QuestionQUESTION: Hi Jim,
I have been helping my cousin with her 70 Spitfire MKIII. The engine needed to be rebuilt so she opted to rebuild a 1500 she had on hand. The long block was professionally built and we put it in the car with the original peripherals. The only improvement on the base engine was the "B" (I think) camshaft.
After the engine was well broken in, we swapped out the single carb for a dual SU unit from a 69 MKIII Spitfire 1296 (HS2 I believe). We also installed a European 1500 exhaust manifold. After a lot of tweaking and figuring out the distributor also had to be swapped because of the difference in the vacuum advance, the engine is running quite well ..... almost. The thing totally lacks power above 3000 RPM and is worse off than with the single carb.
The metering needles in the SUs had to be substantially enriched to get things running as they are now. Could the lack of power be that we need to put in a different set (ie we are running at the edge of these needles)? If so what would they be and where can they be obtained? Are there other things needing to be changed?
She's taking this to the vintage events at Watkins Glen next month!
BTW ... I am a former Sunbeam Tiger SCCA GT2 racer. I miss it but it's too expensive these days!
Thanx much
Keith Bauer jellicat@att.net
ANSWER: Keith,
What I'm suspecting is either the timing not being correct or the carbs not balanced or float bowl levels not correct (or even).
Which vintage/style distributor did you fit? If you're running the twin HS2s from early Spitfires you should have been able to keep the original Mk III distributor.
As to needles, off the top of my head I don't have a good suggestion. One thought that comes to mind is that the float level in the carbs might be too low, a "stock" 1500 barely made more power than a stock 1296 and so it shouldn't draw much more gas.
You might try doing a plug cut. Take the car to your favorite deserted back road with a nice fairly level straight. Do a full acceleration run, turn off the key and coast to a stop. Let the engine cool down for a few minutes and then pull the spark plugs.
Let me know what you discover and we'll keep at it until it's running right. If it all does work out, look up my brothers, they traditionally camp out in the infield over near the end of the back straight.
Cheers,
Jim
---------- FOLLOW-UP ----------
QUESTION: Hi Jim,
The static timing was set to 10BTDC and I note a total advance of an additional 4 to 6 degrees which kicks in very early. It's all in by 2000 RPM or maybe a bit less. I'm not sure what engine was in the car before but it was not the original. It had a single Stromberg. After putting on the dual HS2s I noted the vacuum advance was actually acting in reverse. Hence the swap to the distributor that came with the 69 MKIII that was also the source of the carbs. Now the vacuum advanced matched the carbs and that made a world of difference. I note that the vacuum advance now advances the timing way beyond the timing indicator so I don't know how far it is going.
The distributor now in use is the Delco with the adjustable timing knob at the other end of the vacuum diaphram.
The float level could certainly be a problem. As the carbs looked perfectly fine, and I didn't find any information on what the float level should be, we left that alone. Do you know what the setting should be?
I've thought this could perhaps be an ignition problem. Another part of the upgrade was a Pertronix ignition model and flame thrower coil. I realize now I should put the points back in to see if that makes any difference. One never wants to believe new parts could be faulty but they should not be overlooked!
Certainly a plug cut would be good to try but, I am used to doing that at Watkins Glen where I am running full out then cut the engine and coast into the pits. We are unfortunately in a very congested area where this isn't too practical .... perhaps when we get to Watkins Glen in a few weeks...... although I would hope we solve the problem by then!
I'll try some more things and report in. In the meantime thanx for you input!!!!!
Keith Bauer jellicat@att.net
AnswerKeith,
If the car is a true '70 model then the single Stromberg was the correct carb for that year, so it should have been a large bearing 1296 engine. FK engine prefix.
On the HS2s the correct float level is 1/8" clearance between the top of the float and the chamber lid when it's held with the float upside down.
It sounds like the mechanical advance is working ok, having it all in by 2000 rpm is about right. You need to set the timing with the vacuum unit disconnected and the line from the manifold plugged.
Doing plug cuts at the track is a luxury.. you could almost get it done if there's a large shopping mall nearby... you don't need to be in top gear for the effects to be valid. Just be ready to sweet talk any agents of the law if they get picky.
Cheers,
Jim
