Ralph,

not to go against what Roger says - but I had the same "trapped budgie" whistling and it went away when I replaced the misabs with the old style 'o'-rings.

45's are o.k. and don't whistle as the misabs are the same diameter.

 

the following list in not exhaustive but :

Check that you've got the carbs on nice and square and have not nipped up the top or bottom nuts too much.

Hate to ask this but are you sure that all 8 'o'-rings are properly in their grooves ?

Also check that the balancer bracket is in teh right place and that the balancer screw and the thread that it screws into are in good condition. One or both of these can easily be stripped.

A simple synchrometer would do the balancing job for you. If you have a local 'olde worlde' garage near you then they will have one if you don't fancy buying one. A 5 minute job for the owner. (All MGB owners probably have carb balancers too if you know an owner - those carbs go out of balance when the wind changes direction. Mine used to anyway).

 

 

 

X-flow(er) power now resting. Trying Vx for a change.

[Ralph Morgan]

Dave,

 

Thanks for the tips. I'm pretty sure the O-rngs are properly in place, since after a couple fell out at the start of the refit (onto the floor, fortunately, not down the inlet manifold!) I put a spot of glue in each groove to hold them in place. I suppose they still could have got misaligned - it was a pain of a job as I said, to hold things in place while trying to feel where the nuts should go underneath.

 

I think I have all the nuts tightened correctly - not too tight and the same amount of rubber showing in all the spacers (although it's not that easy to see the lower ones, of course) but I'll double check, and also check the balancer bracket as you say. Otherwise, I know a couple of MGB owners so I'll see if I can tap one of them for a favour before shelling out 40 or 50 quid on a balancer tool .

 

Cheers,

 

Ralph

[ChrisHawkins]

Hi Ralph,

 

I have one of these here It works very well but I would check everything Dave suggests first. I would be interested to know if changing the O ring type stops the noise as mine does it and it is a bit irritating, its a shame they do not do different size misab's as they are a nicer solution.

 

Chris

 

K93 AMJ

[oldbutnotslow]

Going back more years than I care to mention I used to run Escorts with twin 40's. In the very early days there was just 1 O ring that used to expand once petrol came into contact with it and once a thackery washer which as you may know looks like a double spring washer, broke out popped the O ring. then things progressed to 2 O rings with the plate in the middle.

The trick that I used was to smear rubber grease into the grooves on both sides of the centre plate and then insert the O rings. The grease stopped the O rings from falling out. Only use rubber grease not any other grease by the way.

 

Grant Taylor - OBNS Motorsport

😬 183 BHP of Black and 'Stone Chip'. YBNS VX Classic. 😬

 

 

Edited by - oldbutnotslow on 7 Dec 2007 15:49:57

[Mr Locust]

Gunson carb balancer is crude but well up to the job. Generally about £20-25 at shows and I have seen them in Halfords.

 

Would comment the same as others; new misab plates over o-ring carriers every time. Also use proper thackeray spring washers rather than the nasty rubber bobbins which harden and give vibration problems.

 

Ian

 

Green and Silver Roadsport 😬

[andy_h]

*arrowup*I've a Gunson carb balancer if you're interested. Used once, still in original box etc. Blatmail me an offer.

 

 

White road legal 2.0 HPC VX race car 😬

[Nemo]

I've got the "old fashioned" O-ring on my 7 and I'm reasonably sure one or more is leaking. From reading the above am I right in saying that I could replace this type with the "MISAB" spacers. Would this be easier than replacing the individual O-rings arrangement or trickier.

 

The statement - "since after a couple fell out at the start of the refit (onto the floor, fortunately, not down the inlet manifold!)" - by Ralph worried me. Knowing my luck one of the (old or new) O-rings will go down the inlet manifold.

 

Once known as KAP but now I'm no-one

Flared wings to cycle wings and beyond Photos

[Ralph Morgan]

Nemo,

 

I've now got a set of Misabs you can have if you're interested!

[Nemo]

Thanks for the offer Ralph but I've just bought the replacement O rings. I've been told the best way to hold the O rings in place while you're doing the job is grease. Any idea what type of grease is ok to use with rubber? Vaseline any good?

 

Nemo

 

Once known as KAP but now I'm no-one

Flared wings to cycle wings and beyond Photos

[millsn]

I'd have a play with your mixture too. seems unlikely that out of balance running would cause the engine to sound like it's running on too few cylinders. More likely that you were originally running one or two cylinders with lots of fuel to allow for air entering that wasn't supplied by the carbs. Balance the carbs ,then review the mixture again.

[oldbutnotslow]

Rubber grease is freely available from accessory shops and is the best thing to use. If it sounds as if its running on 3 cylinders there is a strong likelihood that the O ring has blown and is sucking air.

Replace the O rings and re set up

 

Grant Taylor - OBNS Motorsport

😬 183 BHP of Black and 'Stone Chip'. YBNS VX Classic. 😬

 

[Nemo]

Thanks Grant .

 

Rubber grease it is then. Let's hope Halfords have some .

 

Nemo

 

Once known as KAP but now I'm no-one

Flared wings to cycle wings and beyond Photos