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MG TD TF 1500 - Non starter after su jet change

I've rebuilt a spare set of carbs and bolted them on, the car ran straight away although a little rough. I spoke to a knowledgable chap who said I should change the jets as well as the needles as they both wear. I've changed the jets and now all I get is a backfire, thinking it's rich I've screwed the jet adjusters right in, but still a backfire, anyone know what to try next?
Cliff Harvey

Cliff,
Did the car run OK with the carbs that were previously on the car??
SPW
Steve Wincze

Hi Steve,
Yes, nothing else has been changed so I know it's carburration, it ran of sorts but with a misfire that I couldn't tune out (not that I've much experience), then with the new jets it simply backfires and refuses to run!
Cliff Harvey

With my MGB and E-Type in the 70's and now the TD I've found that even really poorly tuned SUs will allow a car to run well enough to get home.

I'd start with the basics and work up to the carbs. How old is your exhaust system and are you sure the baffles inside are in good shape? Mufflers with rotted guts can cause all sorts of problems that you will never fix with a screwdriver. Also check the timing, points, coil, points gap, condenser, cap for cracks..... One incredibly small but perfectly placed crack in the distributor cap will make you want to go get your baseball bat.
MAndrus

Where does it backfire? Out the tail pipe or out the carb intakes? Out the tail pipe usually indicates the carbs are too rich, while out the intakes usually indicates the carbs are too lean.
LD Palmer

It backfires out of the exhaust but I've screwed the jet adjusters all the way up to try and clear any excess fuel. I'll pop the old jets back in tomorrow and see what happens ... whatch this space.
Cliff Harvey

Oh, on another point, should TD's have damper springs behind the piston, mine hasn't and neither do two others I know of?
Cliff Harvey

There were, I'm pretty sure, three different type dampers - all brass (older cars I think) with no spring, just the weight of the brass; aluminum weighted with brass or some other heavy stuff and no spring; and all aluminum that used a spring to compensate for not being heavy. Someone with more knowledge that I will have to chime in on how to tell which you have or should have. Jud
J K Chapin

If I remember right only the 1-1/2" H4 carbs of the Mark II and TF had piston springs, not the 1-1/4 carbs.

Tom Lange
MGT Repair
t lange

Cliff,

Copy pasted from the archives, my reply to the piston spring question:

There are 3 types of pistons
1/ Brass.
2/ Aluminum weighted with a steel ring.
3/ Aluminum non weighted.

Only 3/ requires a spring.

John

52 TD
J Scragg

Today I put the old jets back in and it made no difference so l put the new jets back in. I thought I should check the plugs and they were very sooty probably due to all the starting attempts, so after a good clean l tried to start the engine again.
Not only did it start first time but it even responded to my tuning adjustments the new jets seemed to do the trick, thanks folks.
Cliff Harvey

Thanks Lou for the "lean in" / "rich out" info. I'm sure I'll find that piece of advice will prove most useful. Cheers
Peter TD 5801
P Hehir

I stand corrected. Bob Johnson has let me know that the original 1-1/4" carbs were indeed either brass or weighted aluminum, without springs. But when replacement carb pistons were supplied, the later, un-weighted pistons used in MG midgets might be supplied, which require the use of the blue springs.

Tom Lange
MGT Repair
t lange

This thread was discussed between 25/02/2015 and 27/02/2015

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