Spitfire Triumph Spitfire 1300 engine SU carb problem

[sharpe04]

I have nearly completed the full rebuild of a rare (No2 of 9) 1971 TVR Vixen 1300 with a 1300 Triumph Spitfire engine. This is my 3rd rebuild of the car which I have owned since 1986, I have never had any problems before with the SU HS2 twin carbs, which were refurbished professionally 20 years ago and have covered not many miles since. The engine was running a few months ago, albeit requiring fine tuning. I had a fuel leak from the float chamber(s) so replaced the needle valves. This is when the problems started. I have adjusted the carbs as per the SU and Haynes manuals to the basic settings, including checking the float settings and valves and even substituting adjustable floats, all seem nominal.

The engine starts but runs only at 4000 rpm with air filters removed or 3000 with them on. This is without choke and with the butterflies closed, throttle position makes no difference. I cannot see how any fuel/air mixture is actually getting to the engine even though it seems to be running smoothly, frantically and uncontrollably! I'm beginning to think that the carbs are possessed.

The last job I did was to centre the jets as per the SU instructions. The fuel pressure from the standard mechanical pump does not seem to be excessive, there are no obvious air leaks and nothing seems to be at fault with the carburettors or their setting up, they seem to be in good condition. Gaskets are good, the ignition system and timing are fine, they are untouched since the engine was running OK, apart from a confidence check of the static timing.

I am perplexed. Has anybody out there seen the same problem, if so what's wrong? Internet searches have drawn a blank so far. I'm tearing my hair out here. Thanks in anticipation.

 

[charleyf]

The fast idle means that you are getting air/ fuel through the carbs. My first check would be to check the butterflies on the carbs to be sure the butterfly is closing completely. The possibilities here are that the butterflies were mounted incorrectly or the bevel on the butterflies are backwards. Also some linkage may be holding the butterfly open even though the choke knob may be in the off position.
Start by removing the air cleaners and use a strong flashlight to look down the bore. If you do not see any gap around the butterflies, then remove the carbs. Now look at the engine side of the butterflies for any gaps around the edges. There should be no gaps. Next still looking at the engine side of the butterflies and shine your light into the front of the carb.
Note the bevel on those butterflies and make sure they are turned to allow full closure.
Charley

 

[NutmegCT]

Agree on checking the butterflies. Also, air leakage through worn throttle linkage at the carbs - and/or vacuum leaks just about anywhere.

While the engine is running, squirt some water at various parts of the carbs. See if you can pinpoint an air or vacuum leak.

Tom M.

 

[sharpe04]

Thanks for the advice chaps. It all checks out OK on the bench and statically on the car, all the adjustments are nominal, but I've got a borescope so will check it all on the manifold. I've had similar advice today from members of the Dorset TR group here in the UK, an old friend with a TR8 is a member, everything points to an air leak. What really puzzles me is that it was all working fine just a few months ago, then I was out of action for a bit and when I went back to it my problems started. I might blank off the suction pipe to the rocker cover. I'll do some more work and remove the carbs for the nth time and let you know if I make any progress.

As I said I've had this car since 1986 with no carb problems and before that a 73 MG Midget for many years with the same carbs, again with no problems, so I do have lots of "time" on them.

 

[sp53]

Could the needles be pushed into the carb piston too far and letting too much fuel in the manifold?

 

[sharpe04]

Update. I have been delayed in sorting this problem out due to a family problem. However I removed the manifolds to check for air leaks after the carbs. No sign of any cracks or other orifices, reassembled with a new manifold gasket. Tested the floats and needle valves off the car, all worked fine. Fuel pump working ok, filter changed, everything cleaned. Once again checked the carbs including the butterflies, jets, needles, pistons, gaskets, free movement, linkages etc etc. Everything is nominal. Returned the carbs to the basic settings. Statically timed the ignition once again. Checked that a spark was being produced. All this to no avail, in fact the engine will not start at all now. I cannot see how any air is getting into the induction system beyond the carbs, except perhaps:

Although there is no evidence of any oil or water contamination, could the head gasket have been compromised and be admitting air? Seems unlikely as it would have to be a gross breach of the gasket to affect all the cylinders and surely there would be other signs. I'm reluctant to remove the head, but feel that's the next step apart from repeating the above steps all over again.

I've had leaky induction tubes on my aircraft engine (Continental A65) so I know what air getting in after the carb does, the car engine doesn't conform to the aircraft experience. I'm at my wits end!

 

[Sarastro]

Your question about the head gasket tells me that you are really grasping at straws! This is a carb problem; I can't see how it could be anything else. Air leaks can do what you describe, but you still need the carb to provide at least a little fuel. Air alone won't run an engine.

It takes very little fuel to run an unloaded engine at 4000 RPM. So, even a small air leak or throttle misalignment can cause what you are seeing. The butterflies might look like they are closing just fine, but still have an alignment problem and leave a gap. Try looking down the throat with a light source at the other end. With the idle adjustment backed off, you should be able to close the butterflies so that you see no light at all.

Beyond that, I'd suggest just going through the carbs and making certain that all the functions and adjustments are OK: float height is OK, damper piston is free, float valve is closing off, choke function is shutting off properly, and so on. This sounds to me like one of those things where, once you find it, you'll slap yourself upside your head for missing it so long.

 

[sharpe04]

Thanks so much for your response. The butterflies are tight against the carb bodies and the spindles don't seem to have undue play. When the engine was running I adjusted the throttle stops right back and applied pressure to ensure that the butterflies were clearly shut, but still it ran irrespective of the choke position. I agree with you that the symptoms suggest quite a significant air leak, but I can't find any! I'm beginning to think it's possessed, might start calling it Christine. The weather has turned wet and windy and as my garage is full of stuff at the moment I'm working in a car port, not ideal, but I will persevere. The car ran fine for years until laid up and was running ok when I re-installed the engine last year. However I might soon give up and send the carbs for rebuilding, that will cost at least £200 each, so I'm reluctant until I have exhausted all possibilities. I will update again if there's any progress. Cheers.

 

[sp53]

you sure the needles are not push deep into the carb pistons way past the shoulders.

 

[sharpe04]

Thanks for the advice, I'll look at that. So far I've been meticulous in setting up exactly as per the manual, I think I might go "off piste" in an effort to get it sorted before I give up and call in a pro, which is not my style!

 

[mctriumph]

Intake manifold gasket is not sealing. dont know why, but I will bet a 6 pack .It just is
not possible to rev without extra air regardless of hauntings/voodoo or Hollywood scare flics.
Mad dog