QuestionQUESTION: Hello Jim,
I have a Triumph Spitfire Mk IV with an 1300 cc engine. i have modified the engine like this: compresion ratio up to 10:1 , I polished all manifolds and entire cylinder head, I have reduce weight to flywheel, I fixed an Aldon distributor type 106TY1 (without vacuum advance)and a Kent cam (30-70/70-30). I have keeped old twin SU HS2 carburetors. My problem is that I can't lower the rpm under 1500 because engine work with interruptions. Also up to 5000 rpm I fell that my enagine need more fuel. What i must change? What I should do?
Best regards,
Catalin
ANSWER: Hi Catalin,
My name is not Jim but I will answer your question.
Any time you modify an engine you change what is needed for ignition advance and even the ignition advance curve. You also change what is needed for fuel mixture. And even the mixture curve.
A standard Spitfire has SU needles that are profiled (size of the taper)for a standard engine. You have modified your engine a lot so you need to get a completely new taper for your needles.
You need to get the timing correct first because it effects what mixture that is needed. There should be some specs that came with your ignition system to get you in the ball park. Most 4 cylinder engines of this type I have seen run between 35 and 45 deg total advance.
After you get the advance set, try adjusting the mixture at a high RPM so you are 100% sure it is a little too rich. (puffs of black smoke out the tail pipe and a flat black of the edge of the spark plugs)
Always set the high RPM mixture first and always on the rich side. Later when you get a good profile of the needles, then you can slowly lean a little but always work with the high RPM first.
Without an exhaust analyzer you will have to do all of your tuning by plug color, exhaust color and how the engine responds. Another option is to purchase a thermo heat sensor to measure your exhaust headers. Exhaust temperature is an indication of mixture. Providing your ignition timing is correct a high exhaust temp is lean and a lower temp is richer. Keep in mind that this is also dependent on the distance from the head. Most like to measure about 3 to 4 inches from the head. If you purchase one of those lower reading thermo temp guns, that can't read the 1200 to 1500 deg F. you can just read the temp further down the pipe.
This is not as good but it will give you an indication.
Another option is to weld threaded lugs in your header just before they merge that will accept a Zirconium O-2 sensor. A volt meter on the O-2 sensor wire will tell you mixture. Usually .5 volt is good and a lean mixture is a lean volt reading (.2 to .4) and a rich mixture is a rich voltage (.6 to 1v)
Back to tuning with no equipment. If you had "Forged" pistons installed, you can adjust the timing by advancing the timing and reving the engine high to find the advance setting that is the max that you can use. Adjust the mixture jets down in the SUs so it is noticeable that you are getting puffs of black smoke on a quick throttle movement from a 1/4 to 1/2 throttle movement. Keep in mind as much as you have done to this engine, it now has the capabilities of self destruction (like throwing a rod) So if you have stock rods and forged piston only work in the half to 3/4 throttle range. When you get too far advanced you will notice a clear drop in throttle response.(if you have cast piston don't even think about tuning this way)There may of may not be any of the detonation noises at this higher RPM range. But a hard drop in power is always noted when you reach the highest point of advance. At that point you back the timing off about 5 deg and check the advance with a timing light that you have established.
We are going back and forth between timing and mixture because each effects the other.
Even the "Heat Range" of the spark plugs will effect the mixture and timing you use. Generally you should have from one to two ranges COLDER plug than the factory recommended.
This can effect tuning. Even though a colder plug than two ranges is best for high performance, you can't tune very well with a plug that is too cold.
Setting the carbs. -- warm the engine up and put a strong fan in front of the radiator. Set the RPM at about 4500 RPM and run the SU adjusting nut DOWN until the Engine starts to blubber or start running like a 2 cylinder engine. (This is called 8 stroking) and it will also blow black smoke out of the tail pipe. This is the "Too Rich" point and adjust the nuts quickly UP until the black smoke stops and the engine clears up. Set the RPM at about 2000 RPM to clear the plugs up for a minute. Make sure you have engine oil in the top pot of the carbs. Now do a little with the lower RPM ranges. At this point you have the jet adjusted roughly and you can't alter it very much unless you change needles to a different profile.
Now you can roughly check your mid range by holding the RPM at about 3500 RPM and quickly open the throttle a few times while someone watches from the rear of the car at the exhaust. He should see only light puffs of black smoke just at the start of the rev up. If there are no puffs of black smoke than the lower ranges are a little on the lean side. If they are too lean you may get a pop into the intake or a hesitation. If it is too rich you will see a large puff of black smoke. Either one will make it necessary to get a different set of needles. Some companies can sell you a set of needles that will get you close to correct if you tell them exactly what was done to the engine. With some test equipment you can then start reprofiling the needles to get them exact and the most power out of the engine.
If everything went well up to now, you can make a minor adjustment for low RPM and idle.
You should have already synchronized the butterflies of the carbs and set the idle stop screws at the exact same opening. With the engine warmed up set the idle to specs or in your case it may require a higher RPM due to all the modifications. Even with all you have done it should idle at 1000 RPM. If you don't have a Unison unit use a piece of hose to listen to the hiss in the thought of each carb to get the stop screws set exactly the same. Then lift the test pin (under the edge of the top pot) on the front carb and note the reaction. If the mixture of the BACK carb is correct the engine will drop off and start running like it is on two cylinders. Run the adjusting nut up to lean and down to richen. If it drops off and quits you either didn't get the throttle stop screws right or it is too lean. never move the adjusting nut more than three flats and check again. (6 flats = one turn of the nut) If when you lift the pin the engine speeds up and stays fast The OTHER carb is too rich. Contrary to what you may read when you lift the pin on the front carb, you are checking the rear carb and when you lift the pin on the rear carb you are checking the front carb.
If you have to move the adjusting nut more than two turns than you definitely need different needles.
I am sorry it is not "Just go buy an item and turn this or that screw to this or that position and all your troubles are solved" It is a Black art to tuning a modified engine and to do it exactly right it takes test equipment. But if you follow what I said you can get it close.
If you got the high RPM correct and found the lower RPMs off you may need to get a SU needle profile chart and try to pick out one or several that better needles to suit your engine.
You may want to read my tech tip on my website for more insight to tuning a modified engine. http://mg-tri-jag.net/tech12.htm
Let me know how you do.
Howard
---------- FOLLOW-UP ----------
QUESTION: I am sorry for the mistake about the name.
For the adjustments I have used a professional analyzer Prufex. I made the balancing between the 2 SU with an vacuum meter than because I have a sport manifold exhaust I made the same adjustment for each carburetor. I measured to each carburetor 3 % CO. I Know that for obtaining performance the camshaft must correspond with a bigger carburetor (SU HS4). It is correct? On www.triumphspitfire.com I have read that for 90BHP I should use a fastroad 83 camshaft and to made the adjustment a little richer on the same carburetors.(SU HS2). For the engine cooling I have fixed a big water cooler with electric fan and also I fixed a oil cooler with electric fan.
Thank you for your advise but it is more difficult to buy different needles because I am from Romania and from here it is difficult to buy a piece for Triumph. All I have bought till now I bought from Germany.
Please tell me where I could buy this needles.
Best regards,
Catalin
AnswerHi Catalin,
It sounds like you are ahead of most people. It depends on what you want the car to do as to what combination of carbs, cam etc. you use. I found that to get the best results for what ever you want out of the engine, all of the components must match and all must be attended to. For example, I had a customer bring his car to me in the shop and complained that he had put a large carburetor on his car and was disappointed that the performance was not so great. I explained to him that to get the use of a larger carb he needed a larger cleaner ports and larger intake valve, better cam, higher compression ratio and a tuned exhaust system. Each item in this chain depends on all the items in the chain to get the most out of each item.
Because you are working with a fluid called air, you have to keep in mind what is happening when. For example,
At a high RPM air is rushing in the carbs and into the manifold on into the combustion chamber as the piston goes down. Just after the piston reaches the bottom and starts back up, the intake valve closes. But the air is still rushing into the carbs at a high velocity. If the carbs are too large the air slows down, as the valve is closed longer than it is open. The air at the valve has long since stopped and the air charge has slowed in the carb. Then the intake valve opens again and draws that STOPPED air/fuel charge into the combustion chamber. It has mass and does not want to move.
I found that if I leave small carbs on and enlarge the intake valve and cut out as much area just under the valve so I have a large pocket just under the valve. This gave me a poor valve open flow due to my pocket, but it did give me something else. Now my air came in (slightly restricted) at a much higher velocity and when the valve closed my higher speed air/fuel continued coming in to the larger pocket under the valve until it pressurized a large charge of air fuel waiting for the valve to open and since my valve was larger it had what is called "Blow down" into the combustion chamber as the piston started down. I then had more tractable horse power than if I had opened up every thing.
There are limits on everything in the chain. I gained HP in the useable ranges of 3/4 to full throttle but may not have the top HP at the top RPM by what I had done. So keep in mind what you want.
I installed and operated and got to play with a Clayton chassises dyno for a while and I found that you needed to see 5% CO at full throttle, full load to get HP. 3% may make it run smooth and cleaner but I think you are shorting yourself on HP. You can tell by setting the 3% and making three timed acceleration runs and get an average then set it to 5% and do it again and see if you don't get an improvement. Let me know.
With CO equipment you can get a great profile of each needle. If you find (after all of your engine modifications) that your needles are too rich or too lean in any speed range, you can look at the letters on the needle to ID what you have and contact Burlin Company at http://www.burlen.co.uk/default.aspx and tell them what needles you have and where in the range you need it richer or leaner. I think Burlen bought SU and they have every needle available. They should have a profile chart of each needle.
For other parts to the car you should be able to purchase any part from either Engel Imports, The Roadster Factory, Moss Motors or Victoria British.
Let me know your progress.
Howard
