Triumph Repair: Midet (I know its not a spitfire really!) 1500 rough idle, fuel pressure gauge, low pressure gauge

Question
QUESTION: Pior to standing still for 3.5 yrs she had all electrics changed, carbs balanced etc, and she ran as sweet as a nut, unitl the exhaust fell off. 3.5yrs later, replaced exhaust, but didn't tighten everytething up. Spent two weeks getting her up and running and ready for the MoT, still running sweet, tighten up the downpipe, and now she struggles to start, runs rough-ish at idle,until choke pulled/revs increased. Haven't put her on the road yet, so no road trials. What did i do wrong?
ANSWER: Hi Martin,
First thing to try is loosen the exhaust down pipe again. I know it sounds silly but a car that sits for that long can have a stopped up exhaust and if that is the only thing you did when it started running poorly, that is what needs to be checked first. Another method and a positive test, is to drill a hole just a slightly smaller than the OD of a auto brake line and cut a small piece of brake line about an inch long and drive it about 1/4 in. into the hole anywhere in the down pipe. Put a low pressure gauge like a fuel pressure gauge on the nipple sticking out of the down pipe and read back pressure while reving the engine. You should not see any more than 1 1/2 PSI even at a high RPM. Some domestic cars claim they can tolerate up to 3 PSI but I never found any British car that could handle that much.  
Another point is, you said carbs not carb. All Midgets with 1500 engines had one carb thus didn't need any balance work. You didn't say what year this Midget was. If it is a 74 or earlier, it is not a 1500.
Let me know what you find and I will help you diagnose further.
Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,

Many thanx for your swift response.

Yes it does appear to be bunged up, is there anything I can do except replace the silencer?

My midget 1500 1976 does indeed have twin SU HS4 carbs fitted, fitted as standard to the UK midget 1500s, the USA ones had singles I believe.

Regards
Martin

Answer
Cool! the twin SUs were a better carb system.
Depending on your local laws I have seen mechanics take a stopped up muffler off and with a large steel bar and a big hammer, drive holes through the middle of the muffler to open it up. It makes a little more exhaust noise but lets the engine breath. As long as you don't have more than 1.5 PSI back pressure at speed, it will run OK.