SU Carbs problems

[MarkRado]

The summary so far is:
- check that the throttle discs fully open with the pedal floored
- check timing
- check distributor (function of centrifugal advance; specific type/ number)
- make sure needles and piston springs fit your carb type for a 4.2 Jag application http://sucarb.co.uk/
- adjust carbs
- for better fuel economy get the vacuum distributor adjustment working correctly

[malcolm]

I appreciate that the problem doesn't lie just in having two smaller SUs rather than 3 bigger ones, but Jaguar obviously felt that three 2inch SUs were the beasts to go with. So rather than spending time trying to get the current carbs running properly, why not get a proper set up (maybe from Peter?)(I'm not on commission! ) and have the car running the way it should? Not a massive extra cost on what is a valuable car. Or maybe you want to restrict performance if the car is mainly for your wife to drive?

[jagwit]

My Jag mechanic rebuilt my series 2 4.2 liter engine carburetors.

The trouble with rebuilt carbs is that you can assume nothing, NOTHING.

You can not assume that they have the correct parts, or even the same parts. Nothing can be taken for granted.

Just this week I had many conversations with a friend who could not find the trouble on a Mk7 (with rebuilt carbs). Long story short, front carb had larger main jet than rear.

If this car is stalling , the first question should be WHY. Too much fuel or too little fuel?

[johnetype]

I appreciate that the problem doesn't lie just in having two smaller SUs rather than 3 bigger ones, but Jaguar obviously felt that three 2inch SUs were the beasts to go with.

The majority of 4.2 litre XK engines that Jaguar delivered had only two SUs and many of those versions developed a similar or better horsepower than the 4.2 E type installation. Certainly a well set up twin carb engine will generate comparable power to the point where the butt dyno can't tell the difference.

A significant element in the continuation of triple SUs on the E type following on from the triple carb setup on the XK150s was macho marketing.

[PeterCrespin]

It’s news to me if there was any twin carb xk engine for which Jaguar claimed more horsepower than the triple carbed E or Mk X /420G.

I agree with your general premise but ask for your numbers?

[christopher storey]

I agree , Peter. I'm afraid the previous post was , putting it as politely as I can , completely wrong

[malcolm]

If a 4.2 xk engine with 2 smaller SUs produced as much/more power than an E Type with 3 larger SUs, could it be down to other technical differences in the engines (inlet valve size etc?) rather than the change in carbs?

[christopher storey]

Malcolm - it just didn't happen - the only 4.2 litre engines produced after the demise of the S2 E type were the XJ6 ( 2 HD8 and 245 bhp gross- later the fuel injected S3 with bigger valves and probably some more hp ) and the Daimler limousine ( 2 SU and rather less HP )

[PeterCrespin]

I'm afraid the previous post was , putting it as politely as I can , completely wrong

You're such a gentleman, Chris :-)

I almost called you last week when I was over for my ex Mother-in-Law's funeral. I'm back the week after next for my eldest sister's. Would you be aound on the 27th, maybe for a pie and a pint with Tony if he's around? I have your number ending in 3175 assuming it's still the same?

[christopher storey]

Peter

I'm sorry you are having a torrid time - we send our condolences to you both

Yes, 27th looks OK at present

The phone number you have is correct

Chris