Twin carbie issues

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geminilover3236
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Twin carbie issues

Post by geminilover3236 »

So having a bit of a drama with a twin carby setup (su single throat carbies). First of all, it's not on a Gemini but on an old 4cyl MG A.

Carbies have just been rebuilt by a shop.
The MG motor has the inlet and exhaust all on one side, with 3 exhaust ports and 2 inlet ports. Cylinders 2 and 3 go into the middle exhaust port and 1 and 4 have their own individual ones. Inlets are feed 1 & 2 and 3 & 4 respectively.

So, bolted the exhaust manifold, carby manifold and carby all up using new gaskets and gasket compound. However, the thing is only running on cylinders 3 and 4. Take the leads off 1 and 2 and there is practically no noticeable difference. There is plenty of spark and I have switched plugs to make sure it's not the unlikely case of dead plugs. After the car has been running for a while, taking the plugs out of no1 & 2 you notice that they are wet so there is fuel getting in there just not going bang. Spraying aerostart down the throat while the car is running seems to make some difference but not a lot. Ideas are appreciated.

I'll try and get some pictures to help out.

Cheers guys.
TURGEM
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Post by TURGEM »

try swapping a lead and checking under dissy cap for corrosion
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GEMI79
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Post by GEMI79 »

They are su's you said, I would check the slides are properly lubricated. See of the slide is working. Make sure they are synced correctly. Also are the plugs new or old?

Cheers.
my gemini may be s*#t, but ur ugly, at least i can fix my car
geminilover3236
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Post by geminilover3236 »

Im confident it's not a spark issue as we have switched the plugs from 1 and 2 into 3 and 4 with no change and when you take the leads off while the engine is running you can see plenty of spark leaping to the spark plug.

I've tried turning the mixture screw on no 1 carbie from all the way from lean to rich and vis versa whilst running with no change.

I'll check the slide thing later. After I figure out what exactly that is. I assume it is the thing that the choke moves up and down on. There oil levels in the carbies are fine.

keep the ideas coming guys, they are all welcome. trouble shooting seems to be the only way Im gonna get past this.
GEMI79
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Post by GEMI79 »

Check to see when you have it going that the slide is moving to allow air in. Also check for any vacuum leaks.

Cheers Steve.
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marksegec
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Post by marksegec »

How about a fked head gasket or worse, rings / pistons are gone?

No xplosion, plenty of fuel, ...

says to me do a compression test.
geminilover3236
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Post by geminilover3236 »

marksegec wrote:How about a fked head gasket or worse, rings / pistons are gone?

No xplosion, plenty of fuel, ...

says to me do a compression test.
Yer, I suspected maybe a break between 1 and 2. We did a bush mechanic compression test by cranking it and holding a finger over the spark plug hole, there was still compression so not all lost. Would have to get the guage to know for sure though.
GEMI79 wrote:Check to see when you have it going that the slide is moving to allow air in. Also check for any vacuum leaks.

Cheers Steve.
There is a mechanism that allows you to manually move the slide up from underneath. I have moved this up slowly whilst it's running and has made no difference.
GEMI79
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Post by GEMI79 »

Ok then in that case I would pull out a plug put in no.1 lead, earth the plug, turn it over be sure it has spark all the way to the plug. Has fuel, has comp needs good spark.
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Gem WRC
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Post by Gem WRC »

Stupid question :lol: Are the plug leads on in the
correct firing order?
marksegec
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Post by marksegec »

LOL very valid point!
geminilover3236
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Post by geminilover3236 »

Gem WRC wrote:Stupid question :lol: Are the plug leads on in the
correct firing order?
Hurp a durp durp durp.

We have a winna. Im such an idiot. So this is what happened. In an effort to fix the problem I replaced the points in the dissy but when I put it back in I plugged the leads back in coz they'd fallen out and I must have got 1 and 2 back to front.

The actual problem in the first place was that the mixture screws were full in (lean) and thats why it would only run on choke. The points were also screwed but it was a secondary problem.

Now I've just got to figure out every other problem it has. Thanks for the help guys.
TURGEM
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Post by TURGEM »

its absolutely important that they are syncronised...

google; carby syncronisers and you will see what im on about and what theiir purpose is
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geminilover3236
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Post by geminilover3236 »

TURGEM wrote:its absolutely important that they are syncronised...

google; carby syncronisers and you will see what im on about and what theiir purpose is
Yer, I've heard a bit about these, never used one. I have also heard the old 'hose to your hear" trick.

So far Ive just been adjusting the mixture screws to the same position on each. i.e. 3 flats out on the rear, 3 flats out on the front. then idle speed, same deal. Definitely far from a good tune atm. Doesn't help its overheating while idling, so need to sort that out too.

Im getting contradicting clues though as to the tune. Plugs are fouling up (carbon) but the test with the SU's push the choke open just a tad and see if the engine increases revs, increases then drop to same, or drops.

It drops indicating lean mixture tho. More investigating to be done.
ajs991
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Post by ajs991 »

Give it to someone to do. Best money I spent was getting someone who knew what to do to look at them.
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TURGEM
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Post by TURGEM »

To be honest...

I dont think its really worth reading spark plugs much anymore, unless your still running the old (leaded)... Which I doubt,

Temp wise just make sure your timing isnt too retarded as this can cause overheating...

It mite be worth asking if there other SU owners out there, asking whats jets they have to give you a ball park...

P.s. Just because the mixture screws are the same doesn't mean they are... Id definately have a crack at the Hose in your ear trick.
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WASTEGATES - WASTE 8S
IF YOU WANNA FLY, GIVE IT MORE P.S.I 8)
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