PDA

View Full Version : My dizzy



tinman58
12-02-2008, 10:08 PM
My TR3 idels fast 2000 rpms I do have a mild cam and headers the adjustment screws are backed off all the way. I have checked for a vacuum leak and intake leak but found none. I put the timing light on #1 spark plug and reved the motor the timing stayed about the same and I noticed some misfire at higher revs. I have installed the petronics ignition system.
The engine was a basket case when I bought the car so I dont know if there is a problem with the dist or some thing that I am missing?
Dan

prb51
12-02-2008, 10:33 PM
A hot cam may need higher idle rpm but I'd guess no more than 1k. If the strobe isn't jumping about I'd focus on the carbs.
Is the choke cam clear of the screw and fully closed?
Even if you move the dizzy (retard) the idle won't slow?
When tuning did you disconect the vacuum advance?
When you lean the carbs out with the mixture setting nut what happens?
I'd check my carbs visually to see that the throttle discs are closing fully and together.
What number needle are you running (rich, stand, lean) and are they matched/properly set up?

tinman58
12-02-2008, 10:44 PM
The choke cam is clear
If I retard the timing it runs rough.
The vacuum advance is hooked up
If I lean the carbs the idel lowers but again bad performance
The were just rebuilt by Chester in Colorado so I dont know all the info.The strobe dosent jump but misses
Dan

dklawson
12-02-2008, 10:57 PM
Though not a TR3, I had similar behavior with my Mini when running points. I found I had to iteratively adjust the carbs, then the timing, going back and forth between the two. Eventually, the idle settled down to a very respectable low speed.

Still... I'd be looking at your throttle and choke linkages again to make sure the butterflies are fully closed when they are supposed to be and that the jet tubes are fully "up" with the choke pushed in.

Roger
12-02-2008, 11:22 PM
Though not a TR3, I had similar behavior with my Mini when running points. I found I had to iteratively adjust the carbs, then the timing, going back and forth between the two. Eventually, the idle settled down to a very respectable low speed.

Still... I'd be looking at your throttle and choke linkages again to make sure the butterflies are fully closed when they are supposed to be and that the jet tubes are fully "up" with the choke pushed in.

Sorry, but that cannot be right. This old Brit knows better, and I cannot help but quote again an old garage guy who told me, back in 195x, that 99% of carburetter problems are ignition.

Make 100% sure your ignition timing and dwell are right first. Then LEAVE THEM ALONE while you adjust carbs - balance, mixture and throttle opening.

Bouncing from one to another will drive you mad, and won't work anyway!

prb51
12-03-2008, 12:00 AM
Roger has a point.
SU's are pretty simple and once the basics are checked/adjusted they are sound.
How about your dizzy, was it rebuilt or just the pertronix added? The plug wires are the app type for the Pertronix.
Coil issues.
Go thru the dizzy and pertaining systems (swap out elements) to see if there is an issue.
How did it run prior to the Pertronix installation?

dklawson
12-03-2008, 09:00 AM
Yes... I know what you are saying. I have given your advice on setting valves and ignition first myself many times. However, there was a time several years ago when I found I could not get the car to idle at low speed even with the idle screws backed all the way out. I did indeed have to iteratively go trough tweaking both timing and mixture to get things to settle down. I know this runs contrary to logic and normal procedure.

Moseso
12-03-2008, 11:53 AM
I put the timing light on #1 spark plug and reved the motor the timing stayed about the same and I noticed some misfire at higher revs. I have installed the petronics ignition system.This is not how it should be, and points directly to a distributor problem. When you put the timing light on and rev the motor, the timing <span style="font-style: italic">should</span> change. Either the mechanical or vacuum advance mechanisms -- or both -- are not doing their jobs. Further, a stiff, cracked old vacuum advance diaphragm can leak enough air to mess with your fuel/air mixture, making it impossible to properly tune the carbs. I'd start with a deep look into the diz.

JeffS
12-03-2008, 12:33 PM
More precisely, you should see over 20 degrees of advance from the mechanical advance mechanism as you rev the throttle. You should see over 20 more degrees from the vacuum advance if you suck on a hose leading to the advance unit. There's a good chance neither are working properly. Until that is fixed, performance and your idle will be poor at best.

TR3driver
12-03-2008, 01:05 PM
You should see over 20 more degrees from the vacuum advance You sure about that, Jeff? My data shows that, for example, a Lucas 40795 as used on late TR3A and early TR4 should have a 54413565 advance capsule, rated as 2-6-3, meaning it supplies a maximum of 3 camshaft degrees (6 crank degrees) of advance.

I agree entirely with everything else except the vacuum advance number.

Roger
12-03-2008, 01:37 PM
Yes, 40deg advance would be something!

JeffS
12-03-2008, 01:53 PM
Sorry, you're right about the vac advance! Most vac units of that era were in fact 10-12 degree (20-24 crank) units though.

40 degrees of advance is not out of the question, if it includes vacuum advance. In fact, that's a low number. Vacuum advance is ALWAYS eliminated at WOT, so it doesn't play a part in the "total timing" figure you hear about in performance setups. 36-37 degrees of total mechanical advance is very common, even in race applications, plus at least 6-10 degrees of vacuum advance for street cars. TR3s with only 6 degrees of vacuum advance are the lowest made.


Look at some later model stuff if you want to see over advancing! Late model MGBs with 36 crank degrees of advance, plus 6-8 static, plus 24 crank degrees of vacuum advance = roughly 77 degrees of total advance.

Got_All_4
12-03-2008, 08:17 PM
He also said the vacuum advance is hooked up. Sounds like he may not have understood your advise that the vacuum line should be disconnected when adjusting ignition timing.

glemon
12-03-2008, 10:16 PM
I am sorry, 2000 RPM minimum RPM is high. I don't know how something wrong with the dist. timing could make it idle that high, timing will affect RPM, but the primary control is how much air and fuel get, in, I agree most carb problems are electrical, but I would bet this is a case of too much air getting in, throttle discs can be set in wrong so the thing catches as doesn't close, or something else stuck in the linkage or carbs, or a bad leak, but it would have to be bad leak, I have had vacuum hoses off etc, and it doesn't make that much (an extra 1000 RPM or so) difference.

TR3driver
12-03-2008, 10:27 PM
Having the advance connected while setting the timing should actually make no difference at all; but if it did make a difference, it would result in setting the timing too slow; which would cause low idle rpm.

Given the information above, I would be looking for what is holding the throttle butterflies open. They may not be properly centered in the bores; or the throttle linkage may be binding. If 'Chester' replaced the throttle shafts, they may even not be properly drilled (so the stop levers hit the stops even with the screws backed out). If he didn't, they may be so badly worn that the thottle discs hit the side of the throat even with them properly centered.

Having the vacuum and centrifugal advances not working is also a problem; but since they should not be active at idle, that would not cause high idle.

Be nice to know what cam you have, as that will definitely affect idle quality. Moderately late timing (like 5-10 ATDC) should not make it run rough, so I kind of suspect that you were getting below your cam's idle rpm. My "3/4 race" cam didn't like to idle below 1500 (and wasn't particularly smooth even then). Valve lash can also have an effect on idle quality, and most performance cams take more gap than the stock cam does.

Another question is how accurate the tachometer is. Old tachs frequently read high; as the wear in the mechanism lets the magnets droop closer to the cup.

tinman58
12-04-2008, 08:17 PM
Wow thanks for all the info
Randell the cam is a BP270 with a 270 duration intake 410 and exhaust 400.
I did take my 5/16 tube and tried to find a intake leak vacuum leak all around the carbs but found nothing
P.S. I just recived the Panasport rims this afternoon from Moss.
190/65 15 rubber?????
Dan