i've recently had a head gasket and timing belt done,now the car seems to lurch slightly between 40 and 50 mph,it only does it once and is usually but always accompanied with a backfire,i also tow a caravan and it is more severe when towing when the engine is under strain,all the ignition parts were replaced a couple of months ago.It did the same thing but a lot worse last week and on inspection discovered the garage had'nt put the air intake pipe on correctly and it had dropped off at the airbox,i thought i'd cured it but evidently not,could the ignition timing or valve timing be slightly out or is it more likely a vacuum leak.it appears to be backfiring into the exhaust.

Hi James,

I'm no expert and have often needed this Forum to help me with a really unreliable 740, so don't hold me to this, but it sounds as if a valve may be sticking every so often, or there is a worn cam, or maybe the guys who replaced the cambelt put it back a few notches out. Then there's the possibility of a very light crack in the distributor cap which is causing 'tracking' to earth under load I would check this first, together with old HT leads toucng each other or touching earth somewhere. A good idea is to park somewhere dark, start the engine and open the bonnet (that's hood to you cousins over the pond), then see if there is any evidence of blue sparking runs or spots anywhere on the HT system. If so, replace the offending item(s) and tes the car before doing anythng more omlicated.

Hope this helps!

Regards,
Pete. fifiuk@bushinternet.com

I'd be worried the garage may have installed the belt a tooth or two off- I recently replaced the belt on my '86 745T while investigating various suspects for a performance problem. Turns out the belt slipped a tooth during some heavy driving. My problem was a slight misfire at idle and poor acceleration off boost. Ignition timing read as way off spec, and that was my hint- the timing belt slipped at the crank, so the crank pulley timing mark was 'off' 15 degrees. The car ran best at 25 degrees BTDC, when factory was 12. After replacing the belt with all the pulleys where they should be, I found the timing was in fact 10 degrees BTDC. If the garage was off one tooth either at the crank or the cam gear, you would see a drop in performance, and if they reset the ignition timing, it's possible that this would be wrong as well.

Good luck-
Dylan