Carb Problem ???

[kiwimark]

Hi All

I have a miss in the engine on by 72 BGT that only manifests its self on constant speed running. It ranges from mildly annoying to really annoying. When the engine is under load it runs smooth. Only when on the level with a constant throttle position does it manifest its self. It feels as if the car is hitting soft patches of sand as it moves along the road. It is accompanied at times by a crackling sound through the carbs. I have fully rebuilt the carbs with new everything. The needles are centered and I cannot find any vacuum leaks. The timing is set a 14 deg at 1500 rpm and the disributor is new and set correctly. New plugs leads etc. It seems to get worse as the car warms up. It is almost as if the carbs are running lean at high revs. I have tried richening the mixture to no avail. When I set the carbs I did so as per the manual and fine tuned the mixture with a Gunsen Colour tune.

This has me beat. I have even compression and no blow to the crank case.

Your insight would be appreciated.

Regards

Mark

[Jimmy]

I'll be keeping a close eye on this one, as my Beater B is doing the same thing.
I never thought it felt like driving over sand (I assume you mean that there's a hesitation, Mark), but indeed strongly suspected a lean running condition...and the fact that pulling the choke out about halfway "fixes it" did, in my mind, confirm that.
Since I haven't gotten around to doing any troubleshooting, my lazyness will now pay off.

[malcolmr18zoy]

Hey Jimmy,
If you like, I'll fix your car whilst you watch, then you can tell Mark how to sort his car out.
Malcolm

[Jimmy]

Oh, you really do want your hardtop and money back, don't you, Malcolm?

[malcolmr18zoy]

Just a suggestion. I find it easier to do, than to write about, and you're the journalist.
Malcolm

[Jimmy]

Write about?
I was just gonna call Mark and let him know what you did.
But if you insist (bastard), I guess I'd rather write than troubleshoot it myself. Normally I end up doing both the wrenching and the writing, so I guess I'd be ahead no matter what.

PS. Mark, if you come over and watch, I'll really be styling.

[tannyo]

While you're at it, take a few pictures.

[malcolmr18zoy]

Wow, its going to be kind of crowded at my place, all these people and their cameras.
Malcolm

[Jimmy]

At YOUR place?!
Okay, all attendants, be sure your tetanus shots are current before entering Malcolm's premises.

[max71]

Is this the fabled Tune up session that Steve S never scheduled but teased us all with?

[malcolmr18zoy]

Definitely not. Better forget the whole thing. I do not want to do a tech session at my place, and let everyone tell me where I'm going wrong. I'll just tell Jimmy to set his ignition timing, and balance his carbs, and let him do it himself. Then he won't need to go to the expense of tetanus shots.
Malcolm

[max71]

Bummer.

Someone should advise Jimmy of that old saying you get more flies with honey. He seems to subscribe to the more vinegar is better.

And not that tasty vinegar that you put on your fish n chips Malcolm.

[Jimmy]

With horses on the property, there's no need for more flies.

[JerryB]

vacuum leak? Carb bases, phenolic spacers, manifold to head, any vac hoses. Check w/ motor hot.

also use a Mite-Vac on the advance diafam to see if that bleeds down (it should hold steady vacuum) ..apply about 12-14 " to it for a minute or so.....needle shouldnt move.

[kiwimark]

Hi All
solved the problem with the miss in the BGT. Sold the car!!. A buddy wanted it for a V8 conversion so I took his 69 in a swap. This one has a big bore engine, fast road cam, HS6 carbs etc as well as O/D. Runs way better. The body is not quite as good but I still think I have the better end of the deal.

Regards
Mark

[Jimmy]

Okay, Mark, does that mean I can trade the Beater B for your new GT so that I can get my problem(s) solved, too?

[VWNate1]

Almost assuredly this was a lean running problem (I'd have re-checked the timing first though) as I suffered the very same thing on my old 1967 MGB GT after going through the entire engine , re-bushing the HS4's throttle shafts rebuilt dizzy etc., etc. I had both carbys *perfectly* balanced (a mean feat considring my poor hearing these days) so it lit right off and idled perfectly , had gobs of power and whizzed through the A/F ratio tester but in the end , I had to have someone else look & listen to it , he *instantly* said ' it's a tad too lean ' and lowered both jet bridges by maybe two flats , then it ran even better and the odd running @ 3,000 RPM problem dissapeared never to return .

If Malcom ever offers to tweak your engine , GO for it ! .

[kiwimark]

Hi All
I did finally find out what was causing the constant speed miss. The breather on the side plate of the engine was crudded up, causing positive crankcase pressure. When cleaned and hooked back up to the proper carb vacume port the miss was missing. Thanks for all the ideas.

Mark