Forum MG

ttersu a écrit :Pas forcément...J'ai eu à m'en mordre les doigts avec des bougies "glow" de modèle réduits...un avion au tas à cause d'une bougie OS chaude qui ne l'était pas suffisamment comparé à sa cousine de chez ENYA...

yvre a écrit :Ça devient un forum de " cul " ici ...

saby a écrit :quelles bougies utilisez vous?

des NGK BP6ES :-), sans problèmes

Jidé de Grasse a écrit :

saby a écrit :quelles bougies utilisez vous?

des NGK BP6ES :-), sans problèmes

j'ai aussi mis des Beru équivalentes pendant une paire d'année sans constater de différence

JPierre a écrit :Kesako les bougies OS? En recherchant j'ai du des sortes de culots de bougies sans électrodes...

dodo35 a écrit :

JPierre a écrit :Kesako les bougies OS? En recherchant j'ai du des sortes de culots de bougies sans électrodes...

C'était pour répondre à ttrsu
Les bougies OS sont destinées pour des moteurs aéromodélisme 2 temps fonctionnant au méthanol.
Il n'y a pas d'électrodes, juste un filament que l'on fait rougir pour pouvoir démarrer le moteur et qui reste rouge tant que ça tourne.

JPierre a écrit :Ahhh...

Dan, non, je ne serre pas fort mes bougies et je mets toujours de la graisse au cuivre. Peut-être étais-je de traviole...? Elles ont pété sans forcer dessus vraiment facilement.

Je n'ai pas de préférence pour telle ou telle marque de bougie, et je n'avais à ce jour jamais eu de problèmes utilisant des NGK ou Beru sur mes differents véhicules. Là le problème est flagrant: Champion bien, NGK pas bien, c'est pour ça que j'aimerais comprendre. Écartement vérifié : 08

Je verifierai l'avance et l'écartement des rupteurs dans la semaine à la strobo.

Vu que le problème survient vers 2800-3000trs, à combien devrait être l'avance à ce régime?
De mémoire, j'avais réglé 10° à 800 trs environ

MG O1->B69 a écrit : Lorsque vous rencontrez ces "à coups", que se passe t'il au niveau du compte tours?
Avant de partir sur des pistes hypothétiques, souvenez vous que sur HIF4, les pop-up valve peuvent semer le trouble et provoquer des "flat spots" qu' un enrichissement provoqué peut palier ou masquer artificiellement.

J'avoue mon incompétence côté carburation ( SU allumage et autres d'ailleurs)

S'il y avait irrégularité d'allumage au régime, je pense que le problème serait présent aussi avec les Champion je pense.

An Introduction
This page documents a two year long investigation, trying to understand why my SU equipped vehicle stumbled at cruise.

Just because this page is under my MG section, don't for a minute believe this is the only marquee that can have this problem. And it's not just limited to our LBC's. Even the computer controlled vehicles of today can have similar problems.


Forward
Acceleration, and all other modes of throttle were fine. Only under a steady state cruise speed, at any speed, the car would seem to hesitate or stumble. My first impressions were SU carb needles. Why? Well I had converted to K&N filters and was running a 2" Peco free flow header and rear box. Other than that, my 1973 MGBGT was basically stock as far as engine modifications were concerned.

Over the initial two year period of ownership I have gone through quite an exercise, trying to resolve this issue. Some of the experiments, included swapping distributors and playing with the mechanical advance curves. I tested multiple vacuum advance curves (units), had played with ported and manifold vacuum, and finally went as far as to buy brand new SU AUD405 HS4 carbs. Not the newer 'Burlen Fuel Systems' units, but original Skinner Union carbs. (check out Midel Pty Ltd. in Australia. They were nearly $100 cheaper including shipping, than trying to get a pair from Kansas here in the states!) Needles tested included the original AAE, then ADB, then AAA. Then I put in fixed needles and tested #6 and #7 needles. All my needles came from
Joe Curto, Mr. SU himself (718-762-SUSU (7878) ), regarded as one of the best SU rebuilders in existence. I have no financial interest in these two sources. I mention them here directly, as I believe them to be two of the best resources on the planet for SU carburator parts and service.

The thing that made this such a bugger to resolve was the impression that when I pulled out the choke during cruising, the problem disappeared. Hence all the needle and carb experimentation.
After a couple of months now, I'm sure I have the problem resolved.
Trying to find a way to describe everything I went through, everything that was tested and played with, and explain the actual problem, is the reason this page took so long to come to fruition. Trying to figure out how to present it all. So... I believe including the following email threads/conversations, is the easiest way to explain what happened.

Reading ALL the emails I know is boring compared to all the pictures on all the other pages on this site. But if you read them all, I guarantee you will learn something you never knew before, whether directly related to this problem or not.
The details of the problem and it's resolution will reveal themselves as you read the emails.
I'd like to thank everyone (listed here or not) for all the input over the years and their wise cracks as well as their wisdom. Even if someone doesn't hit the nail on the head, it sometimes sparks a thought that resolves an issue. So no matter how wacky the world can seem, .... listen carefully. You may hear your own voice screaming at you!

************* Big clue here folks! ****************

----- written to R.O. Lindsey 'off-list' ---------
Well if you've been following my stumble/surge threads... you're giving everyone a hint as to what was going on my '73 BGT.

Here's the deal. You don't repeat this.. yet (to the lists) And I'll give you a clue on how to determine your answer correctly. I'm still writing up details for my web pages.

To start with.... check
http://www.teglerizer.com/mgstuff/advance_curves.htm
and also not too bad but a bit confusing to read)
http://www.mgbmga.com/tech/mgb21.htm
for a complete list of curves, part numbers, specs.


On your dist. body, you'll find a flat area that has one of the numbers listed on the chart stamped into the body of the dist. This identifies the mech. advance curve.
Around the neck of your vac adv. unit (right near the vac line attachment) will be tiny numbers stamped into it. The long number will match one in the chart as well. Make sure the right vac unit is on the right dist. per the chart. The chart also lists the 'ported' or 'manifold' vac source for setup requirement for that unit.

I had rebuilt everything to '73 specs. Had a '73 Spec distributor with the correct vac unit for the dist. In my case... a 41491 dist with vac unit 54425359 running 10.15.5 vac connected directly to the manifold (See the teglerizer chart)

Apparently both the vac unit and dist model did not match the motor/cam config which WAS causing this stumble at cruise. I was using manifold vac (correct spec for the 18V '73 model engine I 'thought' I had.

Turns out my engine is a 68-70 model as far as cam is concerned and has a HC (high compression) head on it. I put in a spare 41288 dist I had laying around with the correct matching 54411985 vac unit (5.13.10) and hooked up the vac source to the port on the carb.

I initially reset timing to 10... then bumped it up to 14 BTDC at 600 rpm.

BINGO! I can actually set the idle as low as 550 rpm without dying. Starts even easier than before (even in cold weather) I was able to lean out the carbs a bit. and have more power at the low end (upper end seams un effected by the change over)

So .... to tell what you need (what runs best regardless of what you think, or what the books say it should be.... you need to determine what you have...and then test a few variations. I'm sure you can find a friend with a car that has some setup different then yours and experiment.
I didn't even change dist. caps. It took less than ten minutes to swap distributors, hook up the vac lines and retime it.

I was actually a little pissed-off that it was such a simple fix to my problem. In the past I had replaced carbs (brand new ones) tried two different elec. ign. systems as well as points ran no vac/with vac played with mech. advance weights... all kinds of stupid stuff with no change in this problem.
The distributor changed everything. Even my lady friend says it feels like a different car.

I haven't sent this info off to the list.

Paul Tegler

La réponse:

Apparently both the vac unit and dist model did not match the motor/cam config which WAS causing this stumble at cruise. I was using manifold vac (correct spec for the 18V '73 model engine I 'thought' I had

saby a écrit :Si je lache l'accélérateur et rappuie ça repart ou si je mets le starter, ça repart aussi..

carburation trop pauvre, il faut des aiguilles plus riche, dans la zone 2500/3000.
(c’est sans doutes du pipo, mais sur les SU à la reprise il y aurait un effet pompe de reprise)

saby a écrit :Si je lache l'accélérateur et rappuie ça repart ou si je mets le starter, ça repart aussi..

carburation trop pauvre, il faut des aiguilles plus riche, dans la zone 2500/3000.
(c’est sans doutes du pipo, mais sur les SU à la reprise il y aurait un effet pompe de reprise)

c'est que ça fonctionnait très bien avant