Vibration may have backed off a little, plus the engine is beginning to loosen up after ~120kms...can't say I'm loving it though, feels very stiff indeed. A leak in the carby and fuel pump (insulator between pump and block is split...anyone got a spare?) made it very difficult to run in (constant misfire), so went back for repairs and was collected today.

Needs a tune...the guy knows his way around an engine, but not great at setting up twin SU's! Very rough and hestitant at low revs when cold, idle keeps on increasing as the car warms up and needs the odd blip on the gas to encourage it to drop under 1500rpm after a while! Thinking of approaching MG Sales and Service up the road, they should know what they're looking at.

New muffler is next, need to get it sounding like Bruno's!

Looking good there Dwayne.
I had my own share of issues when my engine in Rufus was rebuilt. but all good now, except for minor vibration at 3000 rpm...even after it was balanced.
Look what just arrived for my two cars. They are decent quality SU throttle return springs. Chrome plated, ( thought they may have been stainless steel). Come in packs of 3 for $7.90 from SU Midel in Sydney.
Yes, it is important for the carbies to return to idle easily, and make sure the lower bracket where the bottom end of the spring attaches doesn't move....tighten nut well.

Cheers, Lewis

Hi All
Arr the Black art of Carbies....... Love it

My initial question would be Were these carbies running on the engine fine before the rebuild? If yes then happy days, If no then not so happy.

SU are notorious for off idle stumble. This is usually due to Leaning out which is commonly (and in your instance sounding like) a vaccuum or manifold leak. This would most likely also be why your idle rises.

So where to start
1/ Set base timing approx 12' BTDC. (IF running weber they prefer more timing for interest sake). Note difficult to do if idle at >1500rpm so check and re-check as able.
2/ Check all your linkages are smooth and equal across your carbies (1st step of balancing them) and that the idle stops are engaged with approriate spring tension as per above post.
3/ Grab a large diameter tube ? garden hose or heate hose, cut into approx 80cm, place to your ear and listen for leaks EVERYWHERE.
4/ When you have a suspicion such as carby to manifold, manifold add ons or maifold to head try spraying WD40 and see if engine revs change. Some people say to use Carby cleaner but not good for nice new paint!. If you have a leak the revs should lower.
5/ Repeat, repeat, repeat.

Lets say it is a vaccuum leak and you fix it, happy days. If at a gasket perhaps add high temp Copper silicon but note breaks down with contact of fuel. (could try this with your fuel pump as well)

6/ Remove aircleaners and use same tube to compare induction noise at inlet to carbies. You will hear a large sucking sounds. If all good in the NO Vaccuum Leak Department then they should be the same and adjustable and SENSITIVE/RESPONSIVE to adjustments to throttle stop screws If Not then you are not out of the woods yet. Once sounds are close to equal, the adjustments under the carby effect the fuel mixture through the entire range of operation so differnt kettle of fish and different level of skill.
7/ If still having off idle stumble as I did on my G180ss with 1 3/4 SU's, I pulled the tops off the carbies, removed the pistons, turned them upsidedown and changed the height of the needles. In my instance I lowered then approx 1mm and problem solved, pulled like a Malley Bull

When you have it sorted she will purr like a kitten and roar like a teenage tiger, all this talk of not happy will disappear as you tear up the open road

Hope this helps
Feel free to PM me if you prefer me to chat
Cheers
J

Thanks guys, great advice!

James, the carbies were crap when I took the engine in for a rebuild, with the vacuum hose not even connected...but it was still fine once warmed up.

Well, just drove to work this morning with the car and, yes, revs increased gradually the whole way. The vibration was particularly annoying today, the freakin' panels are going to shake off the thing, and this IS a balanced motor. All Jap Day this Sunday, probably will only have time for a quick wash, so will be snorting, revving and shaking its way through the crowd. Lets hope some carby tweaking will help with at least some of the issues.

OK
Lets assume you dont have any vaccuum leaks (but I would bet my bottom dollar you do)

SU Hitachi's one-on-one

Follow previous steps then

SU's require oil to dampen the piston. The black hand tight screws on top, remove them to see a tube with a bulbous end. On these tubes is a small mark indicating the level of oil to be placed in top of the carbie. Here's the kicker, what oil do I need. When you try to balance the carbies using the listening tube, look down the throats and visualise the pistons, as you rev the motor they should raise up EVENLY. However how fast should they rise..... well not to fast and not too slow. This is determined by the spring tension and what thickness oil you use. If raise too fast try 30W mono oil (or 20-60 engine oil), if not too fast try 15W, if way too slow try Auto transmission oil. I think you may not have oil at all or too thin by the sounds of it and I bet your carbies are not working in unisen hence the Poor performance and worsening vibration. If the pistons raise too quickly then they lean out and stumble or 'fall on its face' as is commonly cited. If raise slowly this is better for torque as is predominately rich through the rev range especially off idle.

Check that the butterflys are also opening evenly. This will be difficult if you have excessive wear in the throttle shafts or the shaft linked to the Throttle linkages. Best you can do is adjust the plastic links to get them as even as possible. If not then one opens before the other then all bets are off with the fuel mix department.

Good luck
J

Havent ever had a GT on the road, but the twin Aisans on my Crown S opened best with sewing machine oil in the chambers. This slowly leaked away, so needed topping up periodically. I filled the chamber.

Duane, I have plenty of fuel pump spacer blocks, send me your address and I will mail you one.
Cheers, Matt.

I used sewing machine oil in my jaguars carbies way back when I had it, seemed to work best for them at the time.

Hi Guys
Found it thanks to our Datsun cousins

http://datsun1200.com/modules/mediawiki ... Adjustment

All you need to know and easy to follow

Re lifting piston with inbuilt plunger - just use a flat blade screwdriver from intake under piston
Revs should raise then Lower as u slightly lift piston = correct mixture

Cheers
James

yes SU tuning is a black art indeed
learnt all about it with trial and error on my twin setup on my 1500 sedan.
and sewing machine oil seemed to work well on my dampener slide of my pistons!

i always did balancing with my eye as the SU is very easy to see how the two pistons are moving.
there is a test button on the base of mine that raise's the piston by about 2mm and that is used for checking if the mixture is rich or lean. Same as using a flat blade screwdriver but much easier. dont know if the GT SU's have the same button.

happy to have a quick look at it tomorrow, Duane. you may be driving a different car home.

OZ

Cheers for all the advice guys, magic! Bruno and Chris helped out a lot on Sunday at AJD too...definitely a carby issue, plus engine mounts are too hard (recently renewed). Not sure what to do about the later, maybe drill a few holes? Or wait?

Thanks Matt, PM coming your way!

Duane