Spitfire Spit Engine Surging

[doates]

Hello Again,

I have swapped carbs on my 70 Spit MK3 from the Zenith to SU HS2's. Went through the SU's a couple of years ago and installed a rebuild kit. I installed them at that time and got the engine idling pretty well. Then I let it sit for a couple of years with gas in the carbs. So after dis assembly and cleaning I can get it to idle at about 800 but after it gets warmed up it surges from 750 to almost 900 in about 1 second intervals. The plugs show sooty black but when I lift a carb piston it stumbles like it is running lean. When I rev it to 2000 or so the surge seems to disappear.

Because of transmission problems (mentioned elsewhere) I can't tell you what it does at speed/under load.

Any suggestions would be greatly appreciated.

Thanks....Dennis

 

[JPSmit]

sounds like you may need to get the shafts rebushed - try wiggling them and see if there is play?

 

[doates]

No wiggling at all. These are new shafts and I bought bushings with the shafts but when I slid the shafts in I decided that the bushings weren't needed. I have sprayed carb cleaner around the shafts (and everywhere there is a seal for that matter) and it doesn't cause a change in idle. Could the needles be that far off? I will pull one this weekend and try to find out which needles I have. Now that I can get it into 2nd gear I will drive around the neighborhood and see how it acts under load.

Thanks...Dennis

 

[71MKIV]

check the timing, mine stumbles like that and it's the wobble in the distributor shaft.

 

[doates]

Thanks

I will play around with distributor. If it is the distributor will I see the timing moving around? My car is very original and I have wanted to be somewhat of a purist about it but the electronic ignition is sounding mighty tempting.

Dennis

 

[tomgt6]

It could be a bad distributor shaft or maybe a bad vacuum in the distibutor or in the line.

 

[71MKIV]

Your timing light will show the timing wandering around. Unfortunetly electronic ignition will not help. The shaft wobbling in the distributor is what's causing this, and nothing short of sending it out for rebuild will help.
Have you oiled the distributor lately? Mine has a little hole labeled "oil" in the point plate, and a couple of drips of oil in there will get into the top bushing and at least help in that the shaft will spin smoothly and not bounce on a dry bush.
I found that out the hard way when the bush seized enough to move the whole distributor, boy did that make it run funny.

 

[tdskip]

If the timing isn't wobbling around you may not want to rule out vacuum leaks -some folks use a hand held torch to check for leaks by listening for an increase in RPM when the gas is released near the various lines.

A Pertronix unit - and this is a matter of some debate here to be fair - I personally believe to be a nice upgrade that is easily reversible. If you decide the distributor needs work Jeff at Advanced Distributors can handle that at the same time as the rebuild.

 

[t_bush]

If the timing is steady then you might need to re-synchronize the carbs. I had a bugeye with the @^#$^ SU H1's on it. It had a wandering idle until I got a color tune and dialed in the two carbs.

- Tim

 

[doates]

OK, I reset carbs to starting point - 1-1/2 turns on idle adjust, 12 flats out on mixture. Took awhile but I got it to start. It was idling fast (maybe 2000) but smooth and almost immediately gas started squirting out of the rear carb overflow. And by squirting I mean 3-4 feet. When I plugged the overflow with my finger the idle slowed down a bit. Took the top off of the fuel pump and everything seemed normal. Put it back together and same thing.

When a fuel pump goes bad can it produce too much pressure?

Thanks for all your responses

Dennis

 

[doates]

Went back out right after sending above. Started the car (easily), idle at 2500, no gas squirting. Turned down idle screws to about 800, balanced carbs by listening with a tube (so close not perfect). Set timing to 6 degrees - it moves very little. Still surging from 800 to 1000 in 1-2 second intervals. Carefully squirted carb shafts, manifold seals and vacuum points with starting fluid and no change in idle.

Help!

 

[71MKIV]

um, this will sound really silly, but disonnect the battery while it is running.

My alternator will drag the engine the engine that much. It's possible you are dealing with the regulator turning on and off. When my electric fan turns on it really drags on the engine.

 

[TR3driver]

um, this will sound really silly, but disonnect the battery while it is running.

Bad idea, IMO. Alternators are quite capable of putting out enough voltage to kill both themselves and you, with the load of the battery suddenly removed.

In fact there was a conversion kit popular back in the 60s & 70s to get 110vdc from your car alternator, to power hand tools and whatnot. The kit basically just rerouted the alternator output to a 110v socket and altered the setpoint of the regulator. Add an inductor and you had a welder.

A safer way to perform this test would be to stop the engine, disconnect the battery, disconnect the alternator (making very sure the wires cannot contact any metal), then reconnect the battery and start the engine.

 

[doates]

Before I read the last two posts I took it down the street because I thought I had the transmission problem licked (wrong). I still could not shift into 2nd but in 1st and 3rd it ran fairly smooth. Seemed to have reasonable power on take off and would even take off in 3rd but when it went to idle it still did the surging.

I had my son (college) available to help me pull the transmission so we did and found a couple of mistakes with my rebuild. I am ordering transmission parts and hopefully will be back to the idle issue next weekend.

This forum is making me feel like I can finally get this thing on the road.

Thanks for your support!

Dennis

 

[t_bush]

Dennis, when the idle varies is it random or is it cyclic? If it happens a regular intervals I would highly suspect the carbs. Random speed changes can be caused by many things. Repeatable evenly spaced changes narrow the problem down. The carbs need to syncronized and matched for throttle opening, idle mixture, and main mixture.
It sounds like you have the throttle opening taken care of. Next is to make sure the float settings are dead even. A float bowl with a higher gas level than the other will run richer, even if the needle positions are identical.
Once you know the float levels are good, run the engine to warm it up and then adjust the jet on one carb. Just a little at a time. Let the engine run a little between each adjustment to get it to settle in to the new setting. If the time between surges gets faster you are going the wrong way. If longer then your getting closer. If teh idle mixure is way rich you might have to adjust both leaner and then go back to adjusting one.
Fuel coming out of the drains means a float needle is sticking or has dirt in it. I do not know of a pump failure mode that increases the fuel pressure. If the drainage occurs when the engine is at speed I would suspect the wrong pump is installed. If it is at idle I would clean out the fuel line and clean and inspect the float bowls. Make sure the needle seat is tight.
Another thing to check is if the carb pistons slide smoothly without any sticky spots. Check that both dampers are straight and both have the same weight and amount of oil.
Once you have the surge worked out you can adjust both Idle mixtures in unison to get Idle mixture right.

SU's, love 'em, Hate 'em, go on to fuel injection.

- Tim

 

[doates]

I have spent a lot of time in the last week reading about SU tuning and realized that I have been setting my timing without plugging the vacuum advance so hopefully this is the problem. If I get my transmission in this weekend I will check it.

Thanks...Dennis