Use of Choke while driving

My 70 runs well to 3500 RPM and then bogs under load and sputters after that. If i put the choke on while it is sputtering it picks up and runs well to 5000 RPM. Am i running her too lean. If i richen her up i may not have enough length on the idle screw to keep the idle comfortable. I have not pulled the plugs yet. so i do not know how they look. Anyone have any ideas. I do not think it is the ignition system because the choke would not effect that.
Oh could it also be because there is no exhaust system, and I am unsure of the air filters they may be partially plugged. have been on the car since 95

I would recommend cleaning the carbs out and resetting them to a known starting point and then tuning them.

here is a downloadable Tuning guide for SU carbs.

http://www.britishcarclubcharleston.com/library/doc_download/38-tuning-su-carburetors

The air filters have been on the car since '95? Sounds like it's time for a minor tune-up.

Another thought is that you have a bad vacuum leak somewhere in the intake.

Get the the ignition and timing set, then the carbs set up, and you will have a baseline. The lack of back pressure from no exhaust may affect it a little. My guess is your carbs are not quite right. Still any MGB will need choke when cold for a little bit, until warm.

I was thinking it may be a vacuum leak the air pump port has duct tape and a clamp over it on the air rail. How do i remove this and fix it? I was just happy it was drivable now i want it running right. I am not sure how to work on the ignition so i will save that for last.

How can I test for Vacuum leaks?

The air cleaners came on the car and the last time it was on the road was 95. The PO bought it from the VOA (Volunteers of America) in 95. and the mileage was the same as when I bought it.

jheilman43402 Wrote:

jheilman43402 Wrote:

Not having the proper air cleaners will make the car run lean.

Not having the exhaust will make the car run not as well.

Take care of these items and then test for leaks using carb cleaner. Replace hoses as necessary. Duct tape is not a solution except on a back road at night, in the rain.

Then test the advances. Being off will make the car run poorer.

Are valves reasonably adjusted.

The carb system is the LAST thing to touch. Absolutely last.

Most likely you will have a distributor issue. But, to get to that, you have to eliminate a lot of stuff.

My guess is that you have 3-5 issues in 10 items. A couple days will get it.

Larry

Also, check the emissions systems for leaks. OR just pull all the emissions hoses off the cabs and plug the ports and see how it runs. My oil filler cap leaks, so my car will start running lean above around 3k if I have the crankcase vent hooked up to the side of the carbs. It will allow enough air to get sucked in through the cap and into the carbs to mess with the mixture.

Hurst

You're going to have to do a much better job of plugging that hole in the intake manifold. No way is that efficiently plugged. I was able to fabricate a piece of bronze that press-fit.

You can't really save the ignition for last. It ALL has bearing on each other.

I usually set the ingition first, then monkey with the carbs.

If you are going to leave that port plugged, I'd make sure it's not leaking.

I'm not really an SU genius, so I'll leave that to the experts, but, read up on adjusting the timing on here, and you'll be FINE.

There is SO MUCH info on here, there's NO REASON why you can;t find what you need, and if you can't just ask.

But, my preference is to usually get the points, condenser, rotor and cap, wires, and plugs all in working order and set properly, THEN I work on any fuel related issues. If you do this the OTHER way, it usually doesn;t end in satisfactory results.

GOOD LUCK,

B

Auto zone, Advance auto and others sell a radiator blanking cap that will fit perfectly over the intake pipe you have duct taped now. That does not look at all air tight. Always adjust the ignition and timing first as a out of adjustment ignition issue can give the same symptoms you describe. If you have standard point type ignition have you cleaned or renewed and set the points? That will give you spluttering and poor performance. Folks here on the site can help you through the process. Jeff Schlemmer is one resident pro on that area, as well as Jimmy Hilton for SU carbs.

I will not have time to work on it untill possibly sunday, but i will check for vacuum leaks, and check the points and condensor. The dizzy has never been out of the car, so I think the timing should be OK. How can I check the Vacuum advance? I will get a Cap for the air pump port . I know the Charcoal canister is not properly attached to the vacuum line on the valve cover. So I will fix that too. Can the Oil cap leak vacuum?

Never assume that the timing is correct. If the points start closing up the timing retards. The spark gets weak,etc. As others have said distributor, valves set properly and then carbs in that order. The carbs are not hard but read the manual on them to understand how they work, BEFORE making any adjustments.

It's the timing and valves that scare me. the carbs i can handle, i have read everything i can on them and figured them out.

jheilman43402 Wrote:

jheilman43402 Wrote: