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MG MGB Technical - carburettor weirdness

I have a weird problem with 2 different carb combos on an 1850 with "butch gilbert cam" & 10:1 CR; permatune CD ign, 155 compression on all 4; timing is approx 30 BTDC at 3000 rpm, 12 static; 3 psi fuel pressure

1. pair of SU 2" from Huffaker, idle & run well up to about 3800, then when throttle is punched the thing bogs badly & can eventually be coaxed up, but doesn't pull well at all above that; also "trailer hitches" or bucks at low rpm in lower gears in response to slight throttle touches; carbs appear to be o.k. upon inspection & fule level is correct

2. so i acquired & tried a weber 45 DCOE with Cannon Y manifold off a different MGB and this is REALLY weird: it is only running on cyls 2 & 3 at idle, if you pull off the plug wires from 1 and or 4 or both it actually runs smoother! At higher rpm it is less noticable but still not hitting on the 2 outboard cyls

Larry Moeller

one more bit of info: with the weber the thing revs quite well all the way to 6000 under load on the road, no bogs, so that tends to eliminate the ign as a source of the problem
Larry Moeller

Larry,
Two things come to mind, the cam and its timing, perhaps the rocker clearance as well. The other is the siamesed intake pulses togather with a "wild" cam. Does the weber and/or the 2" Su manifold have a balance pipe or tube connecting the two runners? Only other idea would be that the cam timing is off.

HTH

Ron
Ron Smith

Larry. Do not know what a "butch gilbert cam" is, but it sounds like you have something in excess of the "basic big bore" engine as described by Peter Burgess. Do you have a copy of his book, "How to Power Tune MGB 4-Cylinder Engines"? I notice that Peter has only gone up to the -6, or 1 6/8" SU carbs on his engines. Thus, his experiments tend to indicate that going larger than a 1.75" venturi does not produce good results. Since your original carbs are larger than this, I wonder if there is insufficient flow through the larger carbs and this is creating your problem.

As to the problems with the DCOE, most of us do not use them. Hence, little experience available. Peter Burgess has done some work with them as has Daniel Wong who posts here from time to time. I do know, from what Danny has written me, that the set up of the DCOE is more complex than the SU or Weber DGV series and that the DCOE seems to need to be set up for a specific engine if it is to perform properly. An e-mail to Peter, through his website, and a post directly to Danny might be in order. Steve S. may also have some input on this as he is also an advanced experimentor. Les
Les Bengtson

I had the same symptoms on my 1995cc/Weber DCOE. The biggest improvement came with fitting a new fuel pump. I'm guessing that weak output in the old one caused the float chamber in the Weber to fill poorly (apparently they're v sensitive to float level so maybe check this too). At any rate, the fuel wasn't atomising well and could be seen rumming along the walls of the inlet manifold at higher revs. Changing the pump brought cylinders 1 and 4 on stronger at higher revs when they'd been useless before. That said, the head/cam/Weber combination means that nothing much does happen when you pull the leads on 1 & 4 unless the engine is under load. Don't know why, that's just how it is. Peter Burgess did give me an explanation at the time but I can't begin to do it justice so I won't try.
Steve Postins

Meant to say, could 3psi be too high and, like Les says, 2" SUs be too big? Sounds a lot for 1850cc. What jets/chokes are on the Weber?
Steve Postins

This thread was discussed on 30/03/2004

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